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Chapter 4: Li2S electrode Figure 4-6. Capacity retention over 100 cycles of a Li2S/SuperP®/PVdF electrode cycled vs. Li at C/20, in the potential window 3.8 V – 1.7 V for the initial cycle, and 2.8 V – 1.7 V for the next cycles. Active material loading of 2.27 mgLi2S cm-2. Error bars in relation to the 3.8 % error due to active mass imprecision (see section 4.2.1.a). 4.2.3.b) Discussion on the coulombic efficiency calculation In parallel to the discharge profile and capacity retention, it is also important to discuss about the coulombic efficiency, and to explain how it is calculated. Two different methods can be employed for efficiency calculations, both providing different information. Calculation method for a Li2S/Li system is proposed according to equation (9): Φ = ( ) !h#$%& (9) ( + 1) !h#$%& This calculation method allows to determine the efficiency of the charge as compared to the prior discharge, and to evidence the occurrence of shuttle mechanism, with an efficiency being usually below 1 after few cycles. One should note that, in the specific case of sulfur system, even if the efficiency is far below 1, this is not necessarily linked to the capacity fading, as commonly observed for Li-ion system. On the contrary, the low coulombic efficiency is mainly linked to the occurrence of shuttle mechanism, which is responsible for longer charge, while not being detrimental for the following discharge in terms of capacity. This calculation method also allows for relatively easy comparison of sulfur and Li2S-based systems, by leaving apart the first charge of Li2S, which is quite different from further cycles. Regarding the first cycle of Li2S electrode, an irreversible capacity can be calculated, using the following formula: 1() !h#$%& − 1() !h#$%& 1() !h#$%& (10) 118PDF Image | Accumulateur Lithium Soufre
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