What is the difference between Tesla 21700 lithium battery and ordinary lithium battery

As one of the core components of electric vehicles, Tesla lithium batteries have been ridiculed and criticized. Even after achieving explosive sales growth, many industry experts still call it "old battery technology" and "no core competitiveness." The reason is that Tesla is the only company that uses 21700 lithium batteries. This type of battery has been used in notebook computers. It is difficult to be included in the "Treasure Hall" of electric vehicles and has potential safety hazards. Is this really the case?

　　 According to reports, as the backing of power output, the role of batteries on electric vehicles is self-evident. Because of its relatively stable performance, high safety factor and many rechargeable cycles, lithium iron phosphate batteries are currently the first choice for power batteries on the market, such as Chevrolet Volt, Nissan Leaf, BYD E6 and FiskerKarma.
　　Use lithium cobalt oxide batteries in cars, Tesla is the first

　　 Tesla is a bit different. His Roadster and ModelS use 21700 lithium batteries and lithium cobalt oxide batteries. Compared with lithium iron phosphate batteries, although this battery has more mature technology, high power, high energy density, and high consistency, it has a lower safety factor, poor thermal and electrical characteristics, and relatively high cost.

　　According to industry insiders, 21700 lithium batteries with external voltage lower than 2.7V or higher than 3.3V will have symptoms of overheating. If the battery pack is large and the temperature gradient within the pack is not well controlled, there will be a great risk of fire. The key to battery technology is precisely voltage, current and heat control. No wonder Tesla has been criticized as unreliable battery technology.

　　 But in fact, the lithium iron phosphate battery, which is considered safer and more reliable, is not foolproof. In the sintering process of the preparation, iron oxide may be reduced to elemental iron in a high temperature environment. Elemental iron can cause micro-short circuit of the battery, which is a very taboo substance in the battery. In addition, lithium iron phosphate batteries have large differences in charge and discharge curves in actual production, poor consistency and low energy density, which directly affects the sensitive battery life of electric vehicles. The latest research report of Haitong International Securities Company shows that the energy density of Tesla batteries (170wh/kg) is approximately twice that of BYD's lithium iron phosphate batteries for electric vehicles.

Why does Tesla insist on using lithium cobalt oxide batteries?

21700 lithium battery used by Tesla
As early as the 1970s, Ms. Whittingham of Binghamton University in the United Kingdom invented 21700 lithium batteries. This type of battery is often used in digital products such as notebook computers and strong light flashlights. However, the 18mm diameter and 65mm high Cylindrical lithium batteries are used in cars, and Tesla was the first to eat crabs.
Kurt Kelty, Tesla’s battery technology director, once said: Tesla initially tried more than 300 types of batteries on the market, including plate and square batteries, and finally chose Panasonic’s 21700 lithium battery. On the one hand, the energy of the 21700 lithium battery The density is greater and the stability and consistency are better. On the other hand, the 21700 lithium battery can effectively reduce the cost of the battery system. In addition, although the size of each battery cell is small, the energy of each cell can be controlled in a small range. Compared with the use of large-size battery cells, even if a cell of the battery pack fails, the failure band can be reduced. Influence. In addition, billions of 21700 lithium battery-type battery cells are produced every year in the world, and the safety level is constantly improving.

　　 Panasonic's official website shows that the lithium battery model NCR21700 lithium battery is a high-energy model (HighCapacity) battery. The battery has a nominal voltage of 3.6V, a nominal minimum capacity of 2750 mAh, and a weight of 45.5 grams. In addition, the 21700 lithium battery used in Tesla's second-generation model ModelS has an energy density 30% higher than that of the previous Roadster.

　　 According to Tesla’s Chief Technology Officer JBStraubel: In the four years of “transition” from Roadster to ModelS, the battery cost has dropped by about 44%, and it will continue to decline. Panasonic invested 30 million US dollars in Tesla in 2010 and became one of its shareholders. And reached a strategic agreement in 2011, will be responsible for Tesla's battery supply for all vehicles in the next five years. According to Tesla's current estimated annual output of 20,000 vehicles, Panasonic 21700 lithium batteries will be assembled on more than 80,000 ModelS.

　　 The magical reorganization of 6831 lithium batteries
　　 It is an indisputable fact that 21700 lithium batteries have safety risks. How did Tesla “solve” this flaw? The secret weapon lies in its battery management system. The solution given is to combine 6831 2Ah Panasonic 21700 lithium battery packs in series and in parallel.

　　To drive an electric car, a large number of 21700 lithium batteries are required. TeslaRoadster's battery system contains 6831 small batteries, and TeslaModelS has as many as 8,000 small batteries. How to arrange and assemble these large numbers of small batteries is particularly important.

　　 Tesla has cooperated with Toyota, Daimler and other manufacturers in the field of electric vehicles, including providing battery power for smart, Mercedes-Benz A-class, Mercedes-Benz B-class and other electric models, and providing battery packs and motors for Toyota RAV4 electric vehicles. Daimler currently holds 4.3% of Tesla and Toyota also holds 2.9% of Tesla. However, due to the short time and small number of Tesla's products officially put on the market, the theoretically perfect Tesla battery system still needs to continue to face practical tests.