hybrid supercapacitor-battery energy storage … · supercapacitor and battery materials are mixed...

Hybrid Supercapacitor-Battery EnergyStorage

Mainul Akhtar and S. B. Majumder

ContentsIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Supercapacitor and Li-Ion Battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Supercapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Lithium-Ion Battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Hybridization Approach: Toward High Energy and Power System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Internal Serial Hybrid (ISH) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Internal Parallel Hybrid (IPH) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Future Direction of the Research on Hybrid Supercapacitor-Battery . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

AbstractHybrid supercapacitor-battery is one of the most attractive material candidates forhigh energy as well as high power density rechargeable lithium (Li) as well assodium ion (Na) batteries. Mostly two types of hybrids are being actively studiedfor electric vehicles and storage of renewable energies. Internal serial hybrid is anasymmetric electrochemical capacitor with one electric double-layer capacitorand another battery-type electrode. On the other hand, in internal parallel hybrids,supercapacitor and battery materials are mixed together to form bi-material-typeelectrode. A brief literature review provides the state of the art of variousasymmetric electrochemical capacitors reported in recent times. Subsequentlywe have described the role of current densities and electrode potential window indesigning the internal serial hybrid electrodes. Mass ratio between the twoelectrodes grossly influences the electrochemical performance of internal serial

M. Akhtar · S. B. Majumder (*)Materials Science Centre, Indian Institute of Technology, Kharagpur, Indiae-mail: [email protected]; [email protected]

© Springer Nature Switzerland AG 2019Y. Mahajan, R. Johnson (eds.), Handbook of Advanced Ceramics and Composites,https://doi.org/10.1007/978-3-319-73255-8_43-1

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hybrids. Theoretical basis of the calculation of voltage, specific capacitance,energy, and power densities of internal serial hybrid has been described in details.The theoretically estimated parameters match quite well with the experimentallyobtained values for activated carbon (AC)//lithium nickel manganese oxide(LNMO) asymmetric electrochemical capacitor made in our laboratory. As com-pared to serial hybrid, limited reports are available on internal parallel hybrid forLi- and/or Na-ion batteries. A brief literature review on this type of hybrids ismade to illustrate the outstanding research issues of this type of hybrids. We havereported excellent electrochemical performance of sodium vanadium phosphate(Na3V2(PO4)3)-activated carbon (AC) bi-material electrodes for lithium-ionrechargeable batteries.

KeywordsLi-ion battery · Supercapacitor · Internal serial hybrid · Internal parallel hybrid ·Energy storage

Introduction

Underwater vehicles for the use of naval defense require high power as well as highenergy rechargeable batteries in order to travel long distances at high speed. Due toboth weight and space limitations, we need novel rechargeable batteries to replacebulky zinc-silver oxide (Zn-AgO) batteries currently used by navy for their under-water vehicles. In addition to its weight and size, imported Zn–AgO batteries areexpensive and have limited wet life (maximum of 9 months since alkali electrolytesare filled), higher self-discharge rate, and long charging time (15–20 h). Alsoformation of Zn dendrite and evolution of hydrogen gas during operation poseserious safety issue, and more importantly these batteries fail to deliver adequateenergy and power densities demanded by modern underwater vehicles [1].

Li-ion rechargeable batteries with higher energy densities, longer life, maintenance-free operation, and lower self-discharge rate could be an effective alternate to Zn–AgObattery. Performance of Li-ion batteries desirable for underwater vehicle applicationsis mostly dependent on characteristics of electrode materials such as capacities, higherpotential for cathode/lower potential for anode, reversible Li+ intercalation, reasonablygood electronic conductivity, stability, and lower cost. To the best of our knowledge,none of the commercially available cathode and/or anode materials (used in Li-ionbatteries for consumer electronic applications) are able to provide both higher energyand power densities required for applications relevant for security and defense.

In addition to develop custom-made battery modules for defense-related applica-tions, the need of sustainable energy storage devices is increasing day by day.Traditionally natural resources (mainly coal, gasoline, petroleum, etc.) are beingused as energy source for high-scale energy applications. These energy resources arenonrenewable, and their use pollutes the ambient. To attain cleaner ambient, exten-sive research is now been pursued to replace fossil fuels with various renewableenergy resources. Solar power and wind energy have emerged as popular clean

2 M. Akhtar and S. B. Majumder

efficient energy resource. But the implementation of these renewable energies dealwith some serious concerns – capital cost and intermittent nature of energy produc-tion as sun does not shine or wind does not blow always [2, 3]. Adequate researchand development is needed to make renewable energy resources as reliable andprimary sources of energy. In such case energy storage plays a crucial role for thegrowth of these technologies to make them commercially viable. Various technolo-gies are used to store renewable energies [4]. Electrochemical energy storage isconsidered one of the most viable technologies for its integration with renewableenergies. In electrochemical energy storage, electrical energy from renewablesources is stored in the form of chemical energy. Rechargeable Li-ion battery andsupercapacitor are considered most useful for electrochemical energy storage [5].

Li-ion batteries store electrical energy in the form of chemical energy, and energyconversion occurs by redox reactions at the anode and cathode. A discharging batteryhas two terminals – positive terminal or cathode (where redox reaction occurs at highervoltage) and negative terminal or anode (where redox reaction occurs at lower voltage).Cathode and anode are relative terms, the more positive electrode is called cathode, andthe more negative electrode is called anode. Li-ion batteries can store high energy(� 200 Whkg�1) as the whole volume of the active mass participates in the redoxreactions. But Li-ion battery takes hours to charge and discharge. The rate of charge anddischarge process is limited by the solid-state diffusion of Li ions in the bulk of theelectrode materials. Since Li+ diffusion is a slow process, it requires several hours tocharge a battery. High rate discharge also deteriorates the capacity of Li-ion cells. Li-ionbatteries are considered as high energy but low power density storage device. Li-ionbatteries are heavily used in portable electronic market for powering mobile phones,computers, laptops, digital cameras, mp3 players, watches, etc. Recently they are alsobeing used in large-scale applications like electric cars (HEV, PHEV, etc.), power backupfor buildings with rooftop solar or windmill installations, to support power grid, etc.

Conventional supercapacitors (termed as electrochemical double-layer capacitor(EDLC)) store electrical energy by electrostatic adsorption and desorption processes,and no redox reaction is involved in energy conversion. On the basis of cell structure,these capacitors may be symmetric (same electrodes on both sides) or asymmetric(two different type electrodes) in nature. On the basis of the nature of electrodes(positive or negative), the electrolyte ions are attracted toward the electrodes; henceelectric double layers are formed at the electrode/electrolyte interfaces. Super-capacitors can be charged or discharged very fast, as charged is stored due to theelectrostatic attraction and no chemical reaction is involved. Supercapacitors cannotstore high energy as the electric charges are accumulated only on the surface of theelectrodes. The full volume of the electrodes cannot be used in this process. Hence,supercapacitors are considered as a high power (~ 3 KWkg�1) and low energydensity (~ 5 Whkg�1) storage device. If high power is repeatedly needed for shortperiod of time, supercapacitor is considered to be the best choice. For example, oneof the interesting uses of supercapacitor is for regenerative breaking (i.e., temporarystorage of energy when a vehicle comes to stop and then reused when it starts tomove again) in automobiles. They are also used in consumer electronics such as inphotographic flashes, portable media players and speakers, etc. Recently they are

Hybrid Supercapacitor-Battery Energy Storage 3

also used in hybrid electric vehicles, electric buses and backup power for windturbines, etc. Supercapacitors can also offer the possibility of designing powersystems more efficiently for soldiers. A soldier relies heavily on batteries forelectrical power in the battlefield. Use of a variety of versatile electronic equipmentin the battlefield demands ever-increasing power of the battery carried by thesoldiers. Soldiers nowadays are carrying numerous types of batteries to powerthese electronic equipments [6]. As a viable solution, one should design a rationalpower distribution system for soldiers utilizing a smarter battery managementsystem (BMS). Such BMS would use supercapacitors in conjunction with highenergy density rechargeable batteries to reduce the dead load of existing batteries.

The electrochemical properties of Li-ion cell and supercapacitor are complemen-tary to each other. According to the specific needs of various consumer electronicdevices, electric vehicles, and storage of renewable energies, Li-ion cells shouldyield appropriate energy and power density values. Like in hybrid electric vehicles,the high energy of battery aids the vehicle to cover long distance, and high powerdrives it at high speed. Li-ion battery and supercapacitor cannot singly fulfil therequirement of high energy and power as Li-ion battery suffers from high power andsupercapacitor from storing high energy. Hence from the last decade, research hasbeen initiated, and various approaches have been persuaded to improve the energyand power in a single device. One of them is the combination of high energy densityLi-ion batteries and high power density supercapacitors in a single device calledhybrid supercapacitor-battery, a novel energy storage system, which is expected toshare the advantageous features of each individual component [7]. Figure 1 showsthe Ragone plots of the energy-storing devices, the X-axis represents how muchenergy system contains, and Y-axis shows how fast that energy can be delivered. Thehybrid battery-supercapacitor system stands in between the energy spectrum ofsupercapacitor and battery and acts as a bridge between them. The energy densityof the hybrid system is greater than supercapacitor and less than battery, and powerdensity is greater than battery and less than supercapacitor. Our chapter is devoted todescribe the state of the art of such supercapacitor – battery hybrids and its alliedvarieties. As and where it is appropriate, we have also described our laboratoryresults on these types of hybrid energy storage systems (Table 1).

