Origin of Capacity Fading in Nano-Sized Co3O4Electrodes: Electrochemical Impedance Spectroscopy Study

Transition metal oxides have been suggested as innovative, high-energy electrode materials for lithium-ion batteries because their electrochemical conversion reactions can transfer two to six electrons. However, nano-sized transition metal oxides, especially Co3O4, exhibit drastic capacity decay during discharge/charge cycling, which hinders their practical use in lithium-ion batteries. Herein, we prepared nano-sized Co3O4with high crystallinity using a simple citrate-gel method and used electrochemical impedance spectroscopy method to examine the origin for the drastic capacity fading observed in the nano-sized Co3O4anode system. During cycling, AC impedance responses were collected at the first discharged state and at every subsequent tenth discharged state until the 100th cycle. By examining the separable relaxation time of each electrochemical reaction and the goodness-of-fit results, a direct relation between the charge transfer process and cycling performance was clearly observed

An update on the reactivity of nanoparticles co-based compounds towards Li

In our comprehensive understanding of reactivity of CoO towards Li, we studied the effect of CoO electrode weight and composition (carbon-free or not) on the cycling performances of CoO/Li half-cells. Capacity and lifetime were measured as a function of the cycling rate and temperature. The lightest electrodes (2 mg/cm2) were shown to behave the best, with sustained capacities as high as 600 mAh/g up to about 250 cycles at 20 °C, and with capacities that peaked up to 1700 mAh/g when cycling was performed at 75 °C. Searching for the origin of this huge "extra capacity" over the normal conversion process (Co2+O → Co0, 715 mAh/g), we dissociated phenomena by testing carbon-loaded and carbon-free CoO-based electrodes in CoO/Li half cells. We unravelled a temperature-driven capacity rate increase similar for both cells, although the initial reversible capacity was quite different. The carbon-free CoO/Li half cell showed limited initial reversible capacity due to the poor efficiency of the conversion process for non-conducting electrodes. This increase appears to be nested in the reversible growth of a polymeric gel-like film resulting from kinetically activated electrolyte degradation. Polymeric layers were also shown to form from the Li electrochemical reduction of numerous binary phases, differentiating either by the nature of the 3d-metal (Cu, Ni, Fe instead of Co) or that of the anion (S, F, N instead of O). Finally, purely coincidental or not, from comparative studies of CoO, CoSb3, and CoSb2 Li half-cells, we found that, to a certain extent, the capacity amplitude associated with the growth of the polymer film scales with the surface developed by Co nano-particles. This fact would imply a possible catalytic role of 3d metals in assisting the electrolyte decomposition. © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved

Structure, texture and reactivity versus lithium of chromium-based oxides films as revealed by TEM investigations

Analytical high resolution transmission electron microscopy has been used to clearly evidence the electrochemical reactivity of Cr2O3 thin films versus lithium. We showed that during the reduction process, chromium sesquioxide (+III) transforms first into chromium monoxide (+II) and then into metallic chromium nanoparticles embedded into a Li2O matrix at 0 V. On the subsequent recharge up to 3 V, we did not convert back to Cr2O3, but only to a chromium monoxide implying a partial re-oxidation process, thus explaining the irreversibility measured during the first cycle. We extended this study to the comprehension of the electrochemical performances of Cr2O3-based electrodes obtained by thermal treatment, under different atmospheres, of chromium rich stainless steel disks. In addition to the characterization of the particles forming the so-obtained electro-active layers, we showed, using nano-probe EDS, that a mixed "Fe-Cr-O" oxide could react versus lithium, through a conversion reaction mechanism, leading to alloyed metallic nanoparticles upon reduction and nanograins of mixed oxide during the following oxidation. © 2006 Elsevier B.V. All rights reserved

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

Identification of Li-based electrolyte degradation products through DEI and ESI high-resolution mass spectrometry

The nature and composition of gel-like organic films forming during the cycling of Li-based cells functioning through a conversion reaction process were investigated. Besides infrared techniques, both desorption electron impact (DEI) and electrospray ionization (ESI) mass spectrometry were used to study the large amounts of films obtained after extended cycling at 55°C. We give direct evidence for the formation, depending on the type of electrolytes used that differ by the nature of either the Li-based salt (LiPF6, LiCF3SO3) or solvents (dimethyl carbonate, propylene carbonate, ethylene carbonate, and their mixtures), of either phosphate-ending PEG-(polyethylene glycol) type chains, PEG chains (CH2-CH 2-O)n, or polypropylene glycol chains (CH(CH 3)-CH2-O)n with n values ranging from 1 to 9, and also trimethyl phosphate. The reaction schemes involving either electrochemical or chemical processes are proposed to describe the formation of such species. © 2004 The Electrochemical Society. All rights reserved

From the vanadates to 3d-metal oxides negative electrodes

With respect to lithium batteries, vanadate-based electrodes display large electrochemical capacities that rapidly decay during the cycling. A full study of the 3d-metal(M)-based vanadates performed using in situ DRX, XAS and TEM has enabled to pinpoint the important role of the 3dmetal cations, and has motivated the study of simple metal oxides MO, (M = Co, Ni, Cu, Fe) in rechargeable Li cells. We found that these materials could reversibly react with a large amount of lithium leading to capacities as high as 800 mAh/g. The reactivity mechanism totally differs from the well established one based on Li insertion-deinsertion or Li alloying reactions but mainly involves the formation of highly reactive metallic nanoparticles that favor Li20 formation-decomposition. Besides, we gave experimental evidence of an electrochemically driven polymerization-dissolution process at low potential, which is highly reversible, and emphasized the importance of the role of the electrolyte on such a process. Finally, the universality of this mechanism to account for the large Li reactivity at low voltage in many 3d-metal (Co, Ni, Fe, Cu, Mn)-based oxides is discussed together with the urgent issues to be solved for such oxide anodes to stand as serious alternative candidates for today's carbon anodes in Li-ion cells

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