Why is lithium iron phosphate battery still the mainstream power battery in the future?

Why is the battery the heart of an electric car? This must start with the history of electric vehicles. 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; today still use the tram is patented in the UK in 1840. The world's first electric car was born in 1881, invented by artificial 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.

As you can see, although electric cars before the car fuel development, and achieved a certain scale in the early days, but in modern times, due to the developing fuel cars, frustrated the electric car in the competition. But the real question is, the past is given priority to with lead-acid batteries of electric cars, subject to the density of lead-acid battery, life, power limitation, there has been no method in the power supply, battery breakthroughs, so that make the electric car development to a halt.

The classification of lithium battery and the advantages and disadvantages

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

Lithium-ion batteries are usually divided into two categories:

Lithium battery: the lithium battery is generally use lithium manganese dioxide as positive pole material, metal or its alloy metal anode materials for batteries, the use of water electrolyte solution.

Lithium ion battery: the lithium ion battery is generally use lithium alloy metal oxide anode material, graphite as the cathode material, using nonaqueous electrolyte battery.

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).

The above all kinds of battery have advantages and disadvantages, summarized as follows:

Lithium iron phosphate:

Advantages: long life, charge and discharge rate, good security, high temperature resistance, elements, harmless, the cost is low.

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

The ternary lithium:

Advantages: high energy density, tap density is high.

Faults: poor safety, high temperature resistance, poor, poor life, high-power discharge, poor, elements, toxic (ternary lithium battery power after the charging and discharging temperature rise sharply, release oxygen combustion extremely easy after high temperature).

Lithium manganese acid:

Advantages: high tap density and low cost.

Disadvantages: high temperature resistance is poor, manganese acid lithium temperature rise sharply after long time use, the battery life attenuation serious (such as Nissan's electric vehicle LEAF).

Cobalt acid lithium:

Usually used for 3 c products, security is very poor, not suitable for power battery.

In theory, we need the battery should be a high energy density, high volume density, good safety, high temperature and low temperature resistance, long cycle life, non-toxic harmless, high power charging and discharging, and gather all the advantages of low cost. But at present there is no such batteries, so in the advantages and disadvantages of different types of batteries will need to choose. And different electric demand for battery point is different, so only based on the plan of the electric car to judge, can help us to correctly judge the battery route choice.

The advantages of lithium iron phosphate batteries

Here will need to be back before two article, we analyzed the future of electric cars should be in small range, fast charging electric cars. The current family car need long range of two-mode hybrid, and a large range of public transport market net charge. Such a car needs what kind of battery?

A, security,

Security is the precondition of cars necessary first. Different from cell phones and computers, car in high speed may encounter many unpredictable factors, such as extrusion and collision accidents caused by batteries. And any adverse factors are likely to produce more elderly. We can see some old car instead of walking to use inferior lead-acid batteries, no security, battery cases of spontaneous combustion, suffers bump combustion everywhere. And closing a year of continuous fire events such as masters, although thanks to Tesla's safety design and there are no casualties. But at the same time also want to see, this event is very slight collision, collision itself, with no damage for cars and people, and the battery is on fire, so if it is a more serious accident?

Second, the high rate discharge life

Ordinary cars last for decades, and an electric car's battery requires at least 3,000 cycles in 10 years. Battery as more expensive components, life is very important to identify with the car, both to ensure the performance of the vehicle and to ensure that the interests of the owners, so as to promote of the market. Currently the world's automakers electric cars, only listed last year BYD "Qin" do the batteries lifetime warranty.

Battery life is the cycle of life, is not a simple battery parameters given by the Numbers. The cycle of the battery and battery life cycle state is closely linked, such as discharge ratio, charge ratio, temperature, etc. Cycle life of battery lab data, usually is 0.3 C constant charging and discharging rate, under the constant best temperature 20 degrees. But in the actual transport process, the rate and temperature are constant. This is also why often laptops, mobile phones, or storage battery car battery, the actual use of live longer than a given vendor data. And the medium and small range of pure electric and long range two-mode hybrid car, because of the battery is less, the discharge requirements will be higher, the impact on the life will be more.

