Solid State Nuclear Magnetic Resonance 42 (2012) 62–70

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Solid State Nuclear Magnetic Resonance

0926-20

doi:10.1

n Corr

Lensfiel

E-m

journal homepage: www.elsevier.com/locate/ssnmr

In situ NMR of lithium ion batteries: Bulk susceptibility effects andpractical considerations

Nicole M. Trease a, Lina Zhou a,b, Hee Jung Chang a, Ben Yunxu Zhu a,b, Clare P. Grey a,b,n

a Stony Brook University, Department of Chemistry, Stony Brook, NY 11794, United Statesb University of Cambridge, Department of Chemistry, Lensfield Road, Cambridge CB2 1EW, United Kingdom

a r t i c l e i n f o

Available online 4 February 2012

Keywords:

In situ NMR

Bulk magnetic susceptibility

Lithium ion battery

40/$ - see front matter & 2012 Elsevier Inc. A

016/j.ssnmr.2012.01.004

esponding author at: University of Cambridg

d Road, Cambridge CB2 1EW, United Kingdom

ail address: cpg27@cam.ac.uk (C.P. Grey).

a b s t r a c t

The application of in situ nuclear magnetic resonance (NMR) to investigate batteries in real time (i.e., as

they are cycling) provides fruitful insight into the electrochemical structural changes that occur in the

battery. A major challenge for in situ static NMR spectroscopy of a battery is, however, to separate the

resonances from the different components. Many resonances overlap and are broadened since spectra

are acquired, to date, in static mode. Spectral analysis is also complicated by bulk magnetic

susceptibility (BMS) effects. Here we describe some of the BMS effects that arise in lithium ion battery

(LIB) materials and provide an outline of some of the practical considerations associated with the

application of in situ NMR spectroscopy to study structural changes in energy materials.

& 2012 Elsevier Inc. All rights reserved.

1. Introduction

1.1. Background

NMR spectroscopy is a useful tool with which to probe thestructural changes that occur in battery electrode materials eitherduring or following electrochemical cycling [1]. Most NMRstudies of materials for LIBs have been performed ex situ, thatis, the battery is cycled to a specific state of charge, taken apart toextract a sample, and then the NMR spectrum of this sample isacquired. This has been an extremely successful approach becauseit allows fast magic angle spinning (MAS) methods to be usedalong with other modern NMR techniques, providing considerableinsight into the different structural and dynamical processes thatcan occur at different states of charge. However, the approachdoes not capture short-lived metastable or reactive phases, whichmay react further before the ex situ NMR spectrum is acquired [2].Furthermore, the wish to be able to quickly monitor the differentlocal structural changes as a function of current and potential, tocomplement in situ X-ray powder diffraction measurements, andto determine the electronic structure, to complement in situ X-rayabsorption experiments, has motivated studies in this area. In situ

NMR, despite a number of challenges, now provides a non-invasive means to study the electrochemically-induced structuralchanges that occur on cycling a LIB.

ll rights reserved.

e, Department of Chemistry,

.

To date, most of the in situ NMR studies have been performedon either diamagnetic or metallic systems, and a major challengenow is to apply these methods to paramagnetic systems. In thispaper we first describe some of the practical aspects associatedwith the acquisition of in situ NMR spectra of LIBs. We thenexplore the nature and origin of the shifts that can arise fromsusceptibility effects due to the metallic and paramagneticcomponents; these can arise from the shape, magnetic properties,and orientation of the different battery components. In theremainder of the introduction, we review the applications ofin situ NMR to batteries and discuss the interactions that lead tothe shifts of the NMR resonances and the susceptibility effectsthat are seen in these systems. We briefly discuss relevant priorNMR results for the materials investigated in the work describedin this paper.

Since the first in situ experiments by Gerald et al. [3,4], therehave been several adaptations of the type of NMR probe and/orbattery design used for in situ experiments. The initial in situ NMRprobe designs by Gerald et al. incorporated a Toroid coil [5]. Theradio frequency (rf) field generated in this type of coil depends onthe distance from the center of the coil, which the authors exploitedto perform both spectroscopy and imaging experiments [3]. Initially,they used a home-built ‘‘near electrode imager’’ probe and cylind-rical batteries. A solid copper rod in the center of the coil/batterywas used as both the current collector of the battery and part of therf circuit. This design allowed the concentration of species asfunction of the distance from the current collector to be monitoredas a function of the reaction time and state of charge. Due to thenon-standard battery design and the associated difficulties infabricating electrodes around the copper rod and compressing solid

N.M. Trease et al. / Solid State Nuclear Magnetic Resonance 42 (2012) 62–70 63

electrodes or solid electrolytes to the cylindrical shape, the sameauthors then designed a ‘‘battery imager’’ probe. Here, the batteryshape was flat, having the form of a coin-cell battery, and a solidcopper plate was used both as part of the NMR detector circuit andthe current collector for the battery. Although the batteries wereeasier to fabricate, the results were still hindered by the inherentlylow signal to noise of the Toroid cavity detector and analysis of thespectra were complicated by the non-linear rf excitation/detection.

The use of Bellcore-type plastic batteries [6] by groups inOrleans and Amiens allowed in situ studies to be carried out in aconventional static NMR probe [7–11]. Although, the use of aconventional NMR probe has simplified the practical implemen-tation of in situ NMR experiments of battery materials, there arestill complications that arise from cycling the flat plastic bag celldesign in a magnet. In particular, susceptibility effects (as dis-cussed below) arising from the flat shape of a bag cell can inducea shift in the resonances, and if care is not taken to understandand correctly interpret these shifts, the spectra can be misleading.Recently, Letellier and co-workers adapted a cylindrical batterycell design for in situ NMR experiments, which eliminates theneed for plastic electrodes and diminishes solvent leakage [12].This cylindrical design will promote the study of materials whichare difficult to fabricate into plastic electrodes and may alsoreduce (but will not eliminate) some susceptibility effects seen forflat bag cells. Although, data has been acquired with this design,with the 1 cm diameter of the cell, the filling factor is poor.

