Modified Poly(vinylidene fluoride-co-hexafluoropropylene) Polymer Electrolyte for Enhanced Stability and Polymer Degradation Inhibition toward the Li Metal Anode

Solid polymer electrolytes (SPEs) are flexible, low cost, and easily scalable for battery manufacturing. These merits make SPEs one of the most practical solid-state electrolytes. Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), which has good mechanical properties and good thermal stability, is considered a promising polymer for SPEs. However, PVDF-HFP is unstable due to the dehydrofluorination side reaction when PVDF-HFP-based electrolytes cycle with Li metal. This reaction leads to the deterioration of the electrolyte–electrode interface. In this work, the PVDF-HFP polymer electrolyte is modified with LiOH during the synthesis of SPEs (pre-dehydrofluorination). The modified electrolyte delivers significantly enhanced stability toward the Li metal anode. The electrochemical stable window is extended from 4.75 to 5.00 V, and the stable cycle time of the galvanostatic test of Li symmetric cells at 0.1 mA cm–2 increases from about 100 h to over 500 h. Ex situ scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analyses indicate that the polymer degradation is suppressed after modification, and a stable interface containing more LiF and inorganic sulfur compounds forms between the electrolyte and Li metal; the interface could effectively suppress Li dendrite growth. Combining electrochemical results with ex situ SEM and XPS analysis, it is observed that the pre-dehydrofluorination is the reason for inhibiting polymer degradation and suppressing Li dendrite growth toward the Li metal anode. These findings will promote the development of safe all-solid-state Li metal batteries.