Smart locks and electric cars are trending on the Internet because the weather in Beijing is too cold!

eric_wang

Smart locks and electric cars are trending on the Internet because the weather in Beijing is too cold! [Copy link]

There is no coldest, only colder. In recent days, the temperature in Beijing has dropped to minus 20 degrees Celsius, setting the coldest record since 1966. With gusts of biting cold wind, the range of electric vehicles has become a mystery. You never know how far you can actually drive. Even some smart fingerprint locks that use batteries face the embarrassing situation of fast power consumption in cold weather.

On January 6, smart fingerprint lock became a hot topic on Weibo because Sina Weibo CEO Wang Gaofei (Weibo ID "Laiqujianjian") complained: I replaced the battery the day before, but because of the cold weather, it ran out of power the next day.

As soon as the post was published, it attracted strong attention from netizens, who joked: "This kind of lock is not for villas... the corridor is not cold; fortunately, I don't have a big villa~ I can't enjoy the feeling of being blocked outside the door by the fingerprint lock; the price of a smart lock, the enjoyment of a Tesla."

The sudden drop in temperature has also made life difficult for some electric car owners. Some Tesla car windows and doors were frozen and they couldn't close the doors. They could only hold the steering wheel with one hand and the door with the other, with the wind blowing in their faces. Some Xiaopeng P7, parked in an open-air parking lot, lost 15km of range overnight. Some BYD's battery dropped from 9% to 0, and it broke down on the road, so they had to ask colleagues to tow the car back to the company...

As we all know, the cruising ability of electric cars and electric bicycles will decline in winter. The reason is that the battery life is shortened in low temperature environments and the power will decay. Similarly, the rapid decay of power in smart locks in cold and low temperature environments is not necessarily due to problems with the smart locks, but may be caused by the battery.

Why are these lithium-containing batteries afraid of cold? This has to start with how they work.

We first need to clarify two issues:

1. Lithium-ion battery is a complex electrochemical system. Look at the constituent materials:

• Solid materials: transition metal oxide positive electrode materials, graphite negative electrode materials, fiber separators, etc.
• Liquid materials: organic electrolyte (including lithium salt, solvent and other additives), etc.

2. The working principle of lithium-ion batteries involves two basic processes: mass transfer of charged particles and electrochemical reactions.

Image from reference [1]

Therefore, we found that both materials and processes in lithium-ion batteries are affected by temperature.

• Solid materials cannot escape the shackles of "thermal expansion and contraction" (ions are neither easy to embed nor to escape, and it is difficult for them to pass through the membrane);
• Liquid materials cannot escape the fate of increasing viscosity or even solidifying at low temperatures (ions cannot move);
• The mass transfer process of charged particles and the speed of electrochemical reactions will inevitably decrease.

The compatibility of the various components of lithium-ion batteries will be greatly reduced at low temperatures.

Therefore, in the face of low temperature environment, lithium-ion batteries will be very "fragile". The resistance of a "cold" lithium-ion battery will increase (resistance increases), and its working efficiency will also decrease (actual capacity decreases rapidly). If it is forced too hard (high current charging and discharging), the resistance will become even greater and the capacity will decrease faster.

Image from reference [2]

Irreversible loss of capacity of low-temperature lithium-ion batteries - low-temperature aging mechanism

Lithium-ion batteries are "cold-averse", which means that low temperatures will not only reduce the working efficiency of lithium-ion batteries, but will also cause more or less damage to the materials of lithium-ion batteries. Just like the human body, a cold or fever can cause cell damage, but the human body has a self-renewal system to repair and renew damaged cells. However, lithium-ion batteries do not have such capabilities. Different from the reversible electrochemical reactions inside the battery, the "irreversible damage" in the question can be mainly divided into irreversible structural damage of materials and permanent loss of active substances (especially circulating lithium).

We may have a question: Will low temperatures cause irreversible damage to lithium-ion batteries when they are not in use?

There are two main aging mechanisms of lithium-ion batteries: calendar aging and cycle aging. Calendar aging refers to aging during static non-use storage. It is mainly affected by temperature and SOC (how much lithium ions are stored in the negative electrode graphite): at high temperature and high SOC, the stability of the electrode/electrolyte interface decreases, and side reactions increase - dissolution of positive electrode metal ions, oxygen evolution, electrolyte decomposition, and thickening of the SEI film on the negative electrode surface.

Therefore, low temperature can inhibit calendar aging to some extent. That is, during the period of non-use, if we do not discuss the mechanical damage caused by cold stress (thermal expansion and contraction), low temperature conditions themselves will not cause irreversible losses of lithium-ion batteries.

——In other words, the aging of lithium-ion batteries under low temperature conditions mainly comes from the cyclic aging caused by the dynamic charging and discharging process.

