Enhancing electrochemical performance of solid-state Li-ion batteries with composite electrolytes of fibrous LLZTO and PVDF-HFP: The role of LLZTO fiber diameter

Composite solid electrolytes (CSEs) are synthesized using poly(vinylidene fluoride-co-hexafluoropropylene) incorporated in a three-dimensional framework of Li6.75La3Zr1.75Ta0.25O12 (LLZTO) ceramic fibers with varying diameters (80 nm, 140 nm, 290 nm, and 6 μm). The influence of LLZTO fiber diameter on the electrochemical performance of the CSEs is investigated. LLZTO fibers of smaller diameter improve the mechanical strength and electrochemical properties of the CSEs, including conductivity, activation energy, and transference number of lithium ion (Li+). Lithium iron phosphate batteries based on different CSEs are evaluated in terms of Li+ polarization, charge-discharge capacity, cycle life, and rate capability. The battery utilizing CSE with 6 μm LLZTO fibers displays an initial discharge capacity of 147 mAh g−1 at 0.2C but has a short charge-discharge life of only 47 cycles, with poor capacity retentions of 64 % and 17 % at 0.5C and 1.0C, respectively. In contrast, the battery with CSE containing 80 nm LLZTO fibers exhibits excellent cycle stability and satisfactory rate capability, maintaining an initial capacity of 161 mAh g−1 at 0.2C for over 200 cycles, with capacity retentions of 95 % and 78 % at 0.5C and 1.0C, respectively. Finite element simulations are employed to elucidate the intricate Li+ flux and trajectory within the ceramic and polymer regions of the composite electrolytes. Such details are beyond the reach of experimental methods alone, thereby enabling us to unravel the mechanism underlying the enhanced electrochemical properties of Li+ in CSEs with thinner LLZTO fibers.