Abstract: The studies of thermal effects of cycling and storage of lithium-sulphur cells have significant value to development of energy storage systems on the base of new electrochemical systems with high specific energy. One of the most promising electrochemical systems is lithium-sulphur system. The mechanisms of electrochemical, physicochemical and chemical processes occurring in the lithium-sulphur cells during cycling significantly differ from the mechanisms of processes in lithium-ion cells. Electrochemical conversion of sulphur and products of its reduction is multi-step process including electrochemical and chemical reactions, also phase transformations (dissolution, precipitation, crystallization, sorption, desorption). To study of thermal effects of electrochemical cells we designed and built specialized instrument “Electrochemical calorimeter”. It simultaneously records thermal and electric characteristics of planar cells under polarisation. The instrument includes calorimetric block, potentiostat and control block. The measurements of thermal effects are carried out in isoperibol mode. The accuracy of temperature stabilization is ± 5∙10 -4 K. The temperature range is 253-363 K. The measurement accuracy of heat flow is 50 µW. The relative error of current stabilization is 0.1 %. The voltage measurement error is 10 μV. We studied the thermal effects of charging and discharging of the lithium-sulphur cells by designed instrument. The test subjects were the lithium-sulphur pouch cells. Lithium foil (99.9%) with a thickness of 100 μm was used as negative electrodes. The working electrodes were sulphur electrodes (70% of Sulphur, 10% of Carbon and 20% of poly(ethylene oxide)). One layer of micro porous membrane Celgard® 3501 was used as a separator. The electrolyte was 1M solution of LiCF 3 SO 3 in sulfolane. There are two maximums of heat generation during discharging of lithium-sulphur cell (Fig. 1). The first maximum has complex shape and correlates to the transition between high and low voltage plateaus of discharge curve. The second maximum is observed at the end of discharging. The comparison between curves of heat generation (Fig. 1c) and internal resistance (Fig. 1b) during discharging of lithium-sulphur cells makes it possible to conclude that first maximum is caused by polarization heat and Joule heat and thermal effects of chemical and electrochemical reactions. The second maximum is caused by the polarization heat and Joule heat. The heat flow curve correlated to the charging of lithium sulphur-cell (Fig. 2c) has more complex shape than the curve correlated to the discharging (Fig. 1c). The analysis of heat flow, internal resistance and charge curves allows to conclude that first maximum of heat generation is caused by the polarization heat and Joule heat. The heat absorption corresponding to the transition between high and low voltage plateaus of charge curve is possible to be caused by physicochemical and chemical process accompany the electrochemical oxidation of low ordered lithium polysulphides. The intensive heat generation is likely to be caused by chemical reaction of sulphur or high ordered lithium polysulphides and metallic lithium at the end of charging of lithium-sulphur cell.

Abstract: The investigations of thermal effects of charging, discharging and storage of batteries is important to solve the fundamental and applied problems of energy conversion in electrochemical systems. They are especially significant to develop and design batteries on the base of electrochemical systems with high energy density (for example Li-S). The thermal effects of charging and discharging batteries include several components: (1) the thermal effects of the electrochemical reactions, (2) the thermal effects of the processes accompanying the electrochemical reactions, (3) Joule heat. In contrast to the batteries with solid active materials (Li-Ion batteries) when lithium-sulphur batteries charging and discharging the complex multistep processes occur and include: - the electrochemical reactions; - the chemical reactions (corrosion, disproportion reactions of lithium polysulphides); - the reactions of solvation/desolvation and complexing of lithium polysulphide generated in solutions; - the phase transformations (dissolution, precipitation, sorption, desorption). All these processes effect on the efficiency of conversion chemical energy to electrical in lithium-sulphur batteries. When changing of conditions of charge/discharge cycling of lithium-sulphur batteries the contribution of different process are changing therefore energy conversion efficiency can be increased or reduced. The effect of current density on the heat generation of lithium-sulphur batteries was studied in this work by special designed and built instrument “Electrochemical calorimeter” The Electrochemical calorimeter simultaneously records thermal and electric characteristics of planar cells under/without polarisation. The instrument includes calorimetric block, potentiostat and control block. The measurements of thermal effects are carried out in isoperibol mode. The accuracy of temperature stabilization is ± 5∙10 -4 K. The temperature range is -20 – +90 °C. The measurement accuracy of heat flow is 50 µW. The relative error of current stabilization is 0.1 %. The voltage measurement error is 10 μV. The test objects were the