On the feasibility of all-solid-state batteries with LLZO as a single electrolyte

Replacement of Li-ion liquid-state electrolytes by solid-state counterparts in a Li-ion battery (LIB) is a major research objective as well as an urgent priority for the industry, as it enables the use of a Li metal anode and provides new opportunities to realize safe, non-flammable, and temperature-resilient batteries. Among the plethora of solid-state electrolytes (SSEs) investigated, garnet-type Li-ion electrolytes based on cubic Li₇La₃Zr₂O₁₂ (LLZO) are considered the most appealing candidates for the development of future solid-state batteries because of their low electronic conductivity of ca. 10^-8 S cm^-1 (RT) and a wide electrochemical operation window of 0-6 V vs. Li⁺/Li. However, high LLZO density (5.1 g cm^-3) and its lower level of Li-ion conductivity (up to 1 mS cm^-1 at RT) compared to liquid electrolytes (1.28 g cm^-3; ca. 10 mS cm^-1 at RT) still raise the question as to the feasibility of using solely LLZO as an electrolyte for achieving competitive energy and power densities. In this work, we analyzed the energy densities of Li-garnet all-solid-state batteries based solely on LLZO SSE by modeling their Ragone plots using LiCoO₂ as the model cathode material. This assessment allowed us to identify values of the LLZO thickness, cathode areal capacity, and LLZO content in the solid-state cathode required to match the energy density of conventional lithium-ion batteries (ca. 180 Wh kg^-1 and 497 Wh L^-1) at the power densities of 200 W kg^-1 and 600 W L^-1, corresponding to ca. 1 h of battery discharge time (1C). We then discuss key challenges in the practical deployment of LLZO SSE in the fabrication of Li-garnet all-solid-state batteries.