Multivalent Energy Storage

• Multivalent energy storage is defined by the use of multicharged ions (e.g., Mg²⁺, Al³⁺) for transferring multiple electrons per ion, achieving higher gravimetric and volumetric capacities than monovalent systems.
• It relies on advanced host frameworks—such as open-tunnel oxides, layered structures, and organic materials—to enable rapid, reversible insertion/extraction with low overpotentials and long cycle lives.
• Recent studies integrate computational screening and AI-driven models to optimize host structures, predict migration barriers, and unlock new design principles for sustainable, high-performance energy storage.

Multivalent energy storage refers to systems in which redox-active ions with valency higher than one—such as Mg$^{2+}$, Zn$^{2+}$, Ca$^{2+}$, Al$^{3+}$, Cr$^{3+}$, and Mn$^{2+}$/Mn$^{4+}$—are exploited to transfer and store multiple electrons per ion. This strategy offers the potential for significantly enhanced gravimetric and volumetric capacities, higher charge storage densities, and material sustainability compared to monovalent (e.g., Li$^{+}$, Na$^{+}$) ion storage. These advances rely critically on the development of host frameworks (inorganic, organic, composite) that can reversibly accommodate multivalent ion insertion/extraction with low overpotential, high rate capability, and long cycle life.

1. Multivalent Redox Mechanisms

Multivalent energy storage relies on the simultaneous transport and redox of multi-charged ions, either as simple ions (e.g., Mg$^{2+}$) or complexed species (e.g., AlCl$_2^+$ in some electrolytes). The principal redox reactions can be classified as:

• Simple insertion/extraction: Direct intercalation of M$^{z+}$ into host lattices (e.g., spinel MgMn$_2$O$_4$: $$\mathrm{Mg}^{2+} + 2\,\mathrm{MnO}_2 + 2\,e^- \rightleftharpoons \mathrm{MgMn}_2\mathrm{O}_4$$).
• Conversion/Faradaic processes: Redox-driven deposition/dissolution, typified by Mn$^{2+}$/MnO$_2$ on conductive supports ($$\mathrm{Mn}^{2+} + 2\,H_2O \rightleftharpoons \mathrm{MnO}_2 + 4\,H^+ + 2\,e^-$$).
• Organic frameworks: Coordination of multivalent ions to “soft” sites (e.g., carbonyls in PTCDA, enabling stepwise insertion of AlCl$_2^+$), or cation–anion co-storage in block-copolymers (e.g., DNVBr).
• Complex-ion mechanisms: In highly acidic or chloroaluminate electrolytes, the electroactive species may be polynuclear or partially reduced (e.g., AlCl$_2^+$ is the mobile carrier for Al$^{3+}$ in EMImCl-AlCl$_3$) (Canever et al., 2021).

The number of electrons transferred per formula unit (the “n” in Faraday’s law) directly impacts theoretical capacity. For example, in PTCDA/AlCl$_2^+$ systems, up to four electrons per PTCDA molecule are involved ($C_\text{theo} \approx 273$ mAh g$^{-1}$) (Canever et al., 2021), whereas in aqueous graphene/MnO$_2$ systems, two-electron processes ($\mathrm{Mn}^{2+} \rightarrow \mathrm{MnO}_2$) yield theoretical capacities up to 4200 mAh g$^{-1}$ (graphene basis) (Chen et al., 2014). In both cases, the key challenge is to enable rapid, reversible multivalent-ion motion with minimal side reactions or host degradation.

