Rational design of mechanically robust Ni-rich cathode materials via concentration gradient strategy

Ni-rich layered oxides (LiNixMnyCozO2, x ≥ 0.7) offer high capacity and cost advantages but suffer from rapid capacity degradation that impedes commercialization. Evidence points to two primary causes: surface chemical instability (leading to parasitic reactions and phase transitions) and mechanical destruction via microcracking that disrupts electronic pathways and exposes fresh surfaces to electrolyte. Operando studies attribute cracking to anisotropic lattice strain and volume changes during (de)lithiation, which are more pronounced at higher Ni content. While coatings and surface protection mitigate chemical degradation, they do little to improve mechanical robustness. Despite widespread use of Mn and Co in Ni-rich cathodes, their intrinsic roles in mechanical properties have been insufficiently understood. This study addresses the knowledge gap by probing how Co and Mn contents affect stiffness and strain evolution, and by testing a rational concentration gradient design aimed at suppressing particle cracking without sacrificing capacity.

Prior work has linked capacity fade in Ni-rich NMC to surface instability, irreversible phase transitions, electrolyte decomposition, and microcrack formation. Protective coatings stabilize surface chemistry but do not prevent cracking, which in turn accelerates parasitic reactions. Operando and modeling studies show anisotropic volume changes and lattice strain drive intergranular and intragranular cracking, exacerbated by deep delithiation and high Ni content. Conventional gradient designs typically reduce Ni and increase Mn from core to surface to improve stability. However, the distinct mechanical impacts of Co versus Mn have been underexplored, and emphasis on Ni content may have obscured optimal compositional solutions. Mn substitution can improve structural reversibility, while the roles of Co in stiffness and electronic conductivity are also recognized. These insights motivate exploring Co/Mn gradients at constant high Ni to optimize mechanical stability and electrochemical performance.

- Materials: Synthesized baseline compositions LiNi0.8Co0.2O2 (NC82) and LiNi0.8Mn0.2O2 (NM82), two concentration-gradient Ni-rich cathodes with identical average composition but converse Co/Mn distributions—NC-NM82: Co-enriched core transitioning to Mn-enriched surface; NM-NC82: Mn-enriched core transitioning to Co-enriched surface—plus a conventional LiNi0.8Mn0.1Co0.1O2 (NMC811) control. All were prepared via coprecipitation in a 4 L reactor using NiSO4·6H2O, CoSO4·7H2O, and MnSO4·H2O precursors, followed by lithiation and calcination. - Structural and compositional characterization: High-energy synchrotron XRD (HEXRD) for phase identification (R−3m) and Rietveld refinement (lattice parameters, Li/Ni disorder); in situ HEXRD during galvanostatic cycling to track phase transitions (H1–M–H2–H3) and c-axis changes; Williamson–Hall analysis for microstrain; 3D X-ray fluorescence (3D XRF) tomography to map spatial Co/Mn/Ni distributions in single secondary particles; nanoscale XRF and scanning X-ray diffraction microscopy (SXDM) on FIB-prepared cross-sections to obtain local composition, d-spacing maps, and lattice tilt/rotation fields. - Morphology and microstructure: SEM for particle morphology and size; ex situ postmortem FIB-SEM cross-sections after cycling to visualize crack evolution; TEM/HRTEM to examine surface reconstruction (rock-salt formation) and internal crack-related transformations. - Electrochemistry: Coin cells evaluated with Li metal counter electrodes. Initial charge/discharge at C/10 within 2.8–4.4 or 2.8–4.5 V windows as specified; rate capability up to 5C; cycling tests at C/2 (after three formation cycles) between 2.8–4.4 V; differential capacity (dQ/dV) analysis to assess structural reversibility and phase transition stability. - Mechanical property inference: Comparative analysis of c-axis lattice parameter evolution versus delithiation state used as a qualitative proxy for Young’s modulus (stiffness) under elastic lattice expansion/contraction, enabling assessment of Co- versus Mn-rich compositions.

