Degradation diagnostics for lithium ion cells

TLDR

Li‑ion battery degradation arises from complex physical and chemical mechanisms that manifest as loss of lithium inventory, loss of active positive electrode material, or loss of active negative electrode material, each presumed to uniquely affect the cell’s open‑circuit voltage, though this has largely been inferred rather than experimentally proven. This study experimentally validates the three canonical degradation modes by testing coin cells deliberately engineered with varying lithium inventory and active electrode material. The authors refine the theory linking degradation modes to OCV and develop a diagnostic algorithm that fits a cell’s OCV to identify and quantify each mode throughout its service life. The resulting algorithm successfully identifies and quantifies the extent of lithium inventory loss, positive electrode loss, and negative electrode loss in Li‑ion cells.

Abstract

Degradation in lithium ion (Li-ion) battery cells is the result of a complex interplay of a host of different physical and chemical mechanisms. The measurable, physical effects of these degradation mechanisms on the cell can be summarised in terms of three degradation modes, namely loss of lithium inventory, loss of active positive electrode material and loss of active negative electrode material. The different degradation modes are assumed to have unique and measurable effects on the open circuit voltage (OCV) of Li-ion cells and electrodes. The presumptive nature and extent of these effects has so far been based on logical arguments rather than experimental proof. This work presents, for the first time, experimental evidence supporting the widely reported degradation modes by means of tests conducted on coin cells, engineered to include different, known amounts of lithium inventory and active electrode material. Moreover, the general theory behind the effects of degradation modes on the OCV of cells and electrodes is refined and a diagnostic algorithm is devised, which allows the identification and quantification of the nature and extent of each degradation mode in Li-ion cells at any point in their service lives, by fitting the cells' OCV.