A recent article published in the Journal of Magnesium and Alloys proposed highly lipophilic Mg-Li-Cu alloys, prepared via a simple and controlled method, for stabilizing Li metal anodes. These alloy-based electrodes enabled highly reversible lithium metal batteries (LMBs).
Study: Mg-Li-Cu alloy anode for highly reversible lithium metal batteries. Image Credit: Kittyfly/Shutterstock.com
Background
Advanced electronic devices and next-generation vehicles require high-power, high-performance energy storage systems. Due to its high theoretical specific capacity and low potential, lithium metal is the most promising anode material for future secondary batteries.
However, its high reactivity and uncontrolled dendrite growth cause undesirable parasitic reactions, low reversibility, poor cycling stability, and safety concerns.
These challenges hinder the practical application of Li-metal anode in high-energy rechargeable LMBs. Consequently, three-dimensional alloys are considered promising to inhibit the growth of dendrites and enhance the cycling stability of Li-metal anodes (LMAs).
Mg-Li alloys with a stable skeletal structure and low melting point are attractive as LMB anodes. However, they require further modifications to prevent parasitic reactions during electroplating solvation. Thus, this study proposes a ternary magnesium alloy (Mg-Li-Cu) anode for high-performance LMBs.
Methods
Mg-Li-Cu alloy electrodes were fabricated by mechanical pressing and melting-induced spontaneous diffusion reaction between an Mg-Li alloy (molar ratio of 5:95) and a copper foil.
The alloys were cut into electrode disks of 15 mm diameter. Additionally, Mg-Li alloy electrodes were prepared for comparison.
The prepared anodes were used in batteries assembled in standard CR2032 coin cells. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry were performed on symmetrical and full cells using both lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and LiPF6 electrolytes for comparison.
Scanning electron microscopy (SEM) was used to investigate the morphology of the prepared electrodes, while their crystal structure was characterized using X-ray diffraction.
Additionally, the surface roughness of the electrodes was examined by laser scanning confocal microscopy, and their chemical composition was determined by X-ray photoelectron spectroscopy.
Theoretical simulations were performed using density functional theory (DFT) on the Vienna Ab-initio Simulation Package (VASP) to understand the lithiophilicity of Mg-Li and Mg-Li-Cu alloys.
The calculations followed the generalized gradient approximation method to describe the exchange-correlation effects. Alternatively, the projected augmented wave method accounted for the core-valence interactions.
Results and Discussion
During the preparation of the Mg-Li-Cu alloy, Cu atoms spontaneously diffused into the Mg-Li alloy, transforming it from delamination to the final Mg-Li-Cu alloy with uniformly dispersed atoms. SEM images of the Mg-Li-Cu alloy anode exhibited a dense morphology.
Additionally, the energy dispersive spectroscopy (EDS) results for the surface and cross-section of the anode demonstrated a uniform distribution of magnesium and copper in Mg-Li-Cu without agglomeration.
The Mg-Li-Cu anode’s interfacial resistance was smaller than that of bare Li and Mg-Li electrodes before and after rate performance tests, further verifying the interfacial changes of the Mg-Li-Cu electrode.
Moreover, the contact angle measurements of bare Li, Mg-Li, and Mg-Li-Cu electrodes with liquid electrolyte revealed the maximum affinity of Mg-Li-Cu with the electrolyte.
Both theoretical calculations and experimental results revealed better lithiophilicity and dendrite suppression ability of Mg-Li-Cu alloys than Mg-Li alloys. This was attributed to the Mg-Li-Cu alloy’s ionic/electronic dual-conductive framework, which delivered high-capacity electrodes with enhanced cycling stability.
Moreover, the Mg-Li-Cu electrodes had a low overpotential (∼28 mV) and negligible voltage fluctuations. Alternatively, the Mg-Li electrode exhibited a significant rise in overpotential after only 189 cycles, while the overpotential of the bare Li metal electrode fluctuated highly at the beginning only.
The symmetric battery comprising Mg-Li-Cu anode exhibited a long lifetime of over 9000 hours at 1 mA/cm. Even under the harsh conditions of low temperatures (−2 °C) and lean electrolyte (20 µL), the symmetrical battery life was up to 400 hours and 700 hours, respectively. Furthermore, the full cell with LiFePO4 (LFP) cathode and Mg-Li-Cu anode exhibited a 96.4% capacity retention rate after 500 cycles.
Conclusion
The researchers successfully fabricated an Mg-Li-Cu alloy anode using a facile process for highly reversible LMBs. Cu self-diffusion into Mg-Li alloy enhanced the delocalization of electrons on the deposited lithium, inhibiting the growth of lithium dendrites in Mg-Li-Cu.
Additionally, uniformly distributed lithophilic sites on the Mg-Li-Cu anode ensured uniform lithium deposition over it. This resulted in excellent long-term cycling stability of the anode by avoiding parasitic reactions with electrolytes.
Symmetric cells with Mg-Li-Cu as the lithium host could be operated for over 9,000 hours at low overpotentials (∼17.9 mV) without significant voltage variations.
Moreover, the Mg-Li-Cu full cell comprising LFP cathode exhibited a high initial capacity of 148.2 mAh/g and a capacity retention rate of 96.4% at 1 C after 500 cycles.
In conclusion, this study demonstrated the feasibility of applying flexible alloy anodes in highly stable LMBs. It may provide novel strategies for preparing and optimizing Mg alloys.
Journal Reference
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