Long-life lithium-sulfur batteries with high areal capacity based on coaxial CNTs@TiN-TiO2 sponge

Lithium-sulfur (Li-S) batteries are highly promising due to their high theoretical energy density (2600 Wh kg⁻¹). However, the 'shuttling effect' of lithium polysulfides (Li2Sx, 4 ≤ x ≤ 8) causes rapid capacity fading and poor cycle life, hindering practical applications. Various sulfur host materials, including porous nanocarbons and polar compounds, have been explored to physically and chemically block polysulfide shuttling. While these methods offer some improvement, the problem isn't completely solved, especially at high sulfur loadings. Recent studies suggest that 'dredging' (catalyzing the conversion) rather than simply 'blocking' polysulfides is a more effective strategy. An ideal catalyst needs high electrical conductivity, appropriate adsorption ability, and catalytic activity to accelerate the conversion of polysulfides to Li2S2/Li2S. While materials like TiO2 offer strong adsorption, their low conductivity is a limitation, and conversely, metal nitrides like TiN have good conductivity but weak polysulfide affinity. Heterostructures combining these materials have shown promise, but traditional fabrication methods are complex and difficult to optimize.

Previous research has demonstrated the effectiveness of heterostructures in enhancing Li-S battery performance. For instance, TiN-TiO2 interlayers have shown high capacity retention, and WS2-WO3 heterostructures have facilitated polysulfide conversion. However, these often involve multi-step synthesis, limiting control over component content and distribution, which are crucial for catalytic ability. This study addresses this limitation by employing ALD for precise control and optimization of the heterostructure.

The researchers fabricated a coaxial CNTs@TiN-TiO2 sponge using a three-step process: 1) Atomic Layer Deposition (ALD) of TiN onto a three-dimensional (3D) carbon nanotube (CNT) sponge, 2) ALD of a TiO2 layer on the TiN, and 3) annealing to promote uniform TiN-TiO2 heterostructure distribution. The 3D CNT sponge acts as an ideal substrate due to its high surface area and interconnected conductive pathways. Instead of loading solid sulfur, the authors infiltrated the sponge with a lithium polysulfide solution, allowing polysulfides to soak into the porous structure. This approach ensures uniform loading and leverages the polarity match between TiO2/TiN and polysulfides for efficient stabilization. The electrochemical performance was evaluated by assembling Li-S cells using the CNTs@TiN-TiO2 sponge as the sulfur host and characterizing various aspects of the batteries and materials. Different TiN and TiO2 layer thicknesses were tested to optimize the heterostructure, along with the effect of annealing. Characterizations included Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS). Lithium polysulfide adsorption was investigated visually and using XPS, and the catalytic ability was assessed using symmetric cells and Li2S precipitation tests. Finally, the Li-S battery's electrochemical performance was analyzed by cyclic voltammetry (CV), galvanostatic charge-discharge tests, and rate capability measurements.

Optimization of the TiN-TiO2 heterostructure was crucial. CNTs@TiN-10 (10 nm TiN) showed the best cycling stability. Adding TiO2 improved polysulfide adsorption, but excessive TiO2 (CNTs@TiN-TiO2-10) hindered performance due to reduced conductivity. The optimal structure, CNTs@TiN-TiO2-5 (10 nm TiN + 5 nm TiO2 after annealing), showed a synergistic effect: TiO2 adsorbed polysulfides, which then diffused to the catalytically active TiN. This resulted in enhanced electrochemical performance. CNTs@TiN-TiO2-5 exhibited the best Li-S battery performance: high specific capacity (1431 mAh g⁻¹ at 0.2C), excellent rate capability (800 mAh g⁻¹ at 5C), and high capacity retention (85% after 500 cycles at 2C). The high porosity of the 3D CNT sponge facilitated high sulfur loading (15 mg cm⁻²) and areal capacity (21.5 mAh cm⁻² at 0.2C), exceeding those of commercial lithium-ion batteries and comparable to state-of-the-art Li-S batteries with similar or higher sulfur loadings. XPS analysis confirmed chemical interactions between Li2S6 and the TiN-TiO2 heterostructure. Li2S precipitation tests showed that CNTs@TiN-TiO2-5 had the highest Li2S precipitation current and capacity, indicating superior catalytic activity. Electrochemical impedance spectroscopy showed that CNTs@TiN-TiO2-5 had the lowest charge transfer resistance, reflecting efficient charge transport.

The findings demonstrate that the rational design of the TiN-TiO2 heterostructure within a 3D conductive framework is highly effective in improving the electrochemical performance of Li-S batteries. The synergistic combination of TiO2's strong adsorption and TiN's catalytic activity, facilitated by the continuous interface of the heterostructure and the conductive CNT sponge, effectively addresses the polysulfide shuttling problem. The high areal capacity achieved is particularly significant, as it is crucial for practical applications. The superior performance compared to other Li-S battery systems highlights the advantages of the ALD fabrication method for creating precisely controlled heterostructures.

This research successfully developed a high-performance Li-S battery based on a coaxial CNTs@TiN-TiO2 sponge fabricated using ALD and annealing. Optimization of TiO2 thickness created a continuous interface facilitating efficient polysulfide adsorption, diffusion, and catalytic conversion. This resulted in enhanced rate performance, cycling stability, and high areal capacity. The method is promising for future development of high-energy-density batteries and other catalytic applications.

While the study demonstrates excellent performance, further investigation is needed to evaluate long-term stability over extended cycles under real-world operating conditions. Scalability and cost-effectiveness of the ALD fabrication method for large-scale production should also be considered. The study focused on a specific electrolyte; testing with alternative electrolytes may reveal further improvements or limitations.