Lithium Sulfide (Li₂S)/Graphene Oxide Nanospheres with Conformal Carbon Coating as a High-Rate, Long-Life Cathode for Li/S Cells

Abstract

In recent years, lithium/sulfur (Li/S) cells have attracted great attention as a candidate for the next generation of rechargeable batteries due to their high theoretical specific energy of 2600 W·h kg(-1), which is much higher than that of Li ion cells (400-600 W·h kg(-1)). However, problems of the S cathode such as highly soluble intermediate species (polysulfides Li2Sn, n = 4-8) and the insulating nature of S cause poor cycle life and low utilization of S, which prevents the practical use of Li/S cells. Here, a high-rate and long-life Li/S cell is proposed, which has a cathode material with a core-shell nanostructure comprising Li2S nanospheres with an embedded graphene oxide (GO) sheet as a core material and a conformal carbon layer as a shell. The conformal carbon coating is easily obtained by a unique CVD coating process using a lab-designed rotating furnace without any repetitive steps. The Li2S/GO@C cathode exhibits a high initial discharge capacity of 650 mA·h g(-1) of Li2S (corresponding to the 942 mA·h g(-1) of S) and very low capacity decay rate of only 0.046% per cycle with a high Coulombic efficiency of up to 99.7% for 1500 cycles when cycled at the 2 C discharge rate.

Li2S nano spheres anchored to single-layered graphene as a high-performance cathode material for lithium/sulfur cells

“…Interestingly, the first charge plateau of all cathodes ( Fig. S8) displayed relatively low overpotentials compared to the results reported by most researchers, and the overpotentials gradually decreased as the amount of graphene increased, which might indicate higher electronic conductivity, good lithium ion diffusivity in the Li 2 S and faster charge transfer at the Li 2 S surface in the #8-Li 2 S/G@C composite [10,36]. The long-term cycling stability was conducted at a high charge and discharge C rate of 2.0 C for 1000 cycles and the results are shown in Fig.…”

“…In addition, another difficulty of the Li/S cells is the use of a metallic lithium anode, which is associated with dendrite formation on the surface of the Li metal anode caused by the inhomogeneous current distribution during cycling, leading to serious safety hazards and cell shorting [30][31][32]. Fully lithiated lithium sulfide (Li 2 S) with a theoretical specific capacity of 1169 mA h g À 1 Li 2 S has become a more desirable cathode material for the Li/S cell due to its capability of pairing with a lithium-free anode such as silicon and some tin compounds which can obviate the safety concerns of the lithium metal anode when using sulfur [10,[33][34][35][36][37][38][39][40][41]. Li 2 S particles, as the end discharge product of the Li/S cell, shrink during charging and generate empty space to accommodate the volume expansion of sulfur particles during lithiation (discharge), thus the mechanical failure of the cathode can be alleviated, resulting in improved cycle life for the Li/S cell.…”

“…Li 2 S particles, as the end discharge product of the Li/S cell, shrink during charging and generate empty space to accommodate the volume expansion of sulfur particles during lithiation (discharge), thus the mechanical failure of the cathode can be alleviated, resulting in improved cycle life for the Li/S cell. Moreover, the higher melting point of Li 2 S (1372°C) [42] enables modifications under high temperature treatment conditions such as chemical vapor deposition (CVD) that can provide a uniform and stable carbon coating on the particle surface [35,36,43]. Sharing the same reaction mechanisms, Li 2 S cathodes inevitably encounter the same problems as for sulfur cathodes such as the dissolution of polysulfides into most liquid electrolytes, and its "shuttle effect" which can induce low utilization of active material, low coulombic efficiency and severe degradation of the cycle life.…”

“…To address the abovementioned problems, many approaches have emerged to improve the electrochemical performance of the Li 2 S cathode, for example employing Li 2 S nanoparticles, and encapsulating Li 2 S with carbon, [1,10,35] conductive polymers or transition metal disulfides [37,45]. In addition, some functional carbon materials such as carbon nanofibers, graphene and GO are also used to enhance the electrical conductivity of the Li 2 S cathode material [36,[46][47][48][49].…”

Redox-Active Supramolecular Polymer Binders for Lithium–Sulfur Batteries That Adapt Their Transport Properties in Operando

“…[1][2][3][4][5][6] Nevertheless, persistent challenges associated with the sulfur cathode must be overcome for Li-S cells to become practical. For example, while sulfur cathodes have been engineered extensively for high energy density and durability, [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] design rules are still lacking for high power while also attaining high specific energy.…”