Lithium batteries are divided into two materials: positive electrode and
negative electrode. The lithium iron phosphate and ternary materials we often
talk about actually refer to the positive electrode of lithium batteries. There
are now thousands of laboratories around the world trying to develop lithium
batteries with better performance. In fact, everyone is just trying to find a
better performance, higher capacity, longer life, or better performance in metal
oxides. It is some positive electrode materials with better charging and
discharging power, or work on the negative electrode. The negative electrode
material of lithium batteries that we usually use is graphite, but in recent
years a new negative electrode material has also appeared, which is lithium
titanate.
When a lithium-ion battery is discharging, the lithium ions escape from the
negative electrode material and then run to the positive electrode. This process
is actually relatively easy to achieve, which means that the discharge power of
the lithium battery can be very large.
But traditionally speaking, charging is not so easy. When charging, lithium
ions have to escape from the positive electrode material and run into the
negative electrode material. This process is relatively challenging for the
negative electrode material, because the lithium ions have to quickly enter the
negative electrode material. , then it will cause damage to the structure of the
negative electrode material and other issues, so generally speaking, the
performance of lithium-ion batteries in terms of fast charging is not good.
However, fast charging is a feature that many people dream of. In the past,
only supercapacitors could perform fast charging. Therefore, to overcome the
inability of lithium batteries to charge quickly, we must start from the
perspective of materials and find ways to improve materials, especially the
negative electrode. How the material behaves when it's charged, that's what you
do on a material or chemical basis.
1. Business Dilemma of Electric Buses
So why should we emphasize the feature of fast charging? I want to start
with the application of electric vehicles. When we were the first to promote
this new energy vehicle in our country, the market that was easiest for the
government to promote was electric buses. Pure buses have been around for six or
seven years from the Olympic Games to the World Expo, and they are used in many
cities. . Looking at a simple set of data, this PPT talks about the difficulties
encountered by electric buses in commercial applications.
Take a 12-meter pure electric bus as an example. A bus typically runs 250
kilometers a day. It runs back and forth between point A and point B. The
typical power consumption is 0.8 kWh per kilometer. If this bus runs during the
day It consumes about 200 kilowatt-hours of power for a whole day. This power
does not seem to be much, but when configuring the battery, two coefficients
must be considered. The first is the battery charge and discharge interval, and
the attenuation of the battery life. In this case, Equipped with 312
kilowatt-hours of electricity, plus the energy consumption of turning on the air
conditioner in summer, the battery needs to be installed with 500 kilowatt-hours
of electricity to meet the full-day operation of the 12-meter bus.
In fact, at the current energy density level of lithium batteries, if
equipped with lithium iron phosphate, a battery of about 500 kWh may weigh 6
tons, and the current cost is at least 1 million. Such a battery is 12 meters in
size and weight. It cannot be installed, so no one can use this solution.
The usual practice is to install almost half of the electricity, that is,
300 kilowatt-hours of electricity, which results in the bus being unable to
operate throughout the day. Generally speaking, in the afternoon, the bus will
return to the charging station for a charge called The work of quickly
replenishing power usually takes an hour to an hour and a half to replenish part
of the power before completing the second half of the operation. However, this
result reduces the effectiveness of its operations for the bus company, because
it cannot operate for two hours during the day.
So the next question is, is it possible for us to use batteries with higher
energy density to complete this work? Among the currently commercialized
batteries, the one with the highest energy density is the Panasonic 18650
battery cell used by Tesla. The energy density of its battery cell reaches 230
watt hours per kilogram. If it really wants to install 500 kWh of electricity,
the weight will be about 3.5 tons, it can meet all-day operations, and can
basically be installed on the bus. However, one problem with using this battery
on a bus is its lifespan. After reassembly, its cycle life may only be 500
times. So we estimate that if it is used like this on public transportation and
charged once a day, it will only last about two to three years.
So we see the use of energy-based batteries on public buses in an attempt
to meet the needs of all-day operations. In fact, they face many engineering and
cost problems. They are either too heavy, too expensive, or their lifespan is
not good enough.
But we have another idea. If we can think about this problem in reverse, we
will not install so many batteries, but install fewer batteries, but use the
fast charging method. Suppose we don’t need to run 250 kilometers, but only need
to run one lap. For example, a bus takes 20 kilometers from point A to point B.
Then I only need to install a battery that can run for 20 kilometers, and then
every time I reach the end point, If you can fully charge the battery in five or
ten minutes and then run for the next trip, is this solution also feasible?
Let's do a simple calculation. If we use an 80 kWh battery or a 120 kWh battery,
we can fast charge four, five, or six times a day, and the charging time should
be short enough to take advantage of the driver's rest time at the terminal.
time. That is to say, if we install fewer batteries, multiple charging methods
can also meet the needs of bus operations, but it poses a challenge to the
battery, that is, the battery must be able to charge quickly, and it will take
about ten minutes to fully charge it. . In addition, because the number of
charging times per day will increase, for example, from once to three, four or
even five times per day, under fast charging conditions, the cycle life of the
battery will be longer than before.
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