Lithium iron phosphate battery is the development direction of electric vehicle battery

The heart of an electric car - battery

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Why is the battery the heart of an electric car? This starts with the history of electric cars. When it comes to electric new energy vehicles, it is easy to classify them into a whole new technology and things. In fact, the history of electric vehicles is much earlier than expected, even before fuel vehicles. American Thomas Davenport built the first DC-driven electric vehicle in 1834; in 1838, the Scottish Robert Davidson invented the electric-driven train; the tram that is still in use today is in 1840. Patents appearing in the United Kingdom. The world's first electric car was born in 1881. The inventor was the French engineer Gustav Truff, a tricycle powered by a lead-acid battery. Then, fuel cells such as lead-acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries, and lithium-ion batteries were used as electric power.

It can be seen that although electric vehicles developed earlier than fuel vehicles and achieved a certain scale in the early stage, in the modern era, due to the vigorous development of fuel vehicles, electric vehicles were frustrated in the competition. But the real problem is that in the past, electric vehicles based on lead-acid batteries were limited by the density, life, power and other aspects of lead-acid batteries. There has been no way to make breakthroughs in the power source, that is, the battery, so that The development of electric vehicles has stagnated.

Lithium battery classification and advantages and disadvantages

This problem was gradually improved after the emergence of lithium batteries and after 20 years of vigorous development.

Lithium batteries are usually divided into two categories:

Lithium metal battery: A lithium metal battery is generally a battery using manganese dioxide as a positive electrode material, metallic lithium or an alloy metal thereof as a negative electrode material, and a nonaqueous electrolyte solution.

Lithium-ion battery: A lithium-ion battery is generally a battery using a lithium alloy metal oxide as a positive electrode material, graphite as a negative electrode material, and a non-aqueous electrolyte.

Although the lithium metal battery has a high energy density, it can theoretically reach 3,860 watts/kg. However, since it is not stable enough and cannot be charged, it cannot be used as a power battery for repeated use. Lithium-ion batteries have been developed as the main power battery due to their ability to be repeatedly charged. However, because of its combination with different elements, the composition of the cathode material varies greatly in various aspects, leading to an increase in the industry's disputes over the cathode material route.

Generally, the most commonly used power batteries are lithium iron phosphate batteries, lithium manganate batteries, lithium cobalt oxide batteries, and ternary lithium batteries (ternary nickel cobalt manganese).

All of the above types of batteries have advantages and disadvantages, roughly summarized as:

Lithium iron phosphate:

Advantages: long life, large charge and discharge rate, good safety, high temperature, harmless elements and low cost.

Disadvantages: low energy density, low tap density (bulk density).

Ternary lithium:

Advantages: high energy density and high tap density.

Disadvantages: poor safety, poor high temperature resistance, poor life, poor power discharge, and toxic elements (the temperature of the ternary lithium battery increases sharply after charging and discharging, and the oxygen is easily burned after high temperature).

Lithium manganese oxide:

Advantages: high tap density and low cost.

Disadvantages: Poor temperature resistance, lithium citrate temperature rises sharply after long-term use, battery life is seriously attenuated (such as Nissan electric car LEAF).

Lithium cobaltate:

Usually used in 3C products, the safety is very poor, not suitable for power batteries.

In theory, the batteries we need should be high energy density, high bulk density, good safety, high temperature and low temperature resistance, long cycle life, non-toxic and harmless, high-power charge and discharge, all the advantages of integration and low cost. However, there is no such battery at present, so there is a trade-off between the advantages and disadvantages of different types of batteries. Moreover, different electric vehicles have different demand points for batteries, so it is only in the long-term judgment of electric vehicles that we can correctly judge the choice of battery routes.

Advantages of lithium iron phosphate battery

Here we need to go back to the previous two articles, we analyzed that the future of electric vehicles should be based on small mileage, fast charging electric vehicles. At present, the family car needs long-life dual-mode hybrid power, as well as the long-life pure electric car in the bus market. So what kind of battery does this car need?

First, security

First of all, safety is a prerequisite for cars. Unlike cars and computers, cars can encounter many unpredictable factors at high speeds, such as battery crushing and impact caused by car accidents. Any unfavorable factor can cause a car to be destroyed. We can see that some old scooters use inferior lead-acid batteries, there is no safety at all, and cases of spontaneous combustion and impact burning of the batteries abound. Another example is Tesla's nearly one-year continuous fire incident, although there is no casualties in Tesla's safety design. But at the same time, we must also see that these incidents are very minor collisions. The collision itself is not harmful to the car and people, but the battery is on fire. If it is a more serious accident?

Second, high rate discharge life

Ordinary cars last for decades, and an electric car's battery requires at least 3,000 cycles in 10 years. As a relatively expensive component, it is very important that the life is equivalent to the car. It is necessary to ensure the performance of the vehicle and to ensure the interests of the owner, so as to benefit the market. At present, the electric vehicles of the world's car companies, only BYD "Qin" listed last year has achieved the battery life warranty.

The life of the battery is also the cycle life, not the number given by the simple battery parameters. The cycle life of the battery is closely related to the cycle state of the battery, such as discharge rate, charge rate, temperature, and the like. Usually, the cycle life obtained from the battery laboratory data is obtained at a constant charge and discharge rate of 0.3 C at a constant temperature of 20 degrees. However, during the actual use of the car, the magnification and temperature are not constant. This is why, in general, whether it is a notebook, a mobile phone, or a battery of a battery car, the life in actual use is far less than the reason given by the manufacturer. The medium and small mileage pure electric and long-life dual-mode hybrid vehicles, because they have fewer batteries, will have higher discharge requirements and will have a greater impact on life.

For example, A123 lithium iron phosphate battery usually has a cycle life of more than 3,000 cycles. However, A123 lithium iron phosphate RC battery is used at a charge rate of 10C and a discharge rate of 5C. The life in the laboratory is shortened to only 600 times, and only about 400 times in actual use, the effect of discharge rate on life can be seen. .

Taking BYD "Qin" as an example, only the 13KWH battery drives a motor with a peak power of 110KW. It can be calculated that when "Qin" is fully charged, its maximum discharge rate is as high as 8.4C. Especially when "Qin" has only 50% electricity, its maximum discharge rate can reach 18C. If the power is low and the discharge rate will exceed 25C, this will greatly shorten the battery life.

Look at the Tesla P85 power, the maximum power of 310KW motor, looks very large, in fact, the battery discharge rate is only 4C. At a charge of only 30%, the maximum discharge rate is only 10C. And Tesla's large-capacity battery, to a great extent, avoids the battery being in a high-powered discharge.

By simple comparison, we can see the superiority of BYD's high-rate discharge life.

Third, temperature adaptability

The impact of extreme cold on the battery is mainly reflected in the low charge-discharge rate and the decrease in capacitance; the effect of extreme heat on the battery is mainly due to the decrease in life, high-temperature safety and the decrease in charge and discharge capacity.

The effect of extreme cold on the battery is relatively light, because the general lithium battery can be used below minus 20 degrees, and the heat itself will be generated during the discharge of the battery, but the increase of energy consumption and the reduction of power are inevitable.