Image from reference [5]

Low temperature cycle aging mainly comes from:

(1) Lithium plating and the growth of lithium dendrites. During the charging process, low temperature causes the lattice to shrink, the negative electrode has insufficient space for lithium embedding, and the charge transfer and solid phase diffusion become slower. Lithium ions that cannot be embedded in the negative electrode can only gain electrons on the surface of the negative electrode, thereby forming a silvery-white metallic lithium element. This is the lithium plating (lithium precipitation) behavior. The uneven growth of low-temperature lithium plating can easily form lithium dendrites. Large lithium dendrites can pierce the diaphragm and even cause functional failure. During the discharge process, the reaction rate between the metallic lithium deposited on the surface of the negative electrode and the electrolyte will also decrease. The closer the lithium element is to the current collector, the more it will dissolve first, leaving the lithium at the top losing its connection with the negative electrode, resulting in "dead lithium". This part of lithium will be a permanent and irreversible loss.

Image from reference [6]

(2) Thickening of SEI film. The lithiation potential of negative electrode materials of lithium-ion batteries is often lower than the reduction decomposition potential of organic electrolytes, so a passivation layer, namely SEI film, is formed. The formation of SEI film runs through the entire use of the battery. The increase in resistance leads to greater polarization, shifting to lower potentials, and lithium plating behavior means lower potentials, so the decomposition of organic electrolytes will be accompanied by lithium plating, forming thicker and thicker SEI films. Circulating lithium is lost in this process, and the impedance will become larger and larger. Therefore, many studies focus on low-temperature charging strategies, suggesting lower current densities to allow lithium insertion behavior to "take it slow."

(3) Local lattice damage of electrode materials. The lattice that shrinks at low temperatures is strongly embedded, which can easily lead to local lattice damage inside the positive and negative electrode materials, which cannot be repaired by themselves.

(4) Polarization decomposition of electrolyte. Under low temperature conditions, electrochemical polarization and concentration polarization are serious, and side reactions are likely to occur at the electrode/electrolyte interface, leading to decomposition of the electrolyte. In addition, during the thickening process of the SEI film, the decomposition of the organic electrolyte is also irreversible damage.

Can solid-state electrolytes solve the irreversible loss at low temperatures?

In all-solid-state lithium-ion batteries, solid electrolytes have strong mechanical properties and can effectively inhibit the growth of lithium dendrites, especially in all-solid-state thin-film lithium-ion batteries, which do not require the addition of conductive agents and binders, and have fewer mechanisms that cause low-temperature performance deterioration. However, existing solid electrolytes still have major problems in constructing a well-contacted "electrode/electrolyte interface."

• Solid-solid interfaces are often not as compatible as solid-liquid interfaces, and the problems of lithium plating and lithium dendrite growth still exist to a greater or lesser extent.
• The interface resistance is high and the interface chemical reaction still exists, resulting in the decomposition of electrolyte components and the loss of circulating lithium;
• The ionic conductivity of solid electrolytes is lower than that of liquid organic electrolytes. At low temperatures, the ion transport rate is reduced and the resistance is increased.
• Local structural damage within the electrode material will not cause fundamental changes due to changes in the electrolyte.

Here is a sentence to explain why the range of electric vehicles will be reduced so much in low temperature environments. Low temperature environments will lead to reduced battery activity, smaller available capacity of the battery, increased internal resistance, and reduced charging and discharging performance. At the same time, turning on the warm air conditioner in a low temperature environment will further increase the loss of battery power. Therefore, the durability of electric vehicles in low temperature environments will inevitably be affected by the combination of these two factors. Finally, it is hoped that companies and scientific research institutions in the industry can increase their exploration and research on the low temperature resistance of batteries and create conditions for batteries to work under low temperature conditions.

Source: Zhihu lithium platinum zinc and other online content.

References:

[1] G. Zhu, K. Wen, W. Lv, X. Zhou, Y. Liang, F. Yang, Z. Chen, M. Zou, J. Li, Y. Zhang, W. He, Materials insights into low-temperature performances of lithium-ion batteries. Journal of Power Sources.

[2] Jaguemont J, Boulon L, Dubé Y, Poudrier D, Low temperature discharge cycle tests for a lithium ion cell. In: Veh power propuls conf.

[3] Kevin L.Gering, Low-temperature performance limitations of lithium-ion batteries.

[4] Zhao Shixi, Guo Shuangtao, Zhao Jianwei, Song Yu, Nan Cewen, Research progress of lithium-ion batteries at low temperature. Journal of the Chinese Ceramic Society.

[5] M.-TF Rodrigues, G. Babu, H. Gullapalli, K. Kalaga, FN Sayed, K. Kato, J. Joyner, PM Ajayan, A materials perspective on Li-ion batteries at extreme temperatures. Nature Energy.

[6] Thomas Waldmann, Bjorn-Ingo Hogg, Margret Wohlfahrt-Mehrens. Li plating as unwanted side reaction in commercial Li-ion cells -A review. Journal of Power Sources.

[7] J. Jaguemont, L. Boulon, Y. Dubé, A comprehensive review of lithium-ion batteries used in hybrid and electric vehicles at cold temperatures. Applied Energy.

chunyang

The first people to know and pay attention to the low-temperature characteristics of lithium batteries were photographers and photography enthusiasts. They went out to take pictures in the cold weather, and just when they took out their cameras, they found that they were out of power. How annoying...

ljh10051

It is hoped that enterprises and scientific research institutions in the industry can increase their exploration and research on the low-temperature resistance of batteries and create conditions for batteries to work under low-temperature conditions.