Nov 10, 2018 Pageview：43
The lithium ion battery is usually divided into two species:
Lithium metal battery: lithium metal battery is a battery using manganese dioxide as anode materials, lithium metal or its alloying metal as cathode materials and non-aqueous electrolyte solution.
Lithium ion battery: lithium ion battery is a battery using lithium alloy oxide as anode material, graphite as cathode material and non-aqueous electrolyte.
Although lithium metal battery has high energy density with theoretical value of 3860 W/kilo, it cannot used as rechargeable power battery because it is not stable enough and non-rechargeable. Lithium ion battery is used as the main power battery due to its rechargeable performance. However, lithium ion battery needs to assort with other materials, and it may has different performances with different anode materials, which leads a heated discussion on anode materials.
Normally, the power batteries refer to lithium iron phosphate battery, lithium manganate battery, lithium cobalt oxides battery and ternary battery (NCM).
The batteries above have their own advantages and disadvantages.
Lithium iron phosphate battery:
Advantages: long cycle life, high charging and discharging rate, good safety performance, good high-temperature performance, harmless elements and low cost.
Disadvantages: low energy density, low tap density (volume density)
Advantages: high energy density, high tap density
Disadvantages: poor safety performance, poor high-temperature resistance, poor cycle life, poor discharging performance under high power, poisonous element (ternary battery heats up rapidly after charging and discharging at high power, which can easily lead to fire after releasing oxygen under high temperature)
Lithium manganate battery
Advantages: high tap density, low cost
Disadvantages: poor high-temperature resistance, lithium manganate battery will heat up rapidly after long-time application, which damages the cycle life of the battery (such as LEAF electric car made in Japan)
Lithium cobalt oxide battery
It is widely used on 3C products with poor safety performance, which is not suitable for power battery.
In theory, the battery we need should have high energy density, high volume density, good safety performance, good high & low-temperature resistance, long cycle life, toxic-free, low cost and can charge and discharge at high power. However, there is no such a battery. We all need to know that different batteries have different advantages and disadvantages. Besides, different electric cars have different requirements on batteries. Only taking a long-term view on electric cars can we judge the direction of battery development in a correct way.
The advantages of lithium ion phosphate battery
It is analyzed that the electric cars in the future need to have small mileage and fast charging, but private cars at present need long mileage with two-mode hybrid, while the electric bus needs large mileage. So what kinds of battery do these cars need?
Safety is the most important point of a car. Differ from mobile phones and computers, cars may suffer from all kinds of unpredicted factors during high speed driving. Only one of the adverse factors can lead to car crash. Some elderly mobility scooter using unqualified lead-acid battery without safety assurance will result in spontaneous combustion and crash combustion. As for continuous fire events of Tesla in the last year, even though there is no casualty due to the safety design, we can also find out that the battery was fire after the slight-crash accident. What if a big accident?
Discharging lifespan at high rate
Normally, the cycle life of a car will be over decades. The battery of an electric car has at least 3000 cycles within 10 years. As the expensive part, it is very important for a car to have a battery with equivalent cycle life, so as to not only make sure the performance of the car, but also assure the benefit of the car owner, which is good for promoting the market. At present, compared to any other electric cars of different automobile enterprises all over the world, only BYD “Qing“ offered a lifetime warranty last year.
The lifespan of a battery means cycle life that is not only a number from the parameter but is closely related to the state of the battery, including discharging rate, charging rate, temperature, etc. Normally, laboratory tests the cycle life at 0.3C constant charging and discharging rate under 20℃optimal constant temperature. However, neither the rate nor the temperature is not constant during actual application, this is the reason why the batteries of laptop, mobile phone or electric car have far lower cycle life than the parameter from manufacturers. HEV (hybrid electric vehicle) with dual-mode of electric medium & small mileage and long battery life has more severe requirements on discharging and more apparent effect on cycle life.
Taking A123 lithium iron phosphate battery as an example, its cycle life usually is over 3000 times. However, the cycle life in lab will shorten to 600 times after apply at 10c charging rate and 5c discharging rate, while it is about 400 times during actual application, which shows that discharging rate is essential for cycle life.
