What Happens When an EV Battery Burns?

When an EV battery burns, it is typically due to a phenomenon called “thermal runaway.” In simplified terms, it’s a chain reaction that begins when a cell in the battery gets overheated for some reason, often because of external physical damage, overheating, or overcharging (referred to as “external harms”). Sometimes, it can be triggered by an internal issue such as manufacturing defects or a short circuit within the battery cell (referred to as “internal worries”).

A burning EV battery can be especially concerning because, unlike a traditional internal combustion engine car, the batteries in EVs often run the length of the vehicle. Once one cell in an EV battery pack catches fire, the heat can cause nearby cells to also catch fire, leading to a chain reaction that can quickly engulf the whole battery pack and potentially, the entire vehicle.

To compound the problem, the most common type of battery used in today’s EVs, the lithium-ion battery, contains flammable organic liquid electrolytes. This makes these batteries more prone to catch fire and explode when damaged or improperly handled. Additionally, there’s a specific risk called “lithium dendrites”, which are tiny, needle-like projections that can develop on the anode during charging. If these dendrites grow large enough, they can pierce the separator, causing a short circuit and possibly leading to a thermal runaway situation.

Therefore, the integrity of the battery structure and the quality of the separators are crucial factors in ensuring the safety of an EV battery. As such, quality batteries undergo various stress tests before leaving the factory, including a “puncture” test that simulates a short circuit caused by the simultaneous damage to the positive and negative electrodes and separator.

Despite the above, it’s important to note that thermal runaway in EVs is relatively rare, and many manufacturers, researchers, and institutions are working diligently to further improve the safety of these batteries. One approach is the development of solid-state batteries, which replace the flammable liquid electrolyte with a non-flammable solid one. However, as of 2023, these batteries are still largely in the research and development stage.

In the early days of August 2023, a NIO ES8 collided with a road pillar in Zhejiang, China, and erupted into flames within seconds, claiming the life of the driver. The incident is still under investigation. Just days prior, in late July, a Tesla Model Y and an Audi sedan collided in Dongguan, Guangdong. The Tesla lost control, struck a guardrail, and burst into flames.

Rewind a bit further, and we find a NIO AUTO battery swap station in Jiangmen, Guangdong, ablaze. The cause? A NIO user’s battery, remotely identified as damaged by external forces, caught fire during inspection upon return to the station.

These are the nightmare scenarios many gasoline enthusiasts resistant to the embrace of electric vehicles (EVs) have imagined, and the hardest to alleviate: the safety of EV batteries. This fear isn’t unfounded; battery fires can be more alarming in EVs than conventional cars. For instance, the battery in an EV is integrated throughout the vehicle, making it prone to total combustion in the event of a fire. Even more unsettling, while conventional vehicle fires are generally associated with traffic accidents, EVs can sometimes spontaneously combust while at rest, making the news more salient.

Common reasons for these “thermal runaway” incidents fall into two categories: external threats and internal worries. External threats involve mechanical abuse, thermal abuse, and electrical abuse, typically due to accidents, high temperatures, overcharging, or discharging. In addition to catching fire upon severe collision during traffic incidents, NIO also reported a spontaneous combustion event of an ES8 EV in 2019 during maintenance due to a short circuit caused by battery pack structure compression following a chassis impact. Nearly all other Chinese EV manufacturers have reported similar cases.

The so-called internal worries are multi-faceted. Current lithium-ion batteries, composed mainly of positive and negative electrodes, separators, and electrolyte, present their own unique hazards. For instance, the phenomenon of lithium plating occurs when lithium ions moving within the battery accumulate on the thin membrane separating the electrodes, forming lithium dendrites. These dendrites can pierce the membrane, causing a short circuit and rapid heat accumulation.

Hence, battery structure integrity and separator quality are crucial determinants of battery safety. High-quality batteries undergo rigorous testing before leaving the factory, including a “nail penetration” test (though not universally mandatory) aimed at short-circuiting by damaging the integrity of the positive and negative electrodes and separator.

With this in mind, the natural pathway to safety improvement seems clear: replace the flammable organic electrolyte with an immobile, non-leaking, thermally stable solid material. Solid-state batteries have become the obvious “next station” in the battery industry’s roadmap for their safety and energy density. However, the journey to widespread adoption has proven elusive. Despite the U.S. Oak Ridge National Laboratory creating the first solid-state battery as early as 1990, consistent technological obstacles have persisted.

In the world of solid-state batteries, there are three mainstream systems for solid electrolyte materials: polymers, oxides, and sulfides. Each has its own strengths and weaknesses, and all must contend with the production scalability and quality control challenges inherent in commercialization.

Skeptics scoff at electric vehicles’ reduced range in the winter due to current liquid batteries’ poor low-temperature performance, while the potential risk of combustion while charging in the summer is also a concern. This underscores the need for a safer, more efficient battery that can handle the demands of all seasons.

Experimentation with 3D printing to create complex structures for solid electrolytes has shown some promise. For example, researchers at the University of Oxford have used 3D printing to construct a three-dimensional framework, filled with a solid electrolyte, to improve mechanical strength and prevent easy fracturing. Similarly, U.S. company Sakuu uses binder jetting technology to deposit required electrode materials and solid electrolyte powders onto a substrate and “solidify” them with liquid reagents.

While 3D printing may offer a means to expand interface contact area and control material porosity, there are still major hurdles to overcome before these experimental techniques can be transformed into a viable, mass-produced solution. Balancing performance and cost, achieving scalability, and maintaining stringent quality control standards are the looming challenges that keep these promising solutions in the lab rather than on the road.

As we race into an increasingly electric future, the inherent risks and the constant pursuit of improved safety measures keep the industry in a state of flux. Despite the daunting challenges, the march towards a safer, more efficient electric vehicle industry continues, fueled by relentless innovation and commitment to a sustainable future. As always, the New Yorker will keep its keen eye on these developments, ready to offer insight and analysis on the journey ahead.