Effect of Current Density on the Heat Generation of Lithium-Sulphur Batteries during Charge and Discharge

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^-4K. 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 LiCF₃SO₃in 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.