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Photo Source: Pixabay doesn't know how long it will take for your current mobile phone to recharge. Some scientists are paying attention to the same thing-the life of lithium-ion batteries. However, using higher-precision imaging techniques and machine learning methods, they "saw" the damage to the battery electrode particles and found a special feature-the group behavior of these particles may be the key to designing and manufacturing long-life batteries.

You may have noticed that once the phone is fully charged (such as 80%), it takes a long time to fully charge. The same is true for phones that now support fast charging. This is actually because charging the phone does not "throw" as much voltage and current as possible into the battery. In fact, the charging of mobile phone lithium-ion battery usually consists of two stages: constant current charging and constant voltage charging.

In general, when a phone with no battery begins to charge, it will be charged at a constant current. At this point, the battery voltage will soar immediately, but the increase in battery capacity (which can be understood as charged electricity) lags slightly. This stage is a critical period when the fast-charging phone can be used, because once the peak voltage (the set threshold) is reached, the phone will enter the constant voltage charging stage. At this point, the voltage will no longer change, the current will decrease, and the rate of increase in battery capacity will slow down, that is, the battery charging speed will slow down in the second stage.

More importantly, prolonged exposure to high voltage is actually "pressurizing" the mobile phone battery, which can lead to electrode damage and reduced battery life. Therefore, not carrying out the second stage of constant voltage charging, or not charging the battery with full charge, may be beneficial to extend the battery life. In fact, most current mobile phone battery management software systems can do this silently. In the case of the iPhone, for example, starting with the iOS 13 system, there is an option called "optimize battery charging" in the battery setting, which allows the phone to stop charging at around 80%, thus preventing the phone from entering the "high voltage zone."

Battery health setting diagram. (photo taken on 5.30) there is a question: if you take good care of the mobile phone battery from now on, will it "live" forever? We may have all experienced a situation in which a new phone can be used for a full day after it is fully charged, but over time, a fully charged phone can only be used for most of the day a year later. If most of the day is enough, then after two years or more, you can't go out without portable battery.

In fact, even if used carefully enough, mobile phone batteries will come to an end one day. We ordinary people can only take good care of the battery in our daily life, and the scientists who are committed to battery research have not been idle, and they are also worried about the life of lithium-ion batteries.

The bottleneck of lithium-ion battery 2019 was a "highlight" moment for the lithium-ion battery industry, when the Nobel Prize in Chemistry was awarded to three scientists and engineers who made outstanding contributions to the invention and development of lithium-ion battery. They are Stanley Stanley Whittingham, John B. Goodenough and Akira Yoshino.

Among them, Professor Gudinav's contribution is that he proposed a kind of cathode material for lithium-ion battery-layered hexagonal compound lithium cobalt around 1980. Until now, the mobile phones in our pockets have been equipped with cathode materials evolved from this material, such as replacing cobalt with elements such as nickel and manganese, or doping nickel, manganese, iron, magnesium and aluminum in lithium cobalt. However, their structures are layered and have one common feature: the ability to store and release lithium ions, so that lithium ions can be reversibly embedded and de-intercalated between the layers of the cathode. and ideally does not destroy the basic structure of the cathode material.

Schematic diagram of lithium-ion battery. (photo Source: nobelprize.com) similarly, battery anodes should have the same capabilities: in 1983, Dr. Richard Yazami confirmed that layered graphite could be reversibly intercalated and deintercalated, making it an excellent anode candidate. However, in order to further improve the performance of lithium-ion batteries, scientists are still looking for and trying new anode materials, such as silicon anode and lithium metal anode.

Although the study of cathode materials has been relatively mature, the optimization of anode materials and the use of solid electrolytes instead of dangerous liquid electrolytes are in trouble. Therefore, it can be said that the development of lithium-ion battery has entered a bottleneck period.

However, some scientists have taken the opportunity to think about another question: can we "squeeze" more electricity on the basis of existing materials?

Recently, researchers from the US SLAC National Accelerator Laboratory, Purdue University, Virginia Institute of Technology and the European Synchrotron radiation Laboratory (ESRF) published a study in the journal Science. On the key question of how to develop long-life lithium-ion batteries, these scientists instead have to look for answers from "Why the battery is deactivated."

One thing to note about "subtracting" an electrode to particles is that when a lithium-ion battery is charged, the cathode material transmits its pre-stored lithium ion through the electrolyte and diaphragm to the anode and stores it between the anode layers. During discharge, the transfer process of lithium ion is reversed, that is, from anode to cathode. In the process of lithium ion moving in and out of the electrode, the structure of the electrode will not be destroyed ideally, but this is not the case, which is the key cause of the gradual deactivation of the battery.

When it comes to electrodes, what comes to mind? An electrode plate? After all, in our textbook, the electrode is a "board" inserted into the electrolyte. But in fact, the electrode is made up of millions of electrode particles stacked one after another. It may be easy to understand that when lithium ions come in or out, they will inevitably collide or interact with the electrode particles, resulting in cracks in the electrode particles. After repeated charge and discharge, the electrode particles will also lose their electrochemical activity.