A major challenge for in situ NMR spectroscopy is to separate theresonances from the different components and to determine whatsignal corresponds to which part of the cell. This is particularlyproblematic because these spectra are acquired in static mode. Mostlithium NMR experiments have been performed with the I¼3/2nucleus 7Li rather than (I¼1) 6Li since the former is more sensitive,due to its higher natural abundance and gyromagnetic ratio, althoughwith enrichment, 6Li experiments are also possible. Both the shiftpositions of 7Li NMR signals and quadrupolar parameters have beenused to extract information on the structural changes that occur inLIBs during the electrochemical processes. Resonances from lithiumin diamagnetic environments such as the electrolyte and lithium ionsin the surface electrolyte interphase (SEI) appear at approximately75 ppm. In contrast, the resonances from Li environments inparamagnetic phases can be shifted by as much as �500 toþ3000 ppm as a result of the (hyperfine) interaction with theunpaired electrons of the paramagnets [1]. Metallic lithium is shiftedto approximately 250 ppm, by the Knight shift, the Knight shiftarising from the interaction of the nuclear spins with the unpaireds orbital electrons located at the Fermi level of the conduction band.To date, most in situ NMR studies have been run as half-cells utilizingLi metal as one of the electrodes. This is convenient because the 7Lishift of metallic lithium appears in a shift range that is distinct fromthose of many other materials that have been investigated.

Since graphitic carbon is the most commonly used anode in acommercial LIB, much of the earlier in situ NMR studies focused oncarbon anodes [4,7–11]. In graphitic carbon, lithium intercalates toform several different phases of LiCx in which Li-filled layersalternate with one or more empty layers. These different phasesare referred to as ‘‘stages’’, the stage number indicating the periodi-city, where a stage 2 compound indicates that every second layer isfilled. For example, the phases seen electrochemically, in order ofincreasing lithium content, are dilute 40 (LiC36), dilute 30 (LiC27),dilute 20 (LiC18), dense 2 (LiC12), and dense 1 (LiC6), although othershave been reported [24,25]. The Knight shifted resonances for stages40, 30, 20, 2, and 1 appear at approximately 2.0, 6.8, 12.2, 45.0, and42.6 ppm, respectively [10,25]. The dilute stages have a LiC9 in-planedensity whereas the dense stages have an in-plane density of LiC6.Since the Li in the LiC6 vs. LiC9 packing is predicted to have astronger interaction with the carbon conduction electron band

(which will also have a greater density of states), this should leadto the greater Knight shift [25]. The different graphite stages are alsoassociated with different 7Li/6Li quadrupolar coupling constants andquadrupole frequencies, nq values, for stage 40, 30, 20, 2, and 1 of 18,18.5, 19, 17, and 22.6 kHz, respectively, have been reported [10].Thus, although stages 1 and 2 have a similar Knight shifts and aredifficult to separate based on shift alone (especially in the staticspectra), Letellier et al. showed that they could be separated basedon their different nq values by measuring the positions of the majordiscontinuities seen in the satellite transition lineshapes [10].

Before cycling, a carbon bag cell contains only lithium metal, acarbon material, a lithium salt electrolyte, and some SEI that mayhave formed on the electrode surfaces (in addition to binders,separators, current collectors etc.). During electrochemicalcycling, resonances for the lithiated graphitic carbon can alsoappear with the electrolyte in the same 75 ppm region, startingwith the Knight-shifted LiC36 (2 ppm) resonance. Greater Knightshifts are seen as more lithium is intercalated, the signal reachingapproximately 45 ppm [10]. If the electrolyte resonance is shiftedor broadened due to susceptibility effects arising from the shapeof the bag cell or bulk magnetic susceptibility (BMS) effects frominteractions with the carbon or lithium metal, overlap of theelectrolyte and LiCx resonances can result. Thus, an understandingof the susceptibility effects on the shifts of the electrolyte isnecessary to interpret and/or deconvolute spectra in the710 ppm region once the battery is cycled. This is illustrated inthe work presented below in Section 3 (ii).

1.2. NMR of paramagnetic and metallic samples

There have been no reported prior in situ NMR studies onparamagnetic lithium metal oxides, despite their role as promis-ing cathode materials for LIBs. Indeed some of these materials arenow commercialized for use in transportation applications. Beforepresenting initial experiments performed utilizing lithium man-ganese spinels in Section 3 (ii), we now briefly describe some ofthe interactions that will complicate the analysis of the spectra ofparamagnetic and metallic materials.

The susceptibility effects seen in diamagnetic samples aretypically small, but are increased in paramagnetic and metallicsamples due to the presence of the unpaired localized anddelocalized electrons, respectively. The NMR interactions formetallic and paramagnetic materials, namely the Knight andFermi contact shifts and dipolar coupling, depend on the couplingof the nuclear spin to the time-averaged value of the electronicspin, /SzS. For metals, the unpaired electrons are located at theFermi level (but there may also be localized unpaired spins insome metals). /SzS is essentially temperature independentand depends on the density of states at the Fermi level. For aparamagnet, /SzS depends on the molar magnetic susceptibility,wm, which is defined by

/SzS¼�B0

NAm0mBge

wm ð1Þ

and

wm ¼m0m2

Bg2e SðSþ1Þ

3kBTð2Þ

where NA is Avogadro’s Number, m0 is the vacuum permeability,mB is the Bohr magneton, ge is the electron g-factor, kB is theBoltzmann constant, and T is the temperature. Note that Eq. (2) isonly valid for an isotropic susceptibility where wm is a scalar. Thevolumetric magnetic susceptibility, wV, is related to wm bywm¼VmwV. N.b., the volume susceptibility is the proportionalityconstant between the magnetization M induced by the unpaired

