Nickel is an important component in redox energy storage, which is used in cathode materials for lithium-ion batteries. How to effectively improve the specific capacity of materials by increasing the nickel content is one of the current research hotspots.
1. High-nickel ternary materials
Generally speaking, high-nickel ternary cathode materials refer to materials in which the mole fraction of nickel is greater than 0.6. Such ternary materials have the characteristics of high specific capacity and low cost, but they also have defects such as low capacity retention and poor thermal stability.
Improvements in the fabrication process can effectively enhance material properties. The micro/nano size and morphology of the particles largely determine the performance of high-nickel ternary cathode materials. Therefore, a current important fabrication method involves uniformly dispersing different raw materials and obtaining nanospheres with large specific surface areas through various growth mechanisms.
Among the many preparation methods, the combination of coprecipitation and high-temperature solid-state method is the current mainstream method. First, coprecipitation is used to obtain a precursor with uniform raw material mixing and uniform particle size. Then, it is calcined at high temperature to obtain a ternary material with regular surface morphology and easy process control. This is an important method in current industrial production.
Spray drying is simpler and faster than co-precipitation, and the resulting material morphology is comparable, showing potential for further research. The drawbacks of high-nickel ternary cathode materials, such as cation mixing and phase transitions during charge and discharge, can be effectively improved through doping and coating modifications. Improving conductivity, cycle performance, rate performance, storage performance, and high-temperature, high-pressure performance while suppressing side reactions and stabilizing the structure will remain a research hotspot.
2. Lithium-rich ternary materials
This material has the characteristic of high voltage, and the first charge and discharge mechanism is different from the subsequent charge: the first charge will cause structural changes, which are reflected in the charging curve with two different plateaus with 4.4V as the boundary. During the second charge, its charging curve is different from the first curve. This is because Li2O is irreversibly extracted from the layered structure of Li2MnO3 during the first charge, and the plateau at around 4.5V disappears.
Lithium-rich ternary cathode materials with different structures can be prepared by solid-state method, sol-gel method, hydrothermal method, spray pyrolysis method and co-precipitation method. Among them, co-precipitation method is more commonly used, and each method has its own advantages and disadvantages.
Lithium-rich ternary materials have shown promising application prospects and are one of the key materials required for the next generation of high-capacity lithium-ion batteries, but large-scale application remains a challenge.
The key areas for future research on this material are as follows:
(1) Insufficient understanding of the lithium insertion/extraction mechanism makes it impossible to explain phenomena such as low coulombic efficiency and large differences in material properties;
(2) Research on doping elements is insufficient and limited;
(3) Poor cycle stability is caused by the erosion of the positive electrode material by the electrolyte under high voltage;
(4) Commercial applications are limited, and the evaluation of safety performance is not comprehensive enough. 3. Single-crystal ternary cathode materials
Under high voltage, as the number of cycles increases, secondary particles or agglomerated single crystals may experience pulverization of the primary particle interface or separation of agglomerated single crystals in the later stages of lithium-ion battery ternary materials, resulting in increased internal resistance, rapid capacity decay, and poor cycle performance.
Single-crystal high-voltage ternary materials can improve lithium-ion transfer efficiency while reducing side reactions between the material and the electrolyte, thereby improving the cycling performance of the material under high voltage. First, a ternary material precursor was prepared using a co-precipitation method, and then single-crystal LiNi0.5Co0.2Mn0.3O2 was obtained under high-temperature solid-state conditions.
This material has a good layered structure. At 3–4.4V, the discharge specific capacity of the coin cell can reach 186.7mAh/g at 0.1V. After 1300 cycles of the full cell, the discharge specific capacity is still 98% of the initial discharge capacity. It is a ternary cathode composite material with excellent electrochemical performance.
The cathode material production line is the first in the world to mass-produce micron-sized single-crystal modified spinel lithium manganese oxide and nickel-cobalt-manganese lithium ternary cathode materials, achieving an annual production capacity of 500 tons.
4. Graphene doping
Graphene has a stable two-dimensional structure with a single-atom thickness and an electrical conductivity of up to 1×10⁶ S/m. Graphene offers the following advantages for use in lithium-ion batteries: ① Excellent electrical and thermal conductivity, which helps improve the battery's rate performance and safety; ② Compared to graphite, graphene has more lithium storage space, which can increase the battery's energy density; ③ Its micro- and nano-scale particle size results in a short lithium-ion diffusion path, which is beneficial for improving the battery's power performance.
5 High-voltage electrolyte
Ternary materials have attracted increasing attention and research due to their high voltage window. However, high-voltage cathode materials have not yet been industrialized because of the low electrochemical stability window of currently commercially available carbonate-based electrolytes.
When the battery voltage reaches approximately 4.5V (vs. Li/Li+), the electrolyte begins to undergo severe oxidative decomposition, preventing the lithium insertion/extraction reaction from proceeding normally. Developing and applying novel high-voltage electrolyte systems or high-voltage film-forming additives to improve the stability of the electrode/electrolyte interface is an effective approach for developing high-voltage electrolytes. In energy storage systems, ionic liquids, dinitrile organic compounds, and sulfone organic solvents are currently the primary electrolytes for high-voltage ternary materials. Ionic liquids, with their low melting point, non-flammability, low vapor pressure, and high ionic conductivity, exhibit excellent electrochemical stability and have been extensively studied.
Replacing all or part of the commonly used carbonate solvents with novel solvents possessing high-pressure stability can indeed effectively improve the oxidation stability of the electrolyte. Furthermore, most of these novel organic solvents have advantages such as low flammability, which promises to fundamentally improve the safety performance of lithium-ion batteries. However, most of these novel solvents also exhibit poor reduction stability and high viscosity, leading to reduced cycle stability of the battery's negative electrode material and decreased rate performance.
