Aqueous Rechargeable Lithium Battery Based on LiNi0.5

Article pubs.acs.org/EF

Aqueous Rechargeable Lithium Battery Based on LiNi0.5Mn1.5O4 Spinel with Promising Performance José Carlos Arrebola, Á lvaro Caballero, Lourdes Hernán,* and Julián Morales Instituto Universitario de Investigación en Química Fina y Nanoquímica, Departamento de Química Inorgánica, Campus de Rabanales, Universidad de Córdoba, 14071 Córdoba, Spain ABSTRACT: By virtue of their different electrode potentials, Li1−xMn2O4 (LMO) and LiNi0.5Mn1.5O4 (LNMO) can act as electrodes in a novel aqueous Li ion battery that exhibits outstanding capacity retention during prolonged cycling. Such an attractive property is related to the stability of both spinels (especially LNMO) under the typical working conditions of the cell. placed in a mill for 30 min and heated at 800 °C for 5 h. The experimental procedure is described elsewhere.26 X-ray diffraction (XRD) patterns were recorded on a Siemens D5000 X-ray diffractometer using non-monochromated Cu Kα radiation and a graphite monochromator for the diffracted beam. The scanning conditions were 5−90° (2θ), a 0.03° step size, and 12 s per step. Scanning electron microscopy (SEM) images were obtained with a Jeol6400 microscope. Electrochemical measurements were made with a three-electrode cell with a Pt wire as a counterelectrode and saturated calomel electrode (SCE) (supplied by CHI Instruments) as a reference electrode. The LNMO and LMO electrodes were prepared by mixing 80 wt % active material with 10 wt % Super P carbon black (Timcal) and 10 wt % polyvinylidene difluoride (PVDF) (Aldrich) with the help of ethanol. After drying, the mixture was pressed into Ni mesh with an active mass loading of 2 mg/cm2. A 3 M LiNO3 aqueous solution was used as an electrolyte. Cyclic voltammetry (CV) was conducted at a different scan rates from 0.1 to 4 mV s−1. The potential range was 0.2− 1.1 V for the cycling of LMO and 0.2−1.0 V for the LMO/LNMO cell, which are outside the region for water decomposition. The CV measurements were with a Solartron 1286 potentiostat−galvanostat. Electrochemical measurements in galvanostatic regime were carried out in a voltage range of 0.2−1.0 V, with a McPile (Biologic) potenciostat−galvanostat system.

1. INTRODUCTION The search for a greener, more sustainable chemistry has reached electrochemical storage energy devices (particularly electrochemical batteries). For example, lithium batteries are more environmentally benign than Pb−acid and Ni−Cd batteries. One drawback of Li ion batteries (LIBs), however, is that they use LiPF6 and organic solvent-based electrolytes. Replacing these chemicals with aqueous solutions would suppress their potential toxicity and make LIBs safer and cheaper. Furthermore, some aqueous rechargeable lithium batteries with higher energy density than those of lithium ion batteries have been reported.1 These attractive properties have boosted research into aqueous rechargeable lithium batteries (ARLBs) in the past few years.2−17 Obviously, this technology is subject to major shortcomings (especially, the reduced limit of water stability and its adverse impact on the amount of energy delivered by the battery). Using an increased number of serially connected batteries could be an effective solution. One other disadvantage is related to the nature of the electrode material. Since their inception,18 V-based oxides have been commonly used as anode materials despite being soluble under the typical operating conditions of the battery.19 A wide variety of cathodic materials, such as LiCoO2,20,21 LiMn2O4,6,22,23 and LiNi1/3Co1/3Mn1/3O2,24 has been tested with somewhat disappointing results, including sustained capacity fading during cycling. This unenthusiastic perception is changing in part because of the recent studies of Wu et al.11−17 Thus, the LiMn2O4 spinel prepared as nanotubes is able to retain 93% of its capacity after 10 000 cycles; even a second-level charge performance has been achieved.25 This paper reports a new aqueous LIB built from two wellknown spinels as electrode materials for organic solvent-based LIBs, namely, Li 1−x Mn 2 O 4 (LMO) and LiNi 0.5 Mn 1.5 O 4 (LNMO), which differ in band structure. The resulting battery can be cycled over the voltage range of 0.2−1.0 V, where it delivers a substantial specific capacity and exhibits excellent capacity retention over 100 cycles. 2. EXPERIMENTAL SECTION

