The full name of lithium battery should be called lithium ion battery (LIB). Sony industrialized lithium battery in the early 1990s. It uses carbon as the negative electrode and lithium containing compounds as the positive electrode; In the process of charge and discharge, there is no metal lithium, only lithium ion, which is the origin of the name of lithium-ion battery.
- Classification of lithium batteries
Lithium batteries are divided into two categories; Non rechargeable and rechargeable.
Non rechargeable batteries are called disposable batteries, which can only convert chemical energy into electrical energy at one time, and can not restore electrical energy back to chemical energy, such as lithium manganese dioxide primary battery and lithium thionyl chloride primary battery.
Rechargeable batteries are called secondary batteries (also known as batteries). It can convert electric energy into chemical energy for storage, and then convert chemical energy into electric energy when in use. It is reversible, such as the common lithium-ion mobile phone battery on the market.
According to appearance: square and cylindrical.
According to outsourcing materials: aluminum shell lithium battery, steel shell lithium battery and soft pack battery.
According to positive and negative electrode materials (additives): lithium cobalt oxide (LiCoO2) battery or lithium manganate (LiMn2O4), lithium iron phosphate battery and disposable lithium manganese dioxide battery.
According to electrolyte materials: polymer lithium ion battery and liquid lithium ion battery. The electrolyte of liquid lithium-ion battery is liquid, and the electrolyte of polymer battery is gel or solid polymer. The reaction principle of polymer lithium-ion battery is the same as that of liquid lithium-ion battery. It is generally in the form of soft package and has strong shape plasticity. In terms of safety, polymer lithium batteries are safer than lithium-ion batteries.
- Main features of lithium battery
Compared with traditional secondary batteries, lithium-ion batteries have outstanding advantages.
(1) The working voltage is high. The working voltage of lithium-ion battery is 3.6V, which is three times that of nickel cadmium and nickel hydrogen batteries.
(2) The specific energy of lithium-ion battery is high. At present, the specific energy of lithium-ion battery has reached 140W · H / kg, which is three times that of nickel cadmium battery and 1.5 times that of nickel hydrogen battery.
(3) Long cycle life. At present, the cycle life of lithium-ion battery has reached more than 1000 times, tens of thousands of times under low discharge depth, which is more than that of other secondary batteries.
(4) The self discharge is small, and the monthly self discharge rate of lithium-ion battery is only 6% ~ 8%.
(5) Without memory effect, it can be charged at any time according to requirements without reducing the performance of the battery.
(6) It can be charged quickly. Lithium batteries can usually be charged with a current of 0.51c, reducing the charging time to 12h. The current safe discharge rate of lithium-ion battery is about 45C; The international excellent lithium polymer battery has been able to achieve a discharge rate of more than 20c.
(7) It can be used in parallel at will.
(8) There is no pollution to the environment. There are no harmful substances in lithium-ion batteries. They are truly “green batteries”.
The cost of lithium-ion battery is high. Compared with other rechargeable batteries, lithium-ion battery is more expensive.
- Structure and working principle of lithium ion battery
Lithium ion batteries usually have cylindrical and rectangular shapes. It is composed of positive electrode, negative electrode, electrolyte and diaphragm.
Positive electrode: the positive electrode is a lithium ion collector composed of transition metal oxide containing lithium and a current collector composed of aluminum film. Commonly used metal oxides include lithium diamond, lithium manganate, ternary materials and lithium iron phosphate.
The cathode material is composed of carbon and graphite particles, which are commonly used as cathode materials for collecting carbon ions.
Electrolyte: an organic electrolyte, mostly composed of lithium hexafluorophosphate (lifl6) and organic solvent (lithium hexafluorophosphate is formed by the reaction crystallization of phosphorus pentachloride and lithium fluoride dissolved in anhydrous hydrogen fluoride).
Diaphragm: a special composite membrane. Its function is to isolate the positive and negative electrodes, prevent electrons from passing through, and allow lithium ions to pass through, so as to complete the rapid transmission of lithium ions between the positive and negative electrodes in the process of electrochemical charge and discharge. At present, it is mainly polyethylene (PE) or polypropylene (PP) microporous membrane.
