Continuous Gradient Composite Solid Electrolyte with Differentiated Functions toward Electrodes for High-Performance Lithium Metal Batteries

In response to the development demand for lithium metal batteries (LMBs) with high energy density and high safety, this study reports a composite solid electrolyte (LPCNE) prepared from poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), cyclopentane-1,1-dicarbonitrile (CPDCN), and Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO). Through gravitational sedimentation, a functionally oriented continuous gradient structure forms inside LPCNE, in which the PVDF-HFP rich layer facing the cathode offers excellent oxidation resistance, while the high modulus LLZTO rich layer facing the lithium metal anode (LMA) effectively suppresses lithium dendrite growth. Such a continuous gradient structure eliminates the interfacial impedance caused by delamination between the organic and inorganic phases, achieving a smooth compositional transition. The incorporation of CPDCN as α-H free nitrile plasticizer increases the ionic conductivity of LPCNE to 0.69 mS cm^–1, and the coordination of CPDCN with metal atoms on the LLZTO surface significantly reduces the concentration of free cyano groups, thereby effectively suppressing side reactions between cyano groups and lithium metal. Consequently, the Li/LPCNE/Li symmetric cell exhibits stable cycling for over 2000 h, and the LiFePO₄/LPCNE/Li cell delivers a discharge specific capacity of 158.3 mAh g^–1 at 0.2 C with 78.4% retention after 650 cycles. Moreover, LPCNE possesses a wide electrochemical stability window of 4.75 V, enabling the LiNi_0.6Co_0.2Mn_0.2O₂/LPCNE/Li cell to deliver a discharge specific capacity of 163.9 mAh g^–1 at 0.1 C with 70% retention after 250 cycles. This study achieves differentiated functionality by adjusting the gradient distribution of the composite solid electrolyte components, providing an innovative solution for high-performance LMBs.