Fig. 1 Ragone plot forsupercapacitor, battery, andsupercapacitor-battery hybriddevices


Definitions

The following are the list of battery definitions and terminologies [8]:

Battery: An electrochemical device that produces electrical energy from storedchemical energy. It consists of two or more electrically connected cells. Cell isthe basic building block of battery. The main components of a cell are anode,cathode, and an electrolyte in between them.

Supercapacitor: The electrochemical capacitors are referred as supercapacitor orultracapacitor. They are of two types – electrochemical double-layer capacitor(EDLC) and pseudocapacitors. The EDLCs store energy electrostatically in anelectric field. The pseudocapacitors store energy by faradaic electron chargetransfer reactions.

Capacity: The amount of energy can be withdrawn at a certain discharge currentfrom a fully charged state of a battery. It is denoted by Ah or mAh or Wh.

Capacitance: It is the ability to store electrical charge of a capacitor. SI unit is farad(F). Capacity can be converted to capacitance by the following equation if thecharge-discharge curve is linear.

Cs F=gð Þ ¼ Specific capacity mAhg�1� �� 3600

dV mVð ÞC-Rate: The measure of the rate at which the battery is charged and discharged. 10C,

1C, and 0.1C rate means the battery will discharge fully in 1/10 h, 1 h, and 10 h.Specific Energy/Energy Density: The amount of energy battery stored per unit mass,

expressed in watt-hours/kilogram (Whkg�1).Specific Power/Power Density: It is the energy delivery rate of battery, expressed in

Watts per kilogram (Wkg�1).

Table 1 Advantages and disadvantages of Li-ion battery and supercapacitor

Advantages Disadvantage

Li-ion battery1. High energy density (100–250 Whkg�1)2. Nominal cell voltage 3.2–3.3 V3. Flat voltage discharge plateau4. Rate of self-discharge very low (<5% per month)5. Less costlier than supercapacitor ($ 0.25 to 1/Wh)

1. Low power density(250–340 Wkg�1)2. Limited cycle life (<1000 cycles)3. Long charging time (>1 h)4. Need protection circuitry from beingovercharged or overheated5. More temperature sensitive thancapacitors

Supercapacitor1. High power density (1000–3000 Wkg�1)2. Long cycle life (500 and higher)3. Very short charging time (within few minutes)4. Excellent temperature performance, due to lowinternal resistance does not heat as much as batteries

1. Low energy density (~5 Whkg�1)2. Low cell voltage (2.5 to 2.7 V)3. Linear discharge voltage4. High self-discharge (40 to 50% permonth)5. High cost (~ $ 10 /Wh)


Supercapacitor and Li-Ion Battery

Electrochemical energy storage devices (viz., fuel cell, battery, supercapacitor, etc.)convert the stored chemical energy or electric potential energy into electrical energyand vice versa. The energy conversation reaction takes place at the interface ofelectrodes and electrolyte. All types of the electrochemical cell comprise of mainlythree components – a negative electrode, a positive electrode, and an electrolyte. Theions and electrons transfer between the negative and positive electrode in anelectrochemical cell; ions transfer through the electrolyte and electron through theexternal circuit [5]. The basic schematic has been shown in Fig. 2. When anelectrochemical cell is connected with the charging circuit, some finite charge isstored in the cell. During discharge of the cell, stored charge is released by providingcurrent through the load (R). On the basis of mechanism of energy storage and energyconversion inside an electrochemical cell, the electrochemical energy storage devicesmay be of different types. The energy storage and energy conversation process insupercapacitor and Li-ion battery will be discussed details in the following section.

Supercapacitor

The electrochemical capacitor sometimes referred to as supercapacitor or ultra-capacitor is a unique energy storage device which bridges the gap between conven-tional capacitor and the batteries in terms of their working mechanism andproperties. The electrochemical capacitors are constructed in a same fashion likebatteries in which two electrodes are immersed in an electrolyte and physicallyseparated by a separator [9]. Based upon the energy storage and energy conversionmechanism and current R&D trends, electrochemical capacitors can be divided intothree general classes – electrochemical double-layer capacitor (EDLC), pseudo-capacitor, and hybrid capacitor. The EDLCs store charge by non-faradaic process(no oxidation-reduction reaction takes place), the pseudocapacitors use faradaicprocess (involves the charge transfer chemical reaction between electrode andelectrolyte), and the hybrid capacitors use a combination of both faradaic andnon-faradaic processes.

Fig. 2 Schematic of a typicalelectrochemical energystorage system


Electrochemical Double-Layer Capacitor (EDLC)Like conventional supercapacitor, EDLCs also store charge electrostatically ornon-faradaic process, but they do not possess any dielectric. Figure 3 schematicallyshows the formation of various electrochemical double layers following Helmholtz,Gouy-Chapman and Stern-Grahame model. The electrochemical double-layermodels are described in the following section [2, 10].

The simplest and first Helmholtz theory was given by Hermann von Helmholtz. Itpostulates that the surface charge is neutralized by an array of opposite signcounterions. The thickness of the counterions (which forms the compact layer) islimited by the size of the solvent molecule. Essentially the double layer is formed bytwo layers of opposite charges separated by distance d. The electrode surfacepotential is linearly dissipated from the surface to the outer Helmholtz plane. Thesecond model, the diffuse layer model or Gouy-Chapman model, was proposed byLouis George Gouy and David Leonard Chapman. It was suggested that counterionsare not rigidly held with the charged surface; rather they diffuse into the electrolytedue to their thermal motion. The concentration variation of the counterions near thecharged surface follows the Boltzmann distribution, and hence the electric potentialexponentially decreases away from the charged surface of the electrolyte (Fig. 3).The third and more reliable model, known as Stern-Grahame model, basicallycombines the Helmholtz and Gouy-Chapman model in series. In this model, someions are especially adsorbed by the surface as suggested by Helmholtz and form theStern layer, while the others diffuse through the bulk of the electrolyte and form the

Fig. 3 Schematic representation of electrochemical double-layer models – (a) Helmholtz model,(b) Gouy-Chapman model, and (c) Stern-Grahame model in which IHP and OHP represent theinner and outer Helmholtz plane


Gouy-Chapman diffuse layer. The potential drops linearly up to the Stern layer.Thereafter exponential potential drop extends to the bulk of the electrolyte.

Figure 4 shows a typical double-layer capacitor in charged condition. When anexternal voltage is applied, the surface of electrodes in EDLC are become charged.The positive charged electrode attracts equal number of negative charges which arediffused across the separator and form anion double layer at positive electrode-electrolyte interface. The same process takes place at the negative electrode tomaintain the charge neutrality of the system, as it attracts the positive ions fromthe electrolyte and forms cation double layer in the opposite side of the cell. So acomplete electrochemical double-layer capacitor possesses two electric doublelayers on the interface of positive and negative electrodes. As the complete capacitoris formed by two capacitors in series, the total capacitance (CT) is calculated asfollows:

1

CT¼ 1

Cþþ 1

C�(1)

In Eq. 1, C+ and C� are the capacitance value of positive and negative electrodes.The double-layer capacitance can be calculated as follows [11].

Cdl ¼ Q

V¼ e0er

A

d(2)

where C is the capacitance of a single electrode, Q is the stored charge at voltagewindow V, e0 and er are dielectric constants of electrolyte and vacuum, A is theexposed surface area of electrode, and d is the charge separation distance. Thecapacitance of capacitor is independent of voltage. Capacitance value depends onthe characteristics of the electrode materials (surface area and pore size distribution).When Cdl is constant for EDLCs, then response current (I ) can be derived as follows[11, 12].

Fig. 4 Schematics of theworking principle ofelectrochemicalsupercapacitor


I ¼ dQ

dt¼ d CdlVð Þ

dt¼ Cdl

dV

dt¼ Cdlυ (3)

For an ideal capacitor, if the voltage scan rate (dV/dt) is kept constant, theresponse current (I ) will also be constant, which results a perfect rectangular-shapedI-V curve (Fig. 5a). Any deviation from the rectangular shape denotes non-idealcapacitor behavior (i.e., some contribution of pseudocapacitance) as shown inFig. 5b. If a double-layer capacitor is charged and discharged at constant currentrate (i.e., I is constant over the charge-discharge period), then according to Eq. 3,dV/dt will be constant (Fig. 6a). This results in a triangular-shaped charge-dischargebehavior for ideal capacitor. For non-ideal capacitor (introduces pseudocapacitivenature), the nonlinearity is introduced in the charge-discharge behavior (Fig. 6b).Carbonaceous materials (viz., activated carbon, carbon nanotube, carbon aerogels,carbon nanofibers, graphene, etc.) are generally used for making electrode ofelectrochemical double-layer capacitors for their high surface area and porousstructure [13].

PseudocapacitorThe other type of electrochemical capacitor is termed as pseudocapacitor (alsonamed as redox capacitor) which stores the energy by faradaic processes but stillbehaves like a EDLC. In this type of capacitor, charge transfer reactions occurbetween the electrode and electrolyte at the surface or near-surface region of theelectrode. The charge transfer reactions include electro-sorption/electro-desorption,oxidation/reduction, and also doping/de-doping, especially in active polymer elec-trodes. Like EDLCs, the accumulation of charge in pseudocapacitor is potentialdependent. But the reactions in pseudocapacitor are faradaic in nature unlike elec-trostatic in EDLCs. In pseudocapacitor, the faradaic reaction occurs over a wide rangeof potential, unlike battery where the redox reaction occurs at constant potential resultsin a flat charge-discharge profile [9, 14]. The electrode materials use pseudocapacitive

Fig. 5 Cyclic voltammograms of (a) ideal capacitor (schematic) and (b) activated carbon(AC) (measured at 2 mVs�1)


energy storage including transition metal oxides (RuO2, MnO2, etc.) and conductingpolymers (polyaniline, polythiophene, polypyrrole, etc.) [15].

