Nowadays, the sky-high hairline of young people sends a message to the
world, "I am quite anxious." The content about worry is just like Tolstoy said
that all happy families are similar, and each unhappy family has its own
misfortune. And young people also have their own worries. New car owners who
have bought new energy vehicles have gradually become anxious about mileage.
There really is no harm without comparison. Compared with traditional internal
combustion engine vehicles, electric vehicles and hybrid plug-in vehicles have
longer driving ranges and longer charging times. Car owners are scratching their
foreheads in worry, and those who have not bought are afraid to buy because of
this problem, which makes marketers want to devote themselves to scientific
research.
Currently, electric vehicles and plug-in hybrid vehicles are mainly charged
in residential and office spaces. The main reason is the long charging time. The
American Society of Automotive Engineers defines Level 1 charging - 120V AC,
maximum charging power 1.9KW (maximum charging current 16A), Level 2 charging -
208-240V AC, maximum charging power 19.2KW (maximum charging current 80A). The
current DC charging pile with a maximum power of 50KW can achieve the advertised
charging time of 20 minutes and travel an extra 160 miles (charging voltage
defined by J1772 is 200-450V, current density is 80A-200A, corresponding to
first-level/secondary charging respectively). The charging power of Tesla’s
“royal” supercharger charging pile can reach 120KW (480V DC, supports high-power
145KW charging), and it can charge for 200 miles in 30 minutes. When the
salesperson confidently introduces these parameters, consumers will only ask: "
Can you please refuel quickly? Brother", so market demand guides the technical
direction. The U.S. Department of Energy proposes Extreme Fast Charging (XFC)
technology as a key technology to increase the purchase and utilization rate of
electric vehicles and plug-in hybrid vehicles, reduce exhaust emissions, and
protect the environment. In other words, as long as it can achieve extremely
fast charging, it will basically smooth out those little knots in the minds of
consumers.
The American Advanced Battery Alliance has proposed a "small goal".
Low-cost/fast-charging power batteries must be charged in 15 minutes to reach
80% power in 2023, and meet other requirements (cell levels $75/KWh, 550Wh/L and
275Wh /Kg). Figure 1 is a diagram of theoretical charging time and corresponding
charging rate. Large battery packs require longer charging time, so the scale of
the charging pile should be designed according to the size of the battery pack.
For example, if the battery pack is relatively large (greater than 90KWh), the
charging pile design must be at least Only 300KW can achieve the 15-minute
charging goal. Of course, a larger battery pack can provide longer cruising
range.
Figure 1 Overview of EV battery fast charging technical requirements: a.
Comparison of currently available charging methods with XFC (Levels 1 and 2 are
standards J1772 developed by the Society of Automotive Engineers. The maximum
charging power of DC motors at fast charging charging piles deployed so far is
about 50 kilowatts) ; The maximum power of the Tesla supercharger reaches 120KW,
which belongs to the second level charging level; And the corresponding charging
rate function curve, the shaded area represents the charging power of
low-cost/fast-charging EV batteries that meet the goals of the American Advanced
Battery Alliance (charging for 15 minutes, 80% of battery capacity)
Wishes are good, but realizing the application of XFC requires wide-ranging
and multi-level research and cooperation, from fast charging structure to
automobile design to single cells, including the balance of grid load during
ultra-fast charging, using reasonable business principles The model implements
the application of charging piles, upgrades the vehicle power system and thermal
management system, limits the cost of system upgrades, etc. Therefore,
lithium-ion batteries are the key technology for fast charging. The current
graphite anode and metal oxide cathode liquid battery system is not conducive to
the performance and safety of the battery if it is to achieve the goal of XFC.
The large polarization caused by high current density will affect the
utilization of active materials and cause the growth of lithium dendrites and
overheating problems.
This review aims at the goal of extremely fast charging, summarizes the
bottleneck issues of current fast charging materials from the perspective of
material transfer and charge transfer, and also talks about the thermal
management issues of batteries adapted to fast charging systems. In addition,
the characterization technology mentioned here will help us further understand
fast charging technology, and strive to provide more reference value information
for the design of fast charging materials.
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