G. Ardel

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.

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).

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

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.

Conduction mechanisms in concentrated LiI-polyethylene oxide-Al{sub 2}O{sub 3}-based solid electrolytes

The ionic conductivity of concentrated LiI-polyethylene oxide P(EO){sub n} high surface area oxide composite polymer electrolytes has been investigated. Two different Arrhenius dependences for concentrated composite polymer electrolytes (CPEs) have been identified. The first one is characterized by an inflection point at about 80 C, and the second, by a conductivity jump. The authors have suggested that in CPEs, where 3T{sub k}orT{sub jump}) and on Ea have been studied (T{sub jump}=temperature of the conductivity jump). The addition of small quantities of ethylene carbonate, poly(methyl methacrylate), and polyacrylonitrile were found to be beneficial while poly(methyl acrylate), poly(butyl acrylate), and poly(vinylidene fluoride) additions made the polymer electrolyte stiffer and less conductive. MgO, Al{sub 2}O{sub 3}, and potassium aluminosilicate muscovite mica based CSEs have similar conductivity. Results clearly demonstrated the depression of CPE crystallinity by addition of fine Al{sub 2}O{sub 3} powder, ethylene carbonate, and poly(ethylene glycol) dimethyl ether, in agreement with the conductivity enhancement of the CPE.

Electrical, thermal and NMR investigation of composite solid electrolytes based on PEO, LiI and high surface area inorganic oxides

Abstract High salt concentration polymer composite electrolytes, containing poly(ethylene oxide) (PEO), LiI and nanoscale Al 2 O 3 or MgO and, in some cases, ethylene carbonate (EC) and poly(methyl methacrylate) (PMMA), were investigate by electrical conductivity, differential scanning calorimetry (DSC) and both wide-line and high resolution solid state 7 Li NMR. The high resolution NMR spectrum of a sample with a PEO:Li ratio of 3:2 is consistent with at least two Li environments, one solvated by the polymer and one in small ionic clusters. A correlation between the observed change in Li environment at elevated temperature and a sudden increase in conductivity is suggested. All results demonstrate that the conduction mechanism in concentrated composite electrolytes is significantly different than in more dilute polymer electrolytes, in which the ion transport process is associated with polymer segmental motion.

Ion-transport phenomena in concentrated PEO-based composite polymer electrolytes

Abstract The enhancement of the ionic conductivity of Li–P(EO) n -based polymer electrolytes by the addition of finely divided inorganic oxides is the subject of considerable discussion. The increase in ionic conductivity in concentrated composite solid polymer electrolytes (CSPE) is related both to the suppression of the formation of crystalline PEO and PEO-salt phases and to interfacial conduction. In the case of polycrystalline or polyparticle solid electrolytes, however, grain-boundary (GB) resistance, which is associated with the crossover of ions from particle to particle across grain boundaries orthogonal to the direction of current flow (or parallel to the field lines), must be considered. The conduction across grain boundaries in polymer appears to be ignored so far. In this work, we provide SEM and ECS experimental data on LiI–P(EO)–Al 2 O 3 CSPEs. The effects of plasticizers, doping by CaI 2 , change in Li/EO ratio, and nanosize alumina concentration on the interior grain ionic conductivity and on the resistance of the orthogonal grain boundaries are addressed.

Lithium polymer electrolyte pyrite rechargeable battery: comparative characterization of natural pyrite from different sources as cathode material

Abstract The thermal and electrochemical behavior of pyrite as an electrode material for rechargeable lithium polymer electrolyte batteries has been investigated. The samples of pyrite from several different sources were characterized by thermogravimetric analysis (TGA), SEM, X-ray photoelectron (XPS) and electrochemical methods. As determined by thermogravimetric measurements, the pyrite samples of “vendors A and G” were highly stable up to 500°C. The weight loss of FeS 2 at 500°C did not exceed 1.3%. The decomposition of the “vendor E” sample, including eight phase transitions, starts at about 100°C and is caused by the surface impurities of pyrite, such as iron oxides, hydroxides and sulfates. These influence the OCV and the first discharge of the Li/CPE/FeS 2 cell. It is noteworthy that the performance characteristics, such as Li/Fe ratio, faradaic efficiency and charge–discharge overpotential of the Li/composite polymer electrolyte (CPE)/10-μm-thick cathode pyrite cells were found to be almost independent of the degree of contamination and, consequently, of the pyrite source during 30 cycles.

Development and characterization of bipolar lithium composite polymer electrolyte (CPE)-FeS2 battery for applications in electric vehicles

A small laboratory prototype of a new lithium battery for electric vehicles and load levelling has been developed. This rechargeable battery consists of thin foils of lithium anode, composite solid electrolyte (CSE) or composite polymer electrolyte (CPE) and a composite FeS2 (pyrite) cathode. The battery has several advantages over other state-of-the-art polymer electrolyte batteries: (i) low-cost cathode, pyrite, is a natural ore, therefore environmentally friendly, (ii) small prototype cells exhibited very high specific energy, projected to be 140 Wh/kg at a C6 to C10 rate (three times larger than that of a lead/acid battery), and more than forty 100% charge/discharge cycles; (iii) this battery has an internal electrochemical overcharge protection mechanism (which is essential for bipolar batteries), and (iv) for both CSE and CPE, the Li/electrolyte interfacial resistance is low and stable up to 3000 h (CPE) and 700 h (CSE) at 120°C. The long-term projected specific energy for this battery is over 200 Wh/kg, five times larger than that of the lead/acid battery and one of the highest among all batteries in progress.

Effect of plasticizers on the CPE conductivity and on the Li-CPE interface

Abstract An enhancement of ionic conductivity to 2 · 10 −3 S cm −1 at 120 °C was achieved by the addition of ethylene carbonate and/or polyethylene glycol dimethyl ether to LiI-based composite polymer electrolyte (CPE). These plasticizers also improved the Li-CPE interfacial contact, so the SEI resistance was low (2 ohm · cm 2 ) and was stable for 1000 h. CPE characterization by differential scanning calorimetry measurements, thermogravimetric analysis, SEM and X-ray methods is presented.

Bulk and interfacial ionic conduction in LiI/Al2O3 mixtures

Abstract We have studied physical mixtures of LiI and nanoscale particles of Al2O3, prepared in pellet form under pressure, by X-ray diffraction, complex impedance, and wide-line 7Li and 127I and high resolution 7Li NMR spectroscopy. Enhancement in ionic conductivity by the addition of Al2O3 is observed, although there is also an increase in grain boundary resistance. The high resolution (MAS) NMR method clearly resolves two or more distinct Li+ sites, one characteristic of bulk LiI and the rest associated with surface/interface regions. The site populations depend strongly on mix composition and temperature, and are correlated with the ionic conductivity behavior.