Pseudocapacitive materials behave more like a capacitor in its electrochemicalsignature (i.e., stored charge varies almost linearly with the voltage in the selectedpotential window), but charge storage involves faradaic process typically exhibitedin battery materials. Oxides such as RuO2 and MnO2 show pseudocapacitivebehavior. The battery materials on the other hand mostly yield flat charge-dischargeplateau, and the oxidation and reduction peaks (during charging and discharging,respectively) are clearly identified in cyclic voltammetry curves. However, insmaller dimensions (viz., as reported in 6-nm-thick LiCoO2 thin film), the batterymaterials might behave like a pseudocapacitor as most of the reactant sites of theelectrode are exposed to the surface in contact with electrolyte [16, 17].

Electrochemical MeasurementFigure 7 shows a typical charge-discharge profile (measured at 50 mAg�1) of anelectrochemical capacitor. The capacitor is charged at t1 seconds and then dischargedat t2 seconds in the voltage window of V2 to V1. The initial direct fall of voltage(from V2 to Vw) during discharge is due to the internal resistance acting like aresistor in series with the capacitor called equivalent series resistance (ESR). Thespecific capacity, specific capacitance, energy density, and power density can becalculated from the V-t graph as follows:

The specific capacity of the capacitor can be expressed as follows:

C mAhg�1� � ¼ i� Δt

3600� m(4)

where i is the constant current in mA, Δt (= t2–t1) is the discharge time in second,and m is the total active mass of electrodes (in mg). Capacity can be directly

Fig. 6 Charge-discharge curves for (a) ideal capacitor (schematic) and (b) activated carbon(AC) (measured at 50 mAg�1)


converted to capacitance, as the charge-discharge curve is linear in nature. Thespecific capacitance of the capacitor is calculated using V-t graph as follows:

CS Fg�1� � ¼ i t2 � t1ð Þ

m V 1 � V 2ð Þ or specific capacity mAhg�1� �� 3600

dV mVð Þ (5)

The energy density is calculated from the area under the V-t graph and expressed as

E WhKg�1� � ¼ 1

2CS Fg�1� �

V 12 � V 2

2

� �V 2� �� 1000

3600

E WhKg�1� � ¼ 1

7:2CS Fg�1� �

V 12 � V 2

2

� �V 2� � (6)

The time rate of energy delivery is the power density and calculated as

P WKg�1� � ¼ E

Δt=3600(7)

Lithium-Ion Battery

The primary functional components of a lithium-ion battery are anode, cathode, andelectrolyte. The materials used as an electrode in battery are capable of intercalatingor reversibly accommodate lithium ions. The most commercially popular negativeelectrode materials are carbon (graphite), Li4Ti5O12, etc. Generally, three types ofmaterials such as layered oxide (viz., lithium cobalt oxide), polyanion (viz., lithiumiron phosphate), and spinel (viz., lithium manganese oxide) are used as cathodematerial in lithium-ion battery. The selection of the electrolyte depends on the

Fig. 7 Typical charge-discharge profile of anelectrochemical capacitor(measured at 50 mAg�1)


potential window of the cell. The electrolyte solution commonly comprises a lithiumsalt (such as LiPF6, LiBF4, or LiClO4) dissolved in a mixture of organic solvents(such as EC, DMC, and DEC). The combination of LiCoO2 as cathode and graphiteas anode is the most common electrode system for lithium-ion battery.

Working Principle of Li-Ion BatteryThe electrodes in lithium-ion battery act as host of lithium ions and provide lithiumions to move in or out of the structure called intercalation (insertion of lithium ions)or de-intercalation (extraction of lithium ions) process. The electrolyte provides thepathway for shuttling of lithium ion ions between the anode and cathode across theseparator. The electron flow is blocked by the separator through the electrolyte. Theelectrons are released from one current collector through the external circuit toanother current collector.

The schematic of the charge-discharge process exemplified by LiCoO2 (cathode)and graphite (anode) systems is shown in Fig. 8. The operative electrochemicalreactions are shown as follows [18].

Anode reaction:

Lix�yC6 þ yLiþ þ ye� $ LixC6

Charge

Discharge(8)

Cathode reaction:

Li1�xþy CoO2 $ Li1�x

Charge

DischargeCoO2 þ yLiþ þ ye� (9)

Overall reaction:

Li1�xþyCoO2 þ Lix�yC6 $ LixC6

Charge

DischargeþLi1�xCoO2 (10)

During charging, lithium ions are extracted from the cathode and conductedthrough the electrolyte to the anode. To compensate the charge in cathode, Co3+

gives up an electron to the outer circuit and converts into Co4+. The lithium ion fromthe electrolyte incorporates into the structure of graphite layer, and hence it acceptsan electron from the outer circuit to pi-electron cloud for charge neutrality. Duringdischarge, the entire process occurs in reverse direction.

The voltage and capacity of an electrochemical cell are dependent on the elec-trode materials used. The voltage is calculated from the chemical potential of theelectrodes. The chemical potential is the thermodynamic quantity describing thechange in Gibbs free energy as a function of change in lithium concentration in the


host matrix. If G is the Gibbs free energy and (x) is the total number of insertedlithium atoms, then chemical potential (μ) of that electrode will be:

μ ¼ μLiC xð Þ � μLiA� �

μ ¼ðx2x1

μLiC xð Þ � μLiA� �

dxLi x2!x1j ¼x= x2 � x1ð Þ

μ ¼ GC Li¼x2ð Þ � GC Li¼x1ð Þ � x2 � x1ð ÞGA Lið Þ� �

Li x2!x1j ¼x=�x2 � x1

μ ¼ @G xð Þ@x

(11)

The change in Gibbs free energy (dG) is the maximum amount of work obtainedfrom an electrochemical cell of open circuit voltage E and can be expressed as

dG ¼ �nFE (12)

where n is the total no of electrons participated in electrode reaction (n= 1 for Li+/Lipair), F is the Faraday constant, and E is the potential difference between theelectrodes. The electric potential (E) of the cell can be obtained from the combina-tion of Eqs. (11 and 12):

�nFE ¼ μC � μAð ÞE ¼ � μc � μað Þ

F

(13)

Fig. 8 Schematic of thecharge-discharge process oflithium-ion cell


where μc and μa are the chemical potentials of cathode and anode. The electricpotential of a cell plays a vital role on selecting an electrolyte. To avoid unwantedreduction or oxidation reactions of electrolyte during charge-discharge process, thebandgap energy (E) of the electrolyte should have the higher value than the redoxenergies of cathode (Ec) and/or anode (Ea). So higher bandgap organic electrolytesare the best choice for lithium-ion battery [19, 20].

Hybridization Approach: Toward High Energy and Power System

As mentioned earlier, the EDLCs can deliver high power, but it lacks high energy.The low energy density of EDLCs limits its use in several high energy applications.The low energy density of EDLCs can be explained as follows [21].

(i) It can’t store large amount of energy, as the charge is stored only at the surfaceof the electrode due to the formation of electrochemical double layer at theelectrode-electrolyte interface. The charge density is very low as compared tothe large ion participation in the bulk of the electrode due to redox reaction inthe pseudocapacitors and Li-ion batteries.

(ii) The charge transfer reactions in EDLCs are voltage dependent, i.e., with theprogression of charge transfer, the potential of the EDLCs changes linearly. Thevoltage decreases progressively during the discharge for EDLCs. So the cellvoltage is low as compared to the Li-ion battery where the voltage is pinned atcertain value during charge and discharge.

(iii) For EDLCs, the energy density also depends on the ion concentration of theelectrolyte. In EDLCs, the opposite ions in the electrolytes are consumed at theelectrodes during charging. In Li-ion battery, at one end lithium ions arecollected from one electrode into electrolyte and depleted to the oppositeelectrode at the other end.

In order to increase the energy density of the EDLCs and simultaneously tomaintain the high power density and long cycleability of the system, pseudo-capacitor or battery-type materials are introduced with EDLCs. Various types ofpossible hybridization approaches for the integration of pseudocapacitors and bat-teries with EDLCs have been shown in Fig. 9. These approaches are summarized asfollows [22, 23].

The readily available supercapacitor and battery devices can be connected exter-nally by wire either in serial or parallel combination resulting in (external) serial andparallel hybrid devices, respectively. Among the two types of external hybridization,the parallel external hybridization is very common in internal combustion enginecracking, hybrid electric vehicles, and pulsed applications. The hybrid device yieldsmore power density as compared to the battery (faradaic type) and more energydensity as compared to the supercapacitor (EDLC type). For pulsed application, thistype of combination is very useful, as it outperforms both the battery and EDLCs. In


this case, the EDLCs provide the power during the pulse, and at rest period thebattery energizes EDLCs.

The same approach of hybridization can be made internally at electrode levelinside an electrochemical cell. Thus, in an electrochemical cell, if one of theelectrodes is of EDLC type and the other one is of faradaic type (viz., pseudo-capacitor or battery), the combination is termed as internal serial hybrid (ISH).These types of systems are also named as hybrid electrochemical capacitor (HEC) orhybrid capacitor. When lithium ions are intercalating species, then such electro-chemical cells are usually termed as lithium-ion hybrid electrochemical capacitor(Li-HEC) or lithium-ion capacitor or lithium capacitor. Alternatively, if an electrodeof an electrochemical cell consists of faradaic and EDLC types of materials, they aretermed as bi-material electrodes. The cells consist of such bi-material electrodes arenamed as internal parallel hybrid. The internal serial and parallel hybrids are thetwo extreme cases of electrode combinations. Apart from these, other intermediatecombinations are also possible. For example, one electrode may be of bi-materialtypes, and the other one is of faradaic or EDLC types. Since a wide spectrum ofelectrode materials (EDLC, pseudocapacitor, and battery types) and their combina-tions can be proposed, further classification of internal hybrid system remains quitecomplex in nature. The state of the art of the internal serial hybrids and internalparallel hybrids is reviewed as follows.

