Reaction Mechanisms of Sodium‐Ion Batteries under Various Charge and Discharge Conditions in a Three‐Electrode Setup

Abstract

Charge‐discharge reaction mechanisms of sodium‐ion batteries under various condition are studied by using a three‐electrode setup of a pouch‐type sodium‐ion battery. The sodium‐ion battery is constructed by using cost‐effective ternary layered Na0.76Ni0.3Fe0.4Mn0.3O2 and commercial hard carbon as the cathode and anode materials, respectively, and 1.0 M NaPF6 in mixed carbonate solvent as the electrolyte. The electrochemical impedance spectroscopy (EIS) results and transmission electron microscopy (TEM) images show that an apparent solid‐electrolyte interphase (SEI) film is formed on anode material surface during the formation (pre‐charging) process, and the potentials for the SEI film formation of the solvents (ethylene carbonate and ethylene carbonate) and the additive (fluorinated ethylene carbonate) in the electrolyte are 2.04 and 2.79 V, respectively. The X‐ray diffraction (XRD) results demonstrate that, during charge, the crystal structure of cathode material changes significantly with the deintercalation of Na+. When the battery is charged to 5.0 V, the diffraction peak corresponding to the (002) plane disappears, as Na+ is further deintercalated, and the structure changes from the hexagonal phase to the monoclinic phase, causing the rapid degradation of the cycle performance. When the battery is overdischarged to 0 V, the EIS results and TEM images show that the SEI film is destroyed completely, and the cycle life performance is significantly deteriorated.

Slope-Dominated N/S Co-Doped Carbon Anode Derived from Pitch for Sodium-ion Batteries

“…33,34 During the charging process, a broad peak (0.2−1.8 V) was observed, which corresponds to the surface desorption of sodium ions and is manifested as the slope region in the charge−discharge curve. 35 After that, the reversible capacities of all samples were evaluated through GCD tests, as shown in Figure 4c, Figure S7a and b. The constant current charge/discharge curves of NSPC6-700 for the first three cycles at 30 mA g −1 were presented in Figure 4b.…”

“…The CV curves of NSPC6-700 during the first three cycles at a scan rate of 0.1 mV s –1 were illustrated in Figure a. During the initial discharge, a broad cathodic peak at approximately 0.79 V was observed, indicating the electrolyte decomposition and formation of a solid-electrolyte interface (SEI) film during the first cycle. , During the charging process, a broad peak (0.2–1.8 V) was observed, which corresponds to the surface desorption of sodium ions and is manifested as the slope region in the charge–discharge curve . After that, the reversible capacities of all samples were evaluated through GCD tests, as shown in Figure c, Figure S7a and b.…”