Winter is here, and the battery power of the phone has turned on its metaphysics mode.

I believe that everyone has encountered the following situation to a greater or lesser degree.

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

Why is my little cellphone turned into a brick when it is frozen outdoors, but once it is warmed up in the room, it can turn on again, and the power is restored.

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

Then you may wonder about life: Is my cell phone battery broken, why is it as confusing as a child’s temper?

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

First of all, you need to know that the lithium-ion battery in 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 the cathode (Athode Electrode) , anode (Anode Electrode) , electrolyte (Electrolyte) , separator (Separator) , and case (Case) .

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

Because the space inside the mobile phone is small, in order to make full use of the space, the positive electrode of the lithium ion battery on the mobile phone is lithium cobaltate (high compaction density, smaller volume and less space) , and the negative electrode is graphite.

The principles of lithium-ion batteries are believed to be well understood by everyone, and they 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.

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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 following picture to know.

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Schematic diagram of lithium-ion battery [1]

During discharge, lithium ions move from the negative electrode to the positive electrode. They 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 pass through the lithium cobaltate. Medium diffusion. 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.

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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) and the ordinate is the voltage U. When you see this, you can’t help but think of a formula:

W = QU (energy = electricity × voltage)

With 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 as the temperature decreases, the energy released by a lithium-ion battery 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 the 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).

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Ethylene Carbonate, melting point: 34 ~ 37 ° C

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Dimethyl Carbonate, melting point: 2 ~ 4 ℃

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 battery is in 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 increase greatly. The terminal voltage of the lithium ion battery (terminal voltage = open circuit voltage-current × battery internal resistance) decreases. When the mobile phone detects lithium ions When the battery is at a low voltage, I think the battery is almost dead, and remind you that the battery is low, and even to protect the battery, it will automatically shut down. 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 to develop electrolyte additives and low-temperature electrolytes, so that lithium-ion batteries can be used normally at low temperatures.

Is there a way for the lithium-ion battery on a mobile phone to keep it from losing power so quickly in winter?

Of course, there are ways. Don’t use your mobile phone in the cold outdoors, find a warm place, such as in a heating room, 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, it should be noted that in winter, do not charge the mobile phone at low temperatures. 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 decrease, and the decline of this capacity is irreversible, even if you take 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 are more reasons to change the phone) .

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Source: giphy.com

References:

[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 from the Institute of Physics, CAS (CAS-IOP)