The tragic accident involving the Xiaomi SU7 has left the families of the three deceased female college students awaiting investigation results over ten days later, and the repercussions of this incident continue to resonate. A warning sign on the highway section in Anhui reads: “Intelligent driving is just an aid; drivers must remain focused at high speeds.” This accident has prompted reflections on the implications for intelligent driving and regulation, while many are also concerned about electric vehicle collisions, their fire risks, and escape times.
Three days after the incident, investors began questioning whether the Xiaomi SU7 used CATL batteries, to which CATL officially denied. Since the rise of electric vehicles (EVs), concerns over the safety of power batteries have been unrelenting. Initially, nearly every instance of spontaneous combustion in a new energy vehicle would trend in the news. Subsequently, manufacturers’ after-sales teams raced against insurance adjusters to be the first on the scene, often covering the vehicles or logos to minimize visibility of such incidents in the media. To alleviate consumer apprehension, many manufacturers have branded their batteries with names suggesting impenetrability, such as “magazine,” “Dayu,” and “amber,” while conducting various safety tests including puncture, gunfire, compression, twisting, and submersion.
A series of technological and marketing efforts have indeed yielded significant results. According to data released by the Passenger Car Association, in 2024, sales of new energy vehicles reached 12.866 million units, accounting for 40.9% of total new car sales. New energy vehicles have now become mainstream in sales. However, even with continuous improvements in battery safety, the sheer number of new vehicles—over ten million annually—ensures that incidents will inevitably occur. If power batteries can withstand gunfire and punctures, why do they still catch fire in collisions? What is the current level of battery safety, beyond superficial measures like covering logos? Are lithium batteries inherently unsafe?
It is widely known that ternary lithium and lithium iron phosphate represent the two most prevalent forms of lithium battery technology today. However, many are unaware that the first electric vehicle was created in 1834 by an American, predating Karl Benz’s first automobile by half a century. Why has it taken until now for electric vehicles to flourish? The answer lies in the lack of usable batteries. After nearly 200 years of failed attempts with Voltaic cells, lead-acid batteries, and nickel-metal hydride batteries, the late 20th century finally saw a viable commercial technology for pure electric vehicles. In 1995, Nissan showcased the first concept car with a lithium-ion battery, the FEV-II, which laid the foundation for the Prairie Joy EV in 1996, the first to use cylindrical lithium batteries. In 1998, Nissan released the Altra, the first mass-produced vehicle with lithium batteries, featuring a pure electric range of only 190 km.
A landmark moment occurred in 2006 when Tesla unveiled the Roadster, which utilized established 18650 cell technology, combining 6831 cells with self-developed packaging and battery management systems (BMS). The first-generation Roadster boasted a battery capacity of 80 kWh and a range of 390 km, which was impressive two decades ago and remains competitive today. Two years later, BYD launched the F3DM, their first pure electric vehicle, employing lithium iron phosphate batteries. This established the initial technological paths for the two major players in the pure electric sector, marking the gradual emergence of the electric vehicle era.
The primary distinction between ternary lithium and lithium iron phosphate lies in their cathode materials. Ternary lithium batteries use a mix of nickel, cobalt, and manganese or nickel, cobalt, and aluminum as cathode materials, while lithium iron phosphate batteries use lithium iron phosphate. This difference in materials leads to varying performance, affecting battery pricing, lifespan, range, and safety. Examining ternary lithium batteries, they offer two main advantages: 1 High energy density ranging from 180-230 Wh/kg, which is 30%-50% higher than lithium iron phosphate, typically used in long-range models; 2 Superior low-temperature performance, which means less reduction in range in cold conditions, making them more suitable for regions north of the Yangtze River. However, they also have two notable drawbacks: 1 Lower safety; ternary lithium batteries are more prone to thermal runaway under high temperatures or external force, increasing fire risk. Manufacturers typically equip vehicles using ternary lithium with more complex BMS to ensure safety. 2 Higher costs due to the use of more precious metals, with material and manufacturing costs significantly elevated. For instance, in 2024, the procurement cost for ternary lithium cathode material was approximately 140,000 yuan/ton, while lithium iron phosphate was less than 50,000 yuan/ton. Consequently, ternary lithium batteries can be a third more expensive than lithium iron phosphate for equivalent capacities.
