The Effect of Amount of Electrolyte on the Efficiency of Sulphur Utilization in 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 lithium-sulphur batteries is significantly low against theoretical expected value. So, specific energy of  prototypes of lithum-sulphur cells is in the range of 150-400 Wh/kg, what is only 8-15% of theoretical value of specific energy, while specific energy of lithium-ion batteries reaches about 30-40% of theoretical value [1-2].

The practical specific energy of batteries is determined by a number of factors. The main factors are an efficiency of utilization of active materials of positive and negative electrodes as well as a weight share of accessory components (electrolyte, separator, current collector, cell body etc.) against the total weight of battery. In lithium-ion batteries an efficiency of utilization of active materials of electrodes reaches 90-95% and the weight share of active materials reaches 35-45% against the total weight of battery. In lithium-sulphur batteries a depth of electrochemical reduction of sulphur is usually in the range of 1250-1400 mAh/g(S) what is 75-85% against theoretical value (1675 mAh/g(S)). The weight share of accessory components in lithium-sulphur batteries is significantly greater than in lithium-ion batteries and reaches 70-80%. Electrolyte makes the main contribution into the weight of lithium-sulphur batteries. Usually weight of electrolyte in lithium-sulphur cells is in the range of 40-60% against in lithium-ion – 15-20%.

The difference in functions of electrolyte in lithium-ion and lithium-sulphur batteries is the reason for the difference in amount of electrolyte. Lithium-sulphur batteries can be classified as batteries with a liquid cathode as far as during charge and discharge of them solid-phase active materials – elementary sulphur (S) and lithium sulphide (Li₂S) – change into well soluble in electrolytes compounds – lithium polysulphides (Li₂S_n):

Discharge:	nS0 + 2Li+ + 2e- →   Li2Sn
Charge:	nLi2S → Li2Sn +   2(n-1)e- + 2(n-1)Li+

The electrolyte in lithium-sulphur batteries has higher functionality than in lithium-ion batteries. In lithium-ion batteries electrolyte performs the function of ionic conductor. In lithium-sulphur batteries the electrolyte provides an ionic transfer between electrodes as well as dissolves the electroactive components (sulphur and lithium polysulphides). 

The aim of the present work is to determine a minimum amount of electrolyte required for good performance 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.% Ketjenblack EC-600JD and 20 wt.% poly(ethylene oxide) (Mw 4x10⁶). The surface capacity of sulphur electrodes was 2 mAh/cm². Lithium metal foil (99.9%) with the thickness of 80 μm was used as the auxiliary electrode. 1M LiCF₃SO₃ in sulfolane was used as the electrolyte. Amount of electrolyte into cells was 1.0; 1.5; 2.0; 3.0 and 4.0 mcl/mAh(S). Porous polypropylene Celgard^® 3501 was used as the separator. The lithium-sulphur cells were assembled by stacking lithium electrode (d=2.55 cm, area=5.10 cm²), separator (d=2.85 cm, area=6.38 cm²) containing 1M LiCF₃SO₃ in sulfolane and sulphur electrode (d=2.85 cm, area=6.38 cm²). 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 is shown that the amount of electrolyte effects on the efficiency of sulphur utilization (fig. 1a-1b) and cycle life of lithium-sulphur cells (fig. 1c). For obtaining of acceptable efficiency of sulphur utilization amount of electrolyte in lithium-sulphur cells should be not less than 1.5-2.0 mcl per 1 mAh of sulphur electrode capacity (fig. 1b). The cycle life of lithium-sulphur cells is practically linearly increasing with increasing of amount of electrolyte.

References

1. 1. Y.V. Mikhaylik and J.R. Akridge, J. Electrochem. Soc., 2004, 151(11), A1969-A1976.

2. M.-Y. Chu, U.S. Patent, US 6,376,123 B1 (2002).