The Dilemma Analysis of Lithium-ion Battery

In 2012, the State Council promulgated the "Development Plan for Energy Conservation and New Energy Vehicle Industry (2012-2020)", which made it clear that the development of new energy vehicles in China will focus on Lithium-ion Battery Pure Electric Vehicles (LIB-EV). In recent years, lithium-ion battery pure electric vehicles have become the absolute mainstream of new energy vehicles in China. Lithium-ion batteries have unified the market of electric vehicle power batteries in China.

In December 2014, Toyota, the world's largest auto company, officially launched the world's first mass-produced fuel cell (PEMFC) electric vehicle Mirai (FC-EV). After that, Honda also unveiled its new fuel cell vehicle, FCV Clarity, in the second half of 2015. Since then, Nissan, Hyundai, GM, BMW and VW have all released their own plans for the industrialization of fuel cell electric vehicles in the past two years.

We can see that China and Japan (and indeed mainstream car companies in South Korea and Europe and the US) have chosen different technological paths for pure electric vehicles. Faced with the reality of small scale commercial production of Toyota, Honda and Hyundai fuel cell vehicles (FC-EV), the first thing we need to think about is why China and Japan have chosen different technology routes in the development of pure electric vehicles. Or what kind of power system would be more suitable for pure electric vehicles?

In essence, secondary batteries, including lithium-ion batteries, are energy storage devices that store and release electrical energy through reversible electrochemical reactions. The fuel cell (PEMFC) is an electrical energy production device that converts the chemical energy in the fuel into electrical energy through an electrocatalytic reaction. Although fuel cells are also called "batteries"(the reason for Chinese translation), their basic mode of operation is somewhat similar to that of internal combustion engines, and it is essentially different from conventional secondary batteries. The nature of the two electrochemical power supply systems is different, which will directly determine their different positioning at the application level.

At present, China basically regards lithium-ion batteries as the only choice in the choice of electric vehicle power source (power cell). And the domestic lithium industry has also been popular with this strange theory that "lithium-ion batteries will replace other secondary batteries" or "lithium-ion batteries are widely used as universal batteries".

I think it is necessary to clarify some basic knowledge about lithium-ion batteries. In the author's view, lithium-ion batteries have two major challenges, making it difficult to become the main power source of large and medium-sized vehicles: safety dilemma and energy density bottleneck.

Safety dilemma: first of all, what I want to emphasize here is that from the most basic thermodynamic point of view, the existing lithium ion battery system is thermodynamically unstable. The reason why it can work stably is that the passivation film on the positive and negative surfaces is dynamically isolated from the further reaction of the highly active positive and negative materials with the electrolyte, and the thermal runaway caused by various factors is the destruction of the positive and negative surfaces. The most direct reason for the passivation film, this scientific question will be crucial to an objective understanding of the safety of lithium-ion batteries.

Any commercial secondary battery needs effective anti-overcharging measures to ensure that the battery reaches full charging state and avoids the safety problems caused by improper overcharging. Whether it is a water system or an organic secondary battery, the charging safety is based on the basic principle of positive limit capacity (negative excess capacity). If this premise disappears, the consequence of overcharging is that the secondary battery of the water system produces hydrogen, and for lithium ion batteries it is negative lithium chromatography.

However, various water secondary batteries can effectively use the decomposition reaction of water to use the principle of "oxygen cycle" to achieve overcharge protection. Although water secondary batteries limit their further increase in energy density due to the decomposition voltage of water, it is important to remember that water also provides a near-perfect and irreplaceable anti-overcharge solution for water secondary batteries. In lithium-ion batteries, once the negative electrode precipitates high-activity metal lithium, safety problems will inevitably arise because the metal lithium can not be eliminated inside the battery.

Therefore, in a sense, lithium-ion batteries in the safety of the problem is no solution! Through the comprehensive application of some technical measures, such as thermal control technology (PTC electrode), positive and negative surface ceramic coatings, overcharge protection additives, voltage-sensitive diaphragm, and flame retardant electrolytes, the safety of lithium electricity can be effectively improved. However, these measures can not fundamentally solve the problem of the safety of lithium electricity, because lithium electricity is a thermodynamically unstable system.

On the other hand, these measures not only increase the cost, but also reduce the energy density of the battery. Limiting the capacity of the power cell monomer core is still a necessary measure to ease the safety. What I want to emphasize here is that BMS can not solve the problem of the safety of lithium-ion power cells. This is determined by the basic working principle of BMS.

If we consider the above factors, we will understand that the "security" of lithium electricity is only relative. In recent years, the domestic lithium-ion industry has been filled with the idea that lithium-ion batteries will unify the rivers and lakes and replace other secondary batteries. This argument is undoubtedly ridiculous from the point of view of safety.

The page contains the contents of the machine translation.