New Material CoNCepts for lithiuM-ioN batteries

New accumulator technologies require adapted, improved or a completely new portfolio of materials.

Chemetall, leading producer of lithium, presents different examples of new technologies and their

impact on the end product in detail. Lithium-ion based technologies are discussed as well as materials

needed for next generation technologies including metal based systems.

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overview

Lithium-ion batteries represent the most promising battery technology for both mobile and stationary applications. This class of batteries is referred to when dis-cussing future mobility concepts due to the comparatively high energy density applica-tions. Considerable expectations are set in this technology. Several optimization efforts to maintain state-of-the-art technologies (evolutionary) and development steps (revo lutionary) are necessary to create a decisive attraction for end-users. An increased energy density of the battery (which is equivalent to the maximum range in terms of automotive applications or duration of power supply for stationary power application [1]) requires new elec-trode materials and suitable electrolytes and in combination with that a standard reproducibility has to be ensured. In addi-tion, reduced packing (energy density on pack level) and reduced costs are com-monly defined as near term targets. An expected development route for new tech-nologies is described and compiled in a roadmap initiated by the Fraunhofer ISI [2]. Some aspects of this roadmap shows ❶.

improved Lithium-ion- teChnoLogieS – Lithium titanate (Lto)

Within the next decade, the share of renew-able energy sources will increase dramati-cally. Within this change, smart energy

dr. ChriStoph hartnig is Project manager Business

Development at Chemetall Gmbh in Frankfurt/main (Germany).

thomaS KrauSe is head of marketing/ Communications at Chemetall Gmbh in Frankfurt/main

(Germany).

Authors

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storage is required to confer reli ability to renewable energy sources and to provide power when the load occurs. Certain draw-backs arise when considering the coinci-dence of renewable power generation and required load: For example, wind genera-tion does not correlate well with load, and often spikes during off-peak periods. To reduce the voltage variations, materials with long life time and cycling numbers which allow for high charge-discharge rates are needed, as this is especially important for load balancing in smart grids. Further requirements are an acceptable price range and a high intrinsic safety. An example for anode materials currently being employed for enhanced lithium-ion batteries is lith-ium titanate (Li

4Ti5O12). The main advan-tage of this material are signifi cantly increased charge-discharge rates exceeding 10 C without lithium plating on the inter-face. These advantages are counterbal-anced by the lower voltage of LTO lead ing to an overall lower energy density which makes them more suitable for stationary systems where packing is not as critical.

Lithium-ion-teChnoL ogieS – high-voLtage SyStemS

Future technologies are commonly de -scribed as generation 3 and 4. Generation 3 includes systems with high-voltage cathodes and generation 4 covers battery technolo-gies with metallic anodes such as lithium-sulphur, lithium-air or zinc-air and is best described as “beyond lithium-ion”.

Still being a future technology high-volt-age materials can be considered the class of future materials closest to application. State-of-the-art materials are layered nickel-man-ganese-cobalt oxides and cobalt-manganese phosphates with varying stoichio metries; the

large-scale production processes of these cathode materials demand suitable base materials in sufficient quantities: The overall quality profile should include well-defined and rather uniform particle sizes; improved drying procedures during the manufacturing of base materials (carbonate, hydroxide) ensure uniform, non-compacting materials. This class of ingredients are usually added to the last step of cathode material produc-tion and represents the best possible adap-tion and amongst other advantages renders additional milling steps unnecessary, ❷.

metaL-baSed teChnoLogieS – generaL aSpeCtS

The main aspect for the development of the next generation of battery technolo-gies (generation 4) is a strongly increased energy density; metal based technologies are considered the most promising candi-dates. lithium based technologies are cur-rently only used for primary applications without the possibility to re-charge the batteries; the main challenge results from a non-uniform deposition of lithium metal during the charging step: caused by a variety of influence factors such as non-uniform current densities, the deposited metal forms dendrites which might be able to penetrate the separating medium and which can potentially lead to short circuits and consequently to the end of life of the cell. In terms of operation safety, these dendrites feature one of most severe issues also on the pack level as a shorting of one cell most likely results in a crucial shut down of the complete pack.

With all the drawbacks in mind, metal based technologies might be able to pro-vide the required high energy densities and once the safety issues are controlled, a real-

istic solution for long-range automotive applications exists. Nevertheless, all the safety and life time requirements demon-strate the need for new, adapted and signifi-cantly changed classes of materials which at the current state still need fundamental development activities. Therefore, applica-tions for use in daily life can hardly be expected in the next decade. The technol-ogy change also affects the demand for lith-ium in terms of new products and more sophisticated base materials which strongly depend on the respective application, the currently most discussed ones are lithium-sulphur and lithium-air; these technologies are elucidated in more detail below. Che-metall shows significant activities in the area of sophisticated base materials both on the R&D and production level.

metaL-baSed teChnoLogieS – Lithium-SuLphur

In charged state a lithium-sulphur battery consists of a lithium metal anode com-bined with a sulphur containing cathode. During discharge, the following reactions take place which can be described in the simplest form as:

eQ. 1

Anode:Li → Li++ e–

Cathode:S8 + 16 e– → 8 S2–

With an overall discharge reaction:16 Li + S8 → 8 Li2S

This reaction scheme underlines the high theoretical capacity of 1675 mAh/g (based on active masses) and an energy density of up to 2900 Wh/g for the lithium-sulphur

