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, Na_0.44MnO₂ 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 Na_0.44MnO₂, 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 Na_0.44MnO₂ 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 Na_0.44MnO₂ tube-liked particles. It is evident from the XRD image (Figure 2) that Na_0.44MnO₂ is formed with absence of nano-Al₂O₃.These results indicate that nano-Al₂O₃ coating is a good method to improve the cyclic performance of Na_0.44MnO₂ positive electrode material. Further work will focus on optimizing the processing conditions and electrochemical performance of nano-Al₂O₃ modified Na_0.44MnO₂.

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 Na_0.44MnO₂ positive electrode material from 61.5 mAh/g to 57.4 mAh/g. However, 4wt% nano-Al₂O₃ modified Na_0.44MnO₂ exhibits better capacity retention after 10 cycles, and long duration cycle testing is under way.

References

[1] Haegyeom Kim, Jihyun Hong, Kyu-Young Park, Hyungsub Kim, Sung-Wook Kim, Kisuk Kang, “Aqueous Rechargeable Li and Na Ion Batteries”, Chem. Rev, 2014, 114 (23), pp 11788–11827.

[2] Wei Wu, Sneha Shabhag, Jiang Chang, Ann Rutt and Jay F. Whitacre, “Relating Electrolyte Concentration to Performance and Stability for NaTi₂(PO₄)₃/Na_0.44MnO₂ Aqueous Sodium-Ion Batteries”, J. Electrochem. Soc. 2015 volume 162, issue 6, A803-A808.

[3] J.F.Whitacre, A.Tevar, S.Sharma, “Na₄Mn₉O₁₈ as A Positive Electrode Material for An Aqueous Electrolyte Sodium-Ion Energy Storage Device”, Electrochemistry Communications, 12 (2010) 463–466.

[4] J.-S.Kima, C.S.Johnsona, J.T.Vaugheya, S.A.Hackneyb, K.A.Walzc, W.A.Zeltnerc, M.A.Andersonc, M.M.Thackeray, “The Electrochemical Stability of Spinel Electrodes Coated with ZrO₂ , Al₂O₃ , and SiO₂ from Colloidal Suspensions”, J. Electrochem. Soc. 2004volume 151, issue 10, A1755-A1761.

[5] Ting-Feng Yi, Yan-Rong Zhu, Xiao-Dong Zhu, J. Shu, Cai-Bo Yue, An-Na Zhou, “A Review of Recent Developments in the Surface Modification of LiMn₂O₄ as Cathode Material of Power Lithium-Ion Battery”,Ionics, 2009, Volume 15, Number 6, Page 779.