A Sharp Contender: BYD’s Blade Battery
By: Eric Y. Zheng
Tags: Blade Battery, Battery, Electric Car, Electric Vehicles

This story is contributed by Xinghua Meng

Edited by Eric Y. Zheng

With the Blade Battery design by BYD Auto, China is reimagining the LFP battery and bringing it back to the forefront of the EV market.

The Blade Battery design significantly improves the safety, cycle life, and energy density, all while lowering the cost.

More and more EV manufacturers and designers are taking another look at LFP, the time-tested chemistry behind the Blade Battery.

BYD Blade Battery on the brands’ flagship EV Han sedan in June, 2020 (source: BYD Auto)

Background

With the aggressive development of the electric vehicle (EV) market, a wave of spontaneous combustion and detonation incidents involving EVs has recently been reported. Due to quality control issues in manufacturing as well as the intrinsic electrochemical and chemical instability of ternary cathode materials, most consumers are still hesitant about purchasing EVs compared with gasoline vehicles.

In the past couple years, the roadmap for power batteries based on ternary batteries (NMC/NCA) versus lithium iron phosphate (LFP) has been widely debated. Given the high mileage demand in the (EVs) market, the former has prevailed for the moment. In 2019, more than 60% of new EVs are equipped with ternary batteries, taking over the main market from LFP. After all, the theoretical specific capacity of LFP is only 170 mAh/g, which is much lower than NMC (275 mAh/g for NMC811) although LFP batteries are much safer than ternary batteries. Academic and industrial researchers and engineers are constantly seeking to improve the performance of LFP at cell level. The state-of-the-art LFP cells in the market can reach 160–165 mAh/g, which is bordering towards its electrochemistry limitation. Any more than 5% improvement is a milestone for developing LFP batteries for EVs, given the relatively low nominal voltage of LFP (3.2 V for LFP vs. 4.2 V for NMC 811). As a result, the overall energy density of the LFP battery pack is much lower than that of the ternary battery pack.

Birth of the Blade Battery

On March 29th, 2020，Fudi Battery Co Ltd, a subsidiary of BYD Auto released a new product applying LFP in the power battery pack and named it the “Blade Battery”.

A 50% increase (40% to 60% in space utilization) of volumetric energy density at the pack level is achieved which is critical in the LFP battery field.

It also suggests that engineering in pack design could move forward the battery market even if the cell level advances are sluggish due to the electrochemical limit of LFP. BYD claims that the release of the Blade Battery will transform the current state of relying on ternary batteries in the EVs market. The safety, lower cost and longer cycling life are the main advantages of the Blade Battery, according to BYD.

What is the Blade Battery?

Figure 1. Structure of the blade cell, cell arrays and battery pack. (source: [1])

Figure 1 provides a detailed structure of the Blade Battery. According to the published BYD patents, the cell unit of the Blade Battery is designed based on a prismatic form factor. Unlike a traditional prismatic cell, however, the length of the Blade Battery is extremely long while the depth and height are relatively short. As a result, the shape of the cell unit is like a blade. The length of this “blade” is adjustable from 600 mm to 2500 mm. The final size of the “blade” in the Blade Battery is not fixed. The three-dimensional parameters: length (L), depth (D) and height (H) are all tunable within the range given by the patents.

High safety

The Blade Battery is currently based on LFP. The electrochemical features of LFP enable the battery to be much safer than those based on NMC, especially Ni-rich NMC 811. In Figure 2, the nail penetration test demonstrated by BYD suggests a very high level of safety in the Blade Battery design. The thermal decomposition temperature of NMC batteries is around 200 degC, which is easily reached when a short circuit is caused by a penetrating nail. During nail penetration, a large amount of oxygen is released, and, as a result, thermal runaway is triggered immediately. LFP cathodes, on the other hand, have phosphate− polyanions, a stable crystal structure which does not decompose easily. Even at ambient temperatures higher than 500 degC, limited oxygen is released during the nail penetration test. Meanwhile, safety valves can help prevent an explosion by releasing electrolyte, gas, and other chemicals when the internal pressure exceeds a specific threshold. Finally, the surface temperature of a regular LFP battery can be kept below 240 degC. In the test, the surface temperature of the Blade Battery cell remained below 60 degC without any signs of smoking, which is much improved over NMC batteries. The Blade Battery possesses not only the relative thermal stability of LFP but also a large specific surface area and a unique ultra-long shape. Consequently, generated heat is easier to dissipate and the short-circuited region is less likely to spread in the cell. In this way, a single cell unit of the Blade Battery is much safer than other geometries of LFP batteries or ternary batteries.

