Lithium metatitanate enhanced solid–solid reaction in a lithium–nitrogen–hydrogen system

Abstract

We have decreased the end temperature of the Li–N–H system, a hydrogen storage material developed in 2002, to below 260 °C, and obtained a lowest peak temperature of 223 °C.

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Lithium metatitanate (Li₂TiO₃) is a technologically important material with many practical applications. It has been used as a double-layer cathode material in molten carbonate fuel cells,¹ and as an electrode material in lithium-ion batteries.^2,3 In lithium-ion batteries, Li₂TiO₃ is usually combined with LiMO₂ (M = Fe, Mn, Cr, Ni) to create a solid-solution, with formulas xLiMO₂ − (1 − x)Li₂TiO₃. This can stabilize the structure of high-capacity cathode materials.^4–7 Li₂TiO₃ is a candidate for solid breeder materials in the blankets of fusion reactors,⁸ and is the preferred material for test blanket modules because of its attractive characteristics, such as high thermal stability, high lithium density, and tritium recovery at low temperatures, compared with lithium zirconate (Li₂ZrO₃) and lithium silicate (Li₂SiO₃).^9–11 Because of its many important applications, the mechanical and thermal properties of Li₂TiO₃ have been thoroughly investigated.

Recently, the diffusion pathways of lithium in the bulk, the occupancy of lithium in the lattice, and the effects of the defects have been investigated by Vijayakumar et al.¹² The lithium diffusion properties of Li₂TiO₃ have been measured by NMR and simulated by potential-based molecular dynamics. Lithium conduction in monoclinic Li₂TiO₃ is three-dimensional, which can provide suitable nearby vacancies. Li₂TiO₃ has been extensively investigated due to its many important applications. Herein, we use Li₂TiO₃ in another area as a catalyst because of its structural characteristics.

Titanium compounds have been doped as catalysts in hydrogen storage materials. Bogdanović and Schwickardi¹³ reported that NaAlH₄ can be used as a solid reversible hydrogen storage material after doping with Ti(Obutyl)₄, reawakening interest in using complex metal hydrides as hydrogen storage materials.¹⁴ One of the most frequently used catalysts is TiCl₃. The active catalytic species is produced during high-energy ball-milling,^14–17 and is highly dispersed within the materials. This is indicates that Ti is suitable for solid-state hydrogen storage materials. Since Chen et al.¹⁸ first reported Li–N–H systems, the reversibility and relatively high H₂ storage capacity of the systems have been investigated. Theoretically, a large amount (6.5 wt%, based on the reaction under experimental conditions, not a tank system) of hydrogen is accessible in this reaction. Hydrogen can be desorbed through the solid–solid reaction in eqn (1).

LiNH₂ + LiH ↔ Li₂NH + H₂ (1)

However, the system is still limited for practical applications because a high desorption temperature is required and the reaction suffers from slow kinetics. Furthermore, NH₃ is a subsidiary product in the dehydrogenation process according to eqn (2). The main reason is LiNH₂ decomposition at higher temperatures.

2LiNH₂ → Li₂NH + NH₃ (2)

Lithium diffusion plays an important role in this solid–solid reaction.^19–21 The kinetics of this reaction are strongly dependent on the crystal structure and its defects. The mobility of lithium ions can be enhanced as the temperature increases.²² In this study, we introduce the structural characteristics of Li₂TiO₃ to the lithium–nitrogen–hydrogen system, which is expected to improve the reaction kinetics. We present X-ray diffraction (XRD), thermogravimetry differential thermal analysis (TG-DTA), and thermal gas desorption mass spectrometry (TDMS) using Li₂TiO₃ as a catalyst in the Li–N–H system.

LiNH₂ (95%), LiH (95%), LiCO₃ (99.997%), and TiO₂ (99%) were purchased from Sigma-Aldrich. Single-phase Li₂TiO₃ was synthesized by the solid-state reaction in eqn (3).²³

Li₂CO₃ + TiO₂ → Li₂TiO₃ + CO₂ (3)

Typically, to ensure homogeneous mixing between the starting materials and the additive, LiNH₂ and LiH powders (300 mg, 1 : 1.1 molar ratio) and Li₂TiO₃ (1 mol%) were ball milled for 2 h (Fritsch P7). Different molar ratios of Li₂TiO₃ (1, 2 and 5 mol%) were doped into the mixtures. During high-energy ball milling, samples were milled at 400 rpm for 2 h under hydrogen gas (99.9999% purity) at a pressure of 1 MPa at room temperature. The milling was interrupted every 30 min for 15 min to prevent frictional heat during the milling process.

