Impact of Oxygen Vacancies in LiCoO 2 on the Electrochemical Performance of Garnet‐Based All‐Solid‐State Li‐Metal Batteries

Abstract Garnet‐structured Li 7 La 3 Zr 2 O 7 (LLZO) is considered as one of the most promising solid electrolytes for high safety all‐solid‐state Li batteries (SSLBs) applications. However, this type of SSLB utilizing LiCoO 2 /LLZO as composite cathode faces high capacity degradation because of delamination between LiCoO 2 (LCO) and LLZO and possible oxygen vacancy‐driven microcrack formation within LCO. Herein, a pure oxygen atmosphere is used for sintering the composite cathode to limit oxygen vacancy formation in LCO. Different sintering temperatures are also used to reduce the effect of sintering atmospheres, which suggests the non‐reversible oxidation peak at ∼3.8 V is not related to Li 2 CO 3 formation. Although the Coulombic efficiencies of the first electrochemical cycle of SSLBs sintered in pure oxygen atmosphere are improved, their electrochemical performances are lower than that of air‐sintered SSLB due to higher cell resistances from the reduction of oxygen vacancies in LCO and possible higher volume change during electrochemical cycling. Also, the lower electrochemical cycling performance and observing tens of micrometers long inter‐granular cracks in the highly dense composite cathode suggests that microstructural optimization is more important than a high relative density. These observations provide guidelines for further improving the electrochemical cycling performance of garnet‐structure‐based SSLBs toward practical applications.