explanation of lithium-ion power battery technology - battery cell

What are battery cells? How many types are there?

The cell of a lithium-ion battery is the core unit for storing and providing electrical energy in a lithium battery system. Each cell stores and releases electrical energy through electrochemical reactions. The components of a lithium-ion secondary rechargeable battery include a cell and a protection circuit board. Among them, the cell is the core part of the rechargeable battery responsible for storing electrical energy, while the protection circuit board is responsible for the safety management of the battery. If the protection circuit board is removed, the remaining part is the cell. The quality of the cell directly determines the overall performance and quality of the rechargeable battery.

1.Anode :

Material : Graphite is usually used.
Function : It acts as the negative electrode when the battery is discharging. Lithium ions are stored in the negative electrode during charging and released during discharging.

2.Cathode :
Materials : Made of lithium compounds such as lithium cobalt oxide (LiCoO₂), lithium iron phosphate (LiFePO₄) or nickel cobalt manganese oxide (NMC).
Function : Acts as the positive electrode when the battery is discharging, stores lithium ions during discharge, and releases lithium ions during charging.

3. Electrolyte :
Mterials : Usually a lithium salt (such as lithium hexafluorophosphate LiPF₆) dissolved in an organic solvent.
Function : Allow lithium ions to move between the negative electrode and the positive electrode. The electrolyte must have high ionic conductivity and electrical insulation.

 

4.Separator :
Material : porous polymer membrane.
Function : Prevents direct contact between the positive and negative electrodes (to avoid short circuits) while allowing lithium ions to pass through.

 

5.Current Collectors :
Material : Copper foil is used for negative electrode and aluminum foil is used for positive electrode.
Function : Collects electrons and transfers them to external circuits to provide power.

The shape and structure of the battery cell:

Lithium-ion batteries have different structural forms according to application requirements :

1. Cylindrical battery cells : such as 18650, 21700, etc., with a sturdy metal shell.

2. Square battery cell (prismatic) : The shape is rectangular, which has higher space utilization rate.

3. Soft-pack battery : uses flexible packaging materials, is light in weight and has high energy density.

1.Introduction
The main content of this article is a description of the lithium-ion power battery technology for new energy vehicles. The article introduces different technical points from materials to Pack structure. The main content is divided into two parts. The upper part first introduces the main materials of lithium battery cells, positive electrode materials, negative electrode materials, electrolytes, and diaphragms; then introduces the structures of different single battery cells; finally, according to different cell structures, the advantages and disadvantages of related characteristics are analyzed.

2.Lithium-ion power battery cell material system

2.1. Cathode Materials

Positive electrode materials are the most critical raw materials for lithium-ion batteries. The upstream of positive electrode materials for lithium batteries are mineral raw materials such as lithium, cobalt, and nickel, which are combined with conductive agents, binders, etc. to make precursors. The precursors are synthesized through a certain process to produce positive electrode materials, which are used in different fields.

The positive electrode material is the decisive factor in the electrochemical performance of lithium batteries, directly determining the energy density and safety of the battery, and thus affecting the overall performance of the battery. Positive electrode materials account for the largest proportion of lithium battery materials, accounting for 45%, mainly including active materials, conductive agents, solvents, adhesives, current collectors, additives, etc. According to the magnetic therapy system, they can generally be divided into lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium iron phosphate (LFP), ternary materials nickel cobalt manganese oxide (NCM) and nickel cobalt aluminum oxide (NCA). Among them, lithium iron phosphate is mainly used in the new energy vehicle and energy storage battery market, while ternary materials are widely used in the new energy passenger car, electric bicycle and power tool battery market.

Different positive electrode materials have different advantages and disadvantages. Potassium cobalt oxide positive electrode material has good electrochemical properties and processing properties, as well as relatively high specific capacity, but potassium cobalt oxide material has high cost (metal cobalt is expensive), low cycle life, and poor safety performance. Compared with lithium cobalt oxide, lithium manganese oxide has the advantages of abundant resources, low cost, pollution-free, good safety performance, and good rate performance; but its low specific capacity, poor cycle performance, especially high-temperature cycle performance, has greatly restricted its application. Lithium iron phosphate is low-priced, environmentally friendly, has high safety performance, good structural stability and cycle performance, but its energy density is low and its low-temperature performance is poor. Nickel cobalt manganese ternary material combines the advantages of lithium cobalt oxide, lithium nickel oxide and lithium manganese oxide, and there is an obvious ternary synergistic effect. Compared with positive electrode materials such as lithium iron phosphate and lithium manganese oxide batteries, ternary materials have higher energy density and longer driving range.