Scientists used synchrotron radiation X-ray imaging technology to observe the damage of electrode particles in the charge-discharge cycle. (photo source: Yang Yang / ESRF) however, most previous studies have focused on the characteristics of individual particles, such as particle size and morphology, but few studies have focused on the group behavior of particles. However, none of the particles is an island, and how the particle network changes with charge and discharge is also very important.

The group behavior of particles in fact, this multi-unit research team published two studies in 2019 through synchrotron radiation X-rays, as well as computational simulations and machine learning to "see" thousands of electrode particles damaged by lithium-ion batteries. Importantly, they found that the electrode particles did not fail at the same time, and that the particles at some locations were deactivated more quickly. For example, particles closer to the battery diaphragm are overused and lose electrochemical activity more quickly than particles closer to the electrode. Moreover, this "non-uniform deactivation" phenomenon is more serious under thicker electrodes and fast charging conditions.

Seeing here, we may not quite know why this non-uniformity has attracted their attention. However, in 2021, the team wrote in the journal Natural Materials Science that it was important that different electrode particles behave differently in charge and discharge. Previously, scientists had thought that lithium ions would flow out or into all electrode particles at the same time and at almost the same speed. But the team found that some particles release lithium ions immediately when charging, but at the same time, some particles do little "work". The researchers say this "non-uniform" behavior puts too much pressure on some "employees" of the electrode, thereby reducing their lifespan. Moreover, in many cycles, these diligent particle "employees" have always been the backbone of the work, and the particles that did not work well at first have not made any progress.

In the new study, recently published in Science, the team rediscovered the importance of "uniformity". This time, they focused on the cathode material of lithium-ion batteries, and chose a nickel-rich composite cathode-composed of multiple layers of lithium-nickel-manganese-cobalt oxide (NMC) particles, as well as conductive carbon and binder, in which the active particles were wrapped in conductive carbon. Through synchrotron radiation X-ray tomography, computational simulation, and computer vision techniques, they studied the changes in the microstructure of the battery cathode-the characteristics of cathode particles after several charge and discharge cycles (10 and 50 times) under fast charging conditions (5C).

A computer vision algorithm developed by the research team can identify particles based on their size, morphology and degree of damage. (photo: Liu Yijin of SLAC National Accelerator Lab) according to Professor Zhao Kejie of Purdue University, one of the study's newsletter authors, these cathode particles are like human beings. At first, everyone goes their own way, and then they meet their companions, so they come together. Therefore, "We need to study not only the electrochemical behavior of individual particles, but also the performance of these particles in the population."

Finally, the researchers identified more than 2000 cathode particles through computer vision, and then calculated and simulated the individual characteristics such as particle size, morphology and surface roughness, and obtained more group features. for example, how these particles come into contact with each other, as well as their morphological differences.

Through the analysis of these characteristics, they found a very special trend: after 10 charge-discharge cycles, the most important factor to determine the damage and failure of the particles is the individual characteristics of the particles, such as the specific surface area of the particles, whether it is spherical or not. However, after 50 charge-discharge cycles, the most important factor is whether these particles have similar "uniform" population characteristics such as particle size, particle arrangement and morphology.

After 10 charge-discharge cycles, the individual characteristics of cathode particles play a key role, while after 50 charge-discharge cycles, it is the group behavior of cathode particles. (photo source: doi: 10.1126 / science.abm8962) We can see that as the charge-discharge cycle goes on, or after the phone has been used for a longer period of time (for example, 1 year), it is the interaction between electrode particles that determines the life of the lithium-ion battery. This is important for scientists and engineers because they can develop technologies to control the mass behavior of particles by designing and manufacturing battery electrodes to "squeeze" more electricity.

Liu Yijin of the SLAC National Accelerator Laboratory in the United States, one of the study's correspondent authors, suggested: "the arrangement between particles can be controlled by magnetic or electric fields." Professor Feng Lin of Virginia Tech, one of the study's newsletter authors, says his lab is currently redesigning battery electrodes to create electrode structures that support fast charging and long life.

Reference link:

Https://www.science.org/doi/10.1126/science.abm8962

Https://vtx.vt.edu/articles/2022/04/science-feng_lin_battery_recharge_lifespan_testing.html

Https://onlinelibrary.wiley.com/doi/10.1002/aenm.201900674

Https://www.sciencedirect.com/science/article/abs/pii/S0022509619303126?via%3Dihub

Https://www.nature.com/articles/s41563-021-00936-1

Https://www.cnet.com/tech/mobile/does-fast-charging-affect-battery-life-6-phone-battery-questions-answered/

Https://batteryuniversity.com/article/bu-409-charging-lithium-ion

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), written by Wang Yibo revision: 27

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