Study On The Influence Of Different Doping Elements On The Structure And Properties Of Lithium Manganese Phosphate As Cathode Material

In production and life,the lithium-ion battery is the secondary battery with the highest energy density,and it is widely used in digital products,mobile communications,new energy vehicles and large-scale energy storage.Li Co O₂ is the most important cathode material for lithium-ion batteries.However,there is a shortage of cobalt resources in China,the market price fluctuates sharply,and cobalt is toxic.Therefore,it is necessary to replace Li Co O₂ with new cathode materials in the industry.The working voltage of Li Mn PO₄ is 4.1 V,and its standard theoretical specific capacity is171 m Ah/g,which is suitable for the existing electrolyte electrochemical window.It also has the advantages of abundant resources,low cost,high security and so on.Therefore,taking Li Mn PO₄ with olivine structure as the research object,aiming at its own structural defects such as low electronic conductivity and ionic conductivity,the synthesis conditions were explored,and then lithium manganese phosphate was modified by carbon coating and doping different elements to improve the electrochemical performance of the material.In this paper,the pure phase Li Mn PO₄ material was prepared by the high-temperature solid phase method,but the battery assembled by it has almost no capacity.In order to achieve the goal of improving the electronic conductivity of the material,Li Mn PO₄/C material was prepared,and the best synthesis conditions were obtained:using glucose as the carbon source,pre-calcination temperature was 350℃and holding for 4 hours,then 10%of the pre-calcined product mass glucose was added and mixed evenly,and sintering temperature was 600℃and holding for 6 hours to obtain the sample.The specific discharge capacity of the sample is 80.48 m Ah/g at 0.1 C.Due to the low ionic conductivity of lithium manganese phosphate cathode materials,the use of different elements doped lithium manganese phosphate materials can obtain good electrochemical performance.First,fluorine element was doped at the phosphate radical site of lithium manganese phosphate,It was found that proper amount of fluorine doping could improve the conductivity and ion mobility of materials of the material,and the electrochemical performance of the synthesized Li Mn（PO₄）_0.975F_0.075/C material was the best,and the initial discharge specific capacity was 87.68 m Ah/g at 0.1 C current charge discharge.Then,on the basis of doped fluorine,the manganese position of lithium manganese phosphate is doped with different contents of iron and magnesium.Iron doping can alleviate the structural degradation and electrochemical performance degradation of the material caused by the Jahn-Teller effect of manganese ion.When the iron content of doped iron is 40%,the Li Mn_0.6Fe_0.4（PO₄）_0.975F_0.075/C material prepared has the best electrochemical performance.The initial discharge specific capacity was 154.37 m Ah/g at 0.1 C current charge discharge.After 20 cycles of charging and discharging,the specific discharge capacity is 152.3 m Ah/g and the capacity retention rate is 98.66%.Discharge specific capacity at 2 C charge discharge 104m Ah/g.It shows that the synthesized material has good cycle performance and magnification performance;In the process of battery charging and discharging,the dissolved manganese ion in the electrolyte will form Mn F₂ and deposit on the negative electrode.Magnesium doping can inhibit the dissolution of manganese ion.When the content of doped magnesium is 40%,the electrochemical performance of the synthesized Li Mn_0.9Mg_0.1（PO₄）_0.975F_0.075/C material is the best,and the initial discharge specific capacity is 105.98 m Ah/g at 0.1 C current charge discharge.Finally,on the basis of fluorine doping,we investigated the doping of 10%different elements in the manganese position of lithium manganese phosphate,and prepared Li Mn_0.9M_0.1（PO₄）_0.975F_0.075/C（M=Ni,Co,Zn,Fe,Mg）cathode materials.The initial discharge specific capacity under 0.1 C charge discharge is 54.98 m Ah/g,94.12 m Ah/g,114.72 m Ah/g,131.15 m Ah/g,105.98 m Ah/g respectively.The doping of transition elements at manganese sites can promote the diffusion of lithium ions and improve the intrinsic conductivity of materials.