Nail penetration safety enhancement in semisolid-state batteries via composite solid-state electrolyte separators

This study develops a composite solid-state electrolyte (SSE) separator for semisolid-state batteries, achieving a 450 Wh/kg lithium metal battery that passes stringent nail penetration tests. The SSE, composed of PEO-LLZTO coated on a polypropylene base, exhibits superior elongation and ionic conductivity enabling effective nail encapsulation during penetration. A custom steel nail with embedded thermocouples was used to monitor the maximum temperature at the penetration site of different pouch cells during the nail penetration test, providing more accurate measurements than surface-mounted thermocouples. A parallel circuit model quantification demonstrated significant differences in peak current and short-circuit internal resistance between liquid batteries and semisolid-state batteries. Semisolid-state batteries with 1.4 g/Ah electrolyte filling demonstrated safety across capacities (3.2–7.5 Ah), whereas liquid batteries failed. The posttest microscope images confirmed the role of SSE in reducing the peak splitting height and fracture area of the electrodes. This work provides a methodological framework for nail penetration analysis, highlighting SSE separators as a solution to balance energy density and safety.