High energy ball milling as a strategy to improve electrochemical performance in sodium titanate anodes

High energy ball milling has emerged as a method for both mechanosynthesis and post processing to control the particle size and crystallinity in Li and Na ion battery materials. Sequentially higher ball milling speeds were applied to commercially available Na₂Ti₃O₇ as a post-processing method, followed by chemical and electrochemical characterisation. At lower milling speeds, the Na₂Ti₃O₇ phase was preserved, but with some degradation of crystallinity and phase. At higher milling speeds, full amorphitisation occurred, followed by recrystallisation into the rarely-observed trigonal Na₄Ti₅O₁₂ phase. The sequential changes in electrochemical properties of the materials following increasing ball mill speeds are reported, with the partially degraded Na₂Ti₃O₇ showing increases in specific capacity which we attribute to a decrease in crystallite size and associated access to kinetically limited sites. Trigonal Na₄Ti₅O₁₂ by contrast, is measured to have superior rate capability (40 mA h g^-1 at 5C), and high cycle stability, alongside a promising specific capacity of 115 mA h g^-1 after 50 cycles. This material is therefore a promising anode candidate for Na-ion batteries.