The Cathode Composition, A Key Player in the Success of Li-Metal Solid-State Batteries

Polymer–ceramic composite electrolytes are major contenders for applications in commercial Li-metal solid-state batteries. However, the cycling performance and energy density of such batteries are not only related to the capabilities of the electrolyte but also to that of the cathode. Here, we investigate the influence of the formulation of a LiFePO₄ cathode, paying close attention to the fraction of the catholyte, on the electrochemical performance of a Li-metal solid-state battery and its theoretical energy density. The ionic conductivity and stripping/plating ability of the polymer–ceramic (PEO–LiTFSI–Li₇La₃Zr₂O₁₂) composite electrolyte are first evaluated before assessing its C-rate performance and cycle life using a Li-metal anode and a LiFePO₄ cathode with a competitive active material loading (≈1 mA h·cm–²). It appears that a fraction of catholyte between 30 and 40 vol % is enough to ensure good interfacial contact and ionic transport throughout the cell. It is estimated that, under certain hypotheses, the cells could provide average energy densities of up to 185 W h·kg–¹ and 345 W h·L–¹, which are well above what are currently reported in the state of the art, because of the high cathode loading targeted in this study. Finally, recommendations for achieving energy density values close to the ones needed for electric vehicle applications (350 W h·kg–¹ and 750 W h·L–¹) are proposed.