Promoting Effect of Amino on Enhancing Cycling Performance at Low Temperatures for Li-S Battery Cathodes
Authors: Zhu Shaoyin , Jin Yongcheng 
Published: February 2020

Comparing to lots of research on electrical performance at room temperature for lithium-sulfur (Li-S) batteries, the low-temperature behaviors of cathodes is outside the scope of concern. The amino group was reported to be a promising candidate to suppress the shuttle through the effective chemisorptions between amino and polysulfides. Herein, we demonstrate an amino-functionalized carbon materials deriving from polyimide wastes. The prepared sulfur cathode (PI-AC-56) displays better low-temperature cycle performance.
As shown in Fig. 1a, an obvious difference of the discharge voltage profiles at the 100th cycle was displayed for two prepared cathodes. In contrast to AC-50 cathode, the typical two-voltage plateaus were displayed for PI-AC-56 cathode at the 100th cycle. As we know, the long-chain polysulfides could easily dissolve into the electrolyte for Li-S battery at room temperature. As reducing the temperature, the viscosity of electrolyte would increase to some extent. And the electrolyte viscosity would further increase following with the dissolution of polysulfide anion to the electrolyte, which would significantly block the migration rate of lithium ion. The further reduction reaction of polysulfides to insolvable discharge products was stemmed. Thus, the low-voltage plateau mainly controlled by the thermodynamics process disappeared for AC-50 electrode and the utilization efficiency of active material also decreased. In reverse, the presence of amino group on surface of carbon material for PI-AC-56 electrode confines the polysulfides dissolution to the electrolyte by the favourable interactions between polysulfides and amino group, which prevents the further increase of electrolyte viscosity to ensure an appropriate migration rate of lithium ion. This insured further reduction reaction for long-chain discharge products. On the other hand, the amino group also insured a homogenous distribution of insoluble discharge products due to the strong interaction between unpaired electrons in the N atom and Li+ in the Li2S, which can prevent the formation of electrochemically inactive large agglomerates. As shown in Fig. 1b, with comparation of AC-50 cathode, better cycle performance and capacity retention were also achieved for PI-AC-56 electrode. The capacity of 368 mAh g-1 after 100 cycles was retained from PI-AC-56 cathode, which was better than the only capacity of 115 mAh g-1 for AC-50 cathode. And with the increase of cycle numbers, the discharge capacity increased gradually first and then decreased, indicating that an activation process was necessary for PI-AC-56 electrode.
CVs results of PI-AC-56 electrode only show a strong reduction peak at 2.31 V (vs Li/Li+) before cycle (Fig. 1c), which was also attributed to the transformation from elemental sulfur (S8) to long-chain lithium polysulfides. In addition to the weak reduction peaks at 2.11 V (vs Li/Li+), another weak reduction peak was observed at about 1.7 V (vs Li/Li+). These two peaks were corresponding to the further reduction reaction of long-chain polysulfides. The similar phenomenon was also obtained for AC-50 cathode at the initial cycle. The difference between the first reduction peak and oxidation peak for PI-AC-56 was smaller than that of AC-50 cathode, indicating the smaller electrode polar for PI-AC-56 electrode. After 100 cycles, an obvious difference was displayed from the CVs curves. A new strong reduction peak relate to the further polysulfide reduction at 1.96 V (vs Li/Li+) appeared for PI-AC-56 electrode, which was corresponding to the presence of low-voltage plateau. Although the similar results also were displayed for AC-50 electrode (Fig. 1d), the whole peak intensity of AC-50 was smaller than that of PI-AC-56 electrode. This result also revealed a fact that the total cell resistance of PI-AC-56 electrode was significantly smaller comparing to AC-50 cathode. These results confirm the positive effect of amino group on improving cycle performance by restricting dissolution of polysulfide to the electrolyte. The electron-donating effect of amino group is favourable to enhancing the reactivity of aromatic carbon rings for sulfur loading by the formation of ammonium polysulphides. Additionally, the lone-pair electrons denoted by the amino group could interact with lithium sulphides by coordination with Li atom. Working together, better cycle performance at different temperatures was achieved by the amino-functionalization.

Figure 1