The electrochemical performance of manganese oxoborate cathodes for lithium-ion batteries: Effect of synthesis method
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 Li1-xNi1–yCoyO2 [5] and Mn to give LiNi1-y-z MnyCozO2 [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.
Section snippets
Synthesis of Mn2OBO3 samples
Manganese oxoborate samples were synthesized by solid-state (SS), hydrothermal (HT), and solution combustion (SC) methods, as we recently reported [28]. HT experiments were performed using a Parr 5500-type mini benchtop reactor. Mn(NO3)2.4H2O (Merck) and Na2B4O7.10H2O (Merck) were introduced in distilled water at B:Mn = 4 and stirred at 50-60 °C for about an hour. The mixture was kept at 220 °C for 24 h in the reactor. The obtained product was washed several times with distilled water, then
Surface structure and morphology of Mn2OBO3
Compared with other polyanion-type electrode materials for rechargeable batteries, borates have high theoretical capacities and energy densities due to the light mass of the boron element [27]. It is well known that the method of synthesis and the morphology of the electrode materials have a significant impact on electrochemical properties [32]. The structure, composition, and morphology of warwickite-type manganese oxoborate samples were examined widely in different experimental conditions in
Conclusion
The purpose of this study was to examine the effects of morphology and surface properties on the electrochemical performances of warwickite-type manganese oxoborates. Mn2OBO3 samples prepared by different experimental methods were used as the cathode material in a LIB for the first time. The morphology, chemical structure, ratio of manganese oxidation states and electrochemical behaviour were confirmed through SEM, XPS and EIS analysis. The quasi-spherical Mn2OBO3 cathode materials prepared via
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by TENMAK-National Boron Research Institute, Turkey under grant code: 2018-30-06-30-005.
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2022, Ceramics InternationalCitation Excerpt :Thus, developing advanced cathodes for high-property lithium-ion batteries still faces huge challenges. Over the past decades, different electrode materials with various advantages have been explored in electrochemical energy storage [4–12]. Among these various cathodes, Li3V2(PO4)3 has regarded as the next-generation electrode for rechargeable lithium energy storage because of the three-dimensional open framework, high potential and high capacity [13–15].