1. Ternary lithium battery structure
The full name of "ternary lithium battery" is "ternary polymer lithium battery". Ternary polymer lithium battery refers to lithium battery that uses lithium nickel cobalt manganese oxide (Li(NiCoMn)O2) ternary cathode material as the cathode material. The ternary composite cathode material precursor product is made of nickel salt, cobalt salt and manganese salt as raw materials. The ratio of nickel, cobalt and manganese can be adjusted according to actual needs. Batteries with ternary materials as cathodes are safer than lithium cobalt oxide batteries.
Structural characteristics of nickel-cobalt-manganese ternary cathode materials:
ternary lithium battery structure diagram
Nickel-cobalt-manganese ternary materials can usually be represented as: LiNixCoyMnzO2, where x+y+z=1.
Based on the different molar ratios (x∶y∶z) of the three elements, they are referred to as different systems. For example, a ternary material with a nickel-cobalt-manganese molar ratio (x∶y∶z) of 1∶1∶1 is simply called the 333 type; a system with a molar ratio of 5∶2∶3 is called the 523 system, and so on. Ternary materials of the 333, 523, and 811 types all belong to the hexagonal crystal system with an α-NaFeO2 type layered rock salt structure.
For example, the NCM811 battery. The 811 battery actually refers to the electrode material of the battery. In terms of the positive electrode material, it mainly uses 80% nickel, 10% cobalt and 10% manganese. Simply put, the electrode material is changed to 8:1:1.
2. Working principle of ternary lithium batteries
In nickel-cobalt-manganese ternary materials, the main valence states of the three elements are +2, +3, and +4, respectively, with Ni being the main active element. The reactions and charge transfer during charging are as follows:
Positive electrode reaction: LiMO2 → Li1-xMO2+xLi++xe-
Negative electrode reaction: nC + xLi + ++xe- → LixCn
Overall battery reaction: LiMO2 + nC → Li1-xMO2 + LixCn
Its anode uses a carbon electrode capable of absorbing lithium ions. During discharge, lithium is converted into lithium ions, which detach from the anode and reach the cathode. The lithium ions move between the anode and cathode without changing the electrode itself. This is the fundamental difference between lithium batteries and metallic lithium batteries. In contrast, the anode of a lithium battery is a graphite crystal, and the cathode is typically lithium dioxide. During charging, lithium atoms in the cathode ionize into lithium ions and electrons, and the lithium ions move towards the anode to combine with electrons to form lithium atoms. During discharge, lithium atoms ionize from the anode surface within the graphite crystal into lithium ions and electrons, and then combine at the cathode to form lithium atoms. Therefore, in this battery, lithium always exists in the form of lithium ions and never in the form of metallic lithium.
Ternary polymer lithium batteries are materials that offer a good balance between capacity and safety, and their cycle performance is better than that of conventional lithium cobalt oxide batteries. In the early days, due to technical reasons, their nominal voltage was only 3.5-3.6V, which limited their application range. However, with continuous improvements in the formula and structure, the nominal voltage of the batteries has now reached 3.7V, and their capacity has reached or exceeded that of lithium cobalt oxide batteries.
3. The correct charging method for ternary lithium batteries
(1) Use the matching charger to fully charge the ternary lithium battery, then charge it for one hour.
(2) Discharge until about 20% remains, then recharge. Over-discharging will damage the ternary lithium battery.
(3) When charging ternary lithium batteries, try to fully charge them in one go.
The maximum charging termination voltage for a single lithium battery is 4.2V. Overcharging is not allowed, otherwise the battery will be ruined due to excessive loss of lithium ions at the positive electrode. When charging lithium batteries, a dedicated constant current and constant voltage charger should be used. First, charge at a constant current until the voltage across the lithium battery reaches 4.2V, then switch to constant voltage charging mode. When the constant voltage charging current drops to 100mA, charging should be stopped.
During discharge, not all lithium ions can migrate to the positive electrode; some must remain at the negative electrode to ensure they can readily intercalate into the channels during the next charge. Otherwise, battery life will be shortened. To ensure that some lithium ions remain in the graphite layer after discharge, the minimum discharge termination voltage must be strictly limited; in other words, lithium batteries cannot be over-discharged.
The discharge termination voltage of a single lithium battery is typically 3.0V, and should not be lower than 2.5V. The battery discharge time depends on the battery capacity and the discharge current. Battery discharge time (hours) = battery capacity / discharge current. Furthermore, the lithium battery discharge current (mA) should not exceed three times the battery capacity. For example, for a 1000mAh lithium battery, the discharge current should be strictly controlled below 3A; otherwise, the battery will be damaged.
