Research and analysis of lithium cobalt oxide material
The most commonly used cathode materials in lithium-ion batteries are lithium cobalt oxide, lithium manganate, lithium phosphate and nickel-cobalt-manganese ternary materials.
Introduction of lithium cobalt oxide material
Lithium cobalt oxide is the first commercial cathode material for lithium-ion batteries in history, it was first produced by the top 10 lithium battery cathode material manufacturers commercially in 1991. Lithium cobalt oxide material has a high theoretical capacity, reaching 274 mAh/g, and the capacity in practical applications reaches 140–180 mAh/g.
It has an open-circuit voltage of 3.6V, an operating voltage of 4.2V, and a high energy density. And lithium cobalt oxide cycle performance is stable, the working temperature range is wide (-20 to 55 °C), and it can adapt to a variety of environments.
Charging and discharging principle of lithium cobalt oxide battery
The cathode material is lithium cobalt oxide (LCO), and the anode material is graphite. The diaphragm and electrolyte not only enable the conduction of lithium ions between the two electrodes, but also prevent them from conducting directly, so that the current can only flow from the cathode to the anode externally.
During charging, lithium ions are precipitated from the lithium cobalt oxide cathode, pass through the diaphragm and electrolyte, and are embedded in the graphite anode.
During the discharge process, lithium ions return from the anode graphite to the cathode lithium cobaltate. It can be seen that the charge-discharge cycle of lithium-ion batteries is achieved by inserting and removing lithium ions between the two electrodes.
So, if we want to increase the capacity of a lithium-ion battery, we only need to use more lithium ions. In order to release more lithium ions into lithium cobaltate, we need to increase its charging voltage, specifically the cut-off voltage, sp as to make lithium cobalt oxide as a high voltage cathode material with better charge and discharge performance.
Structure of lithium cobalt oxide material
The cut-off voltage of lithium cobalt oxide batteries cannot be increased indefinitely. There is a problem here, lithium cobalt oxide material has a very high theoretical specific capacity, reaching 274mAh/g, and the cut-off voltage limit is above 4.75V.
But under normal circumstances, we can only exert half of the lithium capacity, generally about 140mAh/g, and the cut-off voltage is also limited to 4.2V. High theoretical capacity does not mean that it can be used, and the concept of lithium cobalt oxide material needs to be introduced here.
● Crystal structure of lithium cobalt oxide material
Because lithium cobalt oxide has a layered crystal structure, if too many lithium ions are shed, the crystal will phase change. The crystal structure of lithium cobalt oxide is like a hamburger.
Cobalt and oxygen atoms are covalently bonded to form an octahedral structure. We call this the covalent dense layer (also known as the host crystal layer).
Lithium, on the other hand, lies between two cobalt-oxygen layers (also known as the intergranular layer). At the right voltage, lithium ions can be easily embedded and detached between layers, with good ion mobility.
At the same time, lithium ions are in the crystal lattice, supporting the upper and lower cobalt oxygen layers. Positively charged lithium ions bind two negatively charged oxygen layers together by electrostatic attraction.
Therefore, when we need to use more lithium ions in lithium cobalt oxide batteries, it means that more lithium ions are removed from the lithium cobalt oxide material layered crystals, and the lithium ions that can adhere to the cobalt oxide layer will be reduced.
At some key nodes, lithium ions are removed to a certain extent. There are too few lithium ions between the cobalt-oxygen layer, and the entire crystal will undergo irreversible phase changes, which will have a serious impact on the battery cycle, and the battery is easily damaged in this case.
So, we need to keep some lithium ions in their original lattice position. Playing 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 is 4.2V and the capacity is 140 mAh/g.
Charge and discharge of lithium cobalt oxide material
In order to better explain the changes in lithium cobalt oxide material during the charging and discharging processes, we take a lithium cobalt oxide half-battery as the explanation object.
Half-battery means using lithium metal as the anode, lithium metal has the lowest redox potential, and lithium-ion batteries have lithium-rich electrodes. Battery research often uses lithium sheets to make battery anodes, to study other cathode materials.
Lithium cobalt oxide has a very high voltage plateau, starting at about 3.9V and ending until the charging cut-off. And such a high voltage platform also has many advantages in the application.
The energy density of the battery is squared with the voltage platform, and the same high-voltage capacity platform will make the energy density much higher.
For mobile phones, there is no need to design complex circuits to stabilize the voltage, you can obtain a high output voltage, and the use of electricity is more efficient. At the same time, the output power of the battery is higher, which can be applied to higher-powered 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. However, when lithium cobalt oxide is charged and discharged at a high voltage of 4.6V, the capacity of the high voltage battery decreases sharply with the increase in the number of cycles.
At 1C, only 50% of the capacity remains after 100 cycles; after 200 cycles, only 20% remain. Imagine if you charge your phone every day, and your phone battery capacity has to be charged four or five times a day to maintain it half a day after you buy it, which is really unbearable.
Research progress of lithium cobalt oxide material
In order to increase the cut-off voltage of lithium cobalt oxide material, improve the use capacity of the battery, and ensure the stability of the long cycle of the battery, we need to modify the lithium cobalt oxide material appropriately.
In the synthesis of lithium cobaltate, magnesium oxide (MgO), alumina (Al2O3), and titanium dioxide (TiO2) are added. Magnesium and aluminum are doped into the lattice of lithium cobalt oxide material, supporting the lattice like a pillar and inhibiting the harmful phase transition of lithium cobalt oxide material under high pressure.
The titanium element mainly accumulates on the grain boundary 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 at high voltage.
These two methods work synergistically to improve the cycle stability of lithium cobalt oxide material under high pressure, and the 100 cycles at 4.6 volts can reach 87%. However, lithium cobalt oxide material also has some disadvantages.
First, cobalt is highly expensive and is mostly produced in the Congo region of Africa.
Second, cobalt reserves are insufficient, and mineral supplies are unstable, especially compared to lithium iron phosphate, nickel-rich cathode materials, etc.
Lastly, cobalt reserves can not meet the needs. Also, the development of cobalt industry is not perfect causimh the limitation.
At present, we have nickel-cobalt-manganese ternary materials, which have the same layered crystal structure as lithium cobalt oxide, and the application capacity can reach more than 180 mAh/g. However, these are not enough to replace lithium cobaltate, because the most prominent compaction density of lithium cobalt oxide material can achieve 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 efficiency, which is important for the consumer electronics industry.
Finally, it is hoped that in the future, lithium cobalt oxide batteries with a cut-off voltage of 4.6V or even higher can be used, and mobile phones can be left uncharged for a long time, and the battery life will be longer.