Professor Yan Yu's team at USTC utilized the CIQTEK Scanning Electron Microscope SEM3200 to study the post-cycling morphology. It developed amorphous carbon with controllable defects as a candidate material for an artificial interface layer balancing potassiophilicity and catalytic activity.
The research team prepared a series of carbon materials with different degrees of defects (designated as SC-X, where X represents the carbonization temperature) by regulating the carbonization temperature. The study found that SC-800 with excessive defects caused substantial electrolyte decomposition, resulting in an uneven SEI film and shortened cycle life. SC-2300, with the fewest defects, had insufficient affinity for potassium and easily induced potassium dendritic growth. SC-1600, which possessed a locally ordered carbon layer, exhibited an optimized defect structure, achieving the best balance between potassiophilicity and catalytic activity. It could regulate the electrolyte decomposition and form a dense and uniform SEI film.
The experimental results demonstrated that SC-1600@K exhibited long-term cycle stability for up to 2000 hours under a current density of 0.5 mA cm-2 and a capacity of 0.5 mAh cm-2. Even under higher current density (1 mA cm-2) and capacity (1 mAh cm-2), it maintained excellent electrochemical performance with stable cycles exceeding 1300 hours. In full-cell testing, when paired with a PTCDA positive electrode, it maintained 78% capacity retention after 1500 cycles at a current density of 1 A/g, demonstrating outstanding cycle stability.
This research, titled "Balancing Potassiophilicity and Catalytic Activity of Artificial Interface Layer for Dendrite-Free Sodium/Potassium Metal Batteries," was published in Advanced Materials.
Figure 1: The microstructure analysis results of carbon samples (SC-800, SC-1600, and SC-2300) prepared at different carbonization temperatures are presented. Through techniques such as X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and wide-angle X-ray scattering (WAXS), the crystal structure, defect level, and oxygen and nitrogen doping of these samples were analyzed. The results showed that as the carbonization temperature increased, the defects in the carbon materials gradually decreased, and the crystal structure became more orderly.
Figure 2: The current density distribution during potassium metal growth on different composite negative electrodes was analyzed using finite element simulation. The simulation results showed that the SC-1600@K composite electrode exhibited a uniform current distribution during potassium deposition, which helped suppress dendritic growth effectively. Additionally, the Young's modulus of the SEI layer was measured using atomic force microscopy (AFM), and the results showed that the SEI layer on the SC-1600@K electrode had a higher modulus, indicating its stronger firmness and inhibition of dendritic formation.
Figure 3: The electrochemical performance of different composite electrodes (SC-800@K, SC-1600@K, and SC-2300@K) in symmetrical cells is presented. The SC-1600@K electrode exhibited excellent cycle stability and low overpotential at different current densities and capacities. Furthermore, electrochemical impedance spectroscopy (EIS) and Sand's time testing further confirmed the advantages of the SC-1600@K electrode in suppressing dendritic growth and maintaining SEI layer stability.
Figure 4: The structure and composition of the SEI layer on different composite negative electrodes were analyzed using cryogenic transmission electron microscopy (Cryo-TEM) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The results showed that the SC-1600@K electrode had a uniform, thin, and inorganic-rich SEI layer, facilitating fast potassium ion transport kinetics and high Young's modulus. The SEI layers on the SC-800@K and SC-2300@K electrodes exhibited thicker and organic-rich characteristics.
Figure 5: The effects of defect configuration in the carbon layer on potassium ion deposition and SEI formation were explored using density functional theory (DFT) calculations. The results showed that an appropriate amount of defects could enhance the interaction between potassium ions and the carbon layer, reducing the nucleation overpotential, while excessive defects could lead to excessive electrolyte decomposition.
Figure 6: The electrochemical performance of a full cell (PTCDA//SC-1600@K) assembled using the SC-1600@K electrode is presented. This cell exhibited excellent rate performance and long-term cycle stability at different current densities, demonstrating the potential of the SC-1600@K electrode in practical battery applications.
In conclusion, the research team successfully designed and prepared a carbon material (SC-1600) with a locally ordered structure, serving as an artificial interface layer for sodium/potassium metal battery negative electrodes. By precisely controlling the defect content of the material, the team achieved the optimal balance between potassiophilicity and catalytic activity, significantly improving the uniform deposition of potassium ions and promoting the formation of a stable SEI layer. In a potassium symmetrical cell based on SC-1600 in a carbonate electrolyte system, SC-1600@K exhibited excellent cycle stability with a cycle life exceeding 2000 hours. Notably, a full cell assembled with the SC-1600@K negative electrode and PTCDA positive electrode maintained 78% capacity retention after 1500 cycles at a high current density of 1 A/g. This research not only established a model system for optimizing the SEI structure and potassium ion adsorption by controlling the interfacial layer defects but also provided important theoretical guidance and a technological pathway for the rational design of protective interfacial layers in potassium metal batteries.
