Basic principles, main components and overview of lithium batteries

Basic principles, main components and overview of lithium batteries

Lithium ion battery refers to a secondary lithium ion battery that can be repeatedly charged, rather than primary battery which we thrown away when used up.

Lithium-ion batteries are distributed in every corner of our life. Applications include mobile phones, tablets, laptops, smart watches, mobile power supplies, emergency power supplies, razors, electric bicycles, electric cars, electric buses. Tourist cars, 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 without the lithium-ion battery, today's material world can't play (unless we want to go back decades ago). So, what is the ghost of a lithium-ion battery?

This article is not the basic principle and development history of the popular battery. If you are interested, please check Baidu. There are many stories here. The basic theory in the fields of physics and chemistry was basically made by the wave of people before Einstein. The battery is directly related to these two fields. The theory related to batteries has been studied before World War II. Almost, there was no big innovation after World War II. As a kind of battery technology, related theoretical research on lithium-ion batteries has not made any breakthroughs in recent years. Most of the research is focused on materials, formulations, processes, etc., that is, how to improve the degree of industrialization and research performance. A more excellent lithium-ion battery (more energy is stored and used longer).

Many people are using lithium-ion batteries, many people are studying the application of lithium-ion batteries (such as the products mentioned above), but most people know very little about lithium-ion batteries, or always look at the fog, no way. The purpose of writing this article is not for those who want to develop lithium-ion batteries, but for those engineers who use lithium-ion batteries in their products or users of lithium-ion batteries. Therefore, this article strives to be easy to understand, try not to use specialized terminology and formulas, and hope that in the easy reading, it can enhance everyone's understanding of lithium-ion batteries, and play a role in answering questions.

The author is not an expert in the field of lithium-ion batteries. He has not been engaged in the research and development of lithium-ion battery cells, but he has been engaged in the research of lithium-ion battery applications for a long time. Therefore, he hopes to stand in the perspective of "users" to explain the understanding of lithium-ion batteries. Ordinary users usually call lithium-ion batteries directly as lithium batteries. Although the two are not the same, lithium-ion batteries are indeed the absolute main body of current lithium batteries.

Most of the content in the text is not my original, but the existing knowledge, standing on the shoulders of giants, all we must do is stand up straight, raise our heads, and the world is in front of our eyes.

Second, the basic principle of lithium-ion battery

1. How to choose the carrier of energy

First, everyone will ask, why choose lithium as an energy carrier?

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

To be a good energy carrier, store and carry more energy in as small a volume and weight as possible. Therefore, the following basic conditions need to be met:

1) The relative mass of the atom is small

2) Strong ability to gain and lose electrons

3) The electronic transfer ratio is high

Based on these three basic principles, the elements above the periodic table are better than the elements below, and the elements on the left are better than the elements on the right. For initial screening, we can only find materials in the first and second cycles of the periodic table: hydrogen, helium, lithium, cesium, boron, carbon, nitrogen, oxygen, fluorine, and antimony. Excluding inert gases and oxidants, only five elements of hydrogen, lithium, helium, boron, and carbon remain.

Hydrogen is the best energy carrier in nature, so research on hydrogen fuel cells has been in the ascendant, representing a very promising direction in the battery field. Of course, ifspecial fission technology can make major breakthroughs in the next few decades and can be miniaturized or even miniaturized, portablespecial fuel cells will have broad room for development.

The next step is lithium. Choosing lithium as a battery is based on all the current elements of the Earth. We can find a relatively good solution (the reserves of 铍 are too few, rare metals in rare metals). The technical route between hydrogen fuel cells and lithium-ion batteries is in full swing in the field of electric vehicles. Probably because of these two elements, it is a better energy carrier that we can find now. Of course, there are also many commercial interests and even political games. These are not the areas to be discussed in this article.

By the way, the energy that is already in existence in nature and widely used by humans, such as oil, natural gas, coal, etc., is also composed of carbon, hydrogen, oxygen and other elements (in the first and second cycles of the periodic table). Therefore, whether it is a natural choice or a human "design", it is ultimately the same.

2. The working principle of lithium ion battery

Let's talk about the working mechanism of lithium ion batteries. The oxidation reduction reaction is not described here. The chemical foundation is not good, or the person who has already returned the chemical knowledge to the teacher will see dizziness, so we still have a straightforward description. Borrowing a picture here, this picture is easier to understand the principle of lithium-ion batteries.

According to the habit of use, we distinguish the positive (+) and negative (-) according to the voltage difference between charging and discharging. The anode and cathode are not mentioned here, which is time consuming and laborious. In this figure, the positive electrode material of the battery is lithium cobaltate (LiCoO2), and the negative electrode material is graphite (C).

When charging, under the influence of the applied electric field, the lithium element in the LiCoO2 molecule of the positive electrode material is separated and becomes a positively charged lithium ion (Li+). Under the action of the electric field force, it moves from the positive electrode to the negative electrode, and the negative electrode the carbon atom chemically reacts to form LiC6, so that the lithium ions that run out of the positive electrode are "stably" embedded in the graphite layered structure of the negative electrode. The more lithium ions that are transferred from the positive electrode to the negative electrode, the more energy the battery can store.

When the discharge is just the opposite, the internal electric field turns, lithium ions (Li+) are separated from the negative electrode, and in the direction of the electric field, they run back to the positive electrode and become lithium cobaltate molecules (LiCoO2). The more lithium ions that are transferred from the negative electrode to the positive electrode, the more energy the battery can release.

