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Suppressing initial capacity fade in Li-rich Li$_5$FeO$_4$ with anionic redox by partial Co substitution

The increasing relevance of energy storage technologies demands high-capacity cathode materials for Li-ion batteries. Recently, Li-rich defect anti-fluorite Li$_5$FeO$_4$ has emerged as a high-capacity cathode material exhibiting simultaneous anionic and cationic redox without significant oxygen release. But suffers from irreversible structural change and capacity fade during cycling. Herein we investigate the suppressed capacity fade reported in Co substituted Li$_5$FeO$_4$ and establish the optimal performance through tuning the concentration and oxidation state of Co. We have substituted Co at the Fe site and carried out a detailed analysis of structural, magnetic, electronic, and electrochemical properties using first-principles density functional theory calculations. The extended stability and suppressed capacity fading are found only in specific compositions depending on the Co/Li concentration and the oxidation state of Co. The reasons behind suppressed capacity fading in Co substituted systems have been unveiled on the basis of bonding analyses and proposed as a strategy to suppress the voltage fading reported in Li-rich materials due to transition metal migration. From the evaluation of thermodynamic stability and electronic structure, Li$_{5.5}$Fe$_{0.5}$Co$_{0.5}$O$_4$, Li$_5$Fe$_{0.25}$Co$_{0.75}$O$_4$, and Li$_{4.5}$Fe$_{0.5}$Co$_{0.5}$O$_4$ are found to exhibit better electrical conductivity than Li$_5$FeO$_4$. All these systems have voltage in the range of 3 to 5 V and exhibit three dimensional Li diffusion pathway with a diffusion barrier height of around 0.3 eV. From Li$_5$Fe$_{0.25}$Co$_{0.75}$O$_4$, one can delithiate three Li-ions without structural change and oxygen release, therefore expected to acquire a reversible capacity of around 513 mAh/g. Moreover, as in pristine Li$_5$FeO$_4$, the selected Co substituted systems also exhibit simultaneous anionic and cationic redox without significant O2 release.