Lithium-sulfur (Li-S) batteries are a promising next-generation energy storage technology due to their high theoretical energy density and cost-effectiveness. However, the use of lithium metal anodes presents significant challenges that hinder their practical implementation. The primary issues include dendrite formation, interfacial instability, and volume changes during cycling. Addressing these challenges is critical to improving the safety, longevity, and performance of Li-S batteries.

One of the most pressing problems with lithium metal anodes is dendrite formation. During repeated charge-discharge cycles, lithium ions deposit unevenly on the anode surface, leading to the growth of needle-like dendrites. These dendrites can penetrate the separator, causing internal short circuits, thermal runaway, and battery failure. Additionally, dendritic growth consumes active lithium and electrolyte, reducing Coulombic efficiency and cycle life. The high reactivity of lithium metal exacerbates these issues, as it readily reacts with electrolyte components, forming unstable solid-electrolyte interphase (SEI) layers.

Interfacial instability is another major challenge. The native SEI layer on lithium metal is mechanically fragile and chemically heterogeneous, leading to continuous breakdown and reformation during cycling. This process depletes the electrolyte and active lithium, increasing impedance and accelerating capacity fade. Furthermore, the large volume changes of lithium metal during plating and stripping disrupt the SEI layer, exposing fresh lithium to the electrolyte and perpetuating side reactions.

To mitigate these challenges, researchers have developed several protective strategies focusing on the anode side. One approach involves creating artificial SEI layers to stabilize the lithium surface. These engineered interfaces are designed to be mechanically robust, ionically conductive, and chemically stable. For example, inorganic coatings such as lithium fluoride (LiF) or lithium nitride (Li3N) can enhance SEI stability and suppress dendrite growth. Polymer-based artificial SEI layers, such as polycarbonate or poly(ethylene oxide), provide flexibility to accommodate volume changes while maintaining ionic transport. Hybrid organic-inorganic coatings combine the benefits of both materials, offering superior protection against electrolyte decomposition and lithium corrosion.

Another strategy is the use of alloy anodes, where lithium is combined with other metals to form intermetallic compounds. Alloys like lithium-aluminum (Li-Al), lithium-magnesium (Li-Mg), or lithium-silicon (Li-Si) exhibit reduced dendritic growth due to their lower electrochemical potential and improved mechanical properties. These alloys also mitigate volume changes by providing a stable host matrix for lithium deposition. However, challenges remain in achieving uniform alloy formation and maintaining long-term cycling stability without significant capacity loss.

Three-dimensional (3D) host structures have emerged as a promising solution to address lithium anode challenges. These conductive frameworks, made of materials like carbon, copper, or nickel, provide a high-surface-area scaffold for lithium deposition. The porous structure distributes current density more evenly, reducing localized lithium plating and dendrite formation. Additionally, 3D hosts accommodate volume expansion, minimizing mechanical stress on the SEI layer. For instance, carbon-based hosts with lithiophilic coatings, such as zinc oxide (ZnO) or silver (Ag), guide uniform lithium nucleation and growth. Metal foam hosts offer excellent conductivity and structural integrity but require careful optimization to prevent excessive weight and cost.

Despite these advancements, several obstacles remain in implementing protective strategies for lithium metal anodes. Artificial SEI layers must balance thickness and ionic conductivity to avoid impeding lithium-ion transport. Alloy anodes often suffer from slow kinetics and phase separation during cycling, limiting their practical application. 3D host structures face scalability challenges, as their fabrication processes can be complex and expensive. Moreover, integrating these solutions into large-format batteries without compromising energy density remains an ongoing area of research.

In conclusion, lithium metal anodes in Li-S batteries face significant hurdles due to dendrite formation, interfacial instability, and volume changes. Protective strategies such as artificial SEI layers, alloy anodes, and 3D host structures offer promising pathways to overcome these challenges. However, further research is needed to optimize these approaches for commercial viability, ensuring safe and durable high-energy-density batteries. The development of robust lithium metal anodes will be a critical step toward realizing the full potential of Li-S battery technology.