2. Structural Host Requirements and Insertion Thermodynamics

The reversibility and kinetics of multivalent storage depend critically on the choice of host. Key determinants include:

• Open-tunnel or layered frameworks: Structural motifs such as Wadsley–Roth shear phases (NTO), pillared birnessites, and engineered open-tunnel oxides (e.g., trigonal MoV$_3$O$_8$) offer large cross-sectional channels for low-barrier ion diffusion (Datta et al., 2023, Datta et al., 2024).
• Host-ion size matching: Tunnel area must accommodate hydrated or bare multivalent ions, with optimal cross sections typically $\gtrsim15$–$40\,\text{\AA}^2$ for Ca$^{2+}$, Al$^{3+}$ (Datta et al., 2023).
• Binding energy and charge transfer: Strongly coordinated ions may cause over-binding, lattice distortion, or trapping, while weak binding can lead to poor insertion voltage. For MoVO, Bader charge analysis shows higher charge transfer ($\Delta q$) per ion correlates to lower insertion energy; Al$^{3+}$ tends to over-bind in small tunnels, limiting reversibility (Datta et al., 2023).
• Electronic structure: Host band gap should be sufficiently narrow ($<$0.5 eV) to support fast charge propagation upon reduction (e.g., metallicity emerges in PTCDA$\cdot$(AlCl$_2$)$_2$ on discharge) (Canever et al., 2021, Datta et al., 2024).

Adsorption (insertion) voltages and theoretical capacities are determined from DFT-calculated insertion energies and the number of inserted ions per formula unit. For example, MoVO supports Ca$^{2+}$ insertion with voltages of $\sim4.1$–$4.0$ V, and calculated capacities (for two Al$^{3+}$) up to $\sim466$ mAh g$^{-1}$ (Datta et al., 2023).

3. Electrochemical Performance, Transport, and Stability

Multivalent battery systems demonstrate a spectrum of kinetic behaviors:

• Migration Barriers: NEB or approximate-NEB methods reveal Mg$^{2+}$ migration barriers ranging 200 to 1015 meV across candidate hosts, with the lowest (birnessite NaMn$_4$O$_8$, 200 meV) supporting high-rate operation (Rutt et al., 2022). Cr$^{3+}$ mobility in high-entropy alloys is strongly interface-dependent, with Cr$_2$O$_3$/Bi$_2$O$_3$ interfaces offering barriers as low as 0.50 eV (Anjan et al., 10 Jan 2026).
• Rate Capability: Organic hosts such as DNVBr (di-block bipyridinium–naphthalene diimide) exhibit ultrafast kinetics (diffusion coefficient $D\sim10^{-10}$–$10^{-9}$ cm$^2$/s, $R_\mathrm{ct}<50\;\Omega$), and maintain $>95$\% of capacity up to $32$C rate (Perticarari et al., 2019).
• Coulombic Efficiency and Cycle Life: Advanced electrodes achieve $>98$\% efficiency (>6500 cycles), as in DNVBr or MIBs (graphene/MnO$_2$), with high energy density (up to 1200 Wh kg$^{-1}$ for MIB) and minimal hysteresis (Chen et al., 2014, Perticarari et al., 2019). Cr-alloy anodes for Cr-ion batteries sustain 10,000 hours of plating/stripping at $\sim$20 mV overpotential (Anjan et al., 10 Jan 2026).
• Degradation: Practical capacity fade arises from electrolyte solubility (e.g., PTCDA dissolution in chloroaluminate IL, $\sim$5 mg/L at 25°C), Zn dendrite growth, or anode corrosion. Polymer supports and surface coatings are required to mitigate organic cathode dissolution (Canever et al., 2021, Perticarari et al., 2019).

4. AI-Driven Discovery and Computational Screening

Materials discovery for multivalent storage leverages high-throughput computational screening and machine learning:

• Frameworks: Computational pipelines use filtered databases (e.g., Materials Project, $\sim$70,000 entries) and multi-tier criteria: convex-hull stability ($E_\mathrm{hull}<0.2$ eV/atom), reducible high-valent cations, accessible insertion sites, and migration barrier cutoffs ($<650$–$800$ meV) (Rutt et al., 2022).
• AI Generative Models: Crystal Diffusion VAE and fine-tuned LLMs (e.g., LLaMA-3.1) generate, denoise, and filter open-tunnel oxide structures satisfying formation energy ($E_\mathrm{form}<-1.5$ eV/atom), band gap ($<0.5$ eV), and stability criteria. Candidate TMOs such as MgCuO$_2$F$_2$, Ca$_2$InO$_4$, and ZnTi$_2$O$_6$ are validated by DFT and graph-based surrogates (M3GNet) (Datta et al., 2024).
• Interpretable Predictions: Deep learning models (CGCNN) with transfer learning provide voltage predictions (MAE $<0.5$ V) for unexplored multivalent electrodes using only atomic structure, with interpretability analyses (PCA, t-SNE) highlighting atomic covalent radius as the dominant feature (Zhang et al., 2022). Such automated toolkits enable rapid voltage screening with limited data availability.

