Optimized functional additive enabled stable cathode and anode interfaces for high-voltage all-solid-state lithium batteries with significantly improved cycling performance

Functional additives play important roles in stabilizing the interfaces within all-solid-state lithium batteries (ASSLBs), equally vital as in liquid lithium ion batteries (LLIBs). However, they have not received as much attention as in LLIBs; especially the effects of a unique additive on both of cathode and anode interfaces are not clearly understood. Inspired by this idea, the effects of lithium difluoro(oxalate)borate (LiDFOB) and lithium bisoxalatodifluorophosphate (LiBODFP) on the stabilities of the cathode and anode interfaces within the assembled ASSLBs are systematically compared through a series of characterization techniques in this work. Owing to the different degrees of redox kinetics of the LiDFOB and LiBODFP additives, the as-formed cathode solid electrolyte interface (CEI) and anode solid electrolyte interface (SEI) films exhibit drastically different characteristics. Specifically, the LiDFOB-induced CEI film is unevenly distributed and unstable, while a uniform, thin and dense SEI film, delivering an outside-to-inside structure of organic lithium species-layer/LiF-rich layer/Li₂O-rich layer, can be generated in the presence of LiDFOB. By contrast, the formed CEI film induced by the LiBODFP additive exhibits stable, uniformly distributed and thin characteristics. However, the LiBODFP-induced SEI film is flawed due to its slow reduction rate. To take full advantage of the electrochemical activities of LiBODFP and LiDFOB additives, a double-layer PEO-based composite solid electrolyte (CSE) with both additives is designed and fabricated. As a result, the assembled ASSLB with a single crystal LiNi_0.6Co_0.2Mn_0.2 cathode and double-layer CSE shows a high specific capacity and ultra-high capacity retention (87.5% after 1340 cycles at 1C). This novel strategy of stabilizing different electrode/electrolyte interfaces using various functional additives is a promising method to enable ASSLBs with excellent performances.