Designing Multielectron Lithium-Ion Phosphate Cathodes by Mixing Transition Metals

Finding new polyanionic Li-ion battery cathodes with higher capacities than LiFePO4 is currently a major target of battery research. One approach towards this goal is to develop materials capable of exchanging more than one lithium per transition metal. However, constraints on operating voltage due to organic electrolyte stability as well as cathode structural stability have made this target difficult to reach. More specifically, it is very challenging to develop a phosphate-based cathode in which a single element provides +2 to +4 redox activity in a reasonable voltage window: either the +2/+3 couple is too low (e.g., V) or the +3/+4 couple is too high in voltage (e.g., Fe). This makes several appealing structural frameworks such as tavorites difficult to use as practical two-electron systems. Here, we propose a voltage design strategy based on the mixing of different transition metals in crystal structures known to be able to accommodate lithium in insertion and delithiation. By mixing a metal active on the +2/+3 couple (e.g., Fe) with an element active on the +3/+5 or +3/+6 couples (e.g., V or Mo), we show that high capacity multi-electron cathodes can be designed in an adequate voltage window. We illustrate our mixing strategy on LiMP2O7 pyrophosphates as well as LiMPO4(OH) and LiM(PO4)F tavorites, and we use density functional theory computations to evaluate the theoretical capacity, voltage profile and stability of the compounds proposed by our design rules. From this analysis, we identify several new compounds of potential interest as cathode materials.