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What happened to the safety issues with lithium-ion batteries? - Accelerated trends of the adoption of Li-ion batteries for electric vehicles

What happened to the safety issues with lithium-ion batteries?

Accelerated trends of the adoption of Li-ion batteries for electric vehicles

　In the past, the adoption of lithium-ion batteries for automobiles was confined to experimental vehicles. Today, however, the situation is changing. Mitsubishi Motors and Fuji Heavy Industries (Subaru) have employed lithium-ion batteries for their EVs they made commercially available this summer. Following the two, Toyota, Nissan, Honda, GM and Volkswagen are all planning to launch electric vehicles with incorporating lithium-ion batteries within a few years (Fig.1).

　The reason why lithium-ion batteries were not used for automobiles is that they had some safety concerns. Automobile manufacturers and battery manufacturers are putting efforts to ensure the safety of lithium-ion batteries by improving on traditional materials and developing new materials.

Li-ion Batteries have already taken off in consumer electronics

　Soon after the commercialization in early 1990s, lithium-ion batteries became widely adopted as batteries for portable consumer products such as notebook PCs and mobile phones. Employing lithium that has a small specific gravity and high electrochemical reactivity as constituent material, lithium-ion batteries can store two to three times the energy of Ni-Cd or Ni-MH batteries. At present, lithium-ion batteries account for two thirds of all rechargeable batteries used for consumer applications (Fig.2).

　However, the situation in the automotive field is entirely different. Lithium-ion batteries were only used for the experimental vehicles or, on a commercial basis, for hybrid freight vehicles. Automobile manufacturers used to be cautious about the adoption of lithium-ion batteries, but from this year they begun to advance the incorporation of lithium-ion batteries.

　Mitsubishi Motors and Fuji Heavy Industries released EVs employing lithium-ion batteries. Other automobile manufacturers have also made public their plans to launch vehicles incorporating lithium-ion batteries in the next few years (Fig.1).

Safety concerns around lithium-ion batteries

　There have been safety concerns behind the lagged adoption of lithium-ion batteries for cars. With lithium-ion batteries, there are cases that for some reason materials inside raise the temperature and result in abnormal heating or catching fire. In fact, there were many reports on the lithium-ion battery's trouble such as abnormal heating and fire accidents for consumer electronics products such as PCs and mobile phones.

　The abnormal heating and fire caused by lithium-ion batteries were called “thermal runaway”. Some internal parts and materials inside the battery produce heat, and then this causes heat on other parts. This way, battery temperature continues to rise till the battery reaches overheating or catching fire. The main causes for thermal runaway are internal short circuits, overcharging of batteries, or the compounded factors (Fig.3). In case of an internal short circuit, a burst of electric current rushes through the anode, and this generates abnormal heating. The heated anode then heats up the cathode, which develops exothermic reaction.

　Overcharging, on the other hand, leads to the heating of the cathode side. In charging a lithium-ion battery, lithium ions are pulled out of the cathode material and inserted into the anode material (Fig.4). However, in this process, the cathode material which lithium ions are extracted from becomes unstable in terms of crystal structure. In usual circumstances, lithium-ion batteries are controlled so that the amounts of lithium ions extracted from the cathode do not go beyond a certain level. In case of overcharging, an excessive level of lithium ions is pulled out and this leads to the collapse of the crystal structure of the cathode material, during which the cathode develops exothermic reaction.

Safety features required for cars

　Taking advantages of the characteristics of lithium-ion batteries that are light in weight and large in capacity, automobile manufacturers and battery manufacturers are aiming to commercialize the lithium-ion battery for cars. In order to adopt lithium-ion batteries for HEVs/EVs, the batteries need to meet the particular requirements of cars. Theses will include the instantaneous absorption of heavy current for the effective recovery of the energy generated in deceleration, the fast charging from an external power source for the improved convenience of EVs, and the protection against the internal short circuit caused by the deformation of batteries or the compression of internal materials that are caused by external forces such as impacts or shocks to the automobile body.

　Depending on cathode materials, current lithium-ion batteries that Japanese manufacturers plan to mass-produce are largely divided into two categories, a ternary system or a manganese system (Fig.5). The ternary system is considered a variation of a type that is widely used for consumer applications, whose cathode consists of lithium cobalt oxide (LiCoO₂). The ternary system replaces a part of cobalt of lithium cobalt oxide (LiCoO₂) with nickel and manganese (LiNi_1/3Mn_1/3Co_1/3O₂). This combination of materials enhances thermal stability of the cathode. Sanyo Electric and Blue Energy, a joint venture by GS YUASA and Honda plan to release the ternary system for HEVs. Sanyo is going to supply lithium-ion batteries to Volkswagen, and Blue Energy to Honda.

　The manganese system employs lithium manganese oxide (LiMn₂O₄). The crystal structure of it is called the spinel structure, in which the manganese atoms and lithium atoms are located in the gaps between oxygen atoms. Due to such structure, the crystal structure remains stable when lithium ions are extracted in charging. Hitachi Vehicle Energy has already commercialized lithium-ion batteries of this system for hybrid trucks of Mitsubishi Fuso. The manganese system is also incorporated in the batteries of Mitsubishi’s “i-MiEV”, which Lithium Energy Japan produces, and those of Fuji Heavy Industries’ “Plug-in Stella”, which Automotive Energy Supply manufactures.