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The lithium-ion battery referred to here specifically refers to a rechargeable secondary lithium-ion battery, rather than a disposable battery that can be discarded after use.

Lithium ion batteries are distributed in every corner of our lives, and their application areas include mobile phones, tablets, laptops, smartwatches, mobile power supplies (power banks), emergency power supplies, razors, electric bicycles, electric cars, electric buses, tourist vehicles, drones, and other types of electric tools. As a carrier of electricity and a power source for many devices, it can be said that without lithium-ion batteries, the material world today cannot be played around (unless we want to go back decades ago). So, what exactly is lithium-ion batteries?

This article does not popularize the basic principles and development history of batteries. Interested parties can search on Baidu, as there are many stories here. The basic theories in the fields of physics and chemistry were basically mastered by the previous wave of people before Einstein. Batteries are directly related to these two fields, and theories related to batteries have been studied almost before World War II. There has been no significant innovation since World War II. As a type of battery technology, there has been no breakthrough in the theoretical research of lithium-ion batteries in recent years. Most of the research has focused on materials, formulas, processes, and other aspects, that is, how to improve the degree of industrialization and develop lithium-ion batteries with better performance (storing more energy and lasting longer).

Many people are using lithium-ion batteries, and many are researching the product applications of lithium-ion batteries (such as the products mentioned above). However, most people know very little about lithium-ion batteries, or they are always confused and don't get the main idea. The purpose of writing this article is not for those who are engaged in lithium-ion battery research and development, but for engineering and technical personnel or users who use lithium-ion batteries in their products. Therefore, this article strives to be easy to understand and avoid using specialized terminology and formulas. It is hoped that while reading easily, it can enhance everyone's understanding of lithium-ion batteries and play a role in answering questions and solving doubts.

The author himself is not an expert in the field of lithium-ion batteries and has not been engaged in the technology or product development of lithium-ion battery monomers. However, he has long been engaged in research on the application technology of lithium-ion batteries. Therefore, I hope to explain my understanding of lithium-ion batteries from the perspective of "users". 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 mainstay of current lithium-ion batteries.

Most of the content in the article is not original, but existing knowledge. Standing on the shoulders of giants, all we need to do is stand up straight, lift our heads, and the world is right in front of us.

2、 The basic principles of lithium-ion batteries

How to choose the carrier of energy

Firstly, why choose lithium as the energy carrier?

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

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

1) The relative mass of atoms should be small

2) Strong electronic ability for gain and loss

3) The electron transfer ratio should be high

Based on these three basic principles, the elements on the top of the periodic table are better than those on the bottom, and the elements on the left are better than those on the right. Preliminary screening shows that we can only search for materials in the first and second cycles of the periodic table: hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, and neon. Excluding inert gases and oxidants, only hydrogen, lithium, beryllium, boron, and carbon remain.

Hydrogen is the best energy carrier in nature, so research on hydrogen fuel cells has always been thriving, representing a very promising direction in the field of batteries. Of course, if nuclear fission technology can make significant breakthroughs in the coming decades, achieving miniaturization or even miniaturization, then portable nuclear fuel cells will have broad development space.

The next step is lithium. Choosing lithium as a battery is based on the relatively optimal solution we can find among all the current elements on Earth (beryllium has too little reserves and is a rare metal among rare metals). The technology route dispute between hydrogen fuel cells and lithium-ion batteries is raging in the field of electric vehicles, probably because these two elements are currently the best energy carriers we can find. Of course, there are also many commercial interests and even political games involved in this, which are not within the scope of this article.

By the way, energy sources that already exist in nature and are widely used by humans, such as oil, natural gas, coal, etc., are mainly composed of elements such as carbon, hydrogen, oxygen (in the first and second cycles of the periodic table). So whether it's natural selection or human design, they all end up going the same way.

2. Working principle of lithium-ion batteries

The following will discuss the working mechanism of lithium-ion batteries. We will not elaborate on redox reactions here. For those with poor chemical foundations or those who have already returned their chemical knowledge to the teacher, seeing these professional things will make them dizzy. Therefore, let's provide some straightforward descriptions. Here is a picture that makes it easier for people to understand the principle of lithium-ion batteries.

We differentiate the positive electrode (+) and negative electrode (-) based on the voltage difference during charging and discharging according to usage habits. We do not mention the anode and cathode here, which is time-consuming and laborious. On this picture, the positive electrode material of the battery is lithium cobalt oxide (LiCoO2), and the negative electrode material is graphite (C).

When charging, under the influence of an external electric field, the lithium element in the positive electrode material LiCoO2 molecule separates and becomes positively charged lithium ions (Li+). Under the action of the electric field force, it moves from the positive electrode to the negative electrode and reacts chemically with the carbon atoms of the negative electrode to generate LiC6. Therefore, the lithium ions that run out of the positive electrode are stably embedded into the graphite layered structure of the negative electrode. The more lithium ions are transferred from the positive electrode to the negative electrode, the more energy this battery can store.

When discharging, the opposite happens. The internal electric field turns, and lithium ions (Li+) detach from the negative electrode, follow the direction of the electric field, and run back to the positive electrode, becoming lithium cobalt oxide molecules (LiCoO2) again. The more lithium ions are transferred from the negative electrode to the positive electrode, the more energy this battery can release.

During each charge and discharge cycle, lithium ions (Li+) act as carriers of electrical energy, moving back and forth from the positive electrode to the negative electrode, and reacting chemically with the positive and negative electrode materials to convert chemical and electrical energy, achieving charge transfer. This is the basic principle of lithium-ion batteries. Due to the fact that electrolytes, isolation membranes, and other materials are insulators of electrons, there is no back and forth movement of electrons between the positive and negative electrodes during this cycling process. They only participate in the chemical reactions of the electrodes.

