What security issues need to be paid attention to when using mobile phones in daily life? You and your family have a few hits!

In 2010, you had to bring cash with you when you went out

In 2020, masks must be worn when going out

In 2023, you must carry your mobile phone when you go out

Whether it is working or decompressing, eating or sleeping, one person or a bunch of people, mobile phones are with us all the time. From inseparable companions to powerful bombs, are phones really absolutely safe? Friends who are in a hurry to touch the fish can quickly row to the bottom to find the answer

How powerful is the battery?

As everyone's demand for mobile phone battery life is getting higher and higher, the capacity of mobile phone batteries is getting larger and larger. Lithium-ion batteries (LiBs) are used in mobile phones, and LiBs play a very important role in everything from headphones to electric vehicles and energy storage power stations.

At present, lithium cobalt oxide is mainly used for the positive electrode of lithium-ion batteries for commercial mobile phones, because it has the highest volume energy density, so that more energy can be stored in a very small mobile phone, while layered graphite is generally used for the negative electrode. At present, the average output voltage of most mobile phone batteries is about 3.7V, and the capacity is generally 4000mAh, so the energy contained in a fully charged mobile phone is about 14.8Wh, which is equivalent to 53kJ, which is the energy of two grenades?!

Although the probability of a grenade explosion is very small, no one wants to be in the news in this way, right? Therefore, before evaluating the performance of a battery, it is necessary to evaluate its safety, so that consumers can rest assured that they can use it.

The Hexagonal Warrior evolves into a security-focused "Pentagon Core Warrior"

Are electrolytes and diaphragms safe?

Whether it is a battery cell, a module or a large-scale energy storage module, it is actually composed of battery cells connected in series and parallel. The reason for the occurrence of thermal runaway is closely related to the basic structure and working principle of the battery:

Composition of a lithium-ion battery system[1]

The interior of LiBs is generally composed of positive and positive current collectors, electrolytes, diaphragms, negative electrode materials and negative current collectors, gaskets and housings, and the whole system should also include external circuits, which are simply assumed to be composed of power supplies/electrical appliances, switches, and connected by wires.

Among them, the electrolyte of LiBs is generally a flammable and toxic organic solution, which will decompose at high temperatures, so the battery needs to be strictly sealed. If we accidentally disassemble the mobile phone battery, we can only see the positive and negative current collector tabs exposed to the outside, which is used to conduct external circuit electronics (while the cylindrical battery welds the tabs directly into the shell, so it cannot be seen). In addition, the seal can also ensure that the electrolyte will not leak when the battery is pressurized, and it can be used everywhere.

Several common types of lithium-ion batteries

Therefore, the battery is limited from the material to the temperature it works at not too high, not only because the electrolyte is easy to decompose, but also at high temperature, the oxygen inside the lattice of the commercial lithium cobalt oxide cathode will also come out, which not only damages the electrode structure, but also produces oxygen, which reacts directly with the electrolyte to cause thermal runaway, and the working temperature of the general commercial lithium-ion battery should not exceed 55 °C.

So, in the summer, when the phone is hot, stop playing and let it stay where it is cool.

Not only in the manufacturing process, but also when the battery is working, it must always be kept sealed, in the event of collision, extrusion and other extreme working conditions, to ensure that the sealing of the battery is not affected, of course, the battery will be subjected to a series of safety tests before leaving the factory. But if the mobile phone has been seriously bent and deformed, I am really reluctant to change the mobile phone, and I also want to change the battery, otherwise I really hold two grenades in my pocket, and you say that it is not stimulating~

In addition to the electrolyte, the separator of LiBs is also one of the main causes of thermal runaway. The electrolyte and diaphragm of LiBs are almost electronically insulated, preventing electrons from the positive electrode from running directly to the negative electrode, resulting in direct discharge of the internal circuit. If the separator is damaged, such as the separator is punctured, the positive and negative electrodes will come into direct contact when the battery is pressurized, resulting in a short circuit inside the battery, and then the battery will be instantly discharged at a very high current, resulting in thermal runaway of the battery.

Therefore, in the manufacture and use of batteries, it must be ensured that electrons can only go through the outer circuit, and ions can only go through the inner circuit, and the two move at the same time to form a closed loop.

Can the battery be charged indefinitely?

When the LiBs are charged, the cathode material loses electrons, the lithium ions come out, and the cathode potential increases. Driven by an electric field, lithium ions diffuse to the negative electrode in the electrolyte, and with the negative electrode gaining electrons, lithium ions are embedded into the negative electrode material, and the negative electrode potential is reduced, and the electrical energy is converted into chemical energy and stored in the battery.

However, the battery cannot be charged indefinitely, because the positive electrode potential cannot exceed the upper limit of the electrochemical stabilization window of the electrolyte, and the negative battery cannot be lower than the lower limit of the electrochemical stabilization window of the electrolyte, otherwise the composition of the electrolyte will undergo redox reaction with the electrode, resulting in the decomposition of the electrolyte. The decomposition of the electrolyte will produce flammable or combustible gases, which will deform the battery expansion pack and generate heat at the same time, resulting in thermal runaway.

Schematic diagram of electrode potential change during charging of lithium-ion battery[2]

The battery cannot be charged infinitely, and another reason is that the vacancy that the negative electrode can accommodate lithium ions is limited, for example, the theoretical specific capacity of the commercial graphite anode is 372mAh/g, and the actual specific capacity is about 350mAh/g. When the vacancy of the graphite anode has been filled with lithium ions, the diffused lithium ions can only accumulate on the surface of the graphite. If the charging continues, the graphite potential (the lowest lithium intercalation potential is about 0.1V for lithium) continues to decrease, when it is reduced to 0V for lithium, the lithium ion will directly obtain electrons from the negative electrode, and reduce the graphite surface to form metal lithium (dendrite), and the irregular lithium deposition will pierce the separator, resulting in a short circuit of the battery, and then cause a thermal runaway event.

