Investigating S-S bond formation during S2- oxidation in Li-rich sulfide battery materials using Inelastic Neutron Scattering

Layered lithium transition metal oxides continue to be the dominant cathode materials for rechargeable Li-ion batteries. However, these materials suffer from practical issues such as voltage hysteresis and capacity fade over cycling due to the formation of O2 molecules both evolved at the surface and trapped within the bulk. Replacing the oxide ions (O2-) with sulfide ions (S2-) to make Li-rich sulfides offers a promising route to mitigate these undesired anionic redox behaviours and opens up a whole new chemical space to discover high energy density cathodes. However, the fundamental mechanism of S-redox remains poorly understood. The nature of the oxidized sulfur species formed as S2- is oxidised during S-redox needs to be properly understood if we are to fully exploit the potential of anionic redox in next generation high energy density batteries. One of the main hypothesis in the field proposed that (S–S)n- dimers (where 0 < n < 4) are the sulfur oxidized species formed when these materials are charged to 3V, but the precise nature of the S-S bonding is not known. We propose to measure the vibrational properties of the S-S bond directly using Inelastic Neutron Scattering (INS) to conclusively determine the bond order in two battery cathodes, namely Li1.2Ti0.8S2 and Li2FeS2.