Performance comparison of the five major cathode materials for electric vehicle power batteries: Which is better?

    Battery manufacturers are pushing electric vehicles hard, and what will happen if they don't succeed? Like the fuel cells that boomed in the 1990s or the biofuels that have developed over the past decade, they may not survive without heavy government subsidies. The U.S. Department of Energy has also admitted that some key parameters of lithium batteries do not meet demand. The newer nickel-metal hydride batteries are cheaper, safer, and more suitable for electric powertrains than lithium batteries, but because the research is more mature, the research work cannot be funded by the government.

    There is no ideal competitor in the electric powertrain, so lithium batteries are still a good choice. Figure 1 shows five candidate batteries, among which lithium nickel manganese cobalt (NMC) batteries, lithium phosphate batteries, and lithium manganese batteries have certain advantages. The most popular lithium cobalt oxide battery (not listed) in consumer electronics is the "computer battery" with the highest energy density. Although it is used as a power source for electric vehicles by Tesla and Smart Fortwo ED, it is not suitable for large-scale application in general electric vehicles due to its high price and other disadvantages. People need to make a compromise in terms of safety, specific energy, cost, and high temperature performance.

    The performance of five types of batteries is compared in terms of six indicators: safety, specific energy (also known as capacity), specific power (the ability of the battery to charge and discharge at a high rate), high and low temperature performance of the battery, life (including service life and storage life), and cost. However, the figure does not mention the charging time. If a suitable power outlet is available, all batteries in the electric vehicle power system can be charged quickly. Ultra-fast charging is an exception, but most users can still accept a charging time of several hours. Self-discharge performance-this is another battery characteristic that needs to be paid attention to. In general, the self-discharge rate of lithium batteries is low. If the battery is new, the self-discharge is almost negligible. However, battery aging caused by heat will accelerate the self-discharge of the battery and cause problems in battery management. Among the battery types available for electric vehicles, lithium phosphate batteries have a higher self-discharge rate than other battery systems.

    The performance graphs of the five batteries are not exactly the same, but they are similar, and no one battery can stand out. For example, although NCA has high capacity, it is inferior to lithium phosphate batteries in terms of safety, but lithium phosphate batteries have low capacity. There is no perfect battery, and car manufacturers design battery combinations to make up for the lack of performance in certain aspects. Therefore, battery manufacturers generally design batteries according to order requirements and strengthen the important characteristics required for the application.

    Below are a few important metrics for the battery in an electric powertrain.

    Safety: It is one of the most important indicators for choosing electric car batteries. An accident, amplified by the media and public opinion, may make the public reject this electric car. Similar problems also occurred 100 years ago, such as steam engines and gasoline tanks exploding. People are mainly concerned about the thermal runaway of the battery. Using sturdy accessories and well-designed safety circuits should basically solve this problem, but there may still be risks. The battery must ensure its safety after abuse and long-term use.

    Lifespan: reflects the number of cycles and storage life of the battery. Most electric vehicles have a battery life that allows the car to travel 160 kilometers (about 100 miles) on a single charge within 8-10 years of use. Battery capacity loss caused by battery aging is a serious problem (especially in hot climates). Automakers lack data on battery aging under different conditions and climates. Therefore, manufacturers usually increase the size of the battery pack to compensate for the loss of battery capacity to ensure that the car can be used normally within the specified period of time.

    High and low temperature performance of batteries: reflects the performance of batteries in electric vehicles in extremely hot summers and very low temperatures. Unlike internal combustion engines that can operate normally in a large temperature range, electric vehicle batteries have higher temperature requirements. When the vehicle's power source is only the battery, the energy required to regulate the battery temperature and heat and cool the interior of the vehicle will also come from the battery.

    Specific energy: the energy contained in the battery per unit weight, reflecting the mileage of the car. It is well known that the energy output per unit weight of the battery is only 1% of that of fossil fuels. 1kg of gasoline (1.4 liters 0.37 gallons) produces about 12kWh, while 1kg of battery only has about 120Wh. But we must know that the efficiency of electric motors is higher than 90%, while that of internal combustion engines is only about 30%. Despite the huge efficiency difference, it is impossible to compare the energy storage capacity of batteries with that of internal combustion engines without increasing the energy storage capacity by 2-4 times.

    Specific power: This is related to the car's ability to accelerate, and most electric car batteries have a high specific power. Electric motors have a better torque ratio than internal combustion engines of the same horsepower.

    Cost: This is the main disadvantage of electric vehicles. BCG predicts that it is difficult to limit the target cost of batteries to $250-400/KWh. And there are other factors that increase the cost, such as the provisions for safety protection circuits, battery management status, ambient temperature control maintenance, and 8-10 years of warranty. These factors make the price of the battery equivalent to the price of a non-electric vehicle, and basically double the price of an electric vehicle.

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