What is the reason why ternary lithium batteries are afraid of heat and lithium iron phosphate batteries are afraid of the cold?

At the beginning of this year, BYD brought the lithium iron phosphate battery back to people's field of vision through the acupuncture test of the blade battery. Through this acupuncture test, many people have learned that lithium iron phosphate batteries have more advantages than ternary lithium batteries in terms of safety.

At the same time, when the time came to the end of 2020, as the temperature continued to drop, many northern electric car owners began to complain about the Internet about the declining battery life of electric vehicles in winter and slower charging speeds. Among them, electric vehicles that use lithium iron phosphate batteries have a hard time pulling their hips in low-temperature environments.

It can be seen that both ternary lithium batteries and lithium iron phosphate batteries have their own advantages and disadvantages, and one cannot say which battery is good and which battery is not. So what is the reason behind this that causes their performance to be different? Today we will have a good chat.

In power batteries, lithium iron phosphate batteries and ternary lithium batteries are the two most commonly used lithium-ion batteries, and their difference is only in the choice of cathode material. The cathode material of ternary lithium batteries is nickel cobalt manganese (NCM) or nickel cobalt aluminum (NCA), and the cathode material of lithium iron phosphate batteries is lithium iron phosphate. It is precise because of the different cathode materials that they have different fates.

No matter what kind of material is used as the positive electrode, the nature of the battery is still the chemical reaction, and the characteristics of the chemical elements are inherent, which will not change because you make the battery rectangular or cylindrical.

Stable and reliable lithium iron phosphate

No matter what kind of material is used as the positive electrode, the nature of the battery is still the chemical reaction, and the characteristics of the chemical elements are inherent, which will not change because you make the battery rectangular or cylindrical. First of all, from a chemical point of view, lithium iron phosphate is a typical orthorhombic system, each unit cell contains four units, one octahedral FeO4, and one tetrahedral PO4 are co-edges with two octahedral LiO6, and the other is four-sided. The bulk PO4 and the two octahedral LiO6 are co-edges. This structure allows lithium ions to move freely during charging and discharging.

Advantage:

At the same time, high school chemistry knowledge tells us that the P-O covalent bond in lithium iron phosphate has large bond energy, so it is very stable, not easy to decompose, and high temperature or overcharge will not cause its structure to collapse. Just because its structure is difficult to be destroyed, the oxygen atom at the other end of the covalent bond will be very honest and difficult to be oxidized and released.

Therefore, lithium iron phosphate has good high-temperature resistance. Basically, the temperature will reach about 500°C, and it will not destroy the PO covalent bond and release oxygen (when fully charged, the lithium iron phosphate battery needs about 700°C. Thermal decomposition will occur). This explains why the blade battery based on lithium iron phosphate still did not catch fire after acupuncture.

Secondly, when the lithium iron phosphate material is deintercalated with lithium ions, its own crystals will not be rearranged, so it has good reversibility and circulation. This feature allows the cycle life of energy-type lithium iron phosphate batteries to be as long as 3000-4000 times, and the cycle life of rate-type lithium iron phosphate batteries can even reach tens of thousands of times.

Disadvantages:

Because the adjacent FeO6 octahedrons in the structure of lithium iron phosphate are connected by a common vertex, this structure makes its conductivity low; at the same time, the three-dimensional network olivine structure of lithium iron phosphate forms a one-dimensional lithium-ion transmission channel, The diffusion of lithium ions is limited, so its charge and discharge efficiency is affected. In a low-temperature environment, the activity of the material decreases, and the number of lithium ions that can move is reduced. Therefore, lithium iron phosphate performs poorly at low temperatures.

In addition, compared with ternary materials, the discharge-specific capacity of lithium iron phosphate materials is lower, and the average voltage is also lower, so the mass-specific energy of lithium iron phosphate batteries is generally lower than that of ternary lithium. In addition, because the lithium iron phosphate particles are not compact, their tap density and compaction density are low (the compaction density of the lithium iron phosphate pole piece is about 2.3-2.4g/cm, while the ternary pole piece can reach 3.3- 3.5 g/cm). Therefore, in layman's terms, under the same volume conditions, less lithium iron phosphate is installed, the natural capacity is small, and the energy density is also low. And in fact, the consensus in the industry is that the energy density of lithium iron phosphate itself has reached the ceiling, and it is impossible to continue to increase significantly.

