E. Peled

The Electrochemical Behavior of Alkali and Alkaline Earth Metals in Nonaqueous Battery Systems—The Solid Electrolyte Interphase Model

It is suggested that in practical nonaqueous battery systems the alkali and alkaline earth metals are always covered by a surface layer which is instantly formed by the reaction of the metal with the electrolyte. This layer, which acts as an interphase between the metal and the solution, has the properties of a solid electrolyte. The corrosion rate of the metal, the mechanism of the deposition‐dissolution process, the kinetic parameters, the quality of the metal deposit, and the half‐cell potential depend on the character of the solid electrolyte interphase (SEI).

Lithium Sulfur Battery Oxidation/Reduction Mechanisms of Polysulfides in THF Solutions

The redox processes of at a glassy carbon electrode in THF was studied by programmed cyclic voltammetry in the range of +1300 to −2000 mV (vs. polysulfide reference electrode) at sweep rates of 2–200 mV/s. One anodic and up to three cathodic peaks were detected. The anodic peak seems to result from the oxidation of all PSs through the same intermediate to elemental sulfur. The first cathodic peak is caused by the reduction of all PS to in a diffusion controlled reaction. The second reduction peak most likely arises from the reduction of to . This is apparently preceded by a chemical step. The third reduction peak is caused by the reduction of to or S2− or a mixture of both in a diffusion‐controlled reaction. The high Tafel slope of the third peak apparently results from passivation of the electrode by the precipitation of and .

Advanced Model for Solid Electrolyte Interphase Electrodes in Liquid and Polymer Electrolytes

Recent studies show that the SEI on lithium and on Li{sub x}C{sub 6} anodes in liquid nonaqueous solutions consists of many different materials including Li{sub 2}O, LiF, LiCl, Li{sub 2}CO{sub 3}, LiCO{sub 2}-R, alkoxides, and nonconducting polymers. The equivalent circuit for such a mosaic-type SEI electrode is extremely complex. It is shown that near room temperature the grain-boundary resistance (R{sub gb}) for polyparticle solid electrolytes is larger than the bulk ionic resistance. Up to now, all models of SEI electrodes ignored the contribution of R{sub gb} to the overall SEI resistance. The authors show here that this neglect has no justification. On the basis of recent results, the authors propose here for SEI electrodes equivalent circuits which take into account the contribution of grain-boundary and other interfacial impedance terms. This model accounts for a variety of different types of Nyquist plots reported for lithium and Li{sub x}C{sub 6} electrodes in liquid nonaqueous and polymer electrolytes.

Improved Graphite Anode for Lithium‐Ion Batteries Chemically Bonded Solid Electrolyte Interface and Nanochannel Formation

The effects of mild oxidation (burning) of 2 synthetic graphites on the reversible (Q{sub R}) and irreversible (Q{sub IR}) capacities, anode-degradation rate (on cycling) in three different electrolytes and graphite-surface topology have been studied. STM images of both modified graphites show nanochannels having an opening of a few nanometers and up to tens of nanometers. It is believed that these nanochannels are formed at the zigzag and armchair faces between two adjacent crystallites and in the vicinity of defects and impurities. Mild burn-off was found to improve performance in Li/Li{sub x}C cells: Q{sub R} is increased by 10--30%, Q{sub IR} is generally decreased (for less than 6% burn-off) and Li{sub x}C{sub 6} anode degradation rate is much lower. Performance improvement is attributed to the formation of a solid electrolyte interface (SEI) chemically bonded to the surface carboxylic groups at the zigzag and armchair faces, and to accommodation of extra lithium at the zigzag, armchair, and other edge sites and nanovoids.

Electrochemistry of a nonaqueous lithium/sulfur cell

Abstract The development and the electrochemistry of low-rate laboratory prototype Li/S button cells is described. The cell consists of a lithium anode, a porous catalytic current collector which is loaded with sulfur, and an organic solvent containing lithium polysulfide. The case of the cell was made from stainless steel and sealing was accomplished by the use of a combination of organic elastomer and cement (with no crimp). After 3 weeks storage at 60 °C, the button cells lost only about 1 mg of weight. The lithium polysulfide reacts with the Li anode to form a passivating layer which acts as a solid electrolyte interphase (SEI). The e.m.f. of the cells changes from 2.38 to 2.15 V depending on the composition of the solutions. Cells exhibit flat discharge curves at low drains. The energy density of the cells is 730 W h/kg or 900 W h/l at room temperature and 950 W h/kg or 1200 W h/l at 60 °C (calculated on the basis of all cell components, excluding the case). Storage and discharge tests at 60 °C show a capacity loss of 2 – 5% per month depending on solution composition. This indicates a shelf life of at least 10 years at room temperature.

