Potassium-ion batteries (PIBs) have emerged as a promising alternative to lithium-ion batteries due to the abundance of potassium and its similar electrochemical properties. The development of high-performance anode materials is critical for advancing PIB technology. Anode materials for PIBs can be broadly categorized into carbonaceous materials, alloy-based materials, and organic compounds, each with distinct potassium storage mechanisms and challenges.
Carbonaceous materials are widely studied due to their structural diversity, conductivity, and stability. Graphite, the most common carbon anode, exhibits a staging mechanism for K⁺ intercalation, forming KC₈ at full potassiation. However, graphite suffers from limited capacity (around 280 mAh/g) and volume expansion during cycling. Hard carbons, with their disordered structure, offer higher capacity (up to 350 mAh/g) through a combination of intercalation and pore filling. The larger ionic radius of K⁺ (1.38 Å) compared to Li⁺ (0.76 Å) poses challenges for diffusion kinetics, leading to slower charge/discharge rates. Heteroatom doping (e.g., nitrogen or sulfur) can enhance conductivity and introduce defects for additional K⁺ storage sites, improving performance.
Alloy-based anodes, such as those incorporating phosphorus, tin, or antimony, provide high theoretical capacities through conversion or alloying reactions. For example, phosphorus forms KP, delivering a capacity of 865 mAh/g. However, these materials suffer from severe volume expansion (up to 400%) during potassiation, leading to mechanical degradation and rapid capacity fading. Strategies to mitigate this include nanostructuring, composite formation with conductive matrices, and the use of elastic binders. Despite these efforts, achieving long-term cycling stability remains a challenge due to the repeated stress induced by K⁺ insertion/extraction.
Organic compounds represent another class of anode materials, leveraging their structural flexibility and sustainability. Small molecules (e.g., carbonyl-based compounds) and polymers (e.g., polyimides) store K⁺ through reversible redox reactions. Organic anodes often exhibit moderate capacities (200–400 mAh/g) but excel in rate capability and cycling stability due to the absence of structural degradation. Their main drawbacks include low electronic conductivity and solubility in electrolytes, which can be addressed through polymerization or hybridization with conductive additives.
The storage mechanisms in PIB anodes differ significantly from those in Li-ion or Na-ion systems. The larger size of K⁺ results in weaker Lewis acidity, enabling faster ion diffusion in certain materials. However, it also leads to sluggish solid-state diffusion in densely packed structures. The lower redox potential of K⁺/K (−2.93 V vs. SHE) compared to Na⁺/Na (−2.71 V) or Li⁺/Li (−3.04 V) reduces the overall cell voltage, impacting energy density. Additionally, the formation of unstable solid-electrolyte interphases (SEI) on anode surfaces further complicates long-term performance.
Key challenges in PIB anode development include achieving high energy density, improving rate capability, and ensuring cycling stability. The lower energy density stems from the inherent trade-offs between capacity and voltage in potassium-based systems. Strategies such as electrode engineering, electrolyte optimization, and interfacial modification are being explored to overcome these limitations. For instance, ether-based electrolytes have shown promise in reducing SEI resistance and enhancing K⁺ transport.
Future research directions focus on discovering new materials with optimized K⁺ storage properties, understanding degradation mechanisms at the atomic level, and scaling up synthesis methods for practical applications. While carbonaceous materials remain the most viable option for near-term commercialization, alloy and organic anodes offer potential for high-performance PIBs if stability issues are resolved.
In summary, anode materials for potassium-ion batteries present unique opportunities and challenges. Carbon-based materials provide a balance of performance and practicality, while alloys and organic compounds push the boundaries of capacity and sustainability. Addressing the limitations of K⁺ storage mechanisms will be crucial for unlocking the full potential of PIBs in energy storage applications.