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Lithium Battery Bisc Knowledges
- May 18, 2018 -

I. Introduction

The lithium-ion battery mentioned here refers specifically to a rechargeable lithium-ion secondary battery, not a disposable battery.

Lithium-ion batteries are distributed in every corner of our lives. Applications include mobile phones, tablet computers, notebook computers, smart watches, mobile power supplies (rechargeable treasures), emergency power supplies, razors, electric bicycles, electric vehicles, and electric buses. , tourist and sightseeing vehicles, drones, and other types of power tools. As a carrier of electrical energy and a source of power for many devices, it can be said that the lithium-ion battery has left the current world of things (unless we want to go back a few decades ago). So what's the hell of a lithium-ion battery?

This article does not include the basic principles and development history of the battery. If you are interested, please contact Baidu. Here are many stories. Fundamental theories in the field of physics and chemistry were basically made by seventy-eighters by Einstein's previous wave. The battery is directly related to these two fields. The theory related to batteries was studied before World War II. Almost no, after World War II there was no big innovation. As a kind of battery technology, the theoretical research on lithium-ion batteries has not made any breakthroughs in recent years. Most studies have focused on materials, formulations, and processes, that is, how to increase the degree of industrialization and study the performance. Better lithium-ion battery (more energy storage, longer use).

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Many people are using lithium-ion batteries, many people are studying the application of lithium-ion battery products (such as the above mentioned products), but most people know little about lithium-ion batteries, or always look in the fog, no way to lead. The purpose of this article is not to look at people doing lithium-ion battery R&D, but to look at those who use lithium-ion batteries in their products. Therefore, this article strives to be easy to understand and try not to use specialized terminology and formulas. I hope that in addition to easy reading, I can raise everyone’s awareness of lithium-ion batteries and play a role in answering doubts.

The author himself is not an expert in the field of lithium-ion batteries and has not engaged in the research and development of lithium-ion battery cells. However, he has long been engaged in research on the application technology of lithium-ion batteries, so he hopes to stand on the “user” point of view to elaborate on me. Lithium-ion battery awareness. Ordinary users usually refer to lithium-ion batteries directly as lithium batteries. Although the two are not completely equivalent, lithium-ion batteries are indeed the absolute main body of current lithium batteries.

Most of the content of the article is not its own originality but the existing knowledge. Standing on the shoulders of giants, all we have to do is stand still and lift our heads. The world is right in front of us.

Second, the basic principles of lithium-ion batteries

1. How to choose the energy carrier

First of all, we will ask why we choose lithium as an energy carrier.

Well, although we do not want to review the knowledge of chemistry, but this problem must go to the periodic table to find the answer. Fortunately, we still remember the periodic table of elements, right? ! I really don't remember. We'll take a minute to look at the table below.

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To become a good energy carrier, it is necessary to store and carry more energy with the smallest possible size and weight. Therefore, the following basic conditions need to be met:

1) The relative quality of the atom is small

2) strong gains and losses of electronic capabilities

3) High proportion of electronic transfer

Based on these three basic principles, the elements above the periodic table are better than the following elements. The left one is better than the right one. For initial screening, we can only find materials in the first and second cycles of the periodic table: hydrogen, helium, lithium, helium, boron, carbon, nitrogen, oxygen, fluorine, and helium. Excluding the inert gas and oxidant, only hydrogen, lithium, helium, boron, carbon, these five elements.

Hydrogen is the best energy carrier in nature, so the research of hydrogen fuel cells is always on the rise and represents a very promising direction in the field of batteries. Of course, if nuclear fission technology can make major breakthroughs in the next few decades and can be miniaturized or even miniaturized, portable nuclear fuel cells will have ample room for development.

The next thing is lithium. The choice of lithium to make batteries is based on the relative merits of all the elements of the earth. We can find relatively good solutions (there are too few reserves of cesium, rare metals in rare metals). The technical battle between hydrogen fuel cells and lithium-ion batteries is in full swing in the field of electric vehicles, probably because these two elements are the better energy carriers we can find. Of course, there are still many commercial interests and even political games involved. These are not the areas that this article will discuss.

