The origin and development of lithium-ion batteries
In the 1970s, Exxon’s M.S. Whittingham used titanium sulfide as the cathode material and metallic lithium as the anode material to make the first lithium battery.
In 1980, J. Goodenough discovered that lithium cobalt oxide can be used as a cathode material for lithium-ion batteries.
In 982, R.R. Agarwal and J.R. Selman of the Illinois Institute of Technology (the Illinois InsTItute of Technology) discovered that lithium ions have the characteristics of intercalating graphite, and this process is fast and reversible. At the same time, the safety hazards of lithium batteries made of metal lithium have attracted much attention. Therefore, people have tried to make rechargeable batteries using the characteristics of lithium ions embedded in graphite. The first available lithium-ion graphite electrode was successfully trial-produced by Bell Laboratories.
In 1983, M. Thackeray, J. Goodenough, and others discovered that manganese spinel is an excellent cathode material with low price, stability, and excellent conductivity and lithium conductivity. Its decomposition temperature is high, and its oxidation is much lower than that of lithium cobalt oxide. Even if there is a short circuit or overcharge, it can avoid the danger of combustion and explosion.
In 1989, A. Manthiram and J. Goodenough discovered that a positive electrode with a polymer anion would produce a higher voltage.
In 1991, Sony released the first commercial lithium-ion battery. Subsequently, lithium-ion batteries revolutionized the face of consumer electronics.
In 1996, Padhi and Goodenough discovered that phosphates with an olivine structure, such as lithium iron phosphate (LiFePO4), are superior to traditional cathode materials, and therefore have become the current mainstream cathode materials.
Li-ion batteries (Li-ion Batteries) are developed from lithium batteries. So before introducing Li-ion, first introduce lithium batteries. For example, button batteries are lithium batteries. The cathode material of lithium batteries is manganese dioxide or thionyl chloride, and the anode is lithium. After the battery is assembled, the battery has voltage and does not need to be charged. This kind of battery can also be charged, but the cycle performance is not good. During the charge and discharge cycle, lithium dendrites are easily formed, causing an internal short circuit of the battery. Therefore, this kind of battery is generally prohibited from charging.
Later, the Sony Corporation of Japan invented a lithium battery with carbon material as the negative electrode and a lithium-containing compound as the positive electrode. During the charging and discharging process, there is no metal lithium, only lithium ions. This is a lithium-ion battery.
In the early 1990s, Sony Energy Development Corporation of Japan and Moli Energy Corporation of Canada successfully developed new types of lithium-ion batteries, which not only have good performance but also have no pollution to the environment. With the rapid development of information technology, hand-held machinery, and electric vehicles, the demand for high-efficiency power supplies has increased dramatically, and lithium batteries have become one of the fastest-growing fields.
The structure and principle of lithium-ion battery
The main components of lithium-ion batteries:
(1) Positive electrode-the active material mainly refers to lithium cobalt oxide, lithium manganate, lithium iron phosphate, lithium nickelate, lithium nickel cobalt manganate, etc. The conductive current collector generally uses aluminum foil with a thickness of 10--20 microns;
(2) Diaphragm-a special plastic film that allows lithium ions to pass through, but it is an electronic insulator. At present, there are mainly two types of PE and PP and their combination. There is also a type of inorganic solid diaphragm, such as alumina diaphragm coating is an inorganic solid diaphragm;
(3) Negative electrode-active material mainly refers to graphite, lithium titanate, or carbon materials with a similar graphite structure. The conductive current collector generally uses copper foil with a thickness of 7-15 microns;
(4) Electrolyte—generally an organic system, such as a carbonate solvent with lithium hexafluorophosphate dissolved in it, and some polymer batteries use gel-like electrolyte;
(5) Battery case-mainly divided into the hard case (steel case, aluminum case, nickel-plated iron case, etc.) and soft case (aluminum-plastic film).
