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Ion Exchange of Layer-Structured Titanate CsxTi2-x/2Mgx/2O4 (x = 0.70) and Applications as Cathode Materials for Both Lithium- and Sodium-Ion Batteries

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DOI: 10.4236/msa.2019.102012    121 Downloads   190 Views  
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ABSTRACT

Cathode materials for rechargeable batteries have been extensively investigated. Sodium-ion batteries are emerging as alternatives to lithium-ion batteries. In this study, a novel cathode material for both lithium- and sodium-ion batteries has been derived from a layered crystal. Layer-structured titanate CsxTi2-x/2Mgx/2O4 (x = 0.70) with lepidocrocite (γ-FeOOH)-type structure has been prepared in a solid-state reaction from Cs2CO3, anatase-type TiO2, and MgO at 800°C. Ion-exchange reactions of Cs+ in the interlayer space were studied in aqueous solutions. The single phases of Li+, Na+, and H+ exchange products were obtained, and these were found to contain interlayer water. The interlayer water in the lithium ion-exchange product was removed by heating at 180°C in vacuum. The resulting titanate Li0.53H0.13Cs0.14Ti1.65Mg0.30O4 was evaluated for use as cathodes in both rechargeable lithium and sodium batteries. The Li+ intercalation-deintercalation capacities were found to be 151 mAh/g and 114 mAh/g, respectively, for the first cycle in the voltage range 1.0 - 3.5 V. The amounts of Li+ corresponded to 0.98 and 0.74 of the formula unit, respectively. The Na+ intercalation-deintercalation capacities were 91 mAh/g and 77 mAh/g, respectively, for the first cycle in the voltage range 0.70 - 3.5 V. The amounts of Na+ corresponded to 0.59 and 0.50 of the formula unit, respectively. The new cathode material derived from the layer-structured titanate is non-toxic, inexpensive, and environmentally benign.

1. Introduction

We have studied the characterizations of layer-structured titanates with lepidocrocite (γ-FeOOH)-type structure [1] - [7] . In a previous study [6] , we showed that the Li+ exchange product of Li0.60H0.04Cs0.06Ti1.30Fe0.70O4, derived by the ion-exchange reaction from CsxTi2−xFexO4 (x = 0.70) with lepidocrocite-type structure, exhibited discharge and charge capacities of 110 and 92 mAh/g, respectively, for the first cycle in a rechargeable lithium battery in the voltage range 1.5 - 4.2 V. The discharge-charge capacity almost corresponds to a redox reaction of Fe3+/Fe2+ in the titanate. However, the discharge-charge curves showed that there is a small amount of rechargeable capacity corresponding to a Ti4+/Ti3+ redox couple. Recently, we reported that the Li+ exchange product of Li2Ti5O11, derived by the ion-exchange reaction from layer-structured titanate Cs2Ti5O11, exhibited discharge-charge capacities of 120 and 100 mAh, respectively, for the first cycle in a rechargeable sodium battery in the voltage range 0.70 - 4.0 V [8] . These discharge-charge capacities obviously correspond to a Ti4+/Ti3+ redox couple in the layer-structured titanate. In the present study, we showed that the Ti4+/Ti3+ redox couple in the lepidocrocite-type layer structure exhibits considerable discharge-charge capacities by the electrochemical intercalation-deintercalation of both Li+ and Na+.

The crystal structure of CsxTi2−x/2Mgx/2O4 (x = 0.70) is drawn in Figure 1 using the atomic parameters reported by Reid et al. [9] . Each stacking layer consists of a corrugated layer of titanium-oxygen. A portion of the Ti4+ ions (x/2 = 0.35 for formula unit) in the octahedral position (2 for formula unit) is substituted with lower-valent Mg2+ ions. The charge balance is maintained by eight-coordinated interlayer Cs+ ions from oxygen atoms in the layers. The partial occupancy of x = 0.70 by Cs+ in the interlayer positions is attributed to the overcrowding of Cs+ with the large ionic radius [9] .

Figure 1. Structure of CsxTi2−xMgxO4 (x = 0.70) as seen from the direction of the a-axis.

2. Experimental

All chemicals used were High Special Grade (Wako Chemical Industries, Ltd., Japan) and were used without further purification. The layer-structured titanate CsxTi2−x/2Mgx/2O4 (x = 0.70) with lepidocrocite-type structure has been prepared in a solid-state reaction using Cs2CO3, anatase-type TiO2, and MgO at 800˚C, according to a similar method reported by Reid et al. [9] . The mixture with the desired ratio was heated at 800˚C for 20 h, and the resulting powder was ground and heated again at 800˚C for 20 h. Li+ and Na+ exchange were performed using 1.0-mol/L LiNO3 and NaNO3 solutions for 9 d at 60˚C. The solutions were changed every 3 d. The H+ exchange was carried out using 0.05-mol/L H2SO4 solution for 3 d at room temperature between 15˚C - 25˚C, changing the solution every day.

