Chromium-Ion Batteries

• Chromium-ion batteries are rechargeable electrochemical devices that exploit Cr³⁺/Cr⁰ multivalent redox reactions and high-entropy alloy anodes for high volumetric energy capacity.
• The Cr-HEA anode, composed of Cr, Bi, Cu, Sn, and Ni, forms stable multi-oxide heterointerfaces that facilitate reversible Cr³⁺ insertion while preventing passivation.
• Electrochemical tests reveal low overpotentials (~20 mV), exceptional cycle life (>5000 cycles), and competitive performance relative to other multivalent battery systems.

Chromium-ion batteries are a class of rechargeable electrochemical energy storage devices leveraging the multivalent redox chemistry of chromium, specifically Cr³⁺/Cr⁰, for high volumetric capacity. The advent of the first practical chromium-ion battery arose from the utilization of a chromium-rich high-entropy alloy (Cr–Bi–Cu–Sn–Ni, “Cr-HEA”) anode architecture that enables reversible Cr³⁺ insertion and extraction by circumventing the passivating native oxide blockade characteristic of pure chromium anodes. This system, when paired with a sulfur cathode, establishes a robust multivalent battery platform with competitive cycle life and energy storage metrics among multivalent chemistries (Anjan et al., 10 Jan 2026).

1. High-Entropy Alloy Anode: Composition, Structure, and Synthesis

The Cr-HEA anode comprises five elements with atomic fractions Cr₀.₄Bi₀.₁₅Cu₀.₁₅Ni₀.₁₅Sn₀.₁₅ (approximately 40 at.% Cr, 15 at.% each secondary element), yielding a configurational entropy $S_{\mathrm{conf}} \approx 1.504\,R$. The theoretical volumetric capacity is determined as

$$Q_{\mathrm{vol}} = Q_{\mathrm{grav}} \cdot \rho = \frac{n \cdot 26800}{MW} \cdot \rho \approx 4447\,\mathrm{mAh\,cm^{-3}}$$

where $n=3$ (number of exchanged electrons), $MW$ is the effective molecular weight, and $\rho \approx 7.19\,\mathrm{g\,cm^{-3}}$. Synthesis proceeds via scalable powder metallurgy: a ball-to-powder mass ratio of 10:1 in toluene, milling at 600 rpm for 20–40 hours, yielding an average particle size of $1.9 \pm 0.5\,\mu$m, much finer than commercial Cr powders (20–30 µm). Structural analysis (XRD) after ~20 h confirms a single-phase BCC solid solution; compositional uniformity is demonstrated via EDS, STEM-EELS, and APT.

2. Multi-Oxide Native Layer: Compositional and Interfacial Properties

Upon exposure to ambient conditions, the Cr-HEA surface develops a stable, multi-component oxide layer. XPS identifies the primary oxides as Cr₂O₃, Bi₂O₃, CuO, SnO₂, and NiO, with no Cr⁶⁺ species detected. First-principles analysis corroborates the thermodynamic stability of several heterointerfaces—Cr₂O₃∥Bi₂O₃, Cr₂O₃∥SnO₂, Cr₂O₃∥CuO—whereas pure Cr develops only a Cr₂O₃/Cr₂O₃ grain boundary network. The resulting complex interfacial architecture is pivotal for ion-transport selectivity in battery applications.

3. Ionic Transport at Heterointerfaces: Diffusion Barrier Landscape

The migration energy barriers for Cr³⁺ and O²⁻ across these interfaces, as calculated by nudged elastic band (NEB) methods employing ML interatomic potentials (MACE-MP-0), are critical in dictating lithium transport and oxide growth dynamics. Key findings include:

Interface	$E_B$(Cr, eV)	$E_B$(O, eV)
Cr₂O₃/Cr₂O₃ GB	3.83	0.68
Cr₂O₃/Bi₂O₃	0.50	0.95
Cr₂O₃/SnO₂	2.04	1.53
Cr₂O₃/CuO	2.05	3.65

Cr₂O₃/Bi₂O₃ interfaces facilitate rapid Cr³⁺ migration (barrier ~0.5 eV), while Cr₂O₃/CuO and Cr₂O₃/SnO₂ interfaces impose high oxygen barriers (1.53–3.65 eV), effectively arresting further oxide thickening. The resulting heterointerfacial landscape ensures efficient Cr³⁺ ingress/egress with suppressed parasitic passivation.

