Corrosion Processes in Lithium-Sulphur Cells. Isothermal Calorimetry As Method of Their Identification

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^-1at 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.

S₈ + 4Li -> Li₂S_n + Li₂S_k, k+n=8; k & n ≥ 1                                                   (1)

Li₂S_n + 2Li -> Li₂S_m+Li₂S_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.