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

Nickel-rich layered oxides (LiNixMnyCozO2, Ni-rich NMC; x≥0.7) are promising cathode candidates for next-generation lithium-ion batteries due to their high capacity and low cost. However, increasing Ni content exacerbates capacity degradation, hindering commercialization. Capacity degradation is primarily attributed to surface chemical instability and mechanical destruction (microcracks). Strategies to address surface instability, such as coatings, show limited success because cracking blocks electronic transport pathways and exposes fresh surfaces to the electrolyte, leading to further degradation. Therefore, suppressing particle cracking is crucial for improving cycle stability in Ni-rich NMCs. Operando studies reveal that anisotropic volume change and lattice strain initiate mechanical destruction. This heterogeneous volume change breaks contact between primary particles, leading to chemomechanical breakdown. Cracking is more pronounced in Ni-rich cathodes due to severe phase transitions and volume changes. This necessitates understanding the role of Mn and Co on mechanical properties to optimize compositional design for enhanced mechanical stability without sacrificing capacity.

The literature extensively documents the challenges associated with Ni-rich cathode materials, focusing primarily on the negative effects of high Ni content on stability. Many studies have addressed surface chemical instability through coatings and surface modifications, while the impact of Mn and Co on the mechanical properties and their potential to mitigate particle cracking remained less understood. Existing research highlights the relationship between delithiation, phase transitions, and anisotropic volume changes as the driving forces behind microcrack formation and propagation. However, the specific influence of Mn and Co on these processes and the optimal compositional strategies to enhance mechanical robustness has been lacking. This research aims to fill this knowledge gap by comprehensively investigating the individual roles of Mn and Co and implementing a concentration gradient design to improve the mechanical integrity of Ni-rich cathodes.

The study systematically investigated the effects of Co and Mn on the mechanical properties of Ni-rich cathodes using LiNi0.8Co0.2O2 (NC82) and LiNi0.8Mn0.2O2 (NM82) as base materials. In situ and ex situ high-energy synchrotron XRD (HEXRD) was employed to compare structural evolution and calculate Young's modulus. Two concentration gradient Ni-rich cathode materials with identical average compositions but converse Co/Mn distributions were designed: 0.5(LiNi0.8Co0.2O2)bulk0.5(LiNi0.8Mn0.2O2)surface (NC-NM82) and 0.5(LiNi0.8Mn0.2O2)bulk0.5(LiNi0.8Co0.2O2)surface (NM-NC82). LiNi0.8Mn0.1Co0.1O2 (NMC811) served as a control. Materials were synthesized using a co-precipitation method. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and scanning electron microscopy (SEM) characterized the overall composition and morphology. Three-dimensional X-ray fluorescence (3D XRF) mapped the spatial elemental distribution within the particles. Electrochemical performance was evaluated through charge/discharge profiles, rate capability tests, and cycling performance measurements. In situ HEXRD monitored real-time structural evolution during cycling, while ex situ postmortem focused ion beam-scanning electronic microscopy (FIB-SEM) and transmission electron microscopy (TEM) visualized morphological changes. Nanoscale XRF and scanning X-ray diffraction microscopy (SXDM) probed local composition and structure.

The study found that Co-enriched layered structures exhibit lower stiffness (Young's modulus) compared to Mn-enriched structures. In situ XRD showed that NC82 (Co-rich) experienced greater lattice expansion and contraction along the c-axis than NM82 (Mn-rich) under similar stress, indicating lower stiffness for Co-rich materials. NM82 showed significant peak broadening in the highly delithiated state, indicating increased microstrain. The concentration gradient cathodes (NC-NM82 and NM-NC82) exhibited similar initial discharge capacities but differed significantly in cycling performance. NM-NC82 (Co-rich surface, Mn-rich core) showed superior rate capability and capacity retention compared to NC-NM82 (Mn-rich surface, Co-rich core) and NMC811. In situ HEXRD revealed that NM-NC82 exhibited better structural reversibility and less degradation of the H2-H3 phase transition during cycling than NC-NM82. Ex situ FIB-SEM and TEM showed that NC-NM82 suffered severe particle cracking after 100 cycles, exposing the core to the electrolyte and accelerating irreversible phase transitions. NM-NC82 remained largely intact, indicating enhanced mechanical stability. Nanoscale XRF and SXDM showed that the Mn-rich surface of NC-NM82 experienced greater compressive stress, leading to cracking. The Co-rich surface of NM-NC82 exhibited better stability.

The findings directly address the research question by demonstrating the critical role of Co and Mn distribution in determining the mechanical stability of Ni-rich cathodes. The superior performance of NM-NC82 highlights the importance of a less stiff, Co-enriched surface in mitigating particle cracking and improving cycle life. The results contradict previous assumptions that solely focusing on Ni content modification is sufficient for optimal performance. This study introduces a new strategy for enhancing the mechanical robustness of Ni-rich cathodes, offering valuable insights for designing high-performance lithium-ion batteries. The concentration gradient approach allows for independent optimization of surface and bulk properties, offering a path towards achieving both high capacity and extended cycle life.

This research successfully isolated the intrinsic mechanical effects of Co and Mn in Ni-rich cathodes, revealing the significant influence of compositional design on structural stability. The superior performance of the NM-NC82 cathode, with its Co-rich surface and Mn-rich core, demonstrates a novel concentration gradient strategy for mitigating particle cracking and enhancing electrochemical performance. Future research could explore other elemental dopants and refine the concentration gradient design for further improvements in cycle life and energy density.

The study focused on specific Co and Mn concentrations and gradient designs. Further research is needed to explore a wider range of compositions and gradient profiles to optimize the mechanical and electrochemical properties. The study primarily focused on coin cell testing; further evaluation in larger-format cells is necessary to assess the scalability and practical applicability of the concentration gradient approach. The long-term stability beyond 100 cycles warrants further investigation.