Understanding the effects of heterogeneities on battery pack lifetime

Abstract

Practical lithium-ion battery (LIB) systems require parallelisation of tens to hundreds of cells to achieve high capacities, however interconnection resistances, pack architecture and thermal gradients influence start of life (SoL) performance and subsequent degradation. The literature review found that parallel string degradation is a major underexplored area, motivating this work to develop understanding of the effects of heterogeneities on parallel-string degradation.

The tools to perform detailed investigations of the effects of heterogeneities and architecture were developed, including Peltier-based thermally controlled test benches, a quantitative diagnostic model, and an equivalent circuit network model. At SoL, thermal heterogeneities led to state of charge (SoC) deficits and reduction in accessible capacity. Interconnection overpotentials, cell open circuit voltages and cell impedance contribute to the pack current distribution. While coupled and non-linear, cell impedance is the dominant cause of heterogeneity. This study gives a detailed understanding of the coupled causes of current heterogeneities in parallel-strings.

An experimental study of parallel-string lifetime (temperature range 20-45 °C) shows thermal heterogeneities caused divergent capacity fade between parallel-connected cells. While growth of the solid-electrolyte interphase is generally quoted as the dominant degradation mechanism, for the nickel manganese cobalt oxide-graphite cells studied, impedance growth was negatively correlated with temperature, suggesting other dominant mechanisms. Consequently, increasingly heterogeneous current and SoC distributions occur during aging. Quantitative diagnostics indicate the dominant degradation mode transitions from loss of cathode active material at low temperature to loss of lithium inventory at increased temperatures. Therefore, consideration of cathode degradation is critical to understanding degradation within battery packs at intermediate temperature. A mechanistic model describing the observed degradation is proposed whilst the comprehensive aging dataset is made available.

This work thus highlights the importance of cathode polarisation resistance, which was previously not well captured, while developing understanding of lifetime behaviour in LIB packs and providing underpinning insights for developing longer lifetime packs.