High-voltage cathode materials operating above 4.5 V versus Li/Li+ are critical for advancing lithium-ion battery technology, enabling higher energy densities essential for applications such as electric vehicles and grid storage. Among the most promising candidates are lithium nickel manganese oxide (LNMO), high-nickel layered oxides (NMC/NCA), and disordered rocksalt materials. These materials offer substantial improvements in capacity and voltage but face significant challenges, including electrolyte decomposition, transition-metal dissolution, and structural instability. Addressing these issues requires advanced strategies such as surface coatings and tailored electrolyte formulations.

LNMO, with its spinel structure, delivers a high operating voltage of around 4.7 V and a theoretical capacity of 147 mAh/g. Its advantages include low cost, excellent thermal stability, and fast lithium-ion diffusion. However, the high voltage accelerates electrolyte oxidation, leading to rapid capacity fade and gas generation. Transition-metal dissolution, particularly manganese, further degrades performance by destabilizing the solid-electrolyte interphase (SEI) and corroding the anode. Surface modifications using coatings like Al2O3, ZnO, or LiAlO2 have proven effective in suppressing side reactions. These coatings act as physical barriers, reducing direct contact between the cathode and electrolyte while also stabilizing the interface against oxidative decomposition.

High-nickel layered oxides, such as NMC811 (LiNi0.8Mn0.1Co0.1O2), achieve high capacities (>200 mAh/g) but require charging to voltages above 4.5 V to maximize energy output. At these potentials, oxygen loss from the lattice and nickel migration to lithium sites contribute to structural degradation. Cation mixing increases impedance and reduces cyclability. Surface doping with elements like Al or Mg enhances structural integrity by suppressing phase transitions. Additionally, conformal coatings of lithium phosphate or lithium borate improve interfacial stability by scavenging hydrofluoric acid (HF) and preventing transition-metal dissolution.

Disordered rocksalt cathodes represent an emerging class of high-voltage materials that utilize lithium excess to enable high capacities (>300 mAh/g) without relying on traditional layered or spinel structures. Their disordered nature allows for diverse redox-active species, including manganese and vanadium, operating at high voltages. However, oxygen redox activity in these materials can lead to irreversible oxygen release and voltage hysteresis. Strategies such as fluorine substitution and transition-metal redox tuning mitigate these effects by stabilizing the anion framework. Surface passivation with lithium niobate or lithium titanate further enhances cycling stability by preventing surface degradation.

Electrolyte decomposition remains a major challenge for all high-voltage cathodes. Conventional carbonate-based electrolytes oxidize at potentials above 4.3 V, forming resistive layers that increase polarization. Advanced electrolyte formulations incorporate additives like lithium difluorophosphate (LiDFP) and fluoroethylene carbonate (FEC) to form stable CEI (cathode-electrolyte interphase) layers. These additives preferentially oxidize to create protective films that limit further electrolyte breakdown. New salts such as lithium bis(fluorosulfonyl)imide (LiFSI) also improve oxidation stability, though their compatibility with aluminum current collectors requires careful optimization.

Transition-metal dissolution is another critical issue, particularly for manganese- and nickel-based cathodes. Dissolved metal ions migrate to the anode, disrupting SEI formation and accelerating capacity loss. Chelating agents like ethylenediaminetetraacetic acid (EDTA) derivatives can sequester these ions in the electrolyte, though their long-term stability needs further study. Alternatively, electrolyte additives that form stable complexes with transition metals, such as lithium bis(oxalato)borate (LiBOB), reduce dissolution rates without compromising ionic conductivity.

Mechanical degradation due to repeated volume changes during cycling also affects high-voltage cathodes. Particle cracking exposes fresh surfaces to electrolyte attack, exacerbating side reactions. Morphological engineering, such as designing single-crystal or core-shell structures, minimizes strain-induced fractures. For example, single-crystal NMC particles exhibit superior mechanical stability compared to polycrystalline counterparts, leading to longer cycle life.

Thermal stability is another consideration, especially for high-nickel and high-voltage systems. Exothermic reactions at elevated temperatures can trigger thermal runaway, posing safety risks. Doping with thermally stable elements like titanium or coating with thermally conductive materials such as carbon nanotubes improves heat dissipation and reduces reactivity. In-situ polymerization of electrolytes also enhances thermal stability by forming a robust matrix that resists decomposition.

Recent research has explored hybrid approaches combining multiple stabilization strategies. For instance, dual-layer coatings incorporating an inner conductive layer (e.g., carbon) and an outer protective layer (e.g., Al2O3) improve both electronic conductivity and interfacial stability. Similarly, multi-component electrolyte systems with synergistic additives address both oxidation and dissolution challenges simultaneously. These approaches highlight the importance of holistic design in developing commercially viable high-voltage cathodes.

Despite significant progress, challenges remain in scaling up production while maintaining performance. Precise control over coating uniformity and doping distribution is critical for reproducibility. Advanced characterization techniques, such as in-situ X-ray diffraction and electron microscopy, provide insights into degradation mechanisms and guide material optimization. Computational modeling also plays a key role in predicting stable compositions and interfaces, accelerating the discovery of novel materials.

In summary, high-voltage cathode materials offer a pathway to higher energy densities but require careful engineering to overcome stability challenges. Surface coatings, electrolyte additives, and structural modifications are essential for mitigating degradation and enabling long-term cycling. Continued innovation in material design and processing will be crucial for realizing the full potential of these systems in next-generation batteries.