Enhanced Cycling of Ni-Rich Positive Electrodes by Fluorine Modification

Abstract

Ni-rich positive electrodes for Li-ion batteries can provide enhanced initial discharge capacity yet suffer from significant capacity degradation upon cycling. Fluorination of Ni-rich NMC811 positive electrodes results in a capacity retention of more than 90% after 100 cycles upon cycling to 4.4 V Li. The increased cycling stability of F-modified NMC811 can be attributed to the modification of the oxide electronic structures, where density functional theory calculations shows that incorporating fluorine into the oxide lattice decrease the driving force of carbonate dissociation on the oxide surface. In situ infrared (IR) spectroscopy and ex situ X-ray photoelectron spectroscopy (XPS) further supports this argument by showing less carbonate oxidation for F-modified LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) than as-received NMC811 upon charging. The reduced carbonate oxidation is coupled with minimal salt decomposition on the electrode surface, as revealed by XPS. Further comparing in situ IR and XPS with that of the heat-treated NMC811 and Li 2 CO 3 allows for decoupling the solvent decomposition products, where oxides are responsible for the vinylene carbonate formation with two hydrogens removed whereas surface metal carbonates promote dehydrogenated ethylene carbonate with just one hydrogen removed. This work points towards the importance of the anion engineering to increase the cycling stability of Ni-rich NMC positive electrodes.

Synergistic Strategy Using Doping and Polymeric Coating Enables High-Performance High-Nickel Layered Cathodes for Lithium-Ion Batteries

“…These include enhancing the ionic conductivity of the cathode material, fostering electron transfer, promoting lithium-ion diffusion dynamics, stabilizing the surface structure, and preventing direct contact between the cathode material and electrolyte. In the interim, inert compounds like oxides, − fluorides, phosphates, − as well as conductive materials, , such as carbon-based materials and lithium-containing compounds, are primarily used in coating strategies. The elemental doping of the bulk structure, in contrast, is modified at the atomic level.…”

Long-Lasting Ni-Rich NCMA Cathodes via Simultaneous Microstructural Refinement and Surface Modification

“…Once the discharge capacity has stabilized, i.e., no longer increases, the capacity fading (Figure b) suffered by the half cells featuring F-coated cathodes is milder than that suffered by the half cells featuring corresponding uncoated cathodes owing to the surface protection afforded by the F-coating-induced passivation layers. Although not yet fully understood, it is speculated that such stabilization periods, occasionally observed in cells featuring coated or doped cathodes, ,,− are associated with the formation of stable CEI layers on cathode surfaces. For the F-coated cathode cases, the stabilization cycles likely stem from the LiF layer deposited on the cathode surface at the initial stage prior to the formation of a stable CEI layer.…”

Casein from Bovine Milk as a Binder for Silicon Based Electrodes

“…An equivalent circuit diagram for a typical Nyquist plot for Li/ele/Si electrode is shown in Figure 3g. While quantitative analysis of the EIS data for composite electrodes is complicated, [16][17][18] the semicircle, which is in the high to moderate frequency range is typically attributed to the impedance resulting from to the SEI and charge transfer and the semicircle in the low frequency range is attributed to Li-ion diffusion. 19 Surprisingly the electrochemical impedance spectra (EIS) for electrodes containing PVdF have much lower impedance than the electrodes containing casein powder after the first cycle (Figure 3c).…”