David H. Shen

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.

Status of the development of rechargeable lithium cells

Abstract The progress in the development of the ambient temperature lithium-titanium disulfide rechargeable cell under development at the Jet Propulsion Laboratory is described in this paper. Originally aimed at achieving a specific energy of 100 Wh/kg, ‘AA’ cells have demonstrated 125 Wh/kg at the C /3 discharge rate. The results of evaluating cell design parameters are discussed and cycling test data are also included in the paper. Safety-tests results at various overcharge and overdischarge conditions and rates proved to be uneventful. The test results of cell with built-in overcharge mechanism proved the concept was feasible. Replacing the lithium foil electrode with a Li x C resulted in a capacity at 1 mA/cm 2 of 200 mAh/g and 235 mAh/g at 0.167 mA.

Advances in ambient temperature secondary lithium cells

Abstract JPL is carrying out a NASA/OAST sponsored R &D program on the development of ambient temperature secondary lithium cells for future space applications. The goal of the program is to develop secondary lithium cells with a 100 W h kg −1 specific energy and capable of 1000 cycles at 50% DOD. The approach towards meeting these goals initially focussed on several basic issues related to the cell chemistry, selection of cathode materials and electrolytes, and component development. We have examined the performance potential of LiTiS 2 , LiMoS 3 , LiV 6 O 13 and LiNbSe 3 electrochemical systems. Of these four, the LiTiS 2 system was found to be the most promising in terms of achievable specific energy and cycle life. Major advances to date in the development of LiTiS 2 cells are in the areas of cathode processing technology, mixed solvent electrolytes, and cell assembly. This paper summarizes these advances made at JPL on the development of secondary lithium cells.

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.

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.

Examination of design options for 35 A h ambient temperature Li-TiS2 cells

Abstract Some of the design options for a rechargeable 35 A h Li-TiS2 cell have been examined. A specific energy of 80 – 100 W h kg−1 at the 2 h rat

Capacity decline of ambient temperature secondary Li-TiS/sub 2/ cells

The main objective of the study described was to identify the causes responsible for the capacity losses observed during cycling of secondary Li-TiS/sub 2/ cells. Experimental Li-TiS/sub 2/ cells were fabricated and tested for their cycle life performance. The open circuit voltage of the cells was monitored during the rest period between the charging and discharging. The polarization at the Li and TiS/sub 2/ electrodes was also monitored during cycling. Cycled cells were disassembled and the cathodes were analyzed by various analytical techniques. The results of the study indicate that the observed capacity loss is almost entirely due to the increased polarization of the TiS/sub 2/ electrode with cycling. The electrolyte was found to degrade during cycling and the degradation products were found to deposit at the TiS/sub 2/ electrode, which probably lead to the higher polarization.<<ETX>>

Capacity—cycle life behavior in secondary lithium cells

Abstract The practical utilization of high energy density rechargeable lithium cells is dependent upon maintaining high capacity for the duration of the required cycle life. However, a critical, yet generic problem with room temperature lithium systems is that the capacity often declines considerably during the early stages of cycling. We report the results of our studies on electrolyte degradation which we observe after cells have undergone 300 and 700 deep cycles with 3-methylsulfolane- and 2-methyltetrahydrofuran—LiAsF6 electrolytes, respectively.

Elastomeric Binders for Electrodes

The poor mechanical integrity of the cathode represents an important problem which affects the performance of ambient temperature secondary lithium cells. Repeated charge of a TiS2 cathode may give rise to stresses which disturb the electrode structure and can contribute to capacity loss. An investigation indicates that the use of an inelastic binder material, such as Teflon, aggravates the problem, and can lead to electrode disruption and poor TiS2 particle-particle contact. The feasibility of a use of elastomers as TiS2 binder materials has, therefore, been explored. It was found that elastomeric binders provide an effective approach for simplifying rechargeable cathode fabrication. A pronounced improvement in the mechanical integrity of the cathode structure contributes to a prolonged cycle life.

Design concepts of high power bipolar rechargeable lithium battery

Abstract The present study shows that current bipolar Li/TiS2 batteries using a 0.38 mm thick TiS2 bipolar plate, can yield moderate specific power and also high specific energy battery. The computer design studies project that a 100 V, 10 A h bipolar Li/TiS2 battery can achieve 150 W h/kg, 210 W h/l, and 150 W/kg. The unoptimized experimental bipolar Li/TiS2 batteries (3 cells, 90 mA h) exhibited 47 W h/kg, 90 W h/l, and 140 W/kg. Preliminary results on the cycleability of the bipolar batteries were demonstrated. The results also show that enhanced rate capability can be achieved by using pulse discharge and longer rest period between pulses.