Positive Electrode Materials:
Currently, there are three major trends in ternary positive electrode materials: single crystallization, high voltageization, and high nickelization. The development of single crystallization is primarily aimed at improving the battery’s cycle life, while high voltageization and high nickelization are focused on enhancing energy density.
- In the direction of single crystallization, notable positive electrode material companies include Zhuhai Zhenhua New Materials, among others.
- For the high voltage direction, companies like Xiamen Tungsten New Energy and Changyuan Lithium Science are leading the way.
- In the high nickel direction, companies such as Contemporary Amperex Technology and CATL are prominent players.
- These various technological directions often intersect, such as high nickel single crystallization, where companies like Contemporary Amperex Technology are at the forefront.
1. High Nickelization: Advantages
The third major trend in ternary positive electrode materials is high nickelization. The primary objective of high nickelization is to enhance energy density.
From an electronic structure perspective, cobalt (Co) has empty eg orbitals, and the t2g orbitals have significant overlap with oxygen (O) 2p orbitals. This can lead to oxygen evolution and structural collapse during deep lithium extraction. Additionally, the π bonds formed between cobalt’s t2g orbitals and oxygen’s 2p orbitals are relatively weak, making electron transfer easier.
In contrast, nickel (Ni) eg orbitals have minimal overlap with oxygen’s 2p orbitals, theoretically allowing electrons on nickel’s eg orbitals to be completely removed. This results in a higher effective capacity for lithium nickelate.
Regarding manganese, when the nickel content surpasses that of manganese, manganese transitions to a stable 4+ oxidation state. Generally, in the NCM (Nickel-Cobalt-Manganese) system, a higher cobalt content leads to better rate performance, higher nickel content results in greater specific capacity, and higher manganese content contributes to structural stability. In typical 8-series ternary materials, specific capacities exceeding 200mAh/g can be achieved.
Compared to mid-to-low nickel materials, ternary high nickel materials have gradually reduced cobalt content. But they can still excel in terms of conductivity and lithium ion diffusion properties.
- Regarding electrical conductivity, NCM523 exhibits a conductivity of 4.9×10-7s/cm, while 811 can achieve 1.7×10-5s/cm.
- As for lithium ion diffusion coefficients, the 5-series can reach levels on the order of 10-10, and the 8-series can achieve levels on the order of 10-8.
2. High Nickelization: Challenges
In comparison to conventional ternary materials, high-nickel ternary materials exhibit more dynamic performance but also bring forth several new challenges in practical applications.
(1) Reduced Thermal Stability: As nickel content increases, the thermal stability of ternary positive electrode materials decreases.
(2) Decreased Cycle Life: Under the same electrolyte formulation, high-nickel ternary materials may experience faster degradation in cycle performance.
(3) Increased Reactivity with Air: High-nickel materials are more prone to react with water and CO2 in the air during preparation and storage, producing LiOH and Li2CO3.
(4) Additional Impacts: This can further lead to increased viscosity of the positive electrode slurry, uneven coating, reactivity with the electrolyte, and an increase in positive electrode resistance, etc.
3. High Nickelization: Manufacturing Process
In high-nickel materials, nickel primarily exists in its trivalent form. However, nickel starts in its divalent form from nickel salts to precursors. In ternary NCM materials, when manganese content is relatively high, Ni3++Mn3+→Ni2++Mn4+ occurs, and most of the nickel exists in its divalent state (LiNi0.5Mn0.5O2).
However, in high-nickel materials, nickel mainly exists in its trivalent state (LiNiO2). Divalent nickel is challenging to oxidize to trivalent in the presence of air, leading to the following impacts:
(1) High-nickel materials require sintering in an oxygen atmosphere.
(2) High-temperature sintering of lithium carbonate results in CO2 production, affecting the oxidation of divalent nickel.
(3) The switch to lithium hydroxide as a lithium source leads to increased equipment corrosion. In high-nickel materials, a higher nickel content correlates with a lower suitable sintering temperature.
The primary preparation process for high-nickel ternary cathodes is identical to that of conventional ternary cathodes: precursor material preparation upstream, followed by mixing, sintering, crushing, and surface treatment.
In the preparation of high-nickel ternary cathodes:
- A stronger alkaline environment is required during the co-precipitation of precursor materials.
- Lithium hydroxide is typically used as the lithium source, with a lower sintering temperature, and an oxygen atmosphere during sintering.
- Post-sintering processing includes water washing and surface coating.
4. High Nickelization: High-Nickel Single Crystals
Similar to the trend of high voltageization, the use of single crystal particles in high-nickel material systems also holds significant potential.
(1) High-Nickel Single Crystals Typically Exhibit Improved Cycling Performance: High-nickel single crystals generally offer better cycling performance.
(2) Enhanced Safety: During overcharging of lithium-ion batteries, oxygen is produced in large quantities at grain boundaries. The use of single crystals can reduce oxygen production, thereby enhancing safety.
5. High Nickelization: High Nickel and High Voltage
Whether in the high-voltage or high-nickel direction, the primary goal is to enhance the specific capacity of the positive electrode materials. If both aspects are combined, theoretically, the specific capacity of the positive electrode materials can reach higher levels.
However, in practical applications, this may present greater challenges. Achieving high nickel and high voltage is even more difficult. In high-nickel ternary materials operating at high voltage, deterioration not only occurs on the surface of the crystal structure but also extends to the interior of the crystals, posing significant challenges.