Realizing compatibility of high voltage cathode and poly (ethylene oxide) electrolyte in all-solid-state lithium batteries by bilayer electrolyte design

Poly (ethylene oxide) (PEO) is easily oxidized at the cathode interface when coupled with high voltage cathodes (such as LiCoO2), leading to rapid capacity fade, limiting its application in high energy density all-solid-state battery. In this work, a bilayer concept is applied to design two PEO electrolyte layers composited with lithium difluoro (oxalato) borate (LiDFOB) and lithium bis(trifluoromethane sulfonyl) imide (LiTFSI), respec-tively. The thin PEO/LiDFOB layer is introduced by directly dropping the PEO/LiDFOB solution on the LiCoO2 cathode surface to construct a closely contact interphase. Meanwhile, a stable cathode electrolyte interphase (CEI) containing LixBxOy and LiF formed during electrochemical cycling realizes the LiCoO2/PEO interfacial compatibility. The self-generated PEO/LiTFSI layer towards the anode side provides high ionic conductivity and stabilizes the Li/electrolyte interface. As a result, the assembled cell using the bilayer PEO electrolyte achieves good cycling stability, the capacity retention increases from 15% to 75% after 100 cycles at 0.2C. The enhanced electrochemical performance is also achieved in LiNi0.6Co0.2Mn0.2O2/Li cell using this bilayer PEO electrolyte architecture. This work provides a simple strategy to make high-voltage cathode compatible with PEO electrolyte.