The widespread adoption of electric vehicles necessitates higher-energy-density and longer-life cathode materials for Li-ion batteries. LiNiO2 offers a higher energy density at a lower cost than other high-Ni-content cathodes containing additional transition-metal ions. However, detrimental phase transformations and impedance growth, resulting from structural defects formed during synthesis, lead to poor cyclability and limit the practical viability of LiNiO2. Herein, we demonstrate a considerably improved cycle life for LiNiO2 by synthesizing it under a pressurized oxygen environment. The capacity retention in pouch-type full cells with a graphite anode after 1000 cycles is increased from 59 to 76% by applying a mere 1.7 atm of oxygen pressure during the synthesis of LiNiO2. With iodometric titration and inductively coupled plasma optical emission spectroscopy analysis, we provide clear evidence that oxygen pressure during synthesis reduces the occurrence of lattice oxygen vacancies and increases the content of Ni3+ in LiNiO2, improving its structural integrity and cyclability. Post-mortem analysis of the cycled cathodes provides insights into the sources of degradation occurring during long-term cycling. This work demonstrates a practically viable, synthetic approach combined with doping and coating to achieve improved performance with high-Ni layered oxide materials. Furthermore, this work represents the first report of extended cycling of LiNiO2 in pouch full cells with graphite anode and will, therefore, serves as an important benchmark for future research on LiNiO2.
In pursuit of Li-ion batteries with higher energy density, ultrahigh-nickel layered oxides are a leading candidate for next-generation cathode materials. Single-crystalline morphology offers a neat solution to the poor stability of ultrahigh-Ni cathodes; a lower active surface area mitigates electrolyte decomposition at high voltages, and the elimination of grain boundaries improves mechanical resilience and increases volumetric energy density. However, single-crystal cathodes possess their own challenges, several of which originate from synthesis at elevated temperatures meant to induce grain growth. Molten-salt synthesis is an alternative method for obtaining single crystals, accelerating grain growth through the presence of a molten flux without the need for increased temperature. Herein, we offer heuristic guidelines for molten-salt synthesis, discussing key factors for designing reaction mixtures and the necessary exploratory research for novel molten salt/cathode systems. The influence of different salts and synthesis conditions on the morphology and properties of single-crystal LiNiO2 is presented. It is found that oxidative salts, such as Li2O2 and LiNO3, are crucial to supplementing dissolution of gaseous oxygen into the molten phase. Through these discussions, this work aims to provide a set of overarching principles for obtaining higher-quality single-crystal layered oxide cathodes and engender more rigorous and impactful investigation into their fundamental nature and applications.
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 x NbO 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.
Nickel-rich cathode materials are quickly becoming the next commercial cathode for electric vehicles; however, their long-term cycle life retention and air stability remain a barrier to the use of these lower-cost, higher-energy density materials. Surface reactivity and mechanical degradation, especially at high voltages, remain two issues that impede these material’s commercialization. While surface treatments have shown great promise in reducing surface reactivity, mechanical degradation or “cathode cracking” persists yet. In the present work, LiNi0.9Mn0.05Al0.05O2 (NMA) cathode materials are first pulverized into their primary particle constituents and then coated with lithium phosphate via solution-based chemistry with varying concentrations of phosphoric acid. The cathodes are characterized using energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, electrochemical impedance spectroscopy, and electrochemical cycling. After 100 cycles, the pulverized NMA cathodes coated using the lowest concentration of phosphoric acid show delayed voltage decay and double the discharge capacity compared to the pristine material in full cells during high-voltage cycling.
Understanding Li distribution in layered lithium transition-metal oxide (LiTMO) cathodes in Li-ion batteries has been a major challenge at the atomic scale and nanoscale. Li is extremely difficult to study by transmission electron microscopy (TEM) because the high-energy electrons impart significant energy and cause massive migration. Here, we directly map the intrinsic spatial distribution and bonding of Li in LiNiO2-layered cathode materials using low-dose and low-loss electron energy loss spectroscopy (EELS). EELS spectra of the Li–K edge are measured simultaneously with O–K and Ni–L, M3,2 edges from layered, cation-mixed, and rock-salt phases and directly matched with atomic-resolution scanning TEM images to correlate the changes in peak intensities and positions to the stoichiometry changes with continual loss of Li and O. Changes in the Li content in the LiNiO2 particles as a function of electron beam dose are studied by sequential Li spectroscopic mapping. We show that the “intrinsic” Li distribution can be observed using a total dose of less than ∼1.5 × 108 e– nm–2 at an accelerating voltage of 80 kV. The method of nanoscale mapping of Li distribution introduced in this study is applicable to high-Ni LiTMO cathode materials (>89% of Ni) as well as LiNiO2. Further study on the extra peaks of the Li–K edge reveals that the peak at ∼59 eV is from the Li ions intercalated in between NiO2 layers barely interacting with each other with less Li K shell electrons pulled to the L shell electrons in NiO2. The results shown here provide improved low-loss TEM characterization approaches that can be used to understand the intrinsic fundamental behaviors in Li-ion batteries.
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