Organic electrode materials have gained attention as potential candidates for multivalent ion batteries using Mg2+, Al3+, and Zn2+ due to their structural flexibility, environmental sustainability, and potential for high capacity. Unlike conventional inorganic electrodes, organic materials can undergo reversible redox reactions through tailored molecular designs that accommodate multivalent ions. However, challenges such as low operational voltages, poor electronic conductivity, and dissolution in electrolytes must be addressed through careful molecular engineering.

Multivalent ions present unique challenges due to their higher charge density and stronger electrostatic interactions with host materials compared to monovalent ions like Li+. Organic materials must incorporate functional groups capable of chelating these ions while maintaining structural stability. Common strategies include the use of conjugated carbonyl compounds, conductive polymers, and redox-active molecules with heteroatoms (O, N, S) that facilitate ion coordination.

For magnesium-ion batteries, quinone-based compounds are widely studied due to their reversible two-electron redox reactions. The carbonyl groups in quinones form stable chelation complexes with Mg2+, enabling reversible insertion and extraction. Recent work has demonstrated that extending π-conjugation in quinone derivatives improves electronic conductivity and mitigates dissolution. For example, pyrene-4,5,9,10-tetraone exhibits a specific capacity of over 300 mAh/g with a discharge voltage around 1.2 V vs. Mg/Mg2+. However, the low voltage remains a limitation, requiring further modifications to enhance the redox potential.

Aluminum-ion batteries face even greater challenges due to the trivalent nature of Al3+, which leads to strong Coulombic interactions and slow diffusion kinetics. Imide-based compounds, such as perylene-3,4,9,10-tetracarboxylic diimide (PTCDI), have shown promise by leveraging carboxylate groups to stabilize Al3+ insertion. The rigid aromatic structure of PTCDI provides a stable framework, but the discharge voltage typically remains below 1.0 V vs. Al/Al3+. Recent advances include the incorporation of nitrogen-rich heterocycles, which enhance charge delocalization and improve ion mobility.

Zinc-ion batteries benefit from the relatively mild reactivity of Zn2+, making organic materials more viable. Polymers like poly(anthraquinonyl sulfide) (PAQS) exhibit reversible Zn2+ storage through thioether and carbonyl redox centers. The soft Lewis basicity of sulfur atoms facilitates Zn2+ coordination, while the conjugated backbone ensures electronic conductivity. PAQS has demonstrated stable cycling with capacities exceeding 200 mAh/g at voltages around 0.7 V vs. Zn/Zn2+. Another approach involves the use of triazine-based covalent organic frameworks (COFs), where the nitrogen-rich pores provide selective Zn2+ binding sites, improving ion selectivity and reducing side reactions.

A critical challenge for all these systems is the trade-off between ion storage capacity and electrochemical stability. Multivalent ions often induce significant structural distortion during insertion, leading to rapid capacity fading. Molecular design strategies to address this include the introduction of cross-linked polymer networks or the use of π-stacked layered structures that buffer volume changes. For instance, naphthalene diimide-based polymers with flexible alkyl linkers exhibit improved cycle life by accommodating structural strain.

Conductivity limitations are another major hurdle. Organic materials are typically insulators, requiring the integration of conductive additives or intrinsic conductive pathways. Recent progress includes the synthesis of donor-acceptor type polymers where electron-rich and electron-deficient units alternate, promoting intramolecular charge transfer. Thiophene-based conductive polymers, when combined with redox-active carbonyl groups, have shown enhanced rate capability due to improved charge transport.

Dissolution in liquid electrolytes remains a persistent issue, particularly for small-molecule organics. Strategies to mitigate this include polymerization, covalent anchoring to substrates, or the use of solid-state electrolytes. For example, grafting quinone units onto graphene sheets not only prevents dissolution but also enhances electron transport. Solid-state systems employing gel polymer electrolytes have also demonstrated improved stability by physically confining the active material.

Recent breakthroughs in molecular design have pushed the boundaries of performance. Multi-redox systems, where a single molecule undergoes multiple electron transfers, are particularly promising. Hexaazatrinaphthylene (HATN) derivatives, for instance, can store up to six electrons per molecule when paired with Mg2+, significantly increasing theoretical capacity. Similarly, the development of bipolar molecules with separate anionic and cationic redox centers allows for higher voltage outputs, though practical implementations remain challenging.

Despite these advances, several scientific and engineering challenges persist. The low voltages of organic electrodes limit energy density, necessitating further exploration of electron-withdrawing groups to elevate redox potentials. Ion transport kinetics must also be improved through the rational design of pore structures and ion-conducting pathways. Additionally, long-term stability under realistic cycling conditions requires more rigorous testing, particularly for aluminum systems where parasitic reactions are prevalent.

In summary, organic materials for multivalent batteries offer a versatile platform for sustainable energy storage, but their success hinges on precise molecular engineering to address inherent limitations. Recent progress in chelation chemistry, conductivity enhancement, and structural stabilization provides a roadmap for future developments. Continued innovation in synthetic chemistry and electrolyte formulation will be essential to unlock the full potential of these systems for practical applications.