Molybdenum-Based Sodium Ion Batteries Development

A recent study carried out by Yu Jiang et al at Jiangsu Key Laboratory of Thin Films, Soochow University, Suzhou, China has investigated the Molybdenum-based materials for sodium ion batteries (SIBs). SIBs are considered to be one of the most promising candidates for energy storage required for renewable energy sources. The main difficulty is finding a suitable anode material, and molybdenum-based materials are expected to solve this problem.

The over-consumption of fossil fuels and the ensuing environmental pollution make the use of renewable energy sources urgent. However, many renewable energy sources, such as solar and wind, are intermittent, and therefore, the integration of energy storage in these renewable energy systems is essential. Among various energy storage technologies, currently available electrochemical rechargeable batteries and sodium ion batteries are an important solution because sodium resources are almost unlimited and have an electrochemical behavior similar to that of lithium.

Currently, one of the key challenges for SIB applications is the lack of satisfactory anode materials. The radius of sodium ions is large compared to that of lithium ions. Also, the standard electrode potential of Na⁺/Na is higher than that of Li⁺/Li, which reduces the operating voltage and energy of the full cell. To achieve performance comparable to lithium technology, it is imperative to explore Na anode materials with high capacity, fast response kinetics, and long cycle life.

In recent years, anode materials based on transition metal elements have attracted great interest. In particular, materials based on molybdenum have been regarded as promising electrode materials due to their unique structural and performance advantages.

Mo is inexpensive and abundant in the earth's crust. In addition, due to their multiple valence states with different coordination numbers, Mo-based compounds display a rich structural chemistry with various stoichiometries such as oxides (MoO₃, MoO₂), dichalcogenides (MoS₂, MoSe₂) and carbides (Mo₂C, MoC).

Similar to other transition metal compounds, molybdenum-based electrode materials present difficulties such as multiphase conversion, severe electrode pulverization, and slow electron/ion transport. In the past few years, significant progress has been made in this field of research.

Therefore, the research team from Soochow University summarizes the latest progress of Mo-based compounds for sodium storage applications, and this research work can provide guidance for further advances in the development of Mo materials or other transition metal compounds.

Mo-based materials, such as oxides and sulfides, are highly promising because they have much larger capacities than carbonaceous materials and exhibit rich Na-reaction chemistry. However, these materials face several technical problems, such as multi-phase conversion, particle comminution due to volume expansion, and low surface activity during sodization/deiodination. Research has found that material design is essential to address these issues.

Mo-based materials exhibit different sodium storage mechanisms due to differences in structure and composition. Molybdenum oxides and brassides typically undergo insertion and conversion reactions accompanied by multiphase conversions. They usually exhibit poor reversibility because the inserted sodium ions cannot be completely removed. As for the other molybdenum materials, their (de)iodination reactions often involve Na⁺-driven conversion processes. As a result, they exhibit better reversibility upon sodium cycling.

The study titled “Molybdenum-based materials for sodium-ion batteries” has been published in the journal InfoMat 2021(3). 

Although research is progressing, substantial challenges remain and significant efforts are needed. Many molybdenum-based materials store Na⁺ ions through conversion reactions, which are accompanied by substantial volume expansion and drastic structural changes. How to mitigate the volume changes in electrodes is recognized worldwide as a difficult topic. In this regard, atomic and molecular layer deposition offer great opportunities for stabilizing electrodes under harsh conditions. At the same time, a comprehensive understanding of the electronic and structural evolution of molybdenum materials during sodium storage remains crucial.

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