1. Working principle of lithium-ion batteries—Introduction
A lithium-ion battery is a secondary battery that uses two compounds that can reversibly insert and extract lithium ions as positive and negative electrodes, respectively. During charging, lithium atoms at 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 discharging, 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, lithium always exists in the form of lithium ions in this battery, and never in the form of metallic lithium; hence, this type of battery is called a lithium-ion battery.
2. Working principle and structure of lithium-ion batteries
Lithium-ion batteries are a replacement for lithium metal batteries that appeared a few years ago. The main components of a battery are positive and negative electrodes, electrolyte, separator, and casing.
The positive electrode uses a carbon electrode that can absorb lithium ions. During discharge, lithium is converted into lithium ions, which leave the anode and reach the cathode of the lithium-ion battery.
For the negative electrode, materials are selected that can be intercalated into lithium compounds with a potential as close as possible to that of lithium, such as various carbon materials including natural graphite, synthetic graphite, carbon fibers, mesophase microspheres of carbon, and metal oxides.
Electrolyte---a mixed solvent system using alkyl carbonates such as ethylene carbonate, propylene carbonate and low-viscosity diethyl carbonate with LiPF6.
The diaphragm is made of polyolefin microporous membranes such as PE, PP or composite membranes of them. In particular, the PP/PE/PP three-layer diaphragm not only has a low melting point, but also has high puncture resistance, thus playing a role in thermal protection.
The outer casing is made of steel or aluminum, and the cover assembly has an explosion-proof and power-off function.
3. Working principle of lithium-ion batteries
The working principle of a lithium-ion battery refers to its charging and discharging principle.
When a battery is charged, lithium ions are generated at the positive electrode. These lithium ions then move through the electrolyte to the negative electrode. The carbon layer at the negative electrode has a layered structure with many micropores. The lithium ions that reach the negative electrode embed themselves into these micropores. The more lithium ions embedded, the higher the charging capacity. The chemical reaction occurring at the positive electrode at this time is as follows:
Similarly, when the battery is discharged (i.e., when we use the battery), lithium ions embedded in the carbon layer of the negative electrode are released and move back to the positive electrode. The more lithium ions return to the positive electrode, the higher the discharge capacity. The battery capacity we usually refer to is the discharge capacity. The chemical reaction occurring at the negative electrode at this time is:
It's easy to see that during the charging and discharging process of a lithium-ion battery, lithium ions are in a state of motion, moving from the positive electrode to the negative electrode and back to the positive electrode. If we figuratively compare a lithium-ion battery to a rocking chair, with the two ends of the chair representing the two electrodes, then the lithium ions are like excellent athletes running back and forth between the two ends of the chair. Therefore, experts have given lithium-ion batteries another endearing name: rocking chair batteries.
