What Is The First Efficiency of Lithium-ion Batteries?

During the charging process, lithium ions are released from the positive electrode and precipitated at the negative electrode. The charging capacity can be obtained in the test software. During the discharge process, the lithium at the negative electrode loses electrons to form lithium ions, which are embedded in the positive electrode structure through the electrolyte diaphragm. The discharge capacity can be obtained in the test software. (Charging capacity > discharge capacity)

PS: Sometimes we also have a situation where the discharge capacity > charging capacity. For example, the positive electrode material containing Mn is due to the release of Mn ions. This phenomenon will destroy the positive electrode structure.

First efficiency = discharge capacity / charging capacity

(Must be the first charge and discharge)

Half-battery: Since the negative electrode is a lithium sheet and lithium is excessive, the reason why the discharge capacity is less than the charging capacity is mainly due to the destruction of the structure of the positive electrode material, which causes the lithium that has been released to not be fully embedded back.

Full battery: The negative electrode is generally graphite, and all lithium is provided by the positive electrode. A part of the lithium ions is used to form an SEI film on the surface of the negative electrode, so there is less lithium ions returning to the positive electrode. The following figure shows the first charge and discharge curve of a lithium iron phosphate soft pack battery. The charging stage consists of formation-capacitance division.

This is why the first efficiency of a full battery is generally lower than the first efficiency of a half battery.

The first efficiency of the positive electrode (LFP ~98%; LCO ~95%; NCM 80%~90%), the first efficiency of the negative electrode (graphite>90%, there are also poor ones; silicon-based alloy negative electrode: generally less than 90%). Generally, the low first efficiency can be solved by optimizing the material (reducing the specific surface area - appropriately increasing the particle size, adding a passivation layer - electrolyte design or coating); the other is to increase the lithium content by supplementing the positive electrode with lithium.