Synthesis And Performance Of Multivariate Transition Metal Oxide Electrode Materials For Sodium-ion Batteries

Sodium-ion batteries（SIBs）have great potential in large-scale energy storage due to the advantages of their rich reserves and low cost.However,since sodium has larger ion radius and higher transport barrier,the development of ideal electrode materials for sodium ion batteries remains a challenge.The layer-structured P2-type transition metal oxides are promising electrode materials for NIBs because of their stable structure,high ionic mobility and theoretical specific capacity.Because transition metal elements hold various redox couples which enable them to operate under various voltages,they can serve as both cathode and anode materials for SIBs.In this dissertation,several methods were employed to synthesize the transition metal oxides with smaller and more uniform particle size.Surface treatments were used to improve the electrochemical performance.The main results are listed as followed:（1）Solid-phase method and facile sol-gel method were employed to synthesize the Na_2/3Ni_1/6Ti_2/3Co_1/6O₂（NNTC）,the preparing parameters of which were then discussed.Na2CO3,NiO,TiO2 and CoO were used to synthesis the NNTC with diameter of 5 μm and thickness of 1-2 μm through solid-phase method,followed by a calcining process in Ar flow at 950℃ for 12 h.The precursors were synthesized using Butyl titanate,C4H6O4Ni·4H2O、C4H6O4Co·4H2O and Na2CO3 by sol-gel method,which were then calcined in Ar flow at 900℃ for 12 h to prepare NNTC with a diameter of 0.5-1 μm.（2）Carbon-wrapped NNTC（NNTC/C）was prepared to further improve the electric conductivity,and the glucose contents were discussed and optimized.The results showed that the sample with 10 wt% glucose content exhibited better electrochemical performance.As an anode material,the NNTC/C is capable of displaying initial discharge/charge capacity of 122/110 mA h g-1 at the rate of 0.2 C in a voltage range of 0.2 to 2.5 V with capacity retention of almost 82% of its initial particular capacity after 500 cycles.It suggests that only the redox couple Ti4+/Ti3+ is associated with the Na+ insertion and extraction during the charge/discharge process.In addition,ex-situ X-ray diffraction analysis illustrated a minor 1.5 % fluctuation of cell volume in the cycling,suggesting a high level of reversible structural changeability and stability of the NNTC/C.（3）Solid-phase method and coprecipitation method were employed to synthesize the Na_2/3Ni_1/3Mn_2/3O₂（NNM）,the preparing parameters of which were then discussed.Na2CO3,NiO,and Mn O2 were used to fabricate the planchet-shaped NNTC with a diameter of 1-2 μm and a thickness of 0.5-1 μm through solid-phase method,followed by a calcining process in Air at 900℃ for 12 h.The spherical coprecipitate was synthesized using MnSO4·H2O,NiSO4·6H2O and NH4HCO3 by coprecipitation method,which were then calcined in Ar flow at 900℃ for 12 h to prepare NNM.Disadvantages existed in the coprecipitation method which was more complicated and produced relatively larger particles with various shape and size.（4）An environmentally effective molten-salt growth method was used to prepare NNM composite materials（NNM@C）by wrapping NNM with three-dimensional carbon network with the aim of improving their electric conductivity.The results showed that the sample with 10 wt% carbon content exhibited better electrochemical performance.As the cathode materials for sodium ion batteries,the discharge specific capacities of NNM@C and NNM were as high as 93.18 mA h g-1 and 94.85 mA h g-1,respectively,at 0.2C in electrochemical window of 2-3.9V.The capacity retention of NNM@C（87%）was much higher than that of NNM（49%）after 100 cycles.Compared with the unwrapped NNM,there was a notable enhancement in electrochemical performance in the carbon-wrapped NNM.