When the battery is charged, lithium ions are extracted from the positive electrode and embedded in the negative electrode, and vice versa during discharge. This requires an electrode to be in a lithium-intercalation state before assembly. Generally, a lithium-intercalation transition metal oxide with a potential greater than 3V relative to lithium and stable in the air is selected as the positive electrode, such as LiCoO2, LiNiO2, LiMn2O4.
As the material of the negative electrode, choose the intercalated lithium compound whose potential is as close as possible to the lithium potential, such as various carbon materials including natural graphite, synthetic graphite, carbon fiber, mesophase sphere carbon, etc. and metal oxides, including SnO, SnO2, Tin composite oxide SnBxPyOz (x=0.4~0.6, y=0.6~0.4, z=(2 +3x+5y)/2), etc.
The electrolyte adopts a mixed solvent system of LiPF6 ethylene carbonate (EC), propylene carbonate (PC) and low viscosity diethyl carbonate (DEC), and other alkyl carbonates.
The membrane adopts polyolefin microporous membranes such as PE, PP, or their composite membranes, especially the PP/PE/PP three-layer membrane not only has a lower melting point but also has a higher puncture resistance, which plays a role in heat insurance.
The shell is made of steel or aluminum, and the cover assembly has the function of explosion-proof and power-off.
Basic working principle
When the battery is charged, lithium ions are extracted from the lithium-containing compound of the positive electrode, and the lithium ions move to the negative electrode through the electrolyte. The carbon material of the negative electrode has a layered structure. It has many micropores. The lithium ions that reach the negative electrode are embedded in the micropores of the carbon layer. The more lithium ions are inserted, the higher the charging capacity.
Main performance indicators of lithium-ion batteries
Battery capacity
The battery capacity is divided into rated capacity and actual capacity. The rated capacity of the battery refers to the amount of electricity that the battery should provide when the battery is discharged to the final voltage at a rate of 5 hours under the condition of an ambient temperature of 20 ℃ ± 5 ℃, expressed by C5. The actual capacity of the battery refers to the actual power released by the battery under certain discharge conditions, which is mainly affected by the discharge rate and temperature (so strictly speaking, the battery capacity should specify the charge and discharge conditions).
Capacity unit: mAh, Ah (1Ah=1000mAh).
Battery internal resistance
The internal resistance of the battery refers to the resistance experienced by the current flowing through the battery when the battery is working. It is composed of ohmic internal resistance and polarization internal resistance. The large internal resistance of the battery will reduce the battery discharge working voltage and shorten the discharge time. The internal resistance is mainly affected by the battery material, manufacturing process, battery structure and other factors. Battery internal resistance is an important parameter to measure battery performance.
Voltage
Open circuit voltage refers to the potential difference between the positive and negative electrodes of the battery when the battery is in a non-working state, that is, when there is no current flowing in the circuit. Under normal circumstances, the open-circuit voltage of a lithium-ion battery is about 4.1-4.2V after being fully charged, and about 3.0V after being discharged. By detecting the open-circuit voltage of the battery, the state of charge of the battery can be judged.
Working voltage, also known as terminal voltage, refers to the potential difference between the positive and negative poles of the battery when the battery is working, that is, when current flows through the circuit. In the working state of battery discharge, when the current flows through the battery, there is no need to overcome the resistance caused by the internal resistance of the battery, so the working voltage is always lower than the open-circuit voltage, and the opposite is true when charging. The discharge working voltage of lithium-ion batteries is around 3.6V.
When the battery is discharged (the process we use the battery), the lithium ions embedded in the carbon layer of the negative electrode are released and move back to the positive electrode. The more lithium ions returned to the positive electrode, the higher the discharge capacity. What we usually call battery capacity refers to discharge capacity.
During the charging and discharging process of lithium-ion batteries, lithium ions are in a state of movement from positive to negative to positive. This is like a rocking chair. The two ends of the rocking chair are the two poles of the battery, and the lithium-ion moves back and forth on both ends of the rocking chair. Therefore, lithium-ion batteries are also called rocking chair batteries.
Charge and discharge mechanism
The charging process of lithium-ion batteries is divided into two stages: constant current charging stage and constant voltage current decreasing charging stage.
