The electrochemical performance of manganese oxoborate cathodes for lithium-ion batteries: Effect of synthesis method

Abstract

Cathodes for lithium-ion batteries (LIBs) present stability and performance issues; therefore, searching for alternative novel electrode materials or modifications of known ones is one of the main issues. Warwickite-type Mn₂OBO₃ samples synthesized by hydrothermal (HT), solid-state (SS), and solution combustion (SC) methods were investigated for the first time as cathode materials for LIBs. The prepared Mn₂OBO₃ electrodes were analyzed using scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques in order to correlate the surface properties and electrochemical behaviour. X-ray photoelectron spectroscopy (XPS) revealed the existence of +2 and + 3 oxidation states of Mn in the electrodes. The band gaps (E_g) for all electrodes were calculated between 1.75 and 2.04 eV. The electrochemical performance of the Mn₂OBO₃ electrodes (E) prepared with the SC method was significantly improved. The E-SC electrode showed the highest initial discharge capacity of about 155 mAhg^-1 and found to be more stable at a current density of 30 mAg^-1. The results presented here provide a new approach to the utilization of manganese oxoborate compounds as promising cathode materials for high-performance energy storage systems and emphasize that homogeneously structured cathode electrodes with a high surface area are desirable for LIBs.

Introduction

Lithium-ion batteries (LIBs) are commonly used today for portable electronics and electric vehicles. Commercial LIBs work on the principle of obtaining electrical energy as a result of the oxidation-reduction reactions that occur when lithium enters and exits between the electrodes formed from lithium-containing compounds. Positive improvements have been achieved in the past twenty years in increasing the efficiency of these systems [1,2]. However, the production cost is still high, and the energy density of current LiBs does not meet the demands of the growing energy market.

Many studies have shown that the electrochemical performance of the cathode materials depends on the synthesis route, conditions and test parameters. Generally, it has been shown that the optimized grain morphology and suitable size enhance their electrochemical performance [3]. Depending on the choice of electrode materials, the voltage, energy density, life and safety of LIBs can change to a great extent. Layered-, spinel- and polyanion-type transition metal oxides have attracted much attention as cathode materials for LIBs in recent years [4]. Among them, the layered LiCoO₂ remains as one of the best cathodes to date with a high operating voltage of ∼4 V. Other layered LiMO₂’s (M = Ti, V, Mn, Fe, Ni) are not suitable as they suffer from layered to spinel transitions, dissolution problems or synthesis difficulties. To reduce the cost and increase the capacity, LiCoO₂ was substituted with Ni and Mn to give Li_1-xNi_1–yCo_yO₂ [5] and Mn to give LiNi_1-y-z Mn_yCo_zO₂ [6]. Although these materials showed good electrochemical behaviour, their electronic conductivity was still low for a high-rate cathode [7]. Trials have also been carried out with spinel lithium manganese oxides, but efficient results have not been obtained [[8], [9], [10], [11]]. The cobalt and nickel metals used in the conventional electrodes are expensive, and their natural reserves are gradually running out. On the other hand, manganese oxides have attracted great interest in the last two decades as electrodes for LIBs due to their high theoretical capacity [8]. Furthermore, manganese stands out as an abundant, cheap, and less toxic metal than Co and Ni. Additionally, manganese based oxides are achievable by mixed-valence such as Mn²⁺/Mn³⁺, Mn³⁺/Mn⁴⁺, Mn⁴⁺/Mn⁵⁺ redox couple providing potential electrode materials for various electrochemical applications. Significantly, these redox couples play important roles for the charge/discharge properties together with specific capacities in the cathodes [[12], [13], [14]]. Composite electrodes have also been developed by integrating layered and spinel lithium manganese oxides as next-generation high-capacity electrodes [15].

Since the discovery of LiFePO₄ as a candidate cathode material [16], many research groups have tried to improve mixed polyanionic cathodes incorporating XO₄ groups. Much of the work has been centred on the olivine structure, LiMPO₄ (M = Fe, Mn, Ni, or Co), due to low cost and high intrinsic safety [16]. Inspiring by natural olivine, which contains both iron and manganese, the electrochemical performances of LiFePO₄ and LiMnPO₄ have been investigated and shown to be enhanced by Ni and Co doping [1,17]. A theoretical study conducted in 2011 suggested that oxopolyanion compounds with a structure similar to the Tavorite mineral could be promising new generation cathode materials for LIBs [18]. In the Tavorite structure of the general formula “AM(TO₄)X”; A is an alkali or alkaline earth metal, M is a metal, T is a p-block element, and X is O, OH or F. Calculation results for oxophosphates, such as LiVO(PO₄), LiV(PO₄)F and LiFe(SO₄)F, have shown that one-dimensional lithium diffusion occurs at high speed, and two lithium ions are activated reversibly for each redox-active metal ion.

Compared to the studies summarized above for the use of oxophosphate compounds as electrodes, there are very few studies on the use of transition metal oxoborates as electrodes. Norbergite-type iron oxoborate (Fe₃BO₆) showed electrochemical activity in the range of 0.9V–3.0 V against Li /Li⁺ [19,20]. Ludwigite-type Fe₃BO₅ converted into a completely metallic nano-Fe(0) structure in the first cycle. Then the system demonstrated a stable capacity of 345 mAh g⁻¹ in the range 0.75 − 3.0 V, via reversible redox reactions between Fe (0) and Fe (II/III) [21].

No information is available in the literature to use manganese oxoborates as electrode materials, excluding the study published by Li et al. in 2015 [22]. In this study, Warwickite-type Mn₂BO₄ nanowires prepared by the hydro/solvothermal method were used as the anode in LIBs and exhibited high capacity, durability, and reversibility. In warwickite-type homometallic oxoborates (Fe₂OBO₃ and Mn₂OBO₃), metal ions are found in M(II) and M(III) oxidation steps and the metal to boron molar ratio = 2:1, while for ludwigite-type homometallic oxyborates (Fe₃O₂BO₃ and Mn₃O₂BO₃), the ratio is 3:1 [[23], [24], [25], [26]].

Borate-type electrode materials with the simplest B–O frameworks are known as promising cathode materials for LIBs. Transition-metal borates though having the advantages of being lightweight, environmentally friendly and high abundance of the precursor materials, their inadequate conductivities limits their practical applications [27]. In contrast, the development of manganese oxoborate compounds for electrochemical applications is a new and open-ended issue. Given their good electrocatalytic activity and robustness and considering that there appear to be fewer studies on cathode materials for LIBs than the research on anode materials, we focused on investigating the cathodic performance of various manganese oxoborate samples prepared with different synthesis techniques. A discussion is presented by relating the electrochemical performances with the surface/morphology characteristics.