3. RESULTS AND DISCUSSION The XRD patterns for the studied materials, LMO and LNMO, are shown in Figure 1a and correspond to pure spinel phases. The lattice parameters were found to be a = 8.178 for LNMO and a = 8.234 for LMO, both of which are highly consistent with reported values for these spinels.27,28 Figure 1b shows the scanning electron microphotographs for LNMO and LMO. Particles were polyhedral in shape, which is typical of a highly crystalline spinel. The high crystallinity observed at relatively low temperatures can be ascribed to the effect of the polymer used in the synthetic procedure.26 Particle size (around 1 μm) was very uniform, and the Brunauer−Emmett−Teller (BET) surface area, as determined from N2 adsorption measurements, was 3.7 and 4.9 m2 g−1 for LMO and LNMO, respectively. Figure 2a shows the CV curve for the LMO spinel, as recorded in an aqueous LiNO3 solution at a relatively high

The spinels were synthesized mixing Li(CH3−COO)·2H2O, Mn(CH3−COO)2·4H2O, Ni(CH3−COO)2·4H2O, and oxalic acid in appropriate proportions to obtain desired stoichiometry. Polyethylene glycol (PEG) was added to improve crystallinity. The precursor was

Received: April 25, 2013 Revised: November 27, 2013 Published: December 2, 2013

© 2013 American Chemical Society

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and associated to Li ion release from 4a and 4c positions (space group F23), similar to organic solvent-based electrolytes.27 The anodic and cathodic peaks were centered at ca. 0.77 and 0.92 V and at ca. 0.70 and 0.82 V versus SCE, respectively. The two peaks were well-resolved, which provides direct evidence of the ease of Li extraction and insertion, enhanced by the high particle crystallinity of the material. Figure 2b shows the charge and discharge curves for this electrode in different cycles. The two characteristic plateaus, equivalent to the peaks in the CV curves, were well-defined and with small polarization between charge and discharge, as previously observed in the CV curves. The amount of Li removed during the first charge was equivalent to a specific capacity of 120 mAh g−1, similar to a recently reported value but somewhat lower than the theoretical capacity (148 mAh g−1). The recovered capacity was 80 mAh g−1, which is lower than those found in organic solvents; however, this comparison is merely circumstantial because the two systems are not equivalent.29 Thus, the Li+ source in conventional half-cells is Li metal. Here, Li+ comes mainly from spinel deinsertion and the electrolyte. In fact, increasing the electrolyte concentration increases the delivered capacity.30 Capacity retention after the first cycle was acceptable. We chose the spinel charged in the first cycle for use as the anode against the LNMO spinel. A similar study of the LNMO spinel with Ni as the counterelectrode was unfeasible, owing to its high potential (above 1.23 V), which led to the oxidation of the water as the main reaction. This drawback was overcome by testing the electrochemical behavior of the spinel against LMO. This electrode for use as the anode material in the full battery was prepared under the above-described conditions to a cutoff voltage of 1.15 V versus SCE but using a lower scan rate (0.5 mV s−1) to ensure the release of a significant amount of Li+. The calculated value for x was about 0.82 mol of Li+. Figure 3a shows the CV curves of the cell made from LMO and LNMO spinels and SCE as the reference electrode, recorded at a scan rate of 0.5 mV s−1. The structural similarity of two electrodes resulted in enhanced Li+ diffusion. Moreover, these results testify to the outstanding reversibility of Li+ extraction and insertion between the two spinels. The curve profiles retained their shape and degree of polarization over a prolonged cycling. Figure 3b shows the CV curves for the same cell recorded at different rates (0.1, 0.5, 1, 2, and 4 mV s−1). The increase of the CV scan rate produces curves with larger areas, because the anodic or cathodic current is higher.4 As seen, at low rates, the cathodic and anodic curves exhibited two broad and overlapping peaks, consistent with the well-known two steps involved in the Li insertion and deinsertion process for these two spinels. At high rates, the high voltage peak is barely detected as a result of a moderate polarization of the electrodes and associated with the sluggish kinetics. Independent of the scan rate, the similarity between the shape of the cathodic and anodic curves provides direct evidence of the good reversibility of the electrochemical process. To have better knowledge of the cell performance in terms of capacity, electrochemical tests in galvanostatic regime were carried out. Figure 4a shows the variation of the delivered capacity as a function of the number of cycles. These values were obtained at four rates: 2C, 4C, 10C, and 20C (C representing 148 mA g−1). At the second cycle, once the cell was stabilized, the discharge capacity values were 72, 71, 68, and 65 mAh g−1 for 2C, 4C, 10C, and 20C, respectively. This means only a difference of around 10% between 2C and 20C.

