What is the cycle life of a lithium-ion cell?
The cycle life of a battery is ultimately determined by what happens at the micro- and nano- scales inside the electrodes of the cells. In general, capacity degradation occurs in Li-ion cells because of the repeated swelling and shrinking that occurs at the smallest scales as the Lithium ions shuttle between the anode and cathode during charge and discharge. Both the anode and cathode are comprised of microparticles and nanoparticles. During charging, Lithium ions leave the cathode particles, travel through the electrolyte, and enter the anode particles. During discharging, the ions move in the opposite direction. This causes stresses that can lead to a variety of phenomena that reduce capacity including cracking of the electrode materials and peeling of the electrode from the substrate. Capacity degradation can also occur due to errant chemical reactions that occur over time, or at high or low voltages. These reactions include Lithium plating and degradation of the electrolyte. Impurities present in the cell during manufacturing – especially water – can also degrade capacity. Finally, thermal management within the cell is important, because extra stresses can be produced by differential swelling caused by large temperature variations.