High-nickel layered oxide cathodes, such as NMC (LiNi_xMn_yCo_zO₂, where x ≥ 0.6) and NCA (LiNi₀.₈Co₀.₁₅Al₀.₀₅O₂), are critical for achieving high energy density in lithium-ion batteries. However, their widespread adoption is hindered by several degradation mechanisms that accelerate capacity fade and impedance growth. The primary degradation pathways include microcracking, surface reconstruction, and transition-metal dissolution, each exacerbated by electrolyte interactions and mechanical stress. Advanced characterization tools like X-ray diffraction (XRD) and scanning electron microscopy (SEM) have been instrumental in understanding these failure modes.

**Microcracking and Mechanical Degradation**
High-nickel cathodes undergo significant volume changes during charge and discharge due to the anisotropic expansion and contraction of the crystal lattice. These repeated phase transitions generate mechanical stress, leading to the formation of microcracks within secondary particles. Microcracking is particularly severe at high states of charge (SOC > 4.3 V vs. Li/Li⁺), where the H2 to H3 phase transition occurs in NMC811, causing a sharp contraction in the c-axis lattice parameter.

SEM studies reveal that microcracks propagate along grain boundaries, fracturing secondary particles into isolated domains. This fragmentation increases electrode polarization by disrupting electronic and ionic conduction pathways. Additionally, fresh crack surfaces expose new cathode material to the electrolyte, accelerating parasitic reactions. XRD analysis of cycled electrodes shows peak broadening, indicative of strain accumulation and loss of crystallinity due to microcracking.

Mechanical stress is further compounded by electrode-level constraints. The calendering process during manufacturing increases particle-to-particle contact but also introduces residual stress. Operando pressure measurements demonstrate that electrode swelling during cycling exerts additional mechanical load, exacerbating particle fracture.

**Surface Reconstruction and Electrolyte Interactions**
The high reactivity of nickel-rich surfaces leads to reconstruction layers, where the near-surface region transforms from a layered structure to a disordered spinel or rock-salt phase. This reconstruction is driven by oxygen loss, particularly at high voltages, and is facilitated by transition-metal migration. Surface-sensitive techniques, such as high-resolution transmission electron microscopy (HRTEM), confirm the presence of a 5–20 nm thick defective layer after prolonged cycling.

Electrolyte oxidation at the cathode surface further aggravates reconstruction. The decomposition of lithium hexafluorophosphate (LiPF₆) and organic carbonates generates acidic species (e.g., HF), which attack the cathode surface. Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) detect lithium fluoride (LiF) and polycarbonate species on degraded cathodes, confirming electrolyte decomposition.

The reconstructed layer acts as a barrier to lithium-ion diffusion, increasing charge-transfer resistance. Electrochemical impedance spectroscopy (EIS) shows a growing semicircle in the mid-frequency range, correlating with surface passivation. Furthermore, the loss of active lithium due to side reactions reduces reversible capacity.

**Transition-Metal Dissolution and Cross-Effects**
Transition-metal dissolution, particularly nickel and manganese, is a critical degradation pathway. Dissolution occurs via proton exchange in the presence of acidic species, where H⁺ ions from electrolyte decomposition displace Li⁺ in the cathode lattice, releasing transition metals into the electrolyte. Inductively coupled plasma (ICP) analysis of aged electrolytes detects elevated concentrations of dissolved nickel and manganese.

Dissolved transition metals migrate to the anode, where they deposit on the solid-electrolyte interphase (SEI). These metals catalyze further SEI growth, consuming active lithium and increasing cell impedance. Secondary-ion mass spectrometry (SIMS) reveals nickel-rich domains within the SEI, corroborating this cross-talk mechanism.

Mechanical stress exacerbates dissolution by increasing the surface area available for reactions. Microcracks expose fresh material to the electrolyte, while particle fragmentation generates more defects that facilitate metal leaching.

**Characterization and Mitigation Insights**
Advanced tools like synchrotron XRD and operando SEM provide real-time insights into these degradation pathways. For example, synchrotron studies capture the dynamics of phase transitions during cycling, linking H2-H3 phase changes to microcrack initiation. Operando SEM visualizes crack propagation under mechanical load, confirming the role of stress accumulation.

Material modifications, such as core-shell structures and dopants, have shown promise in mitigating degradation. Aluminum and titanium doping stabilize the lattice, reducing phase transition severity. Surface coatings (e.g., Al₂O₃, LiPO₃) limit electrolyte contact, slowing reconstruction and dissolution. However, these strategies require precise control to avoid impeding lithium-ion transport.

In summary, high-nickel cathode degradation is a multifaceted issue involving mechanical, chemical, and electrochemical pathways. Microcracking, surface reconstruction, and transition-metal dissolution are interconnected processes driven by electrolyte interactions and mechanical stress. Advanced characterization techniques are essential for developing robust mitigation strategies, enabling the full potential of high-nickel cathodes in next-generation batteries.