❶ roadmap of lithium-based accumulator technologies

❷ Adapted salts portfolio for high performance materials

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system at a cell voltage of 2.2 to 2.5 V. The cathodic reaction, the reduction of sulphur to form sulfide (S2-), is not a one step proc-ess but a reaction cascade starting with ele-mental sulphur. The sulphur is converted via poly sulfides Li2S8 → Li2S6 → Li2S4 → Li2S3 to the final form lithium sulfide, Li2S. The short polysulfide chains which are formed as intermediates are soluble in standard electrolytes; the diffusion (even through the separator) and deposition on non- conducting spots (or the metallic anode forming insoluble precipitates) rep-resent a severe loss factor for the battery system which is enhanced by the parallel occurring self-discharge by this mecha-nism. However, the solubility of the inter-mediates can also be used to design a liquid cathode representing a partially charged state; based on this technique, the cell can be refreshed during operation, a method that is similar to the one known from the redox flow battery.

Besides the problems on the cathode, the above described formation of dendrites on the anode side bears the largest risk of op -eration failure which has to be overcome.

Future r&d direCtionS – Lithium-SuLphur

For the production of the cells two dif-ferent approaches are commonly used: the cell assembly can be done in a charged state (elemental lithium on the anode and sulphur on the cathode) or in a discharged state (void anode and lith-ium sulfide on the cathode). The first way (assembly in charged state) requires suitable anode materials, which at the same time means that the standard lith-ium foil, which is commonly used in pri-mary batteries, will not be considered in this form any more. New geometries and structures with different coatings will be considered to optimize the dissolution/re-deposition conditions; the cathode has the task to provide a structure where sul-phur is either included (and the soluble polysulfides cannot diffuse away) or trapped in a different way.

In the second way, the cell is assem-bled in a perfectly discharged state: there’s hardly any metallic lithium on the anode side and merely pure lithium sulfide on the cathodic compartment. Standard lithium sulfide usually has two different draw backs: on the one hand, a

low impurity profile is required to ensure a long cycle and calendar life; on the other hand, a very pure lithium sulfide does not exhibit the required conductivity to be used as cathode material. There-fore, carbon composite lithium sulfide compounds are used for cathode materi-als which feature the required conductiv-ity at a concurrently low number of addi-tional impurities [3]. Chemetall is active in the development of different lithium sulfides – ranging from ultra-pure mater-ials which can be also employed for the production of solid electrolytes to carbon composite lithium sul fides for the direct introduction as cathode materials in lith-ium-sulphur batteries.

With all the requirements for the cath-ode side in mind, potential anode prob-lems, i.e., the dendrite formation, must not be neglected. During the discharging step, metallic lithium is dissolved from the anode and migrates to the cathode (where, in the case of the lithium-sulphur system) lithium sulfide is formed. The charging step includes migration of the lithium-ions from the cathode to the anode and the reduction to metallic lith-ium and lithium plating on this electrode. In order to achieve very homogenous lith-ium plating (with a minimum of den-drites being formed) new structures have to be provided to get optimum control of the plating process. lithium powder may potentially be a solution for this problem (which by the way is prevailing for all metal-based technologies, e.g., lithium-sulphur, lithium-air, zinc-air, just to men-tion the most prominent ones). lithium powder can be produced with different particle sizes; the particles can be coated in order to achieve different surface prop-erties, ❸.

metaL-baSed teChnoLogieS – Lithium-air

With a metal based battery technology, dendrite formation is the most significant problem when discussing the cycle life of a lithium-air battery. However, the per-formance of the battery is also severely impacted by the cathode performance: a suitable catalyst has to be determined which shows a bifunctionality as it has to catalyze both the oxygen reduction as well as the oxygen formation during dis-charge and respectively charge:

eQ. 2

Discharge: 2 Li + O2 → Li2O2

Charge: Li2O2 → 2 Li + O2

Besides manganese based compounds platinum-gold catalysts have been dis-cussed recently to promote both reactions. At the same time, the metallic anode exposed to ambient air may lead to per-formance losses caused by side reactions with gases from the ambient air which might enter the cell from the “open” cath-ode. These include: moisture which leads to a reaction of the metal under hydroxide formation; nitrogen and carbon dioxide may also cause the formation of lithium nitrides and carbonates which are both unsoluble precipitates and reduce the overall capacity and performance.

The discharge reaction leads to the for-mation of lithium peroxide as well as lith-ium oxide (as a side product):

eQ. 3 2 Li + O2 → Li2O2

Similar to the above mentioned assembly step of the discharged lithium-sulphur cell a lithium-air cell might be built in the same way. At the same time, creating pro-tected materials for the metallic anodes are another important contribution for both the assembly step as well as for the overall lifetime. As the lithium-air technol-ogy is not anticipated to be realized before 2030, development activities in this field are mainly on the fundamental research level which however are sup-ported by custom-made materials.

reFerenCeS[1] ArPA-e – http://arpa-e.energy.gov/ ProgramsProjects/grids.aspx[2] http://isi.fraunhofer.de/isi-en/index.php[3] takeuchi, t., et al.: materials science Forums Vols. 638-642 (2010), pp. 2184 – 2188

❸ Coated lithium powder forms the basis for next generation anodes

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