Dual-ion batteries represent an emerging energy storage technology where both cations and anions participate in the electrochemical processes. Unlike conventional lithium-ion batteries that rely solely on cation intercalation, dual-ion systems utilize anion storage in the cathode during charging, offering unique advantages in cost and sustainability. The cathode materials in these batteries must efficiently host anions such as PF6⁻, TFSI⁻, or BF4⁻ while maintaining structural integrity and electrochemical performance. This article examines the key cathode materials for dual-ion batteries, focusing on their anion storage mechanisms, stability, and performance characteristics.

Graphite-based cathodes are the most extensively studied materials for anion storage in dual-ion batteries. Graphite's layered structure provides an ideal host for anion intercalation, with the ability to accommodate large anions between its graphene layers. During charging, anions from the electrolyte insert into the graphite matrix, forming staged intercalation compounds. The intercalation mechanism involves the expansion of interlayer spacing to accommodate anions, with stage transitions depending on the anion concentration. For PF6⁻ intercalation, graphite cathodes typically exhibit capacities ranging from 80 to 110 mAh/g, with working potentials around 4.5-5.0 V versus Li/Li⁺. The cycle life of graphite cathodes can exceed 1000 cycles with proper electrolyte optimization, though oxidative decomposition at high voltages remains a challenge. The choice of electrolyte significantly influences graphite's performance, as solvent co-intercalation can lead to exfoliation and capacity fade. To mitigate degradation, researchers have explored modified graphite materials, including expanded graphite, graphene composites, and surface-treated variants. These modifications improve anion diffusion kinetics and reduce irreversible side reactions.

Organic cathode materials offer an alternative to graphite, with advantages in sustainability and structural diversity. Conjugated polymers, carbonyl compounds, and radical polymers have demonstrated anion storage capabilities through redox reactions or electrostatic interactions. Organic materials typically store anions through surface adsorption or bulk insertion, depending on their molecular design. For example, poly(vinylphenothiazine) and poly(viologen) derivatives exhibit reversible anion storage at potentials between 3.8 and 4.3 V, with capacities reaching 120 mAh/g in some cases. The flexibility of organic synthesis allows tuning of redox potentials and anion affinity, though electronic conductivity and dissolution in electrolytes remain persistent issues. Covalent organic frameworks (COFs) have emerged as promising candidates due to their porous crystalline structures that can selectively host anions. Some COFs demonstrate exceptional cycling stability over 2000 cycles with minimal capacity decay, attributed to their robust covalent networks that prevent material dissolution. However, organic cathodes generally exhibit lower volumetric energy density compared to graphite due to their lower packing density.

Beyond graphite and organic materials, other host structures have shown potential for anion storage in dual-ion batteries. Transition metal oxides and sulfides with layered or tunnel structures can intercalate anions, though their high atomic weights limit gravimetric capacity. Materials like MoS₂ and V₂O₅ have demonstrated anion storage capabilities below 50 mAh/g, with working potentials varying based on their electronic structure. Prussian blue analogs represent another class of materials that can reversibly store anions in their open framework structures. These materials offer fast anion diffusion kinetics but suffer from low practical capacity due to the large fraction of inactive framework atoms. Recent research has explored two-dimensional materials beyond graphene, such as MXenes and boron nitride, which show anion adsorption properties through surface functional groups. While these materials exhibit excellent rate capability, their anion storage mechanisms often involve surface reactions rather than bulk intercalation, limiting total capacity.

The performance of cathode materials in dual-ion batteries is typically evaluated through several key metrics. Capacity measures the amount of charge stored per unit mass or volume, with practical values ranging from 50 to 120 mAh/g for most materials. Cycle life reflects the stability of the material over repeated charge-discharge cycles, with graphite and some organic polymers demonstrating the best longevity. Rate capability indicates how quickly the material can store and release anions, which depends on ionic and electronic conductivity within the electrode. Voltage profile determines the energy efficiency of the system, with higher potentials enabling greater energy density but increasing electrolyte decomposition risks. Structural stability during anion insertion is critical, as volume changes can lead to particle cracking and electrical disconnection. Most materials experience some degree of expansion during charging, with graphite showing approximately 10-15% volume change upon full intercalation.

Comparative analysis reveals trade-offs between different cathode materials. Graphite offers the best combination of capacity, cycle life, and rate performance but requires high-voltage electrolytes that remain stable above 4.5 V. Organic materials provide environmental benefits and structural tunability but struggle with low conductivity and dissolution issues. Other host structures often exhibit interesting fundamental behaviors but have yet to match the practical performance of graphite or advanced organic polymers. A critical limitation across all materials is the need for electrolyte formulations that can withstand high oxidation potentials while maintaining good ionic conductivity. Another universal challenge is the relatively low capacity compared to conventional lithium-ion cathodes, as anion storage typically involves only one charge per anion rather than multi-electron redox processes.

Future development of cathode materials for dual-ion batteries will likely focus on improving anion storage capacity while maintaining structural stability. Strategies include designing materials with larger interlayer spacing for easier anion insertion, developing conductive scaffolds for organic materials, and engineering surface chemistries to enhance anion affinity. Understanding the fundamental interactions between anions and host materials at the atomic level will be crucial for rational material design. While significant progress has been made in demonstrating proof-of-concept materials, translating laboratory performance to practical batteries requires addressing scalability, cost, and compatibility with existing manufacturing processes. The unique chemistry of dual-ion systems presents both challenges and opportunities for developing next-generation energy storage technologies with distinct advantages over conventional battery designs.