How do Depth of Discharge, C-rate and Calendar Age Affect Capacity Retention, Impedance Growth, the Electrodes, and the Electrolyte in Li-Ion Cells?

Abstract

Lithium-ion cells testing under different state of charge ranges, C-rates and cycling temperature have different degrees of lithium inventory loss, impedance growth and active mass loss. Here, a large matrix of polycrystalline NMC622/natural graphite Li-ion pouch cells were tested with seven different state of charge ranges (0-25, 0-50, 0-75, 0-100, 75-100, 50-100 and 25-100%), three different C-rates and at two temperatures. First, capacity fade was compared to a model developed by Deshpande and Bernardi. Second, after 2.5 years of cycling, detailed analysis by dV/dQ analysis, lithium-ion differential thermal analysis, volume expansion by Archimedes’ principle, electrode stack growth, ultrasonic transmissivity and x-ray computed tomography were undertaken. These measurements enabled us to develop a complete picture of cell aging for these cells. This then led to an empirical predictive model for cell capacity loss versus SOC range and calendar age. Although these particular cells exhibited substantial positive electrode active mass loss, this did not play a role in capacity retention because the cells were anode limited during full discharge under all the tests carried out here. However, the positive electrode mass loss was strongly coupled to positive electrode swelling and electrolyte “unwetting” that would eventually cause dramatic failure.

Best practices for incremental capacity analysis

“…This publication will address the lower level by providing a guide on how to make sense of the voltage changes. ICA could not only be used to analyze aging (cycle or calendar) but also electrochemical milling (Dubarry et al, 2014), decomposition of additives (Khodr et al, 2020), lithium plating (Anseán et al, 2017;Chen et al, 2022), the impact of temperature (Dubarry et al, 2013;Fly et al, 2022;Gauthier et al, 2022), inhomogeneities (Fath et al, 2019), fast charge (Tanim et al, 2018), hysteresis (Dubarry et al, 2008), overdischarge (Zhang et al, 2022), and many other aspects.…”

Aging‐Driven Composition and Distribution Changes of Electrolyte and Graphite Anode in 18650‐Type Li‐Ion Batteries

“…In contrast, cell aging leads to substantially longer relaxation times. This could be explained with drying out of the aged cell, [ 27,28 ] where liquid electrolyte reacts to solid side products like the solid electrolyte interphase, which are believed to have a lower thermal conductivity than the pristine liquid electrolyte. The thermal gradients in the 18650‐type cells are known to be strongly non‐uniform in radial direction.…”

State of Health Estimation Methods for Lithium-Ion Batteries

“…Some papers discuss the simultaneous measurement of SOH and SOC of a battery. The topic of the equivalent circuit model for evaluating the SOH has previously explained the adaptive filtering approach [82,83]. It is used to process the dynamic behavior of the battery's physical properties.…”