Dual Regulation of Zr-Doped LLTO and Gr-Doped Cathodes: Synergistic Effect of Microstructure on the Cycling Performance of All-Solid-State Lithium Batteries

This study addresses the challenges of low grain boundary conductivity in perovskite-type LLTO solid electrolytes and insufficient cycling stability in LLTO-based full cells by proposing a synergistic strategy combining Zr-doped electrolyte optimization and graphene (Gr)-modified cathodes. A series of Zr-doped LLTO electrolytes were successfully synthesized via a two-step sintering method, and the effects of doping on their microstructures and electrochemical properties were systematically investigated. Results indicate that 2 wt % Zr doping effectively promotes grain growth and homogenization, reduces grain boundary density, and significantly decreases grain boundary resistance. The optimal composition (Li0.33La0.557Ti0.98Zr0.02O3) exhibits excellent overall performance at room temperature: an ionic conductivity of 5.33 × 10–5 S/cm, an extremely low electronic conductivity of 6.28 × 10–9 S/cm, an activation energy of 0.22 eV, and a high lithium-ion transference number of 0.838. To mitigate capacity degradation in LiFePO4|LLTZO|Li full cells, 8 wt % Gr was incorporated into the LiFePO4 cathode to form a composite electrode. The modified full cell demonstrates significantly enhanced cycling stability, achieving a capacity retention of 94.68% after 200 cycles─an 8.41% improvement over the unmodified cell. This study confirms that the synergistic effects of Zr doping and Gr modification provide effective solutions for ion transport and electrode/electrolyte interface stability, offering valuable theoretical and practical insights for the development of high-performance all-solid-state lithium batteries.