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, l'-dimethylferrocene 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.

The two-terminal alternating current impedance of lithium-titanium disulfide (Li/TiS2) 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 Li/TiS2 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 TiS2/electrolyte 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.Of the several high-energy rechargeable lithium battery systems based on lithium-intercalatable cathodes, ~ the one using TiS2 has been advanced to a status of significant technological development. The Li/TiS2 cell consists of an elemental lithium anode (negative electrode), a titanium disulfide cathode (positive electrode), and a lithium salt dissolved in an aprotic nonaqueous solvent as the electrolyte. During discharge of the cell, lithium ions intercalate in the TiS2 cathode forming Li=TiS2 (0 < x < 1), and elemental lithium at the anode is oxidized to lithium ions. These cells can be charged and discharged between 1.6 and 2.7 V. Specific energy values in the range of 75 to 100 Wh/kg and 300 to 500 charge/discharge cycles have been realized. 2-~ During charge/discharge cycling, the Li/TiS2 cell suffers several changes which are irreversible. Some of the processes of cell degradation during cycling include morphological changes at the anode, formation of blocking surface layers, ~' 8 loss of interparticle contact at the cathode, 1 and development of electronic shorts due to dendritic lithium deposits. Destructive physical and chemical analysis during different stages of cycling also confirm degradation of several cell components. 7 The performance and failure properties of the cells are related to the physicochemical processes occurring in the bulk of the electrodes, in the electrolyte, and at the electrode/electrolyte interfaces. Therefore, in the present study, investigation of the interracial processes such as charge-transfer, double-la:~er relaxation, adsorption/desorption, surface layer formation, diffusion of electroactive species in the bulk and pores of the electrode structures, and evalua...

2-Propanol exhibits a substantially higher cell voltage than methanol in a direct liquid fuel cell at a current density less than ca. 100   mA / cm 2 . Although the performance increases with cell temperature, the cell can deliver 690 mV at a current density of 20   mA / cm 2 at room temperature. These features could make 2-propanol an attractive fuel for portable power applications. © 2002 The Electrochemical Society. All rights reserved.

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In situ infrared spectroscopy and mass spectrometry were used to investigate the gas and liquid phases in lithiumsulfur oxyhalide cells driven into anode-limited reversal at 1-5 mA/cm ~. In the lithium-thionyl chloride system the species HC1, CS2, SO~, S~O, SCI~, and SO..,C1._, were identified in the gas phase and HC1, A1CI:,OH-, SOs, SO=,CI.,, and SOC1 + A1C14-in the liquid phase. A species giving rise to three absorption bands at 1337, 1070, and 665 cm-' was observed in the liquid phase of that system during anode-limited reversal only, and in the lithium sulfuryl chloride system during normal discharge and during reversal; this compound was tentatively identified as Li(SO~, SO._,CI~) § AIC14-and is analogous to the well-known complexes involving LiA1C14, SO2, and SOCI.,. The lithium-sulfuryl chloride cell behaved similarly to the thionyl chloride cell, specifically with respect to formation of SO2-and SOCl+-like species--the latter tentatively identified as SO2C1 § Indirect evidence suggests that chlorine may accumulate in both systems at -20~ but at 25~ itsaccumulation in the cells is prevented by its reaction with SO2 to form SO2C12.In the ten years since the feasibility of lithium-sulfur oxyhalide cells was first recognized (1), remarkable progress has been made in hardware development; however, their widespread use has been impeded, owing to the safety hazards associated with their relatively high energy density. Conflicting reports exist as to the cause of several explosions involving the lithium-thionyl chloride system. Explosions have been observed in anode-limited cells driven into reversal (2); a 'hazard also exists in cathode-limited cells since they contain both lithium and sulfur at the end of discharge, and the highly exothermic formation of Li2S may be possible at elevated temperatures (3, 4). In addition, poor mechanical design is suspected in some cases.The lithium-sulfuryl chloride cell which does not form any sulfur during discharge is potentially safer than Li/SOC12 (5). However, the relatively high corrosion rate of lithium in that system limits its use to special high rate reserve applications.Resolution of the unpredictable safety hazards associated with these systems may only be possible by analytical determination of their causes, and several investigations have been conducted along these lines. Istone and Brodd (6) have performed an in situ infrared study of the electrolyte of Li/SOC12 cells and observed that only two absorption bands change in intensity during normal discharge, one at 1335 cm-' due to SO~ formation, the other at 689 cm -1 which they assigned to S~O. Elemental sulfur was also seen to deposit on the spectroscopic cell walls. These authors proposed a mechanism involving formation of SO2 and $20 with subsequent decomposition of the $20 to sulfur and SO2. This mechanism had been previously rejected on the basis of stoichiometry by Schlaikjer et al. (7), who postulated instead the formation of a sulfur monoxide polymer according to the reaction 2nLi + nSOCI~ = 2nLiC1 + (SO)...