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Battery assembly isn't just about adding a protection board; many safety issues depend on the individual battery cells. Today, we're sharing an article about the impact of battery cells on safety.
BMS cannot solve the safety problems of lithium-ion power batteries. This is determined by the basic working principle of BMS. The safety of a power battery system fundamentally depends on the individual battery cells, and the safety issues are amplified and become more prominent after large power batteries are assembled.
2. Comparison of Lithium-ion Batteries and Fuel Cells from a Technological Perspective
The commercial success of an industrial product depends on many factors. If we carefully analyze numerous successful cases of high-tech products worldwide, we will find that technology is often not the most important or decisive factor, such as the well-known Tesla electric vehicles. However, I want to emphasize here that it would be wrong to say the other way around. Technology is not omnipotent, but without technology, nothing is possible!
In this article, I will analyze and compare lithium-ion batteries and fuel cells from several different technological perspectives.
The lifespan and cost of power batteries are also significant factors restricting the industrialization of electric vehicles. However, due to the complexity of the factors affecting lifespan and cost, involving sensitive trade secrets such as electrode materials, production processes and equipment, and cost modeling, this article will not specifically compare and discuss the lifespan and cost of lithium batteries and fuel cells.
2.1 Comparison of Safety
Power batteries are measured by many technical standards and indicators, such as energy density, rate performance, temperature performance, cycle life, etc. Among these technical indicators, I personally believe that safety is the most important, a core element that takes precedence over any other technical indicator.
2.1.1 Safety Issues of Lithium-ion Batteries
In recent years, the burning and explosion of mobile phone and laptop batteries have become commonplace; electric vehicle explosions and fires at lithium battery factories are the real news. However, the widespread battery fires and explosions of the Samsung Galaxy Note 7 last year once again brought the safety issues of lithium-ion batteries to the forefront. Besides external factors related to usage, the safety of lithium-ion power batteries mainly depends on the basic electrochemical system and internal factors such as the structure, design, and manufacturing process of the electrodes/cells. The electrochemical system used in the cell is the most fundamental factor determining battery safety. This article will analyze the safety issues of lithium-ion batteries from several different perspectives.
☞Thermodynamic perspective: Research has confirmed that not only the negative electrode, but also the surface of the positive electrode material is covered with a very thin passivation film. The passivation film covering the positive and negative electrode surfaces has a very important impact on various aspects of the performance of lithium-ion batteries, and this special interface problem only exists in non-aqueous organic electrolyte systems. It is important to emphasize here that, from the perspective of the Fermi level, the existing lithium-ion battery system is thermodynamically unstable. Its stable operation is due to the passivation films on the positive and negative electrode surfaces, which kinetically isolate the positive and negative electrodes from further reactions with the electrolyte. Therefore, the safety of lithium batteries is directly related to the integrity and density of the passivation films on the positive and negative electrode surfaces. Understanding this issue is crucial to understanding the safety of lithium batteries.
☞Heat Transfer Angle:Unsafe behaviors of lithium-ion batteries (including overcharging, over-discharging, rapid charging and discharging, short circuits, mechanical abuse, and high-temperature thermal shock) can easily trigger dangerous side reactions inside the battery, generating heat and directly damaging the passivation films on the surfaces of the negative and positive electrodes. When the cell temperature rises above 130℃, the SEI film on the surface of the negative electrode decomposes, causing the highly active lithium-carbon negative electrode to be exposed to the electrolyte and undergo a violent redox reaction. The heat generated puts the battery into a high-risk state.
When the local temperature inside the battery rises above 200℃, the passivation film on the surface of the positive electrode decomposes, oxygen evolution occurs at the positive electrode, and it continues to react violently with the electrolyte, generating a large amount of heat and forming high internal pressure.
