Sufficient Utilization of Mn~(2+)/Mn~(3+)/Mn~(4+) Redox in NASICON Phosphate Cathodes towards High-Energy Na-Ions Batteries

摘要

Na superionic conductor of Na_3MnTi(PO_4)_3 only containing high earthabundanceelements is regarded as one of the most promising cathodesfor the applicable Na-ion batteries due to its desirable cycling stability andhigh safety. However, the voltage hysteresis caused by Mn~(2+) ions residedin Na+ vacancies has led to significant capacity loss associated with Mnreaction centers between 2.5–4.2 V. Herein, the sodium excess strategybased on charge compensation is applied to suppress the undesirablevoltage hysteresis, thereby achieving sufficient utilization of the Mn~(2+)/Mn~(3+)and Mn~(3+)/Mn~(4+) redox couples. These findings indicate that the sodiumexcess Na_(3.5)MnTi_(0.5)Ti_(0.5)(PO_4)_3 cathode with Ti~(4+) reduction has a lowestMn~(2+) occupation on the Na~+ vacancies in its initial composition, which canimprove the kinetics properties, finally contributing to a suppressed voltagehysteresis. Based on these findings, it is further applied the sodium excessroute on a Mn-richer phosphate cathode, which enables the suppressedvoltage hysteresis and more reversible capacity. Consequently, this developedNa_(3.6)Mn_(1.15)Ti_(0.85)(PO_4)_3 cathode achieved a high energy density over380 Wh kg~(?1) (based on active substance mass of cathode) in full-cell configurations,which is not only superior to most of the phosphate cathodes,but also delivers more application potential than the typical oxides cathodesfor Na-ion batteries.