Degradation of Rechargeable Batteries due to Cycling

An electric battery is essentially a source of DC electrical energy. It converts stored chemical energy into electrical energy through an electrochemical process. This then provides a source of electromotive force to enable currents to flow in electric and electronic circuits. A typical battery consists of one or more voltaic cells. 

The fundamental principle in an electrochemical cell is spontaneous redox reactions in two electrodes separated by an electrolyte, which is a substance that is ionic conductive and electrically insulated.

Degradation of Rechargeable Batteries due to Cycling

Some degradation of rechargeable batteries occurs on each charge–discharge cycle. Degradation usually occurs because electrolyte migrates away from the electrodes or because active material detaches from the electrodes. Manufacturers’ datasheet typically uses the word “cycle life” to specify lifespan in terms of the number of cycles to reach 80% of the rated battery capacity. To avoid danger or premature capacity degradation, it is necessary to follow the manufacturer’s recommendations.

In lithium-ion batteries, degradation and capacity fade are generally attributed to the growth of the solid electrolyte interface (SEI). The solid electrolyte interface is created due to reactions between the electrodes and the electrolyte. These reactions form a film that hinders lithium ions from reacting with the electrodes, and as this film grows in thickness, the cell degrades. 

Low-capacity NiMH batteries (1,700–2,000 mA·h) can be charged some 1,000 times, whereas high-capacity NiMH batteries (above 2,500 mA·h) last about 500 cycles. NiCd batteries tend to be rated for 1,000 cycles before their internal resistance permanently increases beyond usable values. Fast charging increases component changes, shortening battery lifespan. If a charger cannot detect when the battery is fully charged, then overcharging is likely, damaging it.

Most modern 18650 lithium-ion batteries, which are common for laptop batteries, have a typical cycle life of 300 – 500 (charge, discharge cycles). When in C-rate or high DOD situations, this can decrease substantially to 200 cycles.

In real life applications, Li-ion cells experience accelerated degradation due to certain stress factors. Stress factors such as deep DODs, elevated C-rates, high or low temperatures, and operating at high SOCs can have a negative impact on the cell capacity and cause accelerated degradation.

• Temperature. Degradation is strongly temperature-dependent: degradation at room temperature is minimal but increases for batteries stored or used in hot or cold environments. Batteries generate heat when being charged or discharged, especially at high currents. High temperatures during charging may lead to battery degradation, and charging at temperatures above 45 °C will degrade battery performance. Large battery packs, such as those used in electric vehicles, are generally equipped with thermal management systems that maintain a temperature between 15 °C (59 °F) and 35 °C (95 °F).
• Elevated C-rate. High C-rates generate more heat and cause the temperature of the cell to rise invoking the high-temperature degradation mechanisms. C-rate reduces the usable life and capacity of a battery.
• DOD. For lithium-ion batteries, the cycle life of a cell strongly depends on the DOD. The loss of lithium ions and active electrode material is higher for larger DOD cycles. At high DODs, additional degradation mechanisms can occur, resulting in decomposition and dissolution of cathode material and capacity fading.

Other Characteristics

To compare and understand the capability of each battery, some important parameters are characteristic of each battery, also within a type of battery. These parameters are a reference when a battery is needed, and specific qualities are required since batteries are used in all types of devices and for infinite purposes.