By the way, the energy that has already existed in nature and is widely used by mankind, such as oil, natural gas, and coal, is mainly composed of elements such as carbon, hydrogen, and oxygen (in the first cycle and the second cycle of the periodic table). ). Therefore, whether it is a natural choice or the "design" of human beings, it will ultimately lead to the same goal.

2. How Lithium-Ion Batteries Work

The following describes the working mechanism of lithium-ion batteries. Those who do not elaborate on the redox reaction, have a poor chemical basis, or who have already returned chemical knowledge to their teachers, will be dizzy when they see these professional things, so we still have some straightforward descriptions. Here borrow a picture, this figure is easier for people to understand the principle of lithium-ion batteries.

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According to the habit of use, we distinguish the positive (+) and negative (-) electrodes according to the voltage difference at the time of charge and discharge. The anode and the cathode are not described here. It takes time and effort. In this figure, the positive electrode material of the battery is lithium cobalt oxide (LiCoO2), and the negative electrode material is graphite (C).

During charging, under the influence of an applied electric field, the lithium element in the positive electrode material LiCoO2 is released and becomes a positively-charged lithium ion (Li+), which moves from the positive electrode to the negative electrode under the action of the electric field force, and the negative electrode. The carbon atoms react chemically to produce LiC6, so the lithium ions that run from the positive electrode are "stabilized" to be embedded in the graphite layer structure of the negative electrode. The more lithium ions that move from the positive electrode to the negative electrode, the more energy the battery can store.

When the discharge is reversed, the internal electric field turns and lithium ions (Li+) are disengaged from the negative electrode. Following the direction of the electric field, they return to the positive electrode and become lithium cobalt oxide molecules (LiCoO2) again. The more lithium ions that move from the negative electrode to the positive electrode, the more energy that the battery can release.

During each charge-discharge cycle, lithium ions (Li+) act as carriers for the transport of electrical energy. As the cycle moves from the positive electrode to the negative electrode, the positive electrode moves back and forth, reacts with positive and negative electrode materials to chemically convert chemical energy and electrical energy. The transfer of charge is realized, which is the basic principle of "lithium ion battery". Since electrolytes, separators, etc. are all electronic insulators, no electrons move back and forth between the positive and negative electrodes in this cycle. They only participate in the chemical reactions of the electrodes.

3. Basic composition of lithium ion battery

To achieve the above functions, lithium ion batteries need to contain several basic materials: positive active materials, negative active materials, separators, and electrolytes. The following is a brief discussion of what these materials are all about.

The positive and negative electrodes are not difficult to understand. To realize charge transfer, positive and negative electrode materials need to have a potential difference. What is an active material? We know that batteries actually convert electricity and chemical energy to each other in order to store and release energy. In order to realize this process, it is necessary that the materials of the positive and negative electrodes be “easy” to participate in the chemical reaction, be active, be easy to oxidize and reduce, and thus achieve energy conversion, so we need “active material” to make the positive and negative electrodes of the battery.

As mentioned above, lithium is the preferred material for our batteries, so why not use metallic lithium as the active material of the electrode? Isn't it possible to achieve the maximum energy density?

Let us look at the above picture again. Oxygen (O), Cobalt (Co), and Lithium (Li) elements constitute a very stable positive electrode material structure (the ratio and arrangement in the figure are for reference only). Carbon atoms of negative electrode graphite The arrangement also has a very stable layered structure. Positive and negative materials are not only lively, but also have a very stable structure in order to achieve orderly and controllable chemical reactions. What is the result of instability? Considering the burning of gasoline and the explosion of bombs, the energy is released violently. The chemical reaction process cannot be artificially controlled precisely. Therefore, chemical energy becomes thermal energy, which releases energy once and is irreversible.

The lithium elements in the metal form are too "lively," and naughty children are often less obedient and like to engage in destruction. Earlier researches on lithium batteries did focus on lithium metal or its alloys as the negative electrode, but because of safety issues, they had to find other better routes. In recent years, along with people's pursuit of energy density, this research direction has the trend of “full blood resurrection”, which we will talk about later.