Overcharge and discharge of lithium-ion batteries can cause permanent damage to the positive and negative electrodes. The excessive discharge causes the negative carbon sheet structure to collapse, and the collapse will cause the lithium ions to be unable to be inserted during the charging process; overcharging causes too many lithium ions to be embedded in the negative carbon structure, and part of the lithium ions can no longer be released.
The best charging and discharging method for lithium-ion batteries to maintain performance is shallow charge and shallow discharge. Generally 60% DOD is 2 to 4 times the cycle life under 100% DOD conditions.
Discharge platform time
The discharge platform time refers to the discharge time to a certain voltage when the battery is fully charged. For example, measure the 3.6V discharge platform time of a ternary battery, charge it at a constant voltage to a voltage of 4.2V, and stop charging when the charging current is less than 0.02C, that is, after it is fully charged, and then put it aside for 10 minutes at any rate of discharge current The discharge time when the discharge reaches 3.6V is the discharge plateau time at this current.
Because the working voltage of some electrical appliances that use lithium-ion batteries has voltage requirements, if it is lower than the required value, it will not work. Therefore, the discharge platform is one of the important criteria for measuring battery performance.
Charge and discharge rate
The charge-discharge rate refers to the current value required by the battery to discharge its rated capacity within a specified time. 1C is numerically equal to the battery's rated capacity and is usually represented by the letter C. If the nominal rated capacity of the battery is 10Ah, then 10A is 1C (1 rate), 5A is 0.5C, 100A is 10C, and so on.
Self-discharge rate
The self-discharge rate is also called the charge retention capability, which refers to the retention capability of the stored power of the battery under certain conditions when the battery is in an open circuit state. Mainly affected by factors such as the manufacturing process, materials, storage conditions of the battery. It is an important parameter to measure battery performance.
effectiveness
Charging efficiency refers to a measure of the degree to which the electric energy consumed by the battery during the charging process is converted into the chemical energy that the battery can store. It is mainly affected by battery technology, formula and battery working environment temperature. Generally, the higher the environment temperature, the lower the charging efficiency.
Discharge efficiency refers to the ratio of the actual power discharged to the terminal voltage under a certain discharge condition to the rated capacity of the battery. It is mainly affected by the discharge rate, ambient temperature, internal resistance and other factors. Generally, the higher the discharge rate, The lower the discharge efficiency. The lower the temperature, the lower the discharge efficiency.
Cycle life
Battery cycle life refers to the number of charging and discharging times that the battery experiences under a certain charging and discharging system when the battery capacity drops to a certain value. Lithium-ion battery GB stipulates that the capacity retention rate of the battery after 500 cycles under 1C conditions is above 60%.
The main classification of lithium-ion batteries
(1) According to different electrolyte materials used in lithium batteries, lithium batteries can be divided into two categories: liquid lithium battery (lithium-ion battery, referred to as LIB) and polymer lithium-ion battery (referred to as LIP).
(2) According to the charging method, it can be divided into two types: non-rechargeable and rechargeable.
(3) Lithium battery appearance is divided into square type lithium battery (such as commonly used mobile phone batteries) and cylindrical type (such as 18650, 18500);
(4) Lithium battery outsourcing materials are divided into aluminum shell lithium battery, steel shell lithium battery, soft pack battery;
(5) Lithium batteries are divided into positive and negative electrode materials (additives): lithium cobalt oxide (LiCoO2) batteries, lithium manganese oxide (LiMn2O4), lithium iron phosphate batteries, and disposable lithium manganese dioxide batteries
Lithium polymer battery
The positive and negative materials used in polymer lithium batteries are the same as liquid lithium, and the working principles of the batteries are basically the same. Their main difference lies in the difference in electrolytes. Lithium batteries use liquid electrolytes, while polymer lithium batteries are replaced by solid polymer electrolytes. This polymer can be "dry" or "colloidal" At present, most of them use polymer gel electrolyte.
