Behind the electric vehicle fires: The evolution of batteries "kidnapped" by performance
In October, multiple electric vehicle fire incidents have once again pushed the long - discussed topic of battery safety to the forefront.
This time, the protagonists are no longer budget cars, but rather the Xiaomi SU7 Ultra, NIO ET7, Li Auto MEGA, Mercedes - Benz EQE, and Porsche Taycan - these benchmark products priced from 300,000 yuan to millions, equipped with the most advanced batteries.
Over the past decade, in the wave of electrification, automakers and battery manufacturers have jointly promoted the evolution of batteries along two main lines: high energy density and fast charging, to make up for the performance shortcomings of electric vehicles compared to fuel - powered vehicles.
Today, the range of electric vehicles has generally caught up with or even exceeded that of fuel - powered vehicles. The concept of "charging for 10 minutes and getting a 500 - kilometer range" has also turned from a fantasy into reality. However, the other end of the performance "seesaw" - safety, has more often been regarded as a passive bottom - line to be adhered to.
What can prick and wake up the industry are only the unexpected fire incidents.
Climbing Energy Density, Compromised Thermal Stability
The first large - scale "evolution" of power batteries was the shift in chemical materials: from lithium iron phosphate to ternary lithium.
Just from the material characteristics, ternary lithium batteries have a higher energy density and a longer range, but poorer thermal stability.
A single cell consists of a positive electrode, a negative electrode, an electrolyte, and a separator. It relies on the reciprocating movement of lithium ions between the positive and negative electrodes to achieve charging and discharging. The core difference between ternary lithium and lithium iron phosphate batteries lies in the positive electrode material: the former uses three metal elements, nickel, cobalt, and manganese (NCM) or nickel, cobalt, and aluminum (NCA), while the latter uses lithium iron phosphate crystals (LFP).
Lithium iron phosphate is not easy to decompose at high temperatures and does not easily release oxygen, so it is less likely to experience thermal runaway. Its crystal structure is stable, and the electrochemical reaction path is relatively simple, so it has a longer cycle life. The disadvantages are low energy density and poor low - temperature performance.
To make up for the performance shortcomings, ternary lithium batteries with high energy density have become the first choice for mid - to high - end cars. Among its three elements, nickel is responsible for energy density, while cobalt and manganese (aluminum) are responsible for stability. Therefore, the higher the nickel content, the stronger the electrochemical activity, the higher the energy density, and the worse the thermal stability.
High - nickel batteries (NCM 811) were once highly sought after, but problems emerged after mass production. In 2020, the GAC Aion S equipped with CATL's 811 batteries caught fire spontaneously several times. In 2021, General Motors recalled nearly 70,000 cars due to high - nickel battery hazards and claimed $1 billion in compensation from supplier LG Chem. After a series of accidents, the radical high - nickel route was gradually abandoned by the industry in favor of a more balanced solution. Currently, the mainstream ternary lithium batteries in the market usually have a nickel - cobalt - manganese ratio of 5 - 2 - 3 or 6 - 2 - 2.
Lithium iron phosphate materials are still widely used in models priced under 200,000 yuan due to their cost advantages. However, ternary lithium batteries with higher performance have become the standard for mid - to high - end electric vehicles. For example, Tesla uses ternary lithium batteries in its long - range models and lithium iron phosphate batteries in its standard - range models.
Larger Cells and Potential Risks of Thermal Diffusion
In recent years, the evolution of positive electrode materials has become more stable. Automakers and battery manufacturers mainly improve battery energy density by improving the structural design - that is, packing more active chemical materials into a battery pack of the same volume.
Early battery packs used a three - level structure of cells, modules, and battery packs. For example, the first - generation Tesla Model S integrated 444 18650 cylindrical cells in series and parallel into one module, and each module was equipped with an independent BMS (Battery Management System) and cooling pipes. A battery pack could hold 16 modules, with fire - proof materials filled inside and connected to the vehicle's high - voltage system outside. A large amount of space was occupied by structural components and cooling pipes.
Subsequently, the technology evolved towards module - free design. Tesla and Panasonic increased the size of cylindrical cells from 18650 (18mm in diameter and 65mm in height) to 21700 (21mm in diameter and 70mm in height), and now to 4680 (46mm in diameter and 80mm in height). The number of modules was gradually reduced until they were eliminated. The CTP technology (Cell to Pack, module - free technology) that integrates cells into the battery pack and the CTC technology (Cell to Chassis, cell - chassis integration) that combines the battery cover and the vehicle floor and integrates cells directly into the chassis emerged.
Tesla 4680 cells
Domestic square - shell batteries have followed the same path. BYD's Blade Battery has increased the volume utilization rate by 50% by designing the cells into a nearly 1 - meter - long "blade" shape, and the capacity of a single cell has increased from 135 Ah to over 200 Ah. CATL's Qilin Battery has also increased the volume utilization rate to 72% through structural improvements, exceeding that of the 4680 battery (63%). The CTC technologies of the two companies were mass - produced in 2022 and 2023 respectively.
From removing modules to CTP and CTC, automakers and battery manufacturers have successfully injected more energy into the limited chassis space, making up for the range shortcoming. However, an undeniable fact is that these chemical materials that store energy are also fuels themselves.
When a large - capacity cell experiences an internal short - circuit, the thermal diffusion speed will be faster. The energy accumulated inside may form local hotspots, accelerating the reaction chain of thermal runaway. This also explains why in recent fire incidents, the time from smoking to deflagration was extremely short, and the fire was extremely fierce and difficult to extinguish.
