Study On Synthesis Of Layered Sodium-based P2-Oxide And Its Sodium Storage Performance

P2-type layered manganese-based oxide cathode materials have been widely noticed for their advantages such as high discharge capacity and low cost,however,the high content of Mn³⁺leads to Jahn Teller distortion,which can cause irreversible capacity decay in the battery,thereby affecting cycling performance.In this paper,a layered sodium-based P2-oxide Na_0.70Co_0.17Mn_0.83O₂ was designed as the parent material.The parent material was synthesized and doped with Cu²⁺and Si⁴⁺and co-doped with Cu²⁺/Si⁴⁺by high temperature solid phase method,respectively.Effects of Cu²⁺-doping,Si⁴⁺-doping and Cu²⁺/Si⁴⁺co-doping on the structure and sodium storage performance of the pristine have been systematically investigated by a series of structural characterizations and comprehensive tests of the electrochemical performance,and some meaningful results have been obtained.XRD and FE-SEM results demonstrate that Cu²⁺-doping,Si⁴⁺-doping and Cu²⁺/Si⁴⁺co-doping on the parent material does not change the P2-type structure and lamellar morphology of the pristine,respectively.Rietveld refinements indicate that Cu²⁺and Si⁴⁺have entered the transition metal layer to replace Mn ions.The XPS results show that both mono-doping with Cu²⁺and Si⁴⁺and Cu²⁺/Si⁴⁺co-doping can reduce the ratio of Mn³⁺/Mn⁴⁺in the oxide,reducing the Jahn-Teller distortion and improving the cyclic stability of the material.The maximum discharge capacity and the rate capability of the electrodes increase firstly and then decrease with increasing Cu²⁺-doping.Na_0.70Co_0.17Mn_0.83Cu_0.20O₂ electrode achieves a discharge capacity of 150.53 m Ah g^-1 at 0.1 C,which is 125%as high as that of the parent,and the capacity retention of the electrode is 67.7%after 1000 cycles at 5 C,which is significantly higher than 58.9%of the pristine.This is attributed to the improvement of structural stability of the materials and the weakening of Jahn-Teller effect,and also significantly improves the dynamic performance of the material.Si⁴⁺-doping significantly increases the discharge capacity and improves the kinetics of the pristine oxide,which is ascribed to the improvement of structural stability of the oxide and the weakening of the Jahn-Teller effect.Na_0.70Co_0.17Mn_0.79Si_0.04O₂ electrode presents the maximum discharge capacities of 155.6 m Ah g^-1 at 0.1 C and 142.5 m Ah g^-1 at 0.2 C,respectively,which is 29.0%and 27.3%higher than that of the parent oxide electrode at the same rate.The sample electrode doped with Si⁴⁺undergoes a highly reversible P2–P3–P2phase transition during charging and discharging processes.Cu²⁺/Si⁴⁺co-doping significantly improves the cycling stability of P2 oxide at low rate,which is attributed to alleviation of Jahn Teller distortion Na_0.70Co_0.17Mn_0.65（Cu₅Si）_0.03O₂electrode presents a capacity retention rate of 93.0%at 0.1 C after 100 cycles,which is much higher than 82.2%of the parent electrode.and The Cu²⁺/Si⁴⁺co-doping also increases the diffusion coefficient of Na⁺and reduces the charge transfer resistance of the elctrochemical reactions,which significantly improves the dynamic performance of the material.