Uniform Li Deposition through the Graphene-Based Ion-Flux Regulator for High-Rate Li Metal Batteries.

Lithium (Li) metal is considered an ultimate anode owing to its high specific capacity and energy density. However, uncontrolled Li dendrite growth and low Coulombic efficiency have limited the application of Li metal. Among various strategies introduced to address these limitations, the surface modification of polyolefin separators with functional materials has been widely adopted for improving the mechanical and thermal stabilities of polymer separators and to protect the separator from the penetration of Li dendrites. Herein, we report a new functional polymer separator that is surface-altered with a graphene-based Li-ion flux regulator (GLR) to homogenize the Li-ion flux and suppress the growth of sharp dendritic Li in Li metal batteries. The nanopores distributed through the GLR structure serve as channels for ion transport and junctions for electron transfer, facilitating efficient electrolyte penetration and rapid charge transfer between graphene (Gr) sheets. Owing to these favorable features of porous GLR, a Li-Cu cell with the GLR surface-altered polypropylene separator (GLR-PP) delivers excellent cycle and rate performances compared to a Li-Cu cell with a Gr surface-altered polypropylene separator. In addition, among the tested cells, Li-sulfur cells with GLR-PP exhibit the most stable cycle performance over 500 cycles. These results demonstrate that the concept of tailoring the surface of a polymer separator with porous 2D materials is an effective strategy for improving the long-term cycle stability and electrochemical kinetics of Li metal-based batteries and would trigger further relevant studies.

Biomass-derived, activated carbon-sulfur composite cathode with a bifunctional interlayer of functionalized carbon nanotubes for lithium-sulfur cells.

Lithium-sulfur (Li-S) cells are emerging as the dominant constituents of the next generation battery technology, offering high theoretical capacity around 1675 mA h g-1 and the additional advantages of low cost and non-toxic nature. Activated carbon, derived from natural resources is being extensively investigated for applications as electrode materials in high power supercapacitors and for making composite electrodes for designing high energy density electrochemical cells. The present work is aimed at introducing the potential of the composite cathode of sulfur with the biomass-derived, steam activated carbon (AC) along with the free-standing and flexible film of carbon nanotubes as the interlayer for designing efficient Li-S cells. The composite obtained by impregnating sulfur particles into the pores of coconut shell derived and steam activated carbon, subjected to efficient acid washing procedures to attain maximum purity, called as the activated carbon-sulfur (ACS) is used as the composite cathode material. The flexible film of acid-functionalized carbon nanotubes termed as the CNTF placed between the composite cathode and the separator material serves as an active interlayer to boost the performance efficiency of the assembled Li-S cells. The ACS composite is synthesized by the solvothermal method, and the flexible CNTF is obtained by solution casting. The Li-S cells assembled with the ACS composite as the active cathode material and the CNTF as the interlayer are found to exhibit quite impressive discharge capacity and cycling stability. These cells deliver an initial discharge capacity of 1562 mA h g-1 at 0.05 C rate and retain 71% of the initial capacity at 1 C rate after 200 cycles. The conducting and the porous network of the ACS helps to enhance the overall electrical conductivity of the sulfur composite cathode and the highly conducting CNTF interlayer accelerates the electrochemical activity taking place in the cell. The interlayer restricts the polysulfides from migrating to the anode and thereby suppresses the polysulfide shuttle phenomenon. The use of the coconut shell derived, steam activated and acid washed carbon for making the composite cathode with sulfur and the CNTF interlayer, obtained by the acid functionalization of carbon nanotubes is a novel approach to realize Li-S cells with high capacity and excellent cycling stability, which has not yet been pursued in detail.

Freeze-Dried Sulfur-Graphene Oxide-Carbon Nanotube Nanocomposite for High Sulfur-Loading Lithium/Sulfur Cells.

The ambient-temperature rechargeable lithium/sulfur (Li/S) cell is a strong candidate for the beyond lithium ion cell since significant progress on developing advanced sulfur electrodes with high sulfur loading has been made. Here we report on a new sulfur electrode active material consisting of a cetyltrimethylammonium bromide-modified sulfur-graphene oxide-carbon nanotube (S-GO-CTA-CNT) nanocomposite prepared by freeze-drying. We show the real-time formation of nanocrystalline lithium sulfide (Li2S) at the interface between the S-GO-CTA-CNT nanocomposite and the liquid electrolyte by in situ TEM observation of the reaction. The combination of GO and CNT helps to maintain the structural integrity of the S-GO-CTA-CNT nanocomposite during lithiation/delithiation. A high S loading (11.1 mgS/cm2, 75% S) S-GO-CTA-CNT electrode was successfully prepared using a three-dimensional structured Al foam as a substrate and showed good S utilization (1128 mAh/g S corresponding to 12.5 mAh/cm2), even with a very low electrolyte to sulfur weight ratio of 4. Moreover, it was demonstrated that the ionic liquid in the electrolyte improves the Coulombic efficiency and stabilizes the morphology of the Li metal anode.

X-ray Absorption Spectroscopy Characterization of a Li/S Cell.

The X-ray absorption spectroscopy technique has been applied to study different stages of the lithium/sulfur (Li/S) cell life cycle. We have investigated how speciation of S in Li/S cathodes changes upon the introduction of CTAB (cetyltrimethylammonium bromide, CH3(CH2)15N⁺(CH3)3Br-) and with charge/discharge cycling. The introduction of CTAB changes the synthesis reaction pathway dramatically due to the interaction of CTAB with the terminal S atoms of the polysulfide ions in the Na2Sx solution. For the cycled Li/S cell, the loss of electrochemically active sulfur and the accumulation of a compact blocking insulating layer of unexpected sulfur reaction products on the cathode surface during the charge/discharge processes make the capacity decay. A modified coin cell and a vacuum-compatible three-electrode electro-chemical cell have been introduced for further in-situ/in-operando studies.