In recent years, lithium iron phosphate and ternary technology route dispute has never stopped, this paper combines the characteristics of the two anode materials and batteries, their applications in different areas of comparative analysis.
1. Lithium iron phosphate materials and batteries
The three-dimensional spatial mesh olivine structure of LiFePO4 forms a one-dimensional Li+ transport channel, which restricts the diffusion of Li+; at the same time, the octahedral FeO6 is connected to the common top, which makes its electronic conductivity lower and polarization larger in the case of a large multiplicity discharge. In order to solve the LiFePO4 material lower lithium ion diffusion and electronic conductivity, the current technology is mainly improved by means of nanosizing, carbon coating, doping, etc. LiFePO4 material charging and discharging process is mainly in the LiFePO4 and FePO4 two phases of the transition between each other, the volume of the rate of change is small, so the material is extremely stable, and thus the safety and stability of lithium iron phosphate materials and batteries is unquestionable.
Figure 1: Structure of lithium iron phosphate material
Lithium iron phosphate battery has the following characteristics:
(1) Lithium iron phosphate batteries have excellent cycling performance, energy-based battery cycle life can be as long as 3000 to 4000 times, multiplier-type battery cycle even up to tens of thousands of times;
(2) lithium iron phosphate battery has excellent safety performance, even at high temperatures can still maintain a more stable structure, making lithium iron phosphate batteries safe and reliable, and even in the battery deformation damage will not appear smoke, fire and other safety accidents.
On the other hand, lithium iron phosphate raw material resources are more abundant, greatly reducing the use of materials and battery costs, and because of the iron and phosphorus elements of environmental friendliness, lithium iron phosphate materials and batteries on the environment without pollution. However, the structural characteristics of LiFePO4 material determines that the material has a low ionic and electronic conductivity, and as the temperature decreases, the electron transfer impedance and charge migration impedance are rapidly increasing, resulting in poor performance of its battery at low temperatures.
2. Ternary materials and batteries
Li(NixCoyMn1-x-y )O2 materials have attracted great attention from researchers since they were first reported. In order to reduce the cost pressure brought by the price increase of Co, domestic and foreign research has been carried out on the low-Co or even Co-free ternary materials, and such materials may become the mainstream cathode materials in the future.
Li(NixCoyMn1-x-y )O2 and LiCoO2 have similar structures. Take the NCM111-type ternary material as an example, in which Li+ is located at position 3a in the structure, Ni, Mn, and Co are randomly distributed at the position 3b, and lattice oxygen occupies position 6c. The transition metal layer structure consists of Ni, Mn, Co, and is surrounded by six lattice oxygens to form an MO6 (M=Ni, Co, or Mn) octahedral structure, while lithium ions are embedded between the MO6 layers. During the charging and discharging process, lithium ions are de-embedded in the MO6 interlayer structure, and the electric pairs involved in the electrochemical reaction are Ni2+/Ni3+, Ni3+/Ni4+, and Co3+/Co4+, respectively, whereas the Mn element is electrochemically inert and does not contribute to the electrochemical capacity.
According to the proportion of Ni content, ternary materials and batteries can be categorized into conventional and high-Ni types. With the increase of Ni content, the de-embeddable lithium increases, and the material capacity and battery energy density increase, so the high-Ni type ternary materials and batteries are the hotspot of the current research and full of challenges.
First of all, due to the Ni2+ radius and Li+ radius is very close to each other, with the increase of Ni content, high nickel ternary materials in the high temperature sintering preparation of Li/Ni mixing probability increased dramatically, and into the MO6 layer of lithium de-embedding is more difficult to impede the Li + transport capacity, resulting in a reduction in the specific capacity and cycling performance is reduced and is difficult to reverse.
Secondly, with the increase of Ni content, the proportion of Ni3+ in the material also increases, and Ni3+ is very unstable, and it is very easy to react with moisture and CO2 in the air to generate surface residual alkali, leading to the loss of capacity and cycling performance of ternary materials. In addition, too much surface residual alkali will make the ternary battery gas production serious, affecting its cycle performance and safety performance.
Thirdly, the high-valent Ni element also has high catalytic activity and oxidizing property, which leads to the decomposition of the electrolyte and also causes gas generation in the battery. In order to solve the above problems, precursor customization, sintering process personalization, ion doping, surface coating modification, wet processing and production environment control has become a common choice for ternary material manufacturers.
For ternary batteries, their performance characteristics mainly include higher material mass specific capacity, mass and volume specific energy, better multiplication performance and low temperature performance, but due to the stability of the structure, the scarcity of nickel and cobalt resources, etc., they have better cycling performance, general safety performance, and higher cost.
3. Comparative analysis of two materials and batteries
3.1 Energy density
Compared with lithium iron phosphate materials, the discharge specific capacity of ternary materials is higher, and the average voltage is also higher, so 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 materials, smaller particles and carbon coating, the compacted density of the pole piece is about 2.3 to 2.4 g/cm3, while the compacted density of the ternary pole piece can reach 3.3 to 3.5 g/cm3, so the volumetric specific energy of the ternary materials and batteries is also much higher than that of lithium iron phosphate.
3.2 Safety
From the safety point of view, the main structure of lithium iron phosphate material PO4, its bond energy is much higher than the ternary material MO6 octahedral M-O bond energy, the full state of lithium iron phosphate material thermal decomposition temperature of 700 ℃ or so, while the corresponding ternary material thermal decomposition temperature of 200 ~ 300 ℃, so the lithium iron phosphate material is more secure. Comparison from the battery point of view, lithium iron phosphate battery can pass all the safety tests, while the ternary battery pinprick and overcharging and other tests can not be easily passed, need to be improved from the structural components and battery design end.
3.3 Power performance
The activation energy of Li+ of LiFePO4 material is only 0.3~0.5 eV, resulting in its Li+ diffusion coefficient in the order of 10-15~10-12 cm2/s. The Li+ diffusion coefficient is also in the order of 0.5 eV. The very low electronic conductivity and Li+ diffusion coefficient result in poor LFP power performance. In contrast, the Li+ diffusion coefficient of the ternary material is about 10-12 to 10-10 cm2/s and the electronic conductivity is high, so the ternary battery has better power performance.
3.4 Temperature applicability
Influenced by the lower electronic conductivity and ionic conductivity of lithium iron phosphate materials, resulting in poor low temperature performance of lithium iron phosphate batteries. Lithium iron phosphate battery -20 ℃ discharge compared to room temperature, the capacity retention rate is only about 60%, while the same system of ternary batteries can reach more than 70%.
3.5 Cost and environmental factors
Ternary materials contain Ni, Co and other scarce metals, its cost is higher than lithium iron phosphate. With the improvement of materials and battery technology level, the cost of ternary and lithium iron phosphate batteries have dropped significantly, and the current market price of ternary batteries is higher than lithium iron phosphate batteries. At the same time, compared with the environmentally friendly Fe, P elements, ternary materials and batteries in the Ni, Co elements of greater environmental pollution. Combined with the above factors, the environmental control and waste recycling of ternary materials and batteries is more urgent.