It's worth noting that not all battery fires are the fault of the cells. More commonly, automakers purchase cells from battery manufacturers and then encapsulate the battery packs or integrate them into the chassis themselves. The encapsulation process is also crucial. In 2019, NIO recalled 4803 ES8s due to improper routing of voltage wiring harnesses in the battery pack.
Fast Charging, Short Lifespan
In recent years, the rise of high - voltage fast - charging technology has brought new challenges to battery safety management.
The charging speed depends on power, and power = voltage × current. Early electric vehicles were generally based on a 400V platform, and the charging rate was lower than 1C (current = C - rate × battery rated capacity). Tesla increased its peak power from 90 kW of the V1 Supercharger to 250 kW of the V3 by continuously increasing the current, achieving a range of 250 kilometers after 15 minutes of charging, and the charging rate of its on - board battery reached 2 - 2.5C.
Porsche Taycan was the first to raise the vehicle's voltage platform to 800V, achieving a fast - charging power of 270 kW. Although the power is not much higher than that of Tesla's V3, by doubling the voltage and halving the current, it reduces the heat generation during high - power charging and the thermal loss during transmission, improving safety.
Chinese automakers quickly followed up with the 800V platform and pushed the battery rate to 4C or even higher. By increasing both the voltage and the current, they increased the charging power to over 400 kW. In 2023, Li Auto MEGA announced the first use of CATL's 5C Qilin Battery, with a peak charging power exceeding 500 kW. BYD's 10C flash charging is claimed to be able to charge enough for a 600 - kilometer range in 10 minutes. However, some industry insiders said after actual testing that its 10C peak current can only be maintained for a very short time.
CATL 5C Qilin Battery
This high - voltage fast - charging competition has significantly improved the charging experience, but the safety challenges behind it have also increased exponentially: high voltage places extreme requirements on insulation, protection, and arc - extinguishing capabilities. High - rate batteries can provide a larger instantaneous short - circuit current, and the thermal runaway reaction may be more intense. During high - current fast charging, lithium ions are accelerated to embed and detach, which not only generates heat quickly but also easily forms lithium dendrites, reducing the battery life.
Li Bin, the founder of NIO, once said bluntly in an interview in September this year that the current super - charging technology has paid a huge price in pursuit of short - term charging efficiency, one of which is shortening the battery life. NIO's battery swapping stations use slow charging to replenish energy, aiming to achieve 85% battery health without mileage limitations for 15 years.
"Imagine if after 8 years of using the car, you have to spend 80,000 or 100,000 yuan to replace the battery. From the perspective of social resources and users, this is an unacceptable high cost," Li Bin said.
No Absolute Safety, Only Eternal Game
Solid - state batteries that combine high performance and high safety are often regarded as the ultimate form of power batteries. Related research started 30 years ago, but it has not yet achieved industrial implementation.
In terms of R & D and production processes, there are still many challenges in solid - state batteries that have not been perfectly solved. In addition, mass - producing solid - state batteries requires a disruptive transformation of the existing liquid - battery industry, with extremely high costs. Most automakers and battery manufacturers are not ready to make large - scale investments in this regard.
Before the arrival of solid - state batteries, battery companies are also constantly optimizing the safety design of liquid batteries to hedge against the high risks brought by high - performance batteries.
For example, CATL's Qilin Battery increases the heat - exchange area by moving the liquid - cooling plate from the bottom of the cells to between the cells; it arranges the pressure - relief valve at the bottom of the cells, separating it from the positive and negative poles at the top, achieving "thermal - electrical separation". In terms of materials, to support high - voltage fast - charging technology, a graphite coating with smaller particle sizes is used on the negative electrode surface to accelerate the lithium - ion embedding efficiency and reduce the probability of "lithium plating".
The long and thin shape of BYD's Blade Battery is also conducive to heat dissipation. BYD also claims that the close arrangement of multiple blade cells can form a structural support, thus eliminating or reducing traditional support structures such as cross - beams and longitudinal beams. However, the industry has always been concerned that ultra - long cells may bend during a collision, leading to internal short - circuits.
Automakers are also constantly optimizing the BMS system, strengthening real - time monitoring and fault diagnosis of parameters such as voltage, current, and temperature, cutting off the circuit when necessary, and alerting the driver. However, in a high - performance battery system, the instantaneous short - circuit of the battery may exceed its sampling period and response limit.
A well - balanced battery is the sum of materials, structural design, production process, and battery management system, and the final safety is also the result of the superposition of each link. While pursuing high performance, automakers and battery manufacturers must also raise safety to the same level, increase investment in safety, and honestly popularize relevant knowledge to users. Instead of being stingy with investment during R & D, aggressively marketing battery safety when selling cars, and obscuring the suppliers, which makes consumers ignore potential risk differences.
Every power battery needs to go through a large number of experimental verifications before leaving the factory. However, there are countless variables from the laboratory to the real and complex working conditions.
Every fire accident is a heavy warning to the industry and also provides valuable engineering data for technological iteration.
Just as Tesla continuously optimized its BMS system through early spontaneous - combustion incidents and finally became a global leader. Chinese automakers and battery manufacturers are still on the way to applying and improving high - performance batteries.
It must be admitted that there is no absolutely safe battery, only a continuously decreasing accident rate. Currently, the battery failure rate standard of first - tier battery manufacturers has been raised to the ppb (parts per billion) level.
However, a one - in - a - billion probability means 100% for each user.
(Intern Zhao Ruixue also contributed to this article)
This article is from the WeChat official account "Yunjian Insight", author: Wang Hailu, published by 36Kr with permission.