D. Golodnitsky

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.

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 role of anion additives in the electrodeposition of nickel–cobalt alloys from sulfamate electrolyte

Nickel–cobalt alloys have been deposited from sulfamate electrolyte with acetate and citrate-anion additives and evaluated for structure and properties, such as microhardness, tensile strength, internal stress and high-temperature oxidation. XRD data show that at low Co content, the alloys exhibit face-centered cubic (fcc) growth orientations. Above 60% Co, the deposit is completely hexagonal close packed (hcp) with pronounced (100) and (110) lines. It seems likely that the Ni–Co deposits from typical sulfamate electrolyte at pH 5, as well as at current density higher than 5 A/dm2, include metal hydroxides. This is followed by the formation of a more strained structure. The high-temperature oxidation rate of the Ni–Co coating from sulfamate electrolyte at pH 5 is twice that of the alloy deposited from the electrolyte with anion additives. We believe that, citrate complexes of Ni and Co, which are assumed to be involved in alloy deposition, eliminate the incorporation of hydroxides into the deposits and enable low-internal-stress coating. The anion-modified bath offers stability of structure and properties of the alloy over a wide range of acidity and current density.

Pyrite as cathode insertion material in rechargeable lithium/composite polymer electrolyte batteries

Abstract The chemical and physical properties of pyrite have been reviewed with reference to cathode material in lithium/polymer electrolyte batteries. The analysis of d Q /d V curves showed that the charge–discharge process in Li/polymer electrolyte/pyrite battery is more complicated than in non-aqueous and molten media. At least seven domains are distinguished on the d Q /d V discharge curve. The low-voltage step on discharge may be associated with the formation of a new phase causing from the reaction of metallic iron with the electrolyte components. This would explain the capacity fading of the cell. The high-voltage 1.85–2.25 V charge region may be attributed to the insertion of lithium into Li 2− x FeS 2 . However the de-intercalation of lithium from Li 2 FeS 2 in a LiI-CPE cell operating at 130°C is not a pure topotactic one, but rather the Li 2− x FeS 2 undergoes some structural change on cycling. Iron oxides, hydroxides and sulfates contaminate the surface of pyrite from different sources. However the performance characteristics, such as reversible capacity and polarization of the Li/composite polymer electrolyte/FeS 2 cells were found to be independent of the amount of impurities. The thin-cathode cell design has a projected energy density of 130 W h/kg at C/3 rate and specific power of 300 W/kg (on the basis of 5 mA/cm 2 demonstrated in experimental cells). Over 500 100% DOD cycles with a capacity fading rate of less than 0.1%/cycle have been demonstrated in a small laboratory prototype 7 μm-thick modified cathode cell.

Study of phase changes during 500 full cycles of Li/composite polymer electrolyte/FeS2 battery

There is a growing demand for the development of high-energy-density lithium batteries for a number of applications including electric vehicles (EV), energy storage and space. The Li/composite polymer electrolyte (CPE)/pyrite battery, which has a high theoretical energy density (about 810 Wh kg−1 based on 2.8e/FeS2), and is made of cheap, non-toxic and green compounds is a good candidate for EV applications. Materials cost is estimated at 50

Study of Nickel‐Cobalt Alloy Electrodeposition from a Sulfamate Electrolyte with Different Anion Additives

kWh−1 five times lower than that of other lithium and lithium-ion batteries. Over 500 100% DOD cycles (at c3 rate) with a capacity fading rate of less than 0.1% per cycle were carried out in a small (1 cm2 area) laboratory prototype cells with 7 μm-thick cathodes. Charge–discharge processes in the Li/LiI(PEO)nAl2O3-based CPE/pyrite battery during long-term cycle life have been analyzed with the use of dq/dV curves. These studies furnish insights into the electrochemical behavior of pyrite in polymer electrolyte-systems. Up to seven phases have been identified and found to change during the first 50–100 cycles. These phases do not change much over the subsequent 400 cycles. The major phases have been recently identified by EXAFS and NEXAFS measurements. It was proved that reduction of the ferrous disulfide proceeds as a multi-stage process, first to Li2FeS2 and finally to metallic iron. No evidence of FeS was found. When the battery is charged to 2.25 V, Li2−x FeS2 is formed.

