Key Takeaways & Executive Findings
- •• Defect engineering, including vacancy and doping, effectively mitigates the strong Coulomb effect on Mg2+ migration in cathode materials, enhancing diffusion kinetics. • The review systematically analyzes the intrinsic migration mechanisms of Mg2+ and identifies key factors affecting ion diffusion, providing a theoretical foundation for defect design. • Various defect introduction strategies are summarized, demonstrating their positive impact on electrochemical performance of RMB cathodes. • Future directions emphasize the need for advanced characterization and computational methods to precisely control defects for high-performance RMBs.
Abstract
Rechargeable magnesium batteries (RMBs) have been considered a promising “post lithium-ion battery” system to meet the rapidly increasing demand of the emerging electric vehicle and grid energy storage market. However, the sluggish diffusion kinetics of bivalent Mg2+ in the host material, related to the strong Coulomb effect between Mg2+ and host anion lattices, hinders their further development toward practical applications. Defect engineering, regarded as an effective strategy to break through the slow migration puzzle, has been validated in various cathode materials for RMBs. In this review, we first thoroughly understand the intrinsic mechanism of Mg2+ diffusion in cathode materials, from which the key factors affecting ion diffusion are further presented. Then, the positive effects of purposely introduced defects, including vacancy and doping, and the corresponding strategies for introducing various defects are discussed. The applications of defect engineering in cathode materials for RMBs with advanced electrochemical properties are also summarized. Finally, the existing challenges and future perspectives of defect engineering in cathode materials for the overall high-performance RMBs are described.
1. Introduction
Considering energy crisis and environmental pollution are the two major concerns of today’s society, it is urgent to develop sustainable energy storage from intermittent solar and wind sources to replace traditional fossil fuels [1–4]. To date, lithium-ion batteries (LIBs) have been widely dominated in our daily lives ranging over portable electronics, electric vehicles, and smart grids [5–8]. Nevertheless, issues with safety, cost, and resources have hampered the utilization of LIBs in large-scale energy storage systems [9–11]. Therefore, developing the “post LIB” system to meet the rapidly increasing demand has become one of the most important scientific and societal challenges. Among various battery systems, rechargeable magnesium batteries (RMBs) have been considered as a promising candidate due to the apparent metrics of Mg metal anode including Earth’s crust (~2%), less prone to dendrite deposition, and high volumetric capacity (3833 mAh cm−3 for Mg). (Fig. 1a, b) [12–18].
Although great progress has been achieved recently, RMBs still have a long way to their maturity due to the limitations of the irreversibility of Mg anodes, low-efficiency Mg electrolytes, and lacking high-performance Mg2+ host cathode materials [19–29]. Compared to anode and electrolyte, the cathode material lies at the heart of the RMB system to determine the overall energy and power densities. However, the development of RMB cathode materials still faces scientific challenges related to the strong electrostatic interaction between Mg2+ and host anion lattices, which is caused by the inherently strong ionic polarization force of bivalent Mg2+ [30–34]. Therefore, most cathode materials (e.g., Chevrel phase, spinel, layered, and olivine structures) show sluggish diffusion kinetics of Mg2+, further leading to serious voltage polarization/hysteresis and a low magnesiation degree [35]. Moreover, although the ionic radii of Mg2+ and Li+ are comparable (0.72 Å for Mg2+ and 0.76 Å for Li+), Mg2+ has a greater polarizing power (3.85 e Å−2 for Mg2+ and 1.73 e Å−2 for Li+) [36]. Thus, some classical materials can reversibly intercalate Li+ but exhibit poor electrochemical activity against Mg2+ (Fig. 1c) [37–41].
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Zhengqing Fan, Ruimin Li, Xin Zhang, Wanyu Zhao, Zhenghui Pan, Xiaowei Yang (2024). Defect Engineering: Can it Mitigate Strong Coulomb Effect of Mg2+ in Cathode Materials for Rechargeable Magnesium Batteries?. Nano-Micro Letters. https://doi.org/10.1007/s40820-024-01495-1
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Frequently Asked Questions
What is the main challenge for rechargeable magnesium batteries?
The main challenge is the sluggish diffusion kinetics of Mg2+ in cathode materials due to the strong Coulomb effect between the divalent Mg2+ and the host anion lattice, leading to poor rate capability and low energy density.
How does defect engineering help in magnesium battery cathodes?
Defect engineering introduces vacancies or dopants into the cathode lattice, which can reduce the electrostatic interactions, create migration pathways, and enhance Mg2+ diffusion kinetics, thereby improving electrochemical performance.
What types of defects are discussed in the review?
The review discusses vacancy defects and doping (heteroatom substitution) as the main types of defects, along with strategies to introduce them, such as thermal treatment, chemical reduction, and electrochemical activation.
What are the future directions for defect engineering in RMBs?
Future directions include developing advanced characterization techniques to precisely control defect types and concentrations, combining experimental and computational methods to predict optimal defect configurations, and scaling up defect engineering for practical applications.
Why is Mg2+ diffusion slower than Li+ in similar cathode materials?
Mg2+ has a higher polarizing power (3.85 e Å−2) compared to Li+ (1.73 e Å−2), leading to stronger electrostatic interactions with the host lattice, which increases the activation energy for migration and slows down diffusion.
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