Preparation And Performances Research Of Olivine Type LiMPO₄/C?M=Fe?Mn? Cathode Materials

As the rapid consumption of traditional fossil materials and the increasing of environmental pollution,looking for environmentally friendly,renewable raw materials and preparation of high-performance energy materials to replace non-renewable and polluting fossil fuels can meet the demand of the current growing clean energy and sustainable development of urgent need.Lithium iron phosphate?LiFePO₄?of with polyanionic olivine structure can be widely applied to most mobile electronic devices due to its excellent safety performance,long cycle life and non-toxicity in environmental protection,especially for the current rapid development of large-scale hybrids electric vehicles and pure electric.However,LiFePO₄ suffers from disadvantages such as poor electron conductivity,low Li⁺diffusion rate,and low energy density,which seriously restricts the further development and application of this material.Therefore,in view of the above problems,the first and second parts of this paper studied the use of abundant and renewable biomass phytic acid as the phosphorus source in nature,and successfully synthesized carbon coating LiFe PO₄/C and monodisperse large-sized carbon@LiFePO₄/graphene composite microspheres by stepwise method and in situ solvothermal method.In addition,in view of the low energy density of LiFePO₄,combined with the current increasing demand for flexible electrode materials for wearable devices,an integrated flexible self-supporting LiFe_0.8Mn_0.2PO₄/C electrode slice composed of porous nanofibers had been successfully prepared by electrospinning in the third part of this paper.The specific research contents and results are as follows:1.Carbon-coated LiFePO₄/C was successfully synthesized by the fractional step method using abundant renewable biomass phytic acid as a phosphorus source.First,pure LiFePO₄ was synthesized by solvothermal method,and then glucose was used as a carbon source to coat the synthesized material.The experiment explored the effect of different solvothermal reaction time on the morphology and structure of the product,and explored the optimal adding amount of glucose also.Electrical characterization results show that when the solvent thermal time is 10 h and the amount of glucose carbon source added is 5 wt%?the mass ratio of carbon in glucose to that of LiFePO₄?,the synthesized composite has the best performance and cycle stability.Under the current density of 0.1 C,the discharge specific capacity was155.7 mA h g^-1,and after 100 cycles,the capacity was almost not attenuated and the coulombic efficiency of the material during the cycle was close to 100%.2.Large-sized porous glucose derived-carbon?C?and graphene?G?co-modified LiFePO₄?LFP?composite microspheres?C@LFP/G?have been successfully in situ synthesized by one-pot solvothermal and followed annealing process using biological phytic acid as phosphorus source and structure directing agent.The loose and porous microspheres with diameters in the range of 5-12?m are self-assembled by the contaction or connection of carbon coated LFP nano particles with randomly embedded/distributed graphene sheets.The effect of phytic acid dosage and carbon modification on morphology and electrochemical properties were investigated.The results indicated that phytic acid dosage plays an important role in construction of the self-assembled three-dimensional?3D?spherical microstructures during solvothermal process.When the initial solution ratio of Li,Fe and P is 3:1:3,a spherical morphology structure can be formed regardless of the addition of the carbon source.Additionally,the effects of carbon modification were also investigated,which reflected that the addition of glucose can effectively control the size of LiFePO₄ nano-particles and prevent agglomeration among particles and graphene can effectively construct a three-dimensional and multi-space connected conductive network,so the graphene and glucose-derived carbon personate a synergistic effect.A reasonable assembly process elucidating the formation of the different structure is provided based on the experimental results.The typical large-sized C@LFP/G microspheres composite exhibited the best electrochemical properties with the discharge specific capacity of 163.7 mA h g^-1 at 0.1 C and the capacity retention rate of this material is still able to reach 97.8%and the coulomb efficiency is close to 100%during the entire cycle,which is recirculated to 500 cycles at 1C even after 50 cycles under 5 C and 10 C.3.An integrated self-supporting LiFe_0.8Mn_0.2PO₄/C composite electrode sheet with a certain flexibility has been successfully prepared by changing the concentration of PVP,salt concentration,spinning humidity and calcination temperature.The homogeneous and non-precipitated clarified spinning solution adopted inexpensive polyvinylpyrrolidone?PVP?as the structural skeleton and carbon source,clean and environmentally friendly non-toxic harmless water as a solvent,ferrous sulfate heptahydrate as an iron source,manganese sulfate monohydrate as a manganese source,phosphoric acid as a phosphorus source,and lithium hydroxide monohydrate as a lithium source.This pole piece is composed of porous nanofibers,which does not need current collector,binder,conductive agent and brush technology.The electrode material exhibits excellent rate performance and cycle stability:After different charge and discharge current density cycles,the charge-discharge current density returns to 1 C,its capacity still can reache to145.6 mA h g^-1.Besides,battery capacity does not decay after a long-term high current charge and discharge cycles.