Polymer lithium batteries can be divided into three categories:
1. Solid polymer electrolyte lithium battery. The electrolyte is a mixture of polymer and salt. This battery has low ionic conductivity at room temperature and is suitable for high-temperature use.
2. Gel polymer electrolyte lithium battery. That is, additives such as plasticizers are added to the solid polymer electrolyte to increase the ionic conductivity and enable the battery to be used at room temperature.
3. Lithium battery with polymer cathode material. Using conductive polymer as the cathode material, its energy is three times that of existing lithium batteries and is the latest generation of lithium batteries. Since solid electrolytes are used instead of liquid electrolytes, compared with liquid lithium batteries, polymer lithium batteries have the advantages of being thinner, arbitrarily large, and arbitrarily shaped, and will not cause safety problems such as liquid leakage, combustion, and explosion Therefore, the battery casing can be made of aluminum-plastic composite film, which can increase the capacity of the entire battery; polymer lithium batteries can also use polymer as the cathode material, and its mass-specific energy will be more than 50% higher than that of current liquid lithium batteries. In addition, polymer lithium batteries have improved operating voltage and charge-discharge cycle life than lithium batteries.
Advantages of lithium polymer battery:
1. Good safety performance
The polymer lithium battery is structured with aluminum-plastic soft packaging, which is different from the metal shell of the liquid battery. Once a safety hazard occurs, the liquid battery is prone to explode, while the polymer battery will only blow up.
2. The thickness is small and can be made thinner
Ordinary liquid lithium batteries use the method of customizing the shell first, and then plugging the positive and negative materials. The thickness of the battery is below 3.6mm and there is a technical bottleneck. The polymer cell does not have this problem, and the thickness can be below 1mm, which meets the needs of current mobile phones. direction.
3. Lightweight
The weight of a polymer battery is 40% lighter than a steel shell lithium battery of the same capacity and 20% lighter than an aluminum shell battery.
4. Large capacity
Polymer batteries have a capacity of 10-15% higher than steel shell batteries of the same size and 5-10% higher than aluminum shell batteries. They have become the first choice for color screen mobile phones and MMS mobile phones. Nowadays, most of the new color screen and MMS mobile phones on the market are also used Polymer batteries.
5. Small internal resistance
The internal resistance of polymer batteries is smaller than that of ordinary liquid batteries. At present, the internal resistance of domestic polymer batteries can even be below 35mΩ, which greatly reduces the self-consumption of the battery and prolongs the standby time of the mobile phone. Level in line with international standards. This kind of polymer lithium battery that supports a large discharge current is an ideal choice for remote control models, and it has become the most promising product to replace Ni-MH batteries.
6. The shape can be customized
The polymer battery can increase or decrease the thickness of the battery cell according to the needs of customers, and develop new battery cell models, which are cheap and short in the mold opening cycle. Some can even be tailored to the shape of the mobile phone to make full use of the battery shell space and improve the battery. capacity.
7. Good discharge characteristics
Polymer batteries use colloidal electrolytes, which have stable discharge characteristics and a higher discharge platform than liquid electrolytes.
8. The design of the protection board is simple
Due to the use of polymer materials, the battery core does not fire or explode, and the battery itself has sufficient safety. Therefore, the protection circuit design of the polymer battery can be considered to omit the PTC and fuse to save battery costs. Polymer lithium batteries have great advantages in terms of safety, volume, weight, capacity, and discharge performance.
Lithium iron phosphate battery
From the positive and negative materials, lithium batteries are also divided into lithium cobalt oxide (LiCoO2) batteries, lithium manganese oxide (LiMn2O4), lithium iron phosphate batteries
In the first lithium battery launched by Sony, the cathode material is lithium cobalt oxide and the anode material is carbon. Among them, it is the positive electrode material that determines the maximum rechargeable capacity and open-circuit voltage of the battery.
