Solid-State NMR Studies of Ternary Alloys for use in Sodium-ion Batteries

Abstract

Due to the pressures of climate change it is important to produce methods of renewable energy generation and storage which are cheap and can be applied to many industries and consumer needs. Sodium-ion batteries have proved viable alternatives to current Li-ion batteries, however they are unusable with graphite, which is the best negative electrode material we currently use. It is therefore important to develop other negative electrode materials which can perform to the same standards.

This research aims to investigate the performance of TiSnSb, a conversion negative electrode material, against sodium using electro-cycling and Solid-state Magic Angle Spinning Nuclear Magnetic Resonance (MAS ssNMR) to elucidate the chemical reactions which take place within the battery cells. Battery cells were constructed in a glovebox under an argon atmosphere, and data was obtained using a benchtop potentiostat and a Bruker 500.

Data was obtained for TiSnSb at C/2 and 4C rates for the first sodiation (discharge), first de-sodiation (charge), and second de-sodiation. C rate is defined as the number of sodium ions inserted per formula unit of TiSnSb per hour. Data was also collected for a rough mixture of Ti+Sn+Sb.

TiSnSb was found to operate via a conversion mechanism upon sodiation and de-sodiation with starting potentials of 2.6 – 2.8 V achieved consistently for multiple electrodes. In contrast Ti+Sn+Sb operates through an alloying reaction with multiple ‘stages’ of the reaction. The electrochemical rate has a strong effect on the number of Na ions inserted into the structure and the phases formed as evident from the potential profiles and MAS NMR spectra.