4. Electrochemical Characteristics and Cell Performance

4.1 Symmetric Half-Cell (Cr-HEA|Cr-HEA)

• Electrolyte: 2.0 M CrCl₃·6H₂O in DMSO
• Separator: GF/C glass fiber
• Tested current densities: 0.05, 1.0, 2.0 mA cm⁻²; optimum at 2 M salt
• Overpotential: ≈20 mV at 0.2 mA cm⁻² (versus >1.5 V for pure Cr at same conditions)
• Endurance: >5000 cycles (~10 000 h) with negligible voltage hysteresis
• Coulombic efficiency: ≈100%
• High areal capacity (6 mAh cm⁻²): maintains low hysteresis

4.2 Full-Cell with Sulfur Cathode (Cr-HEA‖S)

• Cathode: S/C composite, ≈1.2 mg S/cm² loading
• Anode: Cr-HEA ≈17 mg cm⁻² (≈6.8 mg Cr/cm²)
• Voltage window: 0.05–1.0 V
• CV peaks (0.5 mV s⁻¹): cathodic at ≈0.6 V (S₈→CrₓSₙ), ≈0.1 V; anodic at ≈0.4 and ≈0.8 V
• Initial specific capacity: ≈300 mAh g⁻¹ (S basis)
• After 150 cycles: ≈255 mAh g⁻¹ (83% capacity retention), 94.5% Coulombic efficiency
• Voltage profiles: stable across 50th, 100th, 150th cycles
• EIS: low and stable interfacial resistance (0.1 Hz–0.1 MHz)

5. Redox Processes and Reaction Mechanisms

The fundamental anode and cathode half-cell reactions are:

• Cr anode: $$\mathrm{Cr} \leftrightarrow \mathrm{Cr}^{3+} + 3e^{-}$$
• S cathode: $$3\mathrm{S} + 6e^{-} + 2\mathrm{Cr}^{3+} \leftrightarrow \mathrm{Cr}_{2}\mathrm{S}_{3}$$
• Full-cell: $$2\mathrm{Cr} + 3\mathrm{S} \leftrightarrow \mathrm{Cr}_{2}\mathrm{S}_{3}$$

These processes are fully reversible within the experimental voltage window, as evidenced by electrochemical and impedance metrics.

6. Implications for Multivalent Ion Energy Storage

Chromium-based systems offer a theoretical volumetric capacity ($\sim11\,117\,\mathrm{mAh\,cm^{-3}}$ for Cr³⁺) surpassing other multivalent cations (Al³⁺: ~8 046 mAh cm⁻³; Mg²⁺: ~3 826 mAh cm⁻³; Zn²⁺: ~5 854 mAh cm⁻³). Cr³⁺ ionic charge density ($\sim505\,\mathrm{C\,mm^{-3}}$) also favors superior charge transport. The Cr-HEA platform overcomes the persistent challenge of surface passivation that has impeded rechargeable Cr anodes, yielding performance (20 mV overpotential, >5 000 cycles) competitive with or superior to state-of-the-art Mg-, Al-, and Zn-based batteries—where typical overpotentials exceed 100 mV or cycle life is notably limited.

7. Prospects and Research Directions

The development of Cr-HEA electrodes establishes a methodology for exploiting multi-component oxide heterointerfaces to tailor ion transport while mitigating adverse surface reactions. The demonstrated selectivity—facile Cr³⁺ conduction plus suppressed O²⁻ transport—provides a template for extending high-entropy alloy strategies to other multivalent chemistries. The high cycle life, low polarization, and scalability (via powder metallurgy) point to significant applicability in fundamental and applied multivalent energy storage research, and facilitate high-throughput exploration of multicomponent electrode architectures in future battery design (Anjan et al., 10 Jan 2026).