Battery degradation can be a significant issue for electric vehicle sustainability, so it’s essential to have an in-depth understanding of what causes it and how to prevent it.
Battery degradation is caused primarily by an interaction of electrochemical, thermal and mechanical factors. As such, it involves multiple degradation mechanisms, each with their own feedback loops and interactions.
Battery Life Expectancy
Battery life expectancy is a term manufacturers use to indicate how long an electric vehicle battery will last before it begins to degrade. Estimates vary based on several factors, such as the type of battery, how it’s charged and used, storage/operational temperatures and cycles.
Calculating battery life is usually done by charging the car up to 100 percent and checking its displayed range against what EPA-rated for your car. While this method is imperfect, it provides a useful starting point in determining how long your battery will last.
Some manufacturers, such as Hyundai and Tesla, provide guarantees on the battery lifespan of their electric vehicles (EVs). These warranties usually last eight years or 160,000km – whichever comes first.
In many cases, warranties also state that a battery won’t degrade to less than 70 percent of its original charge capacity during the warranty period. It’s essential to remember, though, that this figure is arbitrary and some vehicles do lose some capacity over time – though usually less than 70 percent.
One study published in the Journal of Cleaner Production revealed that electric vehicle (EV) batteries degrade around 2.3% annually, comparable to how much a normal car’s battery loses over its lifespan (though some types may experience greater losses). Heat and fast-charging were responsible for more degradation than age or mileage; so driving less or reducing how often you charge your EV could help extend its lifespan.
It’s worth noting that where you live has an impact on battery lifespan. If you live in a desert climate or even colder environments, expect shorter battery life for your electric vehicle (EV).
Degradation Causes
Batteries are indispensable in our everyday lives, from laptops and cell phones to medical devices and satellites. They even play an essential role in powering electric vehicles (EVs). Unfortunately, they are not indestructible – meaning we must maintain them carefully or else the power could go out unexpectedly and leave us stranded.
Battery degradation occurs at different rates, depending on the battery type and application environment. It’s an intricate problem involving many electrochemical, thermal, and mechanical modes that are all interrelated. Factors like temperature, electrochemical operating windows, and charge/discharge rates all influence how quickly a battery degrades.
Li-ion batteries typically experience capacity fade due to calendar aging, cycle aging and temperature effects. This results in reduced output power as well as longer charging times due to a decreased amount of lithium available for chemical reactions that consume lithium during charging and discharging cycles.
Due to this reduction in battery capacity, electric vehicle operations require more energy consumption and generate GHG emissions per km driven, leading to an increase in unit electricity GHG emission factor (CO2,eq g km-1).
Calendar aging is the initial stage of battery degradation that takes place over time and with charge-discharge cycles. It has a significant effect on battery performance and life.
Cycle aging is the next stage in battery degradation and it is caused by both high and low cycling conditions. This process also causes a reduction in capacity, but it occurs faster than the calendar aging cycle does.
Temperature has a major role in the durability of batteries. For optimal performance, lithium-ion batteries should be stored and charged between 15degC and 35degC.
When operating outside this temperature range, both electrolyte ionic conductivity and lithium-ion diffusivity at electrodes decrease. This slows down lithium intercalation into the anode, leading to a shorter battery lifespan than when operated at optimal temperatures.
High temperature and excessively high voltages can accelerate battery degradation, while moisture contamination on the electrodes may contribute to further impairment. This leads to hydrogen gas formation inside the battery which is highly hazardous for its health.
Degradation Modes
Battery degradation is a multi-factorial process, and several factors influence its severity. These include temperature, load currents, duty cycles, depth of discharges and cut-off voltages – all of which can inhibit or accelerate specific degradation mechanisms and should be taken into account when developing lifetime prediction models.
One way to approach this problem is through modes of degradation: grouping degradation mechanisms according to their overall impact on a cell’s thermodynamic and kinetic behavior. Loss of active material (LAM) groups mechanisms which lead to reduced active material for redox cycling at both electrodes; loss of lithium inventory (LLI) groups processes that result in less cyclable lithium available for transport between electrodes.
Each of these modes produces operational effects that can be measured through changes in cell impedance and stoichiometric drift. Furthermore, other modes are also detectable at the cell level.
One mode that has gained much attention recently is faradaic rate degradation, which occurs when electrodes fail to react with lithium ions at the same rate they did at startup. This can occur due to various reasons such as SEI growth, pore blockage and electrolyte loss at the interface.
Another mode is stoichiometric drift, which occurs when lithium moves between electrodes and causes a shift in the cell’s redox potential. This has been identified as one cause of power fade in some batteries and may serve as an indication that they have reached their end-of-life.
These mechanisms can be activated through calendar ageing, when the battery is at rest, or cycle ageing during use and charging. Studies have demonstrated that the order in which these events take place has an impact on battery lifetime.
However, to gain full insight into each mode, various techniques are necessary. A particular challenge lies in isolating and capturing their interactions, particularly when using ex situ coin cells as a model.
Degradation Feedback
Degradation in lithium-ion batteries is a complex and path dependent process. Due to the explosive growth of these batteries in recent years, scientists have been studying their physics closely. Unfortunately, many existing models fail to take into account interactions between different degradation mechanisms – particularly SEI growth and Li plating which both contribute significantly to capacity fade when operating these types of systems.
Therefore, it is essential that degradation mechanisms be fully coupled in order to accurately predict battery behaviour under various conditions and cycling protocols. In this paper, an open-source modelling environment is created which enables multiple degradation mechanisms to be implemented together as well as their interactions.
To achieve this goal, the open-source modelling framework Python Battery Mathematical Modelling (PyBaMM) was modified to incorporate four distinct degradation mechanisms that are in direct interaction with each other: SEI layer growth, lithium plating, SEI on cracks and loss of active material (LAM).
Simulating the behavior of these mechanisms using standard cycling protocols with five variations allows us to examine the effect of varying solvent diffusivity on SEI growth rate and other degradation mechanisms, as shown in Figure 1.
Research has demonstrated that altering the solvent diffusion coefficient Dsol can accelerate SEI growth in an experimental cell, though this can be counterbalanced by decreasing charge rate or raising temperature. Ultimately, this alters mechanical stress on electrode particles and causes more cracking – increasing the probability of SEI pore blockage and subsequent lithium plating.
A positive feedback loop occurs, in which SEI growth accelerates over time and decreases cell capacity due to loss of active material. Although this mechanism has only been studied for negative-electrode degradation, it could play an important role in the rapid degradation of lithium-ion batteries.