High-Voltage Cathode Properties of Cr-Containing Fluorophosphate Materials for Sodium-Ion Batteries
Authors: Nishio Akira , Sakamoto Ryo , Ishado Yuji , Nakamoto Kosuke , Minami Hironari , Izumi Hiroaki , Inoishi Atsushi , Okada Shigeto 
Published: December 2020

To satisfy the recent strong demand for large rechargeable batteries, the sodium-ion batteries are attracting attention as an alternative to lithium-ion batteries. Although sodium is much abundant than lithium, the standard electrode potential of sodium is ca. 0.33 V higher than that of lithium, and the cell voltage is therefore lower. In addition, since sodium-ions have twice the ionic volume of lithium-ions, a larger framework is desired for the insertion and desertion of sodium-ions. In order to solve these problems, various polyanionic materials with a NASICON-type framework have been investigated, which can be expected to produce high potentials due to the inductive effect and ensure a large diffusion bottleneck of sodium in the vertex-sharing framework [1]. In particular, it has been reported that Na3Cr2(PO4)3 exhibits the highest potential of 4.5 V vs. Na/Na+, which is the highest among Na-based NASICON-type materials [2]. On the other hand, our group has previously succeeded in raising the charge-discharge potential in fluorophosphate Na3M2(PO4)2F3 [M = Ti, Fe, V] in which a part of the phosphate polyanion is replaced by fluorine, which is highly electro-negative [3]. In this study, a fluorophosphate Na3Cr2(PO4)2F3 based on Na3Cr2(PO4)3 with a part of PO4
3- substituted by F- was synthesized and its high-voltage charge-discharge tests were performed.
Na3Cr2(PO4)2F3 was obtained by mixing NaF and CrPO4 in a molar ratio of 3:2, and then calcinating at 750°C in Ar after pelletizing. To prepare the CrPO4, Cr(NO3)3 and (NH4)2HPO4 were dissolved in water with a molar ratio of 1:1, and evaporated before sintering at 900°C. The electrodes were prepared by mixing Na3Cr2(PO4)2F3:acetylene black:binder = 70:25:5 (by weight) and coating on Al foil. The cathode properties of Na3Cr2(PO4)2F3 were evaluated in a 2032 coin-type cell with 1 M NaPF6 in EC:DEC = 1:1 in vol. electrolyte and Na metal as anode.
Powder X-ray diffraction and its Rietveld analysis revealed a few diffraction peaks of unreacted CrPO4, but the main diffraction peak was identified as the tetragonal Na3Cr2(PO4)2F3 in space group I4/mmm. The charge-discharge curves for the obtained Na3Cr2(PO4)2F3 cathode were shown in Figure 1. The maximum charge capacity was restricted to 127 mAh g-1, which is the theoretical capacity of Na3Cr2(PO4)2F3 based on the two-electron reaction of Cr3+/Cr4+. A charge-discharge plateau of about 4.7 V vs. Na/Na+ was observed at the lower rates of 0.1C (0.01 ~ 0.02 mA cm-2) and 0.05C. On the other hand, the reversible capacity increased at 0.5C and 1C, while the overvoltage increased and no clear plateau was observed in the discharge process. The increase in irreversible capacity at lower rates could be attributed to the fact that the substantially higher voltage promoted the oxidative degradation of the electrolyte due to the reduced overpotential. Although the reversible capacity of Na3Cr2(PO4)2F3 is less than the reported value of Na3Cr2(PO4)3, 80 mAh g-1, the discharge voltage plateau of Na3Cr2(PO4)2F3 was found to be increased 0.2 V higher than that of Na3Cr2(PO4)3 by fluorine introduction.
Reference
[1] K.S. Nanjundaswamy, A.K. Padhi, J.B. Goodenough, S. Okada, H. Ohtsuka, H. Arai, J. Yamaki, Solid State Ionics, 92 (1996) 1.
[2] K. Kawai, W. Zhao, S. Nishimura, A. Yamada, ACS Appl. Energy Mater.
1 (2018) 928-931.
[3] K. Chihara, A. Kitajou, I. D. Gocheva, S. Okada, J. Yamaki, J. Power Sources
227 (2013) 80-85.

Figure 1