3. Basic composition of lithium-ion batteries

To achieve the above functions, lithium-ion batteries need to contain several basic materials inside: positive electrode active material, negative electrode active material, isolation film, and electrolyte. Below is a brief discussion on what these materials are for.

It is not difficult to understand the positive and negative electrodes. In order to achieve charge movement, positive and negative electrode materials with potential differences are needed. So what is an active substance? We know that batteries actually convert electrical and chemical energy to achieve energy storage and release. To achieve this process, it is necessary for the materials of the positive and negative electrodes to be easily involved in chemical reactions, active, and easily oxidized and reduced, in order to achieve energy conversion. Therefore, we need "active substances" to be the positive and negative electrodes of the battery.

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

Let's take a look at the above figure again. Oxygen (O), cobalt (Co), and lithium (Li) form a very stable positive electrode material structure (the proportion and arrangement in the figure are for reference only), and the carbon atom arrangement of negative electrode graphite also has a very stable layered structure. Positive and negative electrode materials not only need to be active, but also have a very stable structure to achieve ordered and controllable chemical reactions. What are the unstable outcomes? Think about gasoline combustion and bomb explosions, where energy is violently released. The process of this chemical reaction is actually precisely controlled by no one, so chemical energy becomes heat energy, releasing the energy all at once, and it is irreversible.

The metallic form of lithium element is too lively, and mischievous children are mostly disobedient and enjoy causing damage. Early research on lithium batteries did indeed focus on metallic lithium or its alloys as negative electrodes, but due to prominent safety issues, other better paths had to be sought. In recent years, with people's pursuit of energy density, there has been a trend of "full blood resurrection" in this research direction, which we will discuss later.

In order to achieve chemical stability during energy storage and release, i.e. the safety and long lifespan of battery charging and discharging cycles, we need an electrode material that is active when needed and stable when needed. After long-term research and exploration, several lithium metal oxides have been found, such as lithium cobalt oxide, lithium titanate, lithium iron phosphate, lithium manganese oxide, nickel cobalt manganese ternary and other materials, as active substances for battery positive or negative electrodes, solving 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. The deintercalation of lithium ions during charging and discharging does not cause lattice collapse. As a side note, there are indeed lithium metal batteries, but compared to lithium-ion batteries, they can be almost negligible. The development of technology ultimately needs to serve the market.

Of course, while solving the stability problem, it also brings serious "side effects", that is, the proportion of lithium element as an energy carrier is greatly reduced, and the energy density is reduced by more than an order of magnitude. There is always a trade-off, it's natural.

The negative electrode usually chooses graphite or other carbon materials as the active material, following the above principles. It requires a good energy carrier, relative stability, and relatively abundant reserves for large-scale manufacturing. Carbon element is a relatively optimal solution. Of course, this is not the only solution. There is extensive research on negative electrode materials, which will be discussed later.

What is electrolyte for? Simply put, it is the "water" in the swimming pool that allows lithium ions to swim freely. Therefore, the ion conductivity needs to be high (with low swimming resistance), the electron conductivity needs to be low (insulation), the chemical stability needs to be good (stability overwhelms everything), the thermal stability needs to be good (all for safety), and the potential window needs to be wide. Based on these principles, after long-term engineering exploration, people have found electrolytes made from high-purity organic solvents, lithium electrolytes, and necessary additives, which are prepared under certain conditions and in a certain proportion. Organic solvents include PC (propylene carbonate), EC (ethylene carbonate), DMC (dimethyl carbonate), DEC (diethyl carbonate), EMC (methyl carbonate) and other materials. Electrolyte lithium salts include materials such as LiPF6 and LiBF4.

The isolation film is added to prevent direct contact between the positive and negative electrode 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 the short circuit becomes a huge risk. In order to prevent short circuits between positive and negative electrode materials, which can cause violent energy release, it is necessary to use a material to "isolate" the positive and negative electrodes, which is the origin of the isolation film. The isolation membrane needs to have good ion permeability, mainly to open channels for lithium ions to pass freely, and also act as an insulator for electrons to achieve insulation between positive and negative electrodes. At present, there are mainly single-layer PP, single-layer PE, double-layer PP/PE, three-layer PP/PE/PP composite membranes on the market.

4. Complete material composition of lithium-ion batteries

In addition to the four main materials mentioned above, in order to transform lithium-ion batteries from an experimental product in the laboratory into a product that can be commercialized, other indispensable materials are also needed.

Let's first look at the positive electrode of the battery. In addition to the active substance, there are also conductive agents and binders, 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 substance on the positive electrode substrate, while the conductive agent should enhance the conductivity between the active substance and the substrate to achieve greater charging and discharging currents. The current collector is responsible for acting as a charge transfer bridge between the inside and outside of the battery.

The structure of the negative electrode is basically the same as the positive electrode, requiring a binder to fix the active material graphite, and copper foil as the substrate and current collector to act as the conductor of the current. However, due to the good conductivity of graphite itself, conductive materials are generally not added to the negative electrode.

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

5. Production process of lithium-ion batteries

The manufacturing process of lithium-ion batteries is quite complex, and here we will only briefly describe some key processes. According to the different assembly methods of the polarizers, there are usually two process routes: winding and stacking.

The lamination process is a manufacturing process that involves cutting the positive and negative electrodes into small pieces and stacking them with isolation films to form small cell monomers. Then, the small cell monomers are stacked and connected in parallel to form a large cell. The general process flow is as follows:

The winding process is to fix the positive and negative electrode plates, isolation film, positive and negative electrode ears, protective tape, termination tape and other materials on the equipment, and the equipment completes the production of battery cells through unwinding.

The common shapes of lithium-ion batteries are cylindrical and square, and depending on the material of the shell, there are also metal shells and soft packaging shells.

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