(A) study on the electrochemical energy storage properties using novel carbon nanostructural materials

Abstract

As large-scale electrochemical energy storage devices for electrified vehicles and utility grids continue to move toward practicality, electrode materials for high energy density lithium ion batteries (LIBs) and su-percapacitors (SCs) are expected to play more critical roles in enabling those emerging applications. However, the most challenging issue is how to prevent the pulverization of electrodes and sustain the high capacitance during long cycle. While many innovated technologies have been considered as electrochemical storage system, the nanostructural transition metal composites have attracted considerable attention as promising elec-trode materials due to their outstanding large capacity and high rate capability caused by fast lithium diffu-sion. These nanomaterials, however, the use of nano-sized metal oxides had been limited because the strain of interspace still exists and poor conductivity of oxide materials leads to low power density. In this thesis, we introduced the nano-structured carbon materials such as graphene, nanotube and carbon nanoshpere into active electrode for overcoming the electrode break and sub-serving the high conductivity. These unique carbon structures using as support materials offer a good stability as well as a superior electron transfer during repeated charge / discharge. First, using nitrogen-doped graphene produced by a simple plasma process, we developed ultracapaci-tors whose capacitances ($~280 F g^{-1}_{electrode}) are about four times larger than those of pristine graphene based counterparts without sacrificing other essential and useful properties for ultracapacitor operations including excellent cycle life (>200,000), high power capability, and compatibility with flexible substrates. While we were trying to understand the improved capacitance using scanning photoemission microscopy with a capability of probing local nitrogen-carbon bonding configurations within a single sheet of graphene, we observed interesting microscopic features of N-configurations: N-doped sites even at basal-planes, distinctive distributions of N-configurations between edges and basal-planes, and their distinctive evolutions with plasma duration. The local N-configuration mappings during plasma treatment, alongside binding energy calculated by density functional theory, revealed that the origin of the improved capacitance is a certain N-configuration at basal-planes. Another issue, in case of the silicon (Si) anodes for LIB, having considered the significance and scala-bility of Si nanoparticles (Si NPs), we developed a Si NP-based electrode structure for highly robust cycling. In particular, Si NPs were embedded in porous carbon nitride spheres (CNSs). The porous nature of CNSs buffers the volume change of silicon and thus resolves critical issues in Si anode operations such as unstable solid-electrolyte-interphase (SEI) formation and vulnerable contacts between Si and carbon. The unique electrode structure exhibits outstanding performance with a gravimetric capacity as high as $1,579 mAh/g^{-1}$ at a C/10 rate based on the mass of both Si and C, a cycle life of 200 cycles with 84 % capacity retention, as well as a rate capability of 6 min discharging while retaining a capacity of $702 mAh/g^{-1}$. Moreover, it is noteworthy that coulombic efficiencies of this structure reach 99.99 %, which is much higher than those of other Si electrodes and is even comparable to those of commercial graphite anodes. Also, all of the materials and procedures are clearly scalable for mass production. We anticipate that further increase in performance can be expected using the novel carbon nanostructured materials that have specific active sites, high conductivity and stability during repeated cycling. Structural changes of metal oxide structures by their volume expansion attributed to lithiation have been known to give short reversible cycles along with the degradation of performance. For alleviating these problems, many research groups have focused on development of small sized active materials at a few na-nometer scales or metal oxide-supporting hybrid materials, which buffer the volume expansion of metal ox-ides. Meanwhile, it is also possible that the lithiation process is considered as an effective way to design the nanomaterias. There was a great effort to pioneer this top-down approach to fabricate the nanoporous struc-tures from metal oxide (Maier et al. Nat. Mater. 2006, 5, 713). Here, we report a novel methodology for rescaling metal oxide nanoparticles into atomic-scale sizes metal oxides on graphene using the lithium pro-cess. Moreover, the blue-shifted band gap of the nickel oxide shows that the nanocrystal is rescaled on the first lithiation/delithiation cycle. We further found out from X-ray photoelectron spectroscopy and in-situ spectroelectrochemical studies that bivalent Ni states of rescaled particles change reversibly to zero-valence states during lithiation/delithiation cycles. Furthermore, rescaled particles exhibit high performance via atom-to-atom surface reactions with the largest capacitance of $2,088 F g^{-1}$, as well as good capacitance retention during over 100,000 charge/discharge cycles. In this case, the graphene, which is one of most famous carbon nanostructural materials, acts as template materials by anchoring the electrochemical active materials during rescaling process. Moreover, the rescaled NiO particles on graphene offer the high capacitance and stability to an asymmetric capacitor system for a practical formation, which shows the energy density as $40.8 Wh/kg^{-1}$ and power density of $18,080 W kg^{-1}$. This thesis has suggested that various carbon nanostructural materials present excellent properties in electrochemical applications by controlling their structure, as well as these unique structures can be used as the mediate materials for enhancing the performances of electrochemical active materials.