1. What Happened When an Iron Ball Was Dropped on a Battery?
The test Yoshino chose was very simple: drop an iron ball on top of the battery and see what happens [5]. First, he tried it on a battery using metallic lithium. The metallic lithium overflowed, and the battery caught fire. However, the lithium-ion battery passed the test, which led to its commercialization, according to Yoshino’s lecture [5]. Yoshino described this event as the moment the lithium-ion battery was truly born [5]. It was not a test of performance, but a test of whether it would burn, that made the battery a real product. Why did the results differ between two batteries that both use lithium? [5]
Note: What was the safety test? Yoshino stated in lectures that the lithium-ion battery was truly born at the "moment of the safety test" [5].
2. Why Batteries Use Carbon Instead of Reactive Metallic Lithium
The original metallic lithium batteries were too reactive. As rechargeable secondary batteries, they were prone to explosion and were not usable [6]. So, Yoshino stopped using reactive metallic lithium for the negative electrode and used a carbon material called petroleum coke instead. This material can take in and release lithium ions. Because of this, the battery became light and could be recharged repeatedly [6]. If we use an analogy, metallic lithium is like "carrying loose gunpowder," while carbon is like "a shelf to store gunpowder." However, inside a real battery, lithium enters between the layers of carbon in the form of ions. The shelf analogy is just an example. Did this make the battery safe? Not entirely. The battery still contained a flammable liquid inside [6].
Note: This does not mean all danger is gone.
3. Why Lithium-Ion Batteries Can Still Burn: The Flammable Liquid Inside
Batteries contain an electrolyte that conducts electricity. This electrolyte is a lithium salt dissolved in an organic solvent, and this organic solvent is flammable. The Yamada Laboratory at the University of Tokyo explains that this is the biggest risk factor for fire and explosion accidents [1]. You might think, "Why not use water? Water doesn't burn." However, ordinary water solutions split into hydrogen and oxygen at a voltage of about 1.2 volts [1]. Batteries work at higher voltages, so water cannot be used as an electrolyte. This is why a flammable liquid is used. Even with a flammable liquid, fire does not usually start. It only becomes a problem when the temperature starts to rise due to some trigger [1].
4. What Is Thermal Runaway in a Battery?
There is a phenomenon where the battery's temperature becomes uncontrollable. This is called "thermal runaway." The Japanese Industrial Standard (JIS C 8715-2) defines it as a state where temperature control is lost due to positive feedback, where heat generation leads to more heat generation [2]. Triggers include internal short circuits, external short circuits, overcharging, over-discharging, and heating [2]. For example, in an internal short circuit, electricity flows rapidly to the negative electrode, causing it to heat up, and that heat spreads to the positive electrode [3]. Heat from specific parts inside the battery causes other parts to generate heat, continuing the temperature rise [3]. Explanatory articles give rough temperature guidelines: around 80°C, the electrodes and electrolyte react; around 140°C, the separator deforms from heat; around 200–300°C, the positive electrode material decomposes; and at even higher temperatures, it may exceed 1000°C [4]. However, these are rough guidelines that vary by battery type and are not fixed values [4]. Is there a way to stop the battery during this chain reaction?
5. How a Thin Membrane Closes Its Pores When It Gets Hot
Inside the battery, there is a membrane with many holes, made of polyethylene or polypropylene, that separates the positive and negative electrodes. This is called the separator [7]. When abnormal heat generation occurs, the pores in the separator close, stopping the current. This safety mechanism is called "shutdown" [7]. The heat causes the separator's pores to close, which stops the flow of electricity. The materials explain that shutdown works when the polyethylene melts [7]. However, the membrane alone cannot prevent everything. Batteries have other layers of protection [7].
6. How Battery Safety Measures Protect Against Each Cause of Fire
Battery safety measures are designed for each trigger of fire [2]. For external short circuits, protection circuits, fuses, and PTC devices are used. For overcharging or over-discharging, protection circuits work. For internal short circuits, manufacturers prevent foreign objects from entering during production, improve inspection methods, and add technology that makes thermal runaway less likely even if a short occurs [2]. In manufacturer examples, IC chips stop charging when full and detect over-discharge, temperature, short circuits, and overcurrent to stop them. Thermal fuses are also used. Portable batteries are tested under UN38.3, based on UN recommendations, checking temperature, vibration, altitude, and other conditions [8]. There are also tricks in the manufacturing site. Electrolytes dislike moisture, so batteries are made in "dry rooms" where dew points are managed [2].
Note: NITE lecture materials organize safety measures by trigger. External short circuits use protection circuits and devices (fuses, PTC). Overcharge/over-discharge uses protection circuits. Internal short circuits use measures against foreign objects in production, optimized inspection, and safety technology to resist thermal runaway [2]. Manufacturer examples show charger control plus IC chips and thermal fuses to detect faults. Battery safety is built by combining materials, circuits, manufacturing, and inspection.
7. Research on Replacing the Flammable Liquid with Water
There is research to reduce the flammability of the electrolyte itself. The Yamada Laboratory at the University of Tokyo found a lithium salt liquid containing water called "hydrate melt." Ordinary water solutions decompose at about 1.2 volts, but hydrate melt does not decompose even at more than 3 volts, despite containing water [1]. It was announced that they succeeded for the first time in operating a lithium-ion battery using this water-based electrolyte [1]. The Yamada Laboratory explains that there is a demand for developing new non-flammable and non-toxic electrolytes [1]. However, this is a research result. Water-based electrolytes are not yet used in commercially sold batteries [1].
8. How to Find Lithium-Ion Batteries Around You
Look for rechargeable batteries at home. On smartphones, mobile batteries, wireless earphones, or their boxes, you may see "Lithium-ion" or "Li-ion" written. If you find one, just look at it. Do not take it apart or apply strong pressure. For those who want to learn more, Yoshino’s lecture record and the Yamada Laboratory’s press release are recommended [5][1]. They show how battery safety has been built up step by step through human ingenuity [5][1].