LiFePO ₄ /Nano-LLZTO Composite Cathodes for Enhanced Performance of Solid-State Lithium Batteries

Solid-state lithium batteries (SSLBs) offer improved safety and stability over conventional liquid-electrolyte systems but often suffer from sluggish ion transport and poor interfacial contact within the cathode. To address these limitations, we investigate the incorporation of nanosized Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (nano-LLZTO) particles into a LiFePO₄ cathode to enhance ionic conductivity and electrochemical performance. Finite element method simulations and experiments reveal that downsizing LLZTO from the microscale to the nanoscale substantially enhances Li⁺ flux uniformity and ionic conductivity (6.48 × 10^-5 S cm^-1 vs 1.03 × 10^-5 S cm^-1), forming more continuous ion-transport networks. The LiFePO₄/nano-LLZTO cathode exhibits reduced polarization, higher Coulombic efficiency (99.6%), and superior high-rate capability compared with the microsized LLZTO counterpart, achieving 144 mAh g^-1 at 1C. Cross-sectional analyses confirm that nano-LLZTO forms homogeneous interfacial coatings, improving ionic percolation and mitigating transport bottlenecks. In situ X-ray absorption near edge structure and extended X-ray absorption fine structure analyses further confirm enhanced redox reversibility and structural stability of Fe sites. Consequently, the LiFePO₄/nano-LLZTO composite cathode retains 95% of its initial capacity (154 mAh g^-1) after 200 cycles at 0.2C and 25 °C. This work demonstrates that reducing LLZTO particle size effectively enhances the cathode ion-transport network and ionic conductivity, thereby improving the rate capability and cycling stability of LiFePO₄-based SSLBs.