Cooperative MnCr redox reconfiguration for stable NASICON sodium-ion cathodes

Mn Cr charge compensation is reconfigured through systematic redox peak shifts. • Cooperative Mn Cr redox suppresses transition-metal charge fluctuation. • XAS and DFT reveal the origin of cooperative redox evolution. • Regulated redox chemistry enhances structural stability and reversibility. • Mechanistic insights guide the design of stable NASICON cathodes. NASICON-type phosphates are promising cathode materials for sodium-ion batteries, owing to their open three-dimensional framework and fast Na + transport kinetics. Among them, Mn-based NASICON compounds are particularly attractive due to their high theoretical capacity, yet their practical performance is often limited by localized redox reactions and structural instability associated with Mn-centered charge compensation. Here, we report that partial substitution of Cr with electrochemically inactive Al in Na 4 MnCr(PO 4 ) 3 induces systematic redox peak shifts, enabling a redistribution of charge compensation between Mn and Cr centers. This redox reconfiguration is verified by Cyclic Voltammetry analysis and X-ray absorption spectroscopy, revealing a coordinated valence evolution of transition metals and modulated electronic structures. In addition, Density Functional Theory calculations indicate minimized fluctuations in the charge states of Mn and Cr during desodiation, suggesting a more balanced and cooperative charge compensation process. Consequently, the Al-doped cathode exhibits enhanced redox reversibility and suppressed structural distortion, delivering improved cycling stability with 90.1% capacity retention after 200 cycles at 1C. These findings highlight that tuning redox provides an effective route to regulate charge compensation and stabilize high-performance Mn-based NASICON cathodes. Download: Download high-res image (126KB) Download: Download full-size image