Ion Exchange of Layer-Structured Titanate CsxTi 2 − x / 2 Mg x / 2 O 4 ( x = 0 . 70 ) and Applications as Cathode Materials for Both Lithium-and Sodium-Ion Batteries

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 lithiumand 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.


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 Li 0.60 H 0.04 Cs 0.06 Ti 1.30 Fe 0.70 O 4 , derived by the ion-exchange reaction from Cs x Ti 2−x Fe x O 4 (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 Fe 3+ /Fe 2+ in the titanate. However, the discharge-charge curves showed that there is a small amount of rechargeable capacity corresponding to a Ti 4+ /Ti 3+ redox couple. Recently, we reported that the Li + exchange product of Li 2 Ti 5 O 11 , derived by the ion-exchange reaction from layer-structured titanate Cs 2 Ti 5 O 11 , 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 Ti 4+ /Ti 3+ redox couple in the layer-structured titanate. In the present study, we showed that the Ti 4+ /Ti 3+ 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 Cs x Ti 2−x/2 Mg x/2 O 4 (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 Ti 4+ ions (x/2 = 0.35 for formula unit) in the octahedral position (2 for formula unit) is substituted with lower-valent Mg 2+ 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].

Experimental
All chemicals used were High Special Grade (Wako Chemical Industries, Ltd., Japan) and were used without further purification. The layer-structured titanate Cs x Ti 2−x/2 Mg x/2 O 4 (x = 0.70) with lepidocrocite-type structure has been prepared in a solid-state reaction using Cs 2 CO 3 , anatase-type TiO 2 , 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 LiNO 3 and NaNO 3 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 H 2 SO 4 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 H 2 SO 4 and HF. The Mg content was determined by gravimetric technique using cupferron (C 6 H 9 N 3 O 2 ) 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 LiPF 6 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 NaClO 4 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/cm 2 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/cm 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 ( corresponds to b/2, increased from 0.852 nm to 0.860 nm. The TGA curve of the product (Figure 3 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)    The XRD pattern of the H + exchange product is shown in Figure 2 Figure 5 shows the discharge-charge curves of Na/Li 0.53 H 0.13 Cs 0.14 Ti  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.

Conclusion
In this study, we showed for the first time that layer-structured titanate

Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.