Powder X-ray diffraction (XRD) patterns were collected on a Rigaku Ultima IV diffractometer over a 2θ range 10˚ - 60˚ using graphite-monochromatized Cu-Kα radiation (λ = 0.15405 nm). The contents of Cs, Li and Na in the samples were determined by the atomic absorption method after dissolving the samples in a mixed-acid solution with H2SO4 and HF. The Mg content was determined by gravimetric technique using cupferron (C6H9N3O2) for the chelating agent. Dehydration processes were studied by TG-DTA at a heating rate of 10˚C/min. A cathode was formed of a mixture of the titanate powder (80 wt%), acetylene black (10 wt%), and PTFE binder (10 wt%), pressed into a stainless-steel grid under a pressure of 100 MPa. The electrolyte of the lithium battery was a 1.0-mol/L LiPF6 solution of 1:1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DME). The electrolyte of the sodium battery was a 1.0-mol/L NaClO4 solution of propylene carbonate (PC). The lithium battery was first discharge and cycled between 1.0 V and 3.5 V at 0.10 mA/cm2 in an Ar-filled glove box at room temperature between 15˚C - 25˚C. The sodium cell was also first discharge and cycled between 0.70 V and 3.5 V at 0.10 mA/cm2.

3. Results and Discussion

3.1. Crystal Structure

The XRD pattern of CsxTi2−x/2Mgx/2O4 (x = 0.70) (Figure 2(a)) was indexed on the basis of an orthorhombic cell of a = 0.3824 (2) nm, b = 1.704 (3) nm, and c = 0.2929 (1) nm (Table 1). The lattice constants of the sample are in good agreement with those prepared by Reid et al. (a = 0.3821 nm, b = 1.7040 nm and c = 0.2981 nm) [9] .

3.2. Ion Exchange

The XRD pattern of the Li+ exchange product is shown in Figure 2(b). The pattern was indexed as a single phase with orthorhombic lattice constants of a = 0.378 nm, b = 1.72 nm, and c = 0.292 nm (Table 1). The lattice constants of a and c were almost unchanged. This shows that the host layer of CsxTi2−x/2Mgx/2O4 (x = 0.70) is maintained through the Li+ exchange. The interlayer spacing which

Figure 2. XRD patterns of (a) Cs0.70Ti1.65Mg0.35O4; (b) Li0.53H0.13Cs0.14Ti1.65Mg0.30O4∙0.92H2O; (c) Na0.56H0.14Ti1.65Mg0.35O4∙1.1H2O; (d) H0.99Cs0.07Ti1.65Mg0.17O4∙1.2H2O; (e) Li0.53H0.13Cs0.14Ti1.65Mg0.30O4 (x: unknown peaks).

Table 1. Compositions and orthorhombic lattice constants of the products.

corresponds to b/2, increased from 0.852 nm to 0.860 nm. The TGA curve of the product (Figure 3(a)) shows a weight loss from 20˚C to 200˚C corresponding to the dehydration of interlayer water. The composition was estimated to be Li0.53H0.13Cs0.14Ti1.65Mg0.30O4∙0.92H2O by chemical analysis and weight loss. It was found that 14% of the Mg in the titanate was leached out in solution during the ion exchange. England et al. [10] also studied the Li+ exchange product and estimated the composition to be Li0.33Cs0.37Ti1.65Mg0.35O4∙0.72H2O by the amount of Cs released into solution, determined by photometric analyses and weight loss from TG analysis. They did not analyze the content of Mg in their Li+-exchanged product.

The Li+-exchange product was heated at 180˚C for 1 h in a vacuum (Figure 2(e)). The XRD pattern was indexed as a single phase with orthorhombic lattice constants of a = 0.371 nm, b = 0.662 nm, and c = 0.300 nm (Table 1). In this case, the lattice constant of b corresponds to the interlayer spacing. The interlayer spacing decreased from 0.860 nm to 0.662 nm because of dehydration of the interlayer water. The dehydrated product of Li0.53H0.13Cs0.14Ti1.65Mg0.30O4 was evaluated for its uses as cathodes in both lithium and sodium batteries.

