Internal resistance in lithium-ion batteries undergoes changes as the battery goes through charge-discharge cycles. These changes are influenced by various factors, including the battery's chemistry, operating conditions, and user behavior.
Key stages of internal resistance evolution during battery usage cycles:
Formation Stage:
During the initial cycles, batteries undergo a formation process where the solid-electrolyte interface (SEI) layer forms and stabilizes. At this stage, the SEI layer is not fully developed, resulting in a higher internal resistance.
Early Cycles:
During the early cycles, internal resistance exhibits notable changes. The active materials within the electrodes undergo structural rearrangement, allowing for improved ion and electron transport. Consequently, internal resistance decreases as the battery's electrochemical pathways become more efficient. However, factors such as electrolyte depletion and electrode degradation may contribute to a slight increase in internal resistance during this stage.
Mid-Life Cycles:
The internal resistance tends to stabilize at a relatively low level. The SEI layer becomes well-formed and provides a stable interface for ion transfer. At this point, the internal resistance remains relatively constant unless external factors, such as extreme temperature conditions or high-rate charging/discharging, accelerate aging mechanisms.
Aging and End of Life:
With extended usage and aging, internal resistance experiences a gradual increase. Irreversible processes, including electrode degradation, loss of active material, and changes in the electrolyte, contribute to this increase. As internal resistance rises, the battery's capacity diminishes, power output decreases, and overall performance declines. High SoCs and elevated operating temperatures further accelerate the aging process and exacerbate the rise in internal resistance.
Internal Resistance Variation with State of Charge (SoC):
Low SoC:
At low SoCs, internal resistance tends to be relatively higher due to the limited availability of active lithium ions. The reduced concentration of lithium ions within the electrodes and electrolyte hinders their efficient transport, resulting in increased resistance.
High SoC:
At high SoCs, internal resistance can also increase, primarily due to factors such as electrode overpotential and electrolyte limitations. The accumulation of unwanted by-products and increased electrochemical stress contribute to the rise in resistance.