Lithium-ion Battery
A lithium-ion battery, also known as the Li-ion battery, is a type of secondary (rechargeable) battery composed of cells in which lithium ions move from the anode through an electrolyte to the cathode during discharge and back when charging.
The cathode is made of a composite material (an intercalated lithium compound) and defines the name of the Li-ion battery cell. The anode is usually made out of porous lithiated graphite. The electrolyte can be liquid, polymer, or solid. The separator is porous to enable the transport of lithium ions and prevents the cell from short-circuiting and thermal runaway.
Chemistry, performance, cost, and safety characteristics vary across types of lithium-ion batteries. Handheld electronics mostly use lithium polymer batteries (with a polymer gel as electrolyte), a lithium cobalt oxide (LiCoO2) cathode material, and a graphite anode, which offer high energy density.
Li-ion batteries, in general, have a high energy density, no memory effect, and low self-discharge. One of the most common types of cells is 18650 battery, which is used in many laptop computer batteries, cordless power tools, certain electric cars, electric kick scooters, most e-bikes, portable power banks, and LED flashlights. The nominal voltage is 3.7 V.
Note that non-rechargeable primary lithium batteries (like lithium button cells CR2032 3V) must be distinguished from secondary lithium-ion or lithium-polymer, which are rechargeable batteries. Primary lithium batteries contain metallic lithium, which lithium-ion batteries do not.
Degradation of Lithium-ion 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. Following the manufacturer’s recommendations is necessary to avoid danger or premature capacity degradation.
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.
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 the decomposition and dissolution of cathode material and capacity fading.
