Initiator-Driven In Situ Formation of Gradient Polymer Electrolytes for High-Performance Lithium Metal Batteries

Polymer electrolytes prepared by an in situ polymerization approach within batteries enable not only substantially simplified manufacturing processes but also significantly improved contact between the polymer electrolytes and the active materials. The initiator is an essential component to trigger an in situ polymerization reaction. Benzoyl peroxide (BPO) and azobis(isobutyronitrile) (AIBN) are the most commonly used initiators for thermally induced free-radical polymerization. However, the utilization of these initiators is not consistent in the literature, and their impacts on the material properties and electrochemical performance of the resulting polymer electrolytes remain unclear. Here, a poly(vinylene carbonate) (PVC)-based polymer electrolyte has been utilized to systematically investigate the impacts of the initiators in different cell formats. We observed that the type of initiator employed significantly influences not only the properties of the polymer electrolytes but also the electrochemical performance of the cells. Especially in full cells, employing BPO has been shown to result in a substantially lower monomer conversion within the cathode compared with cells employing AIBN, leading to a gradient polymer electrolyte within the cell. This gradient polymer electrolyte, synthesized via a single-step in situ polymerization, allows for fast Li⁺ transfer within the cathode and offers sufficient mechanical properties to suppress dendrite formation on the Li metal anode. Consequently, the gradient polymer electrolyte enabled by BPO exhibits significantly improved cycle life and rate capability when compared with the SPE employing AIBN in full cells. This has led to a remarkable cycling performance of the full cells consisting of an NMC622 cathode, a Li metal anode, and an ultrathin polymer electrolyte.