Synthesis And Modification Research Of LiMnPO₄Cathode Materials For Lithium-ion Batteries

The development of cathode materials with the merits of high energy density,safety, low cost has always been the driven force for the large-scale applications oflithium-ion batteries. Olivine-type LiMnPO₄material owning the advantages such ashigh potential （4.1V, vs. Li⁺/Li）, large theoretical energy density (697Wh kg^-1),excellent electrochemical and thermal stability, rich manganese resources, low costand without pollution, is expected to play an active role in the field of power batteries,and thus has caused the research upsurge increasingly. The challenges LiMnPO₄material facing are the poor electronic conductivity due to its insulativity, and theextreme low ionic conductivity as a result of the inherent characteristics of the olivinestructure which limits the Li+only diffusing in the one-dimensional channel. In orderto overcome the above problems, we have carried out the researches on LiMnPO₄bysurface carbon coating, bulk phase single ion doping, and the synthesis of ternarymaterials.Originally, the carbon-coated LiMnPO₄/C composites were obtained throughsolid-state reaction, and the influences of various carbon sources, carbon contents,calcination temperature and time on the morphologies, crystalline structures andelectrochemical performances were discussed, respectively. The results reveal that thesamples calcined at750C for8h using sucrose as carbon source （15wt%carbon）show the best electrochemical performances due to the finest particle size and theoptimal reversibility of electrodes under this condition. The discharge capacities are109.8,106.2and90.3mAh g^-1at0.02C,0.05C and0.2C, respectively. Moreover,the cycling stabilities are the most outstanding under as-stated current densities.On the basis of above optimized condition, a series of solid solution materialsLiMn1-xFexPO4with different Fe²⁺contents （x=0,0.2,0.3,0.5） were synthesized.The doping of Fe²⁺has reduced the electrochemical polarization, resulting in theimprovement in the following aspects such as the reversibility of electrodes, theability of delithiation/lithiation, and the ionic conductivity. The LiMn0.7Fe0.3PO4/Ccomposites have the lowest charge transfer impedance, and the diffusion coefficient ofLi+has increased two orders of magnitudes (from10^-14cm²s^-1in pristine one to10^-12cm²s^-1). The highest capacities are displayed at0.05C,0.1C,0.2C and0.5C, theyare146.8,136.6,127.4, and117.0mAh g^-1, respectively. In addition, the cyclabilitiesare excellent under various rates, a slight increase in capacity (the value reaches119.3 mAh g^-1) is observed when cycled at0.5C after50cycles.Lastly, solid solution LiMn1/3Fe1/3Co1/3PO4/C ternary materials were prepared bysol-gel method with glycine or citric acid as chelating agents. SEM images indicatethat the particles are well distributed with regular morphologies, and the particle sizesare around300nm for both samples. The cyclic voltammetry （CV） andcharge-discharge curves show that the reversibility of Mn²⁺/Mn³⁺couple is improvedapparently, the redox potential corresponding to Fe²⁺/Fe³⁺couple has been increasedwhich is beneficial to the energy density, and the redox potential related to Co²⁺/Co³⁺couple has been declined which could accelerate dynamics of delithiation/lithiationand make the electrochemical reaction more smoothly. The obtained G-LMFCPmaterial with glycine as chelating agent delivers a capacity of151.5,142.6,122.8,97.9mAh g^-1at0.05C,0.1C,0.5C and1C, respectively. And the C-LMFCPmaterial with citric acid as chelating agent delivers a capacity of152.8,135.0,111.5,85.2mAh g^-1, respectively, at above rates. It is worth mentioning that the cyclingperformances are relatively poor for both the G-LMFCP and C-LMFCP, which needimprove in future.