Syntheses And Lithium Storage Properties Of Metal Phosphate Compounds

The research and development of new electrode materials for Lithium-ion batteries（LIBs）is an important way to promote the electrochemical performance of LIBs.As a key component of the battery,the anode materials play an important role for the performance of batteries.Polyanion compounds have shown good structural stability,tunable voltage platform,and excellent cycling stability and safety as the cathode materials for Li-ion batteries.However,the research on polyanion compounds for anode materials of lithium-ion batteries is only in its infancy.Some of the research results have shown the excellent performance of polyanion compounds as anode materials of LIBs.Transition metal phosphate compounds represent the most important family of polyanion compounds.In this thesis,a series of manganese-and copper-based phosphate compounds were synthesized.The crystal structures of as-synthesized compounds were characterized by single-crystal X-ray diffraction analysis.The physicochemical properties of these compounds were investigated.The electrochemical properties of the compounds as anode materials for LIBs and their lithium storage mechanisms were studied.The relationship between the structure and the electrochemical performance of these metal phosphate compounds was discussed.The main results in this thesis are summarized as following:1.Single crystals of the olivine NaMnPO₄ were directly synthesized by a facile hydrothermal method.The crystal structure was characterized using single-crystal X-ray diffraction analysis.The phase transformation between the olivine NaMnPO₄ and maricite NaMnPO₄ was investigated.The electrochemical tests showed that the olivine NaMnPO₄ could provide a reversible specific capacity of 645.0 m Ah g^-1 at a current density of 0.1 A g^-1 as an anode material for LIBs,which is much higher than that of the maricite NaMnPO₄.2.The manganese phosphate compound Mn₅（HOPO₃）₂（PO₄）₂（H₂O）₄（hureaulite）with open-framework structure was synthesized.The crystal structure was characterized by single-crystal X-ray diffraction.This compound exhibited a capacity of 778.7 m Ah g^-1after 300 cycles at a current density of 0.1 A g^-1 and a capacity retention of 88.7%after2000 cycles at 2.0 A g^-1.The three-dimensional open-framework structure provides open channels for the fast transportation of electrons and Li⁺ions,which leads to good rate performance and long-term stability.3.The copper hydroxyphosphate compound Cu₂（OH）PO₄（libethenite）was synthesized and structurally characterized by single-crystal X-ray diffraction.The lithium storage mechanism of the Cu₂（OH）PO₄ anode involves the conversion reaction with Li⁺ions,which produces Cu and Li₃PO₄as the end product at the discharge state.The in situ generated ion conductive Li₃PO₄ is much beneficial to enhance Li⁺diffusion that leads to good rate capability.4.The mixed polyanionic manganese-based phosphate-oxalate compound Mn₄（PO₄）₂（C₂O₄）（H₂O）₂was synthesized.The crystal structure analysis shows that the compound have a layered open-framework structure.The compound exhibits good rate capability and cycling stability as anode material of LIBs.Electrochemical tests show that the compound has a high Li⁺ions diffusion coefficient during charge/discharge cycles.