Abstract
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 LiNixMnyCozO2 (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 NMC622-graphite 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.
Highlights
To cite this article: Roland Jung et al 2019 J
In this work we investigated the transition metal dissolution from NMC622 and the deposition of nickel, manganese, and cobalt on the graphite anode using operando X-ray absorption spectroscopy
We showed that at high potentials Ni, Mn, and Co dissolve nearly stoichiometrically, so that the absolute amounts of Ni deposited on the graphite anode are highest for the nickel-rich NMC622 active material
Summary
To cite this article: Roland Jung et al 2019 J. The dissolution of transition metals, in particular of manganese (Mn), was first identified as a major aging mechanism in LiMn2O4 (LMO) spinel cathodes causing, severe capacity fading in fullcells.[8,9,10,11,12,13] Later on, transition metal dissolution was investigated for the layered NMC materials, showing that this phenomenon is not limited to LMO.[14,15,16,17,18] The detrimental effect of transition metal dissolution can have two origins: i) a loss of CAM due to active material degradation in the course of transition metal dissolution into the electrolyte, and/or ii) subsequent deposition and accumulation of transition metals on the graphite anode The former is of minor importance, as only rather small amounts of the overall transition metal amounts in the CAMs are dissolved. For the in-situ XRD measurement, the NMC cathode was prepared by coating the slurry through a mask (110 μm thick, with a 12 mm diameter hole) onto an aluminum foil (18 μm thickness, MTI Corporation, USA) with a diameter of 42 mm, whereby the aluminum foil serves both as current collector and X-ray window
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