Interestingly, ternary lithium batteries can enhance energy density by adjusting the molar ratios of nickel, cobalt, and manganese in the electrodes. The standard ratio is 1:1:1, known as the NCM111 cell. Variations such as NCM424, NCM523, NCM622, and NCM811 have been developed and mass-produced, with NCM811 being particularly popular; the infamous Tesla 4680 battery utilizes this configuration along with silicon-carbon anodes, achieving an energy density of 300 Wh/kg.
Turning to lithium iron phosphate batteries, they have three main advantages: 1 High safety due to their stable chemical structure, making them less likely to experience thermal runaway or catch fire upon high-temperature exposure or impact; 2 Longer lifespan, with typical cycle counts for ternary lithium being around 1000-1500 cycles, while lithium iron phosphate can exceed 2000 cycles, and some products can exceed 5000 cycles, making them suitable for ride-hailing vehicles; 3 Lower costs due to the absence of precious metals. However, they have significant downsides: 1 Lower energy density, ranging from 90-120 Wh/kg, which is about half that of ternary lithium. This means that for the same storage capacity, the batteries are nearly twice the size. 2 Poor performance in low temperatures; below zero, lithium iron phosphate batteries experience a significant capacity drop, nearly halving, and in even colder regions like the Northeast, the electric range can reduce to 30-40% of summer levels.
While these are theoretical analyses, real-world situations do not always align with theoretical expectations. For instance, the battery in the Xiaomi SU7 involved in the recent accident was theoretically safer lithium iron phosphate. However, the fire was triggered by the impact. It is well-known that numerous lithium ions traverse the electrolyte between the positive and negative electrodes. To isolate the electrodes while allowing lithium ions to pass freely, a separator exists within the battery. Impact can directly damage this separator, causing a short circuit. The sudden temperature rise can also exacerbate the reactions between oxidizers and reducers in the electrolyte, resulting in violent redox reactions. High temperatures may also compromise the isolation between adjacent cell electrodes, leading to a “chain reaction” of battery combustion. As batteries inherently contain oxidizers and reducers, combustion can occur without external oxygen; even submerging a burning battery in water may not extinguish it and could instead produce sparks. Therefore, when facing potential electric vehicle fires, firefighters typically use isolation and cooling methods, allowing the vehicle to burn out before processing it.
However, it is important not to overestimate the safety of lithium iron phosphate batteries; while they have a higher ignition point than ternary lithium batteries, they can still release dangerous flames under specific circumstances like collisions, punctures, or burns. Nevertheless, as established industrial products, manufacturers and battery producers are diligently working on battery safety. For instance, CATL employs battery inversion technology to position pressure relief valves downward, minimizing harm during a fire. They have also introduced five layers of thermal safety measures in their high-end battery series, using materials like mica with a melting point of 1200℃ and aerogels to isolate cell components. BYD has implemented blade battery technology, encapsulating soft-pack cells in rigid aluminum casings to provide effective isolation during fires. Additionally, materials like aerogels, advanced BMS systems, and liquid cooling circuits can help manage thermal runaway, reducing the risk of fires.
As traditional lithium battery performance and safety approach a bottleneck, solid-state batteries have been elevated to new heights. In some manufacturers’ promotional materials, these batteries are touted as a solution to range anxiety and a key to enhancing the safety of new energy vehicles. But is this true? Solid-state batteries differ from traditional lithium batteries primarily by replacing the electrolyte with a solid electrolyte. A simple analogy is: “regular batteries are jelly, while solid-state batteries are cookies.” Solid materials have higher density, and solid-state batteries boast energy densities exceeding 720 Wh/kg, three times that of ternary lithium batteries. Theoretically, solid-state batteries can indeed support pure electric vehicles with ranges of 1000 km on a single charge. Furthermore, the stability of solid electrolytes makes them nearly non-flammable, providing better stability against punctures and impacts that challenge traditional batteries. Even the low-temperature activity issues that plague conventional batteries do not apply here, as solid-state batteries are not affected by temperature fluctuations.
