Battery swelling: Why does it happen and how to prevent it

Today’s consumers expect a premium experience. In consumer electronics, this means ever-thinner, sleeker devices with high runtime, and in automotive, electric vehicles with generous range and powerful charging capabilities. Yet achieving this is easier said than done, and product managers responsible for designing the next, greatest generation of products face constant trade-offs between product design, battery performance and device safety. 

One of the primary concerns when balancing battery attributes to design high-performance batteries is swelling, the expansion of the battery due to a build-up of gasses inside. In the quest to deliver maximum performance in the most attractive form factor, product engineers must ensure they are not inadvertently increasing the possibility of battery swelling, and as a result, impacting the overall safety of the product or end-user experience.

In this article, Breathe Co-founder, Chief Scientist and Chair of our Scientific Advisory Board, Professor Greg Offer, shares his insights on battery swelling, answering key questions including why batteries swell and how can swelling be prevented. 

Why do batteries swell

Batteries can swell for two main reasons. The first, reversible thermal expansion and contraction as batteries warm and cool, is typically minor, predictable in scale and timing, and relatively easily accommodated in product design, for example by designing a volume tolerance in the battery compartment.

The second, irreversible expansion, is more of a challenge because today it is less predictable in its scale and timing. Irreversible expansion always occurs as a result of a degradation mechanism, such as oxygen evolution, dendrite formation, electrode decomposition or others – see “Lithium ion battery degradation: what you need to know” by J. Edge et al. for more background on mechanisms. A degradation mechanism is an unwanted chemical reaction, sometimes triggered by a mechanical process. The most common is known as Solid Electrolyte Interphase (SEI) layer growth, which is where the electrolyte reacts with lithium ions in the negative electrode (anode), typically carbon, to form a passivating layer on the surface of the electrode particles. This SEI is essential to the operation of a lithium-ion battery and can be considered analogous to the oxide layer that forms on aluminium, allowing a highly reactive metal to exist in air, which is a highly oxidising environment. An ideal SEI prevents further degradation reactions but allows lithium ions to diffuse through it, and therefore allows the battery to charge/discharge.

How to control SEI growth

In reality, no SEI is perfect. It continues to grow, particularly at higher temperatures. The rate of SEI layer growth in high-quality batteries can, however, be controlled or slowed down so it is barely noticeable. This is possible through a range of actions available from cell design through to control strategy development at the product and system original equipment manufacturer (OEM) stage. During cell design, cell manufacturers can select swelling-inhibiting electrolyte additives and can make swelling-abating cell design choices. At the product stage, vehicle and device OEMs can select current, voltage and temperature limits, and charge control strategies, to limit swelling. Unfortunately, imposed limits at this stage can sometimes increase cost or reduce useable energy, reducing usable power and charging speeds. Therefore, as product engineers at vehicle and device OEMs, imposition of such limits is undesirable and carries the potential of negatively impacting end-user experiences at one, or both, of the start of life or throughout the product life. 

The link between SEI and swelling

It is the consequences of SEI layer growth that lead users to experience battery swelling. When the lithium ions react with the electrolyte, they are reacting with a solvent molecule, which is commonly an organic molecule such as ethylene carbonate.

Although the reactions in practice can be significantly more complicated, the ethylene carbonate reaction is a good example of the underlying principle. Here, (CH₂O)₂CO, upon reaction with lithium, can very easily form ethylene gas, which is highly reactive, and a lithium salt which becomes part of the SEI. Other gases, such as ethane, CO₂, CO, H₂, and other organic molecules can also form in varying ratios dependent upon conditions and the mix of solvents and additives designed into the battery. 

Degradation beyond SEI

Degradation mechanisms other than SEI layer growth can also directly or indirectly generate gases that contribute towards swelling. Many of those mechanisms do so indirectly by accelerating the SEI layer growth. An example is particle cracking which can happen during fast charging and fast discharging and is exacerbated at relatively lower temperatures. Others, like lithium plating, result in unprotected metallic lithium being exposed to the electrolyte. In this regard, both particle cracking and lithium plating are similar in that they expose a fresh electrode surface on which new SEI forms, leading to accelerated SEI layer growth and subsequent gas generation. At the positive electrode (cathode), degradation that contributes to swelling can also occur.  Different cathodes suffer from oxygen evolution, transition metal dissolution, and acid attack in varying degrees, many of which can result in gas generation.

How to prevent swelling in consumer electronics and EV applications

Managing and preventing swelling is a game of managing degradation. If the degradation mechanisms that are dominating a particular battery in a specific application are well understood, then it is possible to change the future outcomes that the battery experiences, including swelling, in several different ways. Cell designers and manufacturers can adjust the additives and solvents in the electrolyte to reduce the underlying SEI layer growth rates, and other cell design variables to reduce the likelihood of particle cracking, lithium plating and cathode degradation. Product engineers have historically had little available option to prevent swelling short of trading-off battery system performance and end-user experience to impose limits that de-rate the battery and curtail degradation mechanisms.

No swelling, zero trade-offs

Increasingly today, there is another option available to product managers and battery and control engineers in device and vehicle OEMs to prevent swelling, without the need to compromise on performance. This is the use of health-adaptive charge control (“adaptive charging”), which operates with an active awareness of battery health. This technology and approach, as found in physics-based battery management software, can reduce degradation during system operation by actively navigating the battery through its life in a way that delivers both a great end-user experience for charging and longevity and, in real-time, avoids degradation mechanisms that could contribute to swelling.

Avoiding swelling is fundamental to delivering a premium user experience. But swelling is ultimately a result of degradation, and therefore any strategy to reduce degradation will decrease the risk of battery swelling within a product. Product engineers therefore have a choice to make when deciding how to minimise degradation in new products: adapting cell design or imposing system limits to trade performance and longevity, or deploying intelligent control software to existing cells to keep swelling under control without the need to compromise on performance. 

At Breathe, we believe better batteries are key to delivering remarkable end-user experiences. Discover how our adaptive charging software, Breathe Charge and Breathe Life, extract more performance from existing batteries with zero trade-offs or swelling risk.