Lithium‐Sulfur Battery: Evaluation of Dioxolane‐Based Electrolytes

The lithium‐sulfur battery recently developed in our laboratory shows 95%+ sulfur utilization but low rate capability due to its poorly conducting electrolyte, which is based on a THF:toluene solvent mixture. In order to increase the rate capability of this cell, dioxolane‐based electrolytes have been evaluated. The conductivity of electrolytes consisting of mixtures of THF, toluene, and dioxolane were measured in the temperature range −30° to +60°C. The compatibility of lithium with these electrolytes was also studied. It was found that dioxolane‐rich electrolytes are compatible with lithium and have one order of magnitude higher conductivity than do THF:toluene‐rich electrolytes. However, sulfur utilization in dioxolane‐rich electrolytes is only 50%, even at a very low discharge rate. This is due to a different discharge product, namely, .

A Study of Highly Oriented Pyrolytic Graphite as a Model for the Graphite Anode in Li‐Ion Batteries

The mechanisms of oxidation of the basal plane and of the cross-sectional face of highly oriented pyrolytic graphite (HOPG) and the formation of a solid electrolyte interphase (SEI) on HOPG samples that were cycled in ethylene carbonate:diethyl carbonate (EC:DEC 1:2) solutions containing 1 M LiAsF{sub 6} were studied. X-ray photoelectron spectroscopy, energy dispersive spectrometry, and scanning electron microscope techniques were used for the analysis of the surface layer formed on the basal plane and cross section of HOPG. The analysis indicates that the oxidation mechanisms of the basal plane and the cross section are entirely different. The SEI formed in the LiAsF{sub 6} solution is thinner on the basal plane than on the cross section and its composition is different. The SEI formed on the cross section is rich in inorganic compounds whereas the SEI formed on the basal plane is rich in organic compounds. Thus it can be concluded that on the basal plane, the greatest contribution to SEI formation is solvent reduction (EC and DEC), whereas on the cross-sectional face, it is electrolyte salt (LiAsF{sub 6}) reduction.

XPS analysis of the SEI formed on carbonaceous materials

Two carbonaceous materials were produced by chemical vapour deposition of ethylene and by pyrolysis of dehydrated sucrose. Electrochemical cells assembled from these materials and metallic lithium were cycled between 0.00 and 2.00 V vs. Li/Li+ in ethylene carbonate/diethylcarbonate electrolytes containing LiPF6 or LiAsF6. The solid electrolyte interphase (SEI) formed on the carbons was characterised by X-ray photoelectron spectroscopy (XPS). We suggest that the carbon matrix has a more marked effect on the composition and thickness of the SEI than does the nature of the electrolyte. The SEI formed on graphite-like soft carbon in both electrolytes proved to be carbonate-free, its inorganic part consisting almost exclusively of LiF, while the SEI formed on hard (non-graphitizable) carbon was found to be considerably thicker and contained, in addition, phosphorus and arsenic compounds. In the bulk SEI, polymer structures (i.e., solvent-polymerisation products) were abundant in all cases, while carbonates were found only on hard carbon in the presence of LiAsF6.

The sei model—application to lithium-polymer electrolyte batteries

In this work we studied interfacial phenomena in PEO-based composite polymer electrolytes (cpe) which were stabilized by a high-surface-area oxide matrix such as alumina or magnesia. In order to avoid both consumption of the electrolyte salt (by reaction with Li) and anode passivation, we used only thermodynamically stable anions such as I− and Br−. Two types of solid electrolytes have been studied: composite solid electrolytes (cse)—salt-rich electrolytes which have an n to LiI ratio of 2.5–3 (n in P(EO)n), and t+ close to unity and cpes which have an n to LiI ratio of 6–20. Using an ac technique and assuming a simple equivalent circuit, we determined the apparent thickness of the SEI (LSEI), its resistance (RSEI), apparent conductivity (σSEI) and the apparent energy of activation for conduction (EaSEI). The effects of: inorganic oxide matrix, LiX salt, co-polymers and plasticizers on σSEI, EaSEI, LSEI and RSEI were determined. LSEI and RSEI, were found to be low and stable up to 3000 h of storage at 120 °C (less than 10 nm and typically 3–8 Ωcm2).

The electrochemical behavior of polysulfides in tetrahydrofuran

The oxidation-reduction process of sulfur and polysulfides was studied at a glassy carbon electrode in 1M LiClO4-THF solution using a programmed cyclic voltammetry technique. Over the range 1300 to −2000 mV (vs. porous carbon in lithium polysulfide solution), one anodic and up to three cathodic peaks were found. All of them correspond to irreversible processes. Kinetic parameters were evaluated and plausible mechanisms have been suggested.