During each charge-discharge cycle, lithium ion (Li+) acts as a carrier for the electrical energy, and the movement from the positive electrode to the negative electrode to the positive electrode reciprocates back and forth, chemically reacts with the positive and negative materials, and converts chemical energy and electrical energy. The transfer of charge is realized, which is the basic principle of "lithium ion battery". Since the electrolyte, the separator, and the like are all insulators of electrons, no electrons move back and forth between the positive and negative electrodes during this cycle, and they only participate in the chemical reaction of the electrodes.

3. The basic composition of lithium ion battery

To achieve the above functions, the lithium ion battery needs to contain several basic materials inside: a positive electrode active material, a negative electrode active material, a separator, and an electrolyte. Let's briefly discuss what these materials do.

It is not difficult to understand the positive and negative poles. To achieve charge transfer, positive and negative materials with potential difference are needed. What is the active substance? We know that batteries convert electrical energy and chemical energy into each other to achieve energy storage and release. To achieve this process, the material of the positive and negative electrodes needs to be "easy" to participate in the chemical reaction, to be active, to be easily oxidized and reduced, and to achieve energy conversion, so we need "active substances" 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 lithium metal as the active material for the electrodes? Isn't this the maximum energy density that can be achieved?

Let us look at the above picture. Oxygen (O), cobalt (Co), and lithium (Li) constitute a very stable cathode material structure (the ratio and arrangement in the figure are for reference only), and the carbon atoms of the anode graphite. The arrangement also has a very stable layered structure. The positive and negative materials are not only active, but also have a very stable structure to achieve an orderly, controllable chemical reaction. What is the result of instability? Considering the burning of gasoline and the explosion of bombs, the energy is released violently. The process of this chemical reaction is impossible to control precisely, so the chemical energy becomes heat energy, and the energy is released at one time, and it is irreversible.

The lithium element in the form of metal is too "live", and the naughty children are mostly disobedient and like to destroy. Early research on lithium batteries did focus on the direction of using lithium metal or its alloys as a negative electrode, but because of the safety problems, we had to find other better paths. In recent years, with the pursuit of energy density, this research direction has a trend of "full blood resurrection", which we will talk about later.

In order to achieve chemical stability during energy storage and release, that is, the safety and long life of the battery charge and discharge cycle, we need an electrode material that is active when it needs to be active and stable when it needs 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 ternary and other materials, as the active or negative electrode of the battery the substance solves the above problems. As shown in the above figure, the olivine structure of lithium iron phosphate is also a very stable structure of the positive electrode material, and the deintercalation of lithium ions during charging and discharging does not cause lattice collapse. Off-topic, lithium metal batteries are indeed there, but compared with lithium-ion batteries, almost negligible, the development of technology, and ultimately still must 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, the energy density is reduced by more than one order of magnitude, and there must be a loss, the natural way.

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

What is the electrolyte? Popularly speaking, the "water" inside the swimming pool allows lithium ions to swim freely, so the ionic conductivity is high (small resistance to swimming), electronic conductivity is small (insulation), chemical stability to be good (stability overwhelming), thermal stability is good (all for safety), the potential window should be wide. Based on these principles, after long-term engineering exploration, people have found electrolytes prepared from high-purity organic solvents, electrolyte lithium salts, and necessary additives, under certain conditions and at a certain ratio. The organic solvent is PC (propylene carbonate), EC (ethylene carbonate), DMC (dimethyl carbonate), DEC (diethyl carbonate), EMC (ethyl methyl carbonate) and the like. The electrolyte lithium salt has materials such as LiPF6 and LiBF4.

The separator is added to prevent direct contact between the positive and negative materials. We hope to make the battery as small as possible and store as much energy as possible, so the distance between the positive and negative electrodes becomes smaller and shorter. Become a huge risk. In order to prevent the short circuit of the positive and negative materials, resulting in the intense release of energy, it is necessary to "separate" the positive and negative electrodes with a material, which is the origin of the separator. The separator needs to have good ion permeability, mainly to open channels for lithium ions, allowing it to pass freely, and at the same time being an insulator of electrons to achieve insulation between the positive and negative electrodes. At present, the separators 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, in order to turn a lithium-ion battery from an "experimental product" in the laboratory into a product that can be commercialized, other indispensable materials are needed.

Let us first look at the positive electrode of the battery, in addition to the active material, as well as the conductive agent and binder, as well as the substrate and current collector used as the current carrier (the positive electrode is usually aluminum foil). The binder should uniformly "fix" the lithium metal oxide as the active material on the positive base strip, and the conductive agent should enhance the electrical conductivity of the active material and the substrate to achieve a larger charge and discharge current, and the current collector is responsible for acting as a battery. Internal and external charge transfer bridges.

The structure of the negative electrode is substantially 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 conductor of current, but since the graphite itself has good conductivity, the negative electrode generally does not contain a conductive agent material.

In addition to the above materials, a complete lithium-ion battery also includes insulation sheets, cover plates, pressure relief valves, housings (aluminum, steel, composite membranes, etc.), as well as other ancillary materials.

5. Process for manufacturing lithium ion battery

The fabrication process of lithium-ion batteries is complicated, and only a few key processes are briefly described here. Depending on the way the pole piece is assembled, there are usually two routes for winding and lamination.

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

The winding process is to fix the positive and negative electrodes, the separator, the positive and negative electrodes, the protective tape, the termination tape and the like on the device, and the device is completed by unwinding the battery.

The common shapes of lithium-ion batteries are mainly cylindrical and square and with different casing material, there are metal casings and soft casings.

The page contains the contents of the machine translation.