Fig. 9 Schematic representation of plausible approaches of hybridization using EDLC battery andpseudocapacitors


Internal Serial Hybrid (ISH)

As mentioned earlier, internal serial hybrids combine pseudocapacitors or batterymaterials with non-faradaic-type EDLCs. Two different types of electrodes withdifferent electrochemically active materials in ISHs exhibit different voltage profilesduring charging and discharging (linear for EDLC type and usually plateau type inbattery-type electrodes). Proper selection of electrodes in ISHs can offer wideworking voltage window, larger capacity, as well as energy density values[24]. Depending upon the working voltage window, the faradaic electrodes can beused as either positive or negative electrodes. The EDLC-type electrodes are mostlyactivated carbon and derivatives of carbons such as graphene, carbon nanotube,reduced graphene oxide, carbon nanofoam, etc. [25]. As outlined in the followingsection, for ISHs, various types of faradaic electrodes have been explored usingaqueous and nonaqueous electrolyte [23].

Faradaic Materials Used in ISH with Aqueous ElectrolyteTransition metal oxides:Metal oxides are the most common faradaic electrodes usedfor internal serial hybrids. It includes manganese oxide, nickel oxide, rutheniumoxide, cobalt oxide, etc. Sometimes hydroxides of these elements are also used formaking the electrodes for such asymmetric combination (with EDLC-type activatedcarbon (AC)). They are used with AC using mostly aqueous electrolyte. Thepseudocapacitance from these types of compounds in aqueous media originatesdue to the change of oxidation state of the transition metals. The transition metaloxides form interfacial oxycation species [e.g., MOa(OH)b] at different oxidationstate and exchange protons and/or alkali cations (viz., K+, Na+, etc.) with theelectrolyte medium. The associated reactions are as follows:

MOa OHð Þb þ δHþ þ δe� ¼ MOa�δ OHð Þbþδ (14)

where MOa(OH)b andMOa(OH)b are interfacial species for MOx.nH2O at higher andlower oxidation state [26, 27].

Lead compounds: Using the technology of lead-acid battery and replacing thelead with AC, the internal serial hybrid system PbO2//AC has been conceptualized.Several research outputs have been found based on the activated carbon as negativeand mixture of lead oxide and lead sulfate as positive electrode in aqueous sulfuricacid media [28]. The positive side of the asymmetric cell exactly follows the reactionas in lead-acid battery. The following reversible reaction occurs in this type ofelectrochemical cell during charging and discharging process.

PbO2 þ HSO4� þ 3Hþ þ 2e� $ PbSO4

Discharging

Chargingþ2H2O (15)

The faradaic reaction at positive side involves the sulfation of the PbO2 electrodewith the change of Pb(IV)/Pb(II) oxidation states. Simultaneously the recharging of


electric double-layer-type AC occurs on the other side of the cell. The double-layerformation at AC anode can be described as follows:

C�=Hþadð Þ þ HSO4

� $ Hþ þ CþDischarging

Charging= HSO4

�ð Þad þ 2e� (16)

At the charged state of the capacitor, positively charged proton (H+) forms doublelayer with negatively charged carbon electrode surface. During discharging, protonsflow to the electrolyte, and bi-sulfate (HSO4

¯) anions adsorb onto the positivelycharged carbon cathode [29, 30].

Conducting polymers: The π-conjugated conducting polymers have received agreat importance for the use in electrochemical capacitors. The conducting polymersmost commonly used for supercapacitor applications are polypyrrole, polyaniline,and derivatives of polythiophene [31]. The conducting polymers can be positively ornegatively charged by ion insertion in the polymer matrix. It can be p-doped orn-doped according to the insertion of anions or cations, respectively, into it. Thecharging processes of the conducting polymers are as follows:

CP ! CPnþ A�ð Þn þ ne� P-dopingð Þ

CP þ ne� ! CPn� Cþð Þn N-dopingð Þ

The p-dopable polymers are used as positive electrodes and n-dopable as negativeelectrodes. However, n-doped polymer supercapacitors are very rare due to therequirement of very high negative voltage for n-doping. Numerous literature reviewsreported p-doped conducting polymers as positive electrode with double-layer-typeactivated carbon as negative electrode for ISH capacitor [32, 33]. In this type ofhybrid combination, reactions follow the following equations:

Cpð Þn þ nCsurface þ nyCþA� $ CpyþAy�� Charging

Discharging n

þ n Csurfacey�=Cy

þ� �(17)

At charged state, the conducting polymer become fully p-doped (or intercalated)state and AC become polarized state forming cation double layer. During discharge,the anions are de-doped (or de-intercalated) from the conducting polymer, andsimultaneously the activated carbon is depolarized.

Li+-/Na+-ion insertion (battery) type: The Li+ insertion compounds are combinedwith activated carbon to form asymmetric ISHs. The literature reviews reported forhybrid Li insertion system using aqueous electrolyte are mainly based on manganeseoxide [34, 35]. The Li+-ion insertion compound (like LiMn2O4) is used as a positiveelectrode in combination with AC as a negative electrode in Li+ ion aqueouselectrolyte (like Li2SO4). In this hybrid capacitor system, during charging, Li+ ionsare de-intercalated from the insertion cathode and are migrated through the electro-lyte to the negative double-layer-type AC electrode. Simultaneously, Li+ ions are


electrostatically attracted by the negatively charged AC surface and adsorbed onto itto form the electrochemical cation double layer. The electrolyte mainly functionslike an ionic conductor as similar to the electrolyte in battery system; it is notconsumed through the process of charging and discharging. So, this type of systemovercomes the electrolyte depletion problem, as it is found for EDLCs or hybridsystem using metal oxide or hydroxide, Li-based anode (Li4Ti5O12), etc. [36].

Faradaic Materials Used in ISH Electrodes with Organic ElectrolyteThe internal serial hybrid (ISH) consisting of lithium-ion insertion (i.e., battery/faradaic-type) and activated carbon- or other carbon based material (i.e., EDLC-type) electrodes in organic media has been widely discussed by several researchgroups. This type of system is also called lithium hybrid electrochemical capacitor(Li-HEC) or lithium-ion capacitor or simply lithium capacitor. The lithium batteriesare intrinsically low power device with limited cycle life, where the EDLCs are lowenergy devices with excellent cycleability. To mitigate the relative disadvantages oflithium-ion battery and supercapacitor, they are combined in a single cell in non-aqueous (organic) electrolyte medium. The battery-type faradaic electrode may beused as a positive or a negative electrode (see Table 2). Table 2 summarizes theprominent results related to the ISHs using battery- and EDLC-type electrodes innonaqueous (or organic) electrolyte medium. For positive electrode, the intercalationcompound should have high oxidation potential (>3 V vs. Li+/Li) and high specificcapacity. Various types of layered oxides (LiNi1/3Mn1/3Co1/3O2), spinel oxides(LiMn2O4, LiNi0.5Mn1.5O4, etc.), phosphates [LiCoPO4, Li3V2(PO4), etc.], andfluorophosphates (LiVPO4F) have been employed as positive electrode forLi-HECs. For negative electrode, the insertion electrodes should have low reductionpotential (<2 V vs. Li+/Li) with high specific capacity. Numerous insertion com-pounds have been found which intercalates Li ions at potential below 2 V vs. Li+/Li.A wide variety of materials including transition metal oxides [TiO2(B), α-MnO2,Li4Ti5O12, LiCrTiO4, H2T12O25, etc.], carbonaceous materials (graphite, graphene,Li-doped carbon, etc.), hydroxides (FeOOH), polyanions [TiP2O7, LiTi2(PO4)3,Li2CoPO4F, Li3V2(PO4), etc.], silicates (Li2FeSiO4, Li2MnSiO4, etc.), and borates(LiMnBO3, etc.) have been classified for use of negative electrodes for Li-HECs.The faradaic insertion electrodes utilized for Li-HECs should have excellentcycleability and rate capability to adjust with the performance attributed by thehigh surface area EDLC-type carbonaceous materials. This type of hybridizationhas first reported in the battery field in 2001 by Amatucci’s group [37]. They usednanostructured Li4Ti5O12 as negative electrode with activated carbon as positiveelectrode, designed a Li4Ti5O12//AC hybrid system which exhibits a sloping voltageof 3 to 1.5 V with energy density of 20 Whkg�1. Subsequently, several researchworks have been reported on various combinations of insertion-type battery mate-rials with EDLC-type carbonaceous electrodes.

As tabulated in Table 2, in ISHs, mostly battery materials are used as negativeelectrode, whereas EDLC type carbonaceous materials are used as positive elec-trodes. The working principle in Li-HECs (where positive electrode is battery andnegative electrode is EDLC) is different from the Li-HECs (where positive electrode


is EDLC and negative electrode is battery). The role of positive and negativeelectrodes and electrolyte in these two types of Li-HECs has been discussed below.

