Study And Application Of DFT Method On Cathode Materials Of Lithium Ion Battery

Lithium iron phosphate has been widely used as the mainstream LIB cathode material,but its performance is nearly to the limit.The same type of olivine phosphate materials,such as manganese phosphate,are still in the laboratory development stage.The energy density of lithium manganese phosphate is 20 to 30 percent higher than that of iron phosphate.The low-cost,the environment friendship,unnecessary to import Co and other raw materials,make LiMnPO₄ is the ideal cheap energy storage material in the future.The main modifications of LiMnPO₄ are dopings and coatings.Compareing with effect on LiFePO₄ these methods have little effect on the modification of LiMnPO₄.Therefore,we must go deeply into the structure,nature and mechanism of materials,to analyze the differences between them.Early literatures by first-principles calculation on olivine type phosphate gave explain of different performances from the electronic and ionic conductivity.However,we found that the properties of cathode materials are more depending on the properties interface rather than the properties of the bulk crystals.In recent years,the performance of LiMnPO₄ synthesised by the new methods has been greatly promoted,far beyond the explanation of the original theory.Therefore,using the first-principles to calculate and analyze the new interface model is the development direction of electrochemical mechanism research.According to the results calculated by DFT,a small proportion of the solid solution phase in LiFe_xMn_1-x,PO₄ are predicted,and properties were verified by experiments and literatures.The energy of the transition state is higher than the energy of the two separations,leading to the fact that this state is not the mainstream of the LiFe_xMn_1-xPO₄/C synthesized by common methods,and will automatically divide to two independent crystalline phases.However,this mixing-phase state is not nonexistent in thermodynamics.We synthesised LiFe_xMn_1-xPO₄ by solid-phase reaction and verify the existance of transition state,give a explain of the microscopic embedded potential of lithium ions,find that its corresponding potential decreases with the increase of x LiFe_xMn_1-xPO₄.It shows that the transition metal ions are approximate to random arrangement in this local area.The formation of the transition state is a kinetic reaction.For the smaller energy gap than LiMnPO₄ and LiFePO4,this mixing-phase state will have a higher electronic conductivity in theory,which deserves our attention.Another structure of interface of LiMnPO₄/C was set up to describe the interface of LiMnPO₄/C.form Carbon nanosheets are horizontally fixed on the surface of LiMnPO₄ by the C-O-Mn bond.This structure is stable by calculating the adsorb energy.Study of DOS shows that these two models have different conductive mechanism.The traditional model uses C-O-M as the conductive bridge to transfer electrons directly to the carbon layer,The C-O-M bonds of new model only fix carbon layers to crystal,and carbon layers exchange electrons directly with metal ions.In contrast to the lattice structure,we reject the possibility of M-C bonds,and speculate that the conductive mechanism of this interface is the tunneling effect,the conductivity of interface of LiMnPO₄/C is highly correlated with the length of bridge bond.Lithium-rich material aLiNi_xCo_yMn_1-x-yO₂·（1-a）Li₂MnO₃ is a strong competitor of the next generation cathode materials for its high energy density,low dependence upon material importing and environmental friendship.However,the faulty cyclic stability and imperfect rate performance limit its application.Surface coating and doping are often used to improve performance of lithium-rich layered materials.By DFT method,we calculated Li-rich materials of different ratio of Ni-Co-Mn,found that it effected oxidation state and spin of the transition metal ions,which can be examinated by XPS spectra.Samples of different quatities of spinel Li₄Mn₅O₁₂ coating of lithium-rich oxide 0.5LiNi_aCo_bMn_cO₂·0.5Li₂MnO₃ were simulated by DFT methods.We found that the transportion of Li from lithium rich oxide core to spinel surface caused perk shifting on XRD,which had been proved by experiments,and may be detected by XPS or EELS.The results showed that XPS can nondestructively detect the components of lithium-rich precursors and semi-quantitatively calibrate the coating effect of subsequent oxides.