2.2. Anode materials

The performance of negative electrode materials directly affects the power battery, and its excellent performance is reflected in the key performance of the power battery, such as cycle life performance, expansion and deformation problems, and rate performance. According to the cost ratio of lithium-ion batteries, negative electrode materials account for 25% to 28% of the total cost of lithium batteries. Compared with the positive electrode materials of lithium batteries, the research on negative electrode materials is in the ascendant. The more ideal negative electrode materials must meet at least the following 7 conditions:

a.The chemical potential is low, forming a large potential difference with the positive electrode material, thus obtaining a high-power battery;

b.It should have a higher specific circulation capacity;

c. Li+ should be easily embedded and extracted from the negative electrode material, with a high coulombic efficiency, so that a relatively stable charge and discharge voltage can be achieved during the Li+ embedding and extraction process;

d. Have good electronic conductivity and ionic conductivity;

e. Good stability and compatibility with electrolytes;

f. The source of the material should be abundant, cheap and the manufacturing process should be simple;

g.Safe, green and pollution-free.

Negative electrode materials that meet the above conditions basically do not exist at present, so it is imperative to study new negative electrode materials with high energy density, good safety performance, low price and easy availability of materials. This is also a hot topic in the field of lithium battery research at this stage. At present, the negative electrode materials of lithium-ion batteries mainly include carbon materials, transition metal oxides, alloy materials, silicon materials and other silicon-containing materials, lithium-containing transition metal nitrides and lithium titanate materials. According to the composition of the materials, lithium battery negative electrode materials can usually be divided into two categories: carbon materials and non-carbonaceous materials. Carbon material negative electrodes are further classified into natural graphite negative electrodes, artificial graphite negative electrodes, mesophase carbon microbeads (MCMB), soft carbon (such as coke) negative electrodes, hard carbon negative electrodes, carbon nanotubes, graphene, carbon fibers, etc.; other non-carbon negative electrode materials are mainly divided into silicon-based and composite materials, nitride negative electrodes, tin-based materials, lithium titanate, alloy materials, etc.

Carbon material negative electrode is a general term, which can be generally divided into five categories: graphite, hard carbon, soft carbon, carbon nanotubes and graphene. Graphite can be divided into artificial graphite, natural graphite and mesophase carbon microspheres.

 2.3.Electrolyte

The electrolyte is the carrier of ion transmission in the battery. It is generally composed of lithium salt and organic solvent. The electrolyte plays the role of conducting ions between the positive and negative electrodes of the lithium battery, and is the guarantee for the lithium-ion battery to obtain advantages such as high voltage and high specific energy. The electrolyte is generally made of high-purity organic solvents, electrolyte lithium salts, necessary additives and other raw materials, prepared under certain conditions and in a certain proportion. The main electrolytes used in lithium batteries are lithium perchlorate, lithium hexafluorophosphate, etc. However, batteries made of lithium perchlorate have poor low-temperature effects and are prone to explosion. Japan and the United States have banned their use. Batteries made of fluorine-containing lithium salts have good performance, no explosion risk, and strong applicability, especially batteries made of lithium hexafluorophosphate. In addition to the above advantages, the disposal of discarded batteries in the future is relatively simple and friendly to the ecological environment. Therefore, the market prospects of this type of electrolyte are very broad.

The solvent in the electrolyte is electronically insulating and is used to dissolve lithium salts. The basic requirements for the electrolyte solvent system are: 1. Possessing a certain polarity (high dielectric constant) to dissolve lithium salts; 2. Wide electrochemical window (the electrochemical window of the electrolyte is mainly reflected in the electrochemical window of the solvent), resistant to positive electrode oxidation and negative electrode reduction; 3. Low viscosity, easy to wet the electrode and improve low-temperature performance; 4. Heat resistance.

2.4.Diaphragm
The separator is a very important material in lithium-ion power batteries. It is located between the positive and negative electrodes, and plays the role of isolation and preventing direct contact to prevent short circuits and battery failure. It is crucial to the performance and safety of the battery. Common separator materials include polyolefins (such as polypropylene) and polyimide (PI). Polyolefin separators are usually made of polypropylene film, which has high thermal stability and good electrolyte wetting properties. Polyimide separators are made of polyimide film, which has higher thermal stability and lower electrolyte wetting properties, but are more suitable for high temperature and high power applications..

The separator has the following functions in lithium-ion power batteries:

a.Isolate the positive and negative electrodes: The separator can prevent direct contact between the positive and negative electrodes, thereby preventing the battery from short circuiting.

b.Ion conduction: The separator must have sufficient porosity and conductivity so that lithium ions can be transferred between the positive and negative electrodes through the separator to achieve battery charging and discharging.

c. Retention of electrolyte: The diaphragm needs to have a certain pore structure to be able to adsorb and retain the electrolyte to ensure smooth ion transfer between the positive and negative electrodes.

With the advancement of science and technology and the advancement of research and development, the performance and materials of diaphragms will continue to improve, further promoting the development of lithium-ion power batteries, while the performance and safety requirements of diaphragms are also increasing. Some innovative diaphragm materials with self-healing functions are also being studied, which can automatically repair themselves when damaged to improve the service life and reliability of the battery.

 

3.Lithium-ion power battery cell structure

According to the packaging method and the shape of the battery cell, the battery cells can be divided into three categories: cylindrical battery cells, soft-pack battery cells, and square battery cells. The following are the structures of different types of battery cells.

