For Canadians who spend long periods outdoors, winter battery problems are almost inevitable. Phones die faster when used in extremely cold conditions. The range on electric vehicles drops noticeably during sub-zero temperatures. E-bikes and e-scooters charge slowly after being left outside during a cold night. These are some of the issues associated with cold-weather operation of current commercial lithium-ion batteries.
My last blog post explored factors that threaten the longevity of lithium-ion batteries. Another growing concern in battery applications is the operation of these batteries in sub-zero temperatures, which poses numerous safety risks in certain use-cases.
Surprisingly, low temperatures can contribute to major battery safety risks, especially when charged after prolonged exposure to freezing conditions. Overheating may seem like the most intuitive cause of battery fires, while cold temperatures may rarely come to mind as an expected failure trigger. But hang on – let me walk you through the science behind this “perceived” anomaly.
In recent years, cities like Toronto have experienced increased concerns around fires linked to e-scooters, e-bikes and lithium-ion battery-powered devices in public spaces, including the transit systems and subway stations. These concerns were significant enough for the Toronto Transit Commission (TTC) to introduce a seasonal ban on lithium-ion-powered e-scooters and e-bikes on board their transportation fleet during the winter months, due to associated fire risks with cold temperature charging and storage. While battery fires are mostly linked to overheating, their cold weather charging behavior is becoming a part of an important conversation from a geographical point of view.
To understand why cold weather charging has become such an important concern, it is necessary to understand the behavior of lithium-ion batteries at low temperatures.
Lithium-ion batteries operate on a so-called “rocking chair” mechanism, where lithium ions move between two electrodes through an electrolyte – a mixture of salt and solvent. Under normal conditions, this process is largely efficient. However, at colder temperatures, the movement of lithium ions slows down significantly. The electrolyte (which is liquid for most commercial cells) becomes more resistant to the movement of ions, chemical reactions proceed more slowly and the battery temporarily loses power capability and usable energy. Importantly, much of this performance loss is temporary and mostly recovers once the battery warms up.
The real concern emerges during charging. At very low temperatures, lithium ions may not properly insert themselves into the layers of graphite – the commonly used negative electrode material in lithium-ion batteries – during charging. Instead, metallic lithium deposits on the surface of the electrode and not in the designated graphite layers (a process known as lithium plating). Over time this accelerates degradation and creates serious safety concerns.
Combined with poor quality manufacturing, low-cost substandard chargers increase the likelihood of thermal runaway in batteries – the uncontrolled chain reactions associated with rapid heating and fires in batteries and battery-powered devices.
The rapid growth of e-bikes and e-scooters has introduced a more complicated societal challenge. In dense urban environments, people tend to store and charge these vehicles inside apartments, hallways or other enclosed public places. During winter months, riders may bring their e-mobility vehicles indoors from freezing outdoor temperatures and immediately begin charging them, unaware of the additional stress that charging a freezing-cold battery places on it.
Society’s growing dependence on lithium-ion batteries means we must think beyond simply making batteries more energy dense. Questions surrounding safe charging behaviour, product quality standards, public awareness, building regulations and transportation policies are increasingly important.
Lithium-ion batteries are here to stay and policies regarding their deployment in modern mobility applications must evolve alongside the technology – from stronger manufacturing safety standards to better consumer education and charging practices.
As cities continue to adopt electrified transportation, these recurring incidents demonstrate that lithium-ion battery safety will require more than sound engineering. It will require consideration for public infrastructure, consumer education and policy development.
In my next article, I will be discussing existing policies and standards, including potential loopholes that regulatory agencies and manufacturers may consider, to inform safer everyday use of this technology.
Photo by Himiway Bikes on Unsplash
