Preparation And Performance Research Of Electrode Materials Based On Magnesium-lithium Double-ion Batteries

Magnesium-ion batteries are considered to be one of the best candidates for replacing lithium-ion batteries,but only when the problems of cathode materials and electrolyte are solved can magnesium-ion batteries be further developed.Magnesium-lithium dual-ion batteries can effectively solve the slow insertion of Mg²⁺and provide a new direction for the development of magnesium-ion batteries.In this thesis,the cathode materials based on magnesium-lithium dual-ion is studied,and the main content is the modification of titanium-based materials,containing lithium titanate and sodium titanium phosphate,to obtain lithium storage performance.(1)A series of carbon-coated and tin-doped LTO composite materials were synthesized by the sol-gel method.It was found that the materials grew on the conductive carbon skeleton fromed by carbonization of citric acid.The synthesized Sn_0.1-LTO/C composite material has excellent lithium storage performance.At a current rate of 1 C,the initial discharge specific capacity of Sn_0.1-LTO/C is as high as 174.3 m Ah g^-1,and after cycling 200 cycles,the specific capacity is still 167.1 m Ah g^-1.The high conductivity provided by the carbon skeleton can maintain excellent electrochemical performance even if the electrode is prepared without adding a conductive agent.The electrochemical tests show that there is still a discharge specific capacity of 103 m Ah g^-1 at a current rate of 5 C;after 1000 cycles,the capacity retention rate is 81.4%.When the Sn_0.1-LTO/C material is assembled into a magnesium-lithium dual-ion battery(MLIB),at a current rate of 0.5 C,the discharge specific capacity can reach to 159.1 m Ah g^-1 at first lap.The excellent electrochemical performance is attributed to the small amount of Sn doping and the conductive carbon skeleton.The mechanism test shows that the reversible capacity of the battery is mainly attributed to the reversible insertion/de-insertion of Li⁺in the positive electrode and the dissolution and deposition of Mg²⁺in the negative electrode.(2)Sodium titanium phosphate was prepared by hydrothermal method,and the effects of temperature,time,material concentration(based on Ti)and material filling degree on the structure and morphology of NTP were studied.Taking energy consumption and output in production as consideration factors,it is found that when the hydrothermal temperature is180?,the hydrothermal time is 6 h,the concentration is about 1.5 mol/L,and the material filling degree is 50%,the condition is optimization.The NTP product with cubic structure formed by stacking of nanosheets has been prepared.In order to improve the conductivity of the material,the product was modified by carbon coating with citric acid as carbon source.The modified NTP had an initial discharge capacity of 112.6 m Ah g^-1 at a current rate of 1 C.Subsequently,we assembled a magnesium-lithium dual-ion battery.The carbon caoted NTP coated has stable cycle performance and excellent rate performance.The initial discharge capacity at a current rate of 0.5 C is 73.7 m Ah g^-1,after 10 cycles of activation,it reaches 78.5m Ah g^-1and can be cycled stably for 200 cycles with little attenuation of capacity.The better electrochemical performance is attributed to better crystallinity and more carbon content during the secondary carbon coating process.The mechanism test shows that the reversible phase transition between Li₃Ti₂(PO₄)and Li Ti₂(PO₄)₃ occurs with insertion/de-insertion of Li⁺in the positive electrode and Mg dissolves and deposits in the negative electrode after the first cycle of activation.(3)The carbon-coated sodium titanium phosphate-carbon nanotube composite material was prepared with the aid of spray drying method.Sodium citrate containing aliphatic carbon chains in the molecular structure is used as the sodium source,and a small amount of carbon nanotubes are added at the same time.It was found through SEM that the amorphous carbon generated by carbonization of the organic sodium source during the high temperature treatment coats the NTP particles in situ and the NTP particles grows on a conductive network composed of carbon nanotubes.Electrochemical performance test results show that NTP@C-CNTs has better lithium storage performance than NTP@C.At a current rate of 2 C,the average specific discharge capacity of NTP@C-CNTs is 40 m Ah g^-1 higher than that of NTP@C within 200 cycles.At a high current rate of 10 C,the capacity retention rate is78.8%over 2000 cycles.When the MLIB is assembled,the is battery possesses a discharge specific capacity of 110.5 m Ah g^-1in the first lap at a current rate of 1 C.The synergistic strategy of in-situ carbon coating and the use of carbon nanotubes to build a conductive network enalbes the composite material to have better electrochemical performance.