Q.The button cell used in watches functions as follows: $\text{Zn(s)} + \text{Ag}_2\text{O(s)} + \text{H}_2\text{O(l)} \rightleftharpoons 2\text{Ag(s)} + \text{Zn}^{2+}\text{(aq)} + 2\text{OH}^{-}\text{(aq)}$ The half-cell potentials are $\text{Ag}_2\text{O(s)} + \text{H}_2\text{O(l)} + 2e^{-} \rightarrow 2\text{Ag(s)} + 2\text{OH}^{-}\text{(aq)}$, $E^{o} = 0.34$ V, and $\text{Zn(s)} \rightarrow \text{Zn}^{2+}\text{(aq)} + 2e^{-}$, $E^{o} = 0.76$ V. The cell potential will be

A battery is a packaged galvanic cell built to deliver steady direct current, split into primary batteries (non-rechargeable, discarded once discharged) and secondary batteries (rechargeable, since their cell reaction can be driven backward by an external potential greater than their own emf). The Leclanché (dry) cell — Zn anode, graphite/MnO2 cathode, NH4Cl/ZnCl2 paste electrolyte, ~1.5 V — is the classic primary cell; its emf drifts downward over its working life because its overall reaction, $\text{Zn} + 2\text{NH}_4^{+} + 2\text{MnO}_2 \rightarrow \text{Zn}^{2+} + \text{Mn}_2\text{O}_3 + \text{H}_2\text{O} + 2\text{NH}_3$, involves ions whose concentrations genuinely shift as discharge proceeds. The mercury button cell — Zn(Hg) anode, HgO/graphite cathode, KOH/ZnO electrolyte, ~1.35 V — is also primary, but its overall reaction, $\text{Zn} + \text{HgO} \rightarrow \text{ZnO} + \text{Hg}$, involves no ions at all, which is exactly why its voltage stays remarkably flat throughout discharge.

Of the secondary batteries, the lead storage battery (spongy Pb anode, PbO2 cathode, ~38% H2SO4 electrolyte, ~2 V per cell, six cells in series for 12 V) discharges via $\text{Pb} + \text{PbO}_2 + 4\text{H}^{+} + 2\text{SO}_4^{2-} \rightarrow 2\text{PbSO}_4 + 2\text{H}_2\text{O}$ and recharges by driving this reaction exactly backward, regenerating both the metallic electrodes and the sulphuric acid electrolyte. The lithium-ion battery (porous graphite anode, LiCoO2 cathode, Li-salt organic electrolyte) instead works by intercalation: Li⁺ ions shuttle from graphite to LiCoO2 during discharge and back again during charging, embedding themselves between graphite's carbon layers without breaking down the host structure — the mechanism behind its long cycle life and high energy density.