Vertically aligned sulfur-graphene nanowall cathodes for Li-sulfur batteries deliver high capacity and rate performance
A team at Beihang University in China has synthesized cathode materials for Li-sulfur batteries consisting of vertically aligned sulfur–graphene (S-G) nanowalls on electrically conductive substrates. In each individual S-G nanowall, the sulfur nanoparticles are homogeneously anchored between graphene layers; ordered graphene arrays arrange perpendicularly to the substrates, enabling fast diffusion of both lithium ions and electrons.
As reported in their paper in the ACS journal Nano Letters, the cathodes achieve a high reversible capacity of 1261 mAh g–1 in the first cycle and more than 1210 mAh g–1 after 120 cycles with excellent cyclability and high-rate performance (more than 400 mAh g–1 at 8C, 13.36 A g–1). This is the best demonstrated rate performance for sulfur–graphene cathodes to date, according to the team.
Despite the theoretical promise of Li-sulfur batteries (energy density of 2567 Wh kg−1, more than 5x that of commercial Li-ion batteries; sulfur is abundant, nontoxic, and inexpensive), practical application has been hampered by a number of challenges. These include the inherent low electrical conductivity of sulfur, intermediate polysulfide products and final Li2S, which result in low active material utilization efficiency and poor rate capability; and the high solubility of polysulfide intermediates, which can shuttle between the anode and cathode, forming deposits of solid Li2S2 and Li2S on the cathode, causing large loss of the active material and severe degradation of cycle life.
In order to overcome … obstacles of Li−S batteries, one effective strategy is to confine sulfur and polysulfides into carbonaceous matrix and meanwhile improve the electrical conductivity of overall electrodes. In this respect, various carbons including mesoporous carbons, microporous carbons, hollow carbon spheres, carbon nanotubes, and graphene have been investigated to produce sulfur−carbon hybrids and demonstrated the improved electrochemical performances.
In particular, graphene, atomic layer of carbon, exhibits ultrahigh electrical conductivity, good flexibility, large surface area, high chemical and thermal stabilities. This provides new opportunities to design and fabricate sulfur−graphene composites with unique structures. … Very recently, controllably self-assembly of graphene layers into three-dimensional architectures with macroporous or mesoporous structures is becoming an effective approach to harness the intriguing properties of graphene to practical applications. However, until now, how to facilely control the assembly of graphene layers or graphene-guest hybrids remains a big challenge.—Li et al.
The Beihang team devised a simple electrochemical assembly strategy to achieve ordered sulfur−graphene (S-G) nanowalls, which are controllably and vertically aligned onto the surface of electrically conductive substrates during the tunable cyclic voltammetry (CV) processes.
Sulfur nanoparticles were first anchored onto the surface of graphene (or reduced graphene oxide) via the reaction of Na2S with graphene oxide (GO). The team then conducted electrochemical assembly processes in a two electrode system with electrical conductive substrates (copper or nickel sheet) as the work electrode and a platinum sheet as contrast electrode.
Controllable cyclic voltammetry processes were operated in the voltage range of -0.5 to 2V at a scanning rate of 50 mV s-1. Adjusting the scanning numbers results in vertically aligned S-G nanowalls with tunable thickness. The mass areal density of S-G nanowalls fabricated by CV scanning 10 cycles is 1.3 mg cm-2; increasing the CV scanning number to 15 and 20 increased the mass areal density to 1.8 and 2.2 mg cm-2, respectively.
For electrochemical measurement, the team assembled coin cells with the as-prepared S-G nanowalls directly using as electrode without any binder and conductive additive, Li metal as a counter electrode, a separator (Celgard 2300), and an electrolyte of 1 M LiTFSI in a mixture of 1,3-dioxolane (DOL), 1,2-dimethoxyethane(DMC) and tetraethylene glycol dimethyl ether (TEGDME) (volume ratio 5:4:1).
We believe that such a simple and controllable electrochemical assembly protocol will provide a new pathway for the production of various graphene-containing composites with unique structures for catalysis, sensors, and energy storage and conversions.—Li et al.
Bin Li, Songmei Li, Jianhua Liu, Bo Wang, and Shubin Yang (2015) “Vertically Aligned Sulfur–Graphene Nanowalls on Substrates for Ultrafast Lithium–Sulfur Batteries” Nano Letters doi: 10.1021/acs.nanolett.5b00064