Solvent-free single-ion conducting borate network polymer electrolyte for lithium metal battery applications

Abstract

Climate change and fossil fuel shortages have urged society to utilize renewable energy sources (e.g. solar, water and wind). However, lithium-ion batteries, the prevailing clean energy storage technology, face limitations in energy density, cyclability, and safety concerns associated with commercial liquid electrolytes due to their flammability and instability with high energy density anode. Solvent-free single-ion conducting (SIC) polymer electrolytes emerge as a promising alternative, offering enhanced safety and mitigated dendrite growth for extended battery cycle life. In this thesis, we firstly designed a series of SIC borate network polymer electrolytes and enhanced Li + transport conductivity by manipulating their segmental mobilities through alterations in interanionic distance and networking degrees. The anionic network polymer (ANP), possessing rapid segmental mobility, demonstrates a high ionic conductivity (3.02 × 10 −7 S cm −1 ) at room temperature, significant selectivity for lithium-ion conduction (t Li + = 0.979), and a broad working electrochemical window (4.2 V). Besides, the results revealed a substantial (at least 4-fold) increase in ionic conductivity with augmented segmental mobility. However, ANP with flexible side chains experienced constrained ionic transport despite high mobility. Furthermore, we developed a series of microcrack-free borate ANP membranes formed by a facile one-step click reaction, demonstrating superior non-flammability and prolonged cyclability in Li metal batteries under high temperatures (a capacity retention of 92.7% and an average coulombic efficiency of 99.867% at 450 cycles). The microcrack-free borate ANP membranes also exhibited a decent cationic conductivity (3.1 × 10 −5 S cm −1 ) at 88 °C, a wide electrochemical window (up to 5 V), and outstanding resistance to dendrite growth. These enhanced properties are attributed to the presence of tethered borate anions in microcrack-free membranes. Notably, molecular modelling also provided insights into lithium-ion migration behavior within electrolytes, revealing that tethered borate anions accelerate selective Li + cations transport. Our research not only reveals the lithium-ion transport behavior within ANP electrolytes but also provides fundamental design principles for highly conducting electrolytes suitable, which opens avenues for future advancements in next-generation lithium batteries.