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.
High
degrees of delithiation of layered transition metal oxide
cathode active materials (NCMs and HE-NCM) for lithium-ion batteries
(LIBs) was shown to lead to the release of singlet oxygen, which is
accompanied by enhanced electrolyte decomposition. Here, we study
the reactivity of chemically produced singlet oxygen with the commonly
used cyclic and linear carbonate solvents for LIB electrolytes. On-line
gassing analysis of the decomposition of ethylene carbonate (EC) and
dimethyl carbonate (DMC) reveals different stability toward the chemical
attack of singlet oxygen, which is produced in situ by photoexcitation
of the Rose Bengal dye. Ab initio calculations and
on-the-fly simulations reveal a possible reaction mechanism, confirming
the experimental findings. In the case of EC, hydrogen peroxide and
vinylene carbonate (VC) are found to be the products of the first
reaction step of EC with singlet oxygen in the reaction cascade of
the EC chemical decomposition. In contrast to EC, simulations suggested
DMC to be stable in the presence of singlet oxygen, which was also
confirmed experimentally. Hydrogen peroxide is detrimental for cycling
of a battery. For all known cathode active materials, the potential
where singlet oxygen is released is found to be already high enough
to electrochemically oxidize hydrogen peroxide. The formed protons
and/or water both react with the typically used LiPF6 salt
to HF that then leads to transition metal dissolution from the cathode
active materials. This study shows how important the chemical stability
toward singlet oxygen is for today’s battery systems and that
a trade-off will have to be found between chemical and electrochemical
stability of the solvent to be used.
Aprotic lithium-oxygen (Li-O2 ) batteries have attracted considerable attention in recent years owing to their outstanding theoretical energy density. A major challenge is their poor reversibility caused by degradation reactions, which mainly occur during battery charge and are still poorly understood. Herein, we show that singlet oxygen ((1) Δg ) is formed upon Li2 O2 oxidation at potentials above 3.5 V. Singlet oxygen was detected through a reaction with a spin trap to form a stable radical that was observed by time- and voltage-resolved in operando EPR spectroscopy in a purpose-built spectroelectrochemical cell. According to our estimate, a lower limit of approximately 0.5 % of the evolved oxygen is singlet oxygen. The occurrence of highly reactive singlet oxygen might be the long-overlooked missing link in the understanding of the electrolyte degradation and carbon corrosion reactions that occur during the charging of Li-O2 cells.
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