The samples were measured by TG-DTA (Bruker TAPS3000S) combined with TDMS upon heating to 400 °C at a rate of 5 °C min⁻¹. TG-DTA equipment was installed in a glovebox to avoid exposing the sample to air during the measurements. Furthermore, the structural properties were characterized by XRD.

Li₂TiO₃ was synthesized by sintering a mixture of Li₂CO₃ and TiO₂ at 700 °C. The powder XRD profile is shown in Fig. 1. The XRD pattern showed no apparent impurities in single-phase Li₂TiO₃. Single-phase Li₂TiO₃ was off-white, which is consistent with previous results.²⁴ The stability of Li₂TiO₃ is also reflected by the XRD patterns in Fig. 1. After high-energy ball milling and dehydrogenation, the structure of Li₂TiO₃ was the same as that of the single phase of raw Li₂TiO₃. This indicates that the Li₂TiO₃ in the cycled sample remains unchanged.

Fig. 1 XRD patterns for the single phase of Li₂TiO₃ and LiNH₂ + LiH with 1 mol% Li₂TiO₃, ball-milled for 2 h after TG measurements up to 400 °C.

To clarify the thermal desorption properties of the mixture with additives, the desorption gas and weight loss were determined by TG-DTA-TDMS. The TDMS result from the sample with catalyst is shown in Fig. 2. The LiH + LiNH₂ composite with catalyst showed only a sharp H₂ peak. The peak temperature was at 223 °C. No NH₃ was detected by TDMS. Compared with the sample with no catalyst, which released H₂ and NH₃ at higher temperatures,²⁰ this clearly demonstrates that doping Li₂TiO₃ had an effect on the whole system. The results of the sample with no catalyst are shown in ref. 20. Furthermore, the doping of Li₂TiO₃ led to full desorption within the temperature range of 150 to 260 °C. The DTA peak temperature of the Li₂TiO₃-doped sample is lower (223 °C) than other common catalysts, such as TiCl₃, LiTi₂O₄ (Fig. 3), Ti^Nano, TiO₂,²⁵ BN and TiN.²⁶ Accordingly, the lower dehydrogenation temperature showed that the properties of desorption can be improved greatly by doping with Li₂TiO₃. The temperature of reaction range is also shorter (150–260 °C) than that of TiCl₃ (150–300 °C) and LiTi₂O₄ (150–310 °C). Based on these results, Li₂TiO₃ has the highest catalytic efficiency for the Li–N–H system, and this is the first system to have an end temperature below 260 °C.¹⁸

Fig. 2 TDMS and corresponding TG results for the LiNH₂ + LiH mixture ball milled for 2 h with 1 mol% Li₂TiO₃ during dehydrogenation.

Fig. 3 DTA results for the dehydrogenation of samples (left) LiNH₂ + LiH + 1 mol% Li₂TiO₃; LiNH₂ + LiH + 1 mol% TiCl₃; LiNH₂ + LiH + 0.5 mol% LiTi₂O₄. (Right) Details of the peak temperature.

Different amounts of Li₂TiO₃ were doped into the mixtures to identify the optimum amount of Li₂TiO₃ (Fig. 4). According to the TG-DTA data, the peak temperature for H₂ desorption of the 1, 2, and 5 mol% Li₂TiO₃-doped samples was 223 °C. The DTA curves of the samples were similar. The weight losses for the 1, 2 and 5 mol% Li₂TiO₃-doped samples were 5.8, 5.1 and 4.4 wt%, respectively. These values were based on the reaction of materials during experimental conditions. Minor contamination with the starting materials, such as Li₂O, could explain why the experimental hydrogen storage capacity is lower than the theoretical value. These results indicate that there is no simple linear correlation between the catalytic effect and the amount of catalyst. The optimum amount of Li₂TiO₃ is 1 mol% for achieving maximum storage capacity.

