Insights into P-Type Iron- and Manganese-Based Layered Sodium Cathodes

Abstract

Sodium-ion batteries are a low-cost alternative to lithium-ion batteries in large-scale electric energy storage applications due to the natural-abundant sodium resources and similar working principle to LIBs during cycling [1]. Until now, many different cathodes for SIBs have been studied, including layered transition metal oxides (NaxTMO2, x<1), Prussian blue-type compounds, polyanionic and organic compounds [2]. Among them, NaxTMO2 has been extensively reported, due to its layered structure, easy synthesis, promising electrochemical properties, and feasibility for commercial production [3]. According to the occupation of Na ions and oxygen stacking, NaxTMO2 can be classified into P2 and O3 types, where P2 structure contasins trigonal prismatic sites of sodium ions and ABBA oxygen stacking sequence and O3 structure represents octahedral sites of sodium ions and ABCABC oxygen stacking sequence [4]. At low potentials (<2.0V), there is a phase transition P2-P2’ connected with the manganese redox activity and the Jahn-Teller distortion [5]. Compared with O3-type structure, P2-type NaxTMO2 provides higher specific capacity and better structural stability due to the lower diffusion barriers through Na-ion prismatic sites [6].Iron- and manganese-based layered oxides for sodium-ion batteries have attracted renewed interest due to their low cost and environmental friendliness. However, the phase change at high voltage and the Jahn-Teller effect lead to shortening cycle life and poor rate capability. We design a structure optimization of the Na2/3Mn1/2Fe1/2O2 cathode through a partial substitution of Fe3+ to explore the mechanism of redox activity of P2-type Mn/Fe-based layered cathodes. We present the investigation of the effect of simultaneous cationic substitution on the electrochemical performance and structural stability of P2-Na2/3Mn1/2Fe1/2O2. The obtained material exhibits a solid solution mechanism during desodiation/resodiation process and delivers an initial discharge capacity of 168 mA h g-1 at 0.1C and a capacity retention of 80% after 50 cycles. The modified P2 composition has stable cycle performance in the potential window from 1.5V to 4.5V. The main focus here is to understand the fundamental electrochemical mechanism of this layered cathode material by exploring the redox processes of transition metal cations and oxygen anions upon cycling. In situ X-ray synchrotron radiation diffraction is used to explore the structural changes during cycling. In situ X-ray Absorption Near Edge Spectroscopy is used to elucidate the local electronic structure of Fe and Mn atoms as it evolves throughout this electrochemical process. 23Na solid-state nuclear magnetic resonance spectroscopy is used to investigate Na coordination environments during desodiation/resodiation. Ex situ soft X-ray absorption spectroscopy results reveal the participation of oxygen anions in the electrochemical reactions. Thus, we explore the redox reactions of transition metals and oxygen to explain the mechanism and the electrochemical performance for the P2-type Mn/Fe-based layered cathodes.