Lithium iron phosphate battery refers to a lithium battery that uses lithium iron phosphate as a positive electrode material. There are many kinds of cathode materials for lithium batteries, mainly lithium cobalt oxide, lithium manganate, lithium nickelate, ternary materials, lithium iron phosphate, etc. Among them, lithium cobalt oxide is currently the cathode material used in most lithium batteries, while other cathode materials have not been mass-produced in the market due to various reasons. Lithium iron phosphate is also one of the lithium batteries. In terms of material principle, lithium iron phosphate is also an intercalation/deintercalation process, which is exactly the same as lithium cobaltate and lithium manganate. Lithium iron phosphate batteries are used to make lithium secondary batteries. Now the main direction is power batteries, which have great advantages over NI-MH and Ni-Cd batteries.
Characteristics of lithium iron phosphate batteries
1. Super long life
The cycle life of a long-life lead-acid battery is about 300 times, the highest is 500 times, while the cycle life of a lithium iron phosphate power battery is more than 2000 times, and the standard charge (5-hour rate) use can reach 2000 times. Lead-acid batteries of the same quality are "new half a year, half a year old, and half a year for maintenance", which can take up to 1 to 1.5 years, while lithium iron phosphate batteries will reach 7 to 8 years when used under the same conditions. Comprehensive consideration, the performance-price ratio will be more than 4 times that of lead-acid batteries.
2. Safe to use
Lithium iron phosphate completely solves the safety hazards of lithium cobalt oxide and lithium manganese oxide. Lithium cobalt oxide and lithium manganate will explode in a strong collision and pose a threat to the lives of consumers, while lithium iron phosphate has undergone strict Safety tests will not produce an explosion even in the worst traffic accident.
High current 2C can be charged and discharged quickly. With a dedicated charger, the battery can be fully charged within 40 minutes of 1.5C charging, and the starting current can reach 2C. Lead-acid batteries have no such performance now.
3. High-temperature resistance
The peak value of lithium iron phosphate electric heating can reach 350℃-500℃, while lithium manganate and lithium cobaltate are only around 200℃. Wide operating temperature range (-20C--+75C), with high-temperature resistance, lithium iron phosphate electric heating peak can reach 350℃-500℃, while lithium manganate and lithium cobaltate are only around 200℃.
4. Capacity
It has a larger capacity than ordinary batteries (lead-acid, etc.). Rechargeable batteries work under conditions that are often fully charged and not discharged, and the capacity will quickly fall below the rated capacity. This phenomenon is called the memory effect. Like nickel-metal hydride and nickel-cadmium batteries, there is memory, but lithium iron phosphate batteries do not have this phenomenon. No matter what state the battery is in, it can be charged and used at any time without having to discharge it before charging.
The volume of a lithium iron phosphate battery of the same specification and capacity is 2/3 of the volume of a lead-acid battery and its weight is 1/3 of that of a lead-acid battery. The battery does not contain any heavy metals and rare metals (the nickel-hydrogen battery requires rare metals), is non-toxic (SGS certified), non-polluting, meets European RoHS regulations and is an absolute green battery certificate.
5. No memory effect
The performance of lithium power batteries mainly depends on the positive and negative materials. Lithium iron phosphate as a lithium battery material has only appeared in recent years. The domestic development of large-capacity lithium iron phosphate batteries was in July 2005. Its safety performance and cycle life are unmatched by other materials, and these are also the most important technical indicators of power batteries. 1C charging and discharging cycle life reach 2000 times. Single-cell battery overcharge voltage 30V will not burn, a puncture will not explode. Lithium iron phosphate cathode materials make large-capacity lithium batteries easier to use in series. To meet the needs of frequent charging and discharging of electric vehicles. It has the advantages of non-toxic, non-polluting, good safety performance, wide source of raw materials, low price, and long life. It is an ideal cathode material for new generation lithium batteries.
The positive electrode of the lithium battery is lithium iron phosphate material. This new material is not the former lithium battery cathode material LiCoO2; LiMn2O4; LiNiMO2. Its safety performance and cycle life are unmatched by other materials, and these are also the most important technical indicators of power batteries. 1C charging and discharging cycle life reach 2000 times. A single battery will not burn or explode if the overcharge voltage is 30V. Piercing does not explode. Lithium iron phosphate cathode material makes it easier to connect in series to make large-capacity lithium batteries.
