Abstract
Promising theoretical capacities and high voltages are offered by Li-rich disordered rocksalt oxyfluoride materials as cathodes in lithium-ion batteries. However, as has been discovered for many other Li-rich materials, the oxyfluorides suffer from extensive surface degradation, leading to severe capacity fading. In the case of Li2VO2F, we have previously determined this to be a result of detrimental reactions between an unstable surface layer and the organic electrolyte. Herein, we present the protection of Li2VO2F particles with AlF3 surface modification, resulting in a much-enhanced capacity retention over 50 cycles. While the specific capacity for the untreated material drops below 100 mA h g–1 after only 50 cycles, the treated materials retain almost 200 mA h g–1. Photoelectron spectroscopy depth profiling confirms the stabilization of the active material surface by the surface modification and reveals its suppression of electrolyte decomposition.
Highlights
Lithium-ion batteries (LIBs) currently dominate the portable electronics and full electric vehicle markets; the cathode is seen as a “bottle-neck” to their further substantial development
We have shown here that the treatment of the material can achieve stabilization, as well as maintain the same theoretical specific capacity
Scanning transmission electron microscopy (STEM)−energy-dispersive X-ray spectroscopy (EDS) imaging/mapping confirmed the passivation of the active material by a surface layer of up to 10 nm, rich in aluminum and fluorine
Summary
Lithium-ion batteries (LIBs) currently dominate the portable electronics and full electric vehicle markets; the cathode is seen as a “bottle-neck” to their further substantial development. While state-of-the-art cathode materials, such as NMC (LiNi1−x−yCoxMnyO2) and NCA (LiNi1−x−yCoxAlyO2), approach capacities of 200 mA h g−1, significantly greater capacities can be reached by employing the so-called lithiumrich materials.[1,2] Many of such materials have been found to exhibit anionic redox reactions, in addition to the transition metal redox chemistry, to compensate for the excess lithium extracted.[3] Li-rich layered materials, such as Li[Li0.2Ni0.13Co0.13Mn0.54]O2, are often subject to oxygen loss, phase transformations, densification, and metal dissolution during cycling, leading to poor practical performance.[4,5]. One promising family of Li-rich cathode materials is the face-centered cubic-structured disordered rocksalt oxyfluorides, based on Li2VO2F.6,7. Other disordered rocksalt structures have been studied for use as cathode materials.[9−13]
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