Assessment of Applicability of Potentiometric Method to Determine Thermodynamic Properties of Electrochemical Processes in Lithium Sulphur Batteries

The efficiency of chemical energy conversion to electric is a significant characteristic of batteries and it is especially for batteries on base of high power electrochemical systems (for example Li-S). The maximal possible efficiency of energy conversion is determined by thermodynamic properties of the electrochemical systems.

There are two main methods to estimate of the thermodynamic properties of the electrochemical processes: calorimetric and potentiometric.

Heat generation or absorption of the electrochemical processes is possible to register direct by calorimetric methods. However registered heat generation or absorption includes both thermal effects of the electrochemical reaction and Joule heat during polarization of batteries. If some concurrent processes are going in the batteries then their thermal effects will be part of the registered heat generation or absorption by the instrument. One of the other disadvantages of the calorimetric methods is complexity and high price of instruments.

Potentiometric method is much easier in instrument executions than calorimetric method and allows to more accurate estimate the thermodynamic properties of the electrochemical processes in the batteries.  Therefore it is wide used to estimate the thermodynamic properties of active components of positive and negative electrodes of batteries. The main criterion of applicability of the potentiometric method to measure thermodynamic properties is reversibility of material and energy flows. Therefore this method must be used with care and especially for batteries with high self-discharge rate, for example batteries with liquid cathode (Li-S battery).

The aim of present work is an assessment of applicability of the potentiometric method to determine the thermodynamic properties of the electrochemical processes in the lithium sulphur batteries.

The test subjects were the lithium sulphur pouch cell. Lithium foil (99.9%, LE-1, Russia) 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 Polyethylene oxide). One layer of micro porous membrane Celgard^® 3501 was used as a separator. An electrolyte was 1M solution of LiCF₃SO₃in sulfolane.

Electrolyte preparation, lithium electrode manufacture and lithium sulphur batteries assembly were all carried out in dry air filled glove box.

Sulphur and products of its reduction (lithium polysulphides) are able to direct interact with metallic lithium and disturb the reversibility of material and energy flows. It should be noted that reactivity of sulphur and lithium polysulphides toward metallic lithium depends on them chemical composition (number of sulphur atoms per molecule). Therefore to study the effect of depth of discharge (DoD) on the reversibility of material and energy flows in the lithium sulphur batteries we measured temperature dependence of open circle voltage (OCV) of batteries at different values of DoD.  Current density was 0.2 mA cm^-2.

To reach equilibrium (to obtain uniform lithium polysulphide distribution in the bulk of battery) after discharging or charging at the specified DoD batteries were stored 12 hours at constant temperature (30.0 ± 0.1 °C). Then the temperature dependence of OCV was registered in the range of temperature of 10-40 °C. The equilibrium was considered to have been reached when OCV stabilized to less than 0.2 mV h^-1at a constant temperature (±0,1 °C) and OVC-T measurements were completely reproducible at heating and cooling. The thermodynamic properties were calculated by equations:

ΔG = -nFE_P,T                              (1)

ΔH = -nFE_P,T + nFE_P,T(∂E/∂T)_P  (2)

ΔS = nF(∂E/∂T)_P                         (3)

We estimated that thermodynamic properties of electrochemical reduction of sulphur in the lithium sulphur batteries only can be made by the potentiometric method at the first cycle. At the following cycles of lithium sulphur batteries this method can be applied only at the DoD of 33-100 %. At the depth of discharge of 0-33% (high voltage plateau of charge-discharge curves) lithium sulphur batteries cannot achieve equilibrium because of high rate of self- discharge. The OCV was constantly reducing to the value characteristic to the low voltage plateau of charge-discharge curves.

Figure shows the obtained dependences of Free energy (Gibbs energy), enthalpy and entropy changes of the electrochemical reduction of sulphur and lithium polysulphides on the DoD at 1^st cycle.

Thus it can be concluded that the potentiometric method of measuring thermodynamic properties of electrochemical reactions in lithium sulphur batteries can be applied at 1^st cycle in all range of DoD and at the following cycles it can be applied only in the range of DoD of 33-100 %.

The reported study was partially supported by RFBR, research projects No 13-00-14056 and 4-03-31399.