Seong-Yong Park

Silicon carbide-free graphene growth on silicon for lithium-ion battery with high volumetric energy density

Silicon is receiving discernable attention as an active material for next generation lithium-ion battery anodes because of its unparalleled gravimetric capacity. However, the large volume change of silicon over charge–discharge cycles weakens its competitiveness in the volumetric energy density and cycle life. Here we report direct graphene growth over silicon nanoparticles without silicon carbide formation. The graphene layers anchored onto the silicon surface accommodate the volume expansion of silicon via a sliding process between adjacent graphene layers. When paired with a commercial lithium cobalt oxide cathode, the silicon carbide-free graphene coating allows the full cell to reach volumetric energy densities of 972 and 700 Wh l−1 at first and 200th cycle, respectively, 1.8 and 1.5 times higher than those of current commercial lithium-ion batteries. This observation suggests that two-dimensional layered structure of graphene and its silicon carbide-free integration with silicon can serve as a prototype in advancing silicon anodes to commercially viable technology.

Graphene balls for lithium rechargeable batteries with fast charging and high volumetric energy densities

Improving one property without sacrificing others is challenging for lithium-ion batteries due to the trade-off nature among key parameters. Here we report a chemical vapor deposition process to grow a graphene–silica assembly, called a graphene ball. Its hierarchical three-dimensional structure with the silicon oxide nanoparticle center allows even 1 wt% graphene ball to be uniformly coated onto a nickel-rich layered cathode via scalable Nobilta milling. The graphene-ball coating improves cycle life and fast charging capability by suppressing detrimental side reactions and providing efficient conductive pathways. The graphene ball itself also serves as an anode material with a high specific capacity of 716.2 mAh g−1. A full-cell incorporating graphene balls increases the volumetric energy density by 27.6% compared to a control cell without graphene balls, showing the possibility of achieving 800 Wh L−1 in a commercial cell setting, along with a high cyclability of 78.6% capacity retention after 500 cycles at 5C and 60 °C.Here, the authors report a graphene-silica assembly which could be coated onto a nickel-rich cathode via a scalable process for considerably improved electrochemical performance. In the presence of such graphene balls, a full cell benefits from an increased volumetric energy density by 27.6%.

Improving the kinetics and surface stability of sodium manganese oxide cathode materials for sodium rechargeable batteries with Al2O3/MWCNT hybrid networks

We report the design and fabrication of a novel functional material in which protective Al2O3 nanoparticles are merged with highly conductive multi-walled carbon nanotubes (MWCNTs). In this paper, we discuss in detail the effects of the Al2O3/MWCNT hybrid networks on the electrochemical performance of sodium manganese oxide (Na0.44MnO2), which is used as an electrode material in sodium rechargeable batteries. The Al2O3/MWCNT hybrid networks, which are uniformly dispersed on the surface of Na0.44MnO2, change its surface bonding nature, resulting in an improvement in the cycling performance and rate capability of Na0.44MnO2. We ascribe these enhancements in performance to the inhibition of the formation of damaging NaF-based solid-electrolyte interface (SEI) layers during cycling, which enables facile transfer of Na ions through the Na0.44MnO2 electrode/electrolyte interface. Our findings regarding the control of the chemistry and bonding structure of the Na0.44MnO2 particle surfaces induced by the introduction of the Al2O3/MWCNT functional hybrid networks provide insight into the possibilities for achieving sodium rechargeable batteries with high power density and stability.

In-situ TEM characterization of nanomaterials and devices

Electrical properties of nano size devices were directly measured by TEM. Real time observation of phase transition behavior in PRAM revealed that the volume of the crystalline phase is the main factor in determining cell resistance. In the transistor device, we have identified the doping type and area by measuring the I-V curve at the individual nano contact on the specimen. The evolution of the graphene edge structure was controlled and monitored at and up to 1200°C in-situ.