When the battery temperature reaches above 240 ℃, a violent exothermic reaction occurs between the lithium-carbon anode and the binder. It is evident that damage to the SEI film on the anode surface, leading to a violent exothermic reaction between the highly active lithium-intercalated anode and the electrolyte, is the direct cause of the increased battery temperature and subsequent thermal runaway. The decomposition and exothermic reaction of the cathode material is only one part of the thermal runaway reaction, and not even the most significant factor. While lithium iron phosphate (LFP) has a very stable structure and does not undergo thermal decomposition under normal conditions, other hazardous side reactions still exist in LFP batteries. Therefore, the "safety" of LFP batteries is only relative.From the above analysis, we can see the crucial importance of temperature control for lithium battery safety. Compared to small 3C batteries, large power batteries face greater challenges in heat dissipation due to factors such as cell structure, operating mode, and environment. Therefore, thermal management design for large power battery systems is paramount.
☞Flammability of Electrode Materials: The organic solvents used in lithium batteries are flammable and have very low flash points. Unsafe practices leading to thermal runaway can easily ignite these low-flash-point flammable liquid components, causing the battery to burn. The carbon materials of the lithium battery negative electrode, the separator, and the conductive carbon of the positive electrode are also flammable. The probability of a lithium battery burning is higher than the probability of a battery exploding, but a battery explosion is always accompanied by combustion. Furthermore, when the battery cracks and the ambient humidity is high, the moisture and oxygen in the air can easily react violently with the lithium-intercalated carbon negative electrode, releasing a large amount of heat and potentially causing the battery to burn.
The flammability of electrode materials is a major difference between lithium-ion batteries and aqueous rechargeable batteries. **Overcharging and Related Issues with Metallic Lithium:** Any commercially available rechargeable battery requires effective overcharging prevention measures to ensure the battery reaches a fully charged state and avoid safety issues caused by inappropriate overcharging. Overcharging lithium batteries can lead to several serious consequences, such as damage to the crystal structure of the positive electrode material, resulting in a deteriorated cycle life; accelerated oxidation of the electrolyte on the positive electrode surface, leading to thermal runaway; and lithium plating at the negative electrode, causing short circuits/thermal runaway and other safety problems. **Therefore, preventing overcharging is extremely important for the safe use of lithium batteries.** Unlike aqueous rechargeable batteries, controlling the charging voltage is the only overcharge protection measure for lithium-ion batteries. The voltage change during lithium-ion battery charging mainly originates from the positive electrode material approaching a fully delithilated state. It's difficult to detect the completion of the charging process at the graphite negative electrode (because its lithium intercalation potential is very close to that of metallic lithium). To circumvent the difficulty of monitoring the negative electrode voltage, lithium-ion batteries generally employ a positive limiting capacity design. Of course, another major function of the positive limiting capacity is to ensure the negative electrode has sufficient additional capacity to prevent lithium plating. However, three situations can alter the excess capacity of the negative electrode: 1. The capacity decay rate of the graphite negative electrode is higher than that of the positive electrode material, which has been confirmed in almost all positive electrode material combinations. 2. Due to unreasonable electrode structure design, or under improper usage conditions (such as high rate, low temperature, and overcharging), localized lithium plating occurs at the negative electrode.
3. Side reactions of the electrolyte and impurities lead to increased negative electrode charging and a gradual loss of additional lithium storage capacity.
Any of the above situations will result in insufficient negative electrode lithium storage capacity and lithium deposition, and metallic lithium is the main culprit for lithium battery safety issues. These problems are even more serious in high-capacity power batteries, and even the use of a BMS cannot fundamentally solve these problems.
The author wants to emphasize here that the above three factors become more prominent with battery use, meaning that the safety problems of old batteries are more serious than those of new batteries, and this issue has not received enough attention at present.
A hotly debated topic in the past two years has been the "gradient development" of power batteries, which involves reusing power batteries that have reached the end of their service life (theoretically retaining 70% of their capacity) for energy storage. While the gradient utilization of power batteries seems feasible on the surface, a careful analysis of basic electrochemical principles and in-depth research into battery safety and related issues leads me to believe that it is actually a false proposition. Considering the safety hazards of old batteries and the generally poor quality of power batteries from most domestic manufacturers, I do not believe that gradient development of power batteries is practically feasible in the short term.
