Why SoH_C and SoH_R Alone Are Not Enough - A Study on Cyclic Aging of Automotive Lithium-Ion Cells

Abstract

Lifetime validation is crucial to the development of lithium-ion cells, especially in the automotive context as high regulatory and warranty requirements have to be met. As the experimental lifetime validation can get tedious and costly, lifetime modelling is a way to cut down experimental effort and predict cell aging based on accelerated aging experiments with well selected aging conditions.Using these experiments with constant aging conditions, lifetime models are widely built in a path independent way, where the aging is expressed as a function of only the state of health in capacity (SoHC) and resistance (SoHR). The SoH hereby defines the relative capacity and resistance in comparison to their values at begin of life.Following a design of experiment approach, 62 large format NMC/graphite lithium-ion pouch-cells were tested at 28 different aging conditions, coming close to realistic cyclic aging conditions. This experiment was initially set up in order to build such an empirical lifetime prediction model, capable of predicting SoHC and SoHR in dependence of factors such as the state-of-charge window of the cyclation, the temperature and the charging power.Analyzing this accelerated aging experiment, we show that the definition of a state of health based on only capacity and inner resistance is falling short.This is deduced from differential voltage analysis (DVA) and post mortem analysis from cells with comparable SoH. With differential voltage analysis, changes in the electrochemically active parts of the lithium-ion cell that arise from aging and affect the open circuit potential can be visualized and assigned to certain parts of the cell without its destruction.Hereby a published fitting algorithm is used that can estimate losses of active material (LAM) and the loss of active, cyclable lithium (LL) from slow-discharge potential curves. Following the destructive post mortem analysis of a cell, we validate the results of this algorithm with 3-electrode-measurements.Together with further material analytics entirely different internal states become apparent when LAM on anode and cathode side as well as the loss of active lithium and its distribution in the electrodes are compared.Based on these findings we discuss the implication of the underlying variation in internal states at similar SoH of lithium-ion cells to the modelling of cyclic aging with a path-independent damage accumulation approach following Miner’s rule.