Interfacial reactions between high-nickel cathodes and conventional liquid electrolytes are critical to understanding the degradation mechanisms and performance limitations of advanced lithium-ion batteries. High-nickel layered oxides, such as LiNi0.8Mn0.1Co0.1O2 (NMC811) or LiNi0.8Co0.15Al0.05O2 (NCA), offer high energy density but face challenges due to their reactivity with electrolytes. These reactions lead to the formation of a cathode-electrolyte interphase (CEI), which influences cycle life, thermal stability, and rate capability.

The CEI layer forms during initial cycling as a result of electrochemical oxidation of the electrolyte at the cathode surface. Unlike the solid-electrolyte interphase (SEI) on anodes, the CEI is thinner and less understood. Its composition and stability depend on the cathode material, electrolyte formulation, and operating conditions. In high-nickel cathodes, the highly oxidative environment accelerates electrolyte decomposition, leading to CEI growth and subsequent impedance rise.

Electrolyte oxidation generates species such as lithium alkyl carbonates, polycarbonates, LiF, and lithium oxides, which deposit on the cathode surface. These products originate from reactions involving lithium salts (e.g., LiPF6) and organic solvents like ethylene carbonate (EC) and dimethyl carbonate (DMC). LiPF6 decomposition contributes to LiF formation, while solvent oxidation produces polymeric and organic compounds. The CEI’s inorganic components, such as LiF and LixPOyFz, improve ionic conductivity but may also increase interfacial resistance if excessively thick.

High-nickel cathodes exhibit higher surface reactivity due to the instability of Ni4+ states during charging. Ni4+ promotes nucleophilic attacks on electrolyte molecules, accelerating decomposition. Residual lithium compounds (e.g., Li2CO3 and LiOH) on the cathode surface further exacerbate reactions by reacting with LiPF6 to form CO2 and HF. HF etches the cathode surface, leading to transition metal dissolution, particularly nickel. Dissolved Ni2+ migrates to the anode, degrading the SEI and causing capacity fade.

The CEI’s dynamic nature means its composition evolves with cycling. Early cycles produce organic-rich layers, while prolonged cycling leads to thicker, inorganic-dominated films. This growth increases charge transfer resistance and reduces lithium-ion diffusion kinetics. Elevated temperatures exacerbate these reactions, accelerating CEI thickening and transition metal dissolution.

Electrolyte additives are employed to mitigate interfacial degradation. Compounds like vinylene carbonate (VC) and fluoroethylene carbonate (FEC) preferentially oxidize to form stable CEI layers that suppress further electrolyte breakdown. Additives containing boron or phosphorus form protective surface films that reduce transition metal dissolution. However, additive effectiveness depends on their concentration and compatibility with other electrolyte components.

The role of water and acidic impurities cannot be overlooked. Trace H2O reacts with LiPF6 to form HF, which corrodes the cathode surface. Strict moisture control during cell assembly is necessary to minimize these side reactions. Electrolyte formulations with hydrolytically stable salts, such as LiFSI or LiTFSI, are explored but face challenges due to their corrosion of aluminum current collectors.

Operando characterization techniques, such as X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR), reveal CEI composition and evolution. Electrochemical impedance spectroscopy (EIS) tracks interfacial resistance changes, while transmission electron microscopy (TEM) provides nanoscale insights into CEI morphology. These tools help correlate interfacial chemistry with electrochemical performance.

The impact of cycling conditions on CEI formation is significant. Higher upper cutoff voltages (>4.3 V vs. Li/Li+) intensify electrolyte oxidation, while elevated temperatures accelerate parasitic reactions. Conversely, moderate voltages and temperatures promote thinner, more stable CEI layers. Fast charging exacerbates inhomogeneous CEI formation due to localized current densities, leading to uneven degradation.

Strategies to stabilize the interface include cathode surface coatings and electrolyte engineering. Al2O3, TiO2, or Li2ZrO3 coatings act as physical barriers between the cathode and electrolyte, reducing direct contact and suppressing side reactions. These coatings must be thin and ionically conductive to avoid hindering lithium-ion transport. Electrolyte engineering focuses on high-voltage stable solvents (e.g., sulfones or nitriles) and robust lithium salts to widen the electrochemical window.

In summary, interfacial reactions between high-nickel cathodes and conventional electrolytes are complex and multifaceted. CEI formation, transition metal dissolution, and electrolyte decomposition collectively degrade performance. Advances in electrolyte additives, surface modifications, and operational strategies are essential to harness the full potential of high-nickel cathodes while ensuring long-term stability. Understanding these interfacial processes enables the design of more durable high-energy-density batteries.