Unlocking superior Li+ transport and anodic compatibility for solid polymer electrolytes by zwitterionic metal–organic filler-mediated Li+ coordination engineering

Abstract

Residual solvents such as DMF in vinylidene fluoride (VDF)-based solid polymer electrolytes (SPEs) promote lithium salt dissociation but trigger parasitic reactions with the Li metal anode, limiting interfacial stability. Here, we introduce a multifunctional zwitterionic filler (LS) into the poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) matrix to regulate Li⁺ coordination and suppress solvent-induced degradation. The LS filler contains borate groups that compete with DMF for Li⁺ binding, weakening Li⁺–DMF interactions, while Zn²⁺ sites in its metal–organic cage immobilize TFSI⁻ anions and establish a Li⁺-dominated, weakly coordinated solvation structure. This dual regulation accelerates Li⁺ transport and promotes the formation of an inorganic, F- and N-rich solid electrolyte interface (SEI) on the Li metal anode. As a result, the optimized electrolyte (LSPH) delivers a high ionic conductivity of 0.641 mS cm⁻¹ and a Li-ion transference number of 0.83 at room temperature, outperforming the filler-free control. The Li|LSPH|Li symmetric cells exhibit a high critical current density of 3.4 mA cm⁻² and stable cycling for over 1000 h at 1.0 mA cm⁻². Furthermore, the Li|LSPH|NCM811 full cells deliver an outstanding discharge capacity of 136.3 mA h g⁻¹ at 5C and achieve 70% capacity retention after 1000 cycles at 1C and 4.3 V. This strategy effectively overcomes the trade-off associated with residual solvents in VDF-based electrolytes and advances the development of high-performance solid polymer electrolytes for lithium metal batteries.