A Strategy to Mitigate Jahn Teller Effect of Mn-Rich NASICON Framework for Sodium-Ion Batteries

Mn-based sodium superionic conductors have driven attention to the low-cost advanced cathode materials for sodium-ion batteries (SIBs). However, low-rate capability and unsatisfactory cyclic performance due to the Jahn teller effect of Mn³⁺ redox couple which occurs from the change in Mn-O bond length at the octahedral site of crystal structure during charge-discharge, eventually limiting their application. Herein, a disordered and sodium deficient NASICON Na_4-xMn(FeVCrTi)_0.25(PO₄)₃ (termed as Na_4-xMn(HE)) is synthesized to mitigate this Jahn teller effect to achieve high rate and ultrastable cathode material. Interestingly, the as-prepared Na_3.5Mn(HE) shows five reversible electron reactions (i.e., Ti³⁺/Ti⁴⁺, Fe²⁺/Fe³⁺, V³⁺/V⁴⁺, Mn²⁺/Mn³⁺, and Mn³⁺/Mn⁴⁺) and demonstrates 141 mA h g^-1 at 0.2 C with 80% capacity retention at 1 C after 500 cycles which is far superior to its counterparts binary Mn-based materials. The excellent cyclic performance is due to the remediation of the Jahn teller effect in sodium-deficient entropy-stabilized material. The structural reversibility, enhanced kinetics, and electronic properties are further studied in detail by in situ X-ray diffraction (XRD), ex situ X-ray photoelectron spectroscopy (XPS), and first principal calculations. Na_3.5Mn(HE)//HC full cell delivered 89.7 mAh g^-1 capacity at 0.2 C. This work sheds light on designing Mn-based cathodes with superior electrochemical performance for wide energy storage applications.