N.M. Trease et al. / Solid State Nuclear Magnetic Resonance 42 (2012) 62–7064

electrons in a magnetic field and the magnetic field, B0,

M¼ wV B0 ð3Þ

In orbitally non-degenerate systems, anisotropy in wm canarise. This arises because the contribution of the electronic orbitalangular momentum, L, to the susceptibility can no longer beignored. Now, the magnetic susceptibility has a spatial depen-dence wm(Y), and Eq. (2) is no longer valid. In the high tempera-ture approximation, the magnetic susceptibility can be defined bya 2nd rank tensor, X, with components w0,0 and w2,m defined (fortransition metals and in the absence of strong L and S coupling),as

w0,0 ¼m0m2

Bg20,0SðSþ1Þ

3kBTð4Þ

and

w2,m ¼m0m2

Bg22,mSðSþ1Þ

3kBTð5Þ

here gl,m are components of the 2nd rank tensor g.Fermi contact shifts result from the direct transfer of spin

density from the paramagnetic centers to the s orbitals of thenucleus under investigation and/or by the polarization of inter-vening bonds so as to create unpaired spin density at the nucleus.This mechanism should not, to first approximation, cause broad-ening of the resonances. In contrast, the dipolar-coupling betweenthe unpaired electrons and the nucleus can cause severe broad-ening. The dipolar Hamiltonian can be described by a traceless,axially symmetric 2nd rank tensor, which for an isotropic sus-ceptibility is given by

bHD ¼

ffiffiffi2

3

rDsD 3cos2bDZ�1

2

� �B0bIZ ð6Þ

where bDZ is the angle between the principal axis of the dipolarinteraction and the Zeeman frame, and bIZ is the nuclear spinoperator. The broadening, DsD, is dependent on the distancebetween the I and S spin, rI,S,

DsD ¼_gIwm

2pr3I,S

ð7Þ

where gI is the gyromagnetic ratio of the I spin. An overall shift ofthe resonance only results when the magnetic susceptibility isanisotropic. This shift (called the pseudocontact shift) is generallymuch smaller than the overall dipolar broadening and the largerFermi contact or BMS shifts (discussed below) observed in staticspectra, although it may need to be accounted for in MAS spectra.

Bulk magnetic susceptibility shifts (BMS) arise due to particlelevel and larger macroscopic phenomena. First, a BMS shift canarise if the shape of the sample (container) is not spherical.Furthermore, even for symmetric samples, susceptibility effectsarise from the heterogeneous nature of the different components.The BMS effect can be broken down into two contributions (a) theeffect of the individual crystallites within the sample and (b) theshape of the sample container [15]. Considering phenomenainitially (a), a nucleus inside a particular particle will feel adipolar field that is caused by a non-spherical arrangement ofmagnetic ions within this particle, which will vary as a function ofwhere the nucleus is within the particle [15–17]. The nucleus willalso be affected by a second contribution from the dipolar fieldscaused by surrounding particles; the size of this interaction againvaries depending on where the nucleus is within the particle, andthe nature of the packing of the particle [15,16,18]. Thesesusceptibility effects can in principle be simply described andevaluated by calculating (with Eqs. (6) and (7)) the dipolarcouplings between the nuclei and paramagnets within a particleand between the nuclei and the magnetic fields resulting from the

nearby particles. The challenge comes in describing the packing ofthe particles and their non-uniform shapes, particularly within aheterogeneous system such as a battery electrode. As discussedabove, for the local (shorter-range) dipolar interactions, if themagnetic susceptibility of the magnetic ions is anisotropic, thenthe bulk magnetic susceptibility tensor will be anisotropic[18,19]. This effect, which is referred to as the anisotropic bulkmagnetic susceptibility (ABMS), will cause some broadening andshifts of the resonances and by analogy with the pseudocontactshift will not be completely removed by MAS. However theseeffects are much smaller than those arising from the isotropic partof the susceptibility tensor, Eq. (4).

Considering macroscopic shape effects (b), the local fields (andthus shifts) will vary, even in a uniform medium, for a non-spherical object. In general the effects due to the nearby particlesin (a) are greater, particularly if the system contains paramagnetic/magnetic materials, but even for diamagnetic and homogeneousmedia, the effect due to (b) needs to be considered when referen-cing the shifts of diamagnetic compounds [20–23]. Magic anglespinning (MAS) can be used to reduce the anisotropic part of thisBMS tensor. However, for static NMR, the case with these in situ

NMR experiments, the effects are still apparent in the spectra,resulting in demagnetizing fields that can induce a shift and/orbroadening of the NMR resonances [13,14]. These shifts will makeit more difficult to identify the isotropic shift and extract theprincipal values of the paramagnetic shift tensor. We explore theimportance of these effects in the results described below.

2. Experimental

2.1. Carbon electrodes

The graphitic carbon electrodes contained a mixture of 90%mesoporous carbon microbeads (MCMB 2528) (Osaka Gas), 5%polytetrafluoroethane (PTFE) (Sigma Aldrich), and 5% Ketjencarbon black. The mixture was ground and rolled until a smoothself-supporting electrode was obtained.

