Battery degradation refers to the progressive reduction in a battery's ability to store and supply energy as time passes. As the battery deteriorates over time, its capacity to store energy diminishes, resulting in less effectiveness in powering devices. Battery deterioration is an inherent phenomenon that impacts all rechargeable batteries. However, the extent of degradation differs based on factors such as battery chemistry, usage patterns and environmental conditions.

Which types of batteries are most affected by degradation?

Although all rechargeable batteries undergo degradation, certain chemistries are more prone to it than others. Here are some prevalent categories:

• Lithium-ion (Li-ion): This is the most prevalent battery chemistry used in smartphones, laptops, electric vehicles and many other devices. Li-ion batteries are known for their high energy density, but they are also prone to degradation.
• Nickel-metal hydride (NiMH): These batteries were once popular but have been largely replaced by Li-ion. They exhibit a higher self-discharge rate compared to Li-ion, contributing to faster degradation.
• Lead-acid: Commonly used in cars, these batteries are susceptible to sulfation, a process that reduces battery capacity and performance.

What is the effect of degradation on battery components?

• Anode: During charging, Li ions can deposit on the anode as metal instead of intercalating within its structure. This reduces the amount of available Li for future cycles and weakens the anode structure.
• Cathode: The cathode material may deteriorate over time as a result of chemical reactions with the electrolyte. This can result in a decrease in efficiency and an elevation in internal impedance.
• Electrolyte: The electrolyte may deteriorate with time, particularly under elevated temperatures. This can result in the creation of solid deposits that impede the movement of ions and diminish the efficiency of the battery.
• Separator: The separator can become less effective at separating the anode and cathode as it ages. This can lead to internal short circuits and a sudden loss of battery capacity.

Is it possible to assess battery performance and degradation?

State of Charge (SOC) and State of Health (SOH) are two essential indicators for tracking battery performance and deterioration. By monitoring both the SOC and SOH of a battery, one can determine when it is nearing the end of its useful life. This helps to avoid overcharging and deep discharging, which can speed up the battery's deterioration. Additionally, techniques can be developed to optimize the lifespan.

SOC represents the amount of energy currently available in the battery, expressed as a percentage of its maximum capacity. It can be estimated using coulomb counting, which integrates the current flowing into or out of the battery over time to estimate SOC. Voltage measurement can also be used for its measurement. It uses the battery's voltage to estimate SOC based on a pre-determined voltage-SOC curve. Advanced techniques such as the Kalman filter are also repeatedly being used for more accurate SOC estimation.

SOH indicates the maximum capacity of a battery relative to its original capacity, expressed as a percentage. It can be measured using capacity comparison in which the current maximum capacity is compared with the initial rated capacity. Another method monitors the increase in internal resistance, which correlates with degradation. Open circuit voltage (OCV) relaxation method analyzes the voltage recovery after a discharge to estimate SOH. Similarly, discharge curve analysis compares the discharge curve of a new battery with the current one to identify capacity loss.

Which factors lead to battery degradation?

Calendar aging

Calendar aging refers to the degradation of a battery over time, even when it is not being used. This is caused by several factors. First, batteries experience self-discharge, meaning they gradually lose charge due to internal chemical reactions. Second, the electrolyte within the battery can break down, forming a solid-electrolyte interphase (SEI) layer on the anode. This layer consumes Li ions and increases internal resistance. Finally, the battery materials themselves can degrade over time due to exposure to air and moisture.

Cycle aging

Battery cycle aging is caused by the repetitive process of charging and discharging. This process can give rise to many complications. An issue frequently encountered is Li plating, which refers to the deposition of Li metal on the anode instead of undergoing normal intercalation. Dendrites can develop and penetrate the separator, resulting in a short circuit. Furthermore, the electrolyte progressively deteriorates with each cycle, resulting in the thickening of the SEI layer and a subsequent increase in internal resistance. Over time, the anode and cathode might experience active material depletion, resulting in a decrease in the battery's capacity.

Stress-induced degradation

Extreme operating conditions can accelerate battery degradation. High temperatures can speed up chemical reactions, leading to faster electrolyte breakdown and material degradation. Conversely, low temperatures can hinder Li-ion movement, increasing internal resistance and reducing capacity. High current rates can also induce stress, causing Li plating and heat generation. Finally, deep discharging can strain the electrode materials and contribute to capacity loss.

Conclusion

Battery degradation is the gradual loss of a battery's ability to hold and deliver energy. It's assessed by measuring SOC, remaining energy and SOH maximum capacity compared to new. Key degradation mechanisms include calendar aging (deterioration over time), cycle aging (wearing out from charging/discharging), and stress-induced aging (caused by extreme temperatures, currents or depths of discharge). The future holds promise for longer-lasting, safer batteries through advanced chemistries, improved materials and sophisticated thermal management systems.