Why are Lipo batteries rechargeable?

The Lipo battery uses a chemical reaction to achieve charge and discharge, which is reversible and easy to control, so the battery can be rechargeable.

The cathode of Lipo battery is composed of high-purity lithium-containing metal oxide. The more uniform the chemical composition and crystal structure of cathode material, the better performance and the longer lifetime of battery. At the other end is the anode composed of graphite, which is a layered structure of carbon. The battery also contains a conductive medium-electrolyte, which allows the charged Li⁺ to move freely in it. The purity of this electrolyte must be very high and it must be as water-free as possible to ensure efficient charging of the battery. In order to avoid short circuits, an insulating layer diaphragm is added between the two electrodes.

Commercially available Lipo battery cathode materials are generally LiCoO₂ or LiMn₂O₄ while the anode materials are lithium or lithium-carbon compounds. This article takes LiCoO2 and lithium-carbon compounds as examples to introduce the Lipo battery charging and discharging process. 

Principle:

The Li⁺ in the cathode and electrolyte accumulate to the anode to obtain electrons, which are reduced to Li when charging. Because graphite has a perfect crystal structure with a layered plane, the reduced Li will be embedded in the carbon material of anode and stored in between carbon graphite layers.During the discharge process, elemental Li is active and the outermost electrons are easily lost. When the external circuit is turned on, the Co⁺ in the CoO₂ on the anode attract electrons to migrate from the anode through the external circuit, and the electrons are separated from the Li atoms. The Li embedded in the carbon material of the negative electrode loses electrons and enters the electrolyte. A large amount of Li⁺ move toward the cathode to form an internal current, and reach the cathode through the electrolyte to form LiCoO₂ with cobalt and oxygen.

When the Li that has not lost electrons meets the substance on the opposite side of the electrolyte, the chemical reaction will accelerate uncontrollably and cause a fire or a more serious explosion. We find a way to place a non-conductive translucent separator between the cathode and anode, which just allows Li⁺ to pass through. Because graphite and CoO₂ are not good at collecting or distributing electrons. A conductive Cu layer is added next to graphite and a conductive AI layer is added next to CoO₂ for better conductivity.

Why does the maximum energy of Lipo battery decrease over time?

There are two reasons, one of which is that Li⁺ react with organic solvents in the electrolyte to form solid electrolyte crystals (sei compounds), sei irreversibly consumes Li⁺ and electrolyte, thereby reducing Li and the maximum capacity of the battery. Another reason is that when your battery is completely discharged, there may be too much Li⁺ on the cathode, resulting in irreversible consumption of lithium oxide and cobalt oxide, thereby reducing the service life of the battery. This is why it is not recommended to recharge the battery after it is completely discharged. It is best to start charging when the remaining power is 30%~40%.