There are many kinds of lithium-ion batteries, mainly composed of layered structure lithium cobalt oxide, lithium nickelate, nickel cobalt lithium manganate, olivine structure lithium iron phosphate, etc. Among them, layered structure lithium cobalt oxide battery has the highest theoretical density value, and its volume. No other cathode material can surpass the specific energy so far.
With the continuous deepening of research on lithium cobalt oxide, the charging voltage of lithium cobalt oxide has gradually increased from 4.2V to 4.45 V, or even higher voltages, and the specific capacity has reached 180~185mAh/g.
What is a lithium cobalt oxide battery?
The most commonly used cathode materials in lithium-ion batteries are lithium cobalt oxide, lithium manganate, lithium iron phosphate, and nickel-cobalt-manganese ternary materials. In this way, ternary lithium battery is also well known in battery cathode industry. However, lithium cobalt oxide battery still has a high theoretical capacity, reaching 274mAh/g, and the capacity in practical application reaches 140-180mAh/g.
At the same time, the compaction density of lithium cobalt oxide is the highest among many cathode materials, reaching 4.2g/cm3. The greater the compaction density, the smaller the volume of the battery made under the same quality, and it is more convenient to carry, so lithium cobalt oxide cathode materials are generally used in mobile phones.
The use of lithium cobalt oxide battery was developed by Sony in 1991 for commercial production. Its open circuit voltage is 3.6V, the working voltage reaches 4.2V, and the energy density is high. And lithium cobalt oxide has stable cycle performance and wide operating temperature range (-20~55℃), which can adapt to various environments.
The charging and discharging principle of lithium cobalt oxide battery
The diagram above shows how a lithium-ion battery works. The positive electrode material is lithium cobalt oxide LCO, and the negative electrode material is graphite. The separator and electrolyte not only realize the conduction of lithium ions between the two electrodes, but also make them unable to conduct directly, so that the current can only flow from the positive electrode to the negative electrode externally.
During charging, lithium ions are extracted from the lithium cobalt oxide cathode, pass through the separator and electrolyte, and intercalate into the graphite anode. During the discharge process, lithium ions are returned from the negative electrode graphite to the positive electrode lithium cobaltate. It can be concluded that the charge-discharge cycle of Li-ion batteries is achieved by the intercalation and de-intercalation of Li-ions between the two electrodes.
So, if we want to increase the lithium battery capacity, we just need to use more lithium-ion. In order to release more lithium ions in lithium cobalt oxide, its charging voltage needs to be increased, to be precise, the cut-off voltage needs to be increased.
Lithium cobalt oxide battery structure
Lithium cobaltate has a layered crystal structure, and if too many lithium ions are extracted, the crystal will change phase. The crystal structure of lithium cobaltate is like a hamburger. Cobalt and oxygen atoms are covalently bonded to form an octahedral structure. We call this the covalently dense layer (also called the host crystal layer). The lithium element is between the two cobalt oxide layers (also called the intergranular layer).
Under a suitable voltage, lithium ions can be easily inserted and extracted between the layers, with good ion mobility. At the same time, lithium ions are in the lattice, supporting the upper and lower cobalt oxide layers. Positively charged lithium ions bind the two negatively charged oxygen layers together through electrostatic attraction. Therefore, when more lithium ions need to be used in the lithium cobalt oxide battery, it means that more lithium ions are extracted from the lithium cobalt oxide layered crystal, and the lithium ions that can adhere to the cobalt oxide layer will will decrease.
At some key nodes, the extraction of lithium ions has reached a certain level, and there are too few lithium ions between the cobalt oxide layers, and the entire crystal will undergo an irreversible phase transition, which will have a serious impact on the cycle of the battery, and it will not be used a few times. , the battery is damaged. So, we need to keep some of the lithium ions in their original lattice positions.
And half of the theoretical capacity is a relatively stable state for lithium cobalt oxide batteries, which can not only exert more capacity, but also ensure that the battery capacity loss is small after multiple cycles. The cut-off voltage at this time was 4.2 V, and the capacity was 140 mAh/g.
Lithium cobalt oxide battery charge and discharge
Lithium cobalt oxide has a very high voltage plateau, starting at about 3.9V and ending at charge cutoff. And such a high voltage platform also has many advantages in applications. The energy density of the battery has a square relationship with the voltage platform, and a high voltage platform with the same capacity will make the energy density much higher.
At the same time, the output power of the battery is higher and can be applied to higher power devices. When this battery is charged to 4.2V, the reversible discharge capacity is about 140mAh/g; when charged to 4.6V, the discharge capacity is about 220mAh/g. But when lithium cobalt oxide was charged and discharged at a high voltage of 4.6V, the capacity of the battery dropped sharply with the number of cycles.
At 1C, only 50% capacity remains after 100 cycles. And after 200 cycles, only 20% remained. When the researchers synthesized lithium cobaltate, magnesium oxide (MgO), aluminum oxide (Al2O3) and titanium dioxide (TiO2) were added. Magnesium and aluminum are doped into the lattice of lithium cobalt oxide, propping up the lattice like pillars and inhibiting the harmful phase transition of lithium cobalt oxide under high pressure.
The titanium element is mainly concentrated on the grain boundaries and the surface of the lithium cobalt oxide crystal particles, which changes the microstructure of the grains and stabilizes the oxygen atoms on the surface under high voltage. These two methods synergistically improve the cycling stability of lithium cobaltate under high pressure, reaching 87% for 100 cycles at 4.6 V.
First of all, the price of cobalt is very high, and most of it is produced in the Congo region of Africa. Secondly, the reserves of cobalt are insufficient, and the supply of minerals is unstable, especially compared with lithium iron phosphate and nickel-rich cathode materials, the reserves of cobalt cannot meet the demand.
The current nickel-cobalt-manganese ternary materials have the same layered crystal structure as lithium cobalt oxide, and the application capacity can reach more than 180mAh/g. However, these are not enough to replace lithium cobalt oxide, because of the most prominent pressure of lithium cobalt oxide. The feature of high solid density can realize the preparation of smaller volume batteries.
Moreover, lithium cobalt oxide has the highest voltage platform among many cathode materials, which means higher energy density and higher power usage efficiency, which is important for the consumer electronics industry.