Why do batteries become as “afraid of the cold” as you are in winter?

Produced by: Science Popularization China

Author: Salted Fish in the Sea

Producer: China Science Expo

Winter is here again. Although the indoor heating in the north makes it warm enough to eat ice cream every day, it is still a bit scary to face the temperature difference of about 40 degrees when going out.

If the smartphone in your pocket is really a “smartphone”, it certainly doesn’t want to go out in the cold (I’m sure). Why? Because the battery level of the phone’s “life bar” always drops at an alarming rate in this season.

This season, there is one thing that is discounted very decisively - battery life . In the winter in the north, the range of electric cars may be less than 70% of that in the summer. Mobile phones also lose power at a similar rate.

There is a vivid description on the Internet: In the winter, you need to go out urgently. You pick up your phone and see that there is still 50+ battery left. You go out without worry. When you arrive at your destination, you pick up your phone and see that the green battery icon has turned red to 20. You stare at it in surprise, and it turns to 1% right in front of you. You are shocked and breathe on the phone and rub your hands, but you still can't stop the phone from getting colder...

Are we just going to watch our phones run out of power in the cold weather? Before discussing countermeasures, let's first look at how the battery "power loss dilemma" is formed.

1. The dilemma of lithium battery life in the cold winter

Batteries are indispensable in our lives. Batteries are found in every place that cannot be covered by wired power networks. Lithium-ion batteries that can be recharged repeatedly are basically used in mobile phones, laptops, and digital cameras. At present, electric vehicles are developing well. The core component of electric vehicles is also a large lithium-ion battery. Its performance directly determines how fast and how far the car can run.

Electric cars are now making a comeback, thanks in no small part to lithium batteries. Why do we say that electric cars are making a comeback? Electric cars were once popular in our ancestors.

The history of electric cars can be traced back to 1834, more than half a century earlier than internal combustion engine cars. In the late 19th century and early 20th century, electric cars were widely popular in developed countries such as Europe and the United States. The market share of electric cars in the United States was 16% higher than that of internal combustion engine cars. It is amazing that the streets are full of electric cars.

Pictured is the world's first car that reached a speed of over 100 km/h (yes, it was an electric car). This car set a speed record of 105.882 km/h in 1899

(Image source: Wikipedia)

With the development of economy, the construction of highway network is becoming more and more extensive, the scope of people's activities is expanding, and people's requirements for the range of cars are getting higher and higher. Coupled with the rapid development of oil extraction and refining technology and internal combustion engine technology, fuel vehicles have become more advantageous in terms of cost, range, price and performance. From then on, the popularity of electric vehicles gradually declined and gradually faded out of people's sight.

2. Several major challenges caused by low temperature

Electric cars now have a sufficiently long range, with a theoretical range of 600 to 700 kilometers. But in winter, electric cars may run out of power after running 200 to 300 kilometers, and the charging speed is not as fast as usual. It seems that the shortcoming of electric cars' range is still not long enough.

What was the problem? It turned out that the temperature was too low and the battery was "burned out" .

Before exploring the causes of battery internal consumption, let's first understand how batteries work .

The battery has several main components : negative electrode (mostly graphite is used as the negative electrode), positive electrode (taking lithium iron phosphate as an example), diaphragm (lithium ions can pass through but electrons cannot), and battery casing.

(Image source: self-made by the author)

When a lithium-ion battery is charged, lithium ions are released from the lithium iron phosphate at the positive electrode, move through the electrolyte solution to the negative electrode, and embed into the graphite. The graphite at the negative electrode absorbs the lithium ions coming from the positive electrode and the electrons moving through the wires. When discharging, the lithium ions stored in the graphite escape again, pass through the electrolyte and through the diaphragm back to the positive electrode. Electrons cannot pass through the diaphragm and can only return to the positive electrode from the external wire. This action generates current in the wire, driving the electrical appliance to work.

