Structure Modulation And Electrochemical Performance Of P2-phase Manganese-based Cathode Material For Sodium-ion Batteries

As a promising cathode material for sodium ion battery,the manganese-based P2-type materials（Na_xMnO₂）have attracted considerable attention due to their low cost,high theoretical capacity and environment friendliness of Mn.However,their practical application is still limited by the following drawbacks:the Jahn-Teller distortion arising from the manganese（III）,migration of transition metal ion and continuous phase transition during charging/discharging process,which can severely damage the structural stability of the material,resulting in poor rate and cycle performance and voltage decay.Aiming at the above problems,this paper uses the ion doping method to effectively improve the long-term cycle stability and rate capability of the material.In this work,we prepared a high-valent state niobium ion-doped cathode material Na0.7[Ni0.3Co0.1Mn0.6]0.98Nb0.02O2 and explored the effect of niobium doping on the electrochemical performance of the material.The as-prepared Nb⁵⁺doped material demonstrates a high capacity retention of 87.9%after 200 cycle at 0.5C.Even at 2C,a capacity retention of 68.4%is retained after 500 cycles.Specifically,the Nb⁵⁺doped material indicates much lower discharge midpoint voltage decay（0.132 V）than that of pristine one（0.319 V）after 200 cycles.Remarkably,it also demonstrates good performance at low temperature（-20 ^oC）.These results indicate that substitution of Nb can effectively improve the cycle stability of the materials and suppress voltage fading during cycling.This is probably a result from reducing migration of Ni²⁺and dissolution of Mn³⁺in the transition metal layers,and to some extent,restraining the phase transition after Nb doping.We designed a P2 phase manganese-based cathode material with low sodium content and high specific capacity(Na_0.47Co_0.1Mn_0.9O₂)and significantly improves its rate performance by zinc doping.Na0.47Co0.1Mn0.87Zn0.03O2 shows an excellent rate performance of 162.9 mAh g^-1 initial discharge capacity at 0.1C,and even at a high rate of 10C,the material can deliver a capacity of 64.7 mAh g^-1.The doping of Zn also enhances the stability of the layered structure,leading to improved cycling performance of 88.7%capacity retention at 0.5C after 100 cycles.Diffusion kinetics results show that diffusion coefficient of Na⁺in the zinc doping material is much higher than that of undoped material.All results prove that the electrochemical performance of the material is obviously enhanced after zinc doping,which is a result that zinc doping can suppress Jahn-Teller distortion of Mn³⁺and material polarization during charging/discharging process.Thus,this stabilizes the structure of the material,improves the reversibility of the redox reaction and accelerates the diffusion rate of sodium ions.