The quality of metal oxide-based battery active materials is compromised by surface contamination from storage and handling at ambient conditions. We present a detailed analysis of the true nature ...and the quantity of the surface contaminants on two different cathode active materials, the widely used LiNi1/3Co1/3Mn1/3O2 (NCM111) and the Ni-rich LiNi0.8Co0.1Mn0.1O2 (NCM811). We process these materials in three distinct conditions "wet" (excessive exposure to moisture), "dry" (standard drying of as-received materials), and "calcined" (heat-treatment of cathode powders). Surface contaminants are then quantified by thermogravimetric analysis coupled with mass spectrometry (TGA-MS), and their reactivity with an ethylene carbonate-based electrolyte is evaluated using on-line mass spectrometry (OMS). We demonstrate that not only the commonly assumed LiOH and Li2CO3 residues account for NCM performance deterioration upon storage in moisture and CO2 containing atmosphere, but also basic transition metal hydroxides/carbonates formed on the material surface. Eventually, we showcase a thermal treatment that removes these transition metal based surface contaminants and leads to superior cycling stability.
The anodic oxidation stability of battery components like the conductive carbon black (Super C65) and the co-solvent ethylene carbonate (EC) is of great relevance, especially with regards to ...high-voltage cathode materials. In this study, we use On-line Electrochemical Mass Spectrometry (OEMS) to deconvolute the CO and CO2 evolution from the anodic oxidation of carbon and electrolyte by using a fully 13C-isotope labeled electrolyte based on ethylene carbonate with 2 M LiClO4. We present a newly developed two-compartment cell, which provides a tight seal between anode and cathode compartment via a solid Li+-ion conducting separator, and which thus allows us to examine the effect of trace amounts of water on the anodic oxidation of carbon (12C) and ethylene carbonate (13C) at high potentials (> 4.5 V) and 10 to 60°C. Moreover, we report on the temperature dependence of the water-driven hydrolysis of ethylene carbonate accompanied by CO2 evolution. Finally, by quantifying the evolution rates of 12CO/12CO2 and 13CO/13CO2 at 5.0 V, we demonstrate that the anodic oxidation of carbon and electrolyte can be substantial, especially at high temperature and in the presence of trace water, posing significant challenges for the implementation of 5 V cathode materials.
Efficient electrochemical water splitting to hydrogen and oxygen is considered a promising technology to overcome our dependency on fossil fuels. Searching for novel catalytic materials for ...electrochemical oxygen generation is essential for improving the total efficiency of water splitting processes. We report the synthesis, structural characterization, and electrochemical performance in the oxygen evolution reaction of Fe-doped NiO nanocrystals. The facile solvothermal synthesis in tert-butanol leads to the formation of ultrasmall crystalline and highly dispersible Fe x Ni1–x O nanoparticles with dopant concentrations of up to 20%. The increase in Fe content is accompanied by a decrease in particle size, resulting in nonagglomerated nanocrystals of 1.5–3.8 nm in size. The Fe content and composition of the nanoparticles are determined by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy measurements, while Mössbauer and extended X-ray absorption fine structure analyses reveal a substitutional incorporation of Fe(III) into the NiO rock salt structure. The excellent dispersibility of the nanoparticles in ethanol allows for the preparation of homogeneous ca. 8 nm thin films with a smooth surface on various substrates. The turnover frequencies (TOF) of these films could be precisely calculated using a quartz crystal microbalance. Fe0.1Ni0.9O was found to have the highest electrocatalytic water oxidation activity in basic media with a TOF of 1.9 s–1 at the overpotential of 300 mV. The current density of 10 mA cm–2 is reached at an overpotential of 297 mV with a Tafel slope of 37 mV dec–1. The extremely high catalytic activity, facile preparation, and low cost of the single crystalline Fe x Ni1–x O nanoparticles make them very promising catalysts for the oxygen evolution reaction.
Ultrasmall, crystalline, and dispersible NiO nanoparticles are prepared for the first time, and it is shown that they are promising candidates as catalysts for electrochemical water oxidation. Using ...a solvothermal reaction in tert‐butanol, very small nickel oxide nanocrystals can be made with sizes tunable from 2.5 to 5 nm and a narrow particle size distribution. The crystals are perfectly dispersible in ethanol even after drying, giving stable transparent colloidal dispersions. The structure of the nanocrystals corresponds to phase‐pure stoichiometric nickel(ii) oxide with a partially oxidized surface exhibiting Ni(iii) states. The 3.3 nm nanoparticles demonstrate a remarkably high turn‐over frequency of 0.29 s–1 at an overpotential of g = 300 mV for electrochemical water oxidation, outperforming even expensive rare earth iridium oxide catalysts. The unique features of these NiO nanocrystals provide great potential for the preparation of novel composite materials with applications in the field of (photo)electrochemical water splitting. The dispersed colloidal solutions may also find other applications, such as the preparation of uniform hole‐conducting layers for organic solar cells.
