The physical state of the electrodes plays an important part in the practical operation of a battery. The key characteristic of a battery electrode is that its surface area should be large. This lowers the series resistance, increases the area over which the chemical reaction can take place (hence also reducing the mass transport overvotlage). In addition, a large surface area helps ensure that the reactants are not completely covered by the products of the chemical reaction. A complete, uniform coverage of the electrode by the product reaction would prevent the redox reactions from proceeding, since the reactant species could not longer reach the electrode. Moreover, even in the reaction products allow the reactant species through, the reaction products are often not conductive, and therefore electrons evolved or required by the redox reactions could not pass through the reaction productions. A large surface area is typically achieved by using porous materials. The figure below shows the porous lead used in a lead acid battery.

PIX of porous materials.

During charging and discharging, several processes can occur that change the structure or shape of the electrode. In most battery reactions, the electrode materials undergo a physical change during the discharge/charge cycle. The changes to the electrode, both physical changes as the original electrode material is re-formed and chemical changes of the materials on the electrodes give rise to numerous non-idealities. A key non-ideality is that the material may change its morphology, potentially during deposition of the reaction products on the electrode, but more commonly when the electrode material remains unchanged for long periods of time. For example, in lead-acid batteries, lead sulfate, which forms as the battery is discharged, may form large, relatively insoluble crystals over time. These large crystals are difficult to convert back into lead or lead oxide, and hence they reduce battery capacity if the battery is left in its discharged state.  

Other effects that relate to the physical changes experienced by the electrode or electrolyte are that the reactant products seldom have the identical density as the reactants, and hence the electrode undergoes physical changes in its size. If the mechanical stresses are too large, the electrode material may flake off, hence permanently reducing capacity. The relative physical changes in size may be exacerbated at high or low temperatures, as density differences may increase as the temperate changes.

Finally, as the electrode material is re-formed during charging, the electrode may change its shape. In lead acid batteries, this is circumvented by the fact that the solubility of the lead ion Pb2+ is very low, and hence Pb2+ is rapidly converted to Pb in the close physical proximity to where it was dissolved, thus preventing significant changes of shape of the electrode. Alternately, either the products during discharging or the original battery material during charging may form so as isolate regions from charging or discharging, thus permanently reducing battery capacity.