Perspective on Cathode Composite Design: Revising Ceramic Processing Choices Toward Low Interfacial Impedance for Garnet-Based Solid-State Batteries

The critical factors that determine the performance and lifetime of solid-state batteries (SSBs) are driven by the electrode–electrolyte interfaces.1,2 The main challenge in fabricating ceramic cathode composites for garnet-based SSBs has been lowering the thermal processing window in which both good contact and low interfacial resistance can be achieved.3 Here, we report an alternative ceramic processing strategy that enables the fabrication of all-solid composite cathodes at an unusually low processing temperature without the use of sintering additives or a fluid electrolyte (polymer-gel, liquid electrolyte). We present specific examples of LiFePO4 and LiCoO2 cathodes with a Li-garnet (Li7La3Zr2O12, LLZO) solid-electrolyte. An infiltration step is used to directly synthesize the LiCoO2 cathode from metal salts in a porous LLZO scaffold, resulting in the formation of the composite cathode (LiCoO2-LLZO) on top of a dense LLZO solid electrolyte. A promising discharge capacity of 118 mAhg−1 (3–4.05 V) with 62 Ohm·cm2 is realized for LiCoO2 with a lithium anode, whereas critical phase instabilities for LiFePO4 are uncovered. Our findings encourage a move away from synthesis techniques that employ particle mixing and sintering to fabricate composites. We provide guidelines on circumventing adverse interphase reactions according to chemistry and ceramic thermal processing budgets in the preparation of these ceramic interfaces as well as on increasing the number of reaction sites. Ultimately, this work provides key ceramic processing guidelines for the preparation of all-solid and high-performing composite cathodes for future Li-garnet SSBs for safe energy storage. (1) van den Broek, J.; Afyon, S.; Rupp, J. L. M. Interface-Engineered All-Solid-State Li-Ion Batteries Based on Garnet-Type Fast Li+Conductors. Adv. Energy Mater. 2016, 6 (19), 1–11. (2) Janek, J.; Zeier, W. G. A Solid Future for Battery Development. Nature Energy. 2016, 1–4. (3) Samson, A. J.; Hofstetter, K.; Bag, S.; Thangadurai, V. A Bird’s-Eye View of Li-Stuffed Garnet-Type Li7La3Zr2O12 Ceramic Electrolytes for Advanced All-Solid-State Li Batteries. Environ. Sci. 2019, 12, 2957–2975.