Fig. 4 TG and DTA results for mixtures milled for 2 h with different amounts of Li₂TiO₃.

To clarify the catalytic effect on hydrogenation, the reversibility, recyclability, and the hydrogenation rate were investigated. The effect of the addition of Li₂TiO₃ on hydrogen absorption was examined in a series of experiments. The dehydrogenated samples were hydrogenated at 200 °C and a hydrogen pressure of 1 MPa for 10, 100, 200, and 500 min. Fig. 5 shows the corresponding hydrogen uptake for samples. The hydrogenation rate of the samples with and without Li₂TiO₃ is shown in Table 1. The results of the sample without the catalyst are shown in ref. 27. The hydrogenation rate of the sample with Li₂TiO₃ addition is 4.8 times higher than that of the reference sample for 10 min, 2.5 times for 100 min, and 2.0 times for 200 min.

Fig. 5 Extent of rehydrogenation reaction for samples with Li₂TiO₃ at 200 °C, 1 MPa H₂ for 10, 100, 200, and 500 min.

Table 1 Hydrogenation rate, demonstrating the catalytic effect of hydrogenation

Temperature and pressure		200 °C, 1 MPa H2
Hydrogenation time (min)		10	100	200
Hydrogenation rate (%)	W/o catalyst	9	23	34
	W/ catalyst	44	59	70

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Furthermore, the recyclability and durability of the catalytic effect of Li₂TiO₃ was examined by a 10-cycle test (Fig. 6). The weight loss (5.8%) and peak temperature (223 °C) are the same as the first cycle. These results show that the catalyst affects the dehydrogenation and hydrogenation.

Fig. 6 Weight loss of LiNH₂ + LiH + 1 mol% Li₂TiO₃ mixture after 10 cycles of dehydrogenation/hydrogenation (solid line) and differential thermal analysis (dashed line).

Based on these results, Li₂TiO₃ effectively catalyzes the solid–solid reaction between LiH and LiNH₂ upon dehydrogenation and hydrogenation. The catalytic mechanism of Li₂TiO₃ in the Li–N–H system was investigated. During the temperature increase in dehydrogenation, the thermal vibration of the ions and the density of the vacancies increased. This would also increase the lithium-ion mobility. It has been proposed that the solid–solid reaction between LiH and LiNH₂ occurs via Li⁺ migration across the reactive interfaces between the LiH particles and the LiNH₂ particles during high temperature dehydrogenation.^19,20,28 In this study, Li₂TiO₃ was doped into the Li–N–H system. According to the structural characteristics of Li₂TiO₃ (Fig. 7), nonstoichiometric Li₂TiO₃ can cause lithium vacancies in both the LiTi₂ layers and pure Li, and thus lithium diffusion should happen in the xy-plane and along the z-axis. Lithium mobility in monoclinic Li₂TiO₃ is three-dimensional which could provide suitable nearby vacancies during the temperature increase in dehydrogenation.¹² Doping LiNH₂ and LiH with Li₂TiO₃ creates vacancies that help destabilize the lithium ions in the crystal-lattice of LiNH₂ and LiH, which could increase the mobility of lithium ions and make the reaction happen at a lower temperature. Furthermore, the improvement in the kinetics could consume LiNH₂, and restrict the emission of ammonia at high temperatures.

Fig. 7 Crystal structure of monoclinic LiTi₂O₃ (as reported in ref. 29). Lithium atoms are shown in green, oxygen atoms in red, and titanium atoms in yellow.

Conclusions

In summary, we have reported the discovery of Li₂TiO₃ as a catalyst in a solid–solid reaction for hydrogen storage materials. The catalytic effect, appropriate amount and stability of Li₂TiO₃ in the system were confirmed by TG-DTA, TDMS and XRD. This is the first time that the end temperature of the Li–N–H system has been decreased to under 260 °C and the lowest peak temperature occurred at 223 °C. A storage capacity of 5.8 wt% H₂ was obtained during dehydrogenation. The catalytic effect on the hydrogenation was also investigated. The catalytic effect of Li₂TiO₃ probably results from an increase in the mobility of the lithium ions between the LiNH₂ and LiH solid phases. Our results are expected to help achieve deeper insight into solid–solid reactions catalyzed by metal oxide nanoparticles for hydrogen storage.

Notes and references

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