Lithium-metal batteries represent a significant advancement in energy storage technology due to their potential for high energy density, enabled by the use of a lithium-metal anode. However, the choice of cathode material plays a critical role in determining the overall performance, safety, and longevity of these batteries. Among the most promising cathode materials for lithium-metal batteries are high-nickel NMC (LiNi_xMn_yCo_zO₂), sulfur-based cathodes, and conversion-type cathodes. Each of these materials presents unique advantages and challenges, particularly concerning voltage compatibility, interfacial stability, and capacity retention. Recent research has focused on mitigating these challenges through advanced cathode coatings and the development of dual-phase electrolytes.

High-nickel NMC cathodes, such as LiNi₀.₈Mn₀.₁Co₀.₁O₂ (NMC811), are widely studied due to their high specific capacity and relatively high operating voltage, which can exceed 4.3 V versus Li/Li⁺. These cathodes offer a balance between energy density and cost, making them attractive for next-generation lithium-metal batteries. However, high-nickel NMC materials suffer from several drawbacks when paired with a lithium-metal anode. One major issue is interfacial instability, where the high operating voltage accelerates electrolyte decomposition, leading to the formation of a thick cathode-electrolyte interphase (CEI). This CEI increases impedance and reduces cycle life. Additionally, nickel-rich cathodes are prone to structural degradation, including cation mixing and microcrack formation, which further exacerbate capacity fade. To address these challenges, researchers have explored surface coatings such as Al₂O₃, LiNbO₃, and LiAlO₂, which act as protective barriers against electrolyte decomposition and transition metal dissolution. These coatings have been shown to improve capacity retention by up to 20% after 200 cycles in some studies. Another approach involves the use of dual-phase electrolytes, where a solid electrolyte interphase (SEI)-stabilizing additive is incorporated into the liquid electrolyte to simultaneously protect both the lithium anode and the high-voltage cathode.

Sulfur cathodes, while primarily associated with lithium-sulfur batteries, also hold promise for lithium-metal systems when optimized differently. Unlike traditional lithium-sulfur configurations, sulfur cathodes in lithium-metal batteries can leverage alternative designs that minimize polysulfide shuttling while maintaining high capacity. The key challenge lies in achieving stable cycling at moderate voltages, as sulfur operates at around 2.1 V versus Li/Li⁺, which is significantly lower than high-nickel NMC. This voltage mismatch can lead to uneven lithium deposition and increased dendrite formation. Recent work has focused on sulfur-carbon composites with tailored porosity to confine polysulfides and enhance conductivity. Additionally, coatings such as conductive polymers and metal oxides have been employed to improve interfacial stability. While sulfur cathodes offer theoretical capacities as high as 1675 mAh/g, practical systems typically achieve 800–1200 mAh/g due to incomplete utilization and side reactions. Advances in cathode architecture, such as gradient sulfur loading and binder-free designs, have shown promise in pushing these values closer to theoretical limits.

Conversion-type cathodes, including transition metal oxides (e.g., Fe₂O₃, Co₃O₄) and fluorides (e.g., FeF₃, CuF₂), offer another pathway for high-energy lithium-metal batteries. These materials undergo multi-electron redox reactions, enabling capacities that often exceed 500 mAh/g. However, conversion reactions are typically associated with large volume changes, sluggish kinetics, and significant voltage hysteresis, which pose challenges for practical implementation. For instance, FeF₃ exhibits a theoretical capacity of 712 mAh/g but suffers from poor electronic conductivity and irreversible phase transitions during cycling. Recent strategies to mitigate these issues include nanostructuring to reduce diffusion paths and incorporating conductive matrices such as graphene or carbon nanotubes. Additionally, hybrid cathodes combining conversion materials with intercalation-type compounds have been explored to balance energy density with cycling stability. Coatings such as LiF and carbon have also been applied to conversion cathodes to improve interfacial stability and reduce side reactions with the electrolyte.

A critical consideration for all these cathode materials is their compatibility with lithium-metal anodes, particularly regarding voltage profiles and interfacial reactions. High-nickel NMC cathodes require electrolytes that are stable at high voltages, while sulfur and conversion-type cathodes need electrolytes that can suppress polysulfide dissolution or accommodate large volume changes, respectively. Dual-phase electrolytes, which combine liquid electrolytes with solid-state components, have emerged as a promising solution. These systems often incorporate additives such as LiNO₃ or LiFSI to stabilize the SEI on the lithium anode while also forming a protective CEI on the cathode. For example, electrolytes with localized high-concentration salts have demonstrated improved stability with high-nickel NMC cathodes, enabling cycle life extensions of over 300 cycles with minimal capacity fade. Similarly, gel polymer electrolytes have been used with sulfur cathodes to physically block polysulfide migration while maintaining ionic conductivity.

Capacity retention remains a persistent challenge for lithium-metal batteries, regardless of cathode chemistry. High-nickel NMC cathodes typically exhibit capacity retention of 80–90% after 200 cycles when optimized, while sulfur and conversion-type cathodes often show faster degradation, with retention rates dropping below 70% within the same cycle range. The development of advanced diagnostic tools, such as in-situ X-ray diffraction and electron microscopy, has provided deeper insights into degradation mechanisms, guiding the design of more robust cathode materials. For instance, operando studies have revealed that nickel-rich NMC cathodes experience lattice oxygen loss at high voltages, which can be mitigated by doping with elements like Al or Mg. Similarly, sulfur cathodes benefit from real-time monitoring of polysulfide speciation to optimize host materials and electrolyte formulations.

Recent breakthroughs in cathode coatings have further enhanced the performance of lithium-metal batteries. Atomic layer deposition (ALD) has been employed to apply ultra-thin oxide coatings on high-nickel NMC particles, improving interfacial stability without compromising ionic transport. For sulfur cathodes, coatings such as TiO₂ and MoS₂ have been shown to catalyze sulfur redox reactions while trapping polysulfides. Conversion-type cathodes have benefited from conformal carbon coatings applied via chemical vapor deposition, which enhance electronic conductivity and buffer volume changes. These coatings are often combined with electrolyte additives to create synergistic effects, such as reduced CEI growth and improved lithium deposition uniformity.

In summary, the development of cathode materials for lithium-metal batteries requires a multifaceted approach that addresses voltage compatibility, interfacial stability, and capacity retention. High-nickel NMC, sulfur, and conversion-type cathodes each offer unique advantages but also present distinct challenges that must be overcome through material engineering and electrolyte design. Recent advancements in cathode coatings and dual-phase electrolytes have demonstrated significant improvements in cycle life and energy density, bringing lithium-metal batteries closer to commercial viability. Continued research into degradation mechanisms and interfacial engineering will be essential to unlock the full potential of these systems.