Insights into the Enhanced Structural and Thermal Stabilities of Nb-Substituted Lithium-Rich Layered Oxide Cathodes

Optimized In Situ Doping Strategy Stabling Single-Crystal Ultrahigh-Nickel Layered Cathode Materials

Single-crystal Ni-rich cathodes offer promising prospects in mitigating intergranular microcracks and side reaction issues commonly encountered in conventional polycrystalline cathodes. However, the utilization of micrometer-sized single-crystal particles has raised concerns about sluggish Li+ diffusion kinetics and unfavorable structural degradation, particularly in high Ni content cathodes. Herein, we present an innovative in situ doping strategy to regulate the dominant growth of characteristic planes in the single-crystal precursor, leading to enhanced mechanical properties and effectively tackling the challenges posed by ultrahigh-nickel layered cathodes. Compared with the traditional dry-doping method, our in situ doping approach possesses a more homogeneous and consistent modifying effect from the inside out, ensuring the uniform distribution of doping ions with large radius (Nb, Zr, W, etc). This mitigates the generally unsatisfactory substitution effect, thereby minimizing undesirable coating layers induced by different solubilities during the calcination process. Additionally, the uniformly dispersed ions from this in situ doping are beneficial for alleviating the two-phase coexistence of H2/H3 and optimizing the Li+ concentration gradient during cycling, thus inhibiting the formation of intragranular cracks and interfacial deterioration. Consequently, the in situ doped cathodes demonstrate exceptional cycle retention and rate performance under various harsh testing conditions. Our optimized in situ doping strategy not only expands the application prospects of elemental doping but also offers a promising research direction for developing high-energy-density single-crystal cathodes with extended lifetime.

Phase Engineering via Aluminum Doping Enhances the Electrochemical Stability of Lithium-Rich Cobalt-Free Layered Oxides for Lithium-Ion Batteries

Lithium-rich, cobalt-free oxides are promising potential positive electrode materials for lithium-ion batteries because of their high energy density, lower cost, and reduced environmental and ethical concerns. However, their commercial breakthrough is hindered because of their subpar electrochemical stability. This work studies the effect of aluminum doping on Li1.26 Ni0.15 Mn0.61 O2 as a lithium-rich, cobalt-free layered oxide. Al doping suppresses voltage fade and improves the capacity retention from 46% for Li1.26 Ni0.15 Mn0.61 O2 to 67% for Li1.26 Ni0.15 Mn0.56 Al0.05 O2 after 250 cycles at 0.2 C. The undoped material has a monoclinic Li2 MnO3 -type structure with spinel on the particle edges. In contrast, Al-doped materials (Li1.26 Ni0.15 Mn0.61-x Alx O2 ) consist of a more stable rhombohedral phase at the particle edges, with a monoclinic phase core. For this core-shell structure, the formation of Mn3+ is suppressed along with the material's decomposition to a disordered spinel, and the amount of the rhombohedral phase content increases during galvanostatic cycling. Whereas previous studies generally provided qualitative insight into the degradation mechanisms during electrochemical cycling, this work provides quantitative information on the stabilizing effect of the rhombohedral shell in the doped sample. As such, this study provides fundamental insight into the mechanisms through which Al doping increases the electrochemical stability of lithium-rich cobalt-free layered oxides.

Achieving structural stability and enhanced electrochemical performance through Nb-doping into Li- and Mn-rich layered cathode for lithium-ion batteries

Although Li- and Mn-rich layered oxides are attractive cathode materials possessing high energy densities, they have not been commercialized owing to voltage decay, low rate capability, poor capacity retention, and high irreversible capacity in the first cycle. To circumvent these issues, we propose a Li_1.2Ni_0.13Co_0.13Mn_0.53Nb_0.01O₂ (Nb-LNCM) cathode material, wherein Nb doping strengthens the transition metal oxide (TM-O) bond and alleviates the anisotropic lattice distortion while stabilizing the layered structure. During long-term cycling, maintaining a wider LiO₆ interslab thickness in Nb-LNCM creates a favorable Li⁺ diffusion path, which improves the rate capability. Moreover, Nb doping can decrease oxygen loss, suppress the phase transition from layered to spinel and rock-salt structures, and relieve structural degradation. Nb doping results in less capacity contributions of Mn and Co and more reversible Ni and O redox reactions compared to pristine Li_1.2Ni_0.133Co_0.133Mn_0.533O₂ (LNCM), which significantly mitigates the voltage decay (Δ0.289 and Δ0.516 V for Nb-LNCM and LNCM, respectively) and ensures stable capacity retention (82.7 and 70.3% for Nb-LNCM and LNCM, respectively) during the initial 100 cycles. Our study demonstrates that Nb doping is an effective and practical strategy to enhance the structural and electrochemical integrity of Li- and Mn-rich layered oxides. This promotes the development of stable cathode materials for high-energy-density lithium-ion batteries.