The XRD pattern of the Na+ exchange product showed that the product was a mixture of two phases with the interlayer spacing of d = 1.14 nm and d = 0.89 nm. This product was heated at 40˚C for 1 h. Figure 2(c) shows the XRD pattern of the heated product. The pattern was indexed as a single phase with the orthorhombic lattice constants (Table 1) where the 0.110-nm phase disappeared. These constants show that the host layer of CsxTi2−x/2Mgx/2O4 (x = 0.70) is

Figure 3. TGA curves of (a) Li0.53H0.13Cs0.14Ti1.65Mg0.30O4∙0.92H2O; (b) Na0.56H0.14Ti1.65Mg0.35O4∙1.1H2O; (c) H0.99Cs0.07Ti1.65Mg0.17O4∙1.2H2O.

also maintained through the Na+ exchange. The TGA curve (Figure 3(b)) shows two steps of weight loss: 20˚C - 100˚C and 100˚C - 200˚C. Both steps correspond to the dehydration of the interlayer water. The composition was estimated to be Na0.56H0.14Ti1.65Mg0.35O4∙1.1H2O (Table 1). England et al. [10] also studied the Na+ exchange product and determined the composition to be Na0.70Ti1.65Mg0.35O4・0.70H2O.

The XRD pattern of the H+ exchange product is shown in Figure 2(d). The pattern was indexed as a single phase with the orthorhombic lattice constants of a = 0.378 nm, b = 1.77 nm, and c = 0.298 nm (Table 1). This also shows that the host layer of CsxTi2−x/2Mgx/2O4 (x = 0.70) is maintained through the H+ exchange. The TGA curve (Figure 3(c)) shows two steps of weight loss: 20˚C - 150˚C and 150˚C - 450˚C. The former weight loss corresponds to the dehydration of the interlayer water, and the latter corresponds to dehydration of the decomposition due to the combination of the exchanged H+ with the O2− of the host layer. The composition was estimated to be H0.99Cs0.07Ti1.65Mg0.17O4∙1.2 H2O (Table 1). It was found that 51% of Mg in the titanate was leached out in solution during the ion exchange. England et al. [10] studied the H+ exchange product and estimated the composition to be H0.65Cs0.05Ti1.65Mg0.35O4・0.7H2O.

3.3. Lithium Battery

Figure 4 shows the discharge-charge curves of the Li/ Li0.53H0.13Cs0.14Ti1.65Mg0.30O4 cell. The cell voltage decreased rapidly from the rest potential of 3.1 V to 2.0 V and then decreased slowly to the cutoff voltage of 1.0 V. The discharge capacity was 151 mAh/g for the first cycle. The amount of Li+ intercalated in this process was 0.98 for the formula unit. The discharge potentials of Ti4+/Ti3+ in a Li4/3Ti5/3O4 spinel oxide is reported to be 1.55 V, with the insertion of Li+ in the three-dimensional spinel framework [11] . The Li/Li0.53H0.13Cs0.14Ti1.65Mg0.30O4 cell exhibited almost the same voltage as shown in the figure, so we can conclude that the discharge process corresponds to the intercalation of Li+ into the vacant space of the interlayer and the reduction of Ti4+ to Ti3+ in the lepidocrocite structure.

Figure 4. Discharge-charge curves of Li/Li0.53H0.13Cs0.14Ti1.65Mg0.30O4 cell with 0.10 mA/cm2.

The first discharge and charge capacities were 151 mAh/g and 114 mAh/g, respectively. The amounts of Li+ intercalated and deintercalated were 0.98 and 0.74 of the formula unit, respectively. At the 10th cycle, the cell exhibited 73% (110 mAh/g) of the first discharge capacity and 83% (95 mAh/g) of the first charge capacity. At the 20th cycle, the cell exhibited 70% (105 mAh/g) of the first discharge capacity and 82 % (93 mAh/g) of the first charge capacity.

3.4. Sodium Battery

Figure 5 shows the discharge-charge curves of Na/Li0.53H0.13Cs0.14Ti1.65Mg0.30O4 cell. The cell voltage decreased rapidly from the rest potential of 2.7 V to 1.8 V and then decreased slowly to the cutoff voltage of 0.70 V. The discharge capacity was 91 mAh/g for the first cycle. The amount of Na+ intercalated in this process was 0.59 for the formula unit. Recently, we reported that Li2Ti5O11 derived by ion-exchange reaction from the layer-structured titanate Cs2Ti5O11 exhibited discharge-charge capacities of 120 and 100 mAh, respectively, for the first cycle in a rechargeable sodium battery in the voltage range 0.70 - 4.0 V [8] . The discharge potential of Ti4+/Ti3+ in the layer-structured Li2Ti5O11 was approximately 1.2 V with the insertion of Na+. This shows that the discharge process of the Na/Li0.53H0.13Cs0.14Ti1.65Mg0.30O4 cell corresponds to the intercalation of Na+ and the reduction of Ti4+ to Ti3+ in the lepidocrocite structure.

