Santamaría C., Morales E., Rio C.D., Herradón B., Amarilla J.M.
Electrochimica Acta
The influence of the nature of the cathode, the electrolyte and the operating voltage range on the electrochemical performance of sodium cells has been studied. Manganese-rich P2-Na0.67Mn0.67Ni0.33O2 (MN) layered oxide and the magnesium substituted P2-Na0.67Mn0.67Ni0.23Mg0.1O2 (Mg-MN) have been prepared by solid-state reaction and adequately characterized by X-ray diffraction, SEM and EDX techniques. Cycling performances at the low current of 0.2C (40 mAhg-1) of MN and Mg-MN cathodes have been compared in two voltage ranges. We have found a positive effect on cyclability at high voltages (above 4 V) by partial substitution of Ni2+ by Mg2+, which is in line with previous results. In order to study the influence of the electrolyte, and the working voltage, the electrochemical performance of Mg-MN half-cells with an alkyl carbonate-based electrolyte (1 M NaPF6 in PC) and an ionic liquid (IL)-based electrolyte (1 M NaFSI in Pyr14FSI), has been thoroughly studied in three voltage ranges (2.0V-4.0 V, 2.0V-4.5 V and 1.5V-4.0 V). Furthermore, flammability tests indicate a large improvement in safety for the IL-electrolyte when compared with the alkyl carbonate one. Results from galvanostatic cycling at increasing currents have shown that parameters such as the rate capability, and cyclability depend strongly on both the operation voltage range and the nature of the electrolyte. The discharge capacity, the average working voltage and the cyclability have been superior for the Mg-MN half-cells assembled with the non-flammable IL-electrolyte. Overall, the proper combination of the cathode (Mg-doped layered manganese-rich oxide), a pyrrolidinium-based IL electrolyte, and an adequate working voltage (2.0–4.5 V) shows promising results for the development of safe Na-ion batteries. © 2022
