Analysis and comparison of the safety of lithium batteries and fuel cells from several different technical layers
Summary
Battery Management Systems (BMS) do not fully address the safety concerns of lithium-ion batteries. This is rooted in the fundamental working principles of these batteries. The safety of a power battery system is ultimately determined by the individual cells, and when these are grouped together, their safety issues become more pronounced and amplified.
2. Comparison of Technical Aspects Between Lithium-Ion Batteries and Fuel Cells
The commercial success of any industrial product depends on multiple factors. When we look at the success stories of many high-tech products globally, it becomes clear that technology is often not the most decisive factor. For example, Tesla's electric vehicles have succeeded due to a combination of design, marketing, and user experience, rather than just technological superiority. However, it’s important to note that reversing this statement would be incorrect. Technology isn’t omnipotent, but without it, certain goals are simply impossible.
In this article, I will analyze and compare lithium-ion batteries and fuel cells from various technical perspectives. Factors like service life and cost play a significant role in the industrialization of electric vehicles. However, the factors influencing life and cost are complex, involving sensitive materials such as electrode materials, production processes, and cost modeling. Due to confidentiality, I won’t delve into detailed discussions about the life and cost of lithium-ion batteries and fuel cells in this article.
2.1 Safety Comparison
Power batteries are evaluated based on various technical indicators, including energy density, rate performance, temperature performance, and cycle life. Among these, I believe safety is the most critical factor, as it sits at the core of all other technical considerations.
2.1.1 Safety Issues of Lithium-Ion Batteries
In recent years, incidents involving mobile phone and laptop batteries have become less shocking. However, fires and explosions in electric vehicles and lithium-ion battery factories still make headlines. Last year, the Samsung Galaxy Note 7 incident once again brought the safety of lithium-ion batteries into the spotlight. While external factors can contribute to unsafe behavior, the inherent risks of lithium-ion batteries are largely determined by the electrochemical system and the structure, design, and manufacturing process of the electrodes and cells. The electrochemical system itself is the most fundamental determinant of battery safety. Here, I will examine the safety of lithium-ion batteries from multiple angles.
From a thermodynamic perspective, research has shown that both the anode and cathode surfaces are coated with a thin passivation layer. This layer plays a crucial role in battery performance, and this interface issue only exists in non-aqueous organic electrolyte systems. What I want to emphasize is that, from the Fermi level perspective, the current lithium-ion battery system is thermodynamically unstable. It remains stable because the passivation layer on the anode and cathode is dynamic, isolating further reactions between the electrodes and the electrolyte. Therefore, the integrity and compactness of the passivation film directly affect battery safety. Understanding this is key to grasping the safety challenges of lithium-ion batteries.
From a heat transfer standpoint, unsafe behaviors such as overcharging, over-discharging, rapid charging, short circuits, mechanical abuse, and high-temperature thermal shocks can trigger dangerous side reactions inside the battery, leading to heat generation that damages the passivation film on the anode and cathode. When the cell temperature reaches 130°C, the SEI layer on the anode decomposes, exposing the lithium-rich carbon anode to the electrolyte and causing a violent redox reaction, which increases the risk of thermal runaway.
When internal temperatures exceed 200°C, the cathode passivation layer decomposes, releasing oxygen that reacts violently with the electrolyte, generating more heat and increasing internal pressure. At 240°C or higher, a violent exothermic reaction occurs between the lithium carbon anode and the binder.
It is clear that the breakdown of the SEI layer leads to a dangerous exothermic reaction between the highly reactive anode and the electrolyte, directly causing the battery to overheat and potentially go into thermal runaway. The release of the cathode material is just one part of the thermal runaway process and is not the most critical factor. Even though lithium iron phosphate (LFP) has a very stable structure, other dangerous side reactions can still occur in LFP batteries, meaning the "safety" of LFP batteries is relative.
From the above analysis, it is evident how critical temperature control is for the safety of lithium batteries. Compared to small 3C batteries, large power batteries face greater challenges in heat dissipation due to structural, operational, and environmental factors. Therefore, effective thermal management is essential for large power battery systems.
Flammability of electrode materials: The organic solvents used in lithium batteries are flammable, with low flash points. Thermal runaway caused by unsafe conditions can easily ignite these flammable components, leading to battery fires. The carbon anode, separator, and conductive carbon on the cathode are also flammable. While the probability of lithium burning is higher than that of explosion, any explosion involves burning. Additionally, if the battery cracks and the surrounding humidity is high, moisture and oxygen can react violently with the lithium-containing carbon anode, releasing heat and causing the battery to catch fire. The flammability of electrode materials is a major distinction between lithium-ion batteries and aqueous secondary batteries.
