Lithium metal anodes possess extremely high theoretical specific capacity (3860 mA·bg) and the lowest electrochemical potential (-3.040 V (vs. SHE)), making them considered the "holy grail" electrode among many electrode materials. Lithium metal batteries include lithium-sulfur and lithium-oxygen batteries. Lithium-sulfur batteries have an energy density of approximately 2600 W·h/kg, while lithium-oxygen batteries have an energy density of approximately 3500 W·h/kg, roughly 7 and 10 times that of conventional lithium-ion batteries, respectively. Therefore, lithium metal batteries are considered one of the most promising energy storage systems and a top candidate for next-generation battery systems, attracting considerable attention. However, due to the lithium dendrite problem, early lithium metal batteries could only be applied in certain specialized fields, and their commercialization has been delayed.
Rechargeable lithium metal batteries were invented as early as the 1970s and were widely used in watches, calculators, and other electronic devices.
Lithium metal batteries are widely used in electrical appliances and portable medical devices. However, their commercialization has been hampered by certain defects in lithium metal. As a member of Group 1 of the periodic table, lithium atoms have only one electron in their outermost shell, making them highly chemically reactive as they readily lose this electron. When in contact with an organic electrolyte, lithium metal forms a film called the solid electrolyte interface (SEI) on its surface. The main function of this film is to isolate the lithium metal from the electrolyte, preventing further corrosion of the lithium. However, due to the significant volume change of lithium metal during charging and discharging, the SEI film frequently ruptures. The exposed fresh lithium metal surface reacts again with the electrolyte to form a new SEI film. This process not only promotes the growth of lithium dendrites along the cracks but may also penetrate the separator inside the battery, causing a short circuit. When a short circuit occurs, a large amount of heat is generated inside the battery, which in extreme cases may lead to combustion or explosion, seriously affecting the safety performance and marketability of lithium metal batteries. Furthermore, as the number of lithium dendrites increases, they provide more opportunities for the negative electrode to come into contact with the electrolyte, thereby accelerating the rate of side reactions. These irreversible processes consume electrode materials and electrolytes, reducing the battery's energy density and coulombic efficiency. After prolonged use, many lithium dendrites become encased in the newly formed SEI film, unable to participate in normal electrochemical reactions; simultaneously, lithium dendrites near the substrate rapidly decompose, causing "dead" lithium, meaning this portion of lithium becomes electrochemically inactive, significantly weakening overall battery performance. Over the past 40 years, significant progress has been made in the research and simulation of lithium dendrite formation mechanisms.
One of the most common strategies to suppress dendrite growth is to enhance the stability and consistency of the solid electrolyte interface (SEI) layer on the lithium metal surface by adjusting the electrolyte composition and adding specific substances. However, since lithium metal is thermodynamically unstable in organic additives, forming an effective passivation layer on its surface in a liquid electrolyte environment is quite challenging. Besides optimizing the SEI layer, introducing polymers or solid barrier layers with high mechanical strength can also be an effective means of preventing dendrite penetration into the separator. These methods aim to prevent lithium dendrite damage to the separator by improving the mechanical properties of the SEI layer or the separator itself, but they do not fundamentally eliminate the problem of dendrite formation. While a complete overcoming of this challenge is still some time away, and lithium metal anode-based battery products are not yet widely available on the market, researchers have theoretically proposed several conceptual lithium metal battery designs, demonstrating the potential for practical applications. Among these, lithium-sulfur batteries using sulfur as the cathode material and lithium-oxygen batteries using oxygen as the cathode active material have attracted considerable attention due to their unique advantages and are considered two highly commercially promising all-cell systems. Lithium-sulfur batteries possess extremely high energy density (approximately 2600 W·kg) and are widely recognized as promising candidates for next-generation battery energy storage systems. More importantly, elemental sulfur is abundant in nature and environmentally friendly, further highlighting the advantages of lithium-sulfur batteries. Therefore, lithium-sulfur batteries have received worldwide attention in recent years.
Polysulfide intermediates generated during the charging and discharging of lithium-sulfur batteries dissolve in the electrolyte and shuttle to the negative electrode.Therefore, the suppression of lithium dendrites becomes more complex in the presence of polysulfide intermediates, especially when the sulfur cathode loading is high. Polysulfides can penetrate the SEI film and corrode the fresh lithium metal beneath the surface layer, leading to capacity loss. Therefore, preventing polysulfide shuttle is necessary not only for improving cathode capacity during lithium-sulfur battery operation but also for SEI film stability and obtaining a dendrite-free negative electrode. Through continuous efforts, many methods have been developed, including positive limiting domain and adsorption, electrolyte modification, and separator design. However, these methods seem to focus more on suppressing polysulfide shuttle and improving the utilization rate of the sulfur cathode, without directly suppressing dendrite growth in the lithium metal anode. The performance of lithium-sulfur batteries depends on the protection of the lithium metal anode. The synergistic effect of various dendrite growth suppression methods can accelerate the practical application of lithium-sulfur batteries.
Lithium-oxygen batteries are a type of battery that uses oxygen from the air as the positive electrode; they are sometimes called lithium-air batteries. The theoretical energy density of lithium-oxygen batteries is as high as 3500 Wh/kg, far exceeding that of commercial lithium-ion batteries. Therefore, lithium-oxygen batteries have become a revolutionary advancement in the field of energy storage, attracting worldwide attention and being considered a strong contender in next-generation energy storage systems.
Similar to polysulfide intermediates, oxygen cross-linking from the positive electrode to the lithium metal negative electrode in lithium-oxygen batteries can lead to gradual degradation of the lithium metal surface, resulting in electrolyte decomposition and the formation of LiOH and LiCO3 during charging. Therefore, several strategies have been developed to suppress oxygen cross-linking. Besides the positive electrode problem, lithium depletion caused by dendrite growth and damage to the passivation film severely hinder the use of lithium metal in rechargeable lithium-oxygen batteries. The aforementioned strategies for suppressing lithium dendrite growth are also applicable to lithium-oxygen batteries. Through electrolyte additives, separator modification, and negative electrode design, the performance of lithium batteries can be significantly improved.