Changes in battery capacity directly influence their economic benefits, which is why understanding these changes is so important. One factor that affects capacity, especially early in a battery’s life, is the anode overhang effect. Considering this effect in State of Health (SOH) estimations ensures accurate estimation and prognosis of the SOH, as well as effective management strategies. 

Especially for battery energy storage systems (BESS), knowledge of the available capacity is key for optimal market performance. This article explains the anode overhang effect, its influence of the available capacity and how to manage it in the State of Health estimations.

What is the anode overhang effect?

The anode overhang effect refers to the intentional design choice where the anode extends beyond the edges of the cathode in a battery cell thus creating an overhang.This is done to mitigate the risks associated with a cathode overhang, which can lead to increased dendrite growth, accelerated aging, and potential safety concerns.

Two of the most important components of each battery cell are the two electrodes: the anode and the cathode. These thin layers are positioned on top of each other with a separator in between. Since tolerances need to be considered in every production, these layers aren’t perfectly aligned, raising the question of whether a slightly overhanging electrode (anode or cathode) can cause issues. It turns out, the answer is yes; an overhanging cathode can indeed cause trouble. 

Viewed locally at the edges, the cathode being larger than the anode increases the risk of dendrite growth, which accelerates aging and can become a safety concern. To avoid a cathode overhang, it is common practice to allow for anode overhang instead. 

Figure 1 shows a schematic view of the battery’s layers and overhang. The part of the anode opposite the cathode that participates in the charging/discharging reactions is called the “active surface area.” The overhanging surface area of the anode does not have an opposing cathode area and, hence, does not directly participate in the charging and discharging reactions.

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The overhanging anode solves any issues related to overhanging cathodes but introduces its own challenge: the anode overhang effect. Figure 2 illustrates a common scenario at the beginning of a battery cell’s life, assuming the battery was stored at a 50% state of charge (SoC) for a long time before use. 

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If the battery is kept long enough under these conditions, the lithium concentration in the anode evens out (homogenizes) across its entire surface area. If the battery is then operated around 20% SOC, previously inactive lithium from the anode overhang moves into the active area, temporarily increasing capacity. Conversely, if the battery is operated at 80% SoC, lithium moves from the active area to the inactive area of the overhang, temporarily decreasing capacity. All in all, the homogeneity of lithium distribution (HLD) on the anode significantly impacts usable capacity. 

The anode overhang effect isn’t just a concern at the beginning of the operation. If the operating window changes significantly, this effect fluctuates.  For example, batteries connected to photovoltaic systems have low average SoC in winter and high average SoC in summer. This causes capacity gains in winter and reversible losses in summer.

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