Lithium iron phosphate batteries also have their shortcomings: for example, the tap density of lithium iron phosphate cathode materials is small, and the volume of lithium iron phosphate batteries of equal capacity is larger than lithium batteries such as lithium cobalt oxide, so it has no advantage in miniature batteries.
In the post-industrial era, the speed of automobile popularization has greatly exceeded our imagination. While bringing high efficiency and convenience, a large number of exhaust emissions have also added a lot of pressure to the environment. The soaring oil price and carbon dioxide emissions, it has brought greenhouses. The emergence of problems such as effects is an urgent need to find new energy sources to replace traditional energy sources. Liquid hydrogen, fuel cells, etc. are good choices, but there are problems such as high price and immature technology. Common lead-acid batteries have relatively low cost, but they have heavyweight, low energy density, short service life, and potential heavy metals. Pollution and other issues.
1. The security is quite high
To be the power of a car, safety is the overriding primary consideration. Although the safety of ordinary lithium batteries can be basically guaranteed, there is a possibility of fire and explosion under extreme conditions. Lithium iron phosphate battery, as the second-generation product of lithium battery, has stable physical properties, coupled with the built-in protection functions of overvoltage, Undervoltage, overcurrent, and overcharge in the battery pack. It does not explode or fire. It is currently the only absolutely safe product in the world Lithium-Ion Battery. Due to the use of high thermal stability materials and meticulous process design, battery safety and reliability are greatly enhanced. Compared with the explosion that may occur in an improper use of lithium batteries, lithium iron phosphate batteries will not explode even if they are thrown into the fire. The high-temperature stability can reach 400-500°C, which guarantees the inherent high safety of the battery; it will not explode or burn due to overcharge, high temperature, short circuit, or impact. After strict safety testing, there will be no explosion even in the worst traffic accidents.
2. Long life and low cost
As a power battery, the service life (cycle performance) is closely related to the overall cost of use. Compared with the cycle life of an ordinary lithium battery of about 500 times, the lithium iron phosphate battery can be charged and discharged for 1500 cycles at room temperature, and the capacity retention rate is 95% Above, and the cycle life of 50% capacity has reached more than 2000 times, the battery's continuous mileage life is more than 500,000 kilometers, and it can be used for about five years, which is 8 times that of lead-acid batteries and 3 times that of nickel-hydrogen batteries, which is cobalt. About 4 times that of lithium-acid batteries. In addition, the manufacturing cost itself is lower than that of ordinary lithium batteries, which will undoubtedly greatly reduce the use and maintenance costs of electric vehicles.
At the same time, the discharge performance of the lithium iron phosphate battery is also very good, the power curve is stable, and the ability to resist over-discharge is strong. When the ordinary lithium battery cell is lower than 3.2V, the discharge will be over-discharged, which may lead to scrap. However, the lithium iron phosphate battery still has energy release when it is 2.8V, and there is no problem with scrapping under 2.5V.
3. Easy to use and handle
We know that nickel-metal hydride and nickel-cadmium batteries have a strong memory effect, and ordinary lithium batteries also have certain memory effect problems. They need to be "full charge" as much as possible, which will bring inconvenience to the daily use of electric vehicles, while lithium iron phosphate batteries have no This phenomenon, self-discharge is small; there is no memory effect, the battery can be charged and used no matter what state it is in, it does not need to be discharged and then recharged. At the same time, the battery has excellent fast charging characteristics. It can be charged quickly with a special charger and can be sufficient in half an hour. %about.
The disposal of the battery after the end of its life is also worthy of our attention. The lithium iron phosphate battery does not contain any heavy metals and rare metals is non-toxic, non-polluting, and meets regulations. It is an absolutely green battery. There is a large amount of lead in lead-acid batteries. , If it is disposed of improperly, it will cause secondary pollution to the environment, and the lithium iron phosphate material is pollution-free in both production and use.
