A theoretical study on the cyanation strategy for modulating borate performance in the design of electrolytes for high-voltage lithium-ion and lithium metal batteries

Abstract

In the field of lithium batteries, the development of novel electrolyte additives or solvents is crucial for improving the electrochemical performance of batteries, enhancing environmental adaptability, and reducing costs. In high-voltage lithium-ion batteries and lithium metal battery systems, borates and nitriles have been proven to be efficient electrolyte additives or solvents, significantly enhancing electrochemical performance. However, there remains a research gap regarding the influence of cyano group incorporation on borates. In light of this gap, we designed 19 cyano-modified borates based on trimethyl borate (TMB) and triethyl borate (TEB) and performed quantum chemical calculations on traditional solvents—ethylene carbonate (EC), ethyl methyl carbonate (EMC), TMB, and TEB—and their nitrile derivatives, obtaining the following results. First, the calculation results indicate that the oxidation resistance of molecules strengthens as the number of cyano groups increases. Moreover, when the number of cyano groups is the same, molecules with cyano groups evenly distributed across three side chains exhibit stronger oxidation resistance compared to those with cyano groups concentrated on a single side chain. Using the comprehensive descriptor of EC and EMC (consisting of oxidation potential, maximum electrostatic potential, electrochemical stability window, and molecular polarity index) as a benchmark, classify the molecules. The comprehensive descriptor of each cyanated molecule was compared with those of EC and EMC. Molecules with comprehensive descriptors lower than that of EMC were considered as additives, those with comprehensive descriptors higher than that of EC were considered as solvents, and those with descriptors falling between those of EC and EMC were considered to have potential as both solvents and additives. Second, we screened 16 molecules suitable as additives based on six dimensions: binding energies with O₂⁻, F⁻, HF, and Li⁺, the LUMO (lowest unoccupied molecular orbital)–HOMO (highest occupied molecular orbital) gap and the binding energy with Li⁺ of decomposition products. Among them, TMB_33 demonstrated significant potential as an additive. Third, we evaluated the performance of molecules as solvents across four dimensions: binding energy with Li⁺, minimum electrostatic potential, LUMO–HOMO gap, and molecular polarity index, comparing them with some commonly used conventional solvents. The results showed that TMB_111 exhibits significant potential as a solvent. This work explores the influence of the number and distribution position of cyano groups on the properties of borates, providing a reliable reference and theoretical basis for the design of cyanoborate molecules. It also identifies several promising additives and solvents, offering practical strategies for performance improvement in various high-voltage lithium-ion batteries and lithium-metal batteries.