Li-HECs as battery-type positive electrode: Various types of positive insertionelectrodes including but not limited to LiNi0.5Mn1.5O4, LiMn2O4, Li3V2(PO4),LiVPO4F, etc. have been used with high surface area AC as negative electrode inLi-HEC. The charge storage mechanism in this type of ISHs is schematically shownin Fig. 10. During charging, Li+ ions are extracted out (de-intercalate) from theLi-containing insertion-type battery electrode, and simultaneously, to balance thecharge, Li+ ions present in the electrolyte are attracted by the negatively chargedcarbon surface and adsorbed onto the electrode surface forming cation double layer.During discharge, the Li+ ions present in the electrolyte intercalate back intoinsertion-type crystal lattice and are simultaneously desorbed from the surface ofcarbon and suspended back into the electrolyte solution [44]. The high surface areacarbon electrode realized cation double-layer formation during the process. Toprevent electrolyte reduction, the carbon in a half-cell configuration, i.e., (Li//AC),should be tested from open circuit potential to the decomposition (reduction)potential of the electrolyte. The operating mechanism of electrolyte in this type ofISH is similar to Li-ion battery chemistry. The electrolyte works simply as an ioniccarrier for lithium ions. In this system, Li+ ions are introduced to electrolyte solutionfrom one end one electrode and are depleted at other end other electrode. Themolarity of the electrolyte solution remains constant during charge and dischargeprocess. So, at high current charge-discharge, salt concentration polarization is alimiting factor to the rate capability of the electrolyte [45].

Li-HECs as battery-type negative electrode: The ISHs using battery-type elec-trodes as anode with EDLCs as cathodes have been proposed and extensivelyinvestigated by several authors in the last decades. It was initiated in 2001 byAmatucci’s group utilizing the asymmetric combination Li4Ti5O12//AC andWO2//AC, etc. [37]. The mechanism of charge storage in such Li-HECs is explained

Table 2 Characteristics of ISH system using battery- and EDLC-type electrodes in organicelectrolyte

Negativeelectrode

Positiveelectrode

Potentialwindow(V)

Specificenergy(WhKg�1)

Specificpower(WKg�1) Year References

Li4Ti5O12 AC 1.0–3.0 25 – 2001 37

AC LiNi0.5Mn1.5O4 1.5–2.8 55 – 2005 38

ACLi4Ti5O12

LiMn2O4

AC1–31–3

4530

12001300

2011 22

Li4Ti5O12 AC 0–3 69 – 2013 39

Li2CoPO4F AC 0–3 24 – 2013 40

LiNi0.5Mn1.5O4 AC 1.5–3.25 19 – 2015 41

Li3V2(PO4)-CAC

ACLi3V2(PO4)-C

0.5–2.750.5–2.75

2527

325255

2015 42

H2T12O25 AC 1.5–2.8 35–4 179–5383 2015 43

AC LiVPO4F 1–3 30 – 2016 44


schematically in Fig. 11. During charging of the cell, Li+ ions (from the electrolytesolution) are inserted (or intercalated) into the crystal lattice of insertion electrode.Simultaneously, the anions (PF6

¯, BF4¯, etc.) (from the electrolyte solution) are

attracted by the positively charged carbon surface and adsorbed onto the electrodesurface forming anion double layer. During discharging, the Li+ ion is extracted out(or de-intercalated) from the crystal lattice of the battery electrode, and simulta-neously the adsorbed anions (PF6

¯, BF4¯) are desorbed from the negatively charged

counter EDLC electrode to the electrolyte solution. Here, anion double layer formson the positive EDLC electrode. When the EDLC-type electrodes (with high surfacearea) are exposed to high voltage (>4.5 V), the oxidation of nonaqueous electrolytecannot be avoided. In view to this, charge-discharge profile of Li//AC needs to be

Fig. 10 Schematic ofcharging process in Li-HEC(battery as positive electrode)

Fig. 11 Schematic ofcharging process in Li-HEC(battery as negative electrode)


tested in different voltage windows to identify the appropriate potential windowwithout any oxidation of the electrolyte [45].

It is to be noted that the electrolyte (in Li-HECs where battery acts as negativeelectrode) behaves differently as compared to conventional lithium-ion batteriesand/or Li-HEC (where insertion-type electrode is cathode). In Li-HECs (whenbattery-type electrode acts as anode), opposite ions present in the electrolytes (viz.,Li+, PF6

¯) are depleted during the charging process. Consequently, during dischargeanions and cations come back into the electrolyte. Through this process, the molarityof the electrolyte solution experiences a wide swing during each charge and dis-charge. Hence, one needs to optimize the salt concentration of the electrolyte so thatthe ionic conductivity of organic electrolyte does not get affected during charge-discharge. In general salt concentration polarization is a limiting factor for Li-ionbattery and/or Li-HECs (with battery as cathode). During fast discharge in Li-ionbatteries, Li+ ions are extracted out (or de-intercalated) from the anodes and simul-taneously inserted (or intercalated) into the cathodes. This results in local excess ofpositive ions (Li+) and negative ions (PF6

¯, BF4¯, etc.) near anode and near cathode,

respectively. To equilibrate the charge difference, the anions (PF6¯, BF4

¯, etc.) arediffused through the electrolyte to the anode side. Hence, the electrolyte salt isdepleted at cathode and becomes excess at anode side. The process is reversedduring fast charging in Li-ion battery. As opposed to this, in Li-HECs (with batteryas anode and EDLCs as cathode), during fast discharge, a symmetric driving forceattracts the Li+ ions and PF6

¯ (or BF4¯, etc.) ions toward electrolyte, and none of

them (Li+ and PF6¯/BF4

¯) are depleted at one electrode like conventional Li-ionbatteries or Li-HEC (with battery as cathode). Therefore, in Li-HEC (with battery asanode), the rate capability is found to be improved due to reduced concentrationgradient of constituent ions in the electrolyte [37, 45].

Requirement of Electrode Matching in Li-HECIn order to optimize the electrochemical performance of the faradaic- and EDLC-type hybrid electrochemical cell, appropriate matching of the active mass, potentialwindow, and current is required. Matching of the aforesaid parameters is veryimportant to design these asymmetric hybrid cells. For ideal electrode matching,the current passing through anode and/or the capacity of anode should be equal to thecurrent passing through cathode and/or the capacity of cathode. Two dissimilar(faradaic and EDLCs) electrodes of different specific capacities are used togetherto design asymmetric hybrid capacitors. The capacity is balanced by varying themass of the individual electrodes to take the full advantage of the performance ofboth materials in their optimal potential range. To maintain equal capacity (of anodeand cathode) or equal current flowing (through anode and cathode), the mass ratio(x) can be defined by the following equations.

Q ¼ mcaqca ¼ mbaqba; x ¼mba

mca¼ qca

qba(18)


J ¼ mba jba ¼ mca jca; x ¼mba

mca¼ jca

jba(19)

where Q and J are the total charge and total current passing through the positive andnegative electrodes, respectively. mba, qba, and jba are the active mass, specificcapacity, and current density for battery-type electrode, whereas mca, qca, and jcaare the active mass, specific capacity, and current density of EDLC-type electrode.The mass ratio depends on the current density at which the individual electrodes arecharged and discharged. Again, the mass ratio value influences the voltage windowof the asymmetric hybrid capacitor [46, 47].

Current matching: Ideally at a particular specific current, the specific capacities ofthe electrodes should be taken into account to fix the mass ratio. However, it isinconvenient to follow, when the rate performance of the hybrid capacitor is mea-sured. The schematic in Fig. 12 shows the variation of specific capacities withcurrent density for battery- and EDLC-type electrodes. In battery-type electrode,the charge-discharge kinetics is typically limited by the solid-state diffusion oflithium ions into the electrodes. The diffusion of lithium ions into the bulk of activematerials is a slow process. Here during fast charge-discharge, Li+ ions cannotdiffuse fast resulting in sharp capacity loss for battery-type materials (see Fig. 12).In contrast to battery, EDLCs realize charge adsorption/desorption on the surface ofthe electrode. It is a comparatively fast process. Specific capacity is marginallyreduced with the increase in current density in EDLCs. As in Fig. 12, when x = 1,then qca = qba at certain current density (j= jo). If j> jo, then qca > qba; therefore, tomatch the current density between the electrodes, mass of the battery material mustbe reduced proportionately. Similarly, when qca < qba (at j < jo), for currentmatching, mass of the battery material must be increased proportionately.

Potential matching: EDLCs accumulate charge on the electrode/electrolyte inter-face by coulombic attraction force resulting in linear charge-discharge characteris-tics. Li rechargeable batteries store charge by redox reaction throughout the volumeof the electrode. Li-ion batteries usually exhibit flat discharge plateau. During theasymmetric combination between battery and EDLCs, first appropriate EDLC volt-age window is fixed. Accordingly, the mass ratio is adjusted. This has been betterexplained in the next section. If the battery content is further increased, the potentialwindow for EDLC electrode accords with its electrochemical safe window (ESW)(selected based on half-cell measurement), and the increased battery mass does notaffect much the ESW for battery electrode. This yields same voltage window of theasymmetric capacitor, as it was observed for mass balanced asymmetric capacitor. Incontrast, the voltage window of EDLC differs significantly from its ESW when themass content of EDLC exceeds. This alters the voltage window of asymmetricdevice as compared to mass balanced asymmetric capacitor.

