| Energy shortage and environmental pollution have become issues related to human survival and development, renewable energy will be the inevitable choice for the future. However, energy conversion and storage are the keys to make effective use of renewable energy. Among the several existing storage devices, lithium-ion batteries have the advantages of high energy density and long cycle life. But the large scale usage as the electric vehicle battery power will lead to lithium resource shortages. Sodium and lithium elements locate at the same group, but sodium element is abundant in the earth’s crust and seawater, cheaper in price and higher redox potential, moreover, sodium-ion batteries have a similar working mechanism with lithium-ion batteries, as well as outstanding advantages in terms of price, environmental protection and safety performance. Sodium-ion batteries have attracted more and more attentions in recent years. Among the numberous electrode materials, titanium-based materials have caused a great of attentions for their inexpensive and green properties. Though titanium-based materials as anodes for lithium-ion batteries have been extensively studied, however few reports on anodes for sodium-ion batteries, but great scientific potential. The Na2Ti3O7 anode material was firstly reported by Premkumar’s research group, and attracted a lot of attentions for its low sodium storage potential(ca. 0.3Vvs. Na/Na+). We learn from the idea of co-precipitation method to synthesis ternary and lithium-rich materials, synthesized a sheet-like Na2Ti3O7 by solid-state method, but with microsphere Ti O2 as titanium source, the synthesis process and electrochemical reaction mechanism was explored.Experimental results show that a pure phase Na2Ti3O7 sample is obtained when calcined at 900℃ for 20 h. The initial discharge/charge capacities were 424/191 m Ah g-1 with an initial coulombic efficiency of 45% at the rate of 0.1C. After 50 cycles, the reversible capacity is 101 m Ah g-1. CV test showed that the formation of SEI film lead to the low initial coulombic efficiency, and the SEI film degraded in the following cycles when 1mol/L Na Cl O4 electrolyte(EC/DMC as solvent) is used. However, when a 1mol/L Na PF6 electrolyte(EC/DEC as solvent) is used, the initial discharge/charge capacity are 288/160.1m Ah g-1, with an improved initial coulombic efficiency to 56%, but without significant enchancement in cycle performance.Anatase Ti O2, which is inexpensive and environmental friendly, the studies as anode for sodium-ion batteries have been carrying out. Porous nanocrystallites anatase Ti O2 are synthesized via a dilatory hydrolysis-calcination method in this paper. XRD, SEM, TEM, nitrogen adsorption-desorption measurements are taken to study their microstructure and porosity. Porosity studies showed that mostly of the pores sized in 3.7-4.1nm. When applied as anode for sodium-ion battery, the initinal discharge and charge capacities are 512 m Ah g-1 and 195 m Ah g-1 as the rate is 0.1C. After 50 cycles, the reversible capacity still remains as high as 131.8m Ah g-1. Rate test show that a reversible capacity of 63.3m Ah g-1 at 2C is obtained. EIS test proved that the charge-transfer impedance is 409.4?.Since the semiconductor as well as poor electronic conductivity properties of anatase Ti O2. Carbon coating modification are taken to improve surface electronic conductivity and electronic contact between particles, and the influence of the carbon content on electrochemical performance is especially studies in this paper. XRD test show that carbon coating does not change the crystal structure, Raman spectroscopy confirmes the presence of the surface carbon layer, TG analysis showe that the content of carbon in the three as-prepared samples are 7.2wt.%, 9.9wt.%, 15.8wt.%, respectively. TEM test show porous microstructure of the four samples. And porosity studies show most of the pores sized in 41.9-80.2nm in the 9.9wt.% sample. The galvanostatic discharge-charge tests show significant improvement in sodium storage properties by carbon coating. Set the rate at 0.1C, the initial discharge/charge capacities are 501/193 m Ah g-1, 570/230 m Ah g-1, 598/251 m Ah g-1 for 7.2wt.%, 9.9wt.%, 15.8wt.% samples, respectively. After 50 cycles, the reversible capacities still remain as high as 159 m Ah g-1, 191 m Ah g-1, 205 m Ah g-1. However, under the condition of high current density, the capacities are limited enhanced along with more carbon content, reversible capacities of 80 m Ah g-1, 103 m Ah g-1 and 113 m Ah g-1 are obtained for the 7.2wt.%, 9.9wt.%, 15.8wt.% electrodes when the rate is 2C. EIS test proved that the charge-transfer impedance decreased along with more carbon content, namely 310.6?, 300?, 174.8?. |
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