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

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.

Ni-rich layered oxides, like NCM-811, are promising lithium-ion battery cathode materials for applications such as electric vehicles. However, pronounced capacity fading, especially at high voltages, still lead to a limited cycle life, whereby the underlying degradation mechanisms, e.g. whether they are detrimental reactions in the bulk or at the surface, are still controversially discussed. Here, we investigate the capacity fading of NCM-811/graphite full-cells over 1000 cycles by a combination of in situ synchrotron X-ray powder diffraction, impedance spectroscopy, and X-ray photoelectron spectroscopy. In order to focus on the NCM-811 material, we excluded Li loss at the anode by pre-lithiating the graphite. We were able to find a quantitative correlation between NCM-811 lattice parameters and capacity fading. Our results prove that there are no considerable changes in the bulk structure, which could be responsible for the observed ≈20% capacity loss over the 1000 cycles. However, we identified the formation of a resistive surface layer, which is responsible for (i) an irreversible loss of capacity due to the material lost for its formation, and (ii) for a considerable impedance growth. Further evidence is provided that the surface layer is gradually formed around the primary NCM-811 particles.

Washing is a commonly used method to remove surface impurities of cathode materials for lithium-ion batteries. However, a clear mechanistic understanding of the washing process is missing in the literature. In this study, we will investigate the effect of washing and subsequent drying of nickel-rich NCM cathodes (85% nickel) with respect to gassing and impedance of the washed cathodes. By on-line electrochemical mass spectrometry (OEMS), we will show a drastic reduction of the O 2 release above 80% SOC for the NCM washed with deionized water, suggesting the formation of an oxygen-depleted surface layer on the NCM particle surface. The modification of the surface can be confirmed by a strong impedance buildup of cathodes composed of washed NCM (using a microreference electrode in a full-cell), revealing that the impedance increases strongly with increasing drying temperature after washing. Last, we will propose a comprehensive mechanism on the processes occurring during the washing/drying process of nickel-rich NCM materials and identify the drying temperature after washing as the dominant factor influencing the surface properties.

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Significant mass-transport resistances in polymer-electrolyte-fuel-cell catalyst layers (CLs) impose a lower limit on Pt-loading levels, hindering widespread fuel-cell commercialization. The origin of this resistance remains unclear. Minimization of CL mass-transport resistance is imperative to achieve better CL design and performance. In this paper, an operando method based on H 2 limiting current is used to characterize and quantify CL resistance in traditional porous Pt/carbon-based electrodes. CL sub-resistances are isolated using continuum multiscale modeling and experiments, investigating the effects of reactant molecular weight, pressure, and ionomer to carbon weight ratio. The results expose CL resistance including both interfacial and transport components, although the majority of the CL resistance is ascribed to a local resistance close to the Pt reaction sites, which includes interfacial resistance and local transport resistance. Variations in temperature, humidity, and primary particle loading (Pt:C ratio) highlight the impact of operating conditions and CL design parameters on CL sub-resistances. The observed trends guide optimization of CL design to achieve novel low-loaded fuel-cell electrodes.

Replacement of conventional cars with battery electric vehicles (BEVs) offers an opportunity to significantly reduce future carbon dioxide emissions. One possible way to facilitate widespread acceptance of BEVs is to replace the lithium-ion batteries used in existing BEVs with a lithium-sulfur battery, which operates using a cheap and abundant raw material with a high specific energy density. These significant theoretical advantages of lithium-sulfur batteries over the lithium-ion technology have generated a lot of interest in the system, but the development of practical prototypes, which could be successfully incorporated into BEVs, remains slow. To accelerate the development of improved lithium-sulfur batteries, our work focuses on the mechanistic understanding of the processes occurring inside the battery. In particular, we study the mechanism of the charging process and obtain spatially resolved information about both solution and solid phase intermediates in two locations of an operating Li 2 S-Li battery: the cathode and the separator. These measurements were made possible through the combination of a spectro-electrochemical cell developed in our laboratory and synchrotron based operando X-ray absorption spectroscopy measurements. Using the generated data, we identify a charging mechanism in a standard DOL-DME based electrolyte, which is consistent with both the first and subsequent charging processes. Lithium-sulfur (Li-S) batteries are an emerging battery technology that has the potential to meet the energy density and cost requirements of electric vehicles. Recently, several studies have identified that the attainment of areal capacities as high as 4-8 mAh/cm 2 while minimizing the electrolyte content are the key factors in meeting these requirements.1-3 The only currently commercialized Li-S battery has a significantly lower areal capacity of 2.5 mAh/cm 2 and operates in the presence of excess electrolyte, 4 necessitating significant technological breakthroughs to facilitate the possible use of Li-S batteries in the transportation sector. One of the main barriers to achieving such breakthroughs is the lack of fundamental understanding of the mechanism behind the operation of Li-S batteries. 1,5,6 In particular, it is not yet clear how the mechanism of discharge differs from the charge mechanism, 5 and if these two processes might change upon an increase in active material loading or reduction in electrolyte volume. 1 Consequently, there is a pressing need for performing operando characterization of Li-S batteries under a variety of conditions to identify fundamental aspects of the charging and discharging processes.One attractive but insufficiently explored system for a mechanistic characterization of Li-S batteries is the charging process of a Li 2 S cathode, a possible alternative to the conventional S 8 cathode, with a potential to enable batteries with silicon or tin rather than lithium anodes. 7,8 Specifically, it has been recently reported that a Li-S battery, which is assembled in a discharged s...