Electrochemical Performance of ISH-Type AC//LiNi0.5Mn1.5O4 HybridCapacitor Made in Our LaboratoryTypical charge-discharge profiles (in the form of V vs. t) of battery-type (viz.,LiNi0.5Mn1.5O4) cathode and EDLC-type activated carbon (AC) anode, measured


in our laboratory, are shown in Fig. 13a and b, respectively. The electrochemicalsignature of LiNi0.5Mn1.5O4 shows a flat charge-discharge plateau at around 4.7 V,which can be attributed to the valence change of nickel (Ni3+/Ni4+). TheLi//LiNi0.5Mn1.5O4 half-cell is cycled in the voltage window of 3.5 to 4.8 V(@50mAg�1). The LiNi0.5Mn1.5O4 delivered a specific capacity of 125 mAhg�1.In the voltage window of 2.0 to 3.5 V, the charge-discharge profile of AC showslinear-type double-layer capacitance behavior. The electric double-layer forms in ACdue to the electrostatic adsorbing-desorbing of PF6

�1 anions at electrode-electrolyteinterface. The AC delivered a reversible specific capacity of ~ 37 mAhg�1

(@50mAg�1). An internal serial hybrid (ISH) capacitor has been constructedusing LiNi0.5Mn1.5O4 as cathode and AC as anode. A schematic of charge-dischargeprofile of the hybrid electrochemical capacitor (lower panel) along with the charge-discharge profile of battery- and EDLC-type electrode in half-cell configuration(upper panel) has been shown in Fig. 14.

The symbols in Fig. 14 represent:

ΔVba = Potential window of LiNi0.5Mn1.5O4 cathodeΔVca = Potential window of AC anodeVba,ds = Potential at discharging start for LiNi0.5Mn1.5O4 cathodeVba,de = Potential at discharging end for LiNi0.5Mn1.5O4 cathodeVca,ds = Potential at discharging start for AC anodeVca,de = Potential at discharging end for AC anodeVhyb,ds = Potential at discharging start for HECVhyb,de = Potential at discharging end for HEC

In line with the analyses made by Li et al., the theoretical basis of the calculationof voltage, specific capacitance, and energy and power densities of internal serial

Fig. 12 Schematic of specificcapacity vs. current densitycurves for battery- and EDLC-type electrodes


hybrid using LiNi0.5Mn1.5O4 as cathode and AC as anode has been described indetails as follows [47].

Estimation of Li-HEC voltage:The charging voltage (Vhyb,ds) of the hybrid capacitor can be expressed as

Vhyb,ds ¼ ΔVca þ ΔVbað Þ þ Vhyb,de (20)

Fig. 13 Electrochemical charge-discharge profile of (a) LiNi0.5Mn1.5O4 (in the potential window3.5 and 4.8 V vs. Li+/Li) and (b) AC (in the potential window 2.0 and 3.5 V vs. Li+/Li)

Fig. 14 Schematic of the electrochemical charge-discharge profile (V vs. t) of Li-HEC between0 and 2.7 V, LiNi0.5Mn1.5O4 between 3.5 and 4.8 V vs. Li+/Li, and AC between 2.0 and 3.5 Vvs. Li+/Li


Vhyb:de ¼ Vba,de � Vca,ds�� (21)

As the battery-type material (voltage vs. capacity) curve shows a flat plateau, it isbetter to select the battery-type electrode potential at a platform to avoid overchargeor overdischarge of the hybrid capacitor.

(i) If (mba > mca), i.e., active mass of battery-type material is excessive.During charging of the HEC, the battery material will not be fully charged; at themean time EDLC will be over discharged. For EDLC material, ΔVca will bemaximum. For battery material, ΔVba will not be changed appreciably as thepotential curve is flat.

So,Vhyb,ds ’ ΔVca þ ΔVbað Þ þ Vhyb,de

(ii) If (mba < mca), i.e., EDLC-type material is excessive.During charging of the HEC, the battery material will be fully charged, butEDLC will not be fully discharged. The full voltage window of the EDLC willnot be utilized. So,

Vhyb,ds ¼ ΔVca,mod þ ΔVba

� �þ Vhyb,de

ΔVca,mod can be calculated from sp. capacity vs. sp. capacitance relationshipfor linear charge-discharge profile of EDLC:

ΔVca,mod ¼ qcap �3:6

Cca

ΔVca,mod ¼ mba

mcaqba �

3:6

Cca

ΔVca,mod ¼ 3:6qbaCca

x

So, total charged voltage of the hybrid capacitor will be

Vhyb,ds ¼ 3:6qbaCca

xþ ΔVba

� �þ Vhyb,de (22)

Capacitance of Li-HEC:As the battery and EDLC are in series in ISH type of hybrid HEC, the total

capacitance of the asymmetric device can be obtained by the following relationship:

1

Chyb¼ 1

mcaCcaþ 1

mbaCba


The specific capacitance (Chyb,av) can be expressed as follows:

Chyb,av ¼ Chyb

mba þmca1

Chyb,av¼ mba þmcað Þ 1

mcaCcaþ 1

mbaCba

� �1

Chyb,av¼ mba þmcað Þ 1

mcaCcaþ ΔVba

mcaCcaΔVca

� �

� mbaCba ¼ mbaqbaΔVba

¼ mcaqcaΔVba

¼ mcaCcaΔVca

ΔVba

1

Chyb,av¼ 1þmba

mca

� �1þΔVba

ΔVca

� �1

Cca

Chyb,av ¼ 1

1þΔVba

ΔVca

� � 1

1þmba

mca

� �Cca

(23)

Energy density of Li-HEC:The energy density can be calculated from the area under the V-t graph and can be

expressed as

Ehyb,av WhKg�1� � ¼ 1

7:2Chyb,av Fg�1

� �V 2

hyb,ds � V 2hyb,de

� �V 2� �

Ehyb,max ¼ 1

7:2Chyb,av V 2

hyb,ds

� �when,V 2

hyb,de ¼ 0h i

(24)

Power density of Li-HEC:Power is the energy delivery rate and expressed as

Phyb,av WKg�1� � ¼ Ehyb,av WhKg�1

� �Δthyb sð Þ

3600

(25)

The discharge time thyb (s) of the hybrid capacitor can be obtained from thefollowing relationship:

Chyb,av ¼ 1000Jhyb thyb,de � thyb,ds

� �mba þmcað Þ Vhyb,ds � Vhyb,de

� �Δthyb ¼ 1000Chyb,avΔVhyb,ds � mba þmcað Þ

Jhyb

Δthyb ¼ 1000Chyb,avΔVhyb,ds � mba þmcað Þmbajba

J hyb ¼ mbajba ¼ mcajca� �

Δthyb ¼ 1000Chyb,avΔVhyb,ds � 1

jbaþ mca

mba

1

jba

� �jca ¼

mbajbamca

Δthyb sð Þ ¼ 1000Chyb,av Fg�1� �

ΔVhyb,ds Vð Þ � 1

jba mAg�1� �þ 1

jca mAg�1� �

!(26)


The value of energy density (Ehyb,av) and discharge time (Δthyb) of hybridcapacitor is replaced in Eq. 23.

Phyb,av WKg�1� �¼ 3600

1

7:2Chyb,av Fg�1

� �V 2

hyb,ds�V 2hyb,de

� �V 2� �

1000Chyb,av Fg�1� �

ΔVhyb,ds Vð Þ� 1

jba mAg�1� �þ 1

jca mAg�1� �

!

Phyb,av ¼ 1

2

Vhyb,ds�Vhyb,de� �

Vhyb,dsþVhyb,de� �

Vhyb,ds�Vhyb,de� � � jba

1þ jbajca

� � mbajba ¼mcajca;x¼mba

mca¼ jcajba

Phyb,av WKg�1� �¼ 1

2

Vhyb,dsþVhyb,de� �

Vð Þ1þ1

x

� � jba mAg�1� �

(27)

Using the above equations, the performances of hybrid electrochemical capacitorAC//LiNi0.5Mn1.5O4 have been evaluated. The mass ratio was calculated 1:3, con-sidering 105 mAhg�1 (~87% of 120 mAhg�1) of specific capacity of LNMO and35 mAhg�1 of specific capacities for AC electrode at 50 mAh�1 current rates. Thehybrid cell shows slightly sloping discharge curve in the voltage range from 2.5 to1 V, and most of the capacity is obtained in this voltage range. The hybrid celldelivered the reversible discharge capacity of 25 mAhg�1. The maximum energyand power density for the hybrid cell have been estimated as 26.5 Whkg�1 and34 Wkg�1, respectively.

Salient Features Need to Be Addressed for Optimal Performance of Li-HECsThe salient features decide the electrochemical behavior of Li-HEC (ISH-type)devices may be categorized as follows [48].

(i) The overall capacitance of the hybrid device is more influenced by the electrodeof smaller capacitance. The net overall capacitance of the hybrid electrochem-ical capacitor with respect to the capacitance of the individual electrodes isgiven by the following relationship:

1

Chyb¼ 1

Cbatþ 1

Ccap

It is very important that the entire potential window of electrode of smallercapacitance should be used; otherwise the capacitance of the hybrid device fallssignificantly.

(ii) The current rate performance of the hybrid device is limited by the battery-typeelectrodes. The tuning of structural and morphological property of batteryelectrode is necessary to make it compatible with the current rate performanceof capacitive electrode. The rate performance of battery material can be


improved by using advanced synthesized techniques (like electrospinning),surface coating by conducting CNT/graphene, etc.

(iii) The hybrid cell should be operated at capacitive electrode-limited condition toprotect the battery electrode from deep discharge. Thus it yields longer cyclelife. To achieve this, the effective equivalent active mass of the battery electrodeshould be excess in respect to the active mass of capacitive electrode.

