What is the chemistry behind LiFePO4 batteries

Lithium-ion batteries

The most powerful batteries are lithium batteries. They have the highest energy density with the smallest construction. Precisely because of the high energy density of the lithium cells, they are particularly suitable for mobile devices. Lithium batteries are widely used in wearables, smartphones, tablets, digital cameras and notebooks.
However, lithium batteries are expensive and are much more sensitive to incorrect handling than other batteries. They are also absolutely toxic to the environment. But they offer a high level of comfort. They remain functional for about 5 years with a loss of capacity.

The nominal voltage of the lithium cells depends on the electrode material and is 3.6 or 3.7 V. Because of the higher voltage, they are not suitable as a replacement for rechargeable batteries (1.2 V) and batteries (1.5 V) in AA - or AAA design.

The market for small, battery-operated devices shows a trend towards miniaturization. At the same time, the energy requirements of such systems are increasing. That is why research is being carried out on even more powerful batteries, although there are only small development steps here.

Note: Tinkering with lithium cells is not for beginners and electronics beginners. The risk of explosions due to incorrect or faulty charging circuits is far too great.
Because lithium batteries are sensitive to overcharging and deep discharge, electronics are built into the battery pack to protect them from overcharging and overcharging.

Overview: lithium batteries

  • Lithium-ion (Li (NixCoyMnz) O2): widespread, harmful to the environment
  • Lithium iron phosphate (LiFePO4): harmless, memory effect, complicated charging circuit necessary
  • Lithium cobalt oxide (LiCoO2): the most expensive, the highest energy density, dangerous

Construction of lithium-ion cells

A lithium-ion cell consists of a graphite electrode (negative) and a lithium metal oxide electrode (positive). The lithium metal oxide can have variable proportions of nickel, manganese and cobalt (e.g. Li (NixCoyMnz) O2). These NMC materials have a high energy density and are the first choice for small devices. Even if they are more expensive and potentially less secure. The exact composition influences the properties of the lithium-ion battery and varies depending on the manufacturer and grade. Therefore it is not possible to make precise statements about capacity and service life.

The charge carriers are the lithium ions. They are small in size and highly maneuverable. When the cell is charged, they are deposited between the molecular layers of the graphite. When discharging, the lithium ions migrate back to the lithium metal oxide electrode.

Lithium is the lightest metal and reacts violently with water. That is why an anhydrous but flammable solvent is used as the electrolyte. The solvent is the reason why there are occasional reports of exploding or burning lithium batteries. The electrolyte has ignited. The more precise reasons are varied. As a rule, battery recall campaigns occur when faulty lithium batteries are discovered.
The electrodes are separated by a separator to prevent a short circuit between the electrodes. The separator is permeable to the lithium ions. The cathode acts like a sponge. It can hold such a large number of ions. This creates an energy density of 180 Wh / kg and more.

Charge and discharge function

Charging takes place using the I / U charging process, in which the battery is first charged with constant current and then with constant voltage. The lithium ions migrate into the graphite and collect between the molecular levels. When discharging, the lithium ions migrate back to the lithium metal oxide electrode.

The end-of-charge voltage is 4.1 or 4.2 volts and must be adhered to with an accuracy of 50 millivolts. Otherwise the cell will be destroyed. The lowest voltage limit is 2.5 volts. The cell is damaged underneath.
To prevent damage to the cells, each battery pack has its own charging and protection electronics. It monitors compliance with the limit values ​​during loading and unloading. It is adapted to the lithium cells.
Depending on the quality of the lithium-ion battery, it can only cope with a few hundred charging cycles until the storage capacity drops significantly. Regularly charging a half-full battery does not affect the total capacity.

To conserve the battery, it should not be charged over 90 percent or discharged to less than 10 percent if possible. Some notebooks offer a setting option for this. However, it is not possible to estimate exactly how long the battery will last. Maybe you can use it for a year longer.

