High ionic conductivity of a flexible solid-state composite electrolyte for a lithium-ion battery

Abstract

Solid-state electrolytes are revolutionary electrolytes for use in lithium-ion batteries due to their potential to confer high energy density and their nonflammability, nonvolatility, and superior safety compared with liquid electrolytes. Herein, a polyethylene glycol (PEG)/polyethylene oxide (PEO)/Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP)/cellulose nanofibril (CNF) solid composite electrolyte (named the PLC SCE) is proposed that has long pathways for Li-ion transport to boost ionic conductivity and suppress lithium dendrite growth; these qualities result in a long battery lifetime and favorable anticorrosion ability of Li sheets. Increasing the number of carboxyl groups attached to the CNFs leads to higher surface charge density and lower activation energy for Li⁺ conduction by optimizing solvation and facilitating the transport of Li⁺. A freeze-drying process is used to fabricate the PLC SCE from PEO, PEG, and LATP deposited in a well-arranged CNF network; the PLC SCE has Li⁺ transport pathways and achieves conductivity of up to 1.31 × 10⁻⁴ S cm⁻¹ at 30 °C. The tensile stress of the PLC SCE is 0.96 MPa, which is 246% higher than that of a PEO/PEG/LATP SCE (i.e., no CNFs) and considerably inhibits lithium dendrite formation. Moreover, the PLC SCE is fire-resistant and has an electrochemical window of 0–5.5 V (vs. Li⁺/Li). LiFePO₄/Li cells containing the PLC SCE have a maximum discharge capacity of 154.6 mA h g⁻¹ and capacity retention of 95.1% after 200 cycles at 0.2C. A symmetrical lithium-ion battery containing the PLC SCE is stable for 300 h. Highly flexible pouch batteries performed well when subjected to folding, nail pressing, and cutting in tests. Thus, the electrolyte is promising for use in all-flexible and high-performance solid-state lithium-ion batteries.