Li₂CO₃: Insights into Its Blocking Effect on Li-Ion Transfer in Garnet Composite Electrolytes

Garnet-type Li₇La₃Zr₂O₁₂(LLZO) has been widely used as a filler in composite solid electrolytes (CSEs) to achieve high-performance solid-state batteries (SSBs). Unfortunately, moisture-sensitive LLZO suffers from surface Li₂CO₃passivation when being exposed to an ambient atmosphere. The insulated Li₂CO₃layer is thought to reduce the Li⁺transportability of CSEs. However, further studies are still needed to find out the underlying mechanism, which helps to guide future filler modification and electrolyte design. Herein, the role of the Li₂CO₃layer in CSEs is elucidated from different perspectives. The passivate Li₂CO₃layer is verified to prohibit the formation of the high conductive interlayer, change the Li⁺transport pathway, and decrease the carrier concentration in CSEs. Also, the Li₂CO₃layer would reduce the electropositivity of Li_6.4La₃Zr_1.4Ta_0.6O₁₂(LLZTO) particles, which therefore weakens the anchoring effect toward bis(trifluoromethanesulfonyl)imide (TFSI)^-. Accordingly, without Li₂CO₃, the electrolyte of polyethylene oxide/LiTFSI/IL (ionic liquid) with LLZTO-AT (PLILA) displays 2 times higher ionic conductivity and an improved Li⁺transference number of 0.49. Additionally, an excellent cycling performance is presented in Li symmetric cells and full cells with PLLA. This work provides a novel perspective for future research on lithium-ion transport mechanisms and inspires designing better-performance SSBs.