Lithium Battery Knowledge-What is Lithium Battery?

Lithium batteries are already closely related to our lives, and our food, clothing, housing and travel are inextricably linked to them. However, many people do not understand lithium batteries. This is not limited to ordinary consumers. Even the designers of various electrical appliances have a smattering of knowledge about lithium batteries. The following will share a series of articles on lithium battery knowledge. Through these articles, everyone will have a systematic understanding of lithium batteries.

The lithium-ion battery mentioned here refers specifically to the secondary lithium-ion battery that can be recharged repeatedly, rather than the primary battery that is thrown away when it is used up. Lithium-ion batteries are distributed in every corner of our lives. Their application areas include mobile phones, tablet computers, notebook computers, smart watches, mobile power supplies (power banks), emergency power supplies, razors, electric bicycles, electric vehicles, electric buses, tourist vehicles, drones, and other power tools. As the carrier of electrical energy and the source of power for many equipment, it can be said that without lithium-ion batteries, today’s material world will not be able to play around (unless we want to go back decades). So, what is a lithium-ion battery?

This article does not popularize the basic principles and development history of batteries. If you are interested, please check on Baidu. There are many stories here.The basic theories in the fields of physics and chemistry were basically confused by the wave of people before Einstein. Batteries are directly related to these two fields. The theories related to batteries were almost studied before World War II, and there was no major innovation after World War II. As a kind of battery technology, the theoretical research on lithium-ion batteries has not made any breakthroughs in recent years. Most of the research has focused on materials, formulas, processes, etc., that is, how to improve the degree of industrialization and develop lithium-ion batteries with better performance (store more energy and last longer).

How to Choose a Carrier of Energy

First of all, everyone will ask, why choose lithium as an energy carrier? Well, although we don’t want to review the knowledge of chemistry, we have to go to the periodic table to find the answer to this question. Fortunately, everyone always remembers the periodic table, right?！I really don’t remember, let’s just take a minute to take a look at the table below.

If you want to be a good energy carrier, you must store and carry more energy in the smallest possible size and weight. Therefore, the following basic conditions need to be met：

1) The relative mass of atoms is smaller

2) Strong electronic ability to gain and lose

3) The proportion of electronic transfer should be 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 preliminary screening, we can only find 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 are left.

Hydrogen is the best energy carrier in nature, so the research of hydrogen fuel cells has always been in the ascendant, representing 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, then portable nuclear fuel batteries will have broad room for development.

The next step is lithium. The choice of lithium as a battery is based on the relatively optimal solution we can find among all the current elements of the earth (the reserves of beryllium are too small, and it is a rare metal among rare metals). The dispute over the technical route of 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 at present. Of course, there are also many commercial interests and even political games involved. These are not the areas to be discussed in this article.

By the way, the energy sources that already exist in nature and are widely used by mankind, such as oil, natural gas, coal, etc., are also mainly 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 will eventually go the same way.

How Lithium-ion Batteries Work

Let’s talk about the working mechanism of lithium-ion batteries. The redox reaction is not described here. Those with a bad chemical foundation, or those who have returned their chemical knowledge to the teacher, will feel dizzy when they see these professional things, so we should make a straightforward description. Borrow a picture here, this picture is easier for people to understand the principle of lithium-ion batteries.

According to the habit of use, we distinguish the positive electrode (+) and the negative electrode (-) according to the voltage difference during charge and discharge. The anode and cathode are not mentioned here, which is time-consuming and laborious.In this picture, the cathode material of the battery is lithium cobalt oxide (LiCoO2) and the anode material is graphite (C).

When charging, under the influence of an external electric field, the lithium element in the molecule of the cathode material LiCoO2 is detached 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 reacts chemically with the carbon atoms of the negative electrode to generate LiC6, so the lithium ions that run out of the positive electrode are very “stably” embedded in the graphite layered structure of the negative electrode. The more lithium ions that run out of the positive electrode and transfer to the negative electrode, the more energy this battery can store.

When discharging, it is just the opposite. The internal electric field turns, and the lithium ion (Li+) detaches from the negative electrode. Following the direction of the electric field, it runs back to the positive electrode and becomes a lithium cobalt oxide molecule (LiCoO2) again. The more lithium ions that run out of the negative electrode and transfer to the positive electrode, the more energy this battery can release.

During each charge and discharge cycle, lithium ions (Li+) act as the carrier of electrical energy, moving back and forth from the positive electrode to the negative electrode to the positive electrode over and over again, chemically reacting with the positive and negative electrode materials, converting chemical energy and electrical energy to each other, and realizing the transfer of charge. This is the basic principle of “lithium-ion battery”. Since electrolytes, isolation membranes, etc. are all insulators of electrons, there is no movement of electrons back and forth between the positive and negative electrodes during this cycle, they only participate in the chemical reaction of the electrodes.

The Basic Composition of Lithium-ion Batteries

To achieve the above functions, lithium-ion batteries need to contain several basic materials inside: positive electrode active substance, negative electrode active substance, isolation film, electrolyte. Let’s make a brief discussion below, what are these materials for?

It is not difficult to understand the positive and negative electrodes. To achieve charge movement, a positive and negative electrode material with a potential difference is required. So what is an active substance? We know that batteries actually convert electrical energy and chemical energy to each other to achieve energy storage and release. To realize this process, it is necessary that the positive and negative materials are “easy” to participate in chemical reactions, to be active, to be easy to oxidize and reduce, so as to achieve energy conversion, so we need “active substances” to be the positive and negative electrodes of the battery.