In order to achieve chemical stability in the energy storage and release process, that is, the safety and long life of the battery charge and discharge cycle, we need an electrode material that is lively and lively and stable when it is required to be stable. After long-term research and exploration, people have found several lithium metal oxides, such as lithium cobaltate, lithium titanate, lithium iron phosphate, lithium manganate, nickel cobalt manganese triads, etc., as the activity of the battery positive or negative electrode Substances solve the above problems. As shown in the above figure, the olivine structure of lithium iron phosphate is also a very stable positive electrode material structure. Lithium ion deintercalation during charge and discharge does not cause crystal lattice collapse. Digression, lithium metal batteries do exist, but compared with lithium-ion batteries, almost negligible, the development of technology, and ultimately to serve the market.

Of course, while solving the stability problem, it also brings serious "side effects", that is, the proportion of lithium as an energy carrier is greatly reduced, and the energy density is reduced by more than one order of magnitude. what.

The negative electrode usually uses graphite or other carbon materials as the active material. It also follows the above principles. It requires not only a good energy carrier, but also a relatively stable and relatively abundant reserves. It is convenient for large-scale manufacturing and finding carbon. Elements are a relatively good solution. Of course, this is not the only solution. The research on anode materials is very extensive and discussed later.

What is the electrolyte? In general, the "water" inside the swimming pool allows lithium ions to swim freely. Therefore, the ion conductivity is high (the resistance of swimming is small), the electronic conductivity is small (insulation), and chemical stability Better (stability overrides everything else), better thermal stability (for safety), and a wider potential window. Based on these principles, after long-term engineering exploration, people found electrolytes made from high-purity organic solvents, electrolyte lithium salts, and necessary additives, prepared under certain conditions and in certain proportions. Examples of organic solvents include PC (propylene carbonate), EC (ethylene carbonate), DMC (dimethyl carbonate), DEC (diethyl carbonate), and EMC (ethyl methyl carbonate). Lithium electrolyte salts include LiPF6, LiBF4 and other materials.

The separator is added to prevent direct contact between the positive and negative electrodes. We hope to make the battery as small as possible and store as much energy as possible, so that the distance between positive and negative electrodes becomes smaller and shorter. Become a huge risk. In order to prevent short-circuiting of the positive and negative electrode materials and cause a violent release of energy, it is necessary to use a material to “separate” the positive and negative electrodes, which is the origin of the isolation film. The isolation membrane needs to have good ion passage, mainly to open channels for lithium ions, allowing it to freely pass through, and at the same time be an electronic insulator to achieve insulation between the positive and negative electrodes. At present, the diaphragms on the market mainly include single-layer PP, single-layer PE, double-layer PP/PE, and three-layer PP/PE/PP composite film.

 

4. Complete material composition of lithium ion battery

In addition to the four main materials mentioned above, if you want to turn a lithium-ion battery from a "laboratory" in the laboratory into a product that can be used commercially, there are other indispensable materials.

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  Let us first look at the positive electrode of the battery. In addition to the active material, there is also a conductive agent and a binder, and a substrate and a current collector used as an electric current carrier (the positive electrode is usually an aluminum foil). The binder should be used as the active material of lithium metal oxide evenly "fixed" on the positive baseband, the conductive agent is to enhance the conductivity of the active material and the substrate in order to achieve a greater charge and discharge current, the current collector responsible for the battery Internal and external charge transfer bridges.

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The structure of the negative electrode is basically the same as that of the positive electrode, and an adhesive is required to fix the active material graphite. The copper foil is required as a substrate and a current collector to act as a current conductor, but since the graphite itself has good conductivity, the negative electrode generally does not include a conductive agent material.

In addition to the above materials, a complete lithium-ion battery also includes insulation sheet, cover plate, pressure relief valve, housing (aluminum, steel, composite film, etc.), and other auxiliary materials.

5. Lithium-ion battery manufacturing process

Lithium-ion battery manufacturing process is more complex, here only a brief description of some key processes. Depending on the assembly method of the pole pieces, there are usually two routing methods: winding and lamination.

The lamination process is to cut the positive electrode and the negative electrode into small pieces and a separator to form a small cell monomer, and then to stack the small cells in parallel and form a manufacturing process of a large cell. The general process flow is as follows:  

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The winding process is to fix the positive and negative electrodes, separators, positive and negative electrodes, protective tapes, and termination tapes on the equipment, and the equipment is rolled out to complete the manufacture of the batteries.

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The common appearance of lithium-ion batteries is mainly cylindrical and square. According to different shell materials, there are metal shells and soft shells....  


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