Charge-Discharge Behavior of Electrochemically Prepared NaFePO4 Electrodes in Aqueous Sodium Ion Batteries
Authors: Ikeda Yoshitaka , Tsujimoto Shota , Sakamoto Ryo , Lee Changhee , Miyahara Yuto , Miyazaki Kohei , Abe Takeshi 
Published: December 2024

Introduction

Although lithium-ion batteries, which are currently the most widely used batteries, have various advantages such as long life and high energy density, there are safety issues due to the uneven distribution of regions where lithium can be mined and the risk of ignition from organic solvents. Therefore, research and development of aqueous sodium-ion batteries, which have abundant resources and high safety, is underway. However, aqueous batteries have a low energy density, and the working potentials of the positive and negative electrodes must be within the potential window of water. Therefore, NaFePO4, which operates within about −0.6 − 0.6 V (vs. SHE) and has a high theoretical capacity of 154 mAh g−1, is promising. In a previous study, electrochemical properties were evaluated using NaFePO4 obtained by oxidizing LiFePO4 with NO2BF4 and reducing it with NaI, but its charge-discharge capacity was about 50% of the theoretical capacity, which remains challenges [1]. In this study, we prepared ion exchanged NaFePO4 (NFP) by electrochemical redox of LiFePO4 (LFP) and investigated its electrochemical properties to find an appropriate ion exchange method to achieve higher capacity.

Experiments

A mixture of LiFePO4 (LFP) powder as active material, a mixture of equal mass ratios of vapor grown carbon fiber (VGCF) and acetylene black (AB) as conductive additive, and polyvinylidene fluoride (PVDF) as binder with a weight ratio of 7:2:1 was coated on a Ti foil and dried overnight. A beaker cell was assembled with a Pt electrode as the counter electrode and an Ag/AgCl electrode as reference electrode.
Hereafter, the potential is noted with reference to the Ag/AgCl electrode. Since the oxidation of LFP to FePO4 (FP) is known to occur at about 0.3 − 0.4 V and the reduction of FP to NFP at −0.3 − −0.4 V. [2] The LFP electrode was kept at 0.4 V for 6 hours after linear sweep voltammetry (LSV) to produce an oxidized FP electrode. The NFP electrode was then produced by reduction at −0.4 V for the same time. Charge-discharge measurements and X-ray diffraction (XRD) measurements were performed on the FP and NFP electrodes. For the charge-discharge measurements, the electrolyte was 1.0 mol dm−3 Na2SO4 aqueous solution, the current density was 77 mA g−1, and the cutoff potential was −0.4 − 0.4 V.

Results and Discussion

According to the XRD measurements in Fig. 1(a) and Fig. 1(b), the peaks of the fabricated FP and NFP electrodes are generally consistent with the FP and NFP peaks expected from the ICSD database, respectively, but different from the LFP and FP peaks. Therefore, the oxidation from LFP to FP was well advanced, and the reduction of FP to NFP was also well progressive.
Charge-discharge measurements showed two plateaus during the oxidation of NFP, suggesting that a two-step oxidation reaction was taking place. The obtained electrode had a reversible capacity of approximately 105 mAh g−1, which was greater than the reversible capacity of the electrode in a previous study [1]. This may be due to the ion-exchange NFP electrode having more Li+ desorption and Na+ insertion than the solid-state synthesis method.

Conclusion

NaFePO4 electrodes electrochemically ion-exchanged from LiFePO4 showed greater reversible capacities than electrodes prepared by conventional solid-state synthesis. The results show that the NaFePO4 synthesis method of electrochemical ion exchange is superior.

References

[1] A.J. Fernández-Ropero, D. Saurel , B. Acebedo , T. Rojo , M. Casas-Cabanas J. Power Sources, 291, 2015, 40-45.
[2] Wei Tang, Xiaohe Song, Yonghua Du, Chengxin Peng, Ming Lin, Shibo Xi, Bingbing Tian, Jiaxin Zheng, Yuping Wu, Feng Pan and Kian Ping Loh
J. Mater. Chem. A
, 2016,4, 4882-4892 .

Figure 1