Engineering topics
Publications and source records attributed to Surampudi, S..
Viewgraphs of a discussion of design considerations for rechargable lithium batteries. The objective is to determine the influence of cell design parameters on the performance of Li-TiS2 cells. Topics covered include cell baseline design and testing, cell design and testing, cell design parameters studies, and cell cycling performance.
A new liquid feed direct methanol fuel cell has been developed based on a proton exchange membrane electrolyte and Pt/Ru and Pt catalyzed carbon fuel and air/O2 electrodes respectively.
Cyclo aliphatic epoxide based thin gelled fils prepared by UV photo curing were characterized electrochemically. Thylene carbonate (EC) mixed with different organic liquids in different volume ratios were used as solvents.
It is concluded that secondary lithium batteries are suitable for planetary missions requiring high specific energy, long active shelf life, and limited cycle life. Titanium disulfide cathode material meet all the requirements for rechargeable lithium cell, including high intrinsic reversibility and realizable specific energy. Secondary lithium technology is still evolving, although low capacity cells have been demonstrated and greater than 700 cycle life was achieved. Work is in progress to improve the cycle life and safety of the electrolytes, alternate lithium anode, and the separators.
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.
The development of lithium ambient temperature rechargeable cells is discussed. During the development process, we hope to gain a greater understanding of the materials and the properties of the Li-TiS2 cell and its components. The design will meet the requirements of 100 Wh/Kg and 1000 cycles, at 50 percent depth-of-discharge, by 1995.
Ambient temperature rechargeable lithium-titanium disulfide (Li-TiS2) cells are under development at JPL for future NASA missions. The development of this technology has involved assessment and selection of electrolytes, optimization of cell design, fabrication of cells, preparation of TiS2 cathodes, and evaluation of lithium alloys. This effort has resulted in achieving a major milestone, completing 700 cycles at 50 percent depth of discharge in 1-Ah cells fabricated in house.
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. The model has been experimentally verified using 1,1-prime-dimethylferrocene as a redox additive. The theoretical results may be exploited in the design and optimization of overcharge protection by the redox additive approach.
JPL is involved in a NASA sponsored program to develop ambient temperature secondary Lithium - Titanium Disulfide Cells for future space missions. After several years of research on various lithium systems, the Li-TiS2 system was selected for development in view of its practically realizable high specific energy. In the last two years, the efforts were focused on improving the cycle life of the system and optimizing the cell design. A number of approaches, such as the use of mixed solvent electrolytes, the use of alternate anode materials, the operation of cells at low temperature, and the cycling of cells under optimized voltage limits, are examined to improve cycle life performance of this system. Cycling studies in small 150 mAh cell have identified 1.5 M LiAsF6/2-MeTHF, 1.5 M LiAsF6/EC+2-eTHF, and 1.5 M LiAsF6/THF+2-MeTHF+2-MeF as promising electrolytes for Li-TiS2 cells. Li-Al and Li-C were selected for further assessment as candidate anode materials after a detailed theoretical and experimental evaluation. Cycling of the cells at low temperature did not result in improving the cycle life of the cells. Charge and discharge voltage limits were found to have significant influence on the cycle life of the cells activated with 1.5 M LiAsF6/THF+2-MeTHF+2-MeF electrolyte. The influence of design variables, such as ratio of electrode capacity, quantity of electrolyte, pack tightness, cell configuration, etc., on the cycle life performance are being examined as a part of the design optimization study. Spiral-wound 1 Ah cells fabricated for the design studies have completed more than 500 cycles at 50% DOD. This paper summarizes the advances made in the Li-TiS2 technology at JPL since 1989.
The X-ray diffraction patterns of 100 percent discharged Schawinigan black cathodes from Li/SOCl2-LiAlCl4 cells were obtained using a high resolution Guinier camera. The previous assignments of the diffraction lines to Li2O2 and rhombohedral sulfur are all found to be incorrect; all sharp Bragg diffraction lines not assignable to anhydrous LiCl can be assigned to LiCl.H2O.
Chlorinated elastomeric binder improves cell mechanical and electrical characteristics. Substituted for PTFE binder, chlorinated polyethylene rubber allows high cell-discharge rates and higher stored energy per unit volume. In addition, it costs about one-eighth as much as PTFE.