| Lithium-ion batteries play a crucial role in the development of new energy technologies for energy conversion and storage systems.As an important part of lithium-ion batteries,the research and improvement of performance on cathode material are of great significance for its further development.The olivine-type LiMnPO4 cathode is considered to be a promising material for lithium-ion batteries due to its high energy density(701 Wh·Kg-1),high redox voltage（4.1 V vs Li+/Li）,good structural stability,high safety performance,low cost,abundant manganese resources,and environmental friendliness.However,pure LiMnPO4 delivers the disadvantages of low electronic and ionic conductivity,which seriously hinder its commercial application.Therefore,the improvement of electrical and ionic diffusion rate of LiMnPO4 is the key to realizing its excellent electrochemical performance.In this thesis,high performance of LiMnPO4 is successfully synthesized by exploring suitable synthetic methods and preparation processes,and their effects on the physical structure,morphology,and performance of LiMnPO4 are systematically and comprehensively explored by combining modification me asures such as nanoparticles granularity,surface modification by boron-catalyzed carbon coating and metal ions substitution.The main results are as follows.1.Based on the traditional solid-state and sol-gel methods,a two-step method of liquid-solid phase is designed to synthesize LiMnPO4.This method not only has the advantages of good homogeneity and high dispersion in the solution system,but also reduces the surface free energy combined with high-energy ball milling,which significantly improves the activity of the materials.Meanwhile,the substitution of Fe partially replacing Mnsites can obtain LiMn1-xFexPO4 solid solution.When x is 0.2,the material exhibits high structural stability and good reaction kinetics.The in situ coating modification of LiMn0.8Fe0.2PO4 using phenolic resin as the carbon source to improves its electrochemical performance,and LiMn0.8Fe0.2PO4@C material with high electrochemical activity is obtained.2.Using hydrate MnHPO4 intermediate as a precursor,a liquid-solid phase method is used to synthesize LiMn0.8Fe0.2PO4@C cathode material.The topology of metal phosphate layer in the ac face of MnHPO4 is similar to that of the metal phosphate layer in the bc face of LiMnPO4,which facilitates the structural transformation from MnHPO4 to LiMnPO4.By controlling the gradient addition of oxalic acid,the composition ratio of raw materials,mixing concentration,and carbon content can be reasonably adjusted,thus controlling the morphology and grain size of particles.When the addition of oxalic acid is 0.5,the electrical conductivity of LiMn0.8Fe0.2PO4@C is 6.82×10-2 S·cm-1 and the Li+diffusion coefficient is1.106×10-14 cm2·s-1.The discharge capacity is as high as 153.9 mAh·g-1 at 0.05 C,and the capacity retention is 95.84%and 98.62%after 200 cycles at 0.05 C and 1 C,respectively,which has excellent charge/discharge property and cycle stability.3.LiMnPO4@C nanoparticle is prepared by a simple solvothermal method,and the effects of Li addition,reaction time,the ratio of Mnand Fe,and carbon coating on the morphology evolution and electrochemical performance of LiMnPO4 materials are investigated.When the amount of Li OH is 2.6,the solvothermal reaction time is 18 h,and the carbon content is 5 wt.%,the nano-sized LiMn0.8Fe0.2PO4@C materials have excellent reaction kinetic parameters,low charge transfer resistance（152.7Ω）,high exchange current density(0.168 mA·cm-2)and electrical conductivity(1.02×10-2S·cm-1).The Li+diffusion coefficient is on the order of 10-12 cm2·s-1.4.The modification strategy of boron-catalyzed graphitization carbon coating layer is used to enhance the reaction kinetic properties and Li-storage capability of LiMn0.8Fe0.2PO4 nanocrystals for rechargeable lithium-ion batteries.The graphite-like BC3 formed in the boron-catalyzed graphitization coating layers can not only maintain the dynamic stability of LiMn0.8Fe0.2PO4 nanostructure during cycling,but also improve the conductivity and Li+migration kinetics of cathode material.The optimized LiMn0.8Fe0.2PO4@B-C cathode material exhibits the faster intercalation/deintercalation kinetic parameters,higher electrical conductivity(8.41×10-2 S·cm-1),Li+diffusion coefficient(6.172×10-12 cm2·s-1)and Li-storage performance.The in-situ XRD analysis reveal the structural evolution and Li-storage mechanism of the material during electrochemical reaction,further demonstrating its excellent reversibility and structural stability of LiMn0.8Fe0.2PO4@B-C.The boron-catalyzed graphitization carbon coating modification is expected to to improve conductivity and electrode kinetics of LiMn0.8Fe0.2PO4 material.5.The covalent metal ions(Fe2+,Cr2+,Co2+,Ni2+,and Cu2+)are used to implement a modification strategy for the co-doping of the Mnsites of LiMnPO4.modify the Mnsite of LiMnPO4.It was found that the material with Fe and Ni co-doped has excellent properties.The structure,morphology,uniformity and particle size can be optimized by regulating the amount of Ni-doped,thus improving the reaction kinetics and electrochemical performance of the materials.The discharge capacities of LiMn0.8Fe0.15Ni0.05PO4@C material are 144.8 mAh·g-1 and 105.5 mAh·g-1 at 0.1 C and10 C,respectively,and the capacity recovery rate is as high as 98.42%.In addition,the first-principles calculations show that Ni-doped can improve the electrical conductivity and play an important role in the dynamic properties of the materials.In-situ XRD test is used to elucidate the intrinsic storage mechanism of LiMn0.8Fe0.15Ni0.05PO4@C,and the material conforms to the typical two-phase transition and solid-solution reaction mechanism of the olivine-type LiMnPO4,which exhibits good structural stability and excellent reversibility.These strategies greatly expand the design ideas of phosphate cathode material,providing new insights for the large-scale energy storage devices and power battery applications of lithium-ion batteries. |
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