Figure 1. (a) XRD patterns and (b) SEM images for the LMO and LNMO spinels.

Figure 2. (a) Cyclic voltammetric curves (scan rate of 1 mV s−1) and (b) charge/discharge galvanostatic curves for the LMO spinel at 1C rate.

polarization rate (1 mV s−1), which is comparable to 4C (C representing 1 Li+ ion exchanged in 1 h). As expected, the curve exhibited two pairs of redox peaks, which is typical of this spinel 7855

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consistent with that of the CV curves, particularly the difficulty to differentiate the two plateaus associated with the Li insertion/deinsertion process, clearly observed for LMO (see Figure 2b) and for LNMO versus Li metal.26 The good performance of the cell was a result of the stability of the spinel structure during cycling, especially the LNMO spinel and to a lesser extent the LMO spinel. Figure 5 shows

Figure 5. XRD patterns for the spinels after 100 cycles.

Figure 3. CV curves for the LMO/LNMO cell recorded at (a) 0.5 mV s−1 at different cycles and (b) different scan rates.

the XRD patterns for the two spinels after 100 cycles. All peaks assigned to the spinel structure were observed; therefore, both electrodes virtually maintained their structural integrity. No additional peaks were observed for the LNMO spinel, which provides direct evidence of its high stability in water. This good stability also proves its suitability as an electrode in aqueous secondary batteries. In contrast, the XRD pattern for the LMO spinel reveals that it underwent greater deterioration, probably as a result of the considerable amount of Li+ removed. The peaks assigned to the spinel structure became broader and less intense. Also, a new broad, weak peak was observed at around 10° (2θ), the origin of which is unclear but might be the presence of a birnessite-related structure. In any case, this is a minor phase, and most of the particles retained the spinel structure.

4. CONCLUSION An effective aqueous LIB can be built from the spinels LNMO and LMO if some Li+ is previously removed from LMO. The battery showed an outstanding behavior and a very good rate capability. It delivers initial capacity values from 74 mAh g−1 at 2C to 65 mAh g−1 at 20C, hardly 10 mAh g−1 of gap. The capacity retention ranged from 97 to 100% after 100 cycles. The structural stability of these spinels (particularly by LNMO) under the operating conditions accounts for this unexpected performance.

Figure 4. Electrochemical measurement under galvanostatic regime of the LMO/LNMO cell at different charge/discharge rates: 2C, 4C, 10C, and 20C. (a) Variation of the capacity as a function of the number of cycles and (b) profiles of charge/discharge.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +34957218620. Fax: +34957218621. E-mail: [email protected].

After 100 cycles, the capacity retention was outstanding, between 97% (for 2C) and 100% (for the three remaining rates). Figure 4b shows the charge/discharge curves of the LMO/LNMO cell for the second cycle. The shape was

Notes

The authors declare no competing financial interest. 7856

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ACKNOWLEDGMENTS This work was performed with the financial support of the Ministerio de Economiá y Competitividad (Projects MAT200803160 and MAT2011-27110) and Junta de Andaluciá (Group FQM-175).

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REFERENCES

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