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The increasing relevance of energy storage technologies demands high-capacitycathode materials for Li-ion batteries. Recently, Li-rich defect anti-fluoriteLi$_5$FeO$_4$ has emerged as a high-capacity cathode material exhibitingsimultaneous anionic and cationic redox without significant oxygen release. Butsuffers from irreversible structural change and capacity fade during cycling.Herein we investigate the suppressed capacity fade reported in Co substitutedLi$_5$FeO$_4$ and establish the optimal performance through tuning theconcentration and oxidation state of Co. We have substituted Co at the Fe siteand carried out a detailed analysis of structural, magnetic, electronic, andelectrochemical properties using first-principles density functional theorycalculations. The extended stability and suppressed capacity fading are foundonly in specific compositions depending on the Co/Li concentration and theoxidation state of Co. The reasons behind suppressed capacity fading in Cosubstituted systems have been unveiled on the basis of bonding analyses andproposed as a strategy to suppress the voltage fading reported in Li-richmaterials due to transition metal migration. From the evaluation ofthermodynamic stability and electronic structure,Li$_{5.5}$Fe$_{0.5}$Co$_{0.5}$O$_4$, Li$_5$Fe$_{0.25}$Co$_{0.75}$O$_4$, andLi$_{4.5}$Fe$_{0.5}$Co$_{0.5}$O$_4$ are found to exhibit better electricalconductivity than Li$_5$FeO$_4$. All these systems have voltage in the range of3 to 5 V and exhibit three dimensional Li diffusion pathway with a diffusionbarrier height of around 0.3 eV. From Li$_5$Fe$_{0.25}$Co$_{0.75}$O$_4$, onecan 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 systemsalso exhibit simultaneous anionic and cationic redox without significant O2release.

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