The environmental challenges issued by the current development of our societies urge a redesign of the whole energy cycle. The intermittence of renewable energies requires storage, and their use in most transport needs a vector with high energy density.
Today, this is best achieved by lithium batteries, which after enabling the portable device revolution in the past three decades are also suitable for these more ambitious goals. Their main limitations concern cost, energy density, and use of raw materials (mainly cobalt) that characterize the present technology.
Other novel battery chemistries are under study, but the highest market readiness is provided by alternative lithium intercalation materials to be used at the positive electrode. Lithium-rich transition-metal-oxide cathodes are obtained by replacing some of the cobalt and other transition metals by an excess of lithium. In this way they deliver larger capacities by keeping the operation at high voltages. However, their cycle-life remains limited, and individual roles of the transition-metals are still not fully understood.
Using X-ray spectroscopy and microscopy on the compound Li[Li0.2Ni0.16Mn0.56Co0.08]O2, we focused on the behavior of manganese (Mn), generally considered structural and electrochemically inert upon the lithium deintercalation-intercalation process. During charge, when lithium cations are extracted from the compound, while nickel (Ni), cobalt (Co) and oxygen (O) are oxidized, Mn actually appears to reduce. This counterintuitive effect is reverted in discharge, so that Mn always behaves in opposition to the other elements, in particular Ni. By studying also its electronic configuration and spatial distribution we suggest that this behavior functions to stabilize the redistribution of charge and strain in this active material. Controlling these redistributions, by means, for instance, of appropriate doping, seems the most appropriate strategy to improve its charge-discharge cycle life.
Role of Manganese in Lithium- and Manganese-Rich Layered Oxides Cathodes
Laura Simonelli, Andrea Sorrentino, Carlo Marini, Ramanan, Nitya; Heinis, Dominique; Olszewski, Wojciech; Mullaliu, Angelo; Birrozzi, Agnese; Laszczynski, Nina; Giorgetti, Marco; Passerini, Stefano; Tonti, Dino
Journal of Physical Chemistry Letters 10, (12), 3359–3368, 2019
Figure: Semiquantitative molar fractions for different oxidation states obtained from intensities of O and Mn K-edge absorption spectra from bare and coated samples at different charge states along charge (yellow background) and discharge (green) cycles: Ni4+ (a), Mn4+ (b) and Mn4+ vs. Ni 4+.