Low-speed lithium batteries are so hot, but do you really understand lithium batteries?

What is a lithium battery?

Lithium battery is a type of battery that uses lithium metal or lithium alloy as the negative electrode material and uses a non-aqueous electrolyte solution. The earliest lithium battery came from the great inventor Edison. Due to the very active chemical properties of lithium metal, the processing, storage and use of lithium metal have very high environmental requirements. Therefore, lithium batteries have not been used for a long time. Nowadays, lithium battery has become the mainstream, it is compared as the heart of electric vehicles.

Lithium batteries are generally divided into two categories: 1. Lithium metal batteries: Lithium metal batteries generally use manganese dioxide as the positive electrode material, lithium metal or its alloy metal as the negative electrode material, and use a non-aqueous electrolyte solution. 2. Lithium-ion batteries: Lithium-ion batteries generally use lithium alloy metal oxide as the positive electrode material, graphite as the negative electrode material, and non-aqueous electrolyte.

Although the energy density of lithium metal batteries is high, theoretically it can reach 3860 watts/kg. However, due to its insufficient stability and inability to charge, it cannot be used as a power battery for repeated use. The lithium-ion battery has been developed as the main power battery due to its ability to be recharged. However, because of its combination of different elements, the composition of the positive electrode material has great differences in various aspects, which has led to increased disputes over the route of the positive electrode material in the industry.

Usually, the power batteries we talk about the most are mainly lithium iron phosphate batteries, lithium manganese oxide batteries, lithium cobalt oxide batteries, and ternary lithium batteries (ternary nickel cobalt manganese).

The above types of batteries have their advantages and disadvantages, which can be roughly summarized as follows:

Ternary lithium:

Advantages: high energy density, high tap density.

Disadvantages: poor safety, poor high temperature resistance, poor life, poor high-power discharge, and toxic elements (the temperature rises sharply after high-power charging and discharging of ternary lithium batteries, and the release of oxygen after high temperature is very easy to burn).

Lithium iron phosphate:

Advantages: long life, high charge and discharge rate, good safety, good high temperature performance, harmless elements, low cost.

Disadvantages: low energy density, low tap density (bulk density).

Lithium manganese oxide:

Advantages: high tap density and low cost.

Disadvantages: poor high temperature resistance, the temperature rises sharply after long-term use of lithium manganate, and the battery life is seriously attenuated (such as the Nissan electric car LEAF).

Lithium cobalt oxide: usually used in 3C products, with extremely poor safety and not suitable for power batteries.

Now the low-speed electric vehicle industry has appeared in the lithium battery models, mainly using two types of lithium iron phosphate and ternary lithium, so today we will focus on the two types of ternary lithium and lithium iron phosphate batteries.

　01

Lithium iron phosphate battery: mature but not enough

Lithium iron phosphate electrode material is currently the safest cathode material for lithium-ion batteries. In addition, its cycle life can reach more than 2000 times. It can be used for standard charging (5 hours rate) and can reach 2000 cycles. In addition, due to the mature industry And the price technology threshold and the decline of technology brought about, many manufacturers will adopt lithium iron phosphate batteries for various factors. It can be said that the rise of new energy vehicles has an inseparable relationship with lithium iron phosphate batteries.

However, lithium iron phosphate batteries have a fatal shortcoming, that is, poor low-temperature performance, even if they are nano-sized and carbon coated, this problem has not been solved. Studies have shown that if a battery with a capacity of 3500mAh is operated in an environment of -10°C, after less than 100 charge-discharge cycles, the power will decay sharply to 500mAh, which is basically scrapped. This is indeed not a good thing for our country's vast territory and the comprehensive national conditions where there are indeed more low temperatures in winter.

In addition, the cost of material preparation and battery manufacturing are relatively high, the battery yield is low, and the consistency is poor. This is also an important reason why many pure electric vehicles cannot reach the nominal value. Therefore, we can see that many domestic new energy vehicles (whether pure electric or hybrid electric), or some relatively cheap new energy vehicles, will choose lithium iron phosphate batteries for different reasons. It can be said that the use of lithium iron phosphate batteries has an indelible foundation for the mass production and promotion of new energy vehicles.

　02

Ternary polymer lithium battery: a restless future

Ternary polymer lithium battery refers to a lithium battery that uses lithium nickel cobalt manganate (Li (NiCoMn) O2) as the cathode material. The precursor product of the ternary composite cathode material is nickel salt, cobalt salt, and manganese salt. As raw materials, the ratio of nickel, cobalt and manganese inside can be adjusted according to actual needs. Ternary lithium batteries have greater energy density, but their safety is often questioned.

The reason for this is that even though these two materials will decompose when they reach a certain temperature, the ternary lithium material will decompose at a lower 200 degrees, while the lithium iron phosphate material is about 800 degrees. And the chemical reaction of the ternary lithium material is more intense, it will release oxygen molecules, and the electrolyte will burn rapidly under the action of high temperature, causing a chain reaction. To put it simply, ternary lithium materials are more likely to catch fire than lithium iron phosphate materials. But it should be noted that we are talking about materials, not batteries that have become finished products.

Because of the potential safety hazards of ternary lithium materials, manufacturers are also working hard to prevent accidents. According to the easy pyrolysis characteristics of ternary lithium materials, manufacturers will do a lot of overcharge protection (OVP), over discharge protection (UVP), over temperature protection (OTP), and over current protection (OCP). effort. Therefore, the spontaneous combustion incident should consider whether the manufacturer's functions in these links are in place, rather than simply giving up food due to choking.

So what is the current usage of these two batteries? Let's focus on a set of data. In November last year, the installed capacity of electric buses with lithium iron phosphate batteries accounted for 64.9%, and the installed capacity of ternary lithium batteries was only 27.6%. On the contrary, in the pure electric passenger car market, the installed capacity of ternary lithium batteries in November last year exceeded 76%.

Theoretically, the battery we need should have high energy density, high volume density, good safety, high temperature and low temperature resistance, long cycle life, non-toxic and harmless, high-power charging and discharging, and integrating all the advantages and low cost. But there is no such battery at present, so there is a trade-off between the advantages and disadvantages of different types of batteries. Moreover, different electric vehicles have different requirements for batteries, so which battery is more suitable depends on your own choice!