lithium-ion battery. During charging, lithium ions are extracted from the cathode material's crystal lattice and enter the anode material; the reverse occurs during discharging. The reversible capacity and voltage plateau of the cathode material during charging and discharging largely determine the energy density of the lithium-ion battery. Furthermore, because the cathode material contains metals such as lithium, cobalt, and nickel, it constitutes the most significant component of the cost of a lithium-ion battery.
Developing cathode materials with high energy density, high output voltage, long service life, and ease of fabrication is of great significance. An ideal cathode material should meet the following basic conditions.
(1) Possesses a high redox potential, ensuring a high output voltage for the battery.
(2) Can accommodate as many lithium ions as possible, ensuring a high battery capacity.
(3) During the insertion and extraction of lithium ions, the cathode material can maintain its structural stability, thus ensuring a long cycle life for the electrode.
(4) Possesses excellent electronic and ion conductivity, effectively reducing energy loss caused by polarization effects, thereby ensuring the battery's rapid charge and discharge capabilities.
(5) The battery's operating voltage range should be within the electrochemical stability range of the electrolyte, thereby minimizing unnecessary chemical reactions between the electrode material and the electrolyte.
(6) Not only should it have low cost and a simple synthesis process, but it should also exhibit high environmental friendliness.
Furthermore, the cathode material should also demonstrate excellent electrochemical and thermal stability.
Existing cathode materials can be mainly divided into three categories based on their crystal structure differences: ① layered structure, such as lithium cobalt oxide (LiCoO2) and ternary materials (LiNiCo, Mni-x-yO2); ② olivine structure, such as lithium iron phosphate (LiFePO4); ③ spinel structure oxides, such as lithium manganese oxide (LiMn2O4) and lithium nickel manganese oxide (LiNi10.5Mn1.5O4). Different types of cathodes have different energy densities, electrochemical characteristics, and costs, ultimately making them suitable for different fields and application scenarios. Layered structure cathode materials refer to cathode materials with a layered microcrystalline structure, mainly including lithium cobalt oxide, lithium nickel cobalt manganese oxide, and lithium-rich manganese oxide. Among them, lithium cobalt oxide and lithium nickel cobalt manganese oxide are currently the most widely used cathode materials for lithium-ion batteries in digital electronic products and power lithium-ion batteries. They are characterized by high energy density, excellent cycle performance, and good overall performance, but the high proportion of metals such as nickel, cobalt, and manganese leads to higher costs.
Lithium cobalt oxide (LiCoO2) was discovered by American scientist and Nobel laureate in Chemistry, J.B. Goodenough, and first marketed by Sony Corporation of Japan in the 1990s. Even today, lithium cobalt oxide remains one of the cathode materials with the highest volumetric energy density. For this reason, it is widely used in digital pouch cell products that require high volumetric energy density, such as mobile phones, smartwatches, and Bluetooth headsets.
Lithium cobalt oxide (LiCoO2), as one of the earliest commercially available cathode materials, possesses a volumetric energy density unmatched by other cathode materials. Electrodes prepared from LiCoO2 can achieve a compaction density exceeding 4.2 g/cm², and a specific capacity of 185 mA·h/g at high voltage (>4.45V). Furthermore, LiCoO2 exhibits relatively superior electronic and ionic conductivity, power efficiency, and fast-charging characteristics, meeting the requirements of current consumer electronics batteries and thus having a wide range of applications. Based on these properties, LiCoO2 remains one of the best cathode materials to date.
The main synthesis methods for lithium cobalt oxide include high-temperature solid-state synthesis, sol-gel synthesis, and low-temperature coprecipitation. The high-temperature solid-state synthesis involves mixing lithium salts and cobalt-containing oxides or hydroxides in a specific stoichiometric ratio, then calcining the mixture at a suitable temperature for a certain time, followed by cooling, pulverizing, and sieving to obtain the sample. Although the high-temperature solid-state synthesis method is widely used in industrial production, it is time-consuming, requires high synthesis temperatures, and produces large, unevenly homogeneous powders with significant stoichiometric deviations, resulting in a substantial increase in cost.
In 1997, Goodenough et al. first proposed lithium iron phosphate (LiFePO4) as a cathode material for lithium-ion batteries.
Due to its low cost, stable structure, and high safety, this material has gradually become one of the preferred cathode materials for lithium-ion batteries in electric buses and energy storage systems.
Lithium iron phosphate (LiFePO4) shares a similar crystal structure and crystal system with iron phosphate (FePO4). This means that the material experiences minimal volume change during lithium-ion insertion/extraction, effectively preventing lattice damage caused by volume expansion or contraction. Furthermore, this characteristic ensures good electrical contact between the particles and conductive additives, resulting in excellent cycle stability and a long lifespan. In addition, lithium iron phosphate is renowned for its environmental friendliness, cost-effectiveness, excellent safety, high specific capacity (approximately 170 mA·h/g), and stable charge/discharge platform. Given these advantages, lithium iron phosphate is considered an ideal choice for cathode materials in large-scale energy storage applications.
