C.-K. Huang

The Limits of Low‐Temperature Performance of Li‐Ion Cells

The results of electrode and electrolyte studies reveal that the poor low‐temperature (<−30°C) performance of Li‐ion cells is mainly caused by the carbon electrodes and not the organic electrolytes and solid electrolyte interphase, as previously suggested. It is suggested that the main causes for the poor performance in the carbon electrodes are (i) the low value and concentration dependence of the Li diffusivity and (ii) limited Li capacity.

Lithium-7 NMR investigation of electrochemical reaction of lithium with SnO

Abstract We report on solid state 7Li NMR measurements of electrochemically lithiated SnO. At low Li contents (Li/SnO≤2), the results are consistent with the formation of amorphous Li2O. For Li/SnO ratio of 4.3, in addition to the amorphous Li2O phase, there are at least two distinct Sn/Li alloy environments which are similar to those observed in a reference Li2.3Sn alloy. However the Li+ environments in a sample with Li/SnO ratio equal to 6.4 and an Li4.4Sn reference alloy differ markedly. The NMR data are thus consistent with the simple model of Li insertion proposed by other groups, involving the formation of Li2O and LixSn alloys (SnO+xLi→Li2O+Lix−2Sn), at low and intermediate Li contents. At high Li content, however, the Lix−2Sn alloy structure is significantly different in the electrochemically lithiated SnO material, compared to that in the reference alloy.

Lithium ion batteries for Mars exploration missions

The desired performance characteristics and physical requirements of spacecraft batteries for planetary exploration are reviewed, particularly with respect to surface landers and rovers intended for near-future Mars missions. The use of lithium ion batteries is justified in terms of significant savings on mass and volume, and more importantly of superior low temperature performance deemed essential in the Mars lander and rover missions. Use of these batteries in planetary orbiters may require further improvements in their cycle life, especially at low depths of discharge. Various strategies currently being adopted to prolong the cycle life for orbiter applications, as well as, for extending their operating range down to temperatures as low as −30°C for more imminent Mars lander and rover missions are briefly discussed.

The effects of particle size on SnO electrode performance in lithium-ion cells

It was observed that by decreasing the SnO particle size, while maintaining the same grain size, the reversible and initial specific capacities increased. It is believed that as the particle size decreases the number of Li-ion insertion sites increase and Li-ion diffusion distances decrease thus, increasing both the initial and reversible capacities. A reversible specific capacity of 657 mA h/g was achieved for SnO, which is one of the highest to date for insertion anodes.

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.

Structural aspects of electrochemically lithiated SnO: Nuclear magnetic resonance and X-ray absorption studies

Abstract We have compared the local structure of electrochemically lithiated SnO with Sn/Li alloys using 7 Li to study the environment of the Li ion and extended X-ray absorption fine structure to study the environment of the Sn ion. Although a widely accepted simple model suggested that the electrochemically lithiated SnO should be similar to Sn/Li alloys of corresponding composition, we find this is true only at low Li concentrations. The addition of more Li to the structure produces, for both the Li and the Sn, features which are difficult to reconcile with the simple model, and suggest the presence of direct Sn–O interactions.

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.

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>>