A cobalt-free high-voltage Na₂Fe₂(SO₄)₃ cathode material for Na-ion batteries
A. Hanc, W. Zając*, A. Milewska, J. Molenda

AGH University of Krakow, Faculty of Energy and Fuels
*wojciech.zajac@agh.edu.pl


Sodium-ion batteries are regarded as the key energy storage technology for further spread of renewable power sources. Among various candidate cathode materials earth-abundant cobalt-free Na₂Fe₂(SO₄)₃ alluaudite phase enables utilization of Fe³⁺/Fe²⁺ redox couple offering high voltage of 3.8 V vs. Na⁺/Na, theoretical capacity of 120 Ah/kg and remarkable energy density of 450 Wh/kg [1]. Crystal structure of Na2Fe2(SO4)4 alluaudite is presented in Fig. 1a. Sodium sites (marked in blue) are distributed along 1-D tunnels separated with low energy barriers, resulting in good diffusivity, whereas FeO₆ octahedra and SO₄ tetrahedra form a skeleton structure providing electrochemical stability.
Herein we describe a new method of fabrication of Na₂Fe₂(SO₄)₄ sodium iron(II) sulphate exhibiting alluaudite-type crystal structure to be applied as a prospective cathode material for Na-ion batteries [2,3]. A simple fabrication procedure utilizes environmentally benign aqueous solutions of sodium and iron(II) sulfates. After evaporation, the residual is fired at relatively low temperature of 350°C in reducing conditions. Sodium acetate and glucose additives were applied to maximize fraction of the alluaudite phase and create a pyrolytic carbon coating (Fig. 1b).
Electrochemical performance of the obtained material was measured in a 2032 coin cells with a half-cell configuration vs. metallic sodium, as well as in as a full-cell vs. TiO2 anatase nanopowder. The resulting capacity vs. Na exceeded 88% of the theoretical one with good discharge capacity retention of over 94% between 5th and 50th charge/discharge cycles. Importantly, the cell with a TiO₂ anode provided the discharge capacity of 60 mAh/g, which is a fairly promising result taking into account limited amount of sodium in the cell.

References
[1] P. Barpanda, G. Oyama, S. Nishimura, S.-C. Chung, A. Yamada Nature Comm. 2014, 5, 4358
[2] A. Plewa, A. Kulka, E. Hanc, W. Zając, J. Sun, L. Lu, J. Molenda J. Materials Chem. A, 2020, 8, 2728-2740.
[3] A. Plewa, A. Kulka, J. Molenda, “Method for obtaining material for cathodes, intended for reversible sodium cells”, The Patent Office of the Republic of Poland Pat.236442 (application date 2017-11-27).