As already mentioned above, lithium is our preferred material for batteries, so why not use lithium metal as the active substance for electrodes? Isn’t this the maximum energy density that can be achieved?

Let’s look at the picture above again. The three elements oxygen (O), cobalt (Co), and lithium (Li) constitute a very stable cathode material structure (the proportion and arrangement in the figure are for reference only), and the carbon atom arrangement of the anode graphite also has a very stable layered structure. Positive and negative electrode materials must not only be lively, but also have a very stable structure in order to achieve orderly and controllable chemical reactions. What is the unstable result? Think about the burning of gasoline and the explosion of bombs, and the violent release of energy. The process of chemical reaction is actually impossible to accurately control artificially, so the chemical energy becomes heat energy, and the energy is released at once, and it is irreversible.

The lithium element in the metal form is too “lively”, and most naughty children are disobedient and like to destroy it.Early research on lithium batteries did focus on lithium metal or its alloy as the negative electrode, but because of outstanding safety issues, other better paths had to be found.In recent years, with people’s pursuit of energy density, this research direction has a trend of “resurrection with blood”, which we will talk about later.

In order to achieve chemical stability in the process of 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 lively when it needs to be lively and stable when it needs to be stable. After long-term research and exploration, people have found several metal oxides of lithium, such as lithium cobalt oxide, lithium titanate, lithium iron phosphate, lithium manganese oxide, nickel-cobalt-manganese ternary and other materials, as the active substance of the positive or negative electrode of the battery, to solve the above problems.As shown in the figure above, the peridot structure of lithium iron phosphate is also a very stable cathode material structure. The de-embedding of lithium ions during the charging and discharging process does not cause the lattice to collapse. Off topic, lithium metal batteries are indeed available, but compared with lithium-ion batteries, they are almost negligible. The development of technology will ultimately serve the market.

Of course, while solving the stability problem, it also brought serious “side effects”. That is, the proportion of lithium as an energy carrier was greatly reduced, and the energy density was reduced by more than an order of magnitude. Gains and losses, the natural way.

Graphite or other carbon materials are usually used as active substances for negative electrodes. They also follow the above principles. They require not only a good energy carrier, but also relatively stable, and relatively rich reserves, which are easy to manufacture on a large scale. Looking around, carbon is a relatively optimal solution.Of course, this is not the only solution. The research on anode materials is extensive, and it will be discussed later.

What do electrolytes do? In layman’s terms, it is the “water” in the swimming pool that allows lithium ions to swim around freely. Therefore, the ion conductivity should be high (the resistance to swimming is small), the electronic conductivity should be small (insulation), the chemical stability should be good (stability is overwhelming), the thermal stability should be good (all for safety), and the potential window should be wide.Based on these principles, after long-term engineering exploration, people have found electrolytes made of high-purity organic solvents, electrolyte lithium salts, and necessary additives. Electrolytes are formulated under certain conditions and in a certain proportion. Organic solvents include PC (propylene carbonate), EC (vinyl carbonate), DMC (dimethyl carbonate), DEC (diethyl carbonate), EMC (methyl ethyl carbonate) and other materials. Electrolyte lithium salts have LiPF6, LiBF4 and other materials.

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 is getting smaller and smaller, and short circuit has become a huge risk. In order to prevent the positive and negative electrode materials from shorting and causing a violent release of energy, it is necessary to use a material to “isolate” the positive and negative electrodes. This is the origin of the isolation film. The isolation film needs to have good ion permeability, mainly to open a channel for lithium ions to pass freely, and at the same time it is an insulator for electrons to achieve insulation between the positive and negative electrodes. At present, the main diaphragms on the market are single-layer PP, single-layer PE, double-layer PP/PE, three-layer PP/PE/PP composite film, etc.

4.The complete material composition of lithium-ion batteries

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

In addition to active substances, there are conductive agents and binders, as well as substrates and collectors used as current carriers (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, and the conductive agent should enhance the conductivity of the active substance and the substrate to achieve a greater charge and discharge current. The collector is responsible for acting as a charge transfer bridge inside and outside the battery. The structure of the negative electrode is basically the same as that of the positive electrode. A binder is required to fix the active substance graphite, and copper foil is required as the substrate and the collector to act as the conductor of current.

However, because graphite itself has good electrical conductivity, the negative electrode generally does not add conductive agent material. In addition to the above materials, a complete lithium-ion battery also includes an insulating sheet, a cover plate, a pressure relief valve, a housing (aluminum, steel, composite film, etc.), and other auxiliary materials.

Production process of lithium-ion battery

The production process of lithium-ion batteries is more complicated, and only some of the key processes are briefly described here. Depending on the assembly method of the pole piece, there are usually two process routes: winding and laminating.

The lamination process is to cut the positive and negative electrodes into small pieces and stack the isolation film to synthesize small cell monomers, and then stack the small cell monomers in parallel to form a large cell manufacturing process. The general process flow is as follows：

The winding process is to fix the positive and negative electrode sheet, isolation film, positive and negative electrode ear, protective tape, termination tape and other materials on the equipment, and the equipment is unwound to complete the battery cell production.

The common shapes of lithium-ion batteries are mainly cylindrical and square. Depending on the housing material, there are metal housings and soft-packed housings. Today, our sharing is over. We will give more articles about lithium batterirs. Stay tuned

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