Exploring the Interplay between Composite Cathode Design and Cell Performance for Solid-State Lithium Batteries

Abstract

To accelerate the mass market adaptation of electric vehicles (EVs), raising the gravimetric energy density of batteries to > 250 Wh/kg while maintaining a cost target of < $ 120/kWh is of critical importance (the ultimate targets of the U.S. Department and the advanced battery consortium).[1] Solid-state batteries are widely recognized as excellent prospects for application in the next generation of EVs and other energy storage devices, with the promise to realize higher energy densities, superior safety and environmentally friendly characteristics. At the cell level, the development of SSBs has been hampered by challenges with the interface originating from the poor interfacial stability between the solid-state electrolytes (SSE) and the electrodes. By eliminating the liquid electrolyte, conductive pathways throughout the whole thickness of the positive electrode and at the solid-solid interface become less efficient causing large polarization and diminished cell performance. This can be exacerbated by the current urge to use thicker electrodes in an attempt to increase the energy density which is a common practice in the currently widely adapted lithium ion batteries employing liquid electrolytes. Notably, increasing cathode thickness beyond a certain limit leads to a point of diminishing returns in terms of energy density. This is mainly attributed to the dominating effect of porosity at higher cathode thickness.[2,3] For instance, it has been found that cells with NMC cathodes thinner than 155 μm (< 6.5 mAh cm-2) showed excellent cycling stability and no capacity losses for C-rates up to 0.5C.[4] To address this challenge, one approach is to formulate the cathode (positive electrode) using a small fraction of ionic conductors (Catholyte) that are mostly derived from the solid electrolyte formulation. In addition, this approach could be complemented with the application of a 10 μm intermediate SSE coating at the cathode surface. The use of catholytes has a second advantage of replacing the non-green polymer polyvinylidene fluoride (PVDF) which is currently the most commonly used binder. Having fluorine in its structure, PVDF is non-recyclable and a potential source of environmentally harmful fluorocarbons.[5] We have begun studying the interfacial stability between composite cathodes using LiFePO4 (LFP), a safe, environmentally-friendly, 3.4 V cathode material, and different SSEs including ceramic electrolytes, polymer electrolytes and polymer-ceramic composite electrolytes. In this work, an optimally designed DOE was planned to study the impact of varying active material mass loading in composite cathodes, and the significance of intermediate coating on SSB electrochemical performance. The SSE used is a plasticized polymer electrolyte with the following optimized formula: 77 % PEO, 13% LiTFSI and 10% Succinonitrile (SN) while the composite cathode was composed of 86% LFP, 4% C65 conductive carbon and 10% of the aforementioned SSE as the catholyte. The active material mass loading was varied between 1 – 12 mg cm-2 in 1 mg cm-2 increments and all composite cathodes were tested with and without the intermediate coating layer. This powerful and contemporary statistical approach aims to provide an enhanced design strategy for a popular and very promising SSB chemistry (Li/PEO/LFP). Finally, our findings shed light on a new approach that can be adopted to explore other battery chemistry with higher voltages such as oxide cathodes, non-oxide solid or semi-solid electrolytes.