Zihan Shen

Structure design of NiCo2O4 electrodes for high performance pseudocapacitors and lithium-ion batteries

High capacitance/capacity electrodes are in urgent demand to increase the charge storage capability of energy storage devices. However, there are some scientific and technical challenges for a large amount of charges to be transported through the devices because the kinetic resistances and volume change issues may limit the performance of devices. 3D conductive scaffolds are usually developed to build rapid electron/ion pathways and accommodate volume changes. Using a templated electrodeposition and hydrothermal synthesis technique, we developed a composite electrode consisting of NiCo2O4 nanowires on ultralight nickel foam. The NiCo2O4 nanowires provide a large surface for rapid charge transports. The ultralight nickel foam electrically wires NiCo2O4 and accommodates the volume expansion of NiCo2O4 during lithiation. The composite electrode demonstrates a high performance microstructure for ideal pseudocapacitors and lithium ion anodes. It not only enhances the utilization of active materials but also increases the electrode based specific capacitance by one order of magnitude as compared to the widely used nickel foam. More importantly, the ultralight nickel foam supported structure could further be extended to other high capacitance/capacity metal oxide materials for pseudocapacitors and lithium-ion battery applications.

Carbon-Free O2 Cathode with Three-Dimensional Ultralight Nickel Foam Supported Ruthenium Electrocatalysts for Li-O2 Batteries

A new carbon- and binder-free O2 cathode was fabricated by electroplating Ru-nanoparticle-coated ultralight Ni foam, which has good electron-conducting and electrocatalytic properties. This all-metal monolithic structure was able to suppress CO2 evolution and provided 306 times higher capacity than commercial Ni foam-based O2 cathodes.

Electroplating lithium transition metal oxides

Electrodeposition of lithium-ion battery cathodes enables ultraflexible, ultrathick, and high-power rechargeable batteries. Materials synthesis often provides opportunities for innovation. We demonstrate a general low-temperature (260°C) molten salt electrodeposition approach to directly electroplate the important lithium-ion (Li-ion) battery cathode materials LiCoO2, LiMn2O4, and Al-doped LiCoO2. The crystallinities and electrochemical capacities of the electroplated oxides are comparable to those of the powders synthesized at much higher temperatures (700° to 1000°C). This new growth method significantly broadens the scope of battery form factors and functionalities, enabling a variety of highly desirable battery properties, including high energy, high power, and unprecedented electrode flexibility.

Nitrogen‐Doped CoP Electrocatalysts for Coupled Hydrogen Evolution and Sulfur Generation with Low Energy Consumption

Hydrogen production is the key step for the future hydrogen economy. As a promising H2 production route, electrolysis of water suffers from high overpotentials and high energy consumption. This study proposes an N-doped CoP as the novel and effective electrocatalyst for hydrogen evolution reaction (HER) and constructs a coupled system for simultaneous hydrogen and sulfur production. Nitrogen doping lowers the d-band of CoP and weakens the H adsorption on the surface of CoP because of the strong electronegativity of nitrogen as compared to phosphorus. The H adsorption that is close to thermos-neutral states enables the effective electrolysis of the HER. Only -42 mV is required to drive a current density of -10 mA cm-2 for the HER. The oxygen evolution reaction in the anode is replaced by the oxidation reaction of Fe2+ , which is regenerated by a coupled H2 S absorption reaction. The coupled system can significantly reduce the energy consumption of the HER and recover useful sulfur sources.

Low Interface Energies Tune the Electrochemical Reversibility of Tin Oxide Composite Nanoframes as Lithium-Ion Battery Anodes

The conversion reaction of lithia can push up the capacity limit of tin oxide-based anodes. However, the poor reversibility limits the practical applications of lithia in lithium-ion batteries. The latest reports indicate that the reversibility of lithia has been appropriately promoted by compositing tin oxide with transition metals. The underlying mechanism is not revealed. To design better anodes, we studied the nanostructured metal/Li2O interfaces through atomic-scale modeling and proposed a porous nanoframe structure of Mn/Sn binary oxides. The first-principles calculation implied that because of a low interface energy of metal/Li2O, Mn forms smaller particles in lithia than Sn. Ultrafine Mn nanoparticles surround Sn and suppress the coarsening of Sn particles. Such a composite design and the resultant interfaces significantly enhance the reversible Li-ion storage capabilities of tin oxides. The synthesized nanoframes of manganese tin oxides exhibit an initial capacity of 1620.6 mA h g-1 at 0.05 A g-1. Even after 1000 cycles, the nanoframe anode could deliver a capacity of 547.3 mA h g-1 at 2 A g-1. In general, we demonstrated a strategy of nanostructuring interfaces with low interface energy to enhance the Li-ion storage capability of binary tin oxides and revealed the mechanism of property enhancement, which might be applied to analyze other tin oxide composites.