11 For example, Panasonic NCA-based LiBs are used in Tesla Models S and X, whereas GM Bolt and Volkswagen ID3 cars are powered by NMC-based LiBs. Nickel-based layered oxides, i. e., LiO 2 ( a+ b+ c=1 NCM- abc) and LiO 2 (NCA), consolidated their status as the cathode material of choice for passenger EV batteries over the last decade, while gradually phasing out cubic spinel LiMn 2O 4 (LMO) and olivine LiFePO 4 (LFP) based systems. This requires insights into the respective half-cell chemistries. 10 Technically, the influence of SoC must be broken down into contributions from anode and cathode-related side reactions, which in turn are driven by the respective local overpotentials rather than the terminal cell voltage. 6, 9 Calendar ageing evolves at different rates and is strongly influenced by the storage state-of-charge (SoC) and temperature, with those rates being specific to each battery chemistry. In a first approximation, calendar and cycle ageing can be considered additive, although some interactions are expected. Mechanisms behind calendar ageing are nested in the cycle ageing process and continue to operate at every instant the battery is load-free. 8 While calendar ageing is dominated by parasitic reactions resulting from chemical/thermodynamic instabilities of cell components, cycling adds kinetically induced effects, such as concentration gradients and volume variations, the latter causing mechanical fatigue. 7 The impact of (idle) SoC on degradation is also important for grid-related storage, including EVs participating in vehicle to grid (V2G) schemes. Calendar ageing dominates battery degradation in many EVs, given that those might be parked >90 % of the time (undoubtedly even more during the recent COVID-19 pandemic). Typically, degradation studies can be divided into cycle and calendar ageing, in which batteries are either under load or at rest ( i. e., storage conditions, no current flow), respectively. Such data not only inform battery design but also risk calculations for battery-related insurance and warranty products, with an indirect impact on first and second-life battery costs. 4- 6 Quantifying the impact of working and environmental conditions on the battery state-of-health (SoH) is, therefore, vital to accelerate the learning curve that will inform more accurate life-estimation models and ultimately, more efficient strategies to prolong service life. 1- 3 How fast this threshold is reached will vary considerably depending on intrinsic factors, such as chemistry and manufacturing quality, and on extrinsic factors such as operation temperature, exposition to certain voltage/current windows, rest periods, and the presence of efficient battery (thermal) management systems. We discuss how reversible losses of lithium might play a role in alleviating the rate of irreversible losses on commercial cells.įor most applications of lithium-ion batteries (LiBs), such as electric vehicles (EVs), the end of life (EoL) criterion is defined as the decrease of the dischargeable capacity of the battery by as little as 20 % or 30 % of its initial value. Differential voltage analyses, apparent activation energy analysis, and endpoint slippage tracking provide useful insights into the degradation mechanisms and the respective roles of anode and cathode potential. Rates of capacity fade and their temperature dependencies are distinctly different for SoC values below and above 60 %, respectively. Degradation is slowest for cells stored close to 0 % SoC at all temperatures. Cells kept at 100 % SoC do not show the fastest capacity fade but develop internal short circuits for temperatures T≥40 ☌. Degradation is fastest for cells at 70–80 % SoC according to monthly electrochemical check-up tests. Lithium-nickel-cobalt-aluminium oxide (NCA) and graphite with silicon suboxide (Gr-SiO x) form cathodes and anodes of those cells, respectively. We report on the first year of calendar ageing of commercial high-energy 21700 lithium-ion cells, varying over eight state of charge (SoC) and three temperature values.
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