In lead–acid batteries, major aging processes, leading to gradual loss of performance, and eventually to the end of service life, are:••Anodic corrosion (of grids, plate-lugs, straps or posts).••In lead–acid batteries, major aging processes, leading to gradual loss of performance, and eventually to the end of service life, are:••Anodic corrosion (of grids, plate-lugs, straps or posts).••Positive active mass degradation and loss of adherence to the grid (shedding, sludging).••Irreversible formation of lead sulfate in the active mass (crystallization, sulfation).••Short-circuits.The lead–acid battery is an old system, and its aging processes have been thoroughly investigated. Reviews regarding aging mechanisms, and expected service life, are found in the monographs by Bode and Berndt, and elsewhere,. The present paper is an up-date, summarizing the present understanding. New aspects are: interpretation of hydrogen and oxygen evolution on open-circuit in terms of a thermodynamic “open-circuit hydrogen and oxygen over-voltage”, as influenced by acid concentration, and discussion of acid stratification considering the effects of diffusion potentials.The major aging processes in lead–acid batteries are:••Anodic corrosion (of grids, plate-lugs, straps, posts).••Positive active mass degradation (shedding, sludging) and loss of adherence to the grid.••2.1. Positive platesRegarding positive plates, grid corrosion is the “natural” aging mechanism, causing finally “natural” death. Metallic lead in the positive plate is thermodynamically unstable and anodic corrosion is thus practically unavoidable. Fortunately, the formed corrosion film is protecting the metallic substrate, such that corrosion kinetics becomes sufficiently slow, to allow satisfactory service life. Nevertheless, positive grid corrosion is probably still the most frequent, general cause of lead–acid battery failure, especially in prominent applications, such as for instance in automotive (SLI) batteries and in stand-by batteries. Pictures, as shown in Fig. 1 taken during post-mortem inspection, are familiar to every battery technician. One must, however, immediately add the remark, that (occasional) misuse, or abuse conditions, may strongly contribute to accelerated corrosion, as will be discussed in the following.Fig. 1. Corroded positive plate of a starter battery, at the end of 5 years of service life in a passenger car.In fully charged condition of the positive plates, a dense layer of PbO2 protects the positive grid from rapid anodic attack (Fig. 2). The interface between the metallic (lead alloy) grid and the PbO2 corrosion layer is thought to comprise a very thin interlayer, having a nominal composition of PbOx, whereby x has a value of 1–1.5. This interlayer has app. Loss of coherence between individual particles of the positive active mass, or loss of contact between positive active mass and grid, is a dominant aging factor in batteries subjected to cycling regimes. Transformation of PbO2 into PbSO4, during discharge occurs via a dissolution–precipitation mechanism,. PbSO4 has a completely different morphology and crystallographic structure, and occupies considerably more volume, than PbO2. Upon recharge, PbO2 may be re-deposited in a slightly different morphology than the one having existed before discharge. With continued cycling, this may lead to a morphological “shape-change” of the positive active mass. It has been speculated that the “necks” connecting individual PbO2 particles may slowly become thinner, resulting finally in loss of coherence between particles.With prolonged cycling, the positive mass will become softer and softer and will finally be subject to what is called “shedding” or “sludging”. Fig. 6 shows a “post-mortem” picture of a starter battery positive plate having served in a city bus for 6 months. In this application, the battery has experienced about 3000 shallow cycles (5–10% depth-of-discharge). The active material is totally disintegrated.It has been known for many years, that positive plates with antimony-free grid. The phenomenon called “sulfation” (or “sulfatation”) has plagued battery engineers for many years, and is still a major cause of failure of lead–acid batteries. The term “sulfation” described the condition of a battery plate, in which highly crystalline lead sulfate has formed in an practically irreversible manner. This type of lead sulfate cannot, or only partially, be reconverted back to an electrochemically active form, resulting in a corresponding loss of capacity. The danger of re-crystallization of PbSO4 always exists, when the plates remain in a (partly) discharged condition for prolonged periods of time. This can arrive, when batteries are not being charged sufficiently, or not frequently enough. In fact, the active material should, at least from time to time, be completely converted back to the charged state, to Pb in the negative plates, and to PbO2 in the positive plates, in order to avoid sulfation. Long periods of open-circuit stand, or long periods of discharge at very low rates, can result in sulfated plates, especially at elevated temperature. Self-discharge can thus be an important factor regarding sulfation. Charging rates and charging intervals must be such, as to compensate self-discharge.Negative plates have a larger tendency to become sulfated, than positive plates. In order to avoid formation of highly crystalline sulfate during cycling, the negative active mass must contain the so-called “expanders”, that is an additi.