In high-voltage electrolytes, film-forming additives are also an essential component, and common ones include tetraphenylphosphine, LiBOB, lithium difluorodioxane borate, tetramethoxytitanium, succinyl anhydride, and trimethoxyphosphine.
Adding a small amount (<5%) of film-forming additives to carbonate-based electrolytes allows them to preferentially undergo oxidation/reduction decomposition reactions before solvent molecules, forming an effective protective film on the electrode surface and inhibiting subsequent decomposition of the carbonate-based solvent. The film formed by high-performance additives can even inhibit the dissolution of metal ions in the positive electrode material and their deposition on the negative electrode, thereby significantly improving the stability of the electrode/electrolyte interface and the cycle performance of the battery.
6. Surfactant-assisted synthesis
The performance of ternary cathode materials depends on the preparation method. The co-precipitation method is used to prepare ternary cathode materials by using surfactants, ultrasonic vibration and mechanical stirring in synergy. Finally, the prepared sheet-like precursor is annealed with lithium carbonate at high temperature to grow into a ternary layered structure. This is a novel synthesis process for ternary cathode materials.
It was found that using OA and PVP as surfactants can prepare hexagonal nanosheet cathode material precursors with excellent morphology, and the resulting nanosheets have a relatively uniform particle size distribution with a size of about 400 nm. The surfactants have a good shape control effect on the precursors. The assembled battery has an initial discharge specific capacity of 157.093 mAh˙g⁻¹ at a discharge rate of 1C. After 50 cycles at discharge rates of 1C, 2C, 5C and 10C, the capacity retention rate is greater than 92%, demonstrating good electrochemical performance.
7 Microwave Synthesis Method
Among the important methods for preparing ternary cathode materials, solid-state methods, co-precipitation methods, and sol-gel methods all require high-temperature sintering for several hours, which consumes a lot of energy and involves complex preparation processes. Microwave heating, on the other hand, is a volumetric heating method caused by dielectric loss in the material within an electromagnetic field. It offers rapid and uniform heating, and the synthesized materials often possess superior structure and properties, making it a very promising approach for synthesizing cathode materials.
The structure, microstructure, and electrochemical performance of the synthesized material were characterized using XRD, SEM, and charge-discharge methods. Experimental results show that the cathode material synthesized in microwaves with an output power of 1300 W exhibits an initial discharge specific capacity of 185.2 mAh/g and a coulombic efficiency of 84% under 0.2 C charge-discharge conditions. After 30 cycles, it retains 92.3% of its capacity (2.8–4.3 V), demonstrating excellent electrochemical performance and application potential. 8. Infrared Synthesis Method
When an object is heated by infrared radiation, if the wavelength of the emitted infrared radiation matches the absorption wavelength of the heated object, the heated object absorbs the infrared radiation, and the molecules and atoms inside the object resonate, resulting in strong vibration and rotation. This vibration and rotation raise the temperature of the object, thus achieving the purpose of heating.
This heating principle can be used to prepare ternary cathode materials. HSIEH uses a novel infrared heating calcination technology to prepare ternary materials. First, nickel-cobalt-manganese-lithium acetate is mixed evenly with water, then a glucose solution of a certain concentration is added. The powder obtained by vacuum drying is calcined at 350℃ for 1 hour in an infrared chamber, and then calcined at 900℃ under a nitrogen atmosphere for 3 hours. This one-step process yields carbon-coated 333-type ternary cathode material. Within a voltage range of 2.8–4.5V, after 50 discharge cycles at 1C, the capacity retention rate is as high as 94%, the first discharge specific capacity reaches 170mAh/g, and 75mAh/g at 5C. However, the high-rate performance needs further improvement.
When preparing ternary cathode materials using the traditional high-temperature calcination method, the required synthesis temperature is high, the calcination time is long, and the energy loss is large.
Research has found that in a low-temperature plasma environment, the reactants exhibit high chemical activity and rapid reaction rates, enabling the rapid preparation of ternary cathode materials. A homogeneous mixture of nickel-cobalt-manganese oxides and lithium carbonate is placed in a plasma generator and reacted at 600°C for 20–60 minutes under oxygen-bearing conditions to obtain the ternary cathode material Li(Ni1/3Co1/3Mn1/3)O2.
The prepared cathode material has a high initial discharge specific capacity of 218.9 mAh·g⁻¹. At the same time, its cycle stability, rate capability and high temperature performance are also superior to those of materials prepared by traditional methods.
10. Preparation of ternary cathode materials from waste batteries
The cost of cathode materials in lithium-ion batteries accounts for 30%-40%. Therefore, by recycling the cathode materials of used batteries and using the manufacturing process to restore the energy storage performance of the cathode materials, the cost of lithium-ion batteries can be greatly reduced. Moreover, a complete lithium-ion battery industry chain should include the recycling and reuse of lithium-ion batteries.
GEM Co., Ltd. invested 100 million yuan to build my country's largest waste battery and end-of-life battery material processing production line, which recycles more than 4,000 tons of cobalt resources annually, accounting for more than 30% of my country's strategic cobalt resource supply. This has formed GEM's unique recycling route, from waste batteries to new batteries.
The entire production line uses nickel, cobalt, and manganese recycled from waste batteries to form a solution, adds a synthetic agent, and goes through a series of processes to become nickel-cobalt-manganese ternary lithium-ion battery cathode material. Since it went into production in October 2014, it has achieved an output value of nearly 200 million yuan, and it is expected to achieve an output value of 500 to 600 million yuan in the future.