A123 lithium iron phosphate batteries, for example, usually cycle life can reach more than 3000 times. However, A123 model aircraft batteries, lithium iron phosphate to 10 c of charge ratio, 5 c discharge rate, shorten the life in the laboratory to only 600 times, and only about 400 times in the real practical use, visible discharge rate's impact on life.

Take the example of BYD "Qin", only 13 KWH battery peak power of 110kw motor drive. Can be calculated, when the "Qin" full charge the maximum discharge ratio up to 8.4 C, especially when "Qin" only 50% power, the maximum discharge ratio can reach 18 c. If the battery again low discharge rate will be more than 25 c, it 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.

Through a simple contrast, we can see the advantages of high rate discharge of battery life.

Third, temperature adaptability

Extremely cold impact on battery, main performance in low charge and discharge rate and reduce the capacitance; Extreme heat effect on the battery, main show is reduce the service life, high temperature safety and reduced ability to charge and discharge.

Very cold for the influence of the battery is relatively light, because generally lithium batteries can be used below 20 degrees below zero, and in the process of the discharge of the battery itself will generate heat, but the increase of energy consumption and the loss of power is inevitable.

The impact of extreme cold on pure electric cars is different from that of dual-mode hybrids. Because pure electric vehicles do not have other sources of power, in order to reach the right temperature in extremely cold conditions, they must rely on battery discharge heating, which will have a great impact on energy consumption and cruising range. Tesla has significant differences in peace and water in the winter, regardless of the energy consumption of 100 kilometers and the cruising range.

For the two-mode hybrid effect is weaker. Because the hybrid engine as backup power, such as BYD last November in Baotou, "Qin" promotional activities, when temperatures in 15 to 20 degrees below zero at night, very cold in the morning of launch vehicle, the system will automatically switch to HEV mode, engine driven air conditioning, rapidly improve the temperature inside the car, when the temperature is improved and then switch back to the EV model.

Extremely hot are of great influence on the pure electric and hybrid, such as the battery itself, high-power discharge temperature will rise. In ordinary lithium ion battery, for example, discharge of 20 c, the temperature of the battery can be promoted to nearly 50 degrees. Such a high temperature, not only have an impact on battery life, more important is security risk. Such as ternary Tesla battery under the environment of high temperature can release oxygen, and oxygen is flammable objects. Tesla reduces the temperature through a circulating cooling system and wraps the isolated battery in a hard case to prevent oxygen from escaping. But when it comes to impact, it is still inevitable to catch fire.

Fourth, the energy density

The energy density, just as its name implies is the power of the battery unit weight can hold. Energy density is usually judge battery good important index, but in my analysis system, the energy density in the battery performance is not very important.

There are two reasons:

1. The energy density must be combined with other performance. Such as lithium iron phosphate battery energy density is not high. But because of its characteristics of security and stability of high temperature resistant, to lithium iron phosphate batteries of the battery is very simple, do not need much protection auxiliary equipment. And although Tesla ternary cell battery cell density is high, but because of its poor safety is not high temperature resistant, so must be combined with a complex set of battery protection equipment, these devices have increased the weight of the car. Reports after the continuous burning accident, Tesla and ready to upset the battery protection device, it will weaken the energy density advantage of the ternary battery.

2. Weight for the effects of a car, especially for the mainstream trend of the future electric cars, hybrid and small range of pure electric vehicles. We can imagine a comparison of batteries with an energy density of 130kWh/kg and an energy density of 200kWh/kg. Even the largest 80 degrees of electricity, two kinds of difference but 200 kg weight of the battery.

This has a very low impact on a car that is close to 2 tons.

So I think, even though the battery energy density of nature is the bigger the better, but it doesn't need to deliberately pursue maximum. Especially the bigger the energy density is not stable, this is basic common sense. As long as enough level, the energy density is not too important.

Fifth, the cost

The cost is very well understood, and there must be a cost advantage for widespread adoption, which has been calculated in the first part of this series. Small-scale pure electric or hybrid electric vehicles, on the one hand, need to reduce the amount of vehicle batteries to save the cost of batteries, on the other hand need to reduce the cost of battery protection equipment. Therefore, we found that Tesla's battery cost is low, but the overall cost is still high.

Through the above discussion, we know that different lithium ion batteries have natural advantages and disadvantages. But what's important is how to sort out the key elements of future electric vehicle development so that you can choose the right battery.

The page contains the contents of the machine translation.