Yingying Mi

Fabrication of high tap density LiFe0.6Mn0.4PO4/C microspheres by a double carbon coating–spray drying method for high rate lithium ion batteries

Spherical LiFe0.6Mn0.4PO4/C particles with high tap density were successfully synthesized by sintering spherical precursor powders prepared by a modified spray drying method with a double carbon coating process. The obtained secondary spheres were made of carbon-coated nanocrystallines (∼100 nm), exhibiting a high tap density of 1.4 g cm−3. The LiFe0.6Mn0.4PO4/C microspheres had a reversible capacity of 160.2 mAh g−1 at 0.1C, and a volume energy density of 801.5 Wh L−1 which is nearly 1.4 times that of their nano-sized counterparts. This spherical material showed remarkable rate capability by maintaining 106.3 mAh g−1 at 20C, as well as excellent cycleablity with 98.9% capacity retention after 100 cycles at 2C and 200 cycles at 5C. The excellent electrochemical performance and processability of the LiFe0.6Mn0.4PO4/C microspheres make them very attractive as cathode materials for use in high rate battery application.

Ultrathin dendrimer–graphene oxide composite film for stable cycling lithium–sulfur batteries

Significance The promise of lithium–sulfur batteries for future electric transportation and stationary energy storage is being limited by their poor cycling stability. Previous approaches to improvement often involve incorporating additional components with significant dead weight or volume in battery structures. We develop an ultrathin functionalized dendrimer–graphene oxide composite film which can be applied to virtually any sulfur cathode to alleviate capacity fading over battery cycling without compromising the energy or power density of the entire battery. The design provides a new strategy for confining lithium polysulfide intermediates and thus stabilizing lithium–sulfur batteries. It also brings a suitable platform for elucidating the underlying materials and surface chemistry. Lithium–sulfur batteries (Li–S batteries) have attracted intense interest because of their high specific capacity and low cost, although they are still hindered by severe capacity loss upon cycling caused by the soluble lithium polysulfide intermediates. Although many structure innovations at the material and device levels have been explored for the ultimate goal of realizing long cycle life of Li–S batteries, it remains a major challenge to achieve stable cycling while avoiding energy and power density compromises caused by the introduction of significant dead weight/volume and increased electrochemical resistance. Here we introduce an ultrathin composite film consisting of naphthalimide-functionalized poly(amidoamine) dendrimers and graphene oxide nanosheets as a cycling stabilizer. Combining the dendrimer structure that can confine polysulfide intermediates chemically and physically together with the graphene oxide that renders the film robust and thin (<1% of the thickness of the active sulfur layer), the composite film is designed to enable stable cycling of sulfur cathodes without compromising the energy and power densities. Our sulfur electrodes coated with the composite film exhibit very good cycling stability, together with high sulfur content, large areal capacity, and improved power rate.

Strong Metal–Phosphide Interactions in Core–Shell Geometry for Enhanced Electrocatalysis

Rational design of multicomponent material structures with strong interfacial interactions enabling enhanced electrocatalysis represents an attractive but underdeveloped paradigm for creating better catalysts for important electrochemical energy conversion reactions. In this work, we report metal-phosphide core-shell nanostructures as a new model electrocatalyst material system where the surface electronic states of the shell phosphide and its interactions with reaction intermediates can be effectively influenced by the core metal to achieve higher catalytic activity. The strategy is demonstrated by the design and synthesis of iron-iron phosphide (Fe@FeP) core-shell nanoparticles on carbon nanotubes (CNTs) where we find that the electronic interactions between the metal and the phosphide components increase the binding strength of hydrogen adatoms toward the optimum. As a consequence, the Fe@FeP/CNT material exhibits exceptional catalytic activity for the hydrogen evolution reaction, only requiring overpotentials of 53-110 mV to reach catalytic current densities of 10-100 mA cm-2.

Ferrocene-Promoted Long-Cycle Lithium–Sulfur Batteries

Confining lithium polysulfide intermediates is one of the most effective ways to alleviate the capacity fade of sulfur-cathode materials in lithium-sulfur (Li-S) batteries. To develop long-cycle Li-S batteries, there is an urgent need for material structures with effective polysulfide binding capability and well-defined surface sites; thereby improving cycling stability and allowing study of molecular-level interactions. This challenge was addressed by introducing an organometallic molecular compound, ferrocene, as a new polysulfide-confining agent. With ferrocene molecules covalently anchored on graphene oxide, sulfur electrode materials with capacity decay as low as 0.014 % per cycle were realized, among the best of cycling stabilities reported to date. With combined spectroscopic studies and theoretical calculations, it was determined that effective polysulfide binding originates from favorable cation-π interactions between Li+ of lithium polysulfides and the negatively charged cyclopentadienyl ligands of ferrocene.

High-performance Li–S battery cathode with catalyst-like carbon nanotube-MoP promoting polysulfide redox

Despite promising characteristics such as high specific energy and low cost, current Li–S batteries fall short in cycle life. Improving the cycling stability of S cathodes requires immobilizing the lithium polysulfide (LPS) intermediates as well as accelerating their redox kinetics. Although many materials have been explored for trapping LPS, the ability to promote LPS redox has attracted much less attention. Here, we report for the first time on transition metal phosphides as effective host materials to enhance both LPS adsorption and redox. Integrating MoP-nanoparticle-decorated carbon nanotubes with S deposited on graphene oxide, we enable Li–S battery cathodes with substantially improved cycling stability and rate capability. Capacity decay rates as low as 0.017% per cycle over 1,000 cycles can be realized. Stable and high areal capacity (>3 mAh·cm−2) can be achieved under high mass loading conditions. Comparable electrochemical performance can also be achieved with analogous material structures based on CoP, demonstrating the potential of metal phosphides for long-cycle Li–S batteries.

A pomegranate-structured sulfur cathode material with triple confinement of lithium polysulfides for high-performance lithium–sulfur batteries

High-performance lithium–sulfur batteries are widely and intensively pursued, owing to their projected high energy density and low cost. However, realizing the stable cycling of a sulfur cathode with good discharging/charging rate capability under high sulfur content and high sulfur loading conditions remains a major challenge. Confining the dissolvable lithium polysulfide intermediates while addressing the intrinsic low electrical conductivity of sulfur is a key approach toward solving the problem. This work presents the design of a pomegranate-structured sulfur cathode material with high electrochemical performance. To synthesize the material, mesoporous carbon particles with ferrocene decoration are infiltrated with sulfur and then wrapped into secondary particles by dendrimer-linked graphene oxide. In the designed structure, the mesoporous carbon serves as a conductive matrix and porous host for sulfur species; ferrocene provides polar sites to bind lithium polysulfides chemically; the dendrimer-linked graphene oxide encapsulation layers further confine leaching of polysulfides and ferrocene into the electrolyte. With the three components providing triple confinement of the polysulfides, the material with a high sulfur content of 75.7 wt% exhibits excellent cycling stability and good rate capability. A capacity of 826 mA h g−1 can be delivered at 1.0C with an average decay of only 0.010% per cycle over 1200 cycles. With a high S mass loading of 4 mg cm−2, the cathode can still be cycled at 0.5C for 300 cycles with a capacity decay as low as 0.038% per cycle.