Current collectors based on multiwalled carbon-nanotubes and few-layer graphene for enhancing the conversion process in scalable lithium-sulfur battery

Abstract

We investigated herein the morphological, structural, and electrochemical features of electrodes using a sulfur (S)-super P carbon (SPC) composite (i.e., S@SPC-73), and including few-layer graphene (FLG), multiwalled carbon nanotubes (MWCNTs), or a mixture of them within the current collector design. Furthermore, we studied the effect of two different electron-conducting agents, that is, SPC and FLG, used in the slurry for the electrode preparation. The supports have high structural crystallinity, while their morphologies are dependent on the type of material used. Cyclic voltammetry (CV) shows a reversible and stable conversion reaction between Li and S with an activation process upon the first cycle leading to the decrease of cell polarization. This activation process is verified by electrochemical impedance spectroscopy (EIS) with a decrease of the resistance after the first CV scan. Furthermore, CV at increasing scan rates indicates a Li+ diffusion coefficient (D) ranging between 10−9 and 10−7 cm2·s−1 in the various states of charge of the cell, and the highest D value for the electrodes using FLG as electron-conducting agent. Galvanostatic tests performed at constant current of C/5 (1 C = 1675 mA·gS−1) show high initial specific capacity values, which decrease during the initial cycles due to a partial loss of the active material, and subsequently increase due to the activation process. All the electrodes show a Coulombic efficiency higher than 97% upon the initial cycles, and a retention strongly dependent on the electrode formulation. Therefore, this study suggests a careful control of the electrode in terms of current collector design and slurry composition to achieve good electrode morphology, mechanical stability, and promising electrochemical performance in practical Li-S cells.

Toward Sustainable Li–S Battery Using Scalable Cathode and Safe Glyme-Based Electrolyte

“…Prolonged galvanostatic cycling tests are carried out at the constant C/5 and C/3 rates, as reported in terms of capacity trends in Figure c,d, respectively, while the related voltage profiles are displayed in Figures S4 in the Supporting Information. The cell cycled at C/5 (Figure c) delivers an initial capacity of 810 mA h g –1 that decreases and stabilizes at about 610 mA h g –1 during the first 20 cycles likely due to consolidation of electrode/electrolyte interphase with SEI formation and partial loss of active material . On the other hand, the cell delivers 200 cycles with a final capacity of 500 mA h g –1 , which corresponds to a retention of 62% of the initial value and 82% of the steady state and Coulombic efficiency exceeding 99% for the whole test.…”

“…Thus, a satisfactory rate capability is achieved by the S:MWCNTs 90:10 w/w from C/10 to C/3, as also suggested by the final capacity recovering at 922 mA h g –1 by lowering back the current at C/10 in the last five cycles, that is, 84% of the initial capacity and 94% compared to the steady state at the same C-rate. Instead, the very modest capacity of the cell at C/2 can be expected due to the relatively high amount of the active sulfur compared to electrochemically inactive elements such as the conductive carbon both in the composite and in the support, as well as by the laminated configuration of the electrode which can lead in turns to a relevant volumetric energy density . Prolonged galvanostatic cycling tests are carried out at the constant C/5 and C/3 rates, as reported in terms of capacity trends in Figure c,d, respectively, while the related voltage profiles are displayed in Figures S4 in the Supporting Information.…”

“…This favorable process has been attributed to the relatively long ether chain of glymes that improves the stability toward radical species formed during the Li–S conversion process and, at the same time, enhances the lithium transport across the electrolyte bulk through the chelation of the Li + ions by the glyme molecules allowing an efficient interchain hopping mechanism . Further steps toward practical configurations of the Li–S battery are the increase of active material in the sulfur cathode for achieving adequate gravimetric energy density , and the limitation of electrode thickness and of the electrolyte volume to get a satisfactory volumetric energy density. , In this regard, thin-layer current collectors suitable for Li–S battery application have been recently developed by coating aluminum with various carbons, including multiwall carbon nanotubes (MWCNTs), graphene flakes formed by limited number of layers, and amorphous substrates. , These new supports have been used as the current collectors in Li–S cells achieving volumetric energy density comparable to Li-ion ones (i.e., 500 W h L –1 ) and much higher gravimetric energy (480 W h kg –1 ) . In these cells, the use of the most diffused electrolyte based on DOL:DME, LiTFSI salt, and LiNO 3 additive was adopted to form a protective film on the lithium metal surface and limit the direct reaction of the dissolved polysulfides (particularly Li 2 S 8 ) with the alkali metal at the anode side (i.e., the shuttle reaction) .…”

Influence of Ion Diffusion on the Lithium–Oxygen Electrochemical Process and Battery Application Using Carbon Nanotubes–Graphene Substrate

“…The SEM images show an electrode surface mainly formed by MWCNTs (Figure a) with a characteristic morphology including secondary particles with sizes ranging from 10 to 30 μm (Figure b) intimately curling up primary nanotubes . The SEM imaging also evidences the presence of FLG flakes, with sizes ranging from 1 to 10 μm and nanometric thickness, dispersed into the MWCNT framework (Figure b,c) . The cell using the 39BB GDL coated with MWCNTs/FLG as the electrode is cycled at a constant current of 0.66 mA (geometrical areal value: 0.33 mA cm –2 ) by limiting the capacity to 2 mA h (geometrical areal value: 1 mA h cm –2 ) that corresponds to charge and discharge processes of 3 h each.…”

“…Among the various electrochemical energy storage systems, lithium–sulfur (Li–S) and Li–O 2 batteries rely on abundant cathode materials, limiting their environmental and economic impact compared to Li-ion batteries. − Furthermore, Li can electrochemically react with either S or O 2 according to conversion processes involving multiple electrons/ions exchange, leading to practical energy densities above 500 W h kg –1 , outperforming the state-of-the-art Li-ion batteries based on Li + -insertion-type electrodes. , Particular interest has been devoted to rechargeable Li–O 2 batteries operating in organic solvents because of their notable energy density (i.e., ∼3400 W h kg –1 for the schematic reaction Li 2 O 2 ⇄ 2Li + O 2 ) and potentially low life cycle environmental burdens. , A relevant boost to these intriguing systems has been achieved by the use of ad hoc-designed electrolytes, including those based on glymes with the general formula CH 3 O­(CH 2 CH 2 O) n CH 3 characterized by chemical and electrochemical stabilities, as well as by limited cost and low toxicity. , In particular, glymes with sufficiently long chains and low volatility can form in Li–O 2 batteries stable coordination complexes with the reactive peroxide and superoxide radicals during ORR, , and can withstand oxidation at potential as high as 4.8 V vs Li + /Li upon OER . The effect of the Li salt nature and concentration on the operation of the Li–O 2 cell has been investigated by several studies, reporting promising results for cells using lithium trifluoromethanesulfonate (LiCF 3 SO 3 ) and lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI) in glyme-based electrolytes characterized by high Li + transference number and ionic conductivity, e.g., with tetraethylene glycol dimethyl ether (TEGDME) as the solvent. ,, Despite the role of the Li + diffusion to the electrode–electrolyte interphase on the cell performances has been widely investigated for Li-ion − and Li–S batteries, , only a limited deal of studies correlated the kinetics of Li + diffusion to the performances of Li–O 2 batteries . Efficient ORR/OER processes have been suggested for Li–O 2 cells using GDLs, for facilitating the diffusion of involved species, with various substrates which promote the reaction kinetics, e.g., nanosized carbon, ,, metals, − metal oxides, − and conductive polymers .…”