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All four different non-woven felts were tested in coin cells by galvanostatic cycling. However, for simplicity, they were used as-received as current collectors in semi-liquid Li/S cells, so- called ‘catholyte’ cells. Preparation of catholyte solution (1M LiTFSI + 0.1M LiNO3 + 0.5M Li2S6 in TEGDME/DIOX = 1/1 vol ratio) together with a coin cell architecture were described in section 2.2.1. For these tests, coin cells with 40 μL of catholyte solution (+ 60 μL of standard electrolyte) were made. Theoretical capacity of such prepared semi-liquid coin cell (6.43 mAh) was calculated in respect to the amount of sulfur present in the volume of added catholyte solution. Galvanostatic cycling results obtained at C/20 are presented in Figure 3-21. The voltage profiles do not show any upper discharge plateaus, since the active material was introduced in the form of soluble Li2S6. We can see that during initial cycle, double-side NwC’s (C2 and I2C8) have higher capacities. If we look at the voltage profile, it is clear that higher initial capacities are due to the presence of an additional discharge plateau at ~ 1.7 V, which significantly contributes to the final value. This plateau is related with the irreversible reduction of LiNO3 additive on the positive electrode177,178, and was previously observed for the composite electrodes with high carbon/sulfur ratios. In this case, LiNO3 must reduce on the modified side of the C2 and I2C8 NwC layers, where the active surface of carbon is highly developed by the modification with additional microporous layer (BET surface area, see Table 3-5). (a) (b) Figure 3-21. Capacity retention obtained at C/20 (a) and corresponding first discharge profiles (b) of four ‘catholyte’ cells with different NwC felts used as a positive electrode collectors: classic NwC (in black), T10 (in blue), C2 (in green), I2C8 (in red). The electrolyte solution was containing 40 μL of 0.5M Li2S6 + 1M LiTFSI + 0.1M LiNO3 in TEGDME/DIOX = 1/1 vol, resulting in theoretical capacity of 6.43 mAh. One should note that the reduction of LiNO3 at the potential lower than 1.7 V allowed us to question the effective complete passivation of the electrode by Li2S at the end of discharge. Indeed, if all conductive surface area is passivated by an insulating film of Li2S, further reduction of LiNO3 at ~ 1.5 V would be impossible. After ~ 15 cycles, the capacity values Chapter 3: S8 electrode on NwC 97PDF Image | Accumulateur Lithium Soufre
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