An ultrathin, strong, flexible composite solid electrolyte for high-voltage lithium metal batteries

The urgent need for safe, energy-dense electrochemical storage devices calls for high-voltage solid-state lithium metal batteries. However, state-of-the-art solid-state electrolytes are hindered by two critical issues: the narrow electrochemical window, which prevents the pairing of a lithium metal anode with a high-voltage cathode, and the large thickness which leads to a huge internal resistance and a significant sacrifice in energy density. To simultaneously address both the issues, in this work we develop a poly(acrylonitrile) (PAN)-LiClO₄-boron nitrite nanoflake (BNNF) composite electrolyte modified with a BNNF layer (PBCEB). The PAN-LiClO₄-BNNF composite can sustain an oxidation voltage of up to 4.5 Vvs.Li/Li⁺while the BNNF modifying layer prevents the PAN-LiClO₄-BNNF from the reduction reaction with the lithium metal anode. In the meantime, thanks to the BNNFs, the PAN-LiClO₄-BNNF possesses a strong tensile strength (16.0 MPa) and Young's modulus (563.7 MPa), which enables the PBCEB to be as thin as 13.5 μm (12.0 μm for the PAN-LiClO₄-BNNF and 1.5 μm for the BNNF modifying layer). As a result, a Li/PBCEB/LiNi_0.8Co_0.1Mn_0.1O₂full battery delivers a high specific capacity of 173.6 mA h g⁻¹at 0.2C and achieves a remarkable capacity retention of 68.1% after 350 cycles at 1C. This work provides an effective approach to develop high-performance composite solid electrolytes for high-voltage lithium metal batteries.