Zinc-Ion Inserted a-MnO2/Onion like Carbon (OLC) Nanorod Composite Cathode Materials for Aqueous Zn-Ion Battery
Authors: Palaniyandy Nithyadharseni , Nkosi Funeka Phumzile , Raju Kumar , Ozoemena Kenneth I. , Mathe Mkhulu K. 
Published: February 2020

High-energy density lithium-ion batteries (LIBs) are ultimately the key drivers for portable electronic devices, hybrid electric vehicles and smart grid-energy storage systems [1]. However, the potential scarcity of lithium resources, which are used as cathode and organic electrolytes in batteries, as well as their low safety, will be a serious impediment to their large scale production. Thus, for the past decade, many researchers have been looking for alternative batteries such as Na+, K+ and Zn2+ [2]. Recently, an aqueous Zn-ion rechargeable battery (ZIB) has a tremendous exposure with in the research world market because of potential applications in large-scale energy storage. Owing to its various outstanding properties such as reasonable redox potential, high safety, low cost, most abundant and eco-friendly compared with the post lithium-ion batteries. Moreover, Zn metal as an anode delivers high gravimetric and volumetric capacities of 820 mAh g-1 and 5855 mAh cm-3 [3].

It is well known that the manganese oxide (MnO2)cathode material has been widely studied for lithium-ion batteries, sodium-ion batteries and newly discovered Zinc-ion batteries due to its excellent electrochemistry, high abundance and low cost. The previous explorations of the cathode materials such as manganese based oxides (a-b-g-d-MnO2 and todorokite-MnO2) were used for aqueous Zn-ion batteries [4, 5]. Among them, a-MnO2 is very attractive and has become the subject of great interest since it has a large [2×2] tunnel structure along its c-axis composed of four edge-sharing MnO6 octahedral units. However, a-MnO2 has subdued capacity fading during the initial 20 cycles, poor rate capabilities and inadequate cycling performance thus hinders its practical application of Zn-ion batteries. To solve this issue, the carbon based composite materials has been proposed [6].

Hence, for the first time in the present work, the Zn-ion inserted manganese oxide nano-rod composite (Zn1-xMnO2/OLC) cathode materials were evaluated for aqueous ZIBs. Different concentrations of Zn ion inserted MnO2 (Zn1-xMnO2) nano-rods were prepared by one step molten salt method. The Zn1-xMnO2/OLC composite cathode materials have been prepared by appropriate mixing of Zn1-xMnO2 and OLC using ultra-sonication and magnetic stirrer. The structural (XRD) and morphological (FE-SEM and TEM) reveals there are no phase changes after the insertion of Zn in to the host structure and below 50 nm nano-rod morphology. The electrochemistry was demonstrated using Zn-foil as the anode and the Zn1-xMnO2/OLC composite as the cathode. The 1M ZnSO4 with the addition of MnSO4 with different molar concentrations such as (0.1, 0.3 and 0.5) was used as an aqueous electrolyte. The electrochemistry of the fabricated cells was examined by cyclic voltammetry and galvanostatic cycling under a constant current density of 246 mAg-1 with the potential window of 1.0-1.8 V. The Zn1-xMnO2/OLC exhibits highly stable and better capacity retention (95%) compared to pure a-MnO2 cathode materials (55%). The enhanced cycling and capacity retention may be due to the OLC or Zn ion insertion in to the a-MnO2 structure. In this presentation, we will discuss in detail the physical and chemical properties of this new cathode material and their electrochemical performance in aqueous media.

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