Inducing weak and negative Jahn–Teller distortions to alleviate structural deformations for stable sodium storage†

In the quest for efficient supercapacitor materials, manganese-based layered oxide cathodes stand out for their cost-effectiveness and high theoretical capacity. However, their progress is hindered by the Jahn–Teller (J–T) distortion due to the unavoidable Mn⁴⁺ to Mn³⁺ reduction during ion storage processes. Our study addresses this challenge by stabilizing the K_0.5MnO₂ cathode through strategic Mg²⁺ substitution. This substitution leads to an altered Mn³⁺ electronic configuration, effectively mitigating the strong J–T distortion during ion storage processes. We provide a comprehensive analysis combining experimental evidence and theoretical insights, highlighting the emergence of the weak and negative J–T effects with reduced structural deformation during electrochemical cycling. Our findings reveal that the K_0.5Mn_0.85Mg_0.15O₂ cathode exhibits remarkable durability, retaining 96.0% of initial capacitance after 8000 cycles. This improvement is attributed to the specific electronic configurations of Mn³⁺ ions, which play a crucial role in minimizing volumetric changes and counteracting structural deformation typically induced by the strong J–T distortion. Our study not only advances the understanding of managing J–T distortion in manganese-based cathodes but also opens new avenues for designing high-stability supercapacitors and other energy storage devices by tailoring electrode materials based on their electronic configurations.