Analysis of the causes of battery bulging and explosion:
I. Characteristics of Lithium-ion Batteries
Lithium is the smallest and most reactive metal on the periodic table. Its small size results in high capacity density, making it popular with consumers and engineers. However, its highly reactive chemical properties pose a significant risk. When lithium metal is exposed to air, it reacts violently with oxygen, causing an explosion. To improve safety and voltage, scientists have invented materials such as graphite and lithium cobalt oxide to store lithium atoms. The molecular structure of these materials forms nanoscale storage lattices that can store lithium atoms. This way, even if the battery casing breaks and oxygen enters, the oxygen molecules are too large to fit into these tiny storage lattices, preventing the lithium atoms from contacting the oxygen and thus avoiding an explosion. This principle of lithium-ion batteries allows for both high capacity density and safety.
During charging, lithium atoms at the positive electrode lose electrons and are oxidized into lithium ions. These lithium ions then travel through the electrolyte to the negative electrode, enter its storage compartment, gain an electron, and are reduced back to lithium atoms. During discharging, the process is reversed. To prevent a short circuit caused by direct contact between the positive and negative electrodes, a separator with numerous fine pores is added inside the battery. A good separator can also automatically close its pores when the battery temperature gets too high, preventing lithium ions from passing through and thus rendering the battery ineffective and preventing potential hazards.
Protective measures
Overcharging lithium-ion battery cells to above 4.2V will begin to produce side effects. The higher the overcharge voltage, the greater the danger. When the voltage of a lithium-ion battery cell exceeds 4.2V, less than half of the lithium atoms remain in the positive electrode material. At this point, the storage cells often collapse, causing a permanent decrease in battery capacity. If charging continues, since the storage cells of the negative electrode are already full of lithium atoms, subsequent lithium metal will accumulate on the surface of the negative electrode material. These lithium atoms will grow into dendritic crystals from the negative electrode surface in the direction from which the lithium ions originated. These lithium metal crystals can penetrate the separator paper, causing a short circuit between the positive and negative electrodes. Sometimes the battery explodes before the short circuit occurs because during the overcharge process, materials such as the electrolyte decompose to produce gas, causing the battery casing or pressure valve to bulge and rupture, allowing oxygen to enter and react with the lithium atoms accumulated on the negative electrode surface, leading to an explosion. Therefore, when charging lithium-ion batteries, it is essential to set an upper voltage limit to simultaneously consider battery life, capacity, and safety. The ideal upper voltage limit is 4.2V.
Lithium-ion battery cells also have a lower voltage limit during discharge. When the cell voltage drops below 2.4V, some materials begin to degrade. Furthermore, because batteries self-discharge, the voltage decreases the longer they are discharged. Therefore, it's best not to discharge until the voltage reaches 2.4V. During the discharge process from 3.0V to 2.4V, the energy released by a lithium-ion battery only accounts for about 3% of its capacity. Therefore, 3.0V is an ideal discharge cutoff voltage.
In addition to voltage limitations, current limitations are also necessary during charging and discharging. If the current is too high, lithium ions cannot enter the storage cells in time and will accumulate on the material surface. These lithium ions, after gaining electrons, will form lithium atom crystals on the material surface, which, like overcharging, poses a danger. If the battery casing ruptures, it can explode.
Therefore, the protection of lithium-ion batteries should at least include three aspects: an upper limit for charging voltage, a lower limit for discharging voltage, and an upper limit for current. Generally, in addition to the lithium battery cells, a protection board is present in a lithium battery pack, primarily providing these three protections. However, these three protections from the protection board are clearly insufficient, and lithium battery explosions continue to occur frequently worldwide. To ensure the safety of battery systems, a more thorough analysis of the causes of battery explosions is necessary.
II. Causes of battery explosion:
1. Significant internal polarization!
2: The electrode absorbs water and reacts with the electrolyte to form a gas bubble.
3: The quality and performance of the electrolyte itself.
4: The amount of liquid injected during injection does not meet the process requirements.
5. The laser welding process has poor sealing performance, resulting in air leakage and failure to detect leaks.
6: Dust. Dust on the electrode sheet can easily cause micro short circuits, the specific reasons for which are unknown.
7: The positive and negative electrode sheets are thicker than the process range allows, making them difficult to insert into the casing.
8: Liquid injection sealing problem, poor sealing performance of steel balls leads to air bulging.
9: The incoming shell material has an excessively thick shell wall, and the shell deformation affects the thickness.
III. Explosion Type Analysis
Battery cell explosions can be categorized into three types: external short circuits, internal short circuits, and overcharging. Here, "external" refers to the exterior of the battery cell, including short circuits caused by poor insulation design within the battery pack.
When a short circuit occurs outside the battery cell and the electronic components fail to disconnect the circuit, high heat is generated inside the cell, causing some of the electrolyte to vaporize and expanding the battery casing. When the internal temperature of the battery reaches 135 degrees Celsius, a high-quality separator will close the pores, stopping or nearly stopping the electrochemical reaction, causing a sharp drop in current and a gradual decrease in temperature, thus preventing an explosion. However, a separator with poor pore-closing efficiency, or one where the pores do not close at all, will allow the battery temperature to continue to rise, causing more electrolyte to vaporize, eventually rupturing the battery casing, or even raising the battery temperature to the point where the materials burn and explode.
