Comparative study on ternary spinel cathode Zn–Mn–O microspheres for aqueous rechargeable zinc-ion batteries

Highlights

• Facile preparation of Zn–Mn–O based spinel nanostructures with different cation ratios.

• Fabrication of nanoparticle-aggregated microspheres by a solvothermal reaction.

• Highly reversible Zn storage capability in Zn_1.67Mn_1.33O₄ microspheres for aqueous batteries.

Abstract

We demonstrate the cation ratio-controlled synthesis of ZnMn₂O₄ and Zn_1.67Mn_1.33O₄ aggregated microspheres. The carbonate precursor was synthesized by a solvothermal reaction, and then completely converted to oxide by calcination at 600 °C with a controlled cationic ratio. The prepared ternary oxide has a nanoparticle-aggregated morphology and uniform size distribution. The electrochemical properties were investigated by cyclic voltammetry and constant current charge-discharge measurements. The Zn_1.67Mn_1.33O₄ electrode reveals better performance for Zn²⁺ storage than the other, delivering 175 mA h g⁻¹ after 40 cycles. After the electrochemical test, ex situ analysis was conducted to identify the Zn²⁺ storage mechanisms. From these results, we confirm that the Zn_1.67Mn_1.33O₄ cathode is a promising Zn²⁺ storage material for environmental friendly aqueous rechargeable Zn-ion batteries.

Introduction

In recent years, numerous countries have attached great importance to eco-friendly renewable energy to achieve pollution reduction and sustainable development. Typically, to meet the demand of flexible energy consumption and efficient use of energy resources, energy storage devices are an integral part of the system [1]. Li-ion batteries (LIBs) have been playing this role owing to their excellent performance, in terms of high energy density, high efficiency, and low noise characteristics [[2], [3], [4]]. However, LIBs have some safety concerns in terms of the use of an organic electrolyte; there is a tendency for fire and explosions when the cell is operated under improper conditions such as high temperature and external shock, which is caused by a chemical imbalance in the organic liquid electrolyte system [5]. Furthermore, the price of each component in LIBs is increasing owing to the increasing demand and the limited reserves of raw materials [6].

For these reasons, in recent years, the aqueous bivalent metal anode system of Zn-ion batteries (ZIBs) has received more attention than other multivalent ionic systems (Al, Mg) as a result of their advantageous properties. In practical use, Zn has a relatively low redox potential (−0.76 V vs. standard hydrogen electrode) and high theoretical capacity (820 mA h g⁻¹) [[7], [8], [9]]. Elemental Zn is an economical, earth-abundant material that is highly stable in aqueous electrolytes and when exposed to air [10]. Owing to the use of aqueous electrolyte, it has lower toxicity and flame-resistant properties in comparison with other high-energy-density metals such as Li and Mg systems [11]. Based on this, ZIBs are an up-and-coming substitute for economically viable, environmental friendly, and massive-scale energy storage applications.

Over the last decade, considerable research has been reported on cathodic materials for ZIBs for maximized utilization of a high-energy-density Zn metal anode. To date, diverse Zn²⁺/H⁺ storage materials for ZIBs in mild aqueous electrolytes, such as the polymorphs of manganese oxide (α, β, δ, and λ-phases) [9,[12], [13], [14], [15], [16]], Prussian blue analogs [17,18], Ni [19], Co₃O₄ [20], and V₂O₅ [21] have undergone significant development. Among these candidates, the polymorphs of manganese oxide cathodes have some advantageous features, such as cost effectiveness, earth-abundance, high theoretical capacity (308 mA h g⁻¹), and high output voltage [[21], [22], [23]]. Unfortunately, despite these advantages, they are prone to capacity fading due to the dissolution of Mn²⁺ via Mn³⁺ disproportionation, which is caused by the Jahn-Teller distortion [24]. The capacity retention is enhanced by the pre-addition of Mn²⁺ ions in the electrolyte, which retards the Mn²⁺ dissolution; however, the detailed mechanism requires further study.

ZIBs using MnO₂ as a cathode have been extensively studied, mainly in terms of their phase change mechanism under cycling [12,14,15,23] and zinc storage mechanism [14,25]. By comparison, there has been limited research on the ternary oxide cathode thus far. Recently, the first use of spinel ZnMn₂O₄ as a new cathode material with highly reversible capacity was reported [26]. They reported that the Mn-deficient ZnMn₂O₄ shows good capacity retention because the cation-deficient structure of ZnMn₂O₄ permits rapid diffusion of Zn²⁺ owing to the reduced repulsion of cation deficiency of the spinel structure. Similarly, more recent work has reported on the enhanced reversible capacity of the ZnMn₂O₄ cathode using an MnSO₄ additive within the electrolyte, which retards the Mn²⁺ dissolution [27].

Herein, in this respect, we synthesized Zn-excess cubic spinel structure Zn_1.67Mn_1.33O₄ for the first time for use as a cathode material for ZIBs. We further compared the electrochemical performances of two types of zinc-manganese oxides with different cation ratios, namely Zn_1.67Mn_1.33O₄ (ZMO5412) and ZnMn₂O₄ (ZMO124), with aggregated sphere morphology as cathode materials for ZIBs. Both samples were synthesized via a solvothermal route and calcination. We investigated the electrochemical characteristics of both electrodes, which have different crystal structures, by crystal structure and cationic structure analysis.