Preparation And Modification Of Na_0.7Mn_0.8Ni_0.2O₂ Cathode Material For Na-ion Batteries

Since the 1870s,lithium-ion batteries technology has developed rapidly,quickly entering the market from the laboratory.Compared with traditional lead-acid batteries or alkaline batteries,lithium-ion batteries have gradually replaced traditional lead-acid batteries due to their high energy density,low environmental pollution,and high safety.However,lithium amount is relatively low in nature and ranks 27th in abundance on Earth.The distribution of lithium ore resources is extremely uneven,most of which are located in North and South America,and a small amount in other regions.So it cannot fully meet people’s demand.Sodium exists in large amounts in nature,ranking 6^thin the natural element content ranking.And it is widely distributed,from inland saltwater lakes,underground rock formations,to the vast and boundless sea,sodium is found in nature.Therefore,sodium-ion batteries have resource advantages for large-scale applications.P2-type manganese-based transition metal oxides,which have strong application prospects in sodium-ion batteries,are highly expected because of their abundant raw material resources,high specific capacity,simple synthesis methods,and safer use.However,the larger ionic radius of sodium restricts the migration of sodium ions in the material and in the electrolyte.Compared with lithium ions,the intercalation and deintercalation of sodium ions from the crystal structure causes larger volume changes,resulting in irreversible changes in the structure of electrode material.And Mn³⁺in manganese-based oxides increases the solubility of Mn in weakly polar organic electrolytes,due to the Jahn-Teller effect during the charging and discharging process of the battery,resulting in the dissolution of the electrode in the electrolyte,and ultimately leads to the deterioration of the rate performance and cycle performance of the sodium ion battery.In this paper,the methods of material surface coating and internal element doping were used to optimize the P2-type manganese-based cathode material Na_0.7Mn_0.8Ni_0.2O₂.（1）First,Na_0.7Mn_0.8Ni_0.2O₂was prepared by solvothermal combined calcination method,and then Mg F₂was coated on the surface of Na_0.7Mn_0.8Ni_0.2O₂by wet chemical method.After electrochemical performance tests,the first discharge specific capacity of the uncoated Na_0.7Mn_0.8Ni_0.2O₂at a current density of 100 m A/g was104m Ah/g at a voltage window of 2-4 V.After 300 charges and discharges,the battery has a capacity of 48.7 m Ah/g,with a capacity retention rate of 46.8%.The Na_0.7Mn_0.8Ni_0.2O₂coated with 1 wt.%Mg F₂can obtain an initial discharge capacity of110.4 m Ah/g at a current density of 100 m A/g,and a specific capacity of 74.3 m Ah/g after 300 cycles with a capacity retention rate of 67.5%.The coating of Mg F₂can effectively prevent the occurrence of side reactions on the electrode and improve the structure stability of the cathode material.Therefore,the coating of Mg F₂can effectively improve the cycle stability of the batteries.（2）P2-type Na_0.7Mn_0.8Ni_0.2O₂prepared by solvothermal and calcination method was used as the matrix material,and Cu O was coated on the surface of Na_0.7Mn_0.8Ni_0.2O₂by wet chemical method.Cu O has strong electrical conductivity,and the rate performance of the battery can be improved by coating Cu O.Under the voltage window of 2-4 V,3 wt.%Cu O-coated Na_0.7Mn_0.8Ni_0.2O₂exhibited an initial discharge capacity of 101 m Ah/g at a current density of 100 m A/g,After 100 times of charge and discharge,the material can still maintain a capacity of 86 m Ah/g,and the capacity retention rate is about 85.1%.The coating of Cu O can effectively isolate the contact between the organic electrolyte and the manganese-based cathode,reduce the occurrence of side reactions between the cathode material and the electrolyte,and maintain the structural stability of the positive electrode material.（3）Al-doped Na_0.7Mn_0.8Ni_0.2O₂was prepared by sol-gel method,and the effect of different Al doping amount on the properties of the material was studied.Under the voltage range of 2-4 V,Na_0.7Mn_0.785Ni_0.2Al_0.015O₂has the best electrochemical performance.The Na_0.7Mn_0.8Ni_0.2O₂prepared by the sol-gel method has a specific capacity of 107 m Ah/g for the first discharge at a current density of 100 m A/g,However,it is easy to be eroded by electrolyte during the charging and discharging process,which makes the capacity decay rapidly.After the battery completed 100charges and discharges,the capacity of the battery remains 89.4 m Ah/g,which is only83.2%of the capacity of the initial discharge.Na_0.7Mn_0.785Ni_0.2Al_0.015O₂showed excellent cycle stability,under the current density of 100 m A/g,discharge specific capacity is 106 m Ah/g for the first time.After 100 cycles,it still retains 99.4 m Ah/g capacity,and only 6.3%capacity is lost during the all 100 cycles.The incorporation of Al³⁺can stabilize the structure of the cathode material in the cycling process and effectively improve the cycling performance and rate performance of the material.Therefore,the appropriate doping of Al³⁺is an effective method to improve the electrochemical performance of the material.