Spiral wound structure is adopted inside the battery, and safety valve and PTC element are also installed to protect the battery from damage in case of abnormal state and output short circuit.
During charging, driven by the electric field, lithium ions are separated from the positive lattice, passed through the electrolyte and embedded into the negative lattice. When discharging, the process is just the opposite. The lithium ion returns to the positive electrode, and the electrons pass through the electric appliance and reach the positive electrode from the external circuit to compound with the lithium ion.
Taking LiFePO4 as an example, its chemical reaction equation is as follows:
Charging: LiFePO4 – XLI + – XE – → xfepo4 + (1-x) LiFePO4
Discharge: FePO4 + XLI + + XE – → xlifepo4 + (1-x) LiFePO4
The voltage of a single lithium battery is 3.6V, and the capacity cannot be infinite. Therefore, a single lithium battery is often processed in series and parallel to meet the requirements of different occasions.
- Protection of lithium ion battery
The safety of lithium-ion battery power supply equipment is the most concerned problem, so its protection is very important.
The protection of lithium-ion battery mainly includes overcharge protection, over discharge protection, overcurrent and short circuit protection.
Overcharge protection: when the charger overcharges the lithium-ion battery, it is necessary to terminate the charging state in order to prevent the rise of internal pressure caused by temperature rise. Therefore, the protection device needs to monitor the battery voltage. When it reaches the battery overcharge voltage, it will activate the overcharge protection function and stop charging.
Over discharge protection: in order to prevent the over discharge state of lithium-ion battery, when the voltage of lithium-ion battery is lower than its over discharge voltage detection point, activate the over discharge protection, stop the discharge, and keep the battery in the standby mode of low quiescent current.
Overcurrent and short circuit protection: when the discharge current of lithium-ion battery is too large or short circuit occurs, the protection device will activate the overcurrent protection function.
The temperature control of charge and discharge and the temperature control of charge and discharge process also need to be considered by many designers. Charging and discharging lithium-ion batteries at high temperature may cause explosion; Charging and discharging at low temperature will cause damage to the cell.
- Lithium iron phosphate battery
Lithium iron phosphate (LFP) battery refers to a lithium-ion battery using lithium iron phosphate as the cathode material. Its working principle is the same as that of ordinary lithium-ion battery. In addition to the common characteristics of ordinary lithium batteries, there are also some characteristics.
(1) The nominal voltage of LiFePO4 battery is 3.2V (stable discharge platform), the termination charging voltage is 3.6V, and the termination discharge voltage is 2.0V.
(2) Large specific capacity and high efficiency output: the standard discharge is 25C, the continuous high current discharge can reach 10C, and the instantaneous pulse discharge (10s) can reach 20c.
(3) The operating temperature range is wide (- 20 ℃ ~ + 75 ℃), and the performance is good at high temperature: the external temperature is 65 ℃, the internal temperature is as high as 95 ℃, and the temperature at the end of battery discharge can reach 160 ℃. The internal structure of the battery is safe and intact.
(4) Even if the battery is damaged internally or externally, the battery does not burn, explode and has the best safety.
(5) Excellent cycle life. After 500 cycles, its discharge capacity is still greater than 95%; The lithium iron phosphate single cell prepared in the laboratory has a cycle life of up to 2000 times in the cycle test of IC.
(6) There is no damage when the over discharge reaches zero voltage. After 7 days of storage at zero voltage, the battery has no leakage, good performance and 100% capacity; After storage for 30 days, it has no leakage and good performance, with a capacity of 98%; After 30 days of storage, the battery will be charged and discharged for three more cycles, and the capacity will be restored to 100%.
(7) It can be charged quickly with less self discharge and no memory effect: it can be charged and discharged quickly with a large current of 2C. Under the special charger, the battery can be fully charged within 40 minutes after charging at 1.5C, and the starting current can reach 2C.
(8) Lithium iron phosphate battery has poor low temperature performance. The capacity retention rate is about 6070% at 0 ℃, 4055% at – 10 ℃ and 2040% at – 20 ℃. Such low-temperature performance can not meet the use requirements of power supply, but the current low-temperature performance has been improved.
At present, there are three leading enterprises in the field of lithium iron phosphate in the world, namely A123 in the United States, phostech in Canada and valence in the United States, with relatively mature production technology.