Pure electric 5C supercharging has just begun mass production, but HEV batteries have already achieved 50C?

With the development of electric vehicles, energy replenishment, as one of the biggest disadvantages of electric vehicles, has gradually improved with the development of fast charging technology. The power of charging piles is getting bigger and bigger, and the charging speed is getting faster and faster. In the promotion of many electric vehicles, we can see, for example, 4C charging batteries, 5C batteries, etc. The C here represents the battery charge and discharge rate, and the discharge rate = charge and discharge current/rated capacity.

During the charging process, for example, a 1C battery means that the battery can be fully charged within an hour, and a 5C battery can be fully charged in just 1/5 of an hour, or 12 minutes. In terms of discharge rate, since the rated capacity of the battery pack of pure electric vehicles is relatively large, the mainstream is between 50kWh and 100kWh, and the common electric drive power is generally between 150kW and 300kW, so in fact, for the battery pack of pure electric vehicles, the discharge rate requirement is not high. The general pure electric vehicle battery discharge rate is less than 3C, and high-performance models are only around 6C.

However, this situation is different for PHEV (plug-in hybrid), REEV (extended range) and HEV (hybrid electric) models. The rated capacity of the battery pack is relatively small, resulting in different requirements for these battery packs. For example, the discharge rate of some HEV batteries can be as high as 50C. Let's take a look at the differences in the performance index requirements of battery packs for various new energy vehicles.

Trade-offs in battery pack performance indicators

Generally speaking, the performance indicators of battery packs can be divided into 6 parts, including energy density, capacity, charge rate, discharge rate, power density, and cycle life.

Battery energy density The energy density of batteries is limited by the battery chemical system, and the capacity and operating voltage of positive and negative electrode materials are key factors. Improving energy density can be achieved by using high-capacity positive electrode materials such as high-nickel ternary materials (NCM) and lithium-rich positive electrodes, as well as high-capacity negative electrode materials such as silicon-carbon composites or lithium metal negative electrodes. In addition, the use of key technologies such as in-situ solidification technology, ultra-thin solid electrolyte-coated positive electrode materials, and solid electrolyte-coated diaphragms can also significantly improve the energy density of batteries.

However, the charging rate and battery energy density are often only two choices, which is due to some trade-offs in battery design and material selection. In high-energy-density batteries, such as when using NCM and lithium-rich cathode materials, there may be limitations in the ion diffusion rate and electronic conductivity during high-rate charging and discharging, which affects its performance under high-rate conditions.

In order to achieve high energy density, the battery cell may be designed with thicker electrodes, which helps to increase the amount of stored electricity. However, thicker electrodes will increase the transmission distance of lithium ions inside the electrode and reduce their transmission rate during high-rate charging and discharging, resulting in the inability to achieve higher charge and discharge rates. In addition

, cycle life is also a key indicator that limits the current high-energy-density batteries from achieving high charge and discharge rates. First of all, there is internal resistance inside the battery cell. When the rated voltage of the battery pack is constant, to increase the charging rate, the charging current needs to be increased, which means that fast charging will generate more heat inside the battery cell. At the same time, during the fast charging process, stress accumulation and damage to the structure of the negative electrode material, precipitation of lithium dendrites, and other safety hazards may occur.

Therefore, the actual application scenario requirements will make some trade-offs in the various performance indicators of the battery, which is also included in various types of models such as PHEV, REEV, HEV, EV, etc.

Differences in the requirements for battery performance between pure electric and hybrid models

For pure electric models, the battery charging rate of this year's mainstream models is between 1.5C and 3C. Some models that focus on fast charging, such as Ideal MEGA, Lantu Zhiyin, Zeekr 007/001, have achieved a maximum charging rate of 5.5C. The discharge rate is generally within 6C, generally between 2C and 3C. Some extremely powerful models, such as Zeekr 001FR, Yangwang U9, etc., can have a battery discharge rate of more than 10C.

Compared with PHEV, REEV, HEV, etc., the battery capacity of EV pure electric models is often the largest, so they pay more attention to energy density and cost. The battery capacity is large, so for EV models, the calculation method of battery life is not just to look at the number of cycles. For example, if there are two models, one has a battery with a lower cycle life, but a larger capacity, high energy density, and can travel a longer distance per full charge; then compared with a battery with a longer cycle life, but a smaller capacity, and low energy density, the two models may actually have the same total mileage in their service life.

Since REEV essentially adds a range extender to EV, it is equivalent to adding a fuel-driven "power bank" to EV, so the battery is similar to EV, but the relative capacity will be smaller for cost and vehicle weight considerations.

PHEV is a plug-in hybrid electric vehicle, which can be driven directly by a fuel engine or by an electric motor, and can also be charged at the same time, combining the two driving forms of electric motor and internal combustion engine. PHEV generally uses a battery pack with higher power density, that is, the charge and discharge rate is higher than that of EV models, and the battery capacity is also smaller, generally providing a pure electric range of less than 100km, but with the development of the market, the battery pack capacity of PHEV is also getting larger and larger, and can even provide a pure electric range of more than 200km.

Compared with EV and REEV, PHEV has a smaller battery pack capacity, but needs to release energy quickly in electric mode, and can be quickly charged to achieve kinetic energy recovery during braking. This requires that the PHEV battery be designed to withstand larger charge and discharge currents and have a higher charge and discharge rate.

On the other hand, because the battery capacity is small, the number of natural charging times will also be more frequent, so the cycle life is also required to be higher. For example, a short knife battery launched by Honeycomb Energy last year has a pulse discharge rate of up to 15C and a cycle life of more than 3,000 times.

HEV is a more traditional model, that is, it is mainly based on internal combustion engines and supplemented by electricity. It mainly charges the battery through kinetic energy recovery and internal combustion engines driving generators. It uses motor drive in low-speed scenes such as starting, and cannot charge the battery through external charging piles. At the same time, the HEV battery capacity is smaller than that of PHEV, generally less than 2kWh, and some HEV models are even only 0.7kWh. The

extremely small battery capacity, but to withstand high kinetic energy recovery charging and the power used by the motor, means that there must be an extremely high charge and discharge rate and an extremely high cycle life. For example, JEWELL's HEV battery can achieve a cycle life of 40,000 times and a continuous discharge rate of 50C; Xingchuan Technology has even launched an HEV battery with a discharge rate of 135C and a cycle life of more than 30,000 times.

Summary:

While the electric vehicle industry continues to develop, the EV market is also developing unevenly due to the uneven distribution of charging conditions around the world. Therefore, PHEV, HEV and other models can become transitional technologies and gain more opportunities in the global market. We can see that in recent years, PHEV models have followed the popularity of BYD's DM-i technology, and many domestic and foreign car machines have accelerated their advancement. In the future, the battery demand for PHEV and HEV may have a new peak.