Understanding how these mechanisms arise and interact is essential for developing more durable, reliable and safe energy storage systems.
From the outside, battery ageing may seem like a straightforward process: after a certain number of cycles, the battery stores less energy and no longer delivers the same performance. Yet what the user perceives is merely the outward manifestation of a much more complex evolution. Inside the cell, chemical, electrochemical, thermal and mechanical transformations take place simultaneously, affecting the electrodes, electrolyte and interfaces. These processes do not necessarily progress at the same rate or stem from a single cause.
This is why speaking of “degradation” as though it were one single phenomenon can be misleading. The growth of interfacial layers, metallic lithium deposition, electrolyte degradation, changes in electrode volume and structural damage can coexist and reinforce one another. Temperature and operating conditions also influence the rate at which some of these processes evolve, meaning that two identical batteries can reach the same point in their service lives with very different internal states.
Consequently, chronological age or cycle count tells only part of the story. To truly understand why a battery has lost performance, we need to reconstruct how it has been used, what changes have occurred inside it and how those changes affect its behaviour. Reading the signs of ageing in this way allows us to move beyond observing a symptom to identifying its origin.
Every use leaves a trace inside the battery
Battery operation involves ion movement, charge transfer and reactions at the interfaces between different materials. This normal operation gradually changes the state of the cell. Some of these changes are inherent to its operation, while others may become more pronounced under more demanding conditions. Crucially, the outcome depends not only on how much a battery is used, but also on how it is used.
One phenomenon associated with ageing is the growth of the solid electrolyte interphase (SEI), an interfacial layer that forms on the anode and whose evolution can progressively consume active lithium. Although its initial formation is necessary for the stable operation of many lithium-ion batteries, continued growth reduces the amount of lithium available to participate in charge and discharge cycles.
Other mechanisms may occur alongside it, such as lithium plating, in which lithium is deposited as metal on the anode surface instead of being properly inserted into its structure. This phenomenon can reduce battery efficiency and, under certain conditions, affect safety. Electrolyte degradation is another relevant process, as various parasitic reactions can consume active components and contribute to a gradual increase in internal resistance.
Ageing also has a mechanical dimension. The materials that make up the electrodes undergo volume changes during charging and discharging, and repeated expansion and contraction can gradually alter their structure. At the anode, these changes can affect the integrity of the electrode and its interfaces, while at the cathode, mechanical fatigue can encourage crack formation and accelerate performance loss.
The cell is therefore not a static system. Its materials change over time, and these transformations can alter contact between components, ion and electron transport pathways and, ultimately, the overall behaviour of the battery.
Temperature introduces another decisive variable. Rather than always acting as an independent degradation mechanism, it can accelerate many of the reactions already taking place inside the cell. Prolonged exposure to high temperatures can promote parasitic processes, increase the rate of ageing and reduce available thermal safety margins, while other temperature conditions can affect the kinetics of electrochemical reactions.
This is why a battery’s thermal history matters just as much as its cycling history. Similarly, state of charge, cell chemistry, format, applied currents and depth of cycling all influence how the battery evolves over time. A single capacity measurement or a given number of cycles is therefore insufficient to describe its state of health fully.
This perspective changes how we understand service life. Instead of imagining an identical trajectory for every cell, it is more useful to consider different usage histories that leave different accumulated traces. A battery exposed to certain thermal and electrical conditions over several years may evolve differently from another battery using the same technology but operating under a different profile. In this sense, degradation is a physical memory of what has happened throughout the cell’s life.
