Yusheng Ye

Graphene-Based Three-Dimensional Hierarchical Sandwich-type Architecture for High-Performance Li/S Batteries

A multiwalled carbon nanotube/sulfur (MWCNT@S) composite with core-shell structure was successfully embedded into the interlay galleries of graphene sheets (GS) through a facile two-step assembly process. Scanning and transmission electron microscopy images reveal a 3D hierarchical sandwich-type architecture of the composite GS-MWCNT@S. The thickness of the S layer on the MWCNTs is ~20 nm. Raman spectroscopy, X-ray diffraction, thermogravimetric analysis, and energy-dispersive X-ray analysis confirm that the sulfur in the composite is highly crystalline with a mass loading up to 70% of the composite. This composite is evaluated as a cathode material for Li/S batteries. The GS-MWCNT@S composite exhibits a high initial capacity of 1396 mAh/g at a current density of 0.2C (1C = 1672 mA/g), corresponding to 83% usage of the sulfur active material. Much improved cycling stability and rate capability are achieved for the GS-MWCNT@S composite cathode compared with the composite lacking GS or MWCNT. The superior electrochemical performance of the GS-MWCNT@S composite is mainly attributed to the synergistic effects of GS and MWCNTs, which provide a 3D conductive network for electron transfer, open channels for ion diffusion, strong confinement of soluble polysulfides, and effective buffer for volume expansion of the S cathode during discharge.

An effective approach to protect lithium anode and improve cycle performance for Li-S batteries.

Lithium oxalyldifluoroborate (LiODFB) has been investigated as an organic electrolyte additive to improve the cycling performance of Li-S batteries. Cell test results demonstrate that an appropriate amount of LiODFB added into the electrolyte leads to a high Coulombic efficiency. Analyses by energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and the density functional theory showed that LiODFB promotes the formation of a LiF-rich passivation layer on the lithium metal surface, which not only blocks the polysulfide shuttle, but also stabilizes the lithium surface.

Freestanding three-dimensional core-shell nanoarrays for lithium-ion battery anodes.

Structural degradation and low conductivity of transition-metal oxides lead to severe capacity fading in lithium-ion batteries. Recent efforts to solve this issue have mainly focused on using nanocomposites or hybrids by integrating nanosized metal oxides with conducting additives. Here we design specific hierarchical structures and demonstrate their use in flexible, large-area anode assemblies. Fabrication of these anodes is achieved via oxidative growth of copper oxide nanowires onto copper substrates followed by radio-frequency sputtering of carbon-nitride films, forming freestanding three-dimensional arrays with core–shell nano-architecture. Cable-like copper oxide/carbon-nitride core–shell nanostructures accommodate the volume change during lithiation−delithiation processes, the three-dimensional arrays provide abundant electroactive zones and electron/ion transport paths, and the monolithic sandwich-type configuration without additional binders or conductive agents improves energy/power densities of the whole electrode.

Hierarchical mesoporous/macroporous Co3O4 ultrathin nanosheets as free-standing catalysts for rechargeable lithium–oxygen batteries

Hierarchical mesoporous/macroporous Co3O4 ultrathin nanosheets were synthesized as free-standing catalysts for rechargeable Li–O2 batteries. The Co3O4 nanosheets were directly grown on nickel foam through a simple hydrothermal reaction, followed by a calcination process. The impact of solvents used in the hydrothermal reaction on the morphology of catalysts has been investigated. The results showed that the prepared Co3O4 catalyst synthesized with ethylene glycol and deionized water (1 : 1 in volume) presented a much better electrochemical performance with a capacity of 11 882 mA h g−1 under a current density of 100 mA g−1 during the initial discharge and good cycling stability (more than 80 cycles at 200 mA g−1 with the capacity limited to 500 mA h g−1). Meanwhile, the charge potential was significantly reduced to ca. 3.7 V. It is interesting to find that the morphology of the discharge product, Li2O2 could be changed by controlling the shape of catalysts. The impacts of the hierarchical mesoporous/macroporous nanosheet structure on the performance of Li–O2 batteries have been discussed.

Facile Synthesis of Boron-Doped rGO as Cathode Material for High Energy Li–O2 Batteries

To improve the electrochemical performance of the high energy Li-O2 batteries, it is important to design and construct a suitable and effective oxygen-breathing cathode. Herein, a three-dimensional (3D) porous boron-doped reduction graphite oxide (B-rGO) material with a hierarchical structure has been prepared by a facile freeze-drying method. In this design, boric acid as the boron source helps to form the 3D porous structure, owing to its cross-linking and pore-forming function. This architecture facilitates the rapid oxygen diffusion and electrolyte penetration in the electrode. Meanwhile, the boron-oxygen functional groups linking to the carbon surface or edge serve as additional reaction sites to activate the ORR process. It is vital that boron atoms have been doped into the carbon lattices to greatly activate the electrons in the carbon π system, which is beneficial for fast charge under large current densities. Density functional theory calculation demonstrates that B-rGO exhibits much stronger interactions with Li5O6 clusters, so that B-rGO more effectively activates Li-O bonds to decompose Li2O2 during charge than rGO does. With B-rGO as a catalytic substrate, the Li-O2 battery achieves a high discharge capacity and excellent rate capability. Moreover, catalysts could be added into the B-rGO substrate to further lower the overpotential and enhance the cycling performance in future.

