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

Abstract

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 LiNi1/3Co1/3Mn1/3O2 yielding a stable cycling performance at a C-rate of 0.2 C. Therein, numerous modifications were applied to mitigate lithium-proton exchange in the electrode paste, prevent crack formation during drying, and tailor the electrode microstructure for improved lithium ion mobility. Beyond that, the gained insights could be applied for aqueous processing of electrodes based on Ni-rich LiNi x Co y Mn z O2 (x+y+z=1). Compared to high-energy electrodes, slightly different requirements apply for high-power electrodes. Therein, aqueous binder systems can improve the rate capability by reducing the internal resistance and prolong cycle life due to an enhanced high-voltage stability. Moreover, tailoring the binder composition and aqueous electrode paste recipe can greatly influence processing and cycle life of submicron particles as positive and negative active material. In summary, aqueous processing and the associated binder systems are an environmentally benign and inexpensive alternative to the use of toxic organic solvent. Beyond that, aqueous processing offers various opportunities to specifically tune the electrode properties for each application leading to superior properties compared to NMP-based processing of PVdF. [1] J.-H. Lee, U. Paik, V.A. Hackely, Y.-M. Choi, J Electrochem Soc 152 (2005) A1763-A1769. [2] M. Ma, N.A. Chernova, B.H. Toby, P.Y. Zavalij, M.S. Whittingham, J. Power Sources 165 (2007) 517–534.