Internal short circuits are mainly caused by burrs on the copper and aluminum foil piercing the separator, or by dendritic crystals of lithium atoms piercing the separator. These tiny needle-like metal particles can cause micro-short circuits. Because the needles are very thin, they have a certain resistance, so the current may not be very large. The burrs on the copper and aluminum foil are caused during the manufacturing process, and the observable phenomenon is that the battery leaks too quickly; most of these can be detected by the cell manufacturer or assembly plant. Moreover, because the burrs are small, they may sometimes burn out, allowing the battery to return to normal. Therefore, the probability of an explosion caused by a micro-short circuit due to burrs is low.
This argument is supported by statistics showing that while defective batteries with low voltage are frequently found within battery cell manufacturers shortly after charging, explosions are rare. Therefore, explosions caused by internal short circuits are primarily due to overcharging. Overcharging results in needle-like lithium metal crystals scattered across the electrode surfaces, creating numerous micro-short circuits. Consequently, the battery temperature gradually rises, eventually causing the electrolyte to vaporize. Whether the explosion is caused by excessive heat leading to material combustion, or by the outer casing rupturing first, allowing air to enter and violently oxidize the lithium metal, an explosion is inevitable.
However, explosions caused by internal short circuits due to overcharging don't necessarily occur during charging. It's possible that the battery temperature hasn't reached a point where the materials are burning, and the generated gases aren't enough to burst the battery casing, before the consumer stops charging and takes the phone out. In this case, the heat generated by numerous micro-short circuits slowly raises the battery temperature, and an explosion occurs only after a period of time. Consumers typically describe the phone as being very hot when picked up, exploding after being dropped.
Based on the above types of explosions, we can focus on three key aspects of explosion prevention: preventing overcharging, preventing external short circuits, and improving cell safety. Overcharging and external short circuit prevention fall under electronic protection and are closely related to battery system design and assembly. Improving cell safety focuses on chemical and mechanical protection, which is largely dependent on the battery cell manufacturer.
IV. Design Specifications
With hundreds of millions of mobile phones worldwide, achieving safety requires a failure rate of less than one in a hundred million. However, the failure rate of circuit boards is generally much higher than one in a hundred million. Therefore, battery systems must be designed with at least two lines of defense. A common design error is directly charging the battery pack with the adapter. This places the entire responsibility for overcharge protection on the protection board on the battery pack. Although the failure rate of the protection board is low, even with a failure rate as low as one in a million, explosions still occur globally every day.
If a battery system provides two layers of safety protection against overcharging, over-discharging, and overcurrent, and each layer has a failure rate of one in ten thousand, then two layers of protection can reduce the failure rate to one in one hundred million. A typical battery charging system block diagram is shown below, consisting of two main parts: the charger and the battery pack. The charger further comprises an adapter and a charging controller. The adapter converts AC to DC, while the charging controller limits the maximum current and voltage of the DC power. The battery pack consists of a protection board and battery cells, as well as a PTC (Power Transmitter) to limit the maximum current.
Text blocks: Adapter AC to DC; Text blocks: Charging controller current and voltage limiting; Text blocks: Charger; Text blocks: Protection board for overcharge, over-discharge, and overcurrent protection; Text blocks: Battery pack; Text blocks: Current limiter; Text blocks: Battery cell. Taking a mobile phone battery system as an example, overcharge protection uses the charger output voltage set at around 4.2V to achieve the first layer of protection. This way, even if the protection board on the battery pack fails, the battery will not be overcharged and become dangerous. The second layer of protection is the overcharge protection function on the protection board, which is generally set at 4.3V. In this way, the protection board does not need to cut off the charging current under normal circumstances; it only needs to activate when the charger voltage is abnormally high. Overcurrent protection is handled by the protection board and the current limiter, which is also a double layer of protection to prevent overcurrent and external short circuits. Since over-discharge only occurs during the use of electronic products, the circuit board of the electronic product is generally designed to provide the first layer of protection, while the protection board on the battery pack provides the second layer of protection. When the electronic product detects that the supply voltage is lower than 3.0V, it should automatically shut down. If this function is not designed into the product, the protection board will shut down the discharge circuit when the voltage drops to 2.4V.
In summary, battery system design must provide two layers of electronic protection against overcharging, over-discharging, and overcurrent. The protection board is the second layer of protection. If the battery explodes during charging after removing the protection board, it indicates a design flaw.
While the above methods provide two layers of protection, consumers often buy non-original chargers after their own chargers break down. Charger manufacturers, driven by cost considerations, often remove the charging controller to reduce expenses. As a result, inferior chargers drive out superior ones, leading to a proliferation of substandard chargers on the market. This leaves overcharge protection without its first and most crucial line of defense. Since overcharging is the most significant factor causing battery explosions, substandard chargers can be considered the root cause of battery explosions.
Of course, not all battery systems use the design shown in the diagram above. In some cases, the battery pack also incorporates a charging controller. For example, many external battery packs for laptops have a charging controller. This is because laptops typically integrate the charging controller into the computer itself, providing only an adapter to the consumer. Therefore, external battery packs for laptops must have a charging controller to ensure safety when charging with an adapter. Additionally, products that use car cigarette lighter sockets for charging sometimes also integrate the charging controller into the battery pack.
The last line of defense
If all electronic safety measures fail, the last line of defense comes from the battery cell itself. The safety level of a battery cell can be roughly determined by its ability to withstand external short circuits and overcharging. Because lithium atoms accumulating on the surface of the material before a battery explodes can increase its explosive power, and because overcharging protection is often reduced to a single line of defense due to consumers using substandard chargers, the cell's ability to withstand overcharging is more important than its ability to withstand external short circuits.
Safety Comparison of Aluminum-Cast and Steel-Cast Battery Cells: Aluminum-cased cells have a significant safety advantage over steel-cased cells.