Figure 2. Nail penetration test for NMC battery, regular LFP battery and Blade Battery (source: BYD Co. Ltd)

Low cost

The cost of the LFP battery pack is undeniably lower than NMC battery pack. This is the reason most electric buses in the market use LFP batteries and early EVs in China are equipped with LFP batteries despite the shorter operation ranges. With the help of the Blade Battery design, the cost of the battery pack can be decreased by an additional 20%, compared with conventional LFP battery packs. The cost is claimed to be less than $85/kWh, which is lower than the estimated cost of the latest LFP pack from CATL (~$100/kWh) for TESLA Model 3 in China. This low cost enables the pack to be more competitive under the current policy in China. The current policy dictates that LFP battery systems will get fewer subsidies due to the low energy density (<140 Wh/kg), but the Blade Battery pack can achieve 140 Wh/kg, which allows it to receive the same subsidies as most ternary batteries. According to the latest news, the entire 8 GWh yearly capacity of the Blade Battery factory in Chongqing has been reserved by BYD and their clients. A new report from research firm Wood Mackenzie says: “The projected price of battery packs keeps dropping and the price of battery packs will drop below $100 per kilowatt-hour by 2024. Obviously, BYD has achieved this target in 2020.

Ultra-long life

Thanks to an upgraded battery management system (BMS) and other support systems as well as quality control, the Blade Battery can have an ultra-long cycle life of 1.2 million kilometers (0.75 million miles) over 8 years, according to BYD. LPF batteries can reach 2000 cycles, which is sufficient for power batteries. In lab tests, LFP batteries do not begin to fade significantly until after 3500 cycles, and BYD claims 3000 cycles for its Blade Battery. Thus, the Blade Battery is expected to have a longer life than ternary batteries (~1000–1500 cycles) on the market.

High energy density

After increasing the space utilization, the volumetric energy density of the Blade Battery is the highest in the LFP family and is comparable to some NMC batteries (Table 3). BYD may create a new market between traditional LFP and NMC/NCA, making its LFP battery more attractive.

Concerns

However, the debut of the Blade Battery faces several challenges:

1. Limited range for EVs is a fundamental challenge for LFP to overcome due to its lower gravimetric energy density (<200 Wh/kg). In contrast, NMC batteries are still far away from their theoretical limit(~350–400 Wh/kg, based on graphite anode). There is still a long way to go for LFP based batteries to satisfy the increased demand for long-range EVs

2. Low-temperature performance is more challenging for the Blade Battery due to the electrochemistry of LFP cells. At 0 degC, LFP based batteries can retain only 75% of their capacity at room temperature. In contrast, NMC or NCA typically retain 90% of that capacity. Mitigating this poor low-temperature performance would require an additional heat preservation system, which would also consume some energy from the battery pack.

3. Poor manufacturing consistency and yield is difficult to avoid. Due to inherent difficulties in synthesis and process steps, the consistency of electrodes is still worse than NMC/NCA. For the Blade Battery, given the large cell unit, the consistency issue may become a nightmare. If a single cell unit is out of order, the capacity loss will be much more than a traditional LFP battery pack, which is a concern for module maintenance. The cost of changing or fixing the Blade Battery and the potential for sudden range loss may be unacceptable to consumers.

Outlook

Yet such concerns should not obscure the potential promised by BYD’s Blade Battery. The Blade Battery represents a critical step in exploring cell-to-pack (CTP) technology from a leading battery manufacturer. The adjustable design in a single blade cell will open a new window for CTP solutions.[2] The Blade battery offers an innovative new platform for the battery industry.

The Chinese government recently released stricter standards for power batteries, requiring more safety tests, which gives LFP batteries a great opportunity to make their comeback. The new Tesla Model 3 based on CATL’s LFP solution and BYD Han based on the Blade Battery are serving as catalysts to boost market attention on LFP again. To demonstrate the outstanding performance of the Blade Battery, BYD has released a new EV (series as Han) equipped with the 77 kWh Blade Battery pack. Whether the Blade Battery ultimately offers a viable solution for EVs is a question for the market in the near future.

Dr. Xinghua Meng is a battery research scientist in the United States. His research interests focus on next-generation cathode and anode materials for electric vehicles and renewable energy storage systems. He is actively applying nanotechnology in lithium-ion batteries. Dr. Meng earned his Ph.D. Degree at the Department of Chemical Engineering and Materials Science at Wayne State University in 2016.

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References:

1. 王传福, 何龙, 孙华军, 鲁鹏, and 朱燕. 2019. 电池包、车辆和储能装置. China CN110165118A, filed June 21, 2019, and issued August 23, 2019. https://patents.google.com/patent/CN110165118A/en?oq=CN110165118A.

孙华军, 朱燕, 唐江龙, and 曾而平. 2019. 电池包、电动车和储能装置. China CN110165115A, filed June 21, 2019, and issued August 23, 2019. https://patents.google.com/patent/CN110165115A/en?oq=CN110165115A.

2. 王传福, 何龙, 孙华军, 朱建华, 鲁志佩, 朱燕, and 胡世超. 2019. 一种锂离子电池、电池模组、电池包及汽车. China CN110518156A, filed October 23, 2019, and issued November 29, 2019. https://patents.google.com/patent/CN110518156A/en?oq=CN110518156A.

王传福, 何龙, 孙华军, 朱建华, 鲁志佩, 朱燕, and 胡世超. 2019. 一种电池、电池模组、电池包和电动车. China CN110518174A, filed October 23, 2019, and issued November 29, 2019. https://patents.google.com/patent/CN110518174A/en?oq=CN110518174A.