Aging

The aging of lithium-ion batteries is caused by cell oxidation. The electrodes oxidize in the process. These lose the ability to store lithium ions, which are necessary for the flow of electricity. Cell oxidation is influenced by various factors. For example, through the temperature and the charge status of the battery. Cell oxidation develops particularly quickly at high temperatures and with a fully charged battery. This condition occurs e.g. B. in notebooks, this often happens when the battery is fully charged and the device is in operation and becomes warm at the same time. The heat is transferred to the battery.

storage

If you want to store a lithium-ion battery, then you should half charge it. The optimal state of charge is between 50% and 80%. It is stored at room temperature, better protected from moisture in the refrigerator (not in the cooler). Just before you want to use it again, you fully charge it at room temperature.

If a lithium-ion battery has to be stored for a longer period of time, the charge status must be checked regularly. The self-discharge of 1% per month is extremely low, but strongly dependent on temperature. Lithium-ion batteries should be recharged every 3 to 4 months to avoid deep discharge. If a cell reaches a voltage below 2 volts, the cell can be destroyed.

When purchasing lithium-ion batteries, it must always be expected that batteries will give up prematurely. Especially for batteries that come from the Far East or have been on the road for a longer period of time. This also applies to replacement batteries that may have been in storage for a longer period of time. If a battery is broken, it can be repaired. If not, then the battery should be disposed of at the dealer or as hazardous waste.

Battery care

Chemical changes in the electrolyte and oxidation of the electrodes are the main causes of aging. Lithium-ion batteries lose their capacity after 2 to 3 years is just a rule of thumb. Whether a lithium battery only lasts 1 or maybe 5 years depends on the processing, use and operating temperature. Careful handling rewards a lithium battery with a longer service life.

  • Avoid temperatures above 40 ° C
  • Avoid full loading and unloading
  • Charge to over 90 percent of its capacity as rarely as possible
  • to let less than 10 percent idle if possible

However, following these tips all the time is very cumbersome. With regard to the load, some operating systems can be set accordingly. Since electronic devices heat up considerably during operation, the user has less influence on them.
Nevertheless, if you treat a battery with appropriate care, the service life can be extended from maybe 3 to 4 years.

Energy density

Compared to other batteries, lithium batteries have the highest energy density. They store almost twice as much energy as NiMH batteries, which are of the same size and weight.
The energy density of lithium-ion batteries is mainly determined by the cathode material. Cobalt oxide with an energy density of up to 180 Wh / kg is common. With lithium cobalt nickel (LiNiCo) you can even achieve up to 240 Wh / kg. There is a tendency towards a lower energy density of up to 170 Wh / kg. On the other hand, the batteries can withstand significantly more than 500 charging cycles and thus have a longer service life.

Lithium polymer batteries (Li-Pol)

Lithium-polymer batteries are a further development of the lithium-ion batteries. They do not contain any liquid chemicals, only solid or gel-like components. They are therefore leak-proof and you can do without a protective metal housing. This results in even more designs in which cells with a thickness of 1 millimeter are possible. For example, cavities in portable devices can be optimally used. For example with wearables and particularly flat smartphones.

Lithium air battery

Lithium-air batteries can store between 10 to 20 times more energy (11 kWh / kg) than lithium-ion batteries of the same weight.
In lithium-air batteries, oxygen from the air reacts in pores of a mesoporous carbon electrode with a size of a few nanometers with Li + to form lithium peroxide Li2O2. The counter electrode consists of elemental lithium. The electrolyte used is ether, e.g. B. tetrahydrofuran, in which the lithium ions can dissolve.
However, the carbon electrode corrodes after a few charging cycles. In addition, the electrolyte liquid decomposes very quickly. Practical use is therefore limited.

Lithium-ion battery in the future

In recent years, the development has led to lithium-ion and lithium-polymer batteries, which can store more and more energy. However, this energy density is by no means sufficient for the requirements of small mobile devices.
As an alternative, research was carried out on fuel cells with methanol as fuel (DMFC). Unfortunately, the prototypes do not get beyond the development status. Apparently manufacturers are content to develop stationary uninterruptible power supplies (UPS).
So there are enough improvements to the tried and tested lithium-ion and lithium-polymer batteries. Nanotechnology (working with small particles) in particular gives new impulses.
Past successes in the development of lithium-ion batteries indicate that it will be possible to drive 500 kilometers in hybrid vehicles before having to refuel. Then you no longer need fuel cells.

Overview: batteries

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