Alan I. Attia

Electrochemical Impedance Spectroscopy of Lithium‐Titanium Disulfide Rechargeable Cells

The two‐terminal alternating current impedance of lithium‐titanium disulfide rechargeable cells has been studied as a function of frequency, state‐of‐charge, and extended cycling. Analysis based on a plausible equivalent circuit model for the cell leads to evaluation of kinetic parameters for the various physicochemical processes occurring at the electrode/electrolyte interfaces. To investigate the causes of cell degradation during extended cycling, the parameters evaluated for cells cycled five times have been compared with the parameters of cells that have been cycled over 600 times. The findings are that the combined ohmic resistance of the electrolyte and electrodes suffers a ten‐fold increase after extended cycling, while the charge‐transfer resistance and diffusional impedance at the interface are not significantly affected. The results reflect the morphological change and increase in area of the anode due to cycling. The study also shows that overdischarge of a cathode‐limited cell causes a decrease in the diffusion coefficient of the lithium ion in the cathode. The study demonstrates the value of electrochemical impedance spectroscopy in investigating failure mechanisms. The approach and methodology followed here can be extended to other rechargeable lithium battery systems.

Analysis of redox additive-based overcharge protection for rechargeable lithium batteries

The overcharge condition in secondary lithium batteries employing redox additives for overcharge protection, has been theoretically analyzed in terms of a finite linear diffusion model. The analysis leads to expressions relating the steady-state overcharge current density and cell voltage to the concentration, diffusion coefficient, standard reduction potential of the redox couple, and interelectrode distance. The model permits the estimation of the maximum permissible overcharge rate for any chosen set of system conditions. Digital simulation of the overcharge experiment leads to numerical representation of the potential transients, and estimate of the influence of diffusion coefficient and interelectrode distance on the transient attainment of the steady state during overcharge. The model has been experimentally verified using 1,1-prime-dimethyl ferrocene as a redox additive. The analysis of the experimental results in terms of the theory allows the calculation of the diffusion coefficient and the formal potential of the redox couple. The model and the theoretical results may be exploited in the design and optimization of overcharge protection by the redox additive approach.

Composite Solid Electrolyte for Li Battery Applications

Abstract The electrochemical, bulk and interfacial properties of the polyethylene oxide (PEO) based composite solid electrolyte (CSE) comprising LiI, PEO, and Al2O3 have been evaluated for Li battery applications. The bulk interfacial and transport properties of the CSEs seem to strongly depend on the alumina particle size. For the CSE films with 0.05 micron alumina while the bulk conductivity is around 10−4 (mho cm−1) at 103°C, the Li ion transport number seems to be close to unity at the same temperature. Compared to the PEO electrolyte this polymer composite electrolyte seems to exhibit robust mechanical and interfacial properties. We have studied three different films with three different alumina sizes in the range 0.01–0.3 micron. Effects of Al2O3 particle size on the electrochemical performance of polymer composite electrolyte will be discussed. With TiS2 as cathode a 10 mAh small capacity cell was charged and discharged at C 40 and C 20 rates respectively.

A polyacrylonitrile-based gelled electrolyte: electrochemical kinetic studies

The bulk and interfacial properties of thin films of a polyacrylonitrile-based (PAN-based) gelled electrolyte, which was tested in conjunction with lithium electrodes, were evaluated at room temperature. The typical composition of the gelled electrolyte (GE) studied was PAN (9.13 wt %); propylene carbonate, or PC (84.87 wt %); and LiBF4 (6 wt %). A.c. and d.c. measurements were used to determine the bulk conductivity and the interfacial apparent charge-transfer resistance (Ract) of the GE. While the bulk conductivity remains stable at around 10−3 S cm−1, the Ract varies initially before reaching a steady value. The steady Ract value obtained from the electrochemical measurements is near 1000 ω cm2. The plating/stripping efficiency of lithium, from potentiostatic and galvanostatic measurements, is 70 to 80%.

Sodiummetal chloride battery research at the Jet Propulsion Laboratory (JPL)

Sodiummetal chloride batteries have certain distinct advantages over sodiumsulfur batteries such as increased safety, inherent overcharge capability and lower operating temperatures. Two systems, i.e. Na/FeCl2 and Na/NiCl2 were developed extensively elsewhere and evaluated for various applications including electric vehicles and space. Their performance has been very encouraging and prompted a detailed fundamental study on these cathodes here at the Jet Propulsion Laboratory. A brief review of our studies on these new cathode materials is presented here. The initial efforts focussed on the methods of fabrication of the electrodes and their electrochemical characterization. Subsequent studies were aimed at establishing the reaction mechanism, determining the kinetics and identifying the rate-limiting processes in the reduction of metal chloride cathodes. Nickel chloride emerged from these studies as the most promising candidate material and was taken up for further detailed study on its passivation — a rate limiting process — under different experimental conditions. Also, the feasibility of using copper chloride, which is expected to have higher energy density, has been assessed. Based on the criteria established from the voltammetric response of FeCl2, NiCl2 and CuCl2, several other transition metal chlorides were screened. Of these, molybdenum and cobalt chlorides appear promising.

