Lithium iron phosphate and nickel cobalt aluminium batteries

If the motor and control technology is proven and increasingly mature, the most difficult dilemma and the biggest competition for electric vehicles comes from battery technology. The future of electric vehicles is silence and patience. But China and the West at the top of the wave, BYD and Tesla, have something to say.

Tesla in the early electric sports car Roadster, the use of very small 18650 lithium cobalt acid battery, this battery is usually used in mobile phones, laptops and other small electrical appliances. Its main feature is that it has a very high energy density, almost 170 watt-hours/kg. But its thermal stability is also criticised, at around 180 degrees a decomposition phenomenon occurs and oxygen is produced.

Later, in order to compromise energy density, power density and safety, Tesla used modified ternary nickel-cobalt-aluminium batteries in the Model S. This brought the total number of batteries to more than 8,000, more than 1,000 more than in the Roadster, but the cost was reduced by 30%. However, the very limited number of cycles is still a problem that limits the use of such batteries in electric vehicles; with a charging frequency of once every two days, the battery will be dead after about three to four years.

Tesla's solution to this problem is to offer a "no-fault" battery warranty, which means that as long as the battery is not damaged by human error or collision, you get eight years of free warranty. At the end of that period, Tesla will be responsible for recycling and replacing the battery. Such a policy will put a lot of pressure on Tesla as it introduces entry-level models and increases sales. This may be one reason why the company is preparing to build the world's largest battery factory.

By contrast, the lithium-iron-phosphate battery used by BYD is currently a more widely used battery. Its advantage is that its thermal stability is very high, the structure is still relatively stable at 600 degrees, and because the trivalent iron ion is not active, it is difficult to chemically change, which makes its life relatively long, theoretically longer than the life of the vehicle, and the cost of long-term use is low. At the same time, the power density of the Lithium Iron Phosphate Battery is relatively good, and it can be discharged at a high rate and has good acceleration performance.

However, compared with the ternary Lithium Battery, the energy density of lithium iron phosphate battery does not have an advantage, about 100 to 110 watt-hours/kg, which leads to a shorter range under the same weight conditions, want to achieve a higher range, it is inevitable to increase the weight of the battery, increase the cost.

From a comprehensive performance point of view, not all companies have Tesla's software and battery management capabilities, so Lithium Iron Phosphate Batteries are still more optimistic and pragmatic battery types. This may also be one of the reasons why GE is willing to use lithium iron phosphate batteries.

Due to the characteristics of the battery, Tesla has made a very thorough design of the battery layout, thermal management system and battery management system to ensure that each battery unit is monitored and its status data can be fed back and processed at any time. For a single small battery unit, Tesla will be independently enclosed in a steel compartment, while the liquid cooling system can be specific to each battery unit to cool, reduce the temperature difference between each other, but also relatively reduce the risk of spontaneous combustion of the battery.

The Tesla accident was largely caused by the local short-circuit of the power line caused by the puncture of the battery pack. At present, Tesla cannot solve the situation of combustion and explosion caused by extreme damage to the battery pack by the impact force, but the high-intensity protection has won more time for the owner to escape.

In fact, this is almost a common potential hidden danger of electric vehicles, which places very high demands on the functioning of the battery management system. In addition to daily monitoring of the battery temperature and operating status, it is also necessary to immediately disconnect the high-voltage cable in the event of rapid temperature changes or an extreme collision. The improvement of the thermal management system and the battery management system will also shorten the battery charging time and bring higher charging efficiency.

In addition, how to ensure the efficiency of battery charging and use in a low-temperature environment is a problem that needs to be solved by companies involved in the R&D and production of electric vehicles.

In addition, it should be mentioned that Tesla has been promoting pure electric vehicle products, and its high-end route from high to low product ideas also reflects that the market inclusiveness of electric vehicles is far from enough.

BYD's future plans to promote "dual-engine, dual-mode" vehicles is actually to promote plug-in hybrid cars as a transitional product before the electric market really opens up. Compared with traditional petrol cars, hybrid cars are more fuel efficient and reduce battery consumption, and taking into account the policy subsidies for new energy vehicles, the cost of purchasing cars has also been reduced, which is in line with BYD's civilian product ideas.