If I buy an electric vehicle, will its battery catch fire? Statistically such considerations are almost irrelevant. EV battery fires are no more apt to occur than a gasoline blaze in a new combustion-engine vehicle. Most EVs currently in use are admirably reliable and safe. But a spate of highly publicized Tesla fires (some the result of impact or battery puncture) and GM’s $1.8 billion recall of every single one of the 142,000 Bolt EVs and EUVs ever sold, due to a manufacturing defect found in their lithium-ion batteries supplied by LG Energy Solution, is enough for some prospective buyers to think twice about making the jump to EVs.
“It’s given my wife and me pause,” said Kevin Whittle, a Farmington, Michigan-based industrial automation consultant. “We were looking at various electrics, not at Bolt specifically. But EV fires and the recall concern us. We just don’t trust the tech yet.” Whittle mentioned that his confidence in buying a new EV was further sapped when he saw an online video that showed a sign in a San Francisco public parking lot. It read: “For customer safety: CHEVROLET BOLT EVs are STRICTLY PROHIBITED from parking at this facility. Thank you for your compliance!”
“The batteries in those EVs use NCM, or nickel-cobalt-manganese, cathodes,” explained Dr. Zhong (John) Chen, project manager of vehicle engineering at BYD’s northern California headquarters. “With NCM you can easily get a fire. Users of that chemistry have to put a lot of effort behind thermal management and safety, adding cost.”
Enter lithium-iron-phosphate (LFP; LiFePO4), a proven chemistry whose benefits were somewhat bypassed by the industry’s settling on lithium-ion cells dominated by NCM, experts note. Where LFP cells shine primarily is in their inherent stability, typically sacrificing some energy density versus NCM for resistance to thermal runaways. At approximately 200 Wh/kg at the cell level, they’re about 10-15% less energy-dense than most NCM cells (Tesla's 2170s are rated at over 260 Wh/kg).
“LFP batteries are probably most notable for their positive safety profiles relative to other chemistries,” noted Guidehouse Insights research analyst Maria Chavez in a May 2021 report. The chemistries “are generally thermally and chemically stable—plus, the phosphate chemistry has been shown to offer a long lifetime,” she stated.
LFP batteries have demonstrated life cycles in excess of 10,000 cycles, or millions of highway miles. They exhibit lower internal resistance than NCM types, enabling faster power delivery for improved vehicle acceleration. Discharge voltage of LFP is very consistent. Strategically, their chemistry does not include cobalt or nickel. And the lack of those metals makes them inexpensive, with LFP costs under $100/kWh in China.
That cost advantage has prompted Tesla, Ford and Volkswagen to announce a shift to LFP cathodes in order to move away from cobalt and nickel. This is both for ethical/mining reasons as well as to offer significant cost reduction for some EV models that do not require high performance and +200-mile driving ranges (that will be covered by high-manganese content lithium battery chemistries with higher specific energy). Ford, however, has indicated that LFP cathodes may also end up in the batteries of its electric F-Series.
Eliminate the modules
Prior to its emergence as a consumer electronics giant and an automaker, China’s BYD had been developing lithium– and nickel-based battery technologies since 1995. LFP became a major R&D focus, leading to the “Blade” battery, an innovation in lower cost, safer EV battery packs. As Chen explains it, “The blade battery originates from a concept called CTP – cell to pack. CTP technology directly integrates the battery cells into the pack, without the use of modules. BYD is, I believe, the pioneer to make this battery concept available to the market, considering that most competitors still use different sizes of modules.”
Going beyond the CTP concept, BYD recently showed a new battery concept called CTC (cell-to-chassis). In CTC, the inherent safety of the LFP chemistry enables the Blade batteries to be integrated directly into vehicle body structure. BYD’s ‘e-platform 3.0’ is a special pure electric vehicle structure that uses the battery to increase the rigidity of the vehicle.
BYD engineers and chemists began work on the CTP Blade concept about three years ago, Chen said. It was purpose-designed for automotive, to give BYD a competitive weapon against NCM’s superior energy density and cold-weather performance. “We have a big stake in the LFP technology so we sat down to determine how we can compete with NCM, particularly NCM811 which is “a big competitor” to the Blade battery.
The 811 number of the nickel-rich layered oxide NMC811 denotes 80% nickel and 1% each cobalt and magnesium. While the chemistry offers high specific energy density for EVs, its downsides include rapidly-depleting capacity and voltage, battery experts note. In terms of chemistry, “there isn’t a lot of innovation you can do in a short time,” Chen said. “We did patent the cell structure and mainly focused on structure optimization. After a thorough analysis, we decided to remove the module level; that was the main breakthrough.”
Viewing an LFP battery’s Module level as unnecessary, the engineers removed it. “In doing so we eliminate some mechanical parts, reinforcements, and some harnesses,” Chen noted. “With the Blade battery we reduce weight, decrease overall pack volume by about 50 percent, and increase the energy density. This makes a very solid point for our chemistry. It’s more robust than a module-based pack, it’s lighter per cell and it’s about 20 percent cheaper.”
A robust design
The accompanying exploded view of the Blade battery shows its simplicity. Typical dimensions of the compact, single-cell design are 905 x 118 x 13.5 mm (35.6 x 4.6 x .53 in.). The size can be customized. The thin, blade-like cells are inserted into the pack in a blade-type array. BYD engineers have also decreased the cubic volume of the battery installation by 50%, improving overall vehicle packaging.
From top to bottom, there is a layer of cotton thermal insulation. Then there is the thermal management, usually liquid cooling. BYD uses a special thermally-conductive adhesive between the cooling system and the cell stack. The third layer from the top is the cell stack. Protective plates are integrated with the battery’s aluminum housing. “It’s a robust design; a lot of cross bars, bus bars. The BDU and BMS [battery disconnect unit and battery management system] are included; we do the integration,” he said.
BYD uses the Blade battery in its new-for-2021 Tang electric SUV and in its Han EV sedan, among other vehicles. During development, the Blade battery was subjected to a new series of stringent tests, Chen said. Neither a 300°C furnace test or a 260% overcharging test resulted in any indication of fire or explosion. During a nail-penetration ballistics test, the Blade battery’s surface temperature remained with a 30°C-to-60°C range without any smoke or fire. And the battery successfully sustained repeated 80-Hz vibration attenuation, Chen said.
According to BYD, the Blade battery exceeds 1.2 million km after 3,000 charge/discharge cycles. The new Tang SUV delivers a range of 505 km (NEDC; 313 mi.) on a single charge, BYD claims, with 0-100 km/h acceleration of 4.6-seconds. Tang’s battery has demonstrated a recharge capability from 30% to 80% of full SOC in 30 minutes, on 110-kW DC.
BYD is moving to become an EV parts supplier under the ‘FinDreams’ brand, which will include FinDreams Battery, FinDreams Powertrain, etc., said Chen. Beyond mobility applications, he believes BYD’s Blade battery has “significant market potential” in energy storage and various infrastructure/grid applications such as solar and wind buffers, which “have become huge business for us,” he said. BYD has eight manufacturing locations across China that are either already or will be available to produce the Blade battery, with a current production capacity of about 70 GWh, Chen noted.Continue reading »