Degradation of lithium-ion batteries
The success of electric vehicles (EVs) depends largely on their energy storage system. Among electrical energy storage devices, lithium ion (Li-ion) batteries currently feature the best properties to meet the wide range of requirements specific to automotive applications – high energy density, long lifetime, good power capabilities and (relatively) low cost. However, safety and reliability of Li-ion batteries can be problematic if they are not handled appropriately. Exposing Li-ion batteries to extremely high or low temperatures, voltages or excessive currents results in accelerated battery degradation and in the worst case battery failure.
To prevent this, battery management systems (BMS) are implemented to monitor critical parameters and ensure safe operation. Moreover, the BMS has to provide information on current internal battery states to higher level systems in the vehicle. These states include the state of charge (SOC), which is equivalent to the fuel gauge in a battery, the total useful capacity (i.e. the amount of energy that can be stored in the battery) and the electric power that can be delivered at any point during vehicle operation. SOC, useful capacity and power capability are not measureable and must be estimated using elaborate battery models. The development of electrochemical and empirical battery models is part of our current research, pursued by Adrien Bizeray, Robert Richardson and myself (follow the link for example of literature). To further complicate the matter of battery modelling, the behaviour of the battery and its internal states change over time and usage, as the battery degrades. The battery’s power capabilities and useful capacity deteriorate and there is no practical method to measure the extent of this degradation. This is a big issue for EV developers, since reliable and accurate estimates of SOC, capacity and power capability must be available throughout the entire lifetime of the battery.
A large number of physical and chemical processes contribute to battery degradation. Figure 1 shows a summary of common degradation mechanisms in the most widely used Li-ion battery chemistries. These mechanisms depend on many factors, such as battery chemistry, fabrication, operating conditions, usage history, etc. The complexity and interdependence of the physical and chemical processes involved makes it very difficult to model, let alone predict battery degradation in terms of capacity and power fade. Furthermore, not only the symptoms of battery degradation are of interest, but also the physical processes at play must be identified. This is important since some processes, for example lithium plating and dendrite formation on the negative electrode, can cause internal short circuits, which may lead to the destruction of the battery.
Figure 1: Degradation mechanisms in Li-ion batteries.
For a Li-ion battery degradation model to truly advance the state of the art in BMS development, multiple objectives must be addressed; estimation of current useful battery capacity and power capabilities, prediction of the remaining battery life and possible battery failure. It is therefore necessary not only to quantify the symptoms of battery degradation (i.e. capacity and power fade) but also to identify their cause. As part of our current research we are working on establishing the links between physical degradation mechanisms and their effects on performance deterioration in Li-ion batteries. We trust that our efforts will contribute to make lithium-ion batteries a safer and more reliable option for energy storage in EVs.