Regulating the Electrochemical Performance of A₂Ni₂TeO₆ (A = Na, K) as a Cathode of Alkali Metal Ion Battery by 3d Transition Metal Substitution from a Theoretical Perspective

With its unique honeycomb layered structure, P2-type A2Ni2TeO6 (A = Na, K) exhibits remarkable cycling stability and ionic diffusion capability, making it a promising cathode material for sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). However, the high operating voltage of A2Ni2TeO6 (A = Na, K) leading to surface degradation and CEI formation limits the capacity of A2Ni2TeO6 (A = Na, K), where only 2/3 of Na+ and 1/2 K+ can be reversibly extracted at a high charge rate. To enhance the capacity of A2Ni2TeO6 (A = Na, K) while maintaining its cycling stability, we delved into the impacts of 3d transition metal substitution on sodium and potassium storage chemistry through first-principles calculations. Our investigation includes multiple facets: lattice structure, substituting formation energy, electronic properties, ionic diffusion, average open-circuit voltage, transition metal migration, and intermediate phases in the high-voltage region. After comprehensive consideration, Mn- and Fe-substituted Na2Ni2TeO6 and Fe-substituted K2Ni2TeO6 emerged as the most promising candidates, exhibiting exceptional electrochemical performance. Furthermore, we discovered that the energy difference between alkali metal ions occupying substitution sites and active transition metal sites dominates the ionic diffusion behavior in TM-substituted A2Ni2TeO6 (A = Na, K), and the nonuniform distribution of alkali metal ions significantly contributes to the large volume change during ionic extraction. The findings of this work not only underscore the intricate structure–activity relationship of P2-type A2Ni2TeO6 (A = Na, K) substitution but also provide theoretical insights for future application of honeycomb layered transition metal oxides (HLOs) in SIB and PIB cathodes.