Metal-valence-state modulation of transition metal oxides boosts lithium–sulfur battery performance

Lithium–sulfur (Li–S) batteries, boasting a theoretical specific capacity of up to 1675 mAh g^-1, are emerging as a highly promising candidate for next-generation energy storage technologies. However, two critical challenges –– sluggish sulfur conversion kinetics and the polysulfide shuttle effect –– have significantly hindered their practical application.

While catalysis by metal compounds has seen great progress in overcoming the slow kinetics of sulfur conversion, fundamental understandings of how structural and chemical factors such as the compound composition, the valence states of cations and anions, the crystal structures, and the expose facets influence the catalytic behaviors and performance require further investigations. In particular, as the metal valence state has been probed as a key property in conventional heterogeneous catalysis research, the role of single-metal valence state in emerging energy chemical systems such as Li–S batteries remains systematically underexplored.

To address this gap, the research team systematically investigated the impact of metal valence state on Li–S battery performance using two simple oxides of copper: CuO (containing Cu²⁺) and Cu₂O (containing Cu⁺). Experimental results demonstrated that CuO displayed significantly stronger adsorption capacity for lithium polysulfides (LiPSs, key intermediates generated during battery discharging/charging) compared to Cu₂O. This disparity was attributed to the distinct cation-dictated surface chemistries of two oxides, resulting in dissimilar interactions with LiPSs. Combined spectroscopic and computational investigations revealed that CuO promoted LiPS conversion through two complementary mechanisms: surface thiosulfate redox reactions and indirect optimization of Li-bond strength via O p-band center tuning.

Under simulated harsh operating conditions, CuO demonstrated exceptional catalytic performance. A Li–S battery incorporating a CuO catalytic layer retained a reversible capacity of 550 mAh g^-1 at a high current density of 2 C –– more than double that of the Cu₂O-based battery. Furthermore, under extreme conditions of 4.8 mg cm^-2 S loading and 8.8 µL mg^-1 electrolyte-to-S ratio, the CuO-based battery achieved a low-capacity decay rate of < 0.18% per cycle for over 110 cycles, highlighting its robust cycling stability. These results under high-rate, high-sulfur-loading, and low-electrolyte conditions underscore the desirable practical potential of CuO, especially given that Cu is much more earth-abundant and economically affordable than many precious metals like Pt for many industrial heterogeneous catalytic processes.

“This study aims to clarify the critical role of metal valence state in single-metal compounds for catalytic sulfur conversion in Li–S batteries,” stated Hong-Jie Peng, a professor at Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, “Our findings introduce an ‘indirect’ strategy for regulating Li-bond chemistries and related energy conversion processes in batteries. I believe that the fundamental insights could motivate the design and optimization of novel catalysts towards better battery performance.”

Looking ahead, the research team is attempting to expand this strategy to explore metal-valence-state effects in other transition metal compounds and to develop Li–S battery systems that balance high energy density with long cycle life. They hope to bring this technology to the market soon.

The team of researchers from University of Electronic Science and Technology of China published their work in the journal Particuology at https://www.sciencedirect.com/science/article/pii/S1674200125000720. And the team members are Haobo Zhang, Bobo Zou, Xian Zhong, Xinhe Liu, Kaixi You, Xinyan Liu and Hong-Jie Peng.

The research is funded by the National Natural Science Foundation of China (Grant Nos. 22379021 and 22479021) and the Sichuan Science and Technology Program (Grant No. 2023NSFSC0115).

Particuology (IF=4.1) is an interdisciplinary journal that publishes frontier research articles and critical reviews on the discovery, formulation and engineering of particulate materials, processes and systems. Topics are broadly relevant to the production of materials, pharmaceuticals and food, the conversion of energy resources, and protection of the environment. For more information, please visit: https://www.journals.elsevier.com/particuology.