Surface Modification of Na0.44MnO2 Positive Electrode Material Via Nano-Al2O3 for Aqueous Sodium-Ion Batteries
Authors: Wu Jinming , Chang Jiang , Song Haosheng , Wu Wei , Whitacre Jay 
Published: February 2020

Recently, aqueous sodium-ion batteries have attracted great attention for large-scale energy storage system due to its environmental friendliness, outstanding safety and abundance of sodium element [1]. Efficient positive electrode materials are vital to overall performance of Na-ion batteries(NIBs). Oxides are more favorable candidates for positive electrode materials due to its simple synthesis and high energy density. Among these oxides, Na0.44MnO2 gains great interest because of its low cost and intrinsic crystal structure forming large tunnels which facilitates sodium-ion reversible intercalation and transportation [2-3]. However some of our exploratory work shows that these materials can exhibit capacity fade, especially when discharged to lower potentials.
In order to meet the demand of more stable cycling performance through a wide of applied potentials for Na0.44MnO2, we explore the use of nano-Al2O3(Alfa Aesar) for coating material. Some results, reported elsewhere suggest that the surface modification of LiMn2O4 with Al2O3 can enhance its electrochemical stability [4-5]. Herein, we explored a novel surface-modified Na0.44MnO2 positive electrode material via nano-Al2O3 coating during solid state synthesis process with Mn3O4, which presents improvement in the cycle stability. The concentration of nano-Al2O3 were 0 wt%, 2 wt%, 4 wt%, 6 wt%, 8 wt%, 10 wt%, respectively.
SEM image (Figure 1) shows that those nano-Al2O3 particles tightly attached on the surface of Na0.44MnO2 tube-liked particles. It is evident from the XRD image (Figure 2) that Na0.44MnO2 is formed with absence of nano-Al2O3.These results indicate that nano-Al2O3 coating is a good method to improve the cyclic performance of Na0.44MnO2 positive electrode material. Further work will focus on optimizing the processing conditions and electrochemical performance of nano-Al2O3 modified Na0.44MnO2.
In comparing the cyclic voltammetry curves (Figure 3) of the six samples, early data suggests that the additional of nano-Al2O3 slightly decrease the capacity of Na0.44MnO2 positive electrode material from 61.5 mAh/g to 57.4 mAh/g. However, 4wt% nano-Al2O3 modified Na0.44MnO2 exhibits better capacity retention after 10 cycles, and long duration cycle testing is under way.

References

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Figure 1