Lithium-ion battery characteristics

鋰是化學元素週期表中最小、最活潑的金屬。 由於其體積小、容量密度高，廣受消費者和工程師的歡迎。 但是，化學性質過於活躍，帶來了極高的風險。 當鋰金屬暴露在空氣中時，會與氧氣發生劇烈反應並發生爆炸。 為了提高安全性和電壓，科學家發明了石墨和鈷酸鋰等材料來儲存鋰原子。 這些材料的分子結構形成了納米級的小型存儲網格，可用於存儲鋰原子。 這樣，即使電池殼破裂，氧氣進入，氧分子太大，無法進入這些小蓄電池，鋰原子就不會與氧氣接觸，避免爆炸。 鋰離子電池的這一原理，使人們在實現高容量密度的同時實現安全。

電氣防爆試驗

鋰離子電池充電時，正極中的鋰原子失去電子，被氧化成鋰離子。 鋰離子通過電解液游到負極，進入負極的蓄電池，得到一個電子，被還原為鋰原子。 放電時，整個過程反過來。 為了防止電池的正負極直接接觸短路，在電池上加了一層多孔的隔膜紙，防止短路。 好的隔膜紙還能在電池溫度過高時自動關閉氣孔，讓鋰離子無法通過，讓他們可以用自己的武藝來防範危險。

保障

After the lithium battery cell is overcharged to a voltage higher than 4.2V, side effects will begin to occur. The higher the overcharge voltage, the higher the risk. When the voltage of the lithium battery cell is higher than 4.2V, the number of lithium atoms remaining in the positive electrode material is less than half. At this time, the cell often collapses, causing a permanent decrease in battery capacity. If you continue to charge, since the cell of the negative electrode is already filled with lithium atoms, subsequent lithium metal will accumulate on the surface of the negative electrode material. These lithium atoms will grow dendrites from the surface of the negative electrode toward the direction of the lithium ions. These lithium metal crystals will pass through the separator paper and short-circuit the positive and negative electrodes. Sometimes the battery explodes before the short circuit occurs. This is because during the overcharging process, the electrolyte and other materials will crack to produce gas, causing the battery shell or pressure valve to swell and rupture, allowing oxygen to enter and react with the lithium atoms accumulated on the surface of the negative electrode. And then exploded. Therefore, when charging a lithium battery, the upper voltage limit must be set so that the battery life, capacity, and safety can be taken into account at the same time. The most ideal upper limit of the charging voltage is 4.2V. There is also a lower voltage limit when discharging lithium batteries. When the cell voltage is lower than 2.4V, some materials will begin to be destroyed. Also, since the battery will self-discharge, the longer it is left, the lower the voltage will be. Therefore, it is best not to stop when the battery is discharged to 2.4V. During the period when the lithium battery is discharged from 3.0V to 2.4V, the energy released only accounts for about 3% of the battery capacity. Therefore, 3.0V is an ideal discharge cut-off voltage.

充放電時，除了限壓外，還需要限流。 當電流過大時，鋰離子來不及進入蓄電池，會在材料表面堆積。 這些鋰離子獲得電子後，會在材料表面產生鋰原子晶體，與過充一樣，很危險。 如果電池外殼破裂，就會爆炸。

因此，鋰離子電池的保護必須至少包括三項：充電電壓上限、放電電壓下限、電流上限。 一般來說，在一個鋰電池組中，除了鋰電芯外，還會有一塊保護板。 本保護板主要提供這三種保護。 但是，保護板的這三項保護顯然還不夠，全球鋰電池爆炸事件依然頻發。 為了確保電池系統的安全，必須更加仔細地分析電池爆炸的原因。

Explosion type analysis

The types of battery cell explosion can be classified into three types: external short circuit, internal short circuit, and overcharge. The outside here refers to the outside of the battery cell, including short circuits caused by poor internal insulation design of the battery pack.

當電芯外部發生短路，電子元件未能切斷電路時，電芯內部會產生高熱量，導致部分電解液汽化，使電池殼膨脹。 當電池內部溫度高達135攝氏度時，質量好的隔膜紙將孔隙封閉，電化學反應終止或幾乎終止，電流急劇下降，溫度緩慢下降，從而避免爆炸。 但是，閉孔率太差，或者氣孔根本沒有閉上。 隔膜紙會導致電池溫度不斷升高，更多的電解液會蒸發，最後電池外殼會破裂，甚至會導致電池溫度升高到材料燃燒爆炸。 內部短路主要是銅箔、鋁箔的毛刺刺破隔膜，或鋰原子枝晶刺破隔膜造成的。 這些微小的針狀金屬會導致微短路。 由於針很細，有一定的電阻值，所以電流不一定很大。

銅箔和鋁箔毛刺是在生產過程中產生的。 可觀察到的現像是電池漏液過快，大部分可以通過電芯廠或組裝廠篩選。 而且，由於毛刺很小，有時會被燒毀，導致電池恢復正常。 因此，毛刺微短路引起爆炸的概率不高。 這種說法可以從各電芯廠經常出現充電後不久電壓低的壞電池出現，但很少發生爆炸，這是有統計數據支持的。 因此，內部短路引起的爆炸主要是過充引起的。 因為過充後，極片上到處都是針狀的鋰金屬晶體，到處都是穿刺點，到處都是微短路。 因此，電池溫度會逐漸升高，最後高溫會導致電解液氣化。 在這種情況下，無論是溫度過高導致材料燃燒爆炸，還是外殼先破裂，導致空氣進入氧化鋰金屬，都是爆炸。

However, the explosion caused by an internal short circuit caused by overcharging does not necessarily occur at the time of charging. It is possible that when the battery temperature is not high enough to burn the material and the gas generated is not enough to break the battery casing, the consumer will stop charging and take the mobile phone out. At this time, the heat generated by numerous micro-short circuits slowly raises the temperature of the battery, and it explodes after a period of time. The common description of consumers is that when they pick up the phone, they find that the phone is very hot and explode after throwing it away.

Based on the above types of explosions, we can focus on three aspects of explosion protection: the prevention of overcharge, the prevention of external short circuits, and the improvement of cell safety. Among them, overcharge prevention and external short circuit prevention belong to electronic protection, which has a greater relationship with battery system design and battery assembly. The focus of battery cell safety enhancement is chemical and mechanical protection, which has a greater relationship with battery cell manufacturers.