Preparation And Electrochemical Performance Of P2-Na_xMnO₂ Cathode Materials For Sodium-ion Batteries

Lithium-ion batteries have been widely used in the field of portable electronic devices,electric vehicles and other fields,and have achieved great success and rapid growth.However,the low reserves of lithium lead to the increasing cost of lithium-ion batteries,and sodium-ion batteries are expected to replace lithium-ion batteries in large-scale energy storage equipment.Among many cathode materials for sodium ion batteries,layered transition metal oxides,especially sodium manganese oxides,have the advantages of high specific capacity and working voltage,easy to be prepared,environmentally friendly,non-toxic and low cost.P2-type phase structure has higher ionic conductivity and lower diffusion barrier.Thus,P2-type manganese oxide is a promising cathode material for sodium ion batteries.In this paper,the effects of different temperature and high temperature reaction time and Cu²⁺doping on P2-Na_0.67MnO₂ were investigated.P2-Na_0.5Ni_0.25Mn_0.75O₂ with bipolar properties was prepared and modified by Ti⁴⁺doping.The effects of Ti⁴⁺doping on the structure,morphology,electrochemical performance and ion diffusion coefficient of Na_0.5Ni_0.25Mn_0.75O₂ were further studied.The main conclusions are as follows:?1?In the process of preparing P2-Na_0.67MnO₂ by using a high-temperature solid-phase method,the lattice structure of the materials prepared by different heat treatment temperature have not obviously changed.The morphology of the synthetic materials at different temperatures is a micron-sized block structure and has a certain degree of agglomeration.The materials synthesized at 900?and 1000?have similar discharge capacities above 150 mAh g^-1,but the material prepared at 900?has better cycle performance,the capacity retention rate after 100 cycles is 78.1%,and the rate performance is significantly better than 1000?synthesis's material.?2?At 900°C different high temperature reaction time,the materials capacity gradually increases with the extension of reaction time.When the high temperature reaction time is 15h,the electrochemical performance is best,and the initial discharge capacity is 164.8 mAh g^-1.The capacity retention was 84.7%after 100 cycles.When the reaction time is too long for 20 hours,MnO₂impurities are generated in the material and the capacity is lowered.?3?The Cu²⁺doping obviously changes the lattice structure of the materials,and the materials are transformed into a hexagonal system by an orthorhombic system,and CuO impurities are generated when the doping amount is too high.With the increase of the amount of Cu²⁺doping,the stability and the rate performance increase,but the reversible capacity can also be reduced.?4?Ti⁴⁺can be successfully doped into the lattice of P2-Na_0.5Ni_0.25Mn_0.75O₂material,which will increase the lattice parameter c and cell volume,and inhibit the generation of impurities in the P3 phase.After doping,it has no effect on the morphology of the material,but will cause some agglomeration.?5?The electrochemically inactive Ti⁴⁺doping can reduce the discharge capacity of electrode materials from 151 mAh g^-1 to 138 mAh g^-1 in the initial dicharge process,but it can significantly improve the cyclic stability and the rate performance at high current.As cathode and anode electrodes,the cycle retention increases from 62%,57%to 84%,72%,respectively.In addition,Ti⁴⁺doping can effectively reduce the charge transfer impedance by about 35.4%and increase the Na⁺diffusion coefficient in the whole charging and discharging process,especially in the low voltage region.