ИСТИНА

One of the main challenges in sodium-ion batteries developing consists in their limited cycle-life. Therefore, disclosure of mechanism of their degradation is of great importance. In the present work an electrochemical impedance spectroscopy was used for elucidation of degradation features of certain electrodes. Sodium titanate Na2Ti3O7 was synthesized via solid-state method from anhydrous Na2CO3 and mesoporous titanium oxide. The mixture of initial substances was ball-milled for 10 h, and then was sintered at 1073 K for 5 h. The electrodes under study were prepared by a slurry coating method. The active mass contained 80% Na2Ti3O7, 10% carbon black (Timcal) and 10% PVDF. The current collectors were made from stainless steel mesh. The loading of active material was ca. 10 mg cm‒2. The electrodes were pressed at 1000 kg cm‒2 and then dried under vacuum at 120 oC. The electrodes were tested in three-electrode cells with Na counter and reference electrodes, and a 1.0 M NaClO4 solution in a 1:1 (vol.) mixture of ethylene carbonate and propylene carbonate as an electrolyte. Cycling testing was performed using a computer-aided charge-discharge system (Buster, Russia) under the galvanostatic mode at a rate from C/8 to 2 C. EIS study was carried out at 10 mV vs. Na/Na+ with using of impedancemeter 2B-1. The analysis of impedance spectra taken at various potentials and at various cycles gave a possibility to create the equivalent circuit. Here R0 is the electrolyte resistance, R1 and C1 are resistance and capacitance of the passive film (SEI), W1 is diffusion impedance of SEI, R2 and C2 are charge transfer resistance and double layer capacity, W3 is Warburg impedance related to sodium diffusion in titanate, C3 corresponds to intercalation capacity.