While they seem perfect, solid-state batteries face numerous challenges before widespread adoption. First, charging speed is limited; the solid electrolyte restricts rapid electron flow, resulting in inferior conductivity. Even with 800V high-voltage technologies, charging speed lags behind traditional batteries. Second, the lifespan of solid-state batteries, typically made from lithium iron phosphate (LiFePO4) or lithium cobalt oxide (LiCoO2), suffers under frequent charging and discharging, leading to crystallization near the electrodes that impairs functionality. According to Academician Ouyang Minggao, solid-state batteries with an energy density of 500 Wh/kg can currently achieve only a few dozen cycles; this is substantially shorter than the several thousand cycles typical for ternary lithium or lithium iron phosphate batteries. In the author’s view, solid-state batteries are similar to previously hyped graphene batteries, often used as a marketing tool. The reality is that for solid-state batteries to become commercially viable, they need to overcome issues related to lifespan and charge/discharge speeds, which may take considerable time. How long? Academician Ouyang predicts that for solid-state batteries to capture 50% of the market share of liquid lithium batteries, it will require at least 20-30 years.
Additionally, cost remains a significant factor. Data shows that NIO’s semi-solid-state battery costs between 1.7-2.2 yuan/Wh, while current lithium battery costs range from 0.3-0.4 yuan/Wh, making semi-solid-state batteries 6-7 times more expensive merely in semi-finished form. However, due to their in-situ solidification process, the liquid electrolyte content in semi-solid-state batteries is reduced to 5%-10%, enhancing safety to some extent compared to conventional batteries. Nonetheless, during the Second China All-Solid-State Battery Innovation Development Summit on February 15, 2025, Miao Wei, Vice Chairman of the Economic Committee and former Minister of Industry and Information Technology, emphasized that “semi-solid-state batteries still fall within the liquid battery category and should not be confused with solid-state batteries.” Thus, while discussing solid-state batteries as a new concept for electric vehicles is beneficial, their widespread adoption remains a distant prospect.
Enhancing battery safety should prioritize current technologies, reinforcing existing battery safety measures and implementing robust regulations. For example, starting in March 2025, new regulations for annual inspections of new energy vehicles will focus on battery safety. The most significant change in these regulations is the mandatory inclusion of power battery safety charging inspections and electrical safety checks as required items. Additionally, the highly anticipated national mandatory standard for power batteries, titled “Safety Requirements for Power Batteries Used in Electric Vehicles”, was publicly announced by the Ministry of Industry and Information Technology in January 2025. If approved, it is expected to be implemented on July 1, 2026. This new standard represents a significant enhancement in safety requirements regarding thermal runaway. The previous standard (GB 38031-2020) allowed for fire or explosions in the battery system following thermal runaway in a single cell, provided that at least five minutes of passenger evacuation time was ensured. The new draft stipulates that “in cases of thermal runaway in a cell, the battery pack or system must not catch fire or explode” (under test conditions). To validate this requirement, the standard also introduces bottom collision testing to simulate scenarios where vehicles encounter foreign objects from below during actual driving, ensuring that the battery system does not ignite or explode under such impacts. A series of stringent tests and standards aims to enhance the safety of new energy vehicles.
In conclusion, “bullets may not ignite the battery in a lab, but collisions during actual driving can lead to smoke.” The fundamental cause of this phenomenon lies in the extent of damage. Gunfire generally affects only 3-6 cells, whereas a severe collision can damage a much larger area. When the battery safety system is overwhelmed, fire risks become more understandable. With lithium battery technology nearing its limits and solid-state battery mass production still a long way off, the performance of semi-solid-state batteries remains to be validated. The desire to enhance safety must align with the laws of physics. Currently, significantly improving safety through technological advancements is challenging. The focus should be on safety management of existing technologies and regulatory standards to reduce risks.
