• • LIT0.3C achieves an ionic conductivity of 2.19 mS cm−1 at 30 °C and a low activation energy of 0.273 eV, enabling efficient Li+ transport in all-solid-state batteries.
• • Ta5+ doping induces two Li+ vacancies per substitution, doubling vacancy generation efficiency compared to tetravalent doping, without altering the ccp structure.
• • ASSBs with LIT0.3C and Ni90 cathodes retain 72.3% capacity after 1000 cycles at 0.5 C, versus <50% retention after 300 cycles with undoped LIC, demonstrating superior long-term stability.
• • NCM523 ASSBs with LIT0.3C deliver an initial capacity of 171.5 mAh g−1 at 0.1 C with 92.7% initial coulombic efficiency, and retain 84.1% after 500 cycles at 0.2 C and 80.7% after 1000 cycles, confirming practical viability.