Conditioning the Surface and Bulk of High-Nickel Cathodes with a Nb Coating: An In Situ X-ray Study

Artificial Layer Construction via Cosolvent Enables Stable Ni-Rich Cathodes for Enhanced Lithium Storage

Ni-rich cathodes have recently gained significant attention as next-generation cathodes for lithium-ion batteries. However, their relatively high oxidative surface should be reduced to control the high surface reactivity because the capacity retention decreases rapidly in the batteries. Herein, a simple and effective method to pretreat LiNi0.8Mn0.1Co0.1O2 (NMC811) particles using a cosolvent for improving the battery performance is reported. Imitating the interfacial reaction in practical cells, an artificial layer is created via a spontaneous redox reaction between the cathode and the organic solvent. The artificial layer comprises metal-organic compounds with reduced transition-metal cations. Benefiting from the artificial layer, the cells deliver high capacity retention at a high current density and better rate capability, which might result from the low and stable interfacial resistance of the modified NMC811 cathode. Our approach can effectively reduce the high oxidative surface of most oxide cathode materials and induce a long cyclic lifespan and high capacity retention in most battery systems.

Cobalt-free composite-structured cathodes with lithium-stoichiometry control for sustainable lithium-ion batteries

Lithium-ion batteries play a crucial role in decarbonizing transportation and power grids, but their reliance on high-cost, earth-scarce cobalt in the commonly employed high-energy layered Li(NiMnCo)O2 cathodes raises supply-chain and sustainability concerns. Despite numerous attempts to address this challenge, eliminating Co from Li(NiMnCo)O2 remains elusive, as doing so detrimentally affects its layering and cycling stability. Here, we report on the rational stoichiometry control in synthesizing Li-deficient composite-structured LiNi0.95Mn0.05O2, comprising intergrown layered and rocksalt phases, which outperforms traditional layered counterparts. Through multiscale-correlated experimental characterization and computational modeling on the calcination process, we unveil the role of Li-deficiency in suppressing the rocksalt-to-layered phase transformation and crystal growth, leading to small-sized composites with the desired low anisotropic lattice expansion/contraction during charging and discharging. As a consequence, Li-deficient LiNi0.95Mn0.05O2 delivers 90% first-cycle Coulombic efficiency, 90% capacity retention, and close-to-zero voltage fade for 100 deep cycles, showing its potential as a Co-free cathode for sustainable Li-ion batteries.

Concentration-Gradient Nb-Doping in a Single-Crystal LiNi0.83Co0.12Mn0.05O2 Cathode for High-Rate and Long-Cycle Lithium-Ion Batteries

Single-crystalline nickel-rich layered oxides are promising cathode materials for building high-energy lithium-ion batteries because of alleviated particle cracking and irreversible phase transitions upon cycling, compared with their polycrystalline counterparts. Under a high state of charge, parasitic reactions tend to occur at the cathode-electrolyte interface, which could result in sluggish Li-ion diffusion kinetics and quickly faded electrochemical performance of cathodes. In this work, a concentration-gradient niobium-doping strategy was applied to modify the single-crystal LiNi_0.83Co_0.12Mn_0.05O₂ cathode, with Nb concentration decreasing linearly from the surface to the core of the particle. As a result, the Nb-rich surface functions as an electrochemically active protective layer against electrolyte corrosion and transition metal dissolution, while the Nb-deficient core contributes to a higher capacity. The linear concentration gradient also minimizes structural transition from the surface to the core and helps to maintain structural integrity during repeated Li (de)intercalation. In addition, Nb-doping also assists to alleviate Li⁺/Ni²⁺ mixing and increases the interlayer distance to enable faster Li-ion diffusion kinetics. By taking these advantages, the Nb-doped cathode materials (containing 1.0 atom% Nb) demonstrate a high reversible capacity, a high capacity retention, and improved rate capabilities. This work provides a general and facile approach to improve the storage performance of layered-oxide cathode materials by rationally tuning the bulk structure and interface with the electrolyte.

Surface Stabilization of Cobalt-Free LiNiO2 with Niobium for Lithium-Ion Batteries

Lithium nickel oxide (LiNiO₂) is a promising next-generation cathode material for lithium-ion batteries (LIBs), offering exceptionally high specific capacity and reduced material cost. However, the poor structural, surface, and electrochemical stabilities of LiNiO₂ result in rapid loss of capacity during prolonged cycling, making it unsuitable for application in commercial LIBs. Herein, we demonstrate that incorporation of a small amount of niobium effectively suppresses the structural and surface degradation of LiNiO₂. The niobium-treated LiNiO₂ retains 82% of its initial capacity after 500 cycles in full cells with a graphite anode compared to 73% for untreated LiNiO₂. We utilize a facile method for incorporating niobium, which yields Li_xNbO_y phase formation as a surface coating on the primary particles. Through a combination of X-ray diffraction, electron microscopy, and electrochemical analyses, we show that the resulting niobium coating reduces active material loss over long-term cycling and enhances lithium-ion diffusion kinetics. The enhanced structural integrity and electrochemical performance of the niobium-treated LiNiO₂ are correlated to a reduction in the formation of nanopore defects during cycling compared to the untreated LiNiO₂.

Surface Reduction Stabilizes the Single-Crystalline Ni-Rich Layered Cathode for Li-Ion Batteries

The surface of the layered transition metal oxide cathode plays an important role in its function and degradation. Modification of the surface structure and chemistry is often necessary to overcome the debilitating effect of the native surface. Here, we employ a chemical reduction method using CaI₂ to modify the native surface of single-crystalline layered transition metal oxide cathode particles. High-resolution transmission electron microscopy shows the formation of a conformal cubic phase at the particle surface, where the outmost layer is enriched with Ca. The modified surface significantly improves the long-term capacity retention at low rates of cycling, yet the rate capability is compromised by the impeded interfacial kinetics at high voltages. The lack of oxygen vacancy generation in the chemically induced surface phase transformation likely results in a dense surface layer that accounts for the improved electrochemical stability and impeded Li-ion diffusion. This work highlights the strong dependence of the electrode's (electro)chemical stability and intercalation kinetics on the surface structure and chemistry, which can be further tailored by the chemical reduction method.