2.2. Li1.08Mn1.92O4 electrodes

Lithium excess spinel, Li-enriched Li1.08Mn1.92O4, was synthe-sized via a solid state method with Li2CO3 and Mn2O3 as startingmaterials [26]. Li2CO3 (Fisher 99%) and Mn2O3 (Aldrich) weremixed with a Li/Mn ratio of 1.08/1.92, pelletized, and then heatedat 650 1C for 12 h and 850 1C for 24 h with intermediate grindingand pelletization [12]. The heating and cooling rates are both 4 1C/min. The Li1.08Mn1.92O4 film used in Fig. 7 contains a mixture of70% Li1.08Mn1.92O4 and 30% PTFE (Sigma Aldrich). TheLi1.08Mn1.92O4/carbon electrodes used in Fig. 8 contained a mix-ture of 70% Li1.08Mn1.92O4, 15% PTFE (Sigma Aldrich), and 15%Super P Li (Timcal). The mixture was ground and rolled until asmooth-supporting electrode was obtained.

2.3. Electrolyte

The electrolyte used in all studies, unless noted otherwise, was1 M LiPF6 in an ethylene carbonate/dimethyl carbonate (EC/DMC)mixture,1:1 by volume (Novolyte).

2.4. Bag cells

Bag cells were prepared in an argon glove box. The cells wereprepared as described previously [2] as depicted in Fig. 1A.Polyester bags (Kapak Corporation, type 500-24) were hermeti-cally sealed in an argon filled glove box.

Plastic BagCopper (or Aluminum)wire mesh

Copper wire mesh

Cathode Material orLithium metal

Lithium metal

Separator (soaked with electrolyte)

External Battery Cycler

NMR Spectrometer

NM

R m

agne

t

Fig. 1. Schematic of the in situ NMR setup. (A) In a bag cell battery, the two

electrodes are connected to either aluminum or copper mesh with a separator in

between soaked with electrolyte. The bag is hermetically sealed around the cell.

(B) The bag cell is placed in the coil of a NMR probe and cycled in the magnet.

N.M. Trease et al. / Solid State Nuclear Magnetic Resonance 42 (2012) 62–70 65

2.5. 7Li NMR

Spectra were obtained at two different magnetic fields, 4.7 Tand 7 T. 7Li NMR spectra were obtained at a resonance frequencyof 77.5 MHz at 4.7 T using a 901 pulse length of 2 ms with either aTecmag LapNMR or Redstone spectrometer and at a resonancefrequency of 116.6 MHz at 7 T using a 901 pulse length of 2.75 mswith a CMX spectrometer. Pulse delays of 1 s, 15 s, 10 s, and 0.05 swere used for measuring the spectra of lithium metal, electrolyte,lithium intercalated into carbon (LiCx), and Li1.08Mn1.92O4 elec-trodes, respectively.

2.6. In situ NMR

A typical in situ NMR experimental setup used for plastic bagcell batteries is given in Fig. 1B. A Biologic VSP galvanostat/potentiostat (cycler) was used for all electrochemical (EC) experi-ments. A minicircuits 50 MHz low pass filter is connectedbetween the cycler and the battery to reduce noise in the NMRspectra coming from the cycler.

3. Results and discussion

3.1. Practical considerations

3.1.1. Bag cell components

An example of a plastic bag cell battery is given in Fig. 1A.A bag cell consists of a plastic bag that can be hermetically sealedenclosing cathode and anode materials attached to two currentcollectors, with a separator material (containing electrolyte) in

between. A binder is used to hold the electrode materialstogether. Typical binders include polyvinylidene difluoride(PVdF), polytetrafluoroethylene (PTFE), and carboxymethyl cellu-lose (CMC). Typically, copper or aluminum meshes are used ascurrent collectors. The choice of each of these components affectsthe overall performance and longevity of the battery, and, also theNMR experiment.

The use of bag cell batteries imposes several limitations. Themain disadvantage in the performance of a plastic bag batterywhen compared to a commercial battery is a lack of pressure orconsistent pressure across the cell, when the battery is inserted inan NMR coil. This lack of pressure can increase the resistance insidethe battery, and reduce contact between the particles and thecurrent collector, resulting in poor and irreproducible electroche-mical performance. This problem is solved in the Bellcore plasticbattery design [6], used in many in situ studies. The process ofpreparing these batteries involves forming individual laminates ofthe various components, anode, cathode, and separator, by heatingeach component under pressure, so as to melt the PVdF polymerused as a binder, together with the carbon and active material. Thethree components are then laminated together and to the currentcorrectors, again by heating and under pressure. The plasticizerdibutyl phthalate (DBP) is introduced into the mixture used toform the composite; the DBP is then dissolved in ether to introduceporosity into the film. The Li anode cannot be laminated onto theseparator, but instead a glue is added to help adhesion tothe separator. Although this technique is relatively straightforward,the exact ratio of carbon:PVdF:active material needs to be opti-mized for each active material and varies noticeably with particlesize. Furthermore, the removal of the DBP was observed to result inpoorer electrode performance for Si electrodes [2].

Simpler methods involve only partial lamination of somecomponents and the formation of self-supporting electrodes. Todate, we have had the most success with PTFE self-supportingfilms. (Note, one disadvantage is that these films do not ‘‘wet’’ aswell). We have then applied pressure to the battery by placingPTFE plates outside the bag cell and wrapping the cell tightly witha Teflon tape (a stronger string/tape could be used) or by applyingsmall clamps to the plates. A vacuum sealer can be used to sealbag cells containing viscous (e.g., ionic liquid) electrolytes, toproduce bag cells that have a more consistent pressure. However,most aprotic electrolytes will evaporate under vacuum and it isnon-trivial to seal the very small bags used in these experiments.

