Ammonium-Ion Battery - Battery Design

Ammonium-ion batteries (AIBs) are an emerging class of aqueous rechargeable energy storage systems that use the ammonium cation (NH₄⁺) as the charge-carrying ion. Unlike conventional metallic-ion batteries, AIBs employ a non-metallic carrier, which offers inherent advantages in safety, cost, and environmental impact. The technology has attracted significant academic interest since approximately 2018 as a candidate for grid-scale and portable storage applications.

Overview

In an ammonium-ion battery, NH₄⁺ ions shuttle between the anode and cathode during charge and discharge. The chemistry is analogous to lithium-ion or sodium-ion intercalation, but with the substitution of a polyatomic non-metallic ion. This eliminates the use of lithium or sodium metal, which reduces supply-chain sensitivity and removes dendrite-related safety risks associated with metallic anodes.

Aqueous electrolytes are typically used — commonly ammonium sulfate ((NH₄)₂SO₄) or ammonium acetate (CH₃COONH₄) solutions — which further improves safety relative to organic electrolyte lithium-ion cells.

Electrochemistry

NH₄⁺ ions intercalate into layered or open-framework host materials at the cathode during discharge; the process is reversed during charging. Host materials under investigation include:

MnO₂ polymorphs (α, β, δ): manganese dioxide provides a tunnel or layered structure suited to NH₄⁺ insertion, though Mn dissolution at lower pH limits cycle life.
Vanadium oxides (V₂O₅, VO₂): high theoretical capacity but subject to structural degradation over cycling.
Prussian blue analogues (PBAs): open-framework cyanometalates that accommodate the relatively large NH₄⁺ ion (hydrated ionic radius ~1.25 Å) with minimal lattice strain.
MXenes and 2D materials: emerging cathode candidates offering large interlayer spacing and tunable electrochemistry.

The anode most commonly used is zinc metal or activated carbon, though dedicated ammonium-storing anode materials remain an active research challenge.

Key Performance Characteristics

Nominal voltage: 0.8–1.5 V (cell), depending on cathode material
Specific capacity (cathode): 150–300 mAh g⁻¹ (theoretical); practical values lower
Electrolyte: Aqueous ammonium salt solutions (pH 5–7)
Operating temperature: -20°C to 60°C (aqueous electrolyte window)
Cycle life: 500–2000 cycles reported at laboratory scale
Self-discharge: Moderate; comparable to zinc-ion aqueous systems

Advantages

No metallic lithium or sodium: eliminates flammability and dendrite hazards
Low-cost, abundant raw materials: nitrogen is the most abundant element in Earth’s atmosphere
Aqueous electrolyte: non-flammable, low viscosity, high ionic conductivity
Low molar mass of NH₄⁺ (18 g mol⁻¹): comparable to Li⁺; favourable for gravimetric capacity calculations
Small hydrated radius enables fast ionic diffusion through many host lattices

Limitations

Narrow electrochemical stability window of water (~1.23 V) caps cell voltage and energy density
Limited choice of anode materials: no established high-capacity NH₄⁺-storing anode analogous to graphite
NH₄⁺ is a weaker Lewis acid than Li⁺; intercalation thermodynamics into many layered materials are less favourable
Proton co-insertion competes with NH₄⁺ in acidic electrolytes, complicating reaction mechanisms
Technology maturity: no commercial cells; predominantly laboratory-scale demonstrations

Applications

The primary target application is stationary grid-scale energy storage, where low cost, safety, and long calendar life outweigh the energy density limitations. Flexible and wearable electronics are a secondary target, given the benign nature of aqueous electrolytes. AIBs are not suitable for high-energy-density applications such as electric vehicle propulsion.

References

Kulkarni, P. et al. “Toward Safe and Reliable Aqueous Ammonium Ion Energy Storage Systems.” Advanced Energy Materials, 2024. doi:10.1002/aenm.202400702
Sun, W. et al. “Recent Progress in Aqueous Ammonium-Ion Batteries.” ACS Omega, 2022. doi:10.1021/acsomega.2c04118
Zhang, Y. et al. “Challenges and opportunities in 2D materials for high-performance aqueous ammonium ion batteries.” National Science Review, 2024. doi:10.1093/nsr/nwae433
Liang, G. et al. “An acetate electrolyte for enhanced pseudocapacitive capacity in aqueous ammonium ion batteries.” Nature Communications, 2024. doi:10.1038/s41467-024-46317-5
Wu, L. et al. “Research progress of cathode materials for ammonium-ion batteries.” Journal of Energy Storage, 2024. doi:10.1016/j.est.2024.112819