Advantages and disadvantages of two types of lithium batteries

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 and 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 to 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 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 the cases of spontaneous combustion and impact burning of the batteries abound. Another example is Tesla's nearly one-year continuous fire incident, although there was no casualty in the safety design of Tesla. 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. So what 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 battery 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 effect of extreme cold on the battery is mainly reflected in the low charge-discharge rate and the decrease in capacitance; the influence of extreme heat on the battery is mainly manifested by the decrease of life, high temperature safety and the decrease of 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 process of the battery, but the increase of energy consumption and the reduction of power are 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. In the winter, Tesla has significant differences in energy consumption and cruising range of 100 kilometers.

The effect on the dual-mode hybrid is weaker. Because there is a hybrid engine that provides energy as a backup. For example, in November last year, BYD held the “Qin” promotion activity in Baotou. At that time, the temperature was minus 15 to 20 degrees at night. When the vehicle was started in the cold in the morning, the system would automatically switch to HEV mode. The engine would drive the air conditioner and quickly improve the car. The internal temperature is switched back to EV mode when the temperature is raised.

Extreme heat has a great influence on pure electricity and mixing, for example, the battery itself will increase in high power discharge temperature. Taking a common lithium ion battery as an example, the discharge temperature of the battery can be raised to nearly 50 degrees. Such a high temperature not only has an impact on the life of the battery, but more importantly, a safety hazard. For example, Tesla's ternary battery emits oxygen in a high temperature environment, and oxygen is a flammable object. Tesla reduces the temperature through a circulating cooling system and wraps the isolated battery in a hard case to prevent oxygen from escaping. However, it is inevitable to catch fire when encountering an impact.

Fourth, energy density

Energy density, as the name implies, is the energy that a unit of weight can hold. Energy density is usually an important indicator for judging the battery's superiority, but in the author's analysis system, energy density is not very important in battery performance indicators.

There are two reasons for this:

1. Energy density must be combined with other properties. For example, the energy density of lithium iron phosphate batteries is not high. However, because of its safety, stability, and high temperature resistance, the battery composed of lithium iron phosphate is extremely simple, and does not require much protection auxiliary equipment. Tesla's ternary battery has a high density of battery cells, but because of its poor safety and high temperature, it must be combined with a complex battery protection device, which increases the weight of the car. It has been reported that after a continuous combustion accident, Tesla is preparing to thicken the battery protection equipment, which weakens the energy density advantage of the ternary battery.

2. The weight has little effect on the car, especially for the mainstream trend of hybrid electric vehicles in the future and small mileage pure electric vehicles. We can imagine a comparison of batteries with an energy density of 130 kWh/kg and an energy density of 200 kWh/kg. Even with the maximum total power of 80 degrees, the weight difference between the two batteries is only 200KG.

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

Therefore, the author believes that although the energy density of the battery is naturally greater, it is not necessary to deliberately pursue the maximum. In particular, the greater the energy density, the more unstable it is. This is the basic common sense. As long as it is sufficient, the energy density is not too important.

V. 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 battery in the car to save the cost of the battery, on the other hand need to reduce the cost of the battery pack + 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. In summary, considering the factors of safety, life, discharge capacity, temperature adaptation, energy density, cost and other factors, the author believes that lithium iron phosphate battery is most suitable for the future development direction of electric vehicle batteries.

The page contains the contents of the machine translation.