In a recent study published in Nature Communications, researchers introduced a long-life, fast-charging, and high-capacity magnesium@black phosphorous (Mg@BP) composite negative electrode for non-aqueous magnesium batteries.
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Background
The demand for “post-lithium-ion batteries” has increased significantly due to the recent growth in grid energy storage and electric transportation systems. Among the post-lithium-ion battery technologies, non-aqueous magnesium batteries have gained attention because of the inherent properties of magnesium as a negative electrode.
Magnesium is a cost-efficient alternative negative electrode as it is abundant in nature. It also offers attractive specific and volumetric capacity, along with a low reduction potential.
Limitations of Magnesium Negative Electrode
Despite its advantages, the application of magnesium as a negative electrode in batteries with non-aqueous liquid electrolyte solutions is limited. Existing magnesium negative electrode materials often exhibit poor compatibility with various liquid electrolytes.
Additionally, significant volume changes and severe side reactions can lead to structural collapse of the negative electrode and depletion of the electrolyte solution. Collectively, these issues result in low magnesium utilization efficiency and rapid performance degradation during prolonged cycling.
Although metal magnesium negative electrodes are suitable for rechargeable magnesium batteries from a cost and energy density perspective, a thin magnesium foil of less than 25 μm is necessary to maintain an optimal negative/positive ratio for high-energy magnesium batteries.
However, producing ultrathin magnesium foil is challenging due to the densely packed hexagonal lattice structure of magnesium. The uneven magnesium plating behavior at the negative electrode is another challenge that leads to short cycle life and high overpotential.
The Proposed Approach
In this study, researchers developed a Mg@BP composite using black phosphorus as the substrate and used it as a negative electrode for non-aqueous magnesium-based batteries. Specifically, black phosphorus nanosheets were utilized to synthesize the composite negative electrode.
The objective was to create a magnesium composite negative electrode that offers good electrolyte compatibility, fast-charging capabilities, a long lifespan, uniform deposition behavior, and straightforward synthesis.
Mg@BP Composite Negative Electrode Preparation: The black phosphorus slurry was pasted on the copper current collectors and dried for 24 h at 80 °C in a vacuum to create the black phosphorus electrode. On the current collectors, the black phosphorus mass loading was estimated to be ~0.5 mg cm−2.
The thickness of the black phosphorus layer was controlled by adjusting the casting amount of the slurry. A freestanding black phosphorus (F-BP) electrode was also fabricated by applying the slurry to a polytetrafluoroethylene (PTFE) film and peeling it off after drying.
Magnesium was plated onto the synthesized black phosphorus-based electrodes in a magnesium cell with controlled area capacity and current density. The Mg@BP-based composite negative electrode was obtained by disassembling the asymmetric cell after the deposition process.
The synthesized magnesium composite negative electrodes were designated as Mg@F-BP and Mg@BP. Using the same approach, researchers also synthesized control composite negative electrodes. Magnesium was plated onto commonly used current collectors—copper, aluminum, and carbon fabric—to create the corresponding composite magnesium negative electrodes, designated as Mg@Al, Mg@C, and Mg@Cu.
Micro- and nano-copper sulfide (CuS) positive electrodes were also prepared. Researchers investigated the preparation mechanism of the composite negative electrodes through both theoretical and experimental methods, employing state-of-the-art techniques such as ex-situ transmission electron microscopy (TEM), in situ spectroscopies, and theoretical calculations and simulations.
Research Findings
Results showed that Mg²⁺ was initially partially intercalated into the black phosphorus crystal structure, leading to the formation of chemically stable MgₓP intermediates with metallic properties (limited intercalation/Stage I).
Subsequently, the metallic magnesium was deposited electrochemically on the early-formed MgₓP/black phosphorus to synthesize the Mg@BP composite negative electrode (stable plating/Stage II).
This limited intercalation triggered the transition from semiconductor black phosphorus to a metallic compound, endowing the Mg@BP composite negative electrode with fast charge transfer and magnesiophilic properties. This facilitated uniform magnesium growth behavior and nucleation, which are challenging to achieve on pristine metal magnesium negative electrodes and other composite negative electrodes.
The Mg@BP electrode exhibited stable stripping/plating performance for 800 cycles (1600 hours), a Coulombic efficiency of 99.98 %, and a cumulative capacity of 3200 mAh cm⁻² in asymmetric coin cell configuration. The assembly and testing of the Mg@BP | | nano-CuS coin cell demonstrated a discharge capacity of 398 mAh g⁻¹ and an average cell discharge potential of 1.15 V at a specific current of 560 mA g⁻¹, with a low decay rate of 0.016 % per cycle at 25 °C for 225 cycles.
Overall, this work demonstrates the feasibility of using Mg@BP composite negative electrodes for non-aqueous magnesium batteries, indicating potential advancements in energy storage technology.
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Journal Reference
Zhao, Q. et al. (2024). High-capacity, fast-charging and long-life magnesium/black phosphorous composite negative electrode for non-aqueous magnesium battery. Nature Communications. DOI: 10.1038/s41467-024-52949-4, https://www.nature.com/articles/s41467-024-52949-4
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