5. Design Principles and Comparative Perspectives

The fundamental design rules for high-performance multivalent storage encompass:

1. Host Structure Engineering:
   • Open, one-dimensional channels or layered π-stacked motifs to minimize migration barriers and mechanical strain (Datta et al., 2024, Datta et al., 2023).
   • “Soft” coordination sites (electron-rich, e.g., carbonyls, NDI blocks) for multivalent complex stabilization without trapping (Canever et al., 2021, Perticarari et al., 2019).
2. Electro-chemo-mechanical Considerations:
   • Optimized tunnel size and connectivity for target ion radii (e.g., Ca$^{2+}$, Al$^{3+}$), enabling both capacity and rate (Rutt et al., 2022, Datta et al., 2023).
   • Balanced capacity and stability through molecular weight tuning and host backbone selection (Canever et al., 2021).
3. Electronic and Ionic Conductivity:
   • Minimization of host band gap post-insertion; metallicity ("conductivity rise on discharge") ensures high-rate operation (Canever et al., 2021, Datta et al., 2024).
   • Hydrated ion co-insertion and solvent/anion co-storage play roles in buffering and kinetics, especially in aqueous systems (Perticarari et al., 2019).
4. Chemical Stability and Reversibility:
   • Polymer supports and molecular embedding to retard dissolution of organic species (Canever et al., 2021).
   • Alloyed or composite electrodes (e.g., Cr-HEA) to disrupt passivation and permit efficient multivalent exchange (Anjan et al., 10 Jan 2026).

Comparatively, multivalent systems such as MIB/MRR-MOR cells (4200 mAh/g, 1200 Wh/kg practical), Cr-ion cells (volumetric capacity $>10,000$ mAh/cm$^3$), and organic hybrids (DNVBr) outperform or rival Li-ion in density, rate, and longevity, but may require sophisticated synthesis, interface control, or electrolyte selection to achieve practical deployment (Chen et al., 2014, Anjan et al., 10 Jan 2026, Perticarari et al., 2019).

6. Outlook and Open Questions

Despite substantial scientific progress, several technical challenges and research frontiers remain:

• Mechanistic Elucidation: Further atomic-scale interrogation of nucleation, dissolution, and solid-electrolyte interphase formation in multivalent systems is warranted—particularly for complex-ion or anion/cation-coupled redox processes (Canever et al., 2021, Perticarari et al., 2019, Chen et al., 2014).
• Host Scaling and Stability: Robust synthesis of open-tunnel oxides, high-entropy alloys, and low-solubility organics at industrial scale with minimal defectivity remains an engineering bottleneck (Datta et al., 2024, Anjan et al., 10 Jan 2026).
• Electrolyte Compatibility: New electrolyte chemistries (e.g., superconcentrated, nonaqueous, ionic liquids) to support reversible multivalent-ion transfer without passivation, dendrite growth, or corrosion are an active target (Canever et al., 2021, Anjan et al., 10 Jan 2026).
• Multi-Principle Design via Machine Learning: Integration of generative AI models, ML surrogates, and transfer learning is accelerating the inverse design of hosts and electrolytes, but experimental validation is required to confirm survey-stage predictions (Datta et al., 2024, Zhang et al., 2022).

A plausible implication is that as computational screening, interpretable ML, and synthesis advance, new classes of multivalent energy storage devices—spanning aqueous, nonaqueous, inorganic, and organic platforms—will become increasingly accessible for high-density, sustainable energy storage applications.