Silver-Zinc Battery - Battery Design

The silver-zinc (AgZn) battery pairs a silver oxide cathode with a zinc anode in a concentrated potassium hydroxide electrolyte. Its main claim to fame is specific energy: cell for cell, it is still among the highest of any commercially available battery chemistry, rechargeable or otherwise. That has made it a niche but persistent choice wherever mass matters far more than cost or cycle life – crewed spacecraft, torpedoes, submarines, and, more recently, hearing aids and other miniature electronics.

Electrochemistry

A silver-zinc cell is manufactured in a fully discharged state, which is the reverse of how most cells are built. The as-built cathode is metallic silver and the anode is a mix of zinc oxide and zinc powder, sitting either side of a separator (historically cellophane) soaked in a 35–45 wt% potassium hydroxide solution [1].

Charging drives the cathode reaction in two steps: metallic silver first oxidises to silver(I) oxide (Ag₂O), then further to silver(II) oxide (AgO), giving two distinct voltage plateaus around 1.8 V and 1.5 V respectively [1][2]. At the anode, zinc oxide is reduced to metallic zinc via a soluble zincate intermediate. Charging is deliberately stopped once cell voltage approaches roughly 1.55–2.0 V, because pushing further starts to decompose the electrolyte itself and generates oxygen – both a capacity dead-end and a pressure and fire hazard inside a sealed cell [2].

Key Performance Characteristics

Nominal operating voltage: ~1.65 V, bounded by roughly 1.2 V at end-of-discharge and 2.0 V at end-of-charge [1]
Specific energy: among the highest of any commercial rechargeable chemistry at the cell level, though published figures vary a lot by format – large aerospace-era cells and modern miniature button cells are not really comparable on a like-for-like basis [1]
Cycle life: historically poor in large-format cells – GM’s 1960s Electrovair test EV, for example, only got about 100 charge-discharge cycles out of its pack before the batteries were spent [3][4]. Modern miniature rechargeable silver-zinc cells from specialist manufacturers claim much better retention, with one manufacturer reporting over 98% capacity retention past 300 cycles in button-cell testing [1]
Electrolyte: aqueous KOH rather than a flammable organic solvent, which removes the thermal runaway failure mode associated with Li-ion [1]

Parameter	Value / Range	Notes / Context
Nominal Cell Voltage	1.50 – 1.55 V	Initial stage at 1.85 V followed by main plateau at 1.50 V
Specific Energy (Cell)	100 – 130 Wh/kg	High active material mass density partially offsets lower voltage
Energy Density (Cell)	220 – 300 Wh/L	Exceptionally dense electrode stack layout
Operating Temperature	-20 to +60 °C	Excellent power delivery retained at elevated industrial temps
Discharge Rate Capability	1C – 20C continuous	Extremely high ionic conductivity of alkaline KOH aqueous melt
Cycle Life	100 – 300 cycles	Constrained by silver dissolution/dendrites piercing cellulose separators
Float / Standby Life	1 – 2 years (activated)	Silver ions slowly dissolve and diffuse across the membrane
Cathode Mechanism	Dual redox conversion	AgO + H₂O + 2e^– ⇌ Ag + 2 OH^–

Advantages

Very high specific energy and energy density relative to other commercial rechargeable chemistries
Can sustain high discharge rates, which is part of why the chemistry ended up in torpedoes rather than just spacecraft
Aqueous electrolyte gives a reduced flammability risk compared with Li-ion
Performance holds up well even at very small (button-cell) sizes, because the electrodes are intrinsically dense rather than relying on a wound or stacked jelly-roll geometry the way Li-ion does [1]

Limitations

Silver is expensive, which has confined the chemistry to applications where mass and volume savings justify the cost, or to very small cells where the absolute silver content – and hence cost – stays low
Cycle life in large-format cells has historically been short; this is being addressed in miniature formats but hasn’t been demonstrated at automotive or grid scale
Manufactured in a discharged state, so cells need an initial formation charge before use

Applications and History

Primary (non-rechargeable) silver-zinc cells powered some of the earliest orbital and crewed spaceflight hardware: the first Sputnik satellites, the US Saturn launch vehicles, the Apollo Lunar Module, the lunar rover, and the Apollo life-support backpack [5]. In the Apollo Command Module, hydrogen-oxygen fuel cells did most of the work, but silver-zinc batteries covered peak-power demands the fuel cells couldn’t meet and became the Command Module’s sole power source during re-entry after the Service Module separated – and these were the only batteries on board that got recharged in flight. After the Apollo 13 accident, an additional silver-zinc battery was added to the Service Module as a fuel-cell backup [5].

On the military side, silver-zinc cells have been used in Mark 37 torpedoes and on Alfa-class submarines, applications that share the same underlying logic as spaceflight: high power and energy density matter far more than cost or a long service life [5].

The chemistry’s cycle-life limitation is well illustrated by General Motors’ Electrovair, a 1960s experimental electric conversion of the Corvair that used a silver-zinc pack from Eagle-Picher. It worked, but the pack was expensive and wore out after roughly 100 charge-discharge cycles, which was one of the reasons the project didn’t go anywhere near production [3][4].

More recently, rechargeable silver-zinc has found a second life in miniature electronics – hearing aids in particular – where its energy density advantage over Li-ion at small volumes and its non-flammable electrolyte are both genuine selling points, and where the small absolute amount of silver in each cell keeps material cost manageable [1].

References

Ortega, J. & Dueber, R. (ZPower, LLC), Energy Density Comparison of Silver-Zinc Button Cells with Rechargeable Li-Ion and Li-Polymer Coin and Miniature Prismatic Cells, Battery Power Online, 2015
Silver zinc battery, Wikipedia
Rishavy, E.A., Bond, W.D., Zechin, T.A., Electrovair – A Battery Electric Car, SAE Technical Paper 670175, 1967
Murray, C.J., Long Hard Road: The Lithium-Ion Battery and the Electric Car, Purdue University Press, 2022
Dicicco, M., NASA Research Helps Take Silver-Zinc Batteries from Idea to the Shelf, NASA, 2016

Needs review: published specific energy figures for silver-zinc vary considerably by source and cell format (large aerospace-era cells vs. modern miniature button cells aren’t directly comparable), so a single headline Wh/kg number has deliberately been left out above. If you have a preferred authoritative figure or dataset for a specific format, it would be worth adding a proper comparison table here.