Effect of Lithium Salts on the Cycle Life of Lithium Sulphur Cells

The lithium sulphur electrochemical system has high value of theoretical specific energy – 2600 Wh/kg. However a real reached specific energy of prototypes of lithium sulphur batteries is only 6-15% (150-400 Wh/kg) against theoretical expected value. The main reason of that is great share of accessory components – 70-80%, from which an electrolyte share is 40-60%.

Necessity of using a great volume of electrolyte is caused by multifunctionality of electrolyte in lithium sulphur cells:

• ion transport between electrodes;
  
• sulphur dissolution;
  
• lithium polysulphides solvation and dissolution;
  
• stabilization of the necessary molecular and ionic forms of lithium polysulphides.
  

A problem of high rate of capacity fading of lithium sulphur batteries during cycling is another problem which should be solved for commercial application of these batteries.

Both problems are caused by feature of lithium sulphur batteries – transformation of sulphur and lithium sulphide into lithium polysulphides readily dissolves in the electrolytes. Solubility of lithium polysulphides and the forms of their existence in electrolytes are basically determined by electrolyte salt and nature of solvent. Therefore characteristics of lithium sulphur batteries should also depend on electrolyte salt and nature of solvent.

The aim of the present work is to estimate an effect of lithium salts and amount of electrolyte on the cycle life of lithium sulphur cells.

A study of the electrochemical properties of lithium sulphur cells was carried out in Swagelok type cell. The sulphur (working) electrodes consisted of 70 wt.% sulphur, 10 wt.% carbon and 20 wt.% binder. The sulphur electrodes had an average loading of 1.2 mg sulphur per cm². Lithium metal foil (99.9%, LE-1, Russia) with the thickness of 80 μm was used as the auxiliary electrode. 1M LiClO₄ and 1M LiSO₃CF₃ in sulfolane were used as the electrolytes. Amount of electrolyte into cells was 1.0; 1.5; 2.0; 3.0 and 4.0 μl/mAh(S). Porous polypropylene Celgard^® 3501 was used as the separator. The lithium sulphur cells were assembled by stacking lithium electrode, separator containing electrolyte and sulphur electrode. Electrolyte preparation, lithium electrode manufacture and lithium sulphur cells assembly were all carried out in dry air filled glove box.

The charge and discharge performances of the assembled cells were investigated with a PG12-100 potentiostat between 1.5 V and 2.8 V at +30 ^oC. The charge current density was 0.1 mA/cm², the discharge current density was 0.2 mA/cm².

It has shown, that lithium salt has no effect on the shape of discharge curves (fig.a and fig.c), but has an effect on the rate of capacity fading and cycle life of lithium sulphur cells (fig.b and fig.d).

Irrespective of amount of 1M LiClO₄ in sulfolane the discharge capacity of lithium sulphur cells primarily decreases dramatically on about 40-50 %, then rate of capacity fading decreases and capacity stabilizes. Cycle life of these cells does not practically depend on amount of electrolyte (fig.b).

For 1M LiSO₃CF₃ in sulfolane rate of discharge capacity fading of lithium sulphur cells depends on amount of electrolyte. The increase of amount of electrolyte into cells leads to reduction of rate of discharge capacity fading during cycling and to increase of cycle life (fig.d).

This study was carried out under project of Russian Academy of Sciences No. 01201152192 and also partially supported by RFBR, research project No 13-00-14056.