Investigating the Interaction between Degradation and Thermal Behaviour in Lithium-Ion Batteries
Abstract
The overall aim of this thesis is to generate knowledge on the interplay of battery degradation and thermal behaviour. We focused on the temperature dependency of degradation on the one hand and on the effect of degradation on heat generation, thermal transport and temperature distribution inside Lithium-ion Batteries (LiB) on the other.
In Article I., we reviewed studies investigating the capacity loss and resistance increase of commercial LiB due to both cycling and calendric degradation, covering different anode and cathode active materials, geometries and nominal capacities. The sensitivity of degradation behaviour towards operation conditions was examined. The potential thermal effect of C-rate-induced degradation was investigated in the light of a sufficiently simplified thermal battery model. We pointed out the need for more comparable data and experimental studies for deeper analysis - including an estimate of the actual cell temperature rather than the ambient temperature, as we found the difference between ambient temperature and actual cell temperature can vary significantly. We also pointed out the challenge both when comparing different studies and when transferring results from laboratory experiments to real applications, as cooling conditions differ. Therefore, measuring and reporting the actual battery (surface) temperature was found to be crucial to allow for a proper interpretation of results, especially when evaluating charging and discharging at high rates.
Based on the knowledge about the importance of the actual battery temperature for degradation from Article I., we conducted a degradation study focusing on the changes in battery temperature as part of Article II.. This was done by conducting a degradation study for a wide spread of thermal management strategies and charging and discharging rates, with a special focus on measuring and reporting the battery temperature. We assessed the effect of degradation on heat generation and temperature distribution in LiB. We found that for cells with LCO cathodes, entropic heating plays a major role in the overall heat generation - causing large variations in the heat generation rate and therefore battery temperature over the State-of-Charge (SoC). The maximum battery temperature was found to increase when the cell degrades as irreversible heat generation increases. It appeared that a maximum in heat generation during a full discharge was reached at a SoH between 75 % and 85 % for cells cycled with C-rates greater than 1. Temperature variation over the thickness of the cell appeared to be more substantial than in-plane and it was found that the variation was increasing significantly once the cell degraded. Degraded cells also showed significant thickness increases, where higher temperatures resulted in larger thickness increases with clear signs of gassing.
The changes in thermal transport due to degradation were examined in Article III., specifically the effect of thermal management and clamping. The thermal conductivities of electrodes extracted from cells both at Beginning-of-Life (BoL) and End-of-Life (EoL) were measured, as well as the overall cell cooling efficiency. The effective full-cell thermal conductivities were calculated, and the changes due to degradation were compared to measured changes in overall cell cooling efficiency. A reduction of thermal conductivity of up to 65 % compared to BoL was found. The reduction appeared to be the most extreme for dry graphite anodes. It was found that clamping cells during cycling has the potential to mitigate the reduction. The thermal conductivity of graphite anodes cycled at cold temperatures appeared to reduce significantly less than when the cell was cycled at moderate or high temperatures. The overall cell cooling efficiency was reduced by 50 % from BoL to a SoH of 70 to 75 %. It was found that the main reduction in the thermal conductivity of the full cell is due to changes in the anode and a reduction in the electrolyte solvent.
Finally, the role of the ambient temperature on degradation was further examined in Article IV., utilizing incremental capacity analysis (ICA). We showed how incremental capacity (IC) simulations in combination with analyzing Features-of-Interest (FoI) within the IC curves enables the observation of differences that are difficult to identify from the IC curves alone. The importance of differentiating between degradation and capacity loss became very important: Despite the rate of capacity loss appearing to be very similar for clamped cells cycled at 15 ◦C and 25 ◦C, as well as 5 ◦C and 35 ◦C, the degradation modes and mechanisms appeared to be very different. Using ICA, the degradation of cells cycled at 25 ◦C and higher, as well as 15 ◦C and lower, each show similar degradation behaviour. In general, Loss of Lithium Inventory (LLI) appeared to be the main degradation mechanism, especially for cells cycled at higher temperatures, while the cells cycled at lower temperatures indicated a large degree of inhomogeneous degradation within the single cells. With a suitable Features-of-Interest (FoI) versus State-of-Health (SoH) map, the FoI appeared to enable a simple approach to identify if cells have been pressurized during the use phase and narrow down the temperature range a cell has been cycled at. A possible option for applying this process is identifying cells with unknown cycling histories suitable for second-life applications.
Has parts
Paper 1: Spitthoff, Lena; Shearing, Paul Robert; Burheim, Odne Stokke. Temperature, Ageing and Thermal Management of Lithium-Ion Batteries. Energies 2021 ;Volum 14.(5) s. - This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license.Paper 2: Spitthoff, Lena; Wahl, Markus Solberg; Lamb, Jacob Joseph; Shearing, Paul Robert; Vie, Preben Joakim Svela; Burheim, Odne Stokke. On the Relations between Lithium-Ion Battery Reaction Entropy, Surface Temperatures and Degradation. Batteries 2023 ;Volum 9.(5) s. - This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license
Paper 3: Spitthoff, Lena; Wahl, Markus Solberg; Vie, Preben Joakim Svela; Burheim, Odne Stokke. Thermal transport in lithium-ion batteries: The effect of degradation. Journal of Power Sources 2023 ;Volum 577. s. - This is an open access article under the CC BY license.
Paper 4: Spitthoff, Lena; Vie, Preben Joakim Svela; Wahl, Markus Solberg; Wind, Julia; Burheim, Odne Stokke. Incremental capacity analysis (dQ/dV) as a tool for analysing the effect of ambient temperature and mechanical clamping on degradation. Journal of Electroanalytical Chemistry 2023 ;Volum 944. s. - This is an open access article under the CC BY license.