Alternate cathodes for sodium-metal chloride batteries

Various metal chlorides were tested as possible cathode materials for sodium-metal batteries (in addition to Fe and Ni chlorides, which have been already developed to a stage of commercialization), using an electrochemical cell consisting of a pyrex tube, heated to 250 C, with the metal wire as working electrode, concentric Ni foil as counterelectrode, and high-purity Al as reference electrode. In particular, the aim of this study was to identify metal chlorides insoluble even in neutral melts, possible at the interface during overcharge, in order to eliminate the failure mode of the cell through a cationic exchange of the dissolved metal ions with sodium beta-double-prime alumina solid electrolyte. Results indicate that Mo and Co are likely alternatives to FeCl2 and NiCl2 cathodes in sodium batteries. The open circuit voltages of Na/CoCl(x) and Na/MoCl(x) cells at 250 C would be 2.55 V and 2.64 V, respectively.

Effect of cycling on the lithium/electrolyte interface in organic electrolytes

Abstract The successful operation of ambient temperature secondary lithium cells is primarily dependent on the lithium/electrolyte interface properties. In this study, an attempt has been made to study the effect of cell cycling on the lithium/electrolyte interface by nondestructive methods such as a.c. impedance spectroscopy and microcalorimetry. Experimental Li-TiS2 cells were constructed and activated with different electrolytes. The cells delivered 30 to 300 cycles at 100% depth-of-discharge depending on the electrolyte. The reactivity of both uncycled and cycled lithium towards various electrolytes was studied by measuring the heat evolved from the cells under open-circuit condition at 25 °C by microcalorimetry. Cycled cells at the end of charge/discharge showed considerably higher heat output compared with the uncycled cells. After thirty days of storage, the heat output of the cycled cells is similar to that of the uncycled cells. A.c. impedance analysis results indicate that the cell internal resistance increases with cycling, and this is attributed to the degradation of the electrolyte with cycling. The value Rf was found to decrease with cycling. The observed decrease in Rf is probably due to the increase in the surface area of the lithium anode due to cycling. The peak frequency was found to be in the range, 400 to 1000 Hz for both uncycled and cycled cells suggesting that the passivating film composition does not change significantly with cycling.

The development of a sealed lead-acid battery for pulse power applications

The development of a high specific power 50-kW sealed bipolar lead-acid battery is described. This summary includes: 1) a description of several approaches for the development of a useful light-weight bipolar substrate; 2) a description of the final 50-kW battery design; and 3) performance data which clearly demonstrate the high specific power characteristics of this new battery.

Advances in Li-TiS2 cell technology

Abstract JPL is involved in a NASA sponsored program to develop ambient temperature secondary cells for future space missions. After several years of research on various cathode materials, titanium disulfide (TiS 2 ) was selected in view of its intrinsic reversibility and high faradaic utilization. In the last two years, efforts have been focussed on improving the cycle life of the system and developing 1 A h cells. Several approaches including the use of mixed solvent electrolytes, the operation of cells at low temperature, and the cycling of cells under different voltage limits, were initially examined to improve the cycle life performance of the LiTiS 2 system. Spiral wound 1 A h cells fabricated incorporating the improvements from the above studies have delivered more than 600 cycles at 50% DOD. Work is in progress to identify alternate anode materials that can improve the cycle life of the cells to 1000 cycles at 50% DOD. This paper summarizes the advances made in the LiTiS 2 technology at JPL since 1989.

Electrochemical evaluation of alternate anode materials for ambient temperature secondary Li cells

Some progress has been made at JPL to identify alternative anode materials to improve the cycle life and safety of ambient temperature secondary lithium cells. Graphite is one of the materials under investigation. Experimental results showed that graphite was able to accommodate the interstitial addition of 0.16 mole of lithium per mole of graphite, and the amount of Li corresponding to the capacity from x=0.045 to x=0.16 in Li/sub x/ can be reversibly cycled with negligible capacity loss. This reversible composition region is also a region with very low open circuit potential. In addition, electrode fabrication and the intercalation technique are also discussed. This information and electrochemical studies on other materials, such as Li/sub x/Mg/sub 2/Si and C/sub 60/, are presented.<<ETX>>