Light-weight functional layer on a separator as a polysulfide immobilizer to enhance cycling stability for lithium–sulfur batteries

A light-weight boron-functionalized reduced graphene oxide (B-rGO) layer (only 0.2–0.3 mg cm−2) coated on a separator is demonstrated to improve the cycling stability and rate performance of lithium–sulfur batteries. Such an enhanced performance is ascribed to: (i) the boron species in B-rGO can enhance the binding with polysulfides, which helps suppress the shuttle reactions, thus alleviating overcharge and self-discharge; (ii) a certain amount of the boron doped into the graphene matrix can improve the electrical conductivity of the coating layer, thus enhancing the utilization of sulfur and improving the rate performance of the cells. With the B-rGO coated separator, the severe self-discharge of Li–S batteries can be alleviated. More importantly, for the high sulfur loading cathodes (above 4.5 mg cm−2), an improved high areal capacity of 4.71 mA h cm−2 can be achieved using the B-rGO coated separator. The above results demonstrate the potential of the B-rGO coated separator for practical lithium–sulfur batteries, and such a strategy can be extended to other energy storage systems.

Surface Modification of Li-Rich Cathode Materials for Lithium-Ion Batteries with a PEDOT:PSS Conducting Polymer

UNLABELLED Composites of lithium-rich Li1.2Ni0.2Mn0.6O2 and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) ( PEDOT PSS) are synthesized through coprecipitation followed by a wet coating method. In the resulting samples, the amorphous conductive polymer films on the surface of the Li1.2Ni0.2Mn0.6O2 particles are 5-20 nm thick. The electrochemical properties of Li1.2Ni0.2Mn0.6O2 are obviously enhanced after PEDOT PSS coating. The composite sample with an optimal 3 wt % coating exhibits rate capability and cycling properties that are better than those of Li1.2Ni0.2Mn0.6O2, with an excellent initial discharge capacity of 286.5 mA h g(-1) at a current density of 0.1 C and a discharge capacity that remained at 146.9 mA h g(-1) at 1 C after 100 cycles. The improved performances are ascribed to the high conductivity of the PEDOT PSS coating layer, which can improve the conductivity of the composite material. The PEDOT PSS layer also suppresses the formation and growth of a solid electrolyte interface. Surface modification with PEDOT PSS is a feasible approach for improving the comprehensive properties of cathode materials.

Boron-doped microporous nano carbon as cathode material for high-performance Li-S batteries

In this study, a boron-doped microporous carbon (BMC)/sulfur nanocomposite is synthesized and applied as a novel cathode material for advanced Li-S batteries. The cell with this cathode exhibits an ultrahigh cycling stability and rate capability. After activation, a capacity of 749.5 mAh/g was obtained on the 54th cycle at a discharge current of 3.2 A/g. After 500 cycles, capacity of 561.8 mAh/g remained (74.96% retention), with only a very small average capacity decay of 0.056%. The excellent reversibility and stability of the novel sulfur cathode can be attributed to the ability of the boron-doped microporous carbon host to both physically confine polysulfides and chemically bind these species on the host surface. Theoretical calculations confirm that boron-doped carbon is capable of significantly stronger interactions with the polysulfide species than undoped carbon, most likely as a result of the lower electronegativity of boron. We believe that this doping strategy can be extended to other metal-air batteries and fuel cells, and that it has promising potential for many different applications.

A polypyrrole-supported carbon paper acting as a polysulfide trap for lithium–sulfur batteries

Lithium–sulfur (Li–S) batteries with high theoretical capacities and low cost are a strong candidate for future energy storage, but their development is hindered by many shortcomings, such as high-rate capacity decay due to the “shuttle effect”. Herein, we increase the capacity retention and cycle life of the Li–S battery through the addition of an interlayer made of polypyrrole (PPy)-treated carbon paper (CP) in a Li–S battery. We first quantitatively investigate the effect of the thickness of the carbon paper and then optimize a novel interlayer prepared by using PPy adhered to the carbon paper. The results show that 300 μm CP is the best choice among the three thicknesses. The CP-300 samples deliver a reversible capacity of 490 mAh g−1 after 200 cycles with a 0.5 C rate and show the best rate performance. Because of the porous structure and conductivity of the as-prepared PPy interlayer, the battery incorporating the PPy interlayer exhibits more excellent cycle performance and better rate performance than the CP batteries. Surprisingly, the PPy-coated CP-200 battery displays a reversible capacity of 555 mAh g−1 after 200 cycles with a 0.5 C rate. This feasible way to modify a carbon paper interlayer may have promising prospects in the Li–S battery field.

A Praline‐Like Flexible Interlayer with Highly Mounted Polysulfide Anchors for Lithium–Sulfur Batteries

The development of lithium-sulfur (Li-S) batteries is dogged by the rapid capacity decay arising from polysulfide dissolution and diffusion in organic electrolytes. To solve this critical issue, a praline-like flexible interlayer consisting of high-loading titanium oxide (TiO2 ) nanoparticles and relatively long carbon nanofibers is fabricated. TiO2 nanoparticles with a size gradient occupy both the external and internal of carbon fiber and serve as anchors that allow the chemical adsorption of polysulfides through a conductive nanoarchitecture. The porous conductive carbon backbone helps in the physical absorption of polysulfides and provides redox reaction sites to allow the polysulfides to be reused. More importantly, it offers enough mechanical strength to support a high load TiO2 nanoparticle (79 wt%) that maximizes their chemical role, and can accommodate the large volume changes. Significant enhancement in cycle stability and rate capability is achieved for a readily available sulfur/multi-walled carbon nanotube composite cathode simply by incorporating this hierarchically nanostructured interlayer. The design and synthesis of interlayers by in situ integration of metal oxides and carbon fibers via a simple route offers the potential to advance Li-S batteries for practical applications in the future.