How does low temperature affect the operation of lithium-ion batteries ? The materials and operation of the battery are affected by the low temperature environment. Just like fingers exposed to the outside for a long time in winter will become stiff, the materials in the battery will also be the same. In a low temperature environment, it becomes difficult for ions to escape and embed into the material, it is more difficult to pass through the diaphragm, and the movement speed of ions will also be reduced.

When discharging at low temperatures, the rate at which lithium ions are embedded in the positive electrode material slows down. Before the lithium ions in front have time to embed into the material, the lithium ions behind arrive. Lithium ions begin to get stuck in traffic. A large number of lithium ions accumulate on the surface of the electrode material, which will lead to the acceleration of the generation of the passivation layer (called SEI film in the industry. This film will slow down the rate at which lithium ions are embedded in the electrode), making it more difficult to embed. The macroscopic manifestation is that the internal resistance of the battery increases, the battery begins to "consume internal energy", and the amount of electricity output becomes smaller.

When charging at low temperature, lithium ions migrate to the graphite negative electrode, but the speed at which lithium ions are embedded in graphite is also reduced, while electrons can happily reach the negative electrode through the wire. When electrons encounter lithium ions on the surface of the negative electrode, they will generate metallic lithium to form lithium dendrites. Once the lithium dendrites grow, they will pierce the diaphragm, causing the battery to short-circuit and malfunction.

Lithium dendrites under a microscope

(Image source: References)

3. As long as your thinking does not slip, there are always more solutions than problems

The shortcomings of lithium batteries in low temperature performance are a thorn in the side of many scientific research leaders. The battery field has almost explored the periodic table, but a mere low temperature can stump people? Researchers have come up with ideas to overcome difficulties in their research - currently, there are three ways to make up for this shortcoming.

Article 1: Change materials . You can change the material processing method and the battery manufacturing process.

The reason why the battery discharge performance decreases in low temperature environments is that the internal impedance of the battery is too large. By replacing new electrolyte materials and electrode materials, the internal impedance of the battery in low temperature conditions can be reduced as much as possible, thereby improving the low temperature performance of the battery.

Second: Internal preheating . When people feel cold, they stamp their feet or rub their hands to generate heat. Batteries can do the same.

This technical route is called internal preheating. When manufacturing batteries, manufacturers add a thin nickel foil to the battery structure and cover it with an electrically insulating polymer (to prevent the thin nickel foil from causing a short circuit in the battery). Once the battery temperature is too low, the controller forces current to pass through the nickel foil, generating a large amount of heat energy to quickly heat the battery material. This allows the battery to always discharge within a relatively good operating temperature range.

If you want to charge in low temperature conditions, the charging device will first charge the battery at a low power, using the heat generated by the battery itself during charging to preheat the battery, and then perform high-power fast charging when the battery temperature rises to a suitable range.

Article 3: External heating . You can add preheating equipment to the battery (like providing people with heaters or electric stoves).

The battery first supplies a small amount of power to the preheating device, which generates heat to raise the battery temperature. After reaching the appropriate operating temperature, the battery enters the normal working state. Some electric vehicles are equipped with a battery preheating function. In winter, the battery must be preheated before using the car so that the car can enter the normal working state.

How to preheat the battery

(Image source: References)

4. Low-temperature lithium batteries are promising in the future

Recently, the country's first cold-region electric pushboat "Linghang No. 1" using lithium iron phosphate batteries was successfully launched in a tributary of the Songhua River. It can overcome temperatures as low as minus 30 degrees Celsius.

"Linghang No. 1" cold-region electric push boat

(Photo source: China Science Daily)

The performance shortcomings of low-temperature batteries are being made up step by step. Maybe in a few years, electric cars will be able to run in Arctic research stations. By then, batteries that can survive in the "monster level" of the Arctic will definitely be able to perform stably in our daily use environment.

Editor: Guo Yaxin

References:

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