The first discharge and charge capacities were 91 mAh/g and 77 mAh/g, respectively. The amounts of Na+ intercalated and deintercalated were 0.59 and 0.50 of the formula unit, respectively. At the 10th cycle, the cell exhibited 64% (58 mAh/g) of the first discharge capacity and 73% (56 mAh/g) of the first charge capacity. At the 20th cycle, the cell exhibited 38% (35 mAh/g) of the first discharge capacity and 44% (34 mAh/g) of the first charge capacity.

Figure 5. Discharge-charge curves of Na/Li0.53H0.13Cs0.14Ti1.65Mg0.30O4 cell with 0.10 mA/cm2.

The lower discharge-charge capacity of the sodium battery compared with that of the lithium battery may be attributed to the difference in ionic volume of Na+ and Li+. The larger volume of Na+ as compared with Li+ has a disadvantage in the intercalation into the vacant space of the interlayer.

4. Conclusion

In this study, we showed for the first time that layer-structured titanate Li0.33Cs0.37Ti1.65Mg0.35O4 derived from CsxTi2−x/2Mgx/2O4 (x = 0.70) with lepidcrocite-type structure by ion exchange can be a promising candidate for the cathode materials of both sodium and lithium ion batteries. The titanate is non-toxic, inexpensive, and environmentally benign.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

Cite this paper

Ohashi, M. (2019) Ion Exchange of Layer-Structured Titanate CsxTi2-x/2Mgx/2O4 (x = 0.70) and Applications as Cathode Materials for Both Lithium- and Sodium-Ion Batteries. Materials Sciences and Applications, 10, 150-157. doi: 10.4236/msa.2019.102012.

References

[1] Ohashi, M. (1998) Preparation and Lithium Intercalation of Layer Structured Titanate CsxTi2-x/4O4 (x = 0.68). Molecular Crystals and Liquid Crystals, 311, 51-56.
https://doi.org/10.1080/10587259808042365
[2] Ohashi, M. (2000) Ion Exchange of Layer Structured Titanate CsxTi2-x/4O4 (x = 0.68) and Ionic Conductivity of the Products. Molecular Crystals and Liquid Crystals, 341, 265-270.
https://doi.org/10.1080/10587250008026151
[3] Ohashi, M. (2002) Preparation of Layer Structured Crystal CsxTi2-xMnxO4 (x = 0.70) and Application to Cathode for Rechargeable Lithium Battery. Key Engineering Materials, 216, 119-122.
https://doi.org/10.4028/www.scientific.net/KEM.216.119
[4] Ohashi, M. (2002) Preparation of Layer-Structured Crystal KxTi2-xMnxO4 (x = 0.75) and Application as Cathode Material in Rechargeable Lithium Battery. Key Engineering Materials, 228-229, 289-292.
https://doi.org/10.4028/www.scientific.net/KEM.228-229.289
[5] Ohashi, M. (2004) Preparation and Ion Exchange of Layer Structured Cesium Chromium Titanate CsxTi2-xCrxO4 (x = 0.70). Journal of the Ceramic Society of Japan, Supplement, 112-1, S114-S116.
[6] Ohashi, M. (2004) Preparation of Layer Structured Titanate CsxTi2-xFexO4 (x = 0.70) and Application as Cathode Material in Rechargeable Lithium Battery. Solid State Ionics, 172, 31-32.
https://doi.org/10.1016/j.ssi.2004.01.035
[7] Ohashi, M. (2009) Ion Exchange of Layer Structured Crystal KxTi2-xFexO4 (x = 0.70) and Its Application as Cathode Material in a Rechargeable Lithium Battery. Key Engineering Materials, 388, 97-100.
https://doi.org/10.4028/www.scientific.net/KEM.388.97
[8] Ohashi, M. (2018) Novel Cathode Materials for Sodium Ion Batteries Derived from Layer Structured Titanate Cs2Ti5O11·(1 + x)H2O. Materials Sciences and Applications, 9, 526-533
https://doi.org/10.4236/msa.2018.96037.
[9] Reid, A.F., Mumme, W.G. and Wadsley, A.D. (1968) A New Class of Compound M+xA3+xTi2-xO4 (0.60 < x < 0.80) Typified by RbxMnxTi2-xO4. Acta Crystallographica, B24, 1228-1233.
https://doi.org/10.1107/S0567740868004024
[10] England, W.A., Birkett, J.E., Goodenough, J.B. and Wiseman, P.J. (1983) Ion Exchange in the Csx[Ti2-x/2Mgx/2]O4 Structure. Journal of Solid State Chemistry, 49, 300-308.
https://doi.org/10.1016/S0022-4596(83)80007-3
[11] Ohzuku, T., Ueda, A. and Yamamoto, N. (1995) Zero-Strain Insertion Material of Li[Li1/3Ti5/3]O4 for Rechargeable Lithium Cells. Journal of the Electrochemical Society, 142, 1431-1435.
https://doi.org/10.1149/1.2048592

  
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