Sodium batteries become a novel energy storage device due to their higher safety factor and superior comprehensive performance. The existing electrode materials such as Na_xCoO₂, Na_xTiO₂, NaTi₂(PO₄)₃, etc. are investigated for sodium batteries. However, these electrodes have a disadvantage of poor ion/electronic conductivity, restricting their widespread application. Na₂Ti₃O₇ is an ideal alternative titanium-based negative electrode material for sodium batteries due to its Ti³⁺/Ti⁴⁺ variability and suitable redox pairs, as well as its low voltage plateau. Embedding carbon in Na₂Ti₃O₇ electrode for sodium ion batteries can improve their conductivity and electrochemical performance, especially for the large-scale energy storage applications with different energy density and volume requirements. In this paper, Na₂Ti₃O₇/C composite anode material for sodium ion battery was prepared by a special template method, and its electrochemical characteristics were investigated.

Na₂Ti₃O₇/C composite anode material for sodium ion battery was synthesized with natural spirulina as a biological template. Spirulina was added into Na₂Ti₃O₇ precursor solution, heated and stirred in a magnetic stirrer for 1 h, and then put them in a hydrothermal reactor (with a polytetrafluoroethylene inner tank capacity of 100 mL) at 180 ℃ for 18 h. In addition, Na₂Ti₃O₇ precursor solution was also placed into a tank to hydrothermal reaction under the same condition for comparing with the template method. The final products were calcined in an air furnace (with a heating rate of 1 ℃/min from room temperature to 850 ℃) at 850 ℃ for 5 h to form the perfect crystals and remove the spirulina templates. A CR2032-typed buckle battery was assembled in a glove box (GRS-1200, Wuhan Grace New Energy Co., Ltd., China) filled in argon gas and with the synthesized Na₂Ti₃O₇/C or Na₂Ti₃O₇ electrodes as anodes, high purity sodium tablets as counter electrodes.

The microstructure of the product was characterized by a model Quanta 200 environmental scanning electron microscopy (ESEM) (FEI Ltd., Netherlands), and the components were analyzed by an EDAX spectrometer in the ESEM microscope. The crystal structure was determined by a model X'Pert PRO X-ray diffracto meter (XRD, PANalytical B.V. Ltd., Netherlands). The 2θ range of XRD is 5° to 70°. Their electrochemical properties were analyzed by a model CT2001A programmatical-controllable Land battery testing system (Hubei Lanbo New Energy Equipment Co., Ltd., China). The test parameters involved the charge-discharge capacities at a constant current (i.e., 0.5 C=125 mA/g), cycle stability and coulomb efficiency, and discharge capacities after 20 cycles at different rates (i.e., 0.1, 0.2, 0.3, 0.4 C, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 2.0 C, and 5.0 C). The alternating aurrent (AC) impedance was analyzed via a model CS350 electrochemical workstation (Wuhan Kesite Instrument Co., Ltd., China). The test parameters are the total impedance (R₁) of electrode and electrolyte in series, electrode contact resistance (R₂), sodium ion charge transfer resistance (R₃) and semi-infinite diffusion Warburg impedance of Na⁺ in electrolyte (Ws).

The ESEM results show that Na₂Ti₃O₇/C material prepared with the spirulina biological template has a good network strip structure, with a small amount of carbon residue (the mass fraction of carbon of 1.29%). The Na₂Ti₃O₇ materials by a hydrothermal method are nano-particles with the diameters of approximately 10-30 nm. Their XRD patterns of both materials are consistent with the standard spectra of Na₂Ti₃O₇ (JCPDS 31-1329), indicating a perfect crystal structure. The initial charge-discharge capacity of Na₂Ti₃O₇/C electrode material prepared with spirulina template is 109.3 mA·h/g and 117.6 mA·h/g, respectively. The initial charge-discharge capacity is 89.3 mA·h/g and 86.3 mA·h/g of Na₂Ti₃O₇ electrode material by the hydrothermal method, respectively. After 250 cycles, the average retention rate of the charge-discharge specific capacity of Na₂Ti₃O₇/C material is 70%, which is better than that of Na₂Ti₃O₇ material (i.e., 35%), showing a better cyclic stability and a higher Coulomb efficiency. Based on the results of the specific discharge capacities at different rates, the discharge attenuation of both the materials are faster at a lower current, while they are relatively stable at a higher current, which provides a good power support for high-power hybrid electric vehicles. At the same rate, the stability of Na₂Ti₃O₇/C electrode material is better than that of Na₂Ti₃O₇ electrode material. The electrochemical impedance spectra shows that R₁, R₂, R₃ and Ws resistances of Na₂Ti₃O₇/C material are smaller than those of Na₂Ti₃O₇ material.

The specific capacity decay rate of Na₂Ti₃O₇/C electrode obtained by the spirulina template method was slower than that of Na₂Ti₃O₇ electrode obtained by the hydrothermal method after charge-discharge cycles. This was due to the nano-network strip structure prepared by natural spirulina as a template. Na₂Ti₃O₇/C electrode reduced the detachment resistance of sodium ions on the electrode surface and the diffusion resistance in the electrolyte, increased the contact area between the electrolyte and the electrode surface, thereby proved a good channel for the detachment of sodium ions, thus effectively improving the electrochemical performance of electrode materials. Moreover, the residual carbon in calcination improved its conductivity of Na₂Ti₃O₇ electrode material, and enhanced its electrochemical performance, thus providing a basic support for the development and application of novel sodium batteries.