Unlike coin cells or commercial batteries, many bag cells arepermeable to air. Lithium metal, electrolyte salts and solvents,and cathode materials are air sensitive and can all react withdifferent components in air, water, N2, O2, CO2, etc. Over time, asair permeates into the bag cells, the newly formed species cancomplicate spectra (leading to misinterpretation) and degradebattery performance. We have used several types of polymersthat can be hermetically sealed for bag cell batteries. Polyesterand polyethylene bags have the advantage that they are trans-parent, easing the assembly of the battery, but cannot be used forexperiments lasting more than 5 days (less with extremely airsensitive anode materials), as air enters (and electrolyte evapo-rates) from the bag cell. Aluminum coated bags are much lesspermeable to air, but are opaque making assembly difficult.More problematic is the fact that the Al coating shields thematerials inside the bag cell from the rf pulses. The intensity ofthe rf field, o1, is suppressed when penetrating through a metal ofthickness x, by o1(x)¼o1e�x/d, where d is the skin depthpenetration into the metal (see below). The decrease in rf fieldstrength will increase the optimized (p/2) pulse lengths. Even ifthe pulse lengths are re-optimized to account for the decreasedo1, decreases in overall signal intensity are expected since theNMR signal, S0, is proportional to o1.

0º

30º 45º

90º

Vertical (0º) Horizontal (90º)

Bo Bo

Frequency (ppm)150200250300350

Fig. 2. Bulk magnetic susceptibility effect on the 7Li shift of lithium metal. Spectra

are given for lithium metal strips, with their short axes oriented at approximately

01 (vertical), 301, 451, and 901 (horizontal) with respect to B0, at shifts of 272, 267,

254, and 244 ppm, respectively. The schematic shows the orientation of the

lithium metal strip in the NMR coil at 01 (vertical) and 901 (horizontal) with

respect to B0. Spectra were acquired at 77.5 MHz (4.7 T).

N.M. Trease et al. / Solid State Nuclear Magnetic Resonance 42 (2012) 62–7066

The rf field inside the Al bag, o1(x) (where x is the thickness ofthe Al coating), can be approximated by determining the skindepth penetration of an rf pulse into Al metal. The skin depth ofan rf pulse into a metal can be calculated using the approach ofBhattacharyya et al. [27], using the equation

d¼1

pm0

ffiffiffiffiffiffiffiffirmRf

rð8Þ

where r is the resistivity of the metal, mR is the relative permeabilityof the metal and f is the frequency of the rf pulse. The skin depthpenetration of a 7Li rf pulse with f¼77 MHz into Al metal isd¼9.6 mm, using r(Al)¼28.2 nOm and mR(Al)¼1.000222. The thick-ness of the Al coating can vary, but a thickness of 410 mm isrequired to ensure no holes are present. For an Al bag with thickness,x¼1 mm, the rf field inside the bag, o1

0 is, o10(1 mm)¼0.9o1, while

for 10 mm this has dropped to 0.35o1.Some electrochemical systems require cycling at slow rates,

either because the electrochemistry is inherently slow (theelectrodes show poor rate performance), or the NMR signals areweak (because either the concentration of species is low, theresonances are broad (e.g., for paramagnetic materials), or thespecies have long spin-lattice (T1) relaxation values). To detect allof the phase transitions and resolve resonances in the materialduring cycling at slow rates, these materials generally require theuse of aluminum bags [2].

To complete the battery circuit, a current collector is con-nected to both the negative and positive electrodes. The currentcollector in a normal battery is usually a conductive metal plate.For plastic bag cells, however, metal mesh is typically used, due todifficulties in sealing the bag over metal foil and to help overcomeskin depth issues associated with rf penetration through metalfoil (as discussed above for Al bags). Depending on the electrodematerial, difficulty may arise in adhering the material to themesh. For example, it is difficult to adhere a carbon electrode tocopper mesh; to overcome this, carbon coated Cu mesh ispreferably used.

For some materials, where a self-supporting film cannot beproduced or a thin film is necessary, a metal foil is often requiredfor the current collector. It is extremely difficult to make ahermetic seal over metal foil, without electrolyte leaking out ofthe bag cell. In these cases, the foil can be connected to a mesh, sothat the bag can be sealed over the mesh. Depending on theelectrode material, different thicknesses and sizes of the diamondshaped opening of mesh may create better adhesion.

The separator used in these cells can be a solid electrolytematerial, but more commonly is a porous medium containingliquid electrolyte. Plastic separators, such as Celgard membranes orPVdF/silica mixtures, are typically used, but separators made fromborosilicate glass fibers are also suitable. For bag cells, plasticseparators composed of a PVdF/silica mixture, can be laminated tothe electrodes, making a true plastic bag cell battery [6]. Plasticseparators are thin, typically 25–40 mm. Glass fiber separators arethicker, approx. 0.7 mm, and can hold more electrolyte. If theelectrolyte peak overlaps with other resonances in the electrodematerial or if the nature of SEI formation is being investigated, aplastic separator is more suitable, as it gives rise to a less intenseelectrolyte peak. In contrast, if changes in electrolyte are ofinterest, use of a glass fiber separator is advantageous. Anotheradvantage of glass fiber separators is that they increase the spacingbetween electrodes, helping to reduce the BMS effects of oneelectrode on the other.

3.1.2. Cycling inside the magnet

A typical in situ NMR experimental setup used in our labora-tory to acquire data for plastic bag cell batteries is given in Fig. 1B.The EC cables are run down the bore of the magnet and are

connected to the battery inside the NMR coil. Typically, batteriesare cycled (charged/discharged) using a floating (not grounded)connection, positive and negative, for most electrochemical stu-dies. To reduce noise in the NMR spectrum, the battery is cycledin ground mode, positive and negative/ground, and the probe andEC cables are properly grounded to the magnet. A 50 MHz (orlower) low pass filter is placed in line with the EC cables betweenthe cycler and the battery in the NMR coil to reduce noise in thespectra originating in the cycler. However, without propergrounding, the EC cables may still act as an antenna, saturatingthe NMR receiver with noise. As the battery is cycled, theelectrode materials’ conductivity and the changing polarizationof the battery can induce changes in the NMR radiofrequency (rf)circuit. It is possible that these changes cause the tuning andmatching of the frequency of the probe to change and that re-tuning would be required during the in situ experiment. Here, forall in situ experiments, the tuning of the probe did not changesignificantly, thus the probe was not re-tuned during cycling.

