Evaluation of Cathode α-NaFeO2 for Sodium-Ion Batteries
Authors: Imabayashi Koki , Kobayashi Yoshinao , Yasui Shintaro 
Published: December 2024

Lithium-ion batteries(LIBs) are rechargeable batteries that are indispensable to our daily lives, but the supply of lithium-ion batteries is expected to become more difficult as the market expands due to the constraints of lithium being a rare metal and a scarce resource. In recent years, sodium-ion batteries(SIBs) have much attracted attention as a secondary battery that can replace LIBs with less risk to resources and supply. α-NaFeO2 with a simple layered rock salt structure is well known as the cathode active material for SIBs. It does not contain any rare metals, but only iron which is abundant in the earth's crust, so it is expected to be environmentally friendly and low-cost to synthesize. However, a few paper has evaluated sodium ion batteries using α-NaFeO2 cathode, and only basic information such as synthesis methods and reaction potentials have been reported. In this study, we fabricated α-NaFeO2 and investigated the charge-discharge reaction of batteries using α-NaFeO2 in detail. The battery characteristics were evaluated for the prepared SIBs.

α-NaFeO2 was prepared by a solid-state reaction from stoichiometric amounts of Na2O2(Sigma-Aldrich Co.) and Fe2O3(Rare-metalic Co.) as starting materials. Weighed powders were mixed using a mortar and pestle without any wet solution. The mixed powder was heated in an electric furnace at 650℃ for 20 h in air. After that, they was taken out from the furnace at 200℃ for preventing water absorption, and then immediately transferred into an argon-filled glove box. X-ray diffraction was carried out for analysis of crystal structure for prepared materials. Positive electrodes consisted of 70wt% α-NaFeO2, 20wt% acetylene black, and 10wt% PVDF, which were mixed with N-methlpyrrolidone and pasted Al foil as a current collector by doctor blade, and then dried at 120℃ in vacuum oven. After preparing cathode sheet, 10mm-diamater circle-shape cathode was prepared using punching machine. Anode sheet was also prepared using hard carbon and was made circle-shape anode with 16mm-diameter. 2032-type coin-cells were assembled in the argon filled glove box to evaluate electrode performance of α-NaFeO2. Electrolyte solution used was 1 mol/L NaPF6 dissolved in ethylene carbonate and dimethyl carbonate (EC:DMC=1:1 vol). The charge-discharge characteristics of the coin-cells were measured using constant-current mode, with a voltage range of 1.5 to 2.8 V.

Reversible insertion and desorption of sodium was confirmed over three cycles, and the battery was found to have an average working voltage of about 2.3 V and a discharge capacity of about 40 mAh/g at the first cycle. However, after three cycles, the discharge capacity decreased and the battery no longer functioned as a battery. Compared to the previous study, the average working voltage was about 1 V lower and the discharge capacity was half the value. The reason for the low average working voltage should be the high reaction potential of the hard carbon we used. A possible reason for the low discharge capacity and the inability to function as a battery is that the surface of α-NaFeO2 may have reacted with carbon dioxide and water in the atmosphere under preparing situation, and also is SEI construction at the surface of α-NaFeO2. The XRD results of α-NaFeO2 stored in air and the increased resistance in the impedance results suggest this possibility. We are currently verifying the insertion-desorption potential of hard-carbon and the crystal state of α-NaFeO2 after charging and discharging, and are investigating the cause of the decrease in discharge capacity. We will report the results of these experiments.