Overcharge and metal lithium-related issues: All commercial secondary batteries require effective overcharge protection to prevent safety problems caused by improper charging. Overcharging in lithium batteries can lead to serious consequences, such as damage to the cathode structure, reduced cycle life, electrolyte oxidation, and lithium deposition on the anode, which may cause short circuits and thermal runaway.
Thus, preventing overcharge is vital for the safe use of lithium batteries. Unlike aqueous secondary batteries, controlling the charging voltage is the only overcharge protection measure for lithium-ion batteries. The change in charging voltage is mainly due to the cathode approaching full delithiation, making it difficult to monitor the charging state of the graphite anode (since its lithium insertion potential is close to metallic lithium). To avoid monitoring the anode voltage, lithium-ion batteries typically use a positive limit capacity design. Another key purpose of this design is to ensure the anode has sufficient extra capacity to prevent lithium deposition. However, three situations can reduce the excess capacity of the anode:
1. The capacity decay rate of the graphite anode is higher than that of the cathode material, a fact confirmed in almost all cathode material pairing systems.
2. Improper electrode design or usage conditions (such as high-rate charging, low temperatures, or overcharging) can lead to partial lithium deposition on the anode.
3. Side reactions of the electrolyte and impurities can increase the charge level of the anode, gradually reducing its lithium storage capacity.
Any of these conditions can result in insufficient lithium storage capacity, leading to lithium precipitation and triggering safety issues. These problems are even more severe in large-capacity power batteries, and even with BMS, they cannot be fundamentally solved.
What I want to emphasize here is that the three factors mentioned will become more prominent over time, meaning that older batteries are more prone to safety issues than new ones. This problem has not received enough attention. A topic that has been widely discussed in the past two years is the “gradient development†of power batteries, where used power batteries (with about 70% remaining capacity) are repurposed for energy storage. On the surface, this seems feasible, but after careful analysis of the basic electrochemical principles and deep study of battery safety, I personally believe that gradient utilization of power batteries is more of a theoretical concept than a practical solution. Considering the safety risks of old batteries and the generally poor quality of power batteries from many domestic manufacturers, I don’t think that gradient utilization is practically viable in the short term.
We can also compare the safety of aqueous and lithium-ion batteries from another angle. All secondary batteries, whether aqueous or organic, rely on the principle of positive limiting capacity (excess negative electrode capacity). If this premise is removed, overcharging in aqueous batteries produces hydrogen, while in lithium-ion batteries, it results in lithium deposition on the anode. Aqueous electrolytes have a unique property: they can decompose into hydrogen and oxygen during overcharging, which can then recombine on the electrode or catalyst surface. This is why aqueous batteries typically use an “oxygen cycle†to achieve overcharge protection.
Once lithium is deposited on the anode of a lithium-ion battery, it cannot be eliminated within the battery, inevitably leading to safety issues. Although aqueous batteries are limited in energy density due to water’s decomposition voltage, they provide a near-perfect and irreplaceable overcharge protection solution through water. From this perspective, lithium-ion batteries lack a solution for safety issues. While some technical measures—such as thermal control technology (PTC electrodes), ceramic coatings on electrode surfaces, overcharge protection additives, voltage-sensitive separators, and flame-retardant electrolytes—can improve safety, they cannot solve the underlying safety problems. Moreover, these measures increase costs and reduce energy density.
Considering all these factors, it becomes clear that the “safety†of lithium-ion batteries is relative. Some readers may have noticed that in common batteries like alkaline, lead-acid, and nickel-metal hydride, consumers can buy bare cores directly. However, lithium-ion batteries are an exception. According to industry regulations, manufacturers only sell batteries to authorized pack companies, which then assemble them into battery packs with protection boards before selling them to electrical manufacturers. Battery packs must be used strictly with dedicated chargers. This business model is primarily based on safety considerations for lithium-ion batteries.
The shocking Boeing 787 lithium battery fire incident and the recent Samsung Galaxy Note 7 large-scale battery fire and explosion serve as reminders of the ongoing safety concerns with lithium-ion batteries. Compared to Samsung, Apple has taken a more conservative approach regarding battery capacity and charging voltage, which I believe is primarily driven by safety concerns. Apple may sacrifice battery capacity and energy density to ensure safety.
What I want to stress here is that BMS does not solve the safety problem of lithium-ion batteries, as it is determined by the basic working principle of BMS. The safety of a power battery system ultimately depends on the individual cells, and the safety issues of large power batteries become more severe when grouped. In recent years, there has been much debate in the domestic lithium battery industry that lithium-ion batteries will dominate the market and replace other secondary batteries. From a safety perspective, this argument is clearly unfounded.
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