Polymorphic control of anion redox mechanism in thiophosphate cathode materials Hollie Richards 1,2 , Pezhman Zarabadi-Poor 2,3 , Samuel W. Coles 1,2 , M. Saiful Islam 2,3 , Benjamin J. Morgan 1,2 1 Department of Chemistry, University of Bath, BA2 7AY, UK, 2 The Faraday Institution, Harwell Campus, Didcot, OX11 0RA UK, 3 Department of Materials, University of Oxford, Oxford, OX13 3PH, UK The demands of the automotive energy transition and the precarious nature of cobalt and nickel ore supply chains provide strong motivation for the development of new, high energy density cathode materials, using purely Earth abundant elements. Previously, Li-rich oxides have been shown to be able to access a greater energy density than traditional cathodes by activation of the oxide redox couple alongside the transition metal [1] . The practical employment of these materials is prevented by voltage hysteresis due to oxygen loss and concurrent structural rearrangement [2] . Li-excess sulphides have a lower energy density but can exhibit reversible anion redox processes based around reversible sulfur dimerization [3] . Polyanion cathode materials have been hypothesised as possible route to achieve Li-excess oxide cathode voltages with the degree of reversibility seen in Li-excess sulphides. However, the exploration of this huge chemical space is in its infancy. This work has its origin in the recently reported polyanionic cathode material Li 2 Fe 0.8 Mn 0.2 P 2 S 6 [4] , which undergoes anion redox in the form of intramolecular sulfur dimerisation between P 2 S 6 molecular anions. Here, we go deeper by performing a statistically mechanically thorough first principles comparison of two polymorphs Li 2 FeP 2 S 6 [5,6] the archetypical Li-rich thiophosphate cathode system by which to investigate this relationship. Despite having the same compositions, these materials show distinctly different delithiation behaviours and totally different anion redox mechanisms. This polymorphic control of redox mechanism makes clear that there may be more chemical flexibility than we could ever have expected in cathode materials.
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