Revealing the Roles of Interface Design for Ga-Doped Garnet/Polymer Composite Electrolyte of Solid-State Lithium Batteries

Li₇La₃Zr₂O₁₂(LLZO)-based composite solid electrolyte (CSE) are considered to be the most promising electrolytes in solid-state lithium metal batteries (SSLMBs) due to the advantages of high safety and potentially high energy density. However, unsatisfactory interfacial connections of LLZO at both cathode and anode solid junctions are the key bottlenecks. To reduce the internal resistance of SSLMBs, three strategies have been employed in this work. First, Ga concentration in Li_7-3xLa₃Ga_xZr₂O₁₂ (LGLZO) was optimized. The significant impacts of the Ga stoichiometry on the battery performance are investigated. Second, an LGLZO CSE layer is fabricated directly on top of a LiFePO₄ (LFP) cathode using a spin-coating technique. The CSE slurry infiltrates the cathode layer, forming a Li⁺ conduction network and ensuring intimate contact between the CSE and the cathode. The advantages of a spin-coated CSE over a conventional freestanding CSE in terms of the charge transfer resistance and the battery charge-discharge properties are disclosed. Third, an ionic liquid (IL) connectivity layer is added at the Li/CSE interface to improve wettability. The IL interlayer homogenizes Li⁺ flux throughout the interface and prevents hot spots for Li dendrite formation, thus suppressing interface-side reactions. The optimal ionic conductivity of the CSE is ~1 × 10⁻³S cm⁻¹ at 60 °C. With such an ingenious arrangement, a satisfactory LFP capacity of 141 mAh g⁻¹ at 1 C and a great cyclability of 96% capacity retention after 300 cycles are achieved. We believe that the proposed dual interface design is simple and affordable and is an efficient strategy to significantly improve practical SSLMB performance.