3.2. Susceptibility effects

The use of plastic bag cells can cause shifts of the peaks due tosusceptibility effects arising from the flat shape of the compo-nents of the bag cell (Fig. 1A). The BMS of metallic or paramag-netic materials can induce dramatic shifts in the NMR resonances.We now explore the relative contributions of the differentcomponents in a battery to these shifts.

3.2.1. Lithium metal

The lithium metal resonance in a battery experiences anorientation dependent shift (�30 ppm over 901) with respect tothe magnetic field [27]. In order to explore this, we have studiedindividual strips of Li metal sealed in a bag at different orienta-tions with respect to the static magnetic field, B0 (Fig. 2). Spectraare shown for a lithium metal strip that has a similar dimensionto that used in the working battery (10 mm�4 mm�0.38 mm)with the 4 mm dimension at approximate angles of 01 (vertical),301, 451, and 901 (horizontal) with respect to B0 (see Fig. 2). The7Li metal resonance shifts as the bag is rotated, and for 01, 301,451, and 901, the shift appears at 272, 265, 252, and 242 ppm,respectively. The change in shift can be attributed to BMS effectscaused by the temperature independent paramagnetism (TIP) ofthe lithium metal [27], the orientation dependence of the shiftarising from the non-spherical shape of the metallic strip. Sincehalf-cells are typically used, the BMS effects of the lithium metal

N.M. Trease et al. / Solid State Nuclear Magnetic Resonance 42 (2012) 62–70 67

are apparent in most in situ NMR data. Although the intensity ofthe Li metal resonance appears to change with angle, this islargely a consequence of the change in broadening. The normal-ized integrated intensities of the shifts at 01, 301, 451, and 901 are0.90, 1.00, 0.95, and 0.91, respectively. This change in broadeningand intensity is caused (at least in part) by the difficulty inaccurately aligning the strip at specific angles (so that differentparts of the strip may be at slightly different angles with respectto B0), but is also due to the nature of the BMS effect (as discussedbelow). It has also been recently shown that the excitation of ametal plate varies sensitively with orientation: when a Li metalstrip is oriented perpendicular to the rf field direction, B1, nosignal is detected from the center of the plate [28]. In ourexperiments, the Li metal strip is aligned parallel to the B1 field,but as the Li metal strip in Fig. 2 was rotated with respect to B0,small variations in its orientation with respect to B1 may haveoccurred leading to the varying intensity at different angles.

In Fig. 3, spectra of an MCMB graphitic carbon/Li metal bag cellbattery (with a borosilicate glass separator soaked in LiPF6 inEC/DMC (1:1 by volume) electrolyte) before cycling (pristine) areshown. The bag cell was oriented in two different directions withrespect to Bo. BMS effects that are very similar in size are again

Frequency (ppm)-1000100200300

Vertical

Horizontal

270

240 -10

2.5

-1.5

Fig. 3. Static 7Li spectra of an MCMB/Li metal bag cell (with a borosilicate glass

separator soaked in 1 M LiPF6 in EC/DMC (1:1 by volume) electrolyte) at different

orientations with respect to the static magnetic field, B0. Spectra were obtained at

77.5 MHz (4.7 T).

2 ppm0.3 ppm

Fig. 4. Static 7Li spectra acquired in the horizontal position of a bag containing the elect

battery: (a) pure electrolyte, (b) electrolyte and MCMB electrode, (c) electrolyte and a l

77.5 MHz (4.7 T).

seen as the lithium metal resonance shifts from 270 ppm in thevertical position to 240 ppm in the horizontal position. Also, themetal resonance in the vertical position is slightly narrower thanin the horizontal position. Asymmetry in the Li metal peaks are,again, attributed at least in part to difficulties in aligning the bagcell at exact orientations within the coil.

3.2.2. The diamagnetic region

Surprisingly, there are also noticeable shifts of the resonancesseen in the diamagnetic region of the spectra in Fig. 3. Here, thespectra should only contain resonances from diamagnetic Liþ

ions in the electrolyte (LiPF6) and in the SEI in the 75 ppmregion. In the vertical position, the electrolyte resonance is muchsharper and only one strong resonance (�1.5 ppm) is observed,although there are other weaker resonances. In the horizontalposition, two distinct resonances (2.5 and �10 ppm) are seen forthe electrolyte and overall the spectra are much broader. Again,the difference in the positions and broadening of the resonancesare attributed to magnetic susceptibility of the sample, due to theorientation of the electrodes with respect to Bo. These initialspectra also may suggest that to obtain sharper resonances, in situ

data for carbon electrodes should be collected for bag cells in thevertical orientation. Conversely, the multiple peaks seen in thediamagnetic region for the bag cell in the horizontal positionrepresent different species in different environments within thebag, which in principle could provide more information on spatialarrangements of the various components within the electrode.We now explore the sources of the shifts in the diamagneticregion.

