Lithium iron phosphate materials and batteries
The three-dimensional network olivine structure LiFePO4 forms a one-dimensional Li+ transmission channel, which limits the diffusion of Li+; at the same time, the octahedral FeO6 is connected at the top, making its electronic conductivity The rate is lower, and the polarization is larger when discharging at a high rate.
In order to solve the low lithium ion diffusion and electronic conductivity of LiFePO4 materials, current technology is mainly improved through nanotechnology, carbon coating, doping and other means. The charging and discharging process of LiFePO4 material mainly transforms between LiFePO4 and FePO4 phases. The volume change rate is small, making the material extremely stable. Therefore, there is no doubt about the safety and stability of lithium iron phosphate materials and batteries.
Figure 1 Structural model diagram of lithium iron phosphate material
Lithium iron phosphate battery mainly has the following characteristics:
(1) The cycle performance of lithium iron phosphate batteries is excellent. The cycle life of energy-type batteries can be as long as 3,000 to 4,000 times, and the cycle life of rate-type batteries can even reach tens of thousands of times;
(2) Lithium iron phosphate batteries have excellent safety performance, even in It can still maintain a relatively stable structure under high temperatures, making lithium iron phosphate batteries safe and reliable. Even when the battery is deformed and damaged, there will be no safety accidents such as smoke or fire.
On the other hand, lithium iron phosphate raw material resources are relatively abundant, which greatly reduces the cost of materials and batteries. At the same time, because iron and phosphorus elements are environmentally friendly, lithium iron phosphate materials and batteries do not pollute the environment. However, the structural characteristics of the LiFePO4 material determines that the material has low ionic and electronic conductivity, and as the temperature decreases, both the electron transfer resistance and the charge transfer resistance increase rapidly, resulting in poor low-temperature performance of the battery.
Ternary materials and batteries
Since the Li(NixCoyMn1-x-y)O2 material was first reported, it has attracted great attention from researchers. In order to reduce the cost pressure caused by rising Co prices, research on low-Co or even Co-free ternary materials has been carried out at home and abroad. Such materials may become mainstream cathode materials in the future. The structures of
Li(NixCoyMn1-x-y)O2 and LiCoO2 are similar. Taking the NCM111 ternary material as an example, Li+ is located at position 3a in the structure, Ni, Mn, and Co are randomly distributed at position 3b, and lattice oxygen occupies position 6c. The transition metal layer structure is composed of Ni, Mn, and Co, and is surrounded by 6 lattice oxygens to form an MO6 (M=Ni, Co or Mn) octahedral structure, and lithium ions are embedded between the MO6 layers.
During the charge and discharge process, lithium ions are deintercalated in the MO6 interlayer structure. The electric pairs participating in the electrochemical reaction are Ni2+/Ni3+, Ni3+/Ni4+ and Co3+/Co4+, while the Mn element is electrochemically inert and does not Contribute to electrochemical capacity.
Figure 2 Structural diagram of ternary materials without Li/Ni mixing (a) and with Li/Ni mixing (b)
According to the Ni content ratio, ternary materials and batteries can be divided into conventional types and high nickel types. . As the Ni content increases, the deintercalable lithium increases, and the material capacity and battery energy density increase. Therefore, high-nickel ternary materials and batteries are currently a hot research topic and full of challenges.
First of all, since the radius of Ni2+ is very close to the radius of Li+, as the Ni content increases, the probability of Li/Ni mixed arrangement during high-temperature sintering preparation of high-nickel ternary materials increases sharply, and it is more difficult to deintercalate lithium into the MO6 layer. It hinders Li+ transport ability, resulting in a reduction in specific capacity and cycle performance, which is difficult to reverse.
Secondly, as the Ni content increases, the proportion of Ni3+ in the material also increases, and Ni3+ is very unstable. When exposed to the air, it is very easy to react with moisture in the air and CO2 to generate surface residual alkali, resulting in ternary material capacity and cycle performance loss. In addition, too much surface residual alkali will cause serious gas production of ternary batteries, affecting its cycle performance, safety performance, etc.
Third, the high-priced Ni element also has high catalytic activity and oxidation, which causes the decomposition of electrolyte and also causes the battery to produce gas. In order to solve the above problems, precursor customization, sintering process personalization, ion doping, surface coating modification, wet processing and production environment control have become common choices for ternary material manufacturers.
For ternary batteries, its performance characteristics mainly include higher material mass specific capacity, mass and volume specific energy, better rate performance and low temperature performance. However, due to the stability of the structure and the scarcity of nickel and cobalt resources, its performance characteristics The cycle performance is good, the safety performance is average, and the cost is high.
Comparative analysis of two materials and batteries
Energy density:
If the battery PACK is compared to a human body, then the module is the "heart", responsible for storing and releasing energy to provide power for the car. Compared with lithium iron phosphate materials, ternary materials have higher discharge specific capacity and higher average voltage. Therefore, the mass specific energy of ternary batteries is generally higher than that of lithium iron phosphate.
In addition, due to the low true density of lithium iron phosphate material, small particles and carbon coating, the compacted density of its pole pieces is about 2.3~2.4g/cm3, while the compacted density of ternary pole pieces can reach 3.3~3.5 g/cm3, so the volume specific energy of ternary materials and batteries is much higher than that of lithium iron phosphate.
security:
from a security perspective Speaking of, the main structure of the lithium iron phosphate material is PO4, and its bond energy is much higher than the M-O bond energy of the ternary material MO6 octahedron. The thermal decomposition temperature of the fully charged lithium iron phosphate material is about 700°C, while the corresponding ternary material The thermal decomposition temperature of the material is 200~300℃, so the lithium iron phosphate material is safer.
From a battery perspective, lithium iron phosphate batteries can pass all safety tests, while ternary batteries cannot easily pass tests such as acupuncture and overcharge, and need to be improved from the structural parts and battery design.
Power performance:
The activation energy of Li+ of lithium iron phosphate material is only 0.3~0.5eV, resulting in its Li+ diffusion coefficient in the order of 10-15~10-12cm2/s. The extremely low electronic conductivity and lithium ion diffusion coefficient lead to poor power performance of LFP. The Li+ diffusion coefficient of ternary materials is about 10-12~10-10cm2/s, and the electronic conductivity is high, so ternary batteries have better power performance.
Temperature suitability:
is affected by the low electronic conductivity and ionic conductivity of lithium iron phosphate materials, resulting in poor low-temperature performance of lithium iron phosphate batteries. Compared with the normal temperature discharge of lithium iron phosphate batteries at -20°C, the capacity retention rate is only about 60%, while the ternary battery of the same system can reach more than 70%.
Cost and environmental factors:
Ternary materials contain scarce metals such as Ni and Co, and their costs are higher than lithium iron phosphate. With the improvement of materials and battery technology, the costs of ternary and lithium iron phosphate batteries have dropped significantly. The current market price of ternary batteries is higher than that of lithium iron phosphate batteries. At the same time, compared with environmentally friendly Fe and P elements, Ni and Co elements in ternary materials and batteries cause greater environmental pollution. Combined with the above factors, the need for environmental control and waste recycling of ternary materials and batteries has become more urgent.
Table 1 Comprehensive comparative analysis of lithium iron phosphate materials and ternary materials
As can be seen from Table 1, lithium iron phosphate materials and ternary materials have their own advantages, which also determines the respective application fields of the two materials.