Taking BYD “Qing” for example, it only has the 110KW engine with battery having 13KWH peak drive. The maximum discharging rate is 8.4c when “Qing” is fully charged. It will up to 18c when “Qing” has 50% capacity and then will be over 25c if the capacity keeps lessening, which will greatly hurt the cycle life.
Given that Tesla P85 car has an engine with maximum power of 310kW, it seems to have large power, but its discharging rate is only 4c. Even if it has 30% capacity, the maximum discharging rate is 10c. Besides, Tesla battery with large capacity prevents the battery to discharge at large power.
After the analysis above, it is obvious that BYD batteries do well in discharging lifespan at high rate.
Extremely cold ambient tends to result in low charging and discharging rate and loss of capacity, while extremely hot tends to decrease cycle life, high-temperature performance and charging & discharging capability.
Extremely cold has slighter effect to the battery because common lithium ion battery can be used under -20℃. Besides, the battery will heat itself up during discharging process. However, it is inevitable that energy consumption will increase and battery capacity will decrease.
Extremely cold has different effect on dual-mode hybrid vehicles. Electric car has to heat up by battery discharging so as to reach the appropriate temperature under extremely cold environment, for which it has no other power sources. However, this will affect a lot on energy consumption and endurance mileage. Tesla has obvious difference on hundred-kilometer energy consumption and endurance mileage in winter.
Extremely cold environment has weaker effect on dual-mode hybrid vehicle because it has engine as backup. For example, BYD held a “Qing” promotion activity on Baotou in November. It was -15~-20℃ in the evening. The system will switch automatically to HEV mode when start the car on extremely cold morning. Then the engine drives the air conditioning to increase the temperature quickly inside the car and turn back to EV mode when the temperature is properly high.
Extremely hot environment also affects a lot on electric car and hybrid car, which may increase the discharging temperature at large power. Taking common lithium ion battery as example, the temperature of the battery will increase to nearly 50℃ if discharge by 20C, which is bad for the cycle life and safety performance. What’s more, Tesla ternary battery will release oxygen under high temperature. Oxygen is flammable gas. Tesla decreases the temperature by circulating cooling system and prevents the leakage of oxygen by separating the battery with hard coat. However, it still may lead to fire under impact.
Energy density means the capacity on unit weight. It is a key factor on judging the performance of a battery. However, energy density is not so import in this article for battery performance.
There are two reasons.
Energy density is affected by other performance of the battery. Lithium iron phosphate battery does not have high energy density due to its safety performance and high-temperature resistance. The battery made of lithium iron phosphate cell is simplified without so many protective auxiliary devices, while Tesla ternary battery with high density ought to have a set of complicated protective devices due to its poor safety performance and poor high0-temperature resistance. These devices increase the weight of the car. It is reported that Tesla prepares to upgrade the protective devices after a series of combustion accidents, which weakens the energy density of ternary battery.
The weight is not so important for the car, especially electric hybrid vehicle which is the tendency in the future, and electric car with small mileage. We can compare the batteries with energy density of 130 kWh/kilo and 200 kWh/kilo. Even if the batteries have maximum total capacity of 80 kWh, the difference of the weight between these two kinds of batteries is no more than 200KG.
This will have little effect on a 2-ton car.
It has better to have large energy density, but there is no need to pursuit for the maximum. It is all known that the larger the energy density is, the more unstable the battery is, so having proper energy density is enough.
It is necessary to control the cost before widespread popularity. Electric car with small mileage or hybrid electric vehicle not only need to decrease the cost on lessening the amount of vehicle battery, but also reduce the cost of battery pack and protective devices. In this way, we can find out that the total cost cannot cut down although Tesla has low cell cost.
Above all, different lithium ion batteries have their inherent advantages and disadvantages. We can’t find out the proper trend of battery unless work out the correct order of key factors on electric car development in the future. Therefore, lithium iron phosphate battery is more suitable to be the tendency of electric vehicle material in the future, considered on safety performance, cycle life, discharging capability, temperature adaptability, energy density, cost, etc.