Stabilizing Cobalt-free Li-rich Layered Oxide Cathodes through Oxygen Lattice Regulation by Two-phase Ru Doping

The application of Li-rich layered oxides is hindered by their dramatic capacity and voltage decay on cycling. This work comprehensively studies the mechanistic behaviour of cobalt-free Li_1.2 Ni_0.2 Mn_0.6 O₂ and demonstrates the positive impact of two-phase Ru doping. A mechanistic transition from the monoclinic to the hexagonal behaviour is found for the structural evolution of Li_1.2 Ni_0.2 Mn_0.6 O_2, and the improvement mechanism of Ru doping is understood using the combination of in operando and post-mortem synchrotron analyses. The two-phase Ru doping improves the structural reversibility in the first cycle and restrains structural degradation during cycling by stabilizing oxygen (O^2- ) redox and reducing Mn reduction, thus enabling high structural stability, an extraordinarily stable voltage (decay rate <0.45 mV per cycle), and a high capacity-retention rate during long-term cycling. The understanding of the structure-function relationship of Li_1.2 Ni_0.2 Mn_0.6 O₂ sheds light on the selective doping strategy and rational materials design for better-performance Li-rich layered oxides.

Integrating a Ferroelectric Interface with a Well-Tuned Electronic Structure in Lithium-Rich Layered Oxide Cathodes for Enhanced Lithium Storage

Li-rich layered oxides (LLOs) are considered promising candidates for new high-energy-density cathode materials for next-generation power batteries. However, their large-scale applications are largely hindered by irreversible Li/O loss, structural degradation, and interfacial side reactions during cycling. Herein, we demonstrate an integration strategy that tunes the electronic structure by La/Al codoping and constructs a ferroelectric interface on the LLOs surface through Bi_0.5Na_0.5TiO₃ (BNT) coating. Experimental characterization reveals that the synergistic effect of the ferroelectric interface and the well-tuned electronic structure can not only promote the diffusion of Li⁺ and hinder the migration of O^n- but also suppress the lattice volume changes and reduce interfacial side reactions at high voltages up to 4.9 V vs Li⁺/Li. As a result, the modified material shows enhanced initial capacities and retention rates of 224.4 mAh g^-1 and 78.57% after 500 cycles at 2.0-4.65 V and 231.7 mAh g^-1 and 85.76% after 200 cycles at 2.0-4.9 V at 1C, respectively.

Lithium Antievaporation-Loss Engineering via Sodium/Potassium Doping Enables Superior Electrochemical Performance of High-Nickel Li-Rich Layered Oxide Cathodes

Low-cost Mn- and Li-rich layered oxides suffer from rapid voltage decay, which can be improved by increasing the nickel content to derive high nickel Li-rich layered oxides (HNLO) but is normally accompanied by reduced capacity and inferior cycling stability. Herein, Na or K ions are successfully doped into the lattice of high nickel Li-rich Li_1.2-xM_xNi_0.32Mn_0.48O₂ (M = Na, K) layered oxides via a facile expanded graphite template-sacrificed approach. Both Na- and K-doped samples exhibit excellent rate capability and cycling stability compared with the un-doped one. The Na-doped sample shows a capacity retention of 93% after 200 cycles at 1C, which is quite outstanding for HNLO. The greatly improved electrochemical performances are attributed to the increased effective Li content in the lattice via Li antievaporation-loss engineering, the expanded Li slab, the pillaring effect, the increased C2/m component, and the improved electronic conductivity. Different performances by the introduction of sodium and potassium ions may be ascribed to their different ionic radii, which give rise to their different doping behaviors and threshold doping amounts. This work provides a new idea of enhancing electrochemical performance of HNLO by doping proper alien elements to increase the lattice lithium content effectively.

Significant Enhancement of the Capacity and Cycling Stability of Lithium-Rich Manganese-Based Layered Cathode Materials via Molybdenum Surface Modification

Lithium-rich manganese-based layered cathode materials are considered to be one of the best options for next-generation lithium-ion batteries, owing to their ultra-high specific capacity (>250 mAh·g−1) and platform voltage. However, their poor cycling stability, caused by the release of lattice oxygen as well as the electrode/electrolyte side reactions accompanying complex phase transformation, makes it difficult to use this material in practical applications. In this work, we suggest a molybdenum surface modification strategy to improve the electrochemical performance of Li1.2Mn0.54Ni0.13Co0.13O2. The Mo-modified Li1.2Mn0.54Ni0.13Co0.13O2 material exhibits an enhanced discharge specific capacity of up to 290.5 mAh·g−1 (20 mA·g−1) and a capacity retention rate of 82% (300 cycles at 200 mA·g−1), compared with 261.2 mAh·g−1 and a 70% retention rate for the material without Mo modification. The significantly enhanced performance of the modified material can be ascribed to the formation of a Mo-compound-involved nanolayer on the surface of the materials, which effectively lessens the electrolyte corrosion of the cathode, as well as the activation of Mo6+ towards Ni2+/Ni4+ redox couples and the pre-activation of a Mo compound. This study offers a facile and effective strategy to address the poor cyclability of lithium-rich manganese-based layered cathode materials.