Cathode materials in solid-state batteries face unique compatibility challenges with solid electrolytes that differ significantly from liquid electrolyte systems. The interfacial stability between cathodes and solid electrolytes determines performance metrics such as cycle life, energy density, and safety. Among the most studied cathodes for solid-state systems are lithium nickel manganese cobalt oxide (NMC), lithium cobalt oxide (LCO), and sulfur-based cathodes, each presenting distinct interfacial phenomena when paired with solid electrolytes such as oxides, sulfides, or polymers.

The primary degradation mechanisms at the cathode-solid electrolyte interface include space charge layer formation, chemical reactivity, and mechanical detachment. Space charge layers arise due to the difference in electrochemical potential between the cathode and solid electrolyte, leading to lithium-ion depletion or accumulation at the interface. For example, with sulfide solid electrolytes like Li6PS5Cl, the high electronic conductivity of NMC creates a lithium-depleted space charge layer, increasing interfacial resistance. Experimental studies show this can lead to a 50% increase in impedance after 100 cycles in NMC-Li6PS5Cl systems. LCO interfaces with oxide solid electrolytes like LLZO exhibit similar space charge effects, though to a lesser extent due to closer chemical compatibility.

Chemical reactions between cathode materials and solid electrolytes are another critical challenge. Sulfide electrolytes react with high-voltage cathodes like NMC and LCO, forming resistive interphases. X-ray photoelectron spectroscopy studies reveal the formation of Li2S and transition metal sulfides at NMC-sulfide interfaces above 3.8 V vs Li+/Li. LCO reacts with LLZO at elevated temperatures, producing LaCoO3 and Li2CO3, which impede lithium-ion transport. Sulfur cathodes present unique challenges due to polysulfide migration into sulfide solid electrolytes, causing electronic shorts and electrolyte decomposition. This is evidenced by a 30% capacity fade within 50 cycles in Li-S cells with Li3PS4 electrolytes.

Mechanical instability at the interface results from volume changes during cycling. NMC particles expand by 5-7% during delithiation, causing contact loss with rigid solid electrolytes. LCO exhibits less strain but still suffers from microcrack formation at grain boundaries. Sulfur cathodes undergo 80% volume change during conversion reactions, leading to catastrophic interface detachment unless constrained by composite designs.

Buffer coatings have emerged as a primary solution to mitigate interfacial degradation. Lithium niobate (LiNbO3) coatings on NMC particles demonstrate significant improvement in interfacial stability. Studies show 2-5 nm LiNbO3 layers reduce interfacial resistance by 70% in NMC-LLZO systems, maintaining 90% capacity retention after 200 cycles. The coating acts as a lithium-ion conductor while blocking electron transfer, preventing space charge layer formation. For LCO, Al2O3 coatings have proven effective, reducing reaction products at the interface and maintaining 95% capacity retention at 4.3 V vs Li+/Li. Sulfur cathodes benefit from conductive polymer coatings like PEDOT:PSS, which confine polysulfides while accommodating volume changes.

Alternative approaches include composite cathode designs that integrate solid electrolytes into the cathode layer. NMC mixed with Li3PS4 in a 70:30 ratio shows improved interfacial contact, delivering 150 mAh/g at 0.5C with stable cycling. For LCO, the addition of Li3BO3 as a sintering aid improves contact with garnet electrolytes, achieving area-specific resistances below 25 Ω·cm². Sulfur cathodes with Li6PS5Cl as both electrolyte and active material matrix demonstrate enhanced stability, though energy density remains limited by the low conductivity of sulfur.

The thermal stability of cathode-solid electrolyte interfaces presents another consideration. Differential scanning calorimetry measurements show NMC-sulfide interfaces begin exothermic reactions at 200°C, while LCO-LLZO remains stable up to 300°C. Sulfur cathodes with sulfide electrolytes exhibit thermal runaway risks at lower temperatures (150°C) due to polysulfide reactions. These thermal properties dictate battery safety protocols and operating temperature windows.

Recent advances in interface engineering focus on atomic layer deposition (ALD) of ultrathin coatings. Al2O3 ALD layers as thin as 1 nm on NMC particles show promise in preventing side reactions while minimizing lithium-ion diffusion barriers. For LCO, TiO2 ALD coatings demonstrate superior high-voltage stability compared to conventional coatings. Sulfur cathodes benefit from LiF ALD layers that chemically stabilize polysulfides.

The choice of solid electrolyte significantly impacts cathode compatibility. Sulfide electrolytes offer better interfacial contact but higher reactivity, while oxide electrolytes provide chemical stability but require high sintering temperatures. Polymer electrolytes enable flexible interfaces but suffer from low ionic conductivity. Hybrid systems combining multiple electrolyte types attempt to balance these tradeoffs.

Electrochemical performance metrics reveal the impact of interface engineering. NMC622 with LiNbO3 coatings in oxide-based solid-state cells achieves 180 mAh/g at 0.1C with 1.5% capacity loss per cycle. LCO in sulfide systems with Al2O3 coatings reaches 140 mAh/g at 0.2C with 99% Coulombic efficiency. Sulfur cathodes with optimized interfaces demonstrate 800 mAh/g based on sulfur mass, though cycle life remains below 100 cycles in most reports.

Future development directions include in situ interface formation techniques that create stable interphases during initial cycling. Another approach involves graded interface designs where composition gradually transitions from cathode to electrolyte material. Computational modeling plays an increasing role in predicting stable interface compositions and identifying new coating materials.

The compatibility between cathode materials and solid electrolytes remains a defining challenge for solid-state battery commercialization. While buffer coatings and interface engineering have shown progress, fundamental limitations in ionic transport across interfaces and long-term chemical stability require continued research. The performance gap between liquid and solid electrolyte systems continues to narrow, but substantial improvements in interface design are needed to realize the full potential of solid-state batteries.