Stretching-induced conductivity enhancement of LiI(PEO)-polymer electrolyte

Nickel-cobalt alloys exhibit a spectrum of physical properties that have led to the widespread use of these materials in a variety of high-technology applications. The recent emergence of microstructure- and microsystem-fabrication by electroplating through thick three-dimensional complex-shape electroformed molds illustrates the potential for new challenging applications of these alloys. Magnetic recording tapes, composite coatings, and devices for photothermal conversion of solar energy are only a few examples of nondecorative uses of nickel-cobalt electrodeposition. 1-6 The magnetic, mechanical, and corrosion properties of Ni-Co deposits are dictated by the structure and alloy composition. These parameters, in turn, are affected by processing variables such as plating-bath chemistry, pH, temperature, and applied current density. It is well established that the electrodeposition of iron-group alloys is followed by a local pH rise near the electrode surface. This pH rise is favored when H 2 is evolved simultaneously with alloy deposition. 7-11 In addition it was found that in the absence of boric acid in the electrolyte, the oxygen content in electrodeposited nickel and nickel-iron alloys increases with increasing applied current density. 11 This is explained by the surface precipitation and occlusion of hydroxides in the growing deposit resulting from an increase in the pH of the solution adjacent to the cathode (pHs). A near-electrode pH rise influences the reduction of cations that is supposed to be preceded by dehydration or decomposition of Ni 21 and Co 21 complexes. The importance of knowing the surface concentration of the reacting species in the electrochemical reaction, including that of the hydronium ion in the near-electrode layer, was realized long ago. 12 Deposition of Ni-Co alloys with predictable properties, therefore, depends in large part on understanding the effects of electrode polarization and near-electrode phenomena. The kinetics of single iron-group metal deposition has been studied by many authors and excellent reviews are available. 7-11,13,14 The deposition of cobalt is greatly favored over the deposition of nickel. 10,11,14,15-20 This behavior, the opposite of that which would be predicted from thermodynamics alone (E8 Ni21 5

Lithium-7 NMR studies of concentrated LiI/PEO-based solid electrolytes

The effect of stretching of LiI(PEO)20 polymer electrolyte film on its ionic conductivity was studied. Neither change of polymer electrolyte resistance nor visible sample dimensions were observed on stretching the film, having an initial cross-section of about 0.3 8m m 2 under the loads of 0.3‐1.0 kg. At 1.25 kg, the sample began to flow. Flow, accompanied by the reorganization of the polymer electrolyte chains, resulted in about a five-fold increase in ionic conductivity. At the moment of applying extending axial load to a film, a resistance-decrease transient appeared. The relaxation of the polymer electrolyte seemed to be load-independent. According to SEM micrographs, the pristine sample is characterized by clear-grain crystalline structure, whereas the stretched sample shows fibrous structure in the axial (force) direction. DSC thermograms of stretched polymer electrolyte show two endothermic transitions. The first one, at about 60°C, is sharp and similar to that of the pristine sample; an additional broad endotherm is seen at 130°C, indicating the formation of the second crystalline phase, which is absent in the pristine sample. These results support the theory that the main ion transport occurs along the helical axis.

Tissue-like Silicon Nanowires-Based Three-Dimensional Anodes for High-Capacity Lithium Ion Batteries

Abstract Highly concentrated polymer electrolytes based on poly(ethylene oxide) (PEO) and LiI, with EO/Li ratio ≤3, were investigated by differential scanning calorimetry (DSC), powder X-ray diffraction (XRD) and 7Li solid state nuclear magnetic resonance (NMR) methods. The effect of 15-nm particle size Al2O3 additives and in several cases, other constituents ethylene carbonate and poly(methylmethacrylate) on structure and Li+ ion environment was explored. The addition of Al2O3 suppresses the formation of crystalline phases, including free LiI, which is present in EO/Li=1.5 samples without Al2O3. The conductivity jump observed in these concentrated electrolytes at around 80°C is correlated with an NMR-observed transition to a Li+ environment which is similar to that of free ions in a molten phase.