Reversible Mn2+/Mn4+ double redox in lithium-excess cathode materials

Abstract

There is an urgent need for low-cost, resource-friendly, high-energy-density cathode materials for lithium-ion batteries to satisfy the rapidly increasing need for electrical energy storage. To replace the nickel and cobalt, which are limited resources and are associated with safety problems, in current lithium-ion batteries, high-capacity cathodes based on manganese would be particularly desirable owing to the low cost and high abundance of the metal, and the intrinsic stability of the Mn oxidation state. Here we present a strategy of combining high-valent cations and the partial substitution of fluorine for oxygen in a disordered-rocksalt structure to incorporate the reversible Mn/Mn double redox couple into lithium-excess cathode materials. The lithium-rich cathodes thus produced have high capacity and energy density. The use of the Mn/Mn redox reduces oxygen redox activity, thereby stabilizing the materials, and opens up new opportunities for the design of high-performance manganese-rich cathodes for advanced lithium-ion batteries.

Design principles for high transition metal capacity in disordered rocksalt Li-ion cathodes

“…57 The broad peak near 534 eV indicates the presence of surface carbonate species, 58 which are likely to form when the active materials are mixed with carbon black. 21 We cannot see the evolution of the 534 eV feature in the ex situ charged samples because during the sample preparation, the surface carbonate species have been washed away. 59 Upon charging, we observe an increasing intensity around 530 eV up to 320c, corresponding to continuous Mn 3+ formation during the charging process.…”

“…The increase in Li-gettering by F at higher Li-excess levels indicates that fluorine doping may not increase Li capacity within a given voltage window even in the case where fluorine incorporates into the bulk lattice, even though fluorination is beneficial for other properties such as stability on cycling and obtainable metal-redox capacity. 5,6,21 Thus, in broad terms, in a disordered rocksalt oxyfluoride material, one may expect a reduction in accessible Li capacity equal to 0.4-0.8 Li per F in the discharged cathode material.…”

“…6,8,11 The recently realized Mn 2+/4+ couple avoids this limitation while providing high capacity at a high average voltage. 21 However, as the low-valence of the discharged Mn 2+ requires a source of charge compensation, previous reports have mixed the active Mn with redox-inactive elements, as well as fluorine. It is thus possible to obtain higher transition-metal capacity by using a redox-active charge compensator instead.…”

“…A large number of high-performance cathodes have already been reported among Li-excess disordered rocksalts. To ensure that the composition is charge balanced, these materials have either relied on high-valence transition metals as the redox-active center, such as in Li 3 21 Despite the numerous reported compounds however, the question of which composition is optimal for high reversible capacity and energy density remains open. The design of a disordered rocksalt cathode material essentially consists of choosing one or more active redox centers, the Li-excess content and F/O ratio on the anion site, while ensuring that the desired combination of oxidation states is stable and that the compound is synthetically accessible.…”

Ten years left to redesign lithium-ion batteries

“…Earlier this year, a cathode (based on lithium manganese niobium oxyfluoride) was demonstrated in the lab that could almost match the energy storage capacity of cobalt-and nickel-based materials 10 . But a high voltage is needed to charge it, making it unsafe for use in vehicles.…”