Ternary lithium with high density but heat-resistant

The cathode material of the ternary lithium battery is nickel cobalt manganese (NCM) or nickel cobalt aluminum (NCA). For the most common nickel cobalt manganese ternary lithium battery, it uses nickel salt, cobalt salt, and manganese salt as raw materials. After a certain ratio, each element plays an important role. At the same time, the characteristics of each element also restrict the battery's performance.

NCM has an α-NaFeO2 layered rock salt structure similar to LiCoO2, belongs to the hexagonal crystal system, and the space point group is R3m. As can be seen from Figure 1 below, Li in the crystal lattice mainly occupies the 3a position, O occupies the 6c position, forming the MO6 octahedral structure, and the disorder of Ni, Co, Mn occupies the 3b position, and the entire crystal can be regarded as the [MO6] octahedral layer It is stacked alternately with [LiO6] octahedral layers, which is very suitable for the insertion and extraction of lithium ions.

The radius of Ni2+ (0.069 nm) is close to that of Li+ (0.076 nm). Ni2+ can easily enter the middle wafer to occupy the 3a position of Li+, while Li+ enters the main wafer to occupy the 3b position, causing mixed cations (see Figure 2 below), resulting in a unit cell The parameter increases. The radius of Ni2+ in the Li layer is smaller than Li+, which will reduce the thickness of the inter-chip and oxidize to Ni3+ or Ni4+ during charging, causing the local collapse of the inter-chip space, increasing the difficulty of Li+ ion insertion during discharge and reducing the material's reversible capacity.

However, when Li+ enters the transition metal layer, the thickness of the main wafer is enlarged, and it is difficult to deintercalation, which deteriorates the electrochemical performance of the material. Therefore, the smaller the thickness of the inter-wafer, the more difficult it is for Li+ to re-embed. The degree of ion mixing can be characterized by the value of c/a and I(003)/I(104). When c/a>4.9 and I(003)/I(104)>1.2, the degree of mixing is low.

In layman's terms, cobalt (Co) can make the deintercalation of lithium ions easier, improve the conductivity of the material and improve the discharge cycle performance, but too high Co content will lead to higher costs and low-cost performance; nickel (Ni) can improve the material, However, if its content is too high, the cycle performance of the material will deteriorate; manganese (Mn) can improve the safety and stability of the material, and if the content is too high, it will reduce the material capacity.

Advantage:

With the strong structural support of manganese (the structure of the ternary material is not easy to collapse), combined with the increase in the energy of the cathode material, the ternary material has more electricity than lithium iron phosphate in the case of the same volume.

In addition, another outstanding advantage of the ternary material is its low-temperature performance. Objectively speaking, the low-temperature performance of the ternary material is highlighted due to the poor performance of lithium iron phosphate. Because the polarity of lithium iron phosphate PO4 is too strong, the binding ability to Li is large, and the diffusion coefficient is low. However, ternary materials do not have this problem, so in a low-temperature environment, charging and discharging are less affected.

Disadvantages:

Of course, ternary materials also have their own shortcomings. The three elements themselves are not resistant to high temperatures and will release oxygen molecules in extreme cases. At the same time, their own cycle life is also different from that of lithium iron phosphate. It can be seen that ternary is not universal. eat. Thermal stability is indeed a pain point of ternary materials. The element structure makes it less bound to oxygen. This requires special attention to this weakness in the acquired battery design, just like the bumper of a vehicle.

Summary:

The factor of "subsidy decline" makes new energy car companies have to cut costs and make profits, which also brings the lithium iron phosphate battery back to everyone's field of vision. The use of lithium iron phosphate batteries is not a technical retrogression, because the relationship between it and the ternary lithium battery is like self-priming and a turbo engine. There is no distinction between "who is good and who is bad", but the application scenarios are different.

In the future, lithium iron phosphate batteries and ternary lithium batteries will produce a watershed-based on the positioning of the vehicle. In order to weigh the two important indicators of cruising range and price, lithium iron phosphate will gradually revive in low-end products. But on the contrary, mid-to-high-end products must take into account more usage scenarios and performance, and ternary lithium batteries will still be the mainstream power battery technology.