High Cycle Stability of Post-Pinel Compound NaMn2O4 as Cathode of Sodium Ion Battery
Authors: Liu Xizheng , Iyo Akira , Zhou Haoshen 
Published: February 2020

1. Introduction
Room temperature sodium ion batteries (SIB) have been drawing increasing attentions as potential energy storage devices instead of the normally used lithium ion batteries (LIB) due to its advantages of low cost and unlimited sodium resources [1-2]. Design and synthesis of electrode materials for SIB which can meet commercial standard still challenge the materials scientists. Inspired by the material design and development of LIB, popular electrode materials of LIB have been used as SIB electrode by a electrochemical or chemical Li-Na exchange [3]. Manganese-based materials spinel-type LiMn2O4 is widely used in present large-scale LIB because well electrochemical performance and elemental abundance in the Earth. However, the spinel-type NaMn2O4 is a thermodynamically unstable phase and can not be synthesized directly. Electrochemical desertion of Li and followed by an insertion of Na from LiMn2O4 have been investigated in SIB, but a poor cycle performance and structure rearrangement have been reported [4]. Herein, we synthesized a post-spinel structure NaMn2O4 by a high pressure technique and discussed the potential applications as cathode materials for SIB.
2. Experiments
The NaMn2O4 was synthesized using a high-pressure technique. The mixture of Na2O2 and Mn2O3 (with a 5% excess of Na2O2) were sealed in a Au-capsule and heated at 1223 K under a pressure of 4.5 GPa for 1 hour. The synthesized samples was washed by water and post-heated at 623 K for 5 hours. The structure and morphology have been characterized by XRD, SEM and TEM. Electrodes were fabricated using NaMn2O4, acetylene black and PTFE in a mass ratio of 6:3:1. Coin cells consists by a NaMn2O4 cathode, sodium metal anode and NaPF6 electrolyte.
3. Results and discussion
Fig.1 XRD pattern (a); SEM images (b); HRTEM (c) and selected charge/discharge profiles at the voltage range of 2.0-4.0 V.
The XRD patterns of NaMn2O4 is shown in Fig. 1a. All peaks can be index as the previously known orthorhombic structure with a space group of Pnma. No impurities and secondary phase can be found. It showed a rod-like morphology with a diameter of about 100 nm and a length of 3-5 μm as shown in Fig. 1b. The detailed crystal structure of as prepared NaMn2O4 have also been studied by HRTEM and showed in Fig. 1c. The inner figure of 1c is the structure view of post-spinel NaMn2O4. The 1D tunnels which are surrounded by double rutile chains of Mn2O4 are filled with sodium ions. Sodium ions in the post-spinel structures occupied the sites much larger than that of the Li ions in spine LiMn2O4. Materials with this structure characteristic provide a potentials of reversible sodium ion insertion/desertion with easy ion diffusion and stable framework host. First-principles calculation results showed that this compound is stable at ambient conditions and a high mobility of Na+ in post-spinel phase [5].
Fig. 1d showed the selected charge/discharge profiles at a voltage range of 2.0-4.0 V. It is interesting to note that the superior cycle stability of both charge/discharge capacities and voltages profiles. The stable charge/discharge plateaus at about 3 V (vs. Na+/Na) which can be attributed to the redox reaction of Mn4+/Mn3+. Different from other NaMnxOy compounds, this compound showed that it has a relatively stable structure, sub-plateaus can rarely be founded during charge/discharge processes. As we know, the LiMn2O4 electrode suffered a serious capacity fading when cycled at a temperature higher than 55 °C. The main explanations for this is the Jahn-Teller effects of Mn3+ and dissolution of Mn2+ from the cathode materials. Researchers also endeavored to improve the high temperature performance of Mn-base materials by many ways. We also investigated the cycle performance of this post-spinel NaMn2O4 at 55 °C. A very stable cycle performance at 55 °C have been obtained. The reasons for the superior stability both at room temperature and 55 °C are the large barrier to rearrange Mn ion in this post-spinel structure. The detailed electrochemical performance and relations with structures will be presented at the conference.
Reference
[1] R. Berthelot, D. Carlier, C. Delmas, Nature Mater. 10 (2011) 74-80
[2] V. Palomares, M. Casas-Cabanas, E. Castillo-Martinez, M. H. Han, T. Rojo, Energy Environ. Sci. 6 (2013) 2312-2337
[3] H. Pan, Y. Hu, L. Chen, Energy Environ. Sci. 6 (2013) 2338-2360
[4] N. Yabuuchi, M. Yano, S. Kuze, S. Komaba, Electrochimica Acta, 82 (2012) 296-301
[5] C. Ling, F. Mizuno, Chem. Mater. 25 (2013) 3062-3071