lithium-sulphur pouch cells. Lithium foil (99.9%) with a thickness of 100 μm was used as negative electrodes. The working electrodes were sulphur electrodes (70% of Sulphur, 10% of Carbon and 20% of poly(ethylene oxide)). One layer of micro porous membrane Celgard 3501 was used as a separator. The electrolyte was 1M solution of LiCF 3 SO 3 in sulfolane. To get reproducible results 10 form cycles were done before study of influence of current density on the heat generation of the lithium-sulphur cells. The study shows that the shape of curves of heat generation is similar in the investigated range of current density (Fig.). When charging the lithium-sulphur cells there are few characteristic segments at the curve of heat generation. I. At the initial stages of charging (Q 〈 30 mAh g -1 (S)) there is a maximum of heat generation and it is linear increasing with the current density increase. For the same charge state there is a maximum of overvoltage at the charging curves and it is also increasing with the current density increase. It allows concluding that this maximal of heat generation is caused by Joule heat. II. The voltage and heat generation is continuously increasing with the further charging of lithium-sulphur cell (low voltage plateau). III. There is a minimum at the curve of heat flow and it corresponds to the transition between low and high voltage plateau. This minimum is increasing with current density increase. IV. The heat generation of lithium-sulphur batteries is much greater at the high voltage plateau of charging curve than at the low voltage curve. It should be noted it is linear increasing with current increase. The shapes of heat generation are different at charging and discharging of lithium-sulphur cell (Fig.). The heat generation curve can be divided in several characteristic segments (Fig. d): I. At the initial stage of discharging the intensive heat generation is observed. Its fast decrease correlates to reducing of overvoltage at the discharging. II. There is a maximum at the heat generation curve corresponding to the high voltage curve of discharging. This maximum is linear increasing and shifting to the large discharge capacity with current density increase. III. At the transition between high and low voltage plateau at discharge curve there are two inflections at the heat generation curve. They are also shifting to the higher discharge capacity with increasing of current density. IV. The heat generation of lithium-sulphur cell changes insignificantly at the low voltage discharge curve. The overvoltage and the heat generation increase with current density increasing. Also it should be noted that heat generation is greater at discharging of lithium-sulphur cell then at charging. The analysis of data shows that there are maximum at the dependences of efficiency of energy conversion on the current density for charging and discharging of lithium-sulphur batteries. The current density affects stronger on the efficiency of energy conversion at charging than at discharging (especially for high voltage plateau). The reported study was partially supported by RFBR, research projects No 13-00-14056 and 14-03-31399.

Abstract: Electrochemical system of Lithium-Sulphur is very promising because of its high theoretical specific energy (2600 Wh kg -1 ). In addition to sulphur is cheap and available. However the laboratory prototypes of lithium-sulphur batteries (cells) have several significant disadvantages: low practical specific energy (250-300 Wh kg -1 at first cycles), high rates of self-discharge ( 〉 3% per month) and capacity fade ( 〉 0.1% per cycle) during prolong cycling. The main reason of low practical specific energy of lithium-sulphur cells is the large shares of ballast components including greater share of an electrolyte than in lithium-ion cells. Corrosion processes at lithium electrodes cause the high rates of self-discharge and capacity fade. It is related to the solubility of sulphur and lithium polysulphides in the electrolyte systems and their direct chemical interactions with the lithium electrodes. S 8 + 4Li - 〉 Li 2 S n + Li 2 S k , k+n=8; k & n ≥ 1                                                   (1) Li 2 S n + 2Li - 〉 Li 2 S m +Li 2 S k ; k+m=n, k & m ≥ 1                                             (2) Traditionally the effect of different factors on the corrosion processes in lithium-sulphur cells is measured by results of prolong charge/discharge cycling: the values of charge and discharge capacities and rate of their fading during cycling. Such studies are slow because corrosion process occurs slow at the initial cycles and does not affect the results so it is hard to measure the efficiency of suggested methods to suppressed the corrosions processes.   As results show the combine method of isothermal calorimetry and galvanostatic polarisation (cycling) is very informative and convenient method to study corrosion processes in chemical power sources. This method allows us to identify corrosion processes at the initial stages of cycling and estimate their intensity.  The aim of this work is to estimate the corrosion activity of sulphur, lithium polysulphides and factors, affecting their interactions with metallic lithium, by isothermal calorimetry combined with galvanostatic polarisation. The reported study was partially supported by RFBR, research project No 14-03-31399.