Interaction of Ultrathin Films of Ethylene Carbonate with Oxidized and Reduced Lithium Cobalt Oxide—A Model Study of the Cathode|Electrolyte Interface in Li‐Ion Batteries

Abstract

Li-ion batteries (LIBs). [1][2][3][4][5][6][7][8][9] One of the most crucial parts for their performance and lifetime is the interfacial layer between electrodes and electrolyte, the so-called solid|electrolyte interphase (SEI) at the electrode|electrolyte interface (EEI), which is formed during initial battery operation. [10][11][12][13][14][15] It is generated by the decomposition of electrolytes, forming a passivating layer, which is beneficial for the longtime stability of the battery. The detailed atomic/molecular-scale understanding of the SEI formation process is hampered by the complexity of the cell processes participating therein, including the transfer of Li + ions across the interphase, electrolyte reduction/oxidation, and chemical reactions, all of which are expected to play a role in the SEI formation. Furthermore, standard electrolytes usually consist of a mix of several components such as ethylene carbonate (EC), propylene carbonate, diethyl carbonate (DEC), or dimethyl carbonate and a lithium salt such as lithium hexafluorophosphate (LiPF 6 ), [16] which increases the complexity and makes it difficult to identify individual reactions. Furthermore, the processes at the EEI in LIBs are hardly accessible by techniques providing molecularscale information.One possible way to determine the initial stages of the SEI formation at the EEI at the atomic/molecular level involves the use of surface science techniques, studying the interaction of individual components of electrolytes, such as the typical key component EC (or other electrolyte components like ionic liquids), with well-defined model electrodes under idealized ultrahigh-vacuum (UHV) conditions, [17][18][19][20][21][22][23][24][25][26][27][28][29] which is focus of the ongoing work in our laboratory. Following a previous study on the interaction of EC with Li-free and lithiated highly oriented pyrolytic graphite (HOPG) as model for the anode, [17] we here report the results of a similar type of study on the interaction of EC with well-defined LiCoO 2 electrode surfaces, both fully oxidized LiCoO 2 and partly reduced LiCoO 2−δ surfaces, as models for the cathode. These two different types of samples were chosen since they are expected to exist also under application conditions, and since the oxidation state is expected to influence also their reactivity. Aiming at a detailed, molecular-scale understanding of the initial stages of the solid|electrolyte interphase (SEI) formation in Li-ion batteries, the interaction of the common electrolyte solvent component ethylene carbonate (EC)with fully lithiated LiCoO 2 and reduced LiCoO 2−δ films as model electrodes for the cathode is investigated. The results are compared with previous findings for pristine and lithiated highly oriented pyrolytic graphite, serving as model anode. Employing X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy measurements, it is found that vapor deposition of EC on LiCoO 2 and LiCoO 2−δ at 80 K results in molecularly adsorbed EC, both in the mon...

Surface Science and Electrochemical Model Studies on the Interaction of Graphite and Li‐Containing Ionic Liquids

“…At the same time, a new peak appears at 685.8 eV in the F 1s spectrum (F LiF , filled green), which we attribute to LiF . Such a peak was recently obtained also upon stepwise deposition of Li onto an adsorbed IL (sub)monolayer on HOPG under UHV conditions, where details about the decomposition mechanism were revealed by a combined computational analysis . The formation of LiF as decomposition product is also in excellent agreement with ab initio molecular dynamic simulations by Ando and co‐workers, which included electric field effects.…”

“…Such an increase in reductive currents in Li + ‐containing electrolyte at potentials below 0.2 V is usually attributed to Li + intercalation into graphite . Although the intercalation of Li + into the basal plane of HOPG substrates is very slow, it still may take place, for example, via step and edge defects on the surface . In addition to Li + intercalation, the currents observed around 0.2 V have also been attributed both to [TFSI] − decomposition and to [BMP] + intercalation as competing processes .…”

“…The N TFSI peak shows a very low intensity decrease of only 10 %, which is in contrast to the considerable intensity loss of all other anion‐related peaks in the other spectral ranges by around 60 %. Hence, we assume that the decline of the N TFSI signal is compensated by the formation of (Li‐bound) decomposition products of [TFSI] − , such as LiNSO 2 CF 3 , which have a rather similar N 1s BE as [TFSI] − . Interestingly, the cation peak intensity loss is much more pronounced than that of the N TFSI peak (about 80 %), indicating that most of the cations are decomposed and that the extent of [BMP] + decomposition even exceeds that of [TFSI] − decomposition.…”

“…Interestingly, the cation peak intensity loss is much more pronounced than that of the N TFSI peak (about 80 %), indicating that most of the cations are decomposed and that the extent of [BMP] + decomposition even exceeds that of [TFSI] − decomposition. The new peak at 398.4 eV has been ascribed to Li 3 N although other nitrogen‐containing Li‐bound fragments, for example, linear LiC x H y N species resulting from a ring‐opening of the pyrrolidinium molecule, might be possible as well. Despite the strong changes in the individual peak intensities, however, the total peak intensity of the N 1s spectrum remains almost constant, similar to that in the O 1s region, but different from the F 1s signal.…”

“…Owing to its high decomposition temperature, very low vapor pressure, and a large stability window ranging from −2.5 to 3.0 V versus Ag/AgCl (about 0–5 V vs. Li/Li + ), [BMP][TFSI] is a very promising candidate for LIBs. Special interest was placed on the interactions at the substrate|IL interface under UHV conditions by using several model substrates, for example, single‐crystalline metal surfaces, oxide surfaces, and highly oriented pyrolytic graphite . The above studies, which were conducted in the absence of an applied potential and which focused on the structure formation and the decomposition of the ionic liquid, clearly demonstrated that the chemical interaction with the substrate surface and/or with the added lithium is sufficient to cause decomposition of the IL.…”