(Invited) Enabling Aqueous Processing of High-Energy and High-Power Electrodes for Lithium Ion Batteries – Issues and Mitigation Strategies

Research and development in academia and industry are increasingly applying sophisticated material, process, and cell designs to further improve lithium ion technology. While on materials and cell level, high energy density, long cycle life, and enhanced safety properties are considered key for future lithium ion batteries (LIBs), at higher level the continuous reduction of costs and environmental impact is an ultimate goal. In this regard, replacing the toxic and costly N-methyl pyrrolidone (NMP) solvent in processing the state-of-the-art polyvinylidene difluoride (PVdF) binder by aqueous binder systems is highly desirable. Commercially, aqueous binder systems have already widely replaced NMP-processing in production of carbonaceous negative electrodes [1]. Aqueous processing of positive active materials, however, is facing challenges due to side reactions between solvent and active material, like lithium-proton exchange reactions [2] and the associated rise of pH value and current collector corrosion. Thus, to enable aqueous processing of high-energy and high-power electrodes on both the negative and the positive side, specifically tailored and optimized materials (e.g. binder compositions, selected additives, etc.), recipes, and processing are required. With regard to high-energy applications, it is necessary to enhance the volumetric and gravimetric energy density on materials, electrode, and cell level. For negative electrodes, silicon-based composites are getting enormous attention due to their high theoretical specific capacity. However, the huge volume changes during lithiation/delithiation need to be addressed on materials and electrode level. Thus, it could be shown that by using a suitable aqueous binder system, an optimized amount of active/inactive materials and potential processing additives, the properties of the electrode can be optimized to accommodate the mechanical stresses during cycling and significantly improve the cycle life. To fully take advantage of the high specific capacity of Si-based negative electrodes, the positive electrode needs to match the anode capacity even though the specific capacity of the active materials is significantly lower. A straight forward approach is an increase of the electrode thickness, i.e. the active mass loading. Despite the issues associated with aqueous processing of positive active materials, it could be shown that an active mass loading of up to 60 mg cm -2 can be achieved with LiNi 1/3 Co 1/3 Mn