One of the biggest causes of degradation in lithium-ion batteries is elevated temperature. In electric vehicle, particularly hybrid vehicle applications this can be very difficult to manage, and without fundamentally redesigning the cell is the province of the battery pack engineer. Yet how battery pack design affects both cell performance and degradation has been very poorly studied in the past. The latest work of the electrochemical science & engineering group at Imperial College London on understanding how thermal gradients affect performance and degradation, and how thermal techniques can be used to detect and diagnose path dependent degradation will be presented. State-of-the-art models that can reproduce the diagnostic outputs and  can be embedded in battery management systems and could therefore be used for diagnostic and even prognostic purposes in real world battery packs will also be presented.

Recent work exploring the effect of cell surface cooling and cell tab cooling is shown, reproducing two typical cooling systems that are used in real-world battery packs. It is shown that cooling method alone can significantly affect useable capacity. For new cells at C/20 discharge, very little difference in capacity was seen, however, at 6C discharge surface cooling led to a loss of useable capacity of 9.2% compared to 1.2% for cell tab cooling. Furthermore, after cycling the cells for a 1,000 times at 6C discharge and 2C charge, surface cooling resulted in a degradation rate three times higher than cell tab cooling. Using incremental capacity analysis, electrochemical impedance analysis and thermal models of cells, it is shown that this is due to thermal gradients being perpendicular to the layers for surface cooling leading to higher local currents and faster degradation, but in-plane with the layers for tab cooling leading to more homogenous behaviour. Aged cells were put with new cells in a parallel pack configuration with cooling where measuring the current flowing through each individual cell is often impractical. Using a novel diagnostic method based on simple cell surface temperature measurements developed by our group, a diagnosis method capable of quantitatively determining the state-of-health of individual cells simultaneously during both charge and discharge by only using temperature and voltage readings, and whilst cells are being thermally managed, will be shown.

Our latest models account for complex electrochemical phenomena that are usually omitted in battery models such as variable double layer capacitance, the full current-overpotential relation and overpotentials due to mass transport limitations in the double layer. The coupled electrochemical and thermal model accounts for capacity fade via a loss in active species and for power fade via an increase in resistive solid electrolyte passivation layers at both electrodes. The model's capability to simulate cell behaviour under dynamic events is validated against test procedures, such as standard battery testing load cycles for current rates up to 20 C, as well as realistic automotive drive cycle loads, and ageing up to 1,000 cycles including reproducing incremental capacity analysis results.

References:

Module design and fault diagnosis in electric vehicle batteries, Journal of Power Sources, Vol:206, 2012, Pages:383-392

The effect of thermal gradients on the performance of battery packs in automotive applications, Journal of Power Sources, Vol:243, 2013, Pages:544-554

The effect of thermal gradients on the performance of lithium-ion batteries, Journal of Power Sources, Vol:247, 2014, Pages:1018-1025

Differential thermal voltammetry for tracking of degradation in lithium-ion batteries, Journal of Power Sources, Vol:273, 2015, Pages:495-501

Novel application of differential thermal voltammetry as an in-depth state-of-health diagnosis method for lithium-ion batteries, Journal of Power Sources, Accepted 23^rdDecember 2015