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Silver-oxide battery

## Summary

Specific energy Silver oxide cells 130 Wh/kg[1] 500 Wh/L[1] High N/A Low High N/A 1.55V
Several sizes of button and coin cells, some of which are silver-oxide

A silver-oxide battery (IEC code: S) is a primary cell using silver oxide as the cathode material and zinc for the anode. These cells maintain a nearly constant nominal voltage during discharge until fully depleted.[2] They are available in small sizes as button cells, where the amount of silver used is minimal and not a significant contributor to the product cost.

Silver-oxide primary batteries account for 30% of all primary battery sales in Japan (64 out of 212 million in February 2020).[3]

Silver-oxide batteries were used on Apollo program lunar missions for the lunar module and lunar rover power supplies because of their high energy-to-weight ratio.[4][5]

## Chemistry

A silver-oxide battery uses silver(I) oxide as the positive electrode (cathode), zinc as the negative electrode (anode), plus an alkaline electrolyte, usually sodium hydroxide (NaOH) or potassium hydroxide (KOH). The silver is reduced at the cathode from Ag(I) to Ag, and the zinc is oxidized from Zn to Zn(II).

The half-cell reaction at the positive plate:

${\displaystyle {\ce {Ag2O + 2H+ + 2e- -> 2Ag (v) + H2O}}}$, ${\displaystyle (E^{\circ }=+1.17{\text{ V}})}$

The reaction in the electrolyte:

${\displaystyle {\ce {2H2O -> 2H+ + 2OH-}}}$, ${\displaystyle (E^{\circ }=-0.83{\text{ V}})}$

The half-cell reaction at the negative plate:

${\displaystyle {\ce {{Zn}+2OH^{-}->{\overset {Zinc~hydroxide}{Zn(OH)2}}+2e-}}}$, ${\displaystyle (E^{\circ }=-1.22{\text{ V}})}$

Overall reaction:

${\displaystyle {\ce {Zn + H2O + Ag2O -> Zn(OH)2 + 2Ag(v)}}}$, ${\displaystyle (E^{\circ }=+1.56{\text{ V}})}$

Overall reaction (anhydrous form):

${\displaystyle {\ce {Zn + Ag2O ->[{\ce {KOH/NaOH}}] ZnO + 2Ag (v)}}}$

## Mercury content

Silver-oxide battery without added mercury used to power a quartz watch movement

Until recently, all silver-oxide batteries contained up to 0.2% mercury.[citation needed] The mercury was incorporated into the zinc anode to inhibit corrosion in the alkaline environment. Sony started producing the first silver-oxide batteries without added mercury in 2004.[6]