Study On Preparation And Modification Of Carbon-doped LMFP As The Cathode Of Lithium-ion Battery

The commercial olivine structure LiFePO₄ material has good thermal stability,rich raw materials and high safety performance,but has the problem of low energy density.LiMnPO₄material has a high voltage platform of 4.1V,and the energy density is 20%higher than that of LiFePO₄,but its conductivity and lithium ion diffusion rate are lower than LiFePO₄.Therefore,doping a certain proportion of Fe element in LiMnPO₄ material to form LMFP solid solution material is a subject worthy of our research.In this paper,lithium carbonate?Li₂CO₃?,phosphate?H₃PO₄?,trimanganese tetraoxide?Mn₃O₄?,iron oxide?Fe₂O₃?are used as raw materials,deionized water is used as dispersion medium,and liquid acrylonitrile oligomer?LPAN?is used as carbon source.And using polyvinylpyrrolidone?PVP?as a surfactant in the modification process,the precursor of carbon-doped ferromanganese phosphate cathode material is prepared by nano-milling and spray drying.Then the final product was obtained by calcining precursor at a certain calcination temperature.The characterization test is performed by using a synchronous thermal analyzer,an X-ray powder diffractometer,a field emission scanning electron microscope and an energy spectrometer,an integrated electrochemical test system,a nano laser particle size analyzer,a specific surface area tester,and a blue electric tester.Meanwhile,the effects of different LPAN addition amount,calcination temperature,final calcination temperature and ferromanganese?Mn/Fe?molar ratio on LMFP material,and further improvement and performance modification of LMFP material was studied.The main research results are as follows:?1?Using Li₂CO₃,H₃PO₄,Mn₃O₄,Fe₂O₃ as raw materials,LPAN as carbon source,the LMFP lithium ion battery cathode material was successfully prepared by nano-grinding-spray drying and calcination at high temperature.?2?The effect of the amount of LPAN added on the structure and properties of the material was studied,and the amount of LPAN added was 0%,5%,10%,and 15%,respectively.The results showed that when the amount of LPAN added is 10%,the ferromanganese phosphate cathode material and carbon source is doped best.,and the electrochemical performance is optimal.?3?The effects of pre-calcination temperature and final calcination temperature on the structure and properties of the material was studied.The pre-calcination temperatures were300°C,350°C,400°C,450°C,respectively,and the pre-calcination time was 6 h;the final calcination temperatures were 650°C.,700°C,750°C,800°C,respectively,and the final burning time was 6 h.The results showed that when the pre-calcination temperature was 350°C and the final calcination temperature was 750°C,the ferromanganese phosphate cathode material has better crystallinity and the best electrochemical performance.?4?The effects of different molar ratios of Mn/Fe on the structure and properties was investigated.The results showed that comparing to LiMn_xFe_1-xPO₄ with other Mn/Fe ratios?x=0.5,0.6,0.7,0.8?,when the molar ratio of Mn/Fe is 1,the prepared LiMn_0.5Fe_0.5PO₄ has excellent electrochemical performance.?5?After the pre-firing process,the material was subjected to airflow grinding.The tap density of LMFP material was increased from 0.94 g/ml to 1.25 g/ml,and the particle size was reduced from 27.82?m to 8.35?m,so the processing properties of the material was greatly improved.?6?The material is further modified,that is,by adding a surfactant?PVP?,the agglomeration of the particles is effectively suppressed,the degree of polarization is low,the impedance is small,and the conductivity is high.Accordingly,the electrochemical performance of the material was enhanced.The uncharged LMFP material has a specific discharge capacity of only 123.9 mAh/g at the current density of 0.1 C,and 88.7%after 50 cycles;the specific discharge capacities at the 0.1C and 1C after modification were as high as 151.9mAh/g and138.8mAh/g,respectively.After 50 cycles and 100 cycles,the specific capacity retention rate reached 100%.After a series of studies,the carbon-doped ferromanganese phosphate lithium ion battery cathode material prepared by this research has excellent electrochemical performance,which not only overcomes the shortcomings of poor conductivity of lithium manganese phosphate,but also the total battery energy density is 10%higher than the current lithium iron phosphate.In a word,our experiments has provided a new industrial production road for low-cost,simple process,green and environmentally friendly to prepare ferromanganese phosphate.