Ultrasmall, crystalline, and dispersible NiO nanoparticles are prepared for the first time using a solvothermal reaction in tert‐butanol. These nanocrystals can be prepared with sizes tunable from 2.5 to 5 nm and are highly efficient catalysts for electrochemical oxygen generation.
Carbon coatings on cathode materials with low electrical conductivity like phospho-olivines LiMPO4 (M = 3d-transition metal) are known to improve their performance in Li-ion batteries. However, at ...high potentials and in the presence of water, the stability of carbon coatings on high-voltage materials (e.g., LiCoPO4) may be limited due to the anodic oxidation of carbon. In this work, we describe the synthesis of LiFePO4 (LFP) with an isotopically labeled 13C carbon coating (characterized by Raman spectroscopy, electrical conductivity, and charge/discharge rate capability tests) as a model compound to study the anodic stability of carbon coated cathode materials in ethylene carbonate-based electrolytes. We characterize the degradation of the 13C carbon coating by On-line Electrochemical Mass Spectrometry (OEMS) through the 13CO2 and 13CO signals in order to differentiate the anodic oxidation of the coating (13C) from the oxidation of electrolyte, conductive carbon, and binder (all 12C) in the electrode. The oxidation of the carbon coating takes place at potentials ≥ 4.75 V for electrolyte without H2O (< 20 ppm) and ≥ 4.5 V for electrolyte with 4000 ppm H2O, and it is strongly enhanced for H2O-containing electrolyte. The extent of carbon coating oxidation over 100 h at 4.8 and 5.0 V vs. Li/Li+ (25°C) is projected on the basis of our OEMS data, suggesting that carbon coatings have insufficient stability at such high cathodic potentials. Furthermore, our results prove the in situ formation of H2O during the anodic decomposition of ethylene carbonate-containing electrolyte. The H2O formation is monitored via the detection of gaseous POF3, which is formed from the reaction of LiPF6 with H2O.
Layered lithium transition metal oxides are state-of-the-art cathode materials for Li-ion batteries. Nickel-rich layered oxides suffer from high surface reactivity toward ambient air. Besides ...hydroxides, carbonates are known to be the major surface impurities formed. While the decomposition of Li2CO3 in a battery cell has been studied extensively, the mechanistic aspects of its decomposition during cell formation/cycling are still highly controversial.
The decomposition reaction of Li2CO3 in a standard Li-ion battery electrolyte is studied by on-line electrochemical mass spectrometry, employing an electrode only consisting of Li2CO3 and conductive carbon. By modifying the electrode configurations in the cell, we are able to show that the decomposition of Li2CO3 occurs as a chemical process without any direct electrochemical oxidation of the Li2CO3 particles. Their decomposition proceeds by a chemical process via protons that are formed upon anodic oxidation of the electrolyte solvent and of trace impurities in alkyl carbonate based electrolytes. By adding common impurities in Li-ion battery electrolytes as ethanol and ethylene glycol, whose electrochemical oxidation at rather low anodic potentials (≈ 3.5 V vs Li+/Li) results in the formation of protons, the onset of CO2 evolution from Li2CO3 is accordingly shifted to such low potentials. Tracing the proton-induced LiPF6 decomposition products PF5/POF3, the formation of protons can be followed quantitatively and a direct correlation with the CO2 produced by the proton-induced Li2CO3 decomposition is shown. Implications of these findings for transition metal oxide based cathode materials in Li-ion batteries are discussed based on the here found decomposition mechanism.
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We report the synthesis of MOF@lipid nanoparticles as a versatile and powerful novel class of nanocarriers based on metal-organic frameworks (MOFs). We show that the MOF@lipid system can effectively ...store dye molecules inside the porous scaffold of the MOF while the lipid bilayer prevents their premature release. Efficient uptake of the MOF@lipid nanoparticles by cancer cells makes these nanocarriers promising for drug delivery and diagnostic purposes.
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 O2 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 micro-reference 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.
We report the synthesis of MOF@lipid nanoparticles as a versatile and powerful novel class of nanocarriers based on metal-organic frameworks (MOFs). We show that the MOF@lipid system can effectively ...store dye molecules inside the porous scaffold of the MOF while the lipid bilayer prevents their premature release. Efficient uptake of the MOF@lipid nanoparticles by cancer cells makes these nanocarriers promising for drug delivery and diagnostic purposes.
We report the synthesis of MOF@lipid nanoparticles as a versatile and powerful novel class of nanocarriers based on metal-organic frameworks (MOFs).
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