Triethyl borate and tripropyl borate as electrolyte additives for 4.8 V high voltage layered lithium-rich oxide cathode with enhanced self-discharge suppression performance: A comparative study

Trimethyl borate (TMB) has been identified as electrolyte additive to suppress self-discharge for high voltage layered lithium-rich oxide cathode (LRO) for lithium ion batteries in our previous work. To expand pervious findings, its derivatives, triethyl borate (TEB) and tripropyl borate (TPB) with increased alkyl chains, have been considered as electrolyte additives to mitigate self-discharge behavior of LRO cathode for lithium ion batteries and we make a contrast to explain which one is better and why is better. It is found that fully charged (4.8 V, vs. Li/Li⁺) LRO cathode suffers severe self-discharged issue upon storage in standard (STD) electrolyte (1 mol L^-1 LiPF₆-EC/EMC/DEC (3:5:2, in weight)). In contrast, this detrimental self-discharge behavior of LRO cathode at high voltage can be effectively depressed by introducing TEB or TPB into the electrolyte system. Combinations of electrochemical methods and multi spectroscopy techniques have been employed to evaluate effects of TEB and TPB on the LRO cathode surface. They reveal that the B-O and B-F bonds species in the film formed by TEB and TPB prevents electrolyte decomposition and protects LRO from structure transformation. These species in the film formed by TEB is much more than TPB, which explains the better effect for TEB.