Synthesis and redox properties of LixNi2(MoO4)3: a new 3-V class positive electrode material for rechargeable lithium batteries (original) (raw)
Abstract
A new framework type Li_x_Ni2(MoO4)3 [0⩽_x_⩽4] polyanion compound was synthesized via a glycine-nitrate soft-combustion process at low temperature. The annealed powders were characterized by XRD to confirm the phase formation of the stoichiometric product, Ni2(MoO4)3 in its non-lithiated state. The morphology of the annealed product was found to be composed of soft agglomerates embedded by ultrafine spherical grains. Electrochemical redox properties of the synthesized product were confirmed by employing the new material as a cathode in lithium-containing test cells in an aprotic electrolyte environment (1 M LiPF6 in EC
DMC). Slow scan cyclic voltammetry (SSCV) confirmed the redox behavior corresponding to the reduction/oxidation of the transition metals, Ni and Mo between the potential window of 3.5–1.5 V. The lithium insertion/extraction process was confirmed by galvanostatic measurements on the test cells and they exhibited well discernible discharge/charge profiles with a reversible capacity of 170 mAh/g over the potential window of 3.5–1.5 V after the first charge/discharge cycle. Nevertheless, the discharge capacity was found to deteriorate slowly upon repeated cycling, which might presumably be due to disproportion reaction of the host structure beyond the extent of insertion of two lithium ions.
Introduction
Among the available energy storage technologies, lithium-ion batteries with long cycle life, high energy density, low self-discharge rate and environmentally friendly electrode materials have occupied a place of pride in fulfilling the demands of modern society for a variety of consumer electronic gadgets such as cellular phones, note book computers, camcorders, cordless power tools and future EVs and HEVs (hybrid electric vehicles) [1]. In order to obtain better performance in terms of safety, capacity, cost, etc., when compared to the presently used cathode materials, newer materials are being investigated. Recent developments have resulted in promising discoveries and/or improvements giving better and improved prospects for lithium-ion batteries [2], [3], [4], [5], [6].
In addition to the above, in the context of improving the operating voltage and discharge capacity, oxides with polyanions having three dimensional open framework structures have attracted a great deal of attention [7], [8]. Polyanion materials having a NASICON type structure were extensively studied as solid electrolytes in the past [9], [10]. However, recently, they have emerged as an interesting class of electrode materials for battery applications. Because of the open three-dimensional crystal structure that allows easy diffusion of smaller alkali cations like lithium, many of them have been found to have potential applications as cathodes in lithium batteries. A series of lithium containing compounds incorporating larger polyanions based on olivine type Li_x_M′XO4 (M′=transition metal and X=P, Mo, W, S) and NASICON structure type Li_x_M′2(XO4)3 (M′=Ti, Fe, V and X=P, As, Mo, W, S) have been reported [11], [12], [13], [14], [15], [16], [17], [18], [19], [20].
Amongst the systems, which are intriguing in rechargeable lithium ion technology, molybdenum (VI) compounds remain of significant potential as cathode materials due to the existence of the Mo6+/Mo4+ redox couple, which can offer two-electron transfer during an electrochemical reaction. The latter reaction is very attractive for the development of batteries with high capacity and consequently, with high energy density. Manthiram and Goodenough [11], [12] and Reiff et al. [13] studied lithium insertion properties of Fe2(XO4)3 (X=Mo, W and S) nonlithiated phases. They reported that insertion/extraction of Li+ into and from Li_x_Fe2(XO4)3 is topotactic for 0⩽_x_⩽2, and that the insertion of lithium was found to be limited to two Li+/formula unit. Beyond this extent, it led to disproportionation of the host framework. Another lithium rich polyanion material, Li3Fe2(PO4)3 was found to accommodate two additional lithium ions reversibly which yielded a stoichiometry of Li5Fe2(PO4)3 [14]. Further more, Andersson et al. [17] and Gaubicher et al. [18] also investigated the effect of reversible insertion of lithium in Li3Fe2(PO4)3. Similarly, we also discovered a lithiated phase of NASICON type structure, Li2Ni2(MoO4)3 recently and studied its high voltage redox behavior as positive electrode in lithium batteries [19]. Nevertheless, according to the best of our knowledge, lithium insertion/extraction into and from M′(XO4)3 [M′=Mn, Ni and Co and X=Mo and W] nonlithiated phases has never been studied so far.
It is now explicitly understood that the open framework compounds belonging to the family of polyanions with a general formula, M′2(XO4)3 (M′=transition metal) might favor the topotactic insertion/extraction of Li+ within the framework [11], [21]. In the present work, a new polyanion material, Ni2(MoO4)3, belonging to the above polyanion family, was studied in the context of an electrochemical Li+ insertion/extraction reaction as the positive electrode in lithium cells. This paper describes the first report on the synthesis, structure and electrochemical (redox) characteristics of ultrafine powders of Ni2(MoO4)3.
Section snippets
Synthesis of Ni2(MoO4)3
The formation of the Ni2(MoO4)3 framework compound was accomplished through a soft-combustion low temperature synthesis technique, namely, the glycine-nitrate method, the details of which were previously described by us [2]. Nickel nitrate and hexaammonium heptamolybdate (Aldrich, USA) were dissolved in triply distilled water in the appropriate stoichiometric ratio. This solution was then mixed with an aqueous glycine solution that acted as a soft combustion agent (fuel). The stoichiometric
Phase analysis (XRD)
Fig. 2 presents the X-ray diffraction pattern of Ni2(MoO4)3 annealed at 600 °C in an oxygen atmosphere (90 ml/min). It is clear from the diffractogram that the peaks are sharp and refined and there is no difference found either in peak position or intensity between the two samples annealed for 4 h (sample A) and 7 h (sample B) indicating the formation of a good crystalline structure. Sample A was chosen for further investigation due to its lower annealing time which is preferred in order to
Conclusion
A new NASICON type framework structure Ni2(MoO4)3 was synthesized employing a low temperature technique and the new material was characterized by XRD, which reveals a single-phase structure, and SEM, which reveals sub-micrometer grains embedded by a soft aggregate morphology. The electrochemical reversibility was demonstrated by means of SSCV and galvanostatic cycle tests. The material offers a reversible capacity of 170 mAh/g between a potential window of 3.5 and 1.6 V during the first cycle.
Acknowledgements
The authors thank the Motorola Foundation, Malaysia for the partial financial support to carry out this work. The authors thank the referees for their critical review of this paper and their interesting comments, which helped to improve the discussion part of this paper.
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2012, Electrochimica Acta
Today polyanion containing LiFePO4 (olivine) is successfully commercialized. Over the years many such (XO4)n− polyanion containing compounds (X = As, Si, Mo, W) have been intensively studied for their reactivity versus lithium viz. olivine LiCoAsO4 [10], Li2FeSiO4 [11], LixNi2(MoO4)3 [12], LiFe(WO4)2 [13], etc. Recently a large amount of interest has been invoked by the metal titanium oxyphosphate family (MxTiOPO4, M = Ni, Fe, Mn, Co, Mg), as novel negative electrode materials [14,15].
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