The methods include sol-gel processes, coprecipitation techniques, and hydrothermal synthesis. Specifically, hydrothermal synthesis directly generates the target product in an autoclave by increasing temperature and pressure, using readily available iron, lithium, and phosphorus compounds as raw materials. This method is known for its simple operation, small and uniform particle size, and low energy consumption. However, it has limitations for industrial production, primarily due to the need for specially designed pressure-resistant containers. Coprecipitation, on the other hand, is conducted in a solution system, where the precursor morphology is affected by various factors such as concentration, temperature control, pH adjustment, and stirring rate. Given the decisive role these parameters play in the performance of the final sintered LiFePO material, careful selection of experimental conditions is crucial. Products prepared by this method not only possess excellent microstructure characteristics (i.e., small and uniform particle size) but also exhibit superior electrochemical properties; however, it is worth noting that the entire operation process is relatively complex, and filtration challenges and waste management issues may arise during processing.
Lithium manganese oxide and lithium-rich manganese-based cathode materials
In the research of lithium-ion battery cathode materials, another important and commercially available cathode material is the spinel-structured lithium manganese oxide (LiMn₂O₄) cathode material proposed by Thackeray et al. in 1983. Spinel-structured lithium manganese oxide belongs to the cubic crystal system. Its typical chemical composition is LiMn₂O₄. In the LiMn₂O₄ crystal structure, oxygen is in a face-centered cubic close-packed structure, while manganese and oxygen form an octahedral structure, as shown in the figure below.
Manganese is abundant in nature, and the preparation techniques for spinel-type lithium manganese oxide (LiMn2O4) exhibit diverse characteristics. The synthesis route and processing technology of the material directly affect the microstructure and grain development of the final product. Therefore, optimizing these synthesis processes is crucial for improving the electrochemical performance of electrode materials in practical applications. Currently, industry and academia widely employ two main types of methods to prepare LiMn2O4: one is based on the interaction between solid raw materials, such as high-temperature solid-state reactions, microwave-assisted synthesis, and impregnation treatment in molten salt media.
Another category involves chemical transformation in a liquid environment, with typical examples including sol-gel technology, hydrothermal synthesis, and coprecipitation techniques. LiMnzO4 has attracted widespread attention due to its price advantage, excellent thermal stability, strong overcharge resistance, and good environmental benefits. However, this material has shortcomings in cycling and storage performance, especially at high temperatures, where its cycling performance deteriorates significantly, leading to irreversible capacity loss.
lithium-rich manganese-based
Besides lithium manganese oxide, layered lithium-rich manganese-based materials have attracted widespread attention as an emerging cathode material for lithium-ion batteries.
Preparation methods for lithium-rich manganese-based cathode materials include solid-state methods, sol-gel methods, and co-precipitation methods. The solid-state method involves directly mixing metal oxides and metal carbonates or metal hydroxides in a certain proportion, followed by a high-temperature solid-state reaction to obtain layered lithium-rich materials. The advantages of the solid-state method are its ability to synthesize large quantities of layered lithium-rich materials, its relatively simple preparation method, and its low cost. The disadvantages are the poor diffusion coefficient of the solid during solid-state sintering, and the fact that different transition metals have different diffusion rates in the solid-state reaction, making it difficult for particles to diffuse sufficiently. Therefore, the uniformity of the synthesized material is poor, which affects the performance of the cathode material. The sol-gel method involves first adding a transition metal salt solution to an integrator to form a sol, then evaporating the water to make it a gel, and finally drying and calcining it to obtain layered lithium-rich materials. This method yields materials with uniform distribution and high purity, and the electrodes produced exhibit good electrochemical performance. However, its drawbacks include a long fabrication cycle, the need for numerous integrators (organic acids or ethylene glycol), resulting in high costs. Furthermore, the produced layered lithium-rich materials are mostly fine nano/micron particles with low actual density. Therefore, this method is currently primarily used in laboratory settings for fabricating layered lithium-rich materials and is difficult to commercialize.
High-nickel cathode materials
Researchers have long sought high-temperature stability and excellent rate performance as the primary goals when developing cathode
materials for lithium-ion batteries. Among the three major materials - LiCoO₂, LiNi₁ₓ₋ᵧCoₓMnᵧO₂ (NCM), and LiFePO₄ - NCM is considered one of the most promising cathode materials due to its relatively high specific capacity, relatively low raw material cost, superior safety compared to LiCoO₂, and better environmental friendliness and cost advantages over traditional materials.
This type of material has the same -NaFeO₂-type layered crystal structure and belongs to the R-3m space group. This concept was first proposed by Liu et al. in 1999. It cleverly combines the advantages of three cathode materials - lithium cobalt oxide (LiCoO₂), lithium nickel oxide (LiNiO₂), and lithium manganese oxide (LiMnO₂) - and effectively compensates for the shortcomings present in each individual material (as shown in Figure 5-6). By adjusting the ratio of the transition metal elements, the optimal balance among specific capacity, cycle performance, safety, and cost can be further achieved.
The crystal structure of lithium nickel cobalt manganese oxide (NCM) ternary cathode material is basically the same as that of LiCoO2, both belonging to the hexagonal layered structure.