Internal Parallel Hybrid (IPH)

In recent time ISH type electrochemical capacitors have extensively been studied,however, they are still plagued with capacity fading, poor energy as well as powerdensities. The insertion-type battery electrode determines the current rate perfor-mance in Li-HEC. As mentioned earlier, battery-type electrodes have slower chargetransfer kinetics compared to electric double-layer capacitors. The charge transferkinetics of battery electrode must be improved to achieve the maximum performancefrom such asymmetric system [49]. In Li-HEC, lithium pre-doping of negativeelectrode (e.g., graphite) leads to poor reliability. Li pre-doping is also inconvenientfor mass production [50]. The safe potential window of nonaqueous electrolyte is1 to 4.5 V vs. Li+/Li. This suggests charging of Li-HECs beyond 4.5 V, or deepdischarging below 1 V might cause electrolyte decomposition.

To circumvent these problems, novel electrode systems must be designed to yieldsafer hybrid capacitor with higher energy and power densities. Like conductingmaterial is used in conjunction with battery to make bi-material electrode. Variousapproaches have been adopted to design such novel electrodes which can be groupedinto the following categories. Bi-material-type electrodes consist of electrochemi-cally active materials with different charge storage mechanisms; faradaic (e.g.,battery-type) materials use redox reaction, and EDLC (e.g., conducting carbona-ceous type)-type materials use electrostatic double-layer formation to store electricalcharge. Such asymmetric capacitors (using positive as well as negative bi-materialelectrodes) are termed as internal parallel hybrid (IPH) capacitors (see Table 3).Table 3 summarizes few recently reported results of internal parallel hybrid capacitorin nonaqueous electrolyte medium. The charge storage mechanism of IPH capacitorsis illustrated in the following subsection. Till date, hand count reports are availablewhich used only bi-material electrodes both as positive and negative electrodes [22,60]. In most of the literature reports, people have used one bi-material-type electrode(positive or negative side) in conjunction with other electrodes (mostly EDLC type)in hybrid asymmetric configuration. We have reviewed some of the popularbi-material electrodes as described below.

Nanostructured composite or nano-hybrid capacitor (NHC): Nanostructuredultrafast bi-material electrodes are composed of nanostructured intercalation battery(mainly Li4Ti5O12) [61] or pseudocapacitive materials (such as transition metaloxide or conducting polymers), highly dispersed and entangled in a matrix ofnano-carbon. Several research groups have recently reported the electrochemicalperformance of NHCs, where nanostructured battery-type materials are dispersed in


carbonaceous matrices [62]. The combination of nanostructured transition metaloxides (MnO2, RuO2, other oxides, etc.) and carbonaceous nano-materials (viz.,activated carbon, CNTs, carbon nanofibers, graphene, etc.) was actively explored asnanostructured hybrid composite electrodes for Li-HECs (IPH type). The conductingpolymers like polyaniline (PAni), polypyrrole (PPy), polythiophene, etc. hybridizedwith carbon nanostructured materials (AC, CNFs, carbon cloth, CNTs, graphene,etc.) show synergistic effect, as the carbon matrix acts as a framework that preventsthe strain due to the volume changes associated with the polymers during chargingand discharging. Ternary hybrid nanostructured electrodes based on metal oxide,conducting polymers, and conducting carbons have been recently explored for next-generation IPH electrodes.

Table 3 Characteristics of ISH system using battery- and EDLC-type electrodes in organicelectrolyte

Negativeelectrode Positive electrode

Potentialwindow(V)

Electrochemicalperformance Year References

Li4Ti5O12 LiCoO2 + AC 1.6–3.2 Sp. energy40 WhKg�1

2004 51

Li4Ti5O12 LiFePO4 + AC 1–2.6 Sp. capacity40.08 mAg�1

2007 52

Li4Ti5O12 LiMn2O4 + AC 1.0–2.8 Sp. energy16.47 WhKg�1

2009 53

Li4Ti5O12 LiMn2O4 + AC 1.0–3.0 Sp. energy71 Whkg�1

Sp. power12 KWkg�1

2011 22

Li LiMn2O4 + AC 3.3–4.3 Sp. capacity67 mAg�1

2011 54

Li LiMn2O4 + AC 3.3–4.3 Sp. capacity72 mAg�1

2011 55

Li4Ti5O12 + AC AC 0.5–3.5 Sp. energy32 WhKg1

Sp. power6000 WKg�1

2012 56

MCMB LiFePO4 + AC 2.0–3.8 Sp. energy69.02 WhKg�1

Sp. capacity23.80 mAg�1

2012 57

Li LiFePO4 + AC 2.7–4.3 Sp. capacity37 mAg�1

2012 58

Li Li3V1.95Ni0.05(PO4)3 + AC 3.0–4.31.5–3.0

Sp. capacity61 mAg�1

Sp. capacity24 mAg�1

2016 59

Li4Ti5O12 + AC LiMn2O4 + AC 1–3 Sp. capacity56.4 mAg�1

2017 60


Segmented arrangement of battery and EDLC materials: The bi-material elec-trodes can be fabricated in a segmented configuration in which one part is made up ofbattery-type materials and the other is of EDLC-type materials. The setup is made upin such a way that current passing through each individual material can be monitoredby current sensors (see Fig. 15). Individual current contribution can in turn bemonitored as a function of total current as well as the weight ratio of these twocomponents. It has been reported that at high current rate, the electrode exhibitsEDLC-like behavior, whereas at low current rate, it is faradaic in nature. The currentrate shows very little effect on the EDLC part of the electrode. When the segmentedelectrode is pulsed discharged, it was observed that the current is mostly passedthrough the EDLC part (during pulsing) and at rest period faradaic part recharges theEDLC segment. This kind of IPH device has been demonstrated by Lam et al. [63] in2006, and he has patented it naming “UltraBattery™.” In such ultrabattery, activatedcarbon and lead were used in segmented configuration as negative electrode,whereas lead dioxide (PbO2) plate was used as positive electrode. This configurationexhibited better rate performance and pulse charge-discharge capability as comparedto the regular lead-acid battery. Cericola et al. [55] fabricated segmented electrodebased on faradaic type (LiMn2O4) with EDLC type (AC) and demonstrated thatmass ratio and C-rate have dominant influence on current sharing between the twoconstituents of segmented cathode.

Simple mixing of faradaic- and EDLC-type materials: In this type of internalparallel hybrid, the faradaic- and EDLC-type materials are thoroughly mixedtogether (in different weight ratios) to yield bi-material electrodes. Due to synergisticeffect between the faradaic and carbonaceous components, hybrid capacitors yieldimproved electrochemical characteristics as compared to its individual constituents.For example, Cericola et al. [54] reported the individual electrochemical behavior offaradaic-type LiMn2O4 and EDLC-type AC and compared with the bi-materialelectrode made up of LiMn2O4 and AC. As a synergistic effect, the bi-materialelectrode outperformed AC at low current and LiMn2O4 at high current rates. Thebeneficial effect of AC in bi-material electrode of AC and LiFePO4 has been reportedby Bockenfeld et al. [58]. The presence of AC in such hybrid electrode increases the

Fig. 15 Schematic of the segmented bi-material electrode (with current sensor) used in anelectrochemical cell


conductivity of the composite by entangling the LFP particles in the electrode andmodifies the porosity around the LFP particles. Due to the porous nature of thebi-material electrodes, the charge-discharge characteristics are reported to be improvedbecause of better insertion of liquid electrolyte into the electrode.

Charge Storage Process in Bi-Material ElectrodeThe charge storage process in bi-material electrodes are controlled by redox reac-tions (due to the presence of battery) and electrostatic adsorption-desorption process(due to the presence of EDLCs). The active electrochemical potential range offaradaic and EDLC materials should overlap to the entire working potential windowof the bi-material electrode, i.e., the working potential window of EDLC should spanthe redox potential window of the faradaic material [64]. The charge storage processin hybrid bi-material electrode can be illustrated in the line of Dubal et al. [49]. Thecharge storage in bi-material electrode can be divided into three distinct parts asschematically shown in Fig. 16. The EDLC component is first charged due to theformation of electrochemical double layer by electrostatic ions from electrolytesolution. This process continues until q1 charge is stored and the system reachedto the oxidation reaction potential (ΔVb) of faradaic component. The potential profileshows a typical EDLC-type linear curve in this initial charging process. Now thefaradaic component is charged through electrochemical redox reaction maintainingflat voltage plateau until the system stores Qb amount of charge. At the end of thecharging process, the EDLC component is again charged from potential ΔVb, and itcontinues until q2 is stored and system reached to the maximum potential (ΔVbc) ofthe hybrid electrode. The potential profile again shows a typical EDLC-type linearcurve at the final state of the charging process. Thus total charge (Q) is stored by thebi-material electrode due the contribution of faradaic component (Qb), and theEDLC component (Qc = q1 + q2) is as follows:

Q ¼ Qb þ Qc ¼ Qb þ q1 þ q2ð ÞThe plausible working principle of bi-material-type IPH capacitors is illustrated

in the following subsection.

Fig. 16 Schematic of charge storage in hybrid bi-material electrode


Plausible Working Principle of IPH Capacitor with Bi-Material ElectrodesThe schematic of a typical internal parallel hybrid electrochemical capacitor isshown in Fig. 17. The hybrid capacitor consists of two bi-material electrodes ofdifferent electrode combination. The role of the individual constituents of bi-materialelectrodes are as follows: faradaic materials enhances its energy density for havinghigher nominal voltage and capacity. The EDLC type material increases its powerdensity due to their fast electrostatic adsorption-desorption process, which results insuperior electronic conductivity. During charging, Li+ ions are de-intercalated fromthe insertion-type battery (cathode), and simultaneously Li+ ions in the electrolyteare intercalated within the negative insertion-type battery (anode). Additionally,anions (viz., PF6

�, BF4�, etc. present in the electrolytes) are attracted by the positively

charged carbon surface and form anion double layer. Similarly the cations (i.e., Li+

ions present in the electrolyte solution) are attracted by the negatively charged carbonsurface and form cation double layer. During discharge, the Li+ ions are migrated tothe opposite direction, i.e., de-intercalated from the insertion-type battery (anode) andintercalated into the insertion-type battery (cathode). The cation double layer formsnow on positive electrode and anion double layer on the negative electrode. It isobserved that the electrode at which Li+ ions intercalated forms the cation double layerwith Li+ ions during charging and discharging. Affluence of Li+ ions in closeproximity of insertion-type material eventually enhances the intercalation of Li+ ionsin insertion-type materials [65]. As a result it is expected that both energy and powerdensity of these bi-material-type IPH capacitors will be grossly improved bycircumventing the problem of Li+ ion depletion especially during high-rate chargingand discharging.