Actually, we can also compare the safety issues of aqueous secondary batteries and lithium batteries from another perspective. The charging safety of all secondary batteries, whether aqueous or organic, is based on the fundamental principle of positive limit capacity (excess capacity at the negative electrode). If this premise disappears, the consequence of overcharging is hydrogen production in aqueous secondary batteries and lithium deposition at the negative electrode in lithium-ion batteries. However, the aqueous electrolyte used in various aqueous secondary batteries has a unique property: water can decompose into hydrogen and oxygen during overcharging, and hydrogen and oxygen can recombine on the electrode or composite catalyst surface to form water. Therefore, it's easy to understand why aqueous secondary batteries generally use the principle of "oxygen cycling" to achieve overcharge protection.
In lithium-ion batteries, once highly active metallic lithium is deposited at the negative electrode, it will inevitably lead to safety issues because the metallic lithium cannot be eliminated inside the battery.
While the energy density of aqueous secondary batteries is limited by the decomposition voltage of water, it's important to remember that water also provides a near-perfect and irreplaceable solution against overcharge. Comparing lithium-ion batteries and aqueous secondary batteries from this perspective, the organic electrolyte used in lithium batteries lacks reversible decomposition and recovery characteristics, and once highly reactive lithium metal is formed, it cannot be eliminated. Therefore, in a sense, the safety issue of lithium-ion batteries is unsolvable! Through the comprehensive application of various technical measures, such as thermal control technology (PTC electrode), ceramic coatings on the positive and negative electrode surfaces, overcharge protection additives, voltage-sensitive separators, and flame-retardant electrolytes, the safety of lithium batteries can be effectively improved. However, these measures cannot fundamentally solve the safety problem of lithium batteries because lithium batteries are thermodynamically unstable systems. Furthermore, these measures not only increase costs but also reduce the energy density of the battery.If we consider the above factors comprehensively, we will understand that the "safety" of lithium batteries is only relative. Some readers may have noticed that for general batteries such as alkaline manganese, lead-acid, and nickel-metal hydride batteries, consumers can directly buy bare cells in stores, but lithium-ion batteries are an exception. According to lithium battery industry regulations, battery cell manufacturers only sell their cells to authorized pack companies, which then package the cells and protection boards into battery packs and sell them to appliance manufacturers, not consumers. Furthermore, the battery packs must be used strictly according to regulations and paired with a dedicated charger. The logic behind this special business model is primarily based on considerations of lithium battery safety.
The Boeing 787 "Dreamliner" lithium battery fire that shocked the industry, and the recent widespread battery fires and explosions of the Samsung Galaxy Note 7, have once again sounded the alarm regarding the safety of lithium-ion batteries. Compared to Samsung, Apple has always been relatively conservative and prudent in its battery approach, with lower battery capacity and charging voltage limits. I personally believe that Apple's conservative strategy is primarily based on safety considerations; Apple is willing to slightly sacrifice battery capacity and energy density to ensure safety.
It is important to emphasize here that a Battery Management System (BMS) cannot solve the safety problems of lithium-ion power batteries. This is determined by the basic working principle of the BMS. The safety of a power battery system fundamentally depends on the individual battery cells, and safety issues are amplified and become more prominent when large power batteries are assembled into a battery pack. In recent years, the domestic lithium battery industry has been rife with the argument that lithium-ion batteries will dominate the market and replace other rechargeable batteries. From a safety perspective alone, this argument is undoubtedly absurd and laughable. BMS (Battery Management System) cannot solve the safety problems of lithium-ion power batteries; this is determined by the basic working principle of BMS. The safety of a power battery system fundamentally depends on the individual battery cells, and safety issues are amplified and become more prominent when large power batteries are assembled. I think this article is for customers who only look for the protection board when there is a problem. They should read it carefully and learn! Lithium batteries are a very profound subject. Don't think it's simple, that it's just some cells connected with a protection board. Those who know nothing about battery cells and then look for the protection board at the slightest problem need to learn more!
