Resolving Li-F Locking Effect in Disordered Rocksalt Cathodes with Mg-Doping

The discovery of Li-excess cation-disordered rocksalt (DRX) cathodes greatly enlarges the design space beyond the layered NCM-type rocksalt chemistries. Fluorination of DRX cathodes can provide more capacity by reducing the transition metal valence and reduce oxygen loss by protecting the surface. On the other hand, fluorine’s strong bonding with lithium creates Li-F short-range order (SRO) and prevents some lithium from being extracted. We performed a systematic computational study to investigate which inactive doping elements can lower the bonding strength with lithium, and found that substituting a small amount of Li by Mg could be beneficial for generating higher capacity. To investigate this effect, we synthesized two composition with identical Li-excess, fluorination and transition metal amount: Li 1.25 Mn 0.45 Ti 0.3 O 1.8 F 0.2 (LMTF) / Li 1.25 Mg 0.1 Mn 0.45 Nb 0.2 O 1.8 F 0.2 (LMMNF). Electrochemical experiment confirmed an increased capacity from 279 mAh/g (899 Wh/kg) for the undoped sample to 290 mAh/g (905 Wh/kg) for the Mg-doped sample. The inaccessible Li-content after the first cycle reduces from 0.17 to 0.24 per formula unit. In a related group of composition Li 1.33 Mn 0.67 O 1.33 F 0.67 (LMF) and Li 1.28 Mg 0.11 Mn 0.61 O 1.33 F 0.67 (LMMF) the inaccessible Li-ion content was reduced from 0.53 to 0.47 per formula unit, while delivering a similar charge capacity 276 mAh/g and 272 mAh/g. By comparing these results with detailed Monte Carlo simulations of the ionic configurations in the material we conclude that the Li-site distribution plays a more important role than the metal-redox capacity in determining the initial capacity. While fluorination enhances the long-term performance of DRX materials its strong attraction to lithium can make some of those Li-ions unextractable in a reasonable voltage window. Mg-doping can be used to free some of these locked Li-ions within the working voltage range, thus delivering higher charge capacity. This new design strategy should be considered in oxyfluorides cathode materials. Figure 1