What is a lithium polymer battery?
Lithium polymer battery ( Li-polymer battery ), is a chemical battery. Compared with the previous battery, it has the characteristics of high energy, miniaturization and light weight. Lithium polymer batteries have ultra-thin features, and can be made into batteries of different shapes and capacities to meet the needs of some products. The theoretical minimum thickness can reach 0.5mm.
The three elements of a general battery: positive electrode, negative electrode and electrolyte. The so-called lithium polymer battery refers to a battery system in which at least one or more of the three elements uses polymer materials. In the li-polymer battery system, most of the polymer materials are used in the positive electrode and electrolyte. The positive electrode material uses conductive polymer or inorganic compounds used in general lithium ion batteries, the negative electrode often uses lithium metal or lithium-carbon intercalation compounds, and the electrolyte uses solid or colloidal polymer electrolytes, or organic electrolytes. Since there is no excess electrolyte in the lithium polymer, it is more reliable and stable.
Characteristics of lithium polymer batteries
Lithium polymer batteries use file alloys as positive electrodes, polymer conductive materials, polyacetylene, polyaniline or poly-p-phenol as negative electrodes, and organic solvents as electrolytes. The specific energy of the lithium polyaniline battery can reach 350W.h/kg, but the specific power is only 50-60W/kg, the operating temperature is -40-70 degrees, and the life span is about 330 times.
Compared with lithium ion batteries, the characteristics of lithium polymer batteries are as follows:
1. Relatively, the problem of battery leakage is improved, but it has not been completely improved.
2. Can be made into a thin battery: With a capacity of 3.6V 250mAh, its thickness can be as thin as 0.5mm.
3. The battery can be designed into various shapes.
4. It can be made into a single high voltage: the battery with liquid electrolyte can only obtain high voltage by connecting several batteries in series, while the polymer battery can be made into a multi-layer combination within a single battery to achieve high voltage because there is no liquid.
5. The discharge capacity is theoretically 10% higher than that of a lithium-ion battery of the same size.
Lithium polymer batteries are packaged in aluminum plastic, and internal problems can be immediately manifested through the outer packaging. Even if there is a safety hazard, it will not explode, but will only swell. In polymer batteries, the electrolyte plays the dual function of the separator and the electrolyte: on the one hand, it separates the positive and negative materials like a diaphragm to prevent self-discharge and short circuit inside the battery. Conduct lithium ions between. Polymer electrolyte not only has good conductivity, but also has the characteristics of light weight, good elasticity, and easy film formation that are unique to polymer materials. It also conforms to the development trend of light weight, safety, efficiency and environmental protection of chemical power sources.
Safety issues of lithium polymer batteries
All lithium-ion batteries, whether they are old or current, including polymer lithium-ion batteries, lithium iron phosphate batteries, etc., are very afraid of internal short-circuits, external short-circuits, and overcharge. Because the chemical properties of lithium are very active and it is easy to burn. When the battery is discharged and charged, the internal temperature of the battery will continue to heat up. The gas generated during the activation process expands, the internal pressure of the battery increases, and the pressure reaches a certain level, it will rupture, causing leakage, fire, or even explosion.
In order to alleviate the danger of lithium-ion batteries, technicians have added ingredients that can inhibit the activity of lithium elements (such as cobalt, manganese, iron, etc.), but these do not essentially change the danger of lithium-ion batteries.
When ordinary lithium-ion batteries are overcharged, short circuited, etc., the battery may heat up, the positive electrode material decomposes, and the negative electrode and electrolyte materials are oxidized, which will cause gas expansion and increase the internal pressure of the battery. When the pressure reaches a certain level After the degree, an explosion may occur. The polymer lithium ion battery uses a colloidal electrolyte, so it will not produce a large amount of gas due to the boiling of the liquid, thereby preventing the possibility of a violent explosion.
At present, most of the polymer batteries on the market are only soft-pack batteries, using aluminum plastic film as the outer shell, but the electrolyte has not changed. This kind of battery can also be thinner, and its low-temperature discharge characteristics are better than polymer batteries, and the material energy density is basically the same as that of liquid lithium batteries and ordinary polymer batteries, but because of the use of aluminum plastic film, it is better than ordinary liquid lithium batteries. In terms of safety, when the liquid just boils, the aluminum-plastic film of the soft pack battery will naturally bulge or rupture, and it will not explode.
It should be noted that the new battery may still burn or swell and crack, and safety is not foolproof.
Technical performance of lithium polymer battery
The lithium polymer battery electrolyte is a flexible solid polymer, and the metal file foil is sealed in the battery, and it can still work normally at a high temperature of 180°C. The sample of this lithium-ion battery has been proved to be safe after collision test. To prevent overcharge and overdischarge when multiple lithium polymer batteries are connected in series, the fast charging performance of lithium polymer batteries needs to be further addressed.
Lithium polymer batteries can be divided into three categories according to the electrolyte: (1) Gel polymer electrolyte lithium ion batteries, which add additives to solid polymer electrolytes to improve ion conductivity, so that the battery can be used at room temperature; (2) Solid polymer electrolyte lithium ion battery, the electrolyte is a mixture of polymer and salt, the ion conductivity is low at room temperature, suitable for high temperature use; (3) Lithium ion battery with composite gel polymer cathode material, conductive polymer as The positive electrode material has a specific energy 3 times that of existing lithium-ion batteries, and is the latest generation of lithium-ion batteries.
Comparison of ordinary lithium ion battery and lithium polymer battery
(1) The advantages of ordinary lithium-ion batteries:
① The working voltage is high. The working voltage of Li-ion battery is 3.7V, which is three times that of Ni-Cd and Ni-MH batteries.
② Higher specific energy. The specific energy of lithium-ion batteries has reached 140Wh/kg, which is 3 times that of nickel-cadmium batteries and 1.5 times that of nickel-hydrogen batteries.
③ Long cycle life. The cycle life of lithium-ion batteries has reached more than 1,000 times, and can reach tens of thousands of times at low depths of discharge, which exceeds that of other secondary batteries.
④ Small self-discharge. The monthly self-discharge rate of lithium-ion batteries is only 6%-8%, which is far lower than nickel-cadmium batteries (25%-30%) and nickel-hydrogen batteries (30%-40%).
⑤ No memory effect. It can be charged at any time as required without reducing battery performance.
⑥ No pollution to the environment. There are no harmful substances in lithium-ion batteries, and they are veritable "green batteries".
(2) Advantages of lithium polymer battery
Lithium polymer batteries are a new generation of batteries. Lithium polymer batteries are basically the same as lithium ion batteries except that the electrolyte is a solid polymer instead of a liquid electrolyte.
1. There is no battery leakage problem. The battery does not contain liquid electrolyte and uses colloidal solids.
2. Can be made into a thin battery: With a capacity of 3.6V 400mAh battery, its thickness can be as thin as 0.5mm.
3. The battery can be designed in various shapes.
4. The battery can be bent and deformed: the polymer battery can be bent about 90 degrees at most.
5. It can be made into a single high voltage: the battery with liquid electrolyte can only obtain high voltage by connecting several batteries in series. Since polymer battery has no liquid itself, it can be made into a multi-layer combination in a single battery to achieve high voltage.
6. The capacity will be higher than a lithium-ion battery of the same size.