- Intrinsic mechanical roles of Co vs Mn: - NC82 (LiNi0.8Co0.2O2) exhibits larger c-axis expansion/contraction during cycling than NM82 (LiNi0.8Mn0.2O2) at similar delithiation, indicating lower stiffness (smaller effective Young’s modulus) for Co-enriched layered structures. - NM82 shows significant peak broadening and higher microstrain at high delithiation compared with NC82, consistent with increased stiffness and strain accumulation. - Despite higher stiffness, Mn substitution improves structural reversibility and cycling stability relative to Co substitution. - Gradient design at constant high Ni (≈80% Ni) with converse Co/Mn distributions: - Verified by 3D XRF: NM-NC82 has Mn-enriched core and Co-enriched surface; NC-NM82 has Co-enriched core and Mn-enriched surface; average compositions near Ni 80%, Mn 10%, Co 10%. - Electrochemical performance (2.8–4.4 V): Initial discharge capacities at C/10 are 207–208 mAh g−1 for NC-NM82, NM-NC82, and NMC811 (similar capacity, no sacrifice). - Rate capability: NM-NC82 shows best rate performance and lower overpotential, attributed to Co-enriched surface improving electronic conductivity; Mn-enriched surface (NC-NM82) degrades rate performance. - Cycling at C/2 (100 cycles): Capacity retention is 92% (NM-NC82) > 87% (NMC811) > 83% (NC-NM82). - dQ/dV: NM-NC82 exhibits minimal peak shift and intensity decay, indicating superior structural reversibility; NC-NM82 shows synchronous peak decay and large shifts, indicating poor reversibility. - Phase transition stability from in situ HEXRD: - Both gradients undergo H1–M–H2–H3 transitions; NC-NM82 exhibits a more severe H2–H3 transition (sharp c-axis contraction), associated with higher lattice strain and damage. - After extended cycling, NC-NM82 shows markedly degraded H2–H3 process and increased reaction potential; NM-NC82 maintains a stable H2–H3 transition with negligible degradation. - Morphology and microstructure evolution: - FIB-SEM cross-sections: NC-NM82 develops surface-initiated microcracks extending into the core by 50 cycles and extensive cracking throughout by 100 cycles; NM-NC82 particles remain largely intact with only tiny veins after 100 cycles. - TEM/HRTEM: NC-NM82 shows surface rock-salt reconstruction and internal cracks with irreversible phase transition; NM-NC82 shows surface reconstruction but no extended rock-salt or cracking in the bulk, preserving layered crystallinity at boundaries. - Nanoscale structure-stress insights (XRF/SXDM): - Mn-rich regions display larger initial c-lattice parameter; pronounced inward lattice rotations near the surface indicate compressive surface stress in pristine particles. - In a Co-rich core/Mn-rich surface design (NC-NM82), tensile stress from internal expansion and surface compressive stress concentrate on the stiffer Mn-rich surface, aggravating cracking. - In a Mn-rich core/Co-rich surface design (NM-NC82), smaller internal expansion combined with a more compliant Co-enriched surface suppresses cracking. - Overall: A Co-enriched surface (lower stiffness) with a Mn-enriched core (limits internal expansion and improves reversibility) delivers superior mechanical integrity and electrochemical performance without capacity tradeoff.

The study directly addresses whether targeted Co/Mn distributions can mechanically stabilize Ni-rich NMC without compromising capacity. By decoupling mechanical effects (stiffness, strain accommodation) from average composition, the authors demonstrate that a Co-enriched surface accommodates lattice strain during delithiation, reducing crack initiation and propagation, while a Mn-enriched core mitigates internal expansion and enhances structural reversibility. This combination stabilizes the critical H2–H3 phase transition over prolonged cycling, maintains particle integrity, and suppresses parasitic reactions associated with fresh surfaces. The findings clarify the distinct roles of Co (lower stiffness, improved electronic conductivity) and Mn (enhanced structural reversibility, higher stiffness) and show that compositional gradients at constant high Ni can outperform both conventional uniform NMC811 and the converse gradient. These insights provide a mechanistic basis for designing mechanically robust Ni-rich cathodes and emphasize the necessity of aligning local mechanical properties with internal stress gradients induced by (de)lithiation.

The work isolates the intrinsic mechanical impacts of Co and Mn in Ni-rich layered cathodes and translates these insights into a rational concentration gradient design. A Mn-enriched core coupled with a Co-enriched surface yields mechanically robust particles, minimizes cracking, stabilizes the H2–H3 transition, and enhances cycling and rate performance without sacrificing capacity. Conversely, a Mn-enriched surface with a Co-rich core accelerates mechanical degradation. These results establish a design principle linking composition gradients to local stiffness and stress accommodation and can guide the development of next-generation gradient and single-crystal Ni-rich cathodes. Future research could extend this approach to other transition-metal distributions, optimize gradient profiles and thicknesses, quantify mechanical moduli across states of charge, and validate performance under higher-voltage and practical cell formats.

- No explicit numerical Young’s modulus values were measured; stiffness inferences are qualitative based on lattice parameter evolution versus delithiation state. - Surface reconstruction (rock-salt formation) still occurs on both gradient designs, indicating that while cracking is mitigated, surface chemical degradation is not eliminated. - Electrochemical validation was conducted in Li metal coin cells and within specified voltage windows; performance in full cells, at higher cutoff voltages, and under varied thermal/mechanical conditions remains to be demonstrated. - Gradient designs were evaluated for a single average composition (~Ni 80%, Mn 10%, Co 10%); generality across broader compositions and particle morphologies requires further study.