High capacity lithium-ion electrodes made from earth-abundant materials are highly desirable for large-scale energy storage applications. High entropy materials are a relatively new class of materials that have only recently started to be explored in applications ranging from battery electrodes[1],[2] to solid electrolytes[3] and electrochemical catalysts[4]. High entropy materials comprise of five or more elements randomly arranged in the lattice, with equal probability of occupying the same sites.[5] This disorder, or high configurational entropy, endows the materials with unusual chemical and electronic properties that are still being uncovered.[6] In this study, we present two novel high entropy sulfides including wurtzite Cu₇Mg₂Sn₂ZnGeS₁₃ and zinc-blende Cu₃AlGaInZnS₇, as conversion-type lithium-ion cathode materials with extremely high capacities.

Our results show high capacities of ~1000 mAh g^-1 at 50 mAh g^-1 which is ca. 15% higher than the theoretical capacity of the binary sulfide CoS₂, an extensively studied material as a lithium-ion conversion material.[7] The plateaus in the discharge curves of both materials at ~ 1.7 V and ~0.8 V can be attributed to the reduction of the high entropy sulfide to metal and Li₂S, while in the charging curve, the anodic peaks at ~2.0 V and ~ 2.4 V are assigned to a two-step resulfidation process.[8] These processes are largely reversible, with a variance of not more than +/- 10 mA g^-1 between the charge and discharge capacities . At higher current densities, from 100 – 500 mA g^-1, the specific capacity degrades quickly, but unexpectedly recovers in the last five cycles when the current density returns to 50 mA g^-1. The capacity fade can be attributed to lithium-aluminium alloy formation which occurs at 0.23 V – visible as a low-voltage plateau in the final stage of discharge, as well as SEI formation. Current work is focused on replacing the aluminium current collector with copper foil and carrying out galvanostatic charge-discharge tests over a narrower potential window. Further work will focus on characterising the phases being formed and experimenting with different carbon and electrolyte additives to stabilise the SEI.

This study demonstrates versatile novel high entropy materials with high capacities that could lead to the development of more energy dense lithium-ion batteries using earth abundant transition metals. The findings of this study may be applied to a variety of unexplored high-entropy materials for a range of electrochemical energy conversion/storage applications thanks to their desirable properties, tunability and easy synthesis methods.