To investigate the different 7Li resonances in the 710 ppmregion of the spectra in Fig. 3, three bags were studied containingonly: (a) electrolyte, (b) electrolyte with a MCMB carbon film, and(c) electrolyte with a strip of Li metal (Fig. 4). The pure electrolytespectra show a sharp 7Li resonance at 0.3 ppm, whereas, the bagscontaining the carbon and Li metal have broader peaks. Note thatno attempt was made to shim the spectra acquired at differentorientations or with different components. The bag containingelectrolyte and Li metal only, gives a broad spectrum centerednear 0 ppm. The broadening of the electrolyte peak is ascribed tochanges in the magnetic field felt by the electrolyte due to the Limetal. The bag with electrolyte and MCMB carbon has two peaks.Graphite is associated with a large anisotropic diamagneticsusceptibility, due to the delocalized p electron cloud in thehexagonal (C6) sheets [29]. Ions adsorbed on the surface ofcarbon, have been reported to experience large shifts to lowerfrequencies due to the interaction with the electron cloud(shielding) [30]. The resonance at �16 ppm is thus ascribed to

- 16 ppm

rolyte 1 M LiPF6 in EC/DMC (1:1 by volume) sealed with different components of a

ithium metal strip. Only the electrolyte region is shown. Spectra were obtained at

N.M. Trease et al. / Solid State Nuclear Magnetic Resonance 42 (2012) 62–7068

Liþ in the electrolyte in the mesopores of the carbon electrode.The peak at 2 ppm is ascribed to the free electrolyte (Liþ in LiPF6

in the bag that is not in proximity to another material).The electrolyte in the separator can also experience a suscept-

ibility-induced shift. In Fig. 5, spectra for pure electrolyte andelectrolyte in a borosilicate glass separator are given at differentorientations with respect to B0. For the pure electrolyte, a singlenarrow resonance is seen and experiences a slight shift whenrotating the bag from vertical to horizontal. In the horizontalposition, two resonances are seen for the bag with electrolyte andseparator arising from free electrolyte and electrolyte inside theseparator, while only a single, broad resonance is observed whenthe bag is oriented vertically. Note that although two resonancesare seen in the horizontal position, they are only separated by�1 ppm, this BMS shift is much less significant than the shift seenin the carbon material.

-8-6-4-202468Frequency (ppm)

-8-6-4-202468Frequency (ppm)

-0.22

-2.83

VerticalHorizontal

0.19-0.98 -3.72

VerticalHorizontal

Fig. 5. Static 7Li spectra of (a) electrolyte and (b) an electrolyte-soaked borosili-

cate glass separator sealed in a bag at different orientations with respect to B0.

Spectra were obtained at 77.5 MHz (4.7 T).

300 200 100 0 -100 -200 -300Frequency (ppm)

**

Horizontal

Satellite transitions for S8.25 kHz = 1/2 16.5 kHz

~ 46

Vertical

200 100Frequen

Fig. 6. 7Li static spectra of an MCMB/Li bag cell (with a borosilicate glass separator soak

(a) horizontal and (b, c) vertical position. (c) Blow up of (a) showing the satellite trans

Analysis of the spectra in Figs. 4 and 5, allows the relative sizesof the BMS contributions of the various components to beassessed, helping in the assignments of the resonances in thediamagnetic region for the MCMB/Li battery in the horizontalposition in Fig. 3. The resonances are assigned as free electrolyte,LiPF6 (2.5 ppm), (the 2.5 ppm shift arising from the effect of theMCMB carbon by analogy with Fig. 2(b)) and LiPF6 (electrolyte)within the carbon mesopores (�10 ppm). The broad featurebetween these two resonances is attributed to LiPF6 near the Limetal electrode. A distinct resonance for electrolyte in separatoris not apparent in Fig. 3, which may be attributed to the overallbroadening of the electrolyte peak due to the lithium metal in thebattery bag cell compared to the spectra of electrolyte only in theseparator.

A bag cell of a MCMB/Li metal battery (with a borosilicate glassseparator soaked in LiPF6 in EC/DMC (1:1 by volume) electrolyte)was discharged to 80 mV, where the MCMB electrode shouldcontain both stage 1 (LiC6) and stage 2 (LiC12) [24]. The Knightshifted resonances for stage 1 and 2 both appear at �46 ppm,(Fig. 6) in agreement with previously reported values [10,25,31].Spectra of this bag cell were obtained at 7 T to help resolve thequadrupolar structure (by minimizing overlap with the Li metalsignal and to reduce broadening of the satellite transitions), withthe bag cell oriented in both the horizontal and vertical positions(Fig. 6). In the horizontal position, the satellite transitions weredifficult to distinguish in the baseline of the spectrum (Fig. 6a). Inthe vertical position, the spectrum was not as broad and thesatellite transitions indicating the presence of both stages aremore clearly seen (Fig. 6b and c). These two stages are thenresolved based on the their satellite transitions (as discussed inthe Introduction), since stage 1 and stage 2 have quadrupolarfrequencies, nq, 22.6 kHz and 17 kHz, respectively [10].

300 200 100 0 -100 -200 -300Frequency (ppm)

Vertical

* * *

Satellite transitions for Stage 122.5 kHz

tage 2

ppm

0 -100cy (ppm)

ed in 1 M LiPF6 in EC/DMC (1:1 by volume) electrolyte) discharged to 80 mV in the

itions of the two stages. Spectra were obtained at 116.6 MHz (7 T).

Vertical

810

-135-11

Frequency (ppm)05002000 1500 1000 -1000-500

Vertical

757 308

212

-102

108

-7

N.M. Trease et al. / Solid State Nuclear Magnetic Resonance 42 (2012) 62–70 69

3.2.3. Paramagnetic materials

Compared to the susceptibility effects seen in the NMR spectraof Li metal and the LiCx phases of carbon electrodes (Knightshifted lithium signals) or the Liþ in electrolyte salts (diamagneticlithium signals), the effects seen in paramagnets can be muchgreater. The lithium excess spinel, Li1.08Mn1.92O4 is paramagneticand experiences a huge BMS shift of the resonance (Fig. 7). Themost intense isotropic resonance of Li1.08Mn1.92O4 occurs atapproximately 512 ppm for 6Li1.08Mn1.92O4, as seen in its 6LiMAS NMR spectrum (not shown here); a similar shift has beenreported for a slightly different composition 6Li1.05Mn1.95O4 [32].In an electrode film, however, this resonance shifts from approxi-mately �117 to 803 ppm by rotating the film about an angle withrespect to magnetic field from 901 (horizontal) to 01 (vertical) asseen in Fig. 7. Again, a change in shift at different orientations isdue to the BMS effect, now from the paramagnetic nature of thespinel particles in the film.

