High-power lithium-selenium batteries enabled by atomic cobalt electrocatalyst in hollow carbon cathode

Rechargeable lithium-ion batteries (LIBs) are crucial for sustainable energy storage, but their energy density needs improvement for applications like electric vehicles. Lithium-sulfur (Li-S) batteries offer high theoretical energy density, but suffer from low sulfur conductivity and polysulfide shuttle effects. Selenium (Se), in the same group as sulfur, presents an alternative with higher conductivity and theoretical volumetric capacity (3253 mAh cm⁻³). However, Se cathodes also face dissolution issues with lithium selenides and volume expansion during charge/discharge, resulting in low utilization, capacity, and cycle life. Previous strategies to improve Se cathode performance involved incorporating Se particles with conductive materials and encapsulating them in porous carbon matrices. While these composites reduced charge transfer resistance and suppressed polysulfide shuttle effects, high-power Li-Se batteries with long cycling performance remain a challenge. Single-atom catalysts (SACs), characterized by maximum atom utilization, homogeneous active centers, and unique reaction mechanisms, have shown promise in various applications, including metal-air and metal-sulfur batteries. However, their application in Li-Se batteries to overcome the inherent limitations of selenium cathodes remained unexplored. This study aims to demonstrate, for the first time, that SACs can enable highly effective Li-Se cathodes with superior rate capability and long-term cycling performance by using a cobalt single atom electrocatalyst anchored to a nitrogen-doped hollow porous carbon matrix.

Extensive research has explored strategies to enhance the electrochemical performance of selenium cathodes. A common approach involves embedding Se particles within a conductive porous carbon matrix to improve electronic conductivity and suppress polysulfide shuttle effects. Various carbon materials have been investigated, including nanospheres, nanofibers, hierarchical porous carbon, and hollow carbon structures. Despite these efforts, high-power Li-Se batteries with extended cycle life under high currents have not been reported, highlighting the need for novel approaches to address the remaining challenges of selenium dissolution and volume expansion during cycling. The recent emergence of single-atom catalysts (SACs) provides a unique opportunity to overcome these challenges. SACs offer high atom-utilization efficiency, homogeneous active sites, and tunable electronic properties that can potentially activate Se reactivity and immobilize polyselenides during cycling. While SACs have been successfully applied in other battery systems, their utilization in Li-Se batteries has not been reported before this study. This work seeks to bridge this gap and investigate the potential of SACs for enhancing the performance of Li-Se batteries.

The researchers developed a facile method to synthesize cobalt single atoms/nitrogen-doped hollow porous carbon (CoSA-HC). This involved a core-shell ZIF hybrid structure, created by depositing Zeolitic Imidazolate Framework (ZIF) particles onto polystyrene (PS) spheres. Pyrolysis with zinc evaporation transformed the structure into hollow carbon materials. By tuning the Zn/Co ratio, they produced three samples: CoSA-HC (atomic cobalt), HC (nitrogen-doped hollow carbon), and CoNP-HC (cobalt nanoparticles). Selenium was embedded into the hollow carbon structures to create Se@CoSA-HC, Se@HC, and Se@CoNP-HC composites. Characterizations employed techniques such as FESEM, TEM, HAADF-STEM, EDS, XRD, XPS, BET, Raman spectroscopy, TGA, and XAFS to analyze morphology, composition, structure, and elemental distribution. Electrochemical performance was evaluated using CR2032 coin cells with 1 wt% LiNO₃ in 1,3-dioxolane/1,2-dimethoxyethane electrolyte. Galvanostatic charge-discharge cycling, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) were conducted to assess capacity, rate capability, and cycling stability. First-principles calculations using DFT were used to investigate the reaction kinetics and energy barriers of lithium polyselenide reduction on both Co-NC and NC supports. Visual observations were conducted on the electrolyte of the H-type cell using cycled Se@COSA-HC and bare Se@HC cathodes to assess the suppression of the formation of lithium polyselenides.

The synthesized Se@CoSA-HC cathode exhibited significantly enhanced electrochemical performance compared to Se@HC and Se@CoNP-HC. Specifically, Se@CoSA-HC showed a high discharge capacity of 564 mAh g⁻¹ after 100 cycles at 0.1 C, and superior rate capabilities of 385 mAh g⁻¹ at 20 C and 311 mAh g⁻¹ at 50 C. Long-term cycling tests demonstrated excellent stability: 457 mAh g⁻¹ at 0.5 C after 1700 cycles with only 0.011% capacity decay per cycle, and 267 mAh g⁻¹ at 50 C after 5000 cycles with 0.0067% capacity decay per cycle and nearly 100% Coulombic efficiency. HAADF-STEM images confirmed the atomic dispersion of cobalt within the CoSA-HC structure. XPS analysis showed the formation of C-Se bonds after the first charge-discharge cycle, indicating strong interactions between Se and the carbon matrix. Kinetic analysis revealed that 85% of the capacity was contributed by a capacitive process at 0.5 mV s⁻¹, indicating fast lithium-ion transport. DFT calculations showed that the presence of single Co atoms lowered the energy barriers for the reduction of lithium polyselenides and the transformation of Li₂Se, thus accelerating the reaction kinetics. Visual observation experiments confirmed that the CoSA-HC cathode effectively suppressed the dissolution of polyselenides compared to the control sample (Se@HC), indicating enhanced polyselenide immobilization.

The superior performance of the Se@CoSA-HC cathode can be attributed to the synergistic effects of the atomic cobalt electrocatalyst and the hollow carbon structure. The single cobalt atoms act as efficient electrocatalysts, accelerating the redox reactions of lithium polyselenides and promoting the formation of Li₂Se, reducing the energy barriers and improving the kinetics of the charge/discharge processes. The hollow carbon structure provides more accessible active sites, enhances ion and electron transport, and accommodates the volume changes during cycling. This results in high Se utilization, improved rate capability, and excellent cycling stability. These findings demonstrate that single-atom catalysis can significantly enhance the performance of Li-Se batteries and provide a promising approach for developing high-power and long-life energy storage devices.

This research successfully synthesized a high-performance Li-Se battery cathode using atomic cobalt electrocatalysts in a hollow carbon matrix. The Se@CoSA-HC composite achieved unprecedented cycling stability and rate capability, exceeding previous reports. This is attributed to the catalytic effect of atomic cobalt, enhancing the kinetics of polyselenide conversion and minimizing the energy barriers. The hollow carbon structure further facilitated ion and electron transport and mitigated volume expansion. Future research could explore other single-atom catalysts and optimize the carbon structure to further enhance battery performance and investigate the scalability and cost-effectiveness of this technology.

The study primarily focused on the electrochemical performance of the Se@CoSA-HC cathode under specific conditions. Further investigations are needed to evaluate its performance under different temperatures, electrolyte compositions, and electrode loadings. The long-term stability was tested at high rates but extending these tests to even longer durations would provide additional confidence in the long-term stability. A detailed cost-analysis comparing this technology with existing Li-ion technologies is also needed to assess its commercial viability. Finally, while DFT calculations provided valuable insights into the reaction mechanisms, experimental validation of the proposed mechanisms would further strengthen the findings.