Multifunctional polymer electrolyte improving stability of electrode-electrolyte interface in lithium metal battery under high voltage

High-voltage lithium (Li) metal batteries are promising high-performance energy storage systems. However, their practical applications are troubled by unstable electrode-electrolyte interface, the narrow electrochemical window and unsafety of liquid electrolyte. Here, a multifunctional asymmetrical gel polymer electrolyte (a-GPE) with gradient structure is proposed to address the mentioned above issues. The host of a-GPE is consisted of the poly(vinylidene fluoride hexafluoropropylene) (PVDF-HFP) nanofiber membranes containing BaTiO3 (BTO) with gradually varying content along thickness. This design incorporates comprehensive consideration of ionic conductivity, Li-ion transference number and electrochemical stability window, as well as mechanical property, thus achieving smooth and steady Li-ion flux in interior of a-GPE. Significantly, the PVDF-HFP membranes without BTO face the Li-metal to accomplish homogeneous Li deposition/stripping, and the BTO-rich PVDF-HFP membranes are adjacent to cathode to generate an effective protection film for stabilizing interface owing to polarization effect on the migration kinetics of mobile Li ions, especially under high voltage and large current. Consequently, the LiNi0.5Mn0.3Co0.2O2/Li batteries based on a-GPE exhibit superior comprehensive performances, including dramatic cycling performance and excellent rate performance at high voltage, good electrochemical performance under low temperature and high safety, as well as favorable flexibility. These ideal comprehensive performances can be conveniently realized through ingenious design of conventional polymers, rather than expensive synthetic materials and complicated fabrication process. This work will initiate a new development of advanced polymer electrolyte for high-energy and safe Li batteries.