Thermodynamic Activation of Charge Transfer in Anionic Redox Process for Li-Ion Batteries

Anionic redox processes are vital to realize high capacity in lithium-rich electrodes of lithium-ion batteries. However, the activation mechanism of these processes remains ambiguous, hampering further implementation in new electrode design. This study demonstrates that the electrochemical activity of inert cubic-Li2TiO3 is triggered by Fe3+ substitution, to afford considerable oxygen redox activity. Coupled with first principles calculations, it is found that electron holes tend to be selectively generated on oxygen ions bonded to Fe rather than Ti. Subsequently, a thermodynamic threshold is unravelled dictated by the relative values of the Coulomb and exchange interactions (U) and charge-transfer energy (Delta) for the anionic redox electron-transfer process, which is further verified by extension to inactive layered Li2TiS3, in which the sulfur redox process is activated by Co substitution to form Li1.2Ti0.6Co0.2S2. This work establishes general guidance for the design of high-capacity electrodes utilizing anionic redox processes.