Investigation of Path‐Dependent Degradation in Lithium‐Ion Batteries**

Abstract

Models that predict battery lifetime require knowledge of the causes of degradation and operating conditions that accelerate it. Batteries experience two ageing modes: calendar ageing at rest and cyclic ageing during the passage of current. Existing empirical ageing models treat these as independent, but degradation may be sensitive to their order and periodicity-a phenomenon that has been called "path dependence". This experimental study of path dependence probes whether interactions between ageing conditions can impact a battery's state of health. Groups of graphite/NCA 18650 lithium-ion cells were exposed to load profiles consisting of similar proportions of calendar and cyclic ageing applied in various orders. Load profiles at higher Crates exhibited path dependence, which differential voltage analysis correlates with increased anode degradation. These results suggest that more accurate ageing models should include the possible coupling between calendar and cyclic ageing modes.

An efficient and independent modeling method for lithium-ion battery degradation

“…That prominent peaks of the curve at C/10 are annotated as I, II + ①, III and IV, corresponding to the superimposition of the characteristic peaks of the graphite electrode and the broad curve of the NMC electrode [31,32]. Peak I can be attributed to the effect of the active materials and lithium-ion inventory [31], while the other peaks are mainly correlated to the active materials [7]. Obviously, with the increase of discharge rate, peak II + ① shifts to negative direction and covers peaks III and IV forming a composite peak noted as P 2 .…”

“…For lithium-ion batteries, the capacity loss is generally considered the result of loss of active material (LAM) and loss of lithium inventory (LLI) [6][7][8]. For LAM, the reduction of intercalation sites can be the main reason for its occurrence, which is caused by loss of electrical contact, passivating surface layers or structural disordering [9][10][11][12][13].…”

"Knees" in lithium-ion battery aging trajectories

“…166 Note that periodic rests interspersed throughout a cycling test can substantially improve lifetime in some cases. 167 Increased rest periods may delay the onset of knees driven by high overpotentials (e.g., rate-dependent lithium plating at high rates or low temperatures). For knees driven by side reaction product buildup (e.g., porosity decrease, positive electrode resistance growth, etc.…”

Elbows of Internal Resistance Rise Curves in Li-Ion Cells

“…As discussed in-depth in [19], a cell's capacity does not degrade linearly throughout its lifetime: degradation is path-dependent [20], and a strong association exists between capacity and internal resistance [21]. While the cell's capacity typically starts to degrade in a linear manner, there eventually comes a point, called the 'knee-point', after which the rate of capacity degradation increases considerably [22][23][24][25][26][27][28].…”