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and it can be as high as 35 % (example from Figure 4-4). This irreversible capacity in Li2S- based system reflects the capacity loss between first charge and following discharge, mainly due to the structural changes of the electrode after Li2S dissolution, the electrode pulverization, the loss of carbon/active material contact, loss of active material in the electrolyte leading to the uncompleted transformation back to Li2S during following discharge. Another calculation method of efficiency (Ф), inspired by Li-ion systems, can be proposed as follow: Φ = ( ) !h#$%& (11) ( ) !h#$%& This calculation method gives information on the efficiency of the charge process regarding to the following discharge, i.e. structural changes of materials, electrolyte degradation, loss of lithium ions. This method is more correlated to the capacity fading, and rather related to the structural changes of the positive electrode during the first cycle (in case of the sulfur system). For the example presented previously on Figure 4-6, two different calculation methods of coulombic efficieny were applied (equations (9) and (11)) and are shown on Figure 4-7. Figure 4-7. Coulombic efficiency comparison, calculated for a Li/Li2S cell, with two different ways: as a ratio ‘(n) discharge / (n+1) charge’ – which allows to evidence the shuttle phenomenon (in black); as a ratio ‘(n) discharge / (n) charge’, by analogy to classical Li-ion cells (in red). For the black curve, the initial efficiency is above 100 %, and decreases down to ~ 90 %, while for the red curve the efficiency is far below 100 %, and increases up to ~ 90 % after few cycles. The first 5 – 10 cycles show kind of transitory efficiency, as a demonstration of initial capacity fading. Afterwards, coulombic efficiencies calculated in both ways, come to the same point, i.e. of about ~ 90 %, meaning that the presence of shuttle mechanism is dominant while almost no capacity fading is occurring anymore. Chapter 4: Li2S electrode 119PDF Image | Accumulateur Lithium Soufre
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