Li-ion batteries are similar to lead-acid batteries in that there are positive and negative electrodes in the electrolyte, and lithium ions move from negative to positive during discharge and vice versa during charging.
The negative electrode of a conventional lithium-ion battery is composed of carbon, the most common material being graphite. The positive electrode material is a metal oxide such as: lithium diamond oxide, lithium iron phosphate or lithium manganese oxide. The electrolyte is a lithium salt dissolved in an organic solvent and is a non-aqueous solution. There are a variety of salts that can be used in the electrolyte, and the voltage, capacity, lifespan, and safety of a battery depend on the actual salt used in the battery.
Pure lithium is chemically very reactive, and if placed in water, it will chemically react to form lithium hydroxide and hydrogen. Therefore, the electrolyte in the battery is a non-aqueous solution, and it is strictly forbidden to enter the water.
During charging, lithium ions move from the positive electrode to the negative electrode in the transition metal, where they are intercalated between graphite molecules, a process called intercalation, which is defined as the reversible intercalation of a molecule (or group) between two other molecules. between.
Constant voltage current-limited charging occurs when the battery voltage rises to a maximum value of 4.2V, then the current is reduced to maintain that voltage. Some cells are charged to 0 current, while others are charged at a constant voltage until the current is reduced to a certain percentage of the initial charge current. Typically, the charge is terminated at 3% of the initial charge current.
Early Li-ion batteries cannot be charged quickly, it takes at least 2h to fully charge. The new generation of batteries can be fully charged in 45 minutes or less. Some lithium-ion batteries can be charged to 90% in as little as 10 minutes.
The positive half-reaction formula during charging is
The negative half-reaction formula is
The discharge process is reversed.
If the battery is overcharged and the voltage of the cobalt oxide is too high, the following irreversible reactions are formed:
If the battery is over-discharged, the lithium cobalt oxide is converted to lithium oxide in an irreversible reaction as follows;
The battery voltage depends on the electrode material and electrolyte, and the voltage of each battery cell can vary from 3.3 to 4.2 V.
Lithium-ion batteries are fragile and have an upper voltage limit. If overheated or overcharged, lithium-ion batteries can undergo thermal runaway and cell rupture. It can also lead to burning in extreme cases. Therefore each battery must be individually monitored for temperature and voltage during charging and disconnected from the charging device if necessary. Therefore, as with lead-acid batteries, the charging device can be connected to only one of them, rather than the entire battery pack.
Note: Some Li-ion batteries will not charge if the battery temperature is below 0°C.
A deep discharge can cause the battery to short out, in which case it will not be safe to recharge it.
To reduce these risks, lithium-ion battery packs can contain fail-safe circuits that shut down the individual cells when the voltage falls outside the battery’s specified safe voltage range. The safety range is generally 3 to 4.2V per battery cell.
If the battery has a self-protection circuit and the battery is stored for a long time, the battery can discharge below the safe voltage through the self-protection circuit. Generally, the self-discharge is 5% to 10% per month.
Other security features include:
(1) Turn off the separator when the temperature is exceeded.
(2) Pull off the tab when the internal pressure increases.
(3) Ventilation and pressure reduction.
(4) Thermal interruption during overcurrent/overcharge.
These properties are necessary because the anode generates heat during use and the cathode generates oxygen. These devices and improved electrode designs reduce or eliminate the risk of fire or explosion.
A major advantage of lithium-ion batteries is that they weigh less than lead-acid batteries for the same capacity.
②Lithium iron phosphate battery (LFP)
Lithium Iron Phosphate (LiFePO4) battery, also known as LFP battery, is a type of lithium-ion battery. However, it is safer than standard drill-based lithium-ion batteries, capable of delivering higher discharge currents, and has a longer lifespan. For these reasons, the battery has begun to be used in backup power supplies and is therefore suitable for independent power supply systems. Therefore, this section has included this section separately.
The difference between a lithium iron phosphate battery and a standard lithium ion battery is that the positive terminal of a lithium iron phosphate battery is lithium iron phosphate, compared to lithium cobalt oxide (LiCoO2) in a standard lithium ion battery.
The main advantages of LiFePO4 batteries are their higher safety and better thermal and chemical stability compared to standard Li-ion batteries because Fe-P-O bonds are stronger than CO-O bonds.