Reduction of Voltage Delay and Improving the Shelf Life of Li/SOCl2 Battery System by Using Poly-Vinyl Chloride (PVC) as an Electrolyte Additive

The 3.6 V thionyl chloride – lithium system (SOCl2 – Li) is a high-energy system, which is considered as a promising power source because of its high energy density, high operation voltage, long storage life and wide operation temperature range. The Li/SOCl2 cell belongs to a family of liquid oxidant cell systems in which the reductant component (lithium) and the oxidant component (SOCl2) are initially placed in direct contact with each other. The mechanism for the discharge at the anode is the oxidation of lithium to form lithium ions with the release of an electron. The lithium ion moves across the cell to the porous carbon cathode (PCE) where it reacts with SOCl2 to form lithium chloride (LiCl). The electron moves through the external circuit to the cathode where it reacts with SOCl2, which is reduced. It is well known that a LiCl passivating film is formed on the anode surface when it comes in to contact with the SOCl2 electrolytic solution. This film reduces any chemical interaction between the lithium and the SOCl2 and it serves to protect the lithium from discharging on its own when the load is removed from the cell [1]. In addition, it causes a drastic voltage depression under current load, especially when cells are discharged after storage periods [2]. The film of LiCl, which is essentially a high resistance layer between the electrodes, is primarily responsible for the long shelf life ( 10 years) of lithium thionyl chloride cells.Poly-Vinyl chloride as a kind of vinyl polymers with chlorine substituent groups has been evaluated as voltage-delay reducing additives. PVC powder was added to a 1.8 M LiAlCl4/ SOC12 electrolyte at a concentration of 0.2 gL-l and was stirred. All electrochemical characteristics were measured by SCRIBNER 850E test station. The electrochemical measurements were carried out in a designed test cell which made by PTFE and 316 stainless steel. Observations indicate that by adding PVC to electrolyte, which it caused a polymer film formed on the lithium anode, the growth rate of LiCl passivating film is decreased. 271 Figure 9 Impedance variation for cells with different electrolytes: (a) increase in internal resistance as a function of discharge time (b) nyquist diagram of ohmic resistance after complete discharge for PVC added Cell in comparison with PVC non-added Cell PVC, which is added to the electrolyte, is considered to stick to the lithium anode surface and formed a film on it, due to an affinity between the lithium and the chlorine group. This film controls growth rate of the LiCl-passivating film and changes the film morphology. A cell with reduced voltage delay shows more promising discharge characteristics in comparison with a cell without polymer additives. Figure 10 Discharge characteristics for Li/SOCl2 batteries; PVC added in comparison with PVC non- added battery