Study On Modification And Low-temperature Electrochemical Performance Of LiFe_0.8Mn_0.2PO₄ Cathode Material

Cathode material is one of the most important factors that determine the performance of lithium-ion batteries.Olivine LiMPO₄（M=Fe,Mn）cathode material has been successfully applied in electric vehicles and other fields because of its advantages of excellent cycle stability,high safety and low cost.However,the shortcomings of poor fast charge-discharge capability and low-temperature performance of LiMPO₄ material limit the further expansion of its application fields.In this context,the effect of enhancing the conductivity of LiFe_0.8Mn_0.2PO₄（LFMP）materials via the co-modification of carbon coating-phosphorus doping and boron-phosphorus dual-doped carbon coating,and the impact of lithium difluorophosphate（Li PO₂F₂,Li DFP）additive on were investigated in this thesis.The main research contents are as follows:（1）The LFMP cathode material co-modified with carbon coating and phosphorus doping（LFMP/C-P）was prepared via the sol-gel-hydrothermal method.Compared with mono-modified materials（LFMP/C modified by carbon coating and LFMP-P modified by phosphorus doping singly）,LFMP/C-P exhibits obvious advantages in terms of capacity,rate and cycle stability.The excellent performance of LFMP/C-P is mainly attributed to the synergistic effect of carbon coating and phosphorus doping.Firstly,the coated carbon material forms a three-dimensional conductive channel among the LFMP/C-P nanoparticles,which promotes the transmission of ions and electrons and simultaneously enhances the stability of the material structure.Secondly,the doped phosphorus acts as an electron donor in the carbon material to enhance the electron conductivity of material through the"carrier conduction"mechanism,and also plays a role of bridge on connecting the carbon coating layer on the surface and the bulk of the material.（2）The LFMP material modified by the boron-phosphorus dual-doped carbon coatings of LFMP（LFMP@B/P-C）was prepared via the sol-gel-hydrothermal method that Boric acid and triphenylphosphine were used as boron source and phosphorus source,respectively.The results indicate that LFMP@B/P-C exhibits higher capacity,preferable rate and cycle performance compared with the material modified by boron mono-doped carbon coatings（LFMP@B-C）.Additionally,the material also displays better electrochemical performance at low temperature(the discharge capacity of 1 C is 78.2 m Ah g^-1 at-20℃).The superior performance is mainly attributed to the synergistic effect of the boron-phosphorus dual-doped carbon coatings.The co-existence of the phosphorus as electron donor and boron as electron acceptor makes a special B/P pair formed in the carbon material,which allows the electrons intercalate and de-intercalate easily in the material,thereby greatly enhances the conductivity of the material,which brings out good electrochemical performance.（3）To further improve the electrochemical performance of LFMP@B/P-C/Li cells at low temperatures,the electrolyte composition was optimized.The main strategy is to apply electrolyte additive Li DFP to improve the interfacial film properties of lithium hexafluorophosphate（Li PF₆）-based electrolyte to improve the low-temperature performance of cells.The results show that the cells employed the electrolyte containing Li DFP additive exhibit better electrochemical performance at low temperature.It is ascribed that Li DFP could preferentially oxidize and decompose before Li PF₆,forming a continuous and relatively complete interfacial film on the electrode surface,inhibiting further degradation of Li PF₆,thereby enhancing the cycle stability of the electrode material.Besides,the interface film also owns good lithium conductivity,which effectively reduce the electrochemical impedance and improve the low-temperatures performance of the cells.In summary,the employment of the modification of the heteroatom mono/dual doped carbon coatings on the surface of LFMP cathode material and the addition of Li DFP in the electrolyte significantly improve the rate performance of LFMP material and low-temperature performance of cells.This provides a new idea for the research of the enhancement of the conductivity of electrode materials and the improvement of low-temperature performance of cells.