lithium-ion batteries must be able to meet the demands of high-energy-consuming devices, such as pure electric vehicles, plug-in hybrid electric vehicles, and stationary energy storage systems. For novel anode materials under development, capacity is one of the key performance indicators. Based on different reaction mechanisms, some candidate materials with high theoretical capacities include silicon (Si), germanium (Ge), silicon monoxide (SiO), tin (Sn), and its oxide (SnOz), with typical capacities ranging from 783 mA·g (for SnOz) to 4211 mA·h/g (for Si). Although these alloy materials exhibit higher specific capacity advantages compared to traditional graphite (372 mA·bg) and lithium titanate (LTO, 175 mA·Ng), the volume changes and initial irreversible capacity loss they undergo during charge and discharge limit their lifespan. To overcome these problems, researchers have explored various strategies, such as reducing particle size to the nanoscale and attempting to construct composite material systems containing active or inactive metallic lithium components. Among these methods, combining active lithium gold with alloy materials to form a conductive buffer substrate has shown potential in improving cycle performance. Furthermore, employing different morphologies of nanostructures, such as nanowires or nanotubes, has also proven to be an effective approach to achieving ideal anode materials that combine high capacity, good rate performance, and long cycle life.
Si--Silicon-based anode materials
Silicon-based anode materials are mainly composed of pure silicon, silicon oxide, and silicon/carbon composites. Due to their high theoretical capacity, environmental friendliness, and abundant natural reserves, they are widely considered the ideal choice for next-generation high-energy-density lithium-ion battery anodes. Chinese scientists were the first in the world to propose the concept of applying nanoscale silicon to lithium-ion batteries. Given China's abundant silicon resources and world-leading production capacity of elemental silicon, increasing research and development efforts on silicon-based anode materials and their application in lithium-ion batteries is of great significance for mastering the key technologies of future high-performance lithium-ion batteries.
Compared to traditional graphite anode materials, silicon exhibits a higher theoretical specific capacity (4211 mA·h/g) and a relatively lower delithiation potential (0.5V). Notably, silicon's operating voltage is slightly higher than that of graphite. Figure 5-9 illustrates the specific atomic arrangement within a silicon crystal. During charging, using silicon as the anode can reduce surface lithium plating, thereby improving battery safety. Furthermore, silicon is abundant and inexpensive. However, applying silicon to lithium-ion battery anodes also presents some challenges. As a semiconductor material, silicon has poor conductivity. After multiple charge-discharge cycles, the significant volume changes caused by lithium-ion insertion and release can lead to material breakage, affecting structural stability and potentially causing separation of the active material from the current collector, severely impacting the battery's cycle life. Furthermore, this volume expansion also hinders the formation of a stable and effective solid-state electrolyte interface (SED) film on the silicon surface. Uniformly dispersing pure silicon or its compounds within a carbon matrix can alleviate these problems to some extent: on the one hand, it improves the overall electronic conductivity of the composite material; on the other hand, the presence of carbon helps alleviate the stress caused by silicon volume changes, reducing damage to the electrode structure; simultaneously, carbon can promote the stable formation of the SEI film. Therefore, composite materials combining the advantages of silicon and carbon are considered one of the ideal anode candidates for next-generation high-energy-density lithium-ion batteries.
Besides silicon, silicon monoxide (SiO) is also considered a candidate anode material for lithium-ion batteries due to its theoretical capacity exceeding 1600 mA·h/g. Furthermore, lithium-oxygen coordination implies smaller volume changes and lower activation energies during charge and discharge. Potential electrochemical reactions during this process include the conversion of SiO to Si and LiO, followed by the formation of a silicon-lithium alloy with Li; or the direct formation of a silicon-lithium alloy and LixSiO2. It is noteworthy that pure solid SiO is thermodynamically unstable at any temperature and can therefore decompose into Si and SiO2 under specific conditions through a disproportionation reaction. Similar to silicon, SiO undergoes significant volume expansion or contraction during lithium insertion and extraction. In addition, SiO has poor conductivity, resulting in slow lithium-ion ingress and egress rates. To address these issues, enhance reversible capacity, and improve cycle stability, researchers have explored various strategies. Among these, carbon coating technology, electrochemical reduction of lithium in SiO, and reducing SiO particle size are considered particularly effective approaches. In particular, when combined with smaller particles and carbon coatings, the diffusion path of lithium ions can be effectively shortened, while improving electron and ion conduction efficiency, thereby overcoming the aforementioned challenges.
Germanium has attracted significant attention in lithium-ion battery anode material research due to its high lithium storage capacity (1623 mA·h/g) at the Liz2Ge5 stoichiometric ratio and its reversible lithium insertion and extraction process. Although germanium is more expensive than silicon and has a slightly lower capacity, it possesses significant advantages, such as conductivity 10,000 times that of silicon and a band gap of only 0.67 eV. Studies have shown that the diffusion rate of lithium ions in germanium is 15 times faster than in silicon at 360℃ and 400 times faster at room temperature. These properties give germanium excellent high-current discharge performance and higher charge transport efficiency. This high-power performance is particularly important for applications requiring high-performance power output, such as electric vehicles. However, similar to silicon, germanium also faces the problem of volume expansion of up to 300%, which has become an obstacle to its practical application in lithium-ion batteries. By employing nanostructure designs such as nanoparticles, nanowires, or nanotubes, the negative impact of volume changes can be effectively mitigated, thereby improving coulombic efficiency. It is worth noting that the preparation of germanium nanoparticle-conductive substrate composites using simple methods such as solid-state pyrolysis can further optimize the electrochemical performance of the electrodes.
SnO2
Tin dioxide (SnO2), initially developed by Fujifilm, has attracted widespread attention as a negative electrode material for lithium-ion batteries due to its high theoretical capacity and low operating voltage (approximately 0.6 eV, relative to LiLi). In the electrochemical reaction process, it first undergoes a partially irreversible step, where SnO2 is reduced to metallic tin (Sn) and lithium oxide (LiO); subsequently, a reversible phase occurs, involving the formation and decomposition of the tin-lithium alloy. Theoretically, each mole of SnO2 can react with 8.4 mol of lithium, corresponding to a theoretical capacity of 1491 mA·h/g. However, given the low reversibility of the initial reduction reaction, in practical applications, only the effective capacity contributed by the subsequent alloying/dealloying process-approximately 783 mA·h/g-is typically considered, and this value is used as the practical theoretical capacity of SnO2 material. Furthermore, during charge-discharge cycles, this material undergoes significant volume expansion (over 200%), leading to severe capacity loss. To this end, researchers are dedicated to improving the cycling stability of SnO2 and reducing irreversible capacity loss due to volume changes through various methods.