Half-Cell Electrochemical Performance of Bi-Material-TypeNa3V2(PO4)3@C/AC Hybrid Capacitor Made in Our LaboratoryThe faradaic-type Na3V2(PO4)3@C (carbon-coated NVP) and EDLC-type AC weremixed (40:60 weight ratio), and a bi-material electrode (NVC@C/AC) was made in

Fig. 17 Schematic ofcharging process in internalparallel hybrid capacitor


our laboratory. The electrochemical performance of these individual components(Na3V2(PO4)3@C and AC) and bi-material electrode were investigated in half-cellconfiguration using lithium metal foil as counter and reference electrode. Figure 18shows the voltage vs. capacity profiles (in the potential window of 2.5–4.1 V) of thegalvanostatic charge-discharge for battery-type NVP@C, bi-material NVP@C/AC,and double-layer-type AC electrodes at 50 mAg�1. The potential profile for the ACelectrode is found to be linear in the working potential window suggesting typicalcharacteristic of electrochemical double-layer capacitance behavior. The AC deliv-ered a reversible capacity 36 mAhg�1, which corresponds to a specific capacitancevalue of 81 Fg�1. In case of Li//AC half-cell, during charging, Li+ ions are reduced atLi metal-negative electrode, and simultaneously PF6

�1 ions adsorbed onto theactivated carbon-positive electrode forming anion double layer. The charge storedin Li//AC is due to the coulombic attraction force without any redox reaction. TheNVP@C electrode is characterized by typical battery-type potential profile with aflat plateau near ~3.7 V vs. Li+/Li. The reversible discharge capacity for NVP@Cwas estimated to be ~105 mAh/g. For the first charging in Li//NVP@C, two Na+ ionsde-intercalate from the crystal lattice forming a flat voltage plateau around ~3.8 V.

The potential profile of bi-material NVP@C/AC electrode exhibits the character-istic features of constituent materials. During the charge-discharge cycling, a verysmall deviation in voltage plateaus is observed in bi-material NVP@C/AC electrodeas compared to NVP@C. As shown in Fig. 17, the charge profile of bi-materialNVP@C/AC electrode can be divided into three distinct parts [54]. In region(i) below 3.6 V, the potential profile shows a typical capacitor like linear behavior,and capacity in this region is mostly contributed by the AC. In the middle, theplateau (region ii) in the voltage (3.6 to 3.8 V) shows a typical battery feature. Thefaradaic reaction of battery-type NVP@C material is mainly contributing the capac-ity to the bi-material electrode in this region. At the end of the charging process(region iii, above 3.8 V), the linear voltage confirms the battery-type NVP@C isfully oxidized and the capacity is obtained mainly from the electrostatic adsorptionof PF6

�1 to the AC electrolyte/electrode interface. In a reverse way, the dischargefollows the same reaction mechanism as charging.

Fig. 18 Galvanostatic charge-discharge profiles of NVP@C, NVP@C/AC, and AC at 50 mA g�1


The discharge capacities with different current rates (from 50 to 700 mAg�1) ofNVP@C, NVP@C/AC, and AC are shown in Fig. 19. The charging current was keptconstant at 50 mAg�1 during the rate capability measurements. The NVP@C andAC delivered discharge capacities of 104 mAhg�1 and 35 mAhg�1, respectively, atlower current rate of 50 mAg�1. The bi-material NVP@C/AC electrode providesspecific capacity of 67 mAhg�1. The specific capacity of bi-material electrode is inbetween the specific capacities of NVP@C and AC, which is nearly equal to thepredicted value obtained from mixed rule. At high current rate (700 mAg�1), thespecific capacity of NVP@C falls drastically and retains only 34% of its initialcapacity. Though carbon coating is done on NVP@C particles, slow Li+ ion kineticsmainly hinders the capacity of NVP@C at higher rate. The NVP@C/AC and ACelectrodes show exceptional capacity retention at higher currents and retain 86% and77% of its initial capacity at 700 mAg�1. Interestingly at higher current rate of700 mAg�1, the NVP@C/AC delivers specific capacity of 58 mAhg�1 whichexceeds the specific capacities of 35 mAhg�1 and 27 mAg�1 estimated forNVP@C/AC and AC, respectively. These results indicate that bi-materialNVP@C/AC electrode can be used as a potential cathode material for Li-ion battery.The improved electrochemical performance of bi-material electrode can be attributedto the synergistic effect between NVP@C and AC materials.

Future Direction of the Research on Hybrid Supercapacitor-Battery

• As mentioned earlier, as compared to ISH type, research on IPH-type capacitor isat its infancy. For this type of hybrids, good understanding on the interfacialchemistry between faradaic and capacitive components (in bi-material electrodes)needs to be developed.

• Achieving higher energy density together with power density still remains amajor drawback of this kind of capacitors. In view to this, we feel that various

Fig. 19 The capacityretention characteristics ofNVP@C, NVP@C/AC, andAC at different current rates


combinations of faradaic and capacitive components need to be used. In otherwords, unlike presently explored bi-material electrodes, the constituent compo-nents should not be limited to two only.

• Nanostructured electrodes need to be explored to provide open structure,maintaining connectivity between faradaic and capacitive components might bea major issue, and to circumvent it, encapsulation with graphene or makingcomposite with carbon nanotube might be a fruitful approach.

• As demonstrated in the present work, we have undertaken a novel approach to usesodium vanadium phosphate (NVP, a cathode material used for Na-ion battery)for Li-ion intercalation. In half-cell configuration (using Li as negative electrodeand lithium salt-based electrolyte), after the first charge, Na-ions de-intercalateand come into the electrolyte. During discharge Li+ intercalates into NVP. Sincethe ionic radius of Na+ is larger than Li+, Li+ intercalation is easier in NVP ascompared to any lithium-based electrode. Such approach was found to yieldsuperior power density of the bi-material electrodes. This kind of bi-materialelectrodes needs to be further characterized in full-cell configuration.

• For commercial adaptation of ISH and IPH capacitors, extensive research shouldbe undertaken to develop suitable current collectors, separator, and electrolytematerials.

Conclusion

The chapter describes the state of the art of novel rechargeable hybrid battery-supercapacitor-type electrochemical storage device useful for security and defense,electric vehicles, and renewable energy storage. These hybrids are demonstrated tobe most attractive material candidates for high energy as well as high power densityrechargeable lithium (Li) batteries. We have described two types of hybrids for theaforesaid applications. Internal serial hybrid is an asymmetric electrochemicalcapacitor with one electric double-layer capacitor and another battery-type electrode.On the other hand, in internal parallel hybrids, supercapacitor and battery materialsare mixed together to form bi-material-type electrode. We have reviewed the state ofthe art of various asymmetric electrochemical capacitors reported in recent times.Subsequently, deriving relevant numerical equations, we have illustrated the role ofcurrent densities and electrode potential window in designing the internal serialhybrid electrodes. It was highlighted that the mass ratio between the two electrodesgrossly influences the electrochemical performance of internal serial hybrids. Usingthe derived equations, we have estimated the potential window, capacity, capaci-tance, and energy/power densities for activated carbon (AC)-lithium nickel manga-nese oxide (LiMn1.5Ni0.5O4(LMNO)) asymmetric electrochemical capacitorfabricated in our laboratory in coin-cell configuration. We have reported that thelithium hybrid electrochemical capacitor (ISH configuration) delivered reversibledischarge capacity of 25 mAhg�1. The maximum energy and power density havebeen estimated to be 26.5 Whkg�1 and 34 Wkg�1, respectively. As compared toserial hybrids, limited reports are available on internal parallel hybrid for Li-ionbatteries. A brief literature review is made to illustrate the outstanding research


issues of this type of hybrids. Finally, we have reported excellent electrochemicalperformance of sodium vanadium phosphate (Na3V2(PO4)3(NVP))-activated carbon(AC) bi-material electrodes for lithium-ion rechargeable batteries. The bi-materialNVP@C/AC electrode outperforms the other electrodes at high specific currents andshows better capacity retention than the pure battery NVP@C and capacitor-type ACelectrodes. At higher current rate of 700 mAg�1, the NVP@C/AC delivers a specificcapacity of 58 mAhg�1 which exceeds the specific capacities of 35 mAhg�1 and27 mAg�1 estimated for NVP@C/AC and AC, respectively.

Acknowledgments The work was supported financially by project grants vide sanction letter F.No.: 3-18/2015-T.S-I (Vol. IV) dated 17-05-2017 (IMPRINT, supported byMHRD and DRDO) andsanction EMR/2016/007537, dated 16-03-2018 (supported by SERB, DST). Mr. Mainul Akhtarwishes to thank Mr. Kirtan Sahoo (Materials Science Centre) for fruitful discussion.

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hybrid supercapacitor-battery energy storage … · supercapacitor and battery materials are mixed...

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