Reduction of Voltage Delay and Improving the Shelf Life of Li/SOCl2 BatterySystem 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 isconsidered as a promising power source because of its high energy density, high operationvoltage, long storage life and wide operation temperature range. The Li/SOCl2 cell belongs to afamily of liquid oxidant cell systems in which the reductant component (lithium) and the oxidantcomponent (SOCl2) are initially placed in direct contact with each other. The mechanism for thedischarge at the anode is the oxidation of lithium to form lithium ions with the release of anelectron. The lithium ion moves across the cell to the porous carbon cathode (PCE) where itreacts with SOCl2 to form lithium chloride (LiCl). The electron moves through the externalcircuit to the cathode where it reacts with SOCl2, which is reduced. It is well known that a LiClpassivating film is formed on the anode surface when it comes in to contact with the SOCl2electrolytic solution. This film reduces any chemical interaction between the lithium and theSOCl2 and it serves to protect the lithium from discharging on its own when the load is removedfrom the cell [1]. In addition, it causes a drastic voltage depression under current load, especiallywhen cells are discharged after storage periods [2]. The film of LiCl, which is essentially a highresistance 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 chlorinesubstituent groups has been evaluated as voltage-delay reducing additives. PVC powder wasadded to a 1.8 M LiAlCl4/ SOC12 electrolyte at a concentration of 0.2 gL-l and was stirred. Allelectrochemical characteristics were measured by SCRIBNER 850E test station. Theelectrochemical measurements were carried out in a designed test cell which made by PTFE and316 stainless steel. Observations indicate that by adding PVC to electrolyte, which it caused apolymer film formed on the lithium anode, the growth rate of LiCl passivating film is decreased.PVC, which is added to the electrolyte, is considered to stick to the lithium anode surface andformed a film on it, due to an affinity between the lithium and the chlorine group. This filmcontrols growth rate of the LiCl-passivating film and changes the film morphology. A cell withreduced voltage delay shows more promising discharge characteristics in comparison with a cellwithout polymer additives.

کلیدواژه ها:

Lithium thionyl chloride battery ، Passivating film ، Polyvinylchloride ، Dischargecharacteristics ، Impedance analysis