Identifying the components of the solid–electrolyte interphase in Li-ion batteries

The importance of the solid–electrolyte interphase (SEI) for reversible operation of Li-ion batteries has been well established, but the understanding of its chemistry remains incomplete. The current consensus on the identity of the major organic SEI component is that it consists of lithium ethylene di-carbonate (LEDC), which is thought to have high Li-ion conductivity, but low electronic conductivity (to protect the Li/C electrode). Here, we report on the synthesis and structural and spectroscopic characterizations of authentic LEDC and lithium ethylene mono-carbonate (LEMC). Direct comparisons of the SEI grown on graphite anodes suggest that LEMC, instead of LEDC, is likely to be the major SEI component. Single-crystal X-ray diffraction studies on LEMC and lithium methyl carbonate (LMC) reveal unusual layered structures and Li+ coordination environments. LEMC has Li+ conductivities of >1 × 10−6 S cm−1, while LEDC is almost an ionic insulator. The complex interconversions and equilibria of LMC, LEMC and LEDC in dimethyl sulfoxide solutions are also investigated

Identification of Li battery electrolyte degradation products through direct synthesis and characterization of alkyl carbonate salts

Aiming toward the identification of carbonate-based electrolyte degradation species formed during high-temperature cycling of Li/M-O cells, we have embarked in the synthesis and characterization of lithium-based alkyl carbonates. Through the reaction of commercial or synthesized lithium alkoxides with carbon dioxide, we succeeded in preparing lithium methyl, ethyl, propyl mono carbonates, and therefore we have extended our work to the synthesis of lithium ethandiol-bis carbonate. Their analytical characterization ( 1H and 13C nuclear magnetic resonance, electrospray ionization-mass spectroscopy, Fourier transform: infrared/attenuated total reflection) is described. Furthermore, to our surprise, we managed to demonstrate that these well-known alkyl carbonates show some electrochemical reactivity toward Li. © 2005 The Electrochemical Society. All rights reserved

Characterization of lithium alkyl carbonates by X-ray photoelectron spectroscopy: Experimental and theoretical study

Lithium alkyl carbonates ROCO2Li result from the reductive decomposition of dialkyl carbonates, which are the organic solvents used in the electrolytes of common lithium-ion batteries. They play a crucial role in the formation of surface layers at the electrode/electrolyte interfaces. In this work, we report on the X-ray photoelectron spectroscopy (XPS) characterization of synthesized lithium methyl and ethyl carbonates. Using Hartree-Fock ab initio calculations, we interpret and simulate the valence spectra of both samples, as well as several other Li alkyl carbonates involved in Li-ion batteries. We show that Li alkyl carbonates can be identified at the electrode's surface by a combined analysis of XPS core peaks and valence spectra. © 2005 American Chemical Society

Mass spectrometry investigations on electrolyte degradation products for the development of nanocomposite electrodes in lithium ion batteries

In the continuing challenge to find new routes to improve the performance of commercial lithium ion batteries cycling in alkyl carbonate-based electrolyte solutions, original designs, and new electrode materials are under active worldwide investigation. Our group has focused on the electrochemical behavior of a new generation of nanocomposite electrodes showing improved capacities (up to 3 times the capacity of conventional electrode materials). However, moving down to "nanometric-scale" active materials leads to a significant increase in electrolyte degradation, compared to that taking place within commercial batteries. Postmortem electrolyte studies on experimental coin cells were conducted to understand the degradation mechanisms. Structural analysis of the organic degradation products were investigated using a combination of complementary high-resolution mass spectrometry techniques: desorption under electron impact, electrospray ionization, and gas chromatography coupled to a mass spectrometer equipped with electron impact and chemical ionization ion sources. Numerous organic degradation products such as ethylene oxide oligomers (with methyl, hydroxyl, phosphate, and methyl carbonate endings) have been characterized. In light of our findings, possible chemical or electrochemical pathways are proposed to account for their formation. A thorough knowledge of these degradation mechanisms will enable us to propose new electrolyte formulations to optimize nanocomposite-based lithium ion battery performance. © 2006 American Chemical Society

XPS identification of the organic and inorganic components of the electrode/electrolyte interface formed on a metallic cathode

X-ray photoelection spectroscopy (XPS) was used to determine the nature and composition of electrode/electrolyte interfaces forming during the 55°C cycling of Li-based cells in ethylene carbonate:dimethyl carbonate LiPF 6 electrolyte using a heat-treated stainless steel substrate as the positive electrode. From a classical analysis of the XPS C 1s, O 1s, F 1s, P 2p, and Li 1s core peak spectra complemented by an unusual detailed interpretation of XPS valence spectra, we could follow, as a function of the cell cycling history, the evolution and nature of the species constituting the organic/inorganic layer as well as determine its approximate composition. We have shown that this surface layer mainly consists of PEO oligomers (-CH 2-CH2-O-)n, carbonates Li2CO 3 and/or CH3OCO2Li, LiPF6 salt, and of degradation products of the salt such as LiF and phosphates. Moreover, we give evidence that this layer does not only grow but also becomes richer in CH3OCO2Li and LiF species upon cycling. © 2005 The Electrochemical Society. All rights reserved

Combining electrochemistry and metallurgy for new electrode designs in Li-ion batteries

To benefit from the large electrochemical capacity advantages offered by Li-driven conversion reactions and to overcome poor kinetics, a new electrode configuration concept is reported. The originality of this electrode design is nested in metallurgical aspects of stainless steel, namely, the appearance of temperature-driven surface microstructures that enable the growth of a nanostructured, electrochemically active, chromium-rich oxide surface layer in close contact with a current collector. The thickness of the oxide layer can reach hundreds of nanometers and is shown to be rooted in the preferential migration of Cr toward the sample surface. We further show that chemical etching of the stainless steel surface, prior to high-temperature annealing, enables reversible capacities as high as 750 mAh/g of chromium-rich oxide for at least 800 cycles. On the basis of modeling, several scenarios involving stainless steel/chromium-based oxides current collectors of various porosities show how this new electrode configuration could boost the electrode capacity beyond that of today's carbon negative electrodes used in Li-ion cells by a factor of 2 or 3. © 2005 American Chemical Society