Conversion-type anode materials mainly include metal oxides, phosphides, sulfides, and nitrides. In electrochemical processes, these materials promote the formation or decomposition of lithium compounds through the reduction or oxidation reactions of the metals. Because they can participate in multi-electron redox processes, anodes based on these materials exhibit reversible capacities as high as 1000 mA·bg.
FeOₓ
Due to their low cost, relatively low toxicity, abundant natural reserves, and especially high theoretical specific capacity, iron oxide materials have been extensively studied as anode materials for lithium-ion batteries. Common iron oxide compounds include α-Fe₂O₃, γ-Fe₂O₃, and Fe₃O₄. These compounds exhibit theoretical specific capacities of approximately 1007 mA·h/g and 926 mA·h/g, respectively. However, iron oxide faces many problems during practical application. The slow kinetics of electron/ion transport and severe volume expansion/contraction during repeated charge–discharge processes lead to rapid capacity decay and poor rate performance of iron oxide electrodes. In addition, bulk iron oxide materials have inherently low electrical conductivity. To address these issues, researchers have mainly adopted strategies such as morphology and structure control, carbon coating, and the construction of composite materials with highly conductive substrates. These approaches often achieve synergistic effects through a combination of multiple strategies, and some progress has been made.
CoOₓ
Cobalt oxides (CoOₓ), such as Co₃O₄ and CoO, have also been widely studied as anode materials for lithium-ion batteries due to their high theoretical specific capacities. Like iron oxides, CoOₓ suffers from the same challenges: large volume changes during the charge–discharge process, poor intrinsic electrical conductivity, and slow reaction kinetics, resulting in rapid capacity decay and poor cycling stability. Guan et al. synthesized single-phase eight-sided Co₃O₄ nanodisks using oxygen as the reaction precursor. These nanodisks had a particle size of 100–200 nm and delivered a reversible specific capacity of approximately 474 mA·h/g when cycled at high current density. This result indicates that morphology and particle size have a significant impact on the electrochemical performance of CoOₓ. Wang et al. prepared Co₃O₄ nanoneedles grown directly on a titanium substrate using a hydrothermal method. These nanoneedles not only exhibited excellent electrical contact with the current collector but also effectively buffered volume expansion. After 30 cycles at 0.2C, they still maintained a high reversible capacity of 1015 mA·h/g.
For CoOₓ composite systems composed of two or more components, the synergistic effect between the components can further improve overall electrochemical performance. For example, combining cobalt oxide with highly conductive carbon-based materials or other metal oxides can significantly enhance rate performance and cycling stability. This has led to increasing attention toward the design and development of composite systems in this field.
ZnO
Zinc oxide has also attracted extensive attention as an anode material for lithium-ion batteries due to its relatively high theoretical specific capacity, low cost, ease of preparation, and diverse morphologies. ZnO reacts with lithium through a combined mechanism of alloying (forming the Li–Zn alloy) and conversion (forming Li₂O). Its theoretical specific capacity can reach 978 mA·h/g, which is significantly higher than that of graphite anodes. However, zinc oxide suffers from poor electrical conductivity, severe volume expansion/contraction during repeated charge–discharge cycles, and the generation of large amounts of inactive Li₂O during cycling. These factors lead to rapid capacity decay, poor rate performance, and short cycle life of ZnO electrodes. To address these issues, researchers have mainly adopted strategies such as morphology and structure control, carbon coating, doping with heteroatoms, and constructing ZnO-based composites with highly conductive substrates. These methods often achieve better lithium storage performance by combining multiple modification strategies, and some metal zincate compounds also exhibit excellent electrochemical performance.
4. MPₓ
Metal phosphides have also attracted widespread attention in the application of anode materials for lithium-ion batteries in recent years. These compounds generally react with lithium through a conversion mechanism and often have very high theoretical specific capacities due to the multi-electron transfer reactions per formula unit. However, they generally suffer from large volume expansion during lithiation/delithiation, leading to pulverization and loss of electrical contact between active particles and the current collector, which severely limits their practical application.
Among them, iron, cobalt, nickel, and copper-based phosphides have been intensively studied in recent years. Taking iron phosphides as an example, their theoretical specific capacities can reach 500–1800 mA·h/g. Additionally, metal phosphides generally exhibit higher lithium storage voltages (usually 0.5–1 V vs. Li⁺/Li) than metal oxides and metal sulfides, which helps reduce the risk of lithium dendrite formation during fast charging. Moreover, metal phosphides generally exhibit higher electrical conductivity than the corresponding metal oxides, which is beneficial for improving rate performance. Therefore, the rational design of metal phosphide nanostructures and their composites with carbon-based materials has become an important research direction in this field. Examples include Ni₂P, NiP₂, NiP₃, Ni₅P₄, CoP, Co₂P, CoP₃, FeP, FeP₂, Cu₃P, etc. These compounds have all demonstrated excellent lithium storage performance in research, showing great potential for practical application. Ni₂P and Li–Ni–P ternary compounds can even achieve ultrafast lithium-ion intercalation/deintercalation reactions due to their unique structures and high electrical conductivity.