I believe most people have heard that the lithium battery rule of 30-80% can ensure the health of the battery. Not only the lithium batteries in your mobile phones and computers, but also electric vehicles mostly follow this 30-80% charge and discharge rule, which is the fastest and healthiest. rules, otherwise it will damage the battery. But do you know what the scientific principle behind it is?

To know this, you must first learn about the working principle of lithium batteries. According to the 557th issue of "Scientific Development" in May 2019, lithium-ion batteries are mainly composed of four parts: positive electrode, negative electrode, electrolyte and separator; the positive electrode is a transition metal oxide containing lithium metal, and the negative electrode is mainly carbon material , the electrolyte is composed of lithium salt and organic carbonate solvent, and the isolation film mainly prevents the positive and negative electrodes from contacting and allows lithium ions to pass through.

When a lithium-ion battery is being charged, lithium ions from the positive electrode material are detached and transferred to the negative electrode material through the electrolyte. The lithium ions can be embedded in the carbon layer of the negative electrode material and stored. During discharge, lithium ions will be deintercalated from the negative electrode and embedded in the positive electrode material through the transfer of electrolyte. The higher the ratio of lithium ions that can return to the positive electrode, the higher the stability and life of the battery.

The key lies in the lithium battery isolation film
To put it simply, the principle of charging and discharging lithium batteries is to let lithium ions swim back and forth between the positive and negative electrodes. Why lithium batteries must obey the "30-80% rule" is actually most related to the isolation film of lithium batteries!

As mentioned earlier, the electrolyte between the positive and negative electrodes of lithium batteries is in a liquid state. However, if the positive and negative electrodes are not isolated and the "lithium ion flow rate" is controlled, the battery will generate a very large current in an instant and form a short circuit. High temperature is generated according to Joule's law, so lithium batteries must be separated by an isolation film.

If the power of the lithium battery is lower than 30%, or higher than 80%, the "internal pressure" (the specific physical term is stress, which includes mechanical, thermal, and electrical) generated by the battery charging and discharging at this time will It is greater than usual. Once the internal pressure is too high and not controlled well, it may cause damage to the isolation membrane and cause a short circuit between the different electrolytes between the positive and negative electrodes. This is why we often hear that when the battery level is higher than 80%, charging should be changed to "trickle" charging to prevent excessive stress inside the battery. The reason why we often heard about the Samsung Galaxy Note 7 mobile phone explosion disaster in 2016 was because one of the battery suppliers easily deformed the lithium battery during assembly, so that the isolation film could not fully isolate the positive and negative electrodes.

Reference: https://www.inside.com.tw/article/32578-The-scientific-principle-of-lithium-battery-explosion