In addition to overcharging, when the battery is charged at low temperatures, the graphite anode will also cause the problem of lithium dendrites piercing the separator. At low temperature, the rate of lithium intercalation in the graphite anode slows down, and if it is less than the speed of lithium ion diffusion in the electrolyte, the lithium ions enriched on the surface of the anode may directly combine with electrons and be irregularly deposited on the surface of the graphite anode to form lithium dendrites. Especially when charging at low temperatures, the kinetic diffusion speed inside the electrode is more demanding, and lithium deposition is more likely to occur. Therefore, the use temperature of LiBs is generally more than -20°C. And it's -20°C, so it's better to put the phone in the gazi nest to warm it up first.

Combined with the previous temperature, the temperature range of LiBs operation in the temperature range of -20~55°C will be safer, and the kinetic performance of the battery is also better.

The real shot of lithium dendrites pierced the diaphragm

When the battery is fully charged, or when the battery is in a state of charge (SOC >0%), an external circuit is connected, and the battery will be discharged spontaneously, which is exactly the opposite process to that of charging. The negative potential rises and the positive potential decreases, moving closer to the middle (but not flat) like the water level at both ends of a U-tube. The battery converts the chemical energy of the electrodes into electrical energy (and heat) to power the external circuit. Battery discharge is a spontaneous process, like a waterfall "flying down 3,000 feet", and your phone prompts 10% before you have played much.

Of course, when the manufacturer designs the battery, it has carried out various safety protection and tests to ensure that we will not have problems when we use it normally or occasionally overuse, such as overcharging and stopping charging, automatic shutdown at high and low temperatures, explosion-proof testing, etc., so that we don't have to worry about it all the time when we use the mobile phone. However, in order to prevent this danger from happening around us, we must pay attention to the following points:

Use batteries produced by regular manufacturers, preferably the original manufacturer

When the phone or battery is seriously deformed, replace the battery in time

Do not use the battery continuously at high or low temperatures

The battery can be equipped with a better heat dissipation and heating system

Do not overcharge or discharge

From the roots: play less on your phone

Safer batteries?

In addition, in order to obtain a safer lithium-ion battery, we can also have many means in material design, to name a few:

If the stability of the lithium cobalt oxide cathode is not enough, the cathode will be replaced. The use of a more stable and safe cathode - lithium iron phosphate: the structure is more stable, decomposition will occur at 800 °C, and the cycle life is also longer, up to 10 years under normal use. In addition, lithium iron phosphate does not contain precious metals and is less costly, making it suitable for many scenarios where energy density is not required.

If the organic electrolyte is unstable, change the electrolyte. Use a more stable and safe electrolyte - aqueous electrolyte: replace the solvent of the electrolyte with water, which is non-toxic and non-flammable, and can even be flame retardant. However, the reason why aqueous lithium-ion batteries are not included in the commercial system is that the electrochemical window of pure water is relatively narrow (1.23 V), and there are few choices of positive and negative electrodes, resulting in low output voltage and energy density. However, its safety is unquestionable, and for large-scale wind-solar integrated storage and power stations in the future, water-based batteries are expected to occupy a place.

If the diaphragm is not strong enough, change the diaphragm? Remove the diaphragm directly! An all-solid-state lithium-ion battery is used, and the liquid electrolyte and separator are replaced by a solid-state electrolyte, replacing combustibles, making the battery inherently safe. Solid-state lithium-ion batteries are currently developing rapidly, and major electric vehicle companies are vying to be the first to deploy.

Since the positive electrode, electrolyte and negative electrode are all solid-state, the poor mechanical contact of the solid-solid interface is a problem that needs to be solved, but the solid-state battery can use lithium metal with higher energy density as the negative electrode, and use a stable and reliable solid-state electrolyte, so that the energy density and safety performance of lithium battery are improved again, and it is a strong competitor for power batteries in the future.

Structural composition of an all-solid-state lithium-ion battery[5]

Hand rub the battery together!

Presumably, all the future outstanding young people and thousands of people who are watching are already eager to try and want to rub the battery by hand, so hurry up and join the E01 group of the Institute of Physics of the Chinese Academy of Sciences (together with the original) and create your own battery tivat here.

Finally, there is no such thing as absolute security, only absolute security awareness. However, the safest thing is to play less with mobile phones. I hope you can use your mobile phone correctly, instead of becoming a mobile phone playmate like Xiaobian.

So travelers, every day, ask yourself: What are you turning on your phone for?

bibliography

[1] Goodenough J B, Park K-S. The Li-Ion Rechargeable Battery: A Perspective[J]. Journal of the American Chemical Society,2013, 135 (4): 1167-1176.

[2] Goodenough J B, Kim Y.Challenges for Rechargeable Li Batteries[J]. Chemistry of Materials,2010, 22 (3): 587-603.

[3] Doughty D H, Roth E P.A General Discussion of Li Ion Battery Safety[J]. The Electrochemical Society Interface,2012, 21 (2): 37.

[4] Armand M, Tarascon J M.Building better batteries[J]. Nature,2008, 451 (7179): 652-657.

[5] Janek J, Zeier W G.Challenges in speeding up solid-state battery development[J]. Nature Energy,2023, 8 (3): 230-240.

Edit: TT