Lecture
Lithium-ion battery (Li-ion) is a type of electric battery that is widely used in modern consumer electronics and finds its application as an energy source in electric vehicles and energy storage devices in energy systems. This is the most popular type of battery in devices such as cell phones, laptops, electric cars, digital cameras and camcorders. Sony released the first lithium-ion battery in 1991.
The characteristics of lithium-ion batteries depend on the chemical composition of the components and vary within the following limits:
Chargers support the final voltage in the range of 4.05-4.2 V.
Due to over-voltage while charging, the battery can catch fire, therefore a special miniature electronic board is built into the battery case, which protects the battery from over-charging voltage. Also, this board can optionally control the temperature of the battery, turning it off when overheating, to limit the depth of discharge and the current consumption. Nevertheless, it must be borne in mind that not all batteries are protected. In pursuit of cost or capacity protection may not be put.
Lithium batteries have special requirements when connecting multiple cans in series. Chargers for such multi-canister batteries are equipped with a cell balancing circuit . The point of balancing is that the banks are slightly different, and some will reach a full charge before others. At the same time it is necessary to stop the charge of this bank, while continuing to charge the rest. This function is performed by a special battery balancing assembly. It shunts a charged jar so that the charge current goes past it.
A lithium-ion battery consists of electrodes (cathode material on aluminum foil and anode material on copper foil) separated by porous separators impregnated with electrolyte. The package of electrodes is placed in a sealed enclosure, the cathodes and anodes are connected to the current collector terminals. The housing has a safety valve that relieves internal pressure in emergency situations and the violation of operating conditions. Lithium-ion batteries differ in the type of cathode material used. The charge carrier in a lithium-ion battery is a positively charged lithium ion, which has the ability to penetrate (intercalate) into the crystal lattice of other materials (for example, in graphite, oxides and metal salts) to form a chemical bond, for example: in graphite to form LiC6, oxides (LiMO 2 ) and salts (LiM R O N ) of metals.
Initially, metallic lithium was used as negative plates, then coal coke. Later, graphite was used. The use of cobalt oxides allows the batteries to operate at significantly lower temperatures, increases the number of discharge / charge cycles per battery. The distribution of lithium-ferrum-phosphate batteries due to their relatively low cost. Lithium-ion batteries are used in conjunction with the monitoring and control system - IED or BMS (battery management system) and a special charge / discharge device.
Currently, three classes of cathode materials are used in mass production of lithium-ion batteries:
Electro-chemical circuits of lithium-ion batteries:
Due to the low self-discharge and a large number of charge-discharge cycles, Li-ion batteries are most preferred for use in alternative energy. In addition to the system BMS (SKU), they are equipped with inverters (voltage converters).
Li-ion batteries of the first generation were subject to explosive effects. This was due to the fact that they used an anode of metallic lithium, on which, during multiple charge / discharge cycles, spatial formations (dendrites) occurred, leading to the closure of the electrodes and, as a consequence, to fire or explosion. This problem was finally solved by replacing the anode material with graphite. Similar processes took place on the cathodes of lithium-ion batteries based on cobalt oxide in case of violation of operating conditions (recharge). Lithium-ferro-phosphate batteries are completely devoid of these shortcomings. In addition, all modern lithium-ion batteries are equipped with an integrated electronic circuit that prevents overcharging and overheating due to overcharging.
Loss of storage capacity [1] :
| Temperature, ⁰C | With 40% charge,% per year | With 100% charge,% per year |
|---|---|---|
| 0 | 2 | 6 |
| 25 | four | 20 |
| 40 | 15 | 35 |
| 60 | 25 | 40% for three months |
In accordance with all applicable regulations for the storage and operation of lithium-ion batteries, to ensure long-term storage, they must be recharged to the level of 70% of capacity once every 6–9 months.
According to the results of research by scientists at the Paul Scherer Institute (Switzerland), it was found that lithium-ion batteries have a memory effect [2] . What ultimately deprived this type of battery of one of the main advantages, but at the same time, it allows you to really understand the mechanisms of operation of batteries and solve some problems with their capacity and durability [3] .
Researchers at the Swiss Scherrer Institute in Switzerland, along with colleagues from Toyota Research in Japan, found that the widely used type of lithium-ion battery is still subject to a negative “memory effect”.
The study showed that frequent cycles of incomplete charging and subsequent discharge lead to the emergence of individual "microeffects of memory", which are then summed up. This is because the basis of the battery are the processes of release and reuptake of lithium ions, the dynamics of which becomes far from optimal in the case of incomplete charging.
During the charge process, lithium ions one after another leave particles of lithium ferrophosphate, the size of which is tens of micrometers. Cathode material begins to be divided into particles with different lithium content.
Battery charge occurs against the background of increasing electrochemical potential. At a certain point, it reaches the limit value. This accelerates the release of the remaining lithium ions from the cathode material, but they do not change the total voltage of the battery.
If it is not fully charged, then a certain number of particles close to the boundary state will remain on the cathode. They almost reached the barrier to the release of lithium ions, but did not manage to overcome it.
When discharged, free lithium ions tend to return to the site and recombine with ferrophosphate ions. However, on the surface of the cathode they are also met by particles in the boundary state, which already contain lithium. Reuptake is difficult, and the microstructure of the electrode is disrupted.
Currently, there are two ways to solve the problem: making changes to the algorithms of the battery management system and developing cathodes with an increased surface area.
The temperature of the charge of lithium-polymer and lithium-ion batteries affects their capacity: the capacity decreases when charging in cold or heat. Deep discharge completely disables the lithium-ion battery. Also on the life cycle of the battery affects the depth of its discharge before charging and charging currents above regulated by the manufacturer. They are extremely sensitive to charge voltage. If it is increased by only 4%, then the batteries will lose twice the capacity from cycle to cycle. The optimal storage conditions for Li-ion batteries are achieved with a 40 percent charge from the battery capacity and a temperature of 0 ... 10 ° C. Lithium batteries age, even if not used. After 2 years, the battery loses about 20% of its capacity. Accordingly, there is no need to buy a battery “in reserve” or overly indulge in “saving” its resource. When buying, be sure to look at the production date to know how much this power source has already been in stock. If more than two years have passed since the date of manufacture, it is better to refrain from purchasing.
Comments
To leave a comment
Power supplies for electronic equipment
Terms: Power supplies for electronic equipment