The orientation dependence comes out of simple analysis of theBMS effects, made by considering a specific particle A beingsurrounded by other paramagnetic particles in an essentially two-dimensional (2D) film. Each nucleus within the particle feels theeffect of the dipolar fields caused by the surrounding n paramag-netic particles [17], in addition to the dipolar fields caused by theparamagnetic ions within the particle, as discussed in the Introduc-tion. We only consider the interparticle interactions, since theintraparticle interactions should not vary as a function of wherethe particle is within the film. Considering a nucleus located at thecenter of particle A, each nuclear–particle dipolar interaction has, asnormal, a geometric dependence of (3 cos2 y�1) as given in Eq. (6)for the coupling between two spins, where y is now the anglebetween the interparticle vector and the magnetic field. wm in Eq. (7)should be replaced by xwm, where x is the number of paramagneticions in the particle. The total interparticle dipolar field felt at thecenter of particle A is then a sum over all the n individualparamagnetic particles within the film. This calculation shouldbe repeated for nuclei located in all the particles within the film.For a horizontal film, y is 901 for all nuclear–particle interactions,

Frequency (ppm)-100003000 2000 1000 -3000-2000

20º

35.3º

45º

54.7º (Magic Angle)

75º

90º (Horizontal)

0º (Vertical)

803

152

363

448

663

-117

-53

Fig. 7. 7Li static spectra of a Li1.08Mn1.92O4 film at different orientations with

respect to B0. (Film size: 12 mm�3 mm�0.5 mm). Spectra were obtained at

116 MHz (7 T).

independent of the particle being considered. Thus this (geometric)term is negative and this orientation must by definition produce themost negatively shifted resonance. For the vertical orientation, thevalue of y depends on where the nucleus and particle underconsideration are within the film, but an integral over all theparticles in a 2D object (i.e., the film) must result in a BMS that ispositive (since the integral of (3 cos2 y�1)dy is 40 in 2D). Whenrotating from the vertical to the horizontal orientation, all the yvalues monotonically increase, so the vertical orientation must

Frequency (ppm)-5005003000 2500 -2000-150010001500 -10000

Frequency (ppm)-5005003000 2500 -15001000 -10001500 0

Vertical923

303180

2000

2000

Fig. 8. 7Li static spectra acquired in the vertical position of a bag containing

Li1.08Mn1.92O4 film with different components of a battery: (a) Li metal, borosi-

licate glass separator and electrolyte, (b) electrolyte and borosilicate glass

separator, and (c) Li metal and borosilicate separator. Spectra were obtained at

77.4 MHz (4.7 T).

N.M. Trease et al. / Solid State Nuclear Magnetic Resonance 42 (2012) 62–7070

result in the largest (positive) shift during this rotation. A moredetailed analysis is outside the scope of this paper and will bepublished in a separate report [33].

The 7Li static spectrum of a bag cell in the vertical position thatcontains Li1.08Mn1.92O4 film, electrolyte and a strip of Li metal isshown in Fig. 8a. In order to completely resolve the peak forLi1.08Mn1.92O4 and the peaks for electrolyte and Li metal, the bagcell was placed in the vertical position with respect to themagnetic field. Analysis of the spectra shows a broad peak atapproximately 757 ppm, which is ascribed to Li1.08Mn1.92O4. It is,however, difficult to assign the other peaks, since they do notappear at the expected shifts for Li metal and electrolyte. Toaddress this, two bags placed in the vertical position werestudied, containing (b) Li1.08Mn1.92O4 film, separator, and electro-lyte (Fig. 8b) and (c) Li1.08Mn1.92O4 film, separator, and a strip of Limetal (Fig. 8c). The 7Li static spectrum of bag cell (b) indicatesthat both at peaks around �11 ppm and �135 ppm are due toLiþ components of the electrolyte, the spinel causing significantchanges to a component of the electrolyte. We tentatively assignthe �135 ppm resonance to electrolyte in or in close vicinity tothe electrode film, and the peak at �11 ppm to Liþ in theseparator. Two Li metal peaks are also seen (Fig. 8c) at 303 ppmand 180 ppm. We tentatively assign the resonance at 303 ppm tothe surface of the Li metal that is furthest from the Li1.08Mn1.92O4

electrode. Qualitatively similar resonances are seen in the spec-trum of the full cell, and we can on this basis assign the peaksseen in Fig. 8a at �7 and �102 ppm to the electrolyte and thepeaks at 308 ppm and 212 ppm to Li metal. The weak peak at108 ppm in Fig. 8a, cannot be assigned based on comparison toFig. 8b and c. It is possible that it is due to edge effects but furtherexperiments are needed, to determine its origin.

4. Conclusions

Some of the practical aspects associated with in situ NMRmeasurements of lithium ion bag cells are described, theapproach representing a relatively straightforward method fordetecting changes in Li local environments as a function of stateof charge. The spectra are, however, strongly affected by suscept-ibility effects, which are particularly severe for paramagneticsamples. Susceptibility effects can be detected via their strongorientation dependence of the bag cell with respect to the staticmagnetic field. A practical approach to the assignment of theresonances to the different components in the cell, by isolatingthe different components, is demonstrated. Magnetic ResonanceImaging experiments are currently in progress to help confirmexactly where the different shift components are located, as areexperiments that utilize the spatial dependence of the signals ofthe different components.

Acknowledgments

Research was supported by as part of the NECCES, an EnergyFrontier Research Center funded by the U.S. Department ofEnergy, the Office of Basic Energy Sciences, under AwardDE-SC0001294 (in situ NMR developments) and by GeneralMotors (carbon materials).

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