Transition metal dissolution from the cathode active material and its deposition on the anode causes significant cell aging, studied most intensively for manganese. Owing to their higher specific energy, the current focus is shifting towards nickel-rich layered LiNi x Mn y Co z O 2 (NMC, x + y + z = 1) with x > 0.5, so that the effect of Ni dissolution on cell degradation needs to be understood. This study investigates the dissolution of transition metals from a NMC622 cathode and their subsequent deposition on a graphite anode using operando X-ray absorption spectroscopy. We show that in NMC622-graphite cells transition metals dissolve nearly stoichiometrically at potentials >4.6 V, highlighting the significance of investigating Ni dissolution/deposition. Using NMC622graphite full-cells with electrolyte containing the bis(trifluoromethane)sulfonimide (TFSI) salts of either Ni, Mn, or Co, we compare the detrimental impact of these metals on cell performance. Using in-situ and ex-situ XRD, we show that the aging mechanism induced by all three metals is the loss of cycleable lithium in the solid electrolyte interface (SEI) of the graphite. This loss is larger in magnitude when Mn is present in the electrolyte compared to Ni and Co, which we ascribe to a higher activity of deposited Mn towards SEI decomposition in comparison to Ni and Co.

Operando X-ray absorption spectroscopy studies of transition metal dissolution in Li-ion batteries.

The underlying mechanism of lithium-sulfur batteries is still not fully established because it involves a series of both chemical and electrochemical reactions as well as the formation of soluble polysulfide intermediates. To improve the mechanistic understanding of lithium-sulfur batteries, this study investigates chemical reactions between the Li 2 S cathode and more oxidized sulfur species, such as S 8 and polysulfides, during the electrochemical charge of the battery. By combining the electrochemistry with X-ray absorption spectroscopy, we show that chemical reactions and, in particular, the resulting accumulation of solution species in the electrolyte are essential to oxidize Li 2 S at a low overpotential. Additionally, by efficiently separating the anode and cathode compartments of a battery with a lithium ion-exchanged Nafion interlayer, we establish the adverse effect of the anode on the buildup of solution intermediates. In the absence of the interlayer, polysulfide intermediates can diffuse through the separator and react at the anode's surface, while the addition of the interlayer allows the intermediates to accumulate in the separator of the cathode compartment and facilitate the oxidation of Li 2 S.

Molybdenum oxides and sulfides were studied using resonant inelastic X-ray scattering (RIXS). The 2p 3/2 3d Mo RIXS planes show a rich structure with considerably more spectral information than in conventional Xray absorption near edge structure (XANES) spectroscopy. The spectra were simulated using FEFF9 giving generally good agreement and detailed electronic information can be derived. The reference materials serve as a starting point for detailed electronic and geometric investigations of a broad range of compounds. In particular, this can provide insights in the properties and performance of unknown and changing materials like those in catalysis. Figure 4. Experimental Lα 1 RIXS plot (a) for Na 2 MoO 4 with theoretical RIXS plot calculated using FEFF9 (b) and the CTM4XAS multiplet code (c) for the same compound.