This article is from the WeChat public account: Institute of Physics, Chinese Academy of Sciences (ID: cas-iop) , author: Frions, from FIG title: pexels

Winter is here, and the phone ’s battery has turned on its metaphysics mode.

I believe that everyone has encountered the following.

Why does my little phone clearly show that there is still 20% of battery power, but I just took it out and prepared to scan a shared bicycle, then it showed 2% of battery power, and then automatically shut down, leaving me in the north wind. Messy and shivering me.

Why did my little cell phone freeze and become bricks outdoors, but once I returned to the room to warm up, it turned on again, and the power was restored.

Or why does my small cell phone run out of battery power in winter, and I need to carry a charger when I go?

Then you may be wondering about life: Is my cell phone battery broken? Why is it as confusing as a child ’s temper?

Actually, your cell phone battery is not bad. It is born with this temper and can only follow it.

First of all, you must know that the lithium-ion battery inside the mobile phone is right, that is the lithium-ion battery that won the Nobel Prize in Chemistry this year.

Lithium-ion batteries are mainly composed of positive electrode (Cathode Electrode) , negative electrode ( Anode Electrode) , electrolyte (Electrolyte) , diaphragm (Separator) , (Case) .

Structure of cylindrical and soft-pack lithium-ion battery | Source: futurecar.com

Because of the small space in the mobile phone, in order to make full use of the space, the positive electrode of the lithium ion battery on the mobile phone is lithium cobaltate Small, less space) , the anode is graphite.

I believe everyone already understands the principle of lithium-ion batteries, which is to rely on the migration of lithium ions between the positive and negative electrodes to store and release energy. Lithium ion migration behavior is similar to rocking chair, also known as “rocking chair battery”. During charging, lithium ions are released from the positive electrode, and then re-embedded in the graphite layer of the negative electrode through the electrolyte, and then combine with the electrons from the external circuit. During discharge, the movement direction of lithium ions and electrons is opposite to that during charging.

The above picture is only for the convenience of explaining the principle of the lithium ion battery, and the movement of the lithium ion is drawn as simple as that. In fact, the movement of the lithium ion battery can be more complicated than this. See the picture below to know.

Schematic diagram of lithium-ion battery [1]

During discharge, the movement of lithium ions from the negative electrode to the positive electrode must first diffuse in the graphite layer, pass through the interface between graphite and the electrolyte, then migrate in the electrolyte, pass through the interface between the electrolyte and lithium cobaltate, and finally Diffusion in lithium cobaltate. This step by step, it can really be said that it is across the mountains and the sea, it is not easy to “move home” for lithium ions.

Having said all this, why does the lithium-ion battery run out so quickly in winter?

Don’t worry, let’s look at a picture first. This is the discharge curve of lithium-ion batteries at different temperatures.

Discharge curves of lithium-ion batteries at different temperatures [2]

When you get a picture, the first thing to look at is its coordinate axis. The abscissa is the capacity. Q (electricity) , vertical The coordinate is the voltage U. Seeing this, I can’t help but think of a formula:

W = QU (energy = electricity × voltage)

Using a little knowledge of calculus, we can know that the area enclosed by the curve and the coordinate axis in the figure is the energy. This picture tells us that the lithium-ion batteryAs it decreases, the energy it can release is decreasing. Not only that, but its capacity is decreasing and its voltage is getting lower.

This is why lithium-ion batteries are not durable in winter. The cute little curiosity is definitely not satisfied with just knowing this phenomenon, but also want to know the underlying reason.

As mentioned earlier, when a lithium-ion battery is discharged, lithium ions migrate from graphite to the positive electrode, crossing mountains and the sea. When the temperature of a lithium-ion battery decreases, the migration of lithium ions becomes more difficult. When the temperature decreases, the diffusion coefficient of lithium ions in graphite and lithium cobaltate decreases, while the viscosity of the electrolyte increases, and the migration of lithium ions in the active material and the electrolyte is greatly hindered.

This doesn’t sound very intuitive, let’s take a look at the ingredients without added electrolyte to see what is going on. The solvent of the electrolytic solution is a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC).

Ethylene Carbonate, melting point: 34 ~ 37 ° C

Dimethyl Carbonate, melting point: 2 ~ 4 ℃

Did you see the melting points of these two substances? The ethylene carbonate solidified at room temperature, and the dimethyl carbonate became a solid at 0 ° C. Then when the lithium-ion batteryIn a low temperature environment, the viscosity of the electrolyte will increase, and it will even partially solidify. Lithium ions migrate in a frozen solid electrolyte, which is not easy to think about.

At low temperatures, the migration of lithium ions is hindered. The most significant effect is that the internal resistance of the battery will greatly increase. = Open circuit voltage-current × battery internal resistance) drops. When the mobile phone detects that the lithium-ion battery is at a low voltage, it thinks that the battery is almost out of power. . In fact, the lithium-ion battery obviously has so much power, but it can’t work.

In order to overcome this problem, scientists are also working hard to develop electrolyte additives and low-temperature electrolytes, so that lithium-ion batteries can be used normally at low temperatures.

So is there a way to keep the lithium-ion battery on your phone from losing power so quickly in winter?

Of course, you do n’t have to use your mobile phone in the cold outdoors. Find a warm place, such as in a heating room, to keep your small cell phone intact. In other words, isn’t it good to play with mobile phone in heating room? If you really want to use your mobile phone in the cold outdoors, then use it quickly, and quickly carry it in your pocket after using it. Do n’t worry if the phone is frozen and silly, it can be restored and used normally in a warm place.

In fact, in winter, you should pay attention not to charge your mobile phone at low temperature. As mentioned earlier, the migration rate of lithium ions at low temperatures will decrease. When charging lithium ion batteries at low temperatures, the rate of lithium ions being embedded in the graphite layer is slower, and there will be too few lithium ions to be embedded in the graphite layer, and directly The process of obtaining electrons on the graphite surface to form metallic lithium is called “lithium precipitation”. Part of the precipitated metallic lithium becomes inactive, that is, it does not participate in subsequent lithium ion migration and becomes “dead lithium.”

You must have thought that if you are smart, there will be less lithium ions involved in the charge and discharge process, so the capacity of the lithium-ion battery will be reduced.The attenuation is irreversible. Even if you bring the battery back to a warm place, it can’t recover. At this time, there are only two solutions-change the battery or change the phone. (there is another reason to change the phone) .

Source: giphy.com


Reference:

[1] XU K. Nonaqueous liquid electrolytes for lithium-based rechargeable batteries [J]. Chem Rev, 2004, 104 (10): 4303-417.

[2] Li-ion Battery and Gauge Introduction


This article is from WeChat public account: Institute of Physics, Chinese Academy of Sciences (ID: cas-iop) , author: Frions