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

• A bilayer PEO electrolyte with different lithium salts is constructed. • The design of bilayer electrolytes facilitates interfacial compatibility with the cathode at high voltage. • The assembled ASSLBs have achieved highly improved cycling stability. Poly (ethylene oxide) (PEO) is easily oxidized at the cathode interface when coupled with high voltage cathodes (such as LiCoO 2 ), 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), respectively. The thin PEO/LiDFOB layer is introduced by directly dropping the PEO/LiDFOB solution on the LiCoO 2 cathode surface to construct a closely contact interphase. Meanwhile, a stable cathode electrolyte interphase (CEI) containing Li x B x O y and LiF formed during electrochemical cycling realizes the LiCoO 2 /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 0.2 C. The enhanced electrochemical performance is also achieved in LiNi 0.6 Co 0.2 Mn 0.2 O 2 /Li cell using this bilayer PEO electrolyte architecture. This work provides a simple strategy to make high-voltage cathode compatible with PEO electrolyte.