The cylindrical battery cell structure includes: a cap/positive electrode cap, an outer packaging shell/negative electrode, a gasket, a positive electrode sheet, a negative electrode sheet, a diaphragm, an electrolyte, a shell, a waste exhaust valve, a current blocking device, and a diaphragm.

The soft-pack battery cell structure includes: aluminum-plastic packaging film, insulating sheet, positive electrode ear, negative electrode ear, positive electrode, negative electrode, separator, and electrolyte.

The square battery cell structure includes: positive electrode sheet, negative electrode sheet, diaphragm, spacer, insulating plate, cover sealing plate, collector, pressure relief port, negative electrode, gasket, sealing cap, liquid filling port, outer shell/positive electrode, and electrolyte.

4.Lithium battery cell structural characteristics

The representative enterprise of cylindrical batteries is Panasonic Battery. Panasonic has entered the battery field since 1923 and launched dry batteries, alkaline batteries, nickel-cadmium batteries and other consumer batteries for daily household appliances. From dry batteries to power batteries, Panasonic has always been one of the earliest players. Common 18650, 21700, 26700, 4680 and other models of batteries belong to cylindrical batteries. Cylindrical batteries have a "long history" and adopt a winding process. The outer shell is mostly made of steel and aluminum shells. It has the advantages of mature production process, flexible grouping, high yield and consistency. However, due to its small size and small monomers, a large number of cylindrical batteries are required for grouping, and the space utilization rate is low. A large number of batteries require a more complex battery management system, which has higher requirements for BMS collection and logic processing. Tesla is a leader in BMS. The Model S battery pack has 16 modules, each with 444 modules, and a total of about 7104 18650 cells. Tesla has a very deep binding with Panasonic batteries and uses a large number of Panasonic cylindrical NCA batteries, which occupy most of its production capacity.

The representative company of soft-pack batteries is LG Chem. LG Chem was founded in 1947. It started to develop lithium batteries in 1995, mass-produced consumer battery products in 1999, produced the world's first batch of PHEV soft-pack batteries in 2010, and produced BEV soft-pack batteries in 2011. After 25 years of development, it has occupied a place in the global battery industry, and on December 1, it split its battery business - LG New Energy. LG New Energy has layouts in both soft-pack and cylindrical (LG New Energy provides Tesla with 21700 cylindrical batteries), and its soft-pack accounts for the largest proportion, making it the king of soft-pack batteries. Soft-pack power batteries are simply large-sized mobile phone batteries. The production process of soft-pack batteries is complicated, including stacking, folding, lamination, cutting and exhausting. It has a thinner volume. The outer shell is made of aluminum-plastic film material, which has the characteristics of flexible design, light weight, high energy density, and low internal resistance (the theoretical energy density of soft-pack batteries is higher than that of square batteries and cylindrical batteries). However, with the development of battery technology and new energy vehicles, the disadvantages of soft packs have gradually become apparent.

Disadvantage 1: Safety. Due to the unique aluminum-plastic film packaging of the soft-pack, the soft-pack battery cell cannot guarantee the explosion or heat conduction direction after internal thermal runaway. At the same time, the aluminum-plastic film cannot share the external extrusion force. When extrusion occurs, it is easy to cause the internal core to deform and cause thermal runaway.

Disadvantage 2: Reliability. There is a risk of leakage in the long-term use of soft-pack batteries. Because the aluminum-plastic film is basically composed of a three-layer structure of PET/AL/PP, the thickness of the AL layer is only 40μm, which is easily pierced by small metal particles and causes leakage. At the same time, the PP layer of the soft-pack packaging will creep once it is stressed. After long-term use, the internal chemical system will produce gas and easily rupture the packaging area.

Disadvantage 3: Grouping efficiency. The soft-pack battery cell itself is not rigid enough and needs to add more structural parts, so the grouping efficiency is lower than that of the square shell battery cell.

Most of the representative companies of square batteries are domestic Tier 1, led by CATL, BYD, and Honeycomb Energy. Taking CATL as an example, its product line covers square, soft pack, and cylindrical, but square cells account for most of its shipments, which is its specialty. Square batteries use winding or lamination technology, with steel or aluminum shells as shells, and have the characteristics of good heat dissipation, good reliability, high space utilization, compact structure and not easily damaged by external forces. Car companies can customize the size of square batteries according to the needs of vehicle models, but the disadvantage is that there are many models and it is difficult to unify the process; its higher space utilization also requires higher requirements for the layout of the cooling system, which will further increase the design cost of the battery pack. However, the square shell can be made into a large module, which can reduce the number of battery cells in the Pack group, and the management of BMS will be more precise. CATL has also launched the CTP high-integration battery development platform, which bypasses the module and is directly integrated into the battery pack. Compared with traditional battery packs, the volume utilization of CTP battery packs is increased by 15%-20%, the battery pack components are reduced by 40%, and the energy density is increased from 180wh/kg to 260wh/kg. In addition, the CTC (Cell to Chassis) technology that CATL is developing will further improve the integration level, which can be understood as a further extension of CTP (Cell to Pack).

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