RESUMO
For Li-S batteries, the interlayer between the separator and sulfur cathode preventing lithium polysulfide (LiPS) travel across the membrane is a research hotspot. The good blocking ability for LiPSs indicates that these interlayers can promote the electrochemistry performance with high S loading. However, most of these interlayers are just used as a simple blocking wall. Such a blocking wall, for example, the lower Li+ ion conductivity, would often reduce the electrochemical performance, especially under large current density. Here, we report a multifunctional ion-sieve made by three two-dimensional (2D) sheets, graphitic carbon nitride (g-C3N4), boron nitride (BN), and graphene. A g-C3N4 sheet which possesses orderly channels with a size of 3 Å in the crystalline structure can effectively prevent polysulfides from passing through but allow lithium ions to pass freely, whereas a BN sheet acts as an excellent catalyst for sulfur redox, and graphene acts as an extended collector, which can promote the conductivity of the sulfur electrode region. Benefiting from the synergistic effect among these 2D materials, the ion-sieve interlayer makes the Li-S battery show excellent performance at a large rate with both high sulfur loadings and high sulfur content. In addition, the host materials are not necessary in these cells. The ion-sieve liberated a discharge capacity of about 600 mA h g-1 after 500 cycles at 1 C, and the capacity attenuation was less than 0.01% per cycle with a 6 mg cm-2 areal S-loading (pure S as the active material). The reversible capacity could be maintained at more than 400 mA h g-1 at 2 C, which amounts to an area current density of 26.88 mA cm-2.
RESUMO
For Li-S batteries, the catalysis for S redox reaction is indispensable. A lot of multifunctional sulfur electrode support materials with have been investigated widely. However, most of these studies were carried out at room temperature, and the interaction between different components in the matrix is not often paid enough attention. Here, we report a graphene supported BN nanosheet composite in which the synergistic effect between BN and graphene greatly enhanced the adsorption for polysulfides, thus leading to excellent performance in a wide temperature range. When used as a host material of sulfur, it can make the Li-S battery apply to a wide range of temperatures, from -40 to 70 °C, delivering a high utilization of sulfur, an excellent rate capability, and outstanding cycling life. The capacity can stabilized at 888 mAh g-1 at 2 C after 300 cycles with a capacity attenuation of <0.04% per cycle at 70 °C, and the battery can deliver a capacity above 650 mAh g-1 at -40 °C.
RESUMO
High utilization and loading of sulfur in cathodes holds the key in the realization of Li-S batteries. We here synthesized a Co4N mesoporous sphere, which was made up of nanosheets, via an easy and convenient method. This material presents high affinity, speedy trapping, and absorbing capacity for polysulfides and acts as a bifunctional catalysis for sulfur redox processes; therefore it is an ideal matrix for S active material. With such a mesoporous sphere used as a sulfur host in Li-S batteries, extraordinary electrochemistry performance has been achieved. With a sulfur content of 72.3 wt % in the composite, the Co4N@S delivered a high specific discharge capacity of 1659 mAh g-1 at 0.1 C, almost reaching its theoretic capacity. Also, the battery exhibited a large reversible capacity of about 1100 mAh g-1 at 0.5 C and 1000 mAh g-1 at 1 C after 100 cycles. At a high rate of 2 C and 5 C, after 300 cycles, the discharge capacity finally stabilized at 805 and 585 mAh g-1. Even at a 94.88% sulfur content, the cathode can still deliver an extremely high specific discharge capacity of 1259 mAh g-1 with good cycle performance.
