Seong-Min Bak

Spinel LiMn2O4/reduced graphene oxide hybrid for high rate lithium ion batteries

A well-crystallized and nano-sized spinel LiMn2O4/reduced graphene oxide hybrid cathode material for high rate lithium-ion batteries has been successfully synthesized via a microwave-assisted hydrothermal method at 200 °C for 30 min without any post heat-treatment. The nano-sized LiMn2O4 particles were evenly dispersed on the reduced graphene oxide template without agglomeration, which allows the inherent high active surface area of individual LiMn2O4 nanoparticles in the hybrid. These unique structural and morphological properties of LiMn2O4 on the highly conductive reduced graphene oxide sheets in the hybrid enable achieving the high specific capacity, an excellent high rate capability and stable cycling performance. An analysis of the cyclic voltammogram data revealed that a large surface charge storage contribution of the LiMn2O4/reduced graphene oxide hybrid plays an important role in achieving faster charge/discharge.

Solid-state microwave irradiation synthesis of high quality graphene nanosheets under hydrogen containing atmosphere

High quality graphene nanosheets were fabricated within 1 min by solid-state microwave irradiation of a mixture of graphite oxide and graphene nanosheets under a hydrogen atmosphere. The graphene nanosheets in the mixture acted as an effective microwave susceptor under microwave irradiation synthesis, and could provide sufficiently rapid heating for the effective exfoliation of graphite oxide. A hydrogen containing atmosphere played important roles in improving the quality of the graphene nanosheets by increasing the level of reduction and preventing the formation of defects in the graphene nanosheets. The graphene nanosheets thus obtained exhibited a specific surface area of 586 m2 g−1 and an outstanding carbon/oxygen ratio of 18.5.

Mesoporous nickel/carbon nanotube hybrid material prepared by electroless deposition

A mesoporous nickel/carbon nanotube (Ni/CNT) hybrid material was synthesized by electroless deposition in the presence of lyotropic liquid crystal (LLC) of the nonionic surfactant Brij 56 through the heterogeneous nucleation and growth of mesoporous Ni on the outer surface of the CNT template. The Brunauer–Emmett–Teller (BET) surface area of the hybrid material was 112.8 m2 g−1 with a total pore volume of 0.356 cm3 g−1. Its electrochemical properties were studied by cyclic voltammetry (CV), which revealed it to be a promising candidate for electrochemical energy storage applications.

Phase transition behavior of NaCrO2 during sodium extraction studied by synchrotron-based X-ray diffraction and absorption spectroscopy

The structural evolution of layered NaCrO2 cathodes for sodium-ion batteries during charge was investigated using synchrotron-based in situ X-ray diffraction and ex situ X-ray absorption spectroscopy. Three solid solution phases with expanding ‘c’ and contracting ‘a’/‘b’ lattice parameters were observed. The coordination changes of Cr and Na during sodium extraction were also studied.

Investigating Local Degradation and Thermal Stability of Charged Nickel-Based Cathode Materials through Real-Time Electron Microscopy

In this work, we take advantage of in situ transmission electron microscopy (TEM) to investigate thermally induced decomposition of the surface of Li(x)Ni(0.8)Co(0.15)Al(0.05)O2 (NCA) cathode materials that have been subjected to different states of charge (SOC). While uncharged NCA is stable up to 400 °C, significant changes occur in charged NCA with increasing temperature. These include the development of surface porosity and changes in the oxygen K-edge electron energy loss spectra, with pre-edge peaks shifting to higher energy losses. These changes are closely related to O2 gas released from the structure, as well as to phase changes of NCA from the layered structure to the disordered spinel structure, and finally to the rock-salt structure. Although the temperatures where these changes initiate depend strongly on the state of charge, there also exist significant variations among particles with the same state of charge. Notably, when NCA is charged to x = 0.33 (the charge state that is the practical upper limit voltage in most applications), the surfaces of some particles undergo morphological and oxygen K-edge changes even at temperatures below 100 °C, a temperature that electronic devices containing lithium ion batteries (LIB) can possibly see during normal operation. Those particles that experience these changes are likely to be extremely unstable and may trigger thermal runaway at much lower temperatures than would be usually expected. These results demonstrate that in situ heating experiments are a unique tool not only to study the general thermal behavior of cathode materials but also to explore particle-to-particle variations, which are sometimes of critical importance in understanding the performance of the overall system.

High‐Surface‐Area Nitrogen‐Doped Reduced Graphene Oxide for Electric Double‐Layer Capacitors

A two-step method consisting of solid-state microwave irradiation and heat treatment under NH3 gas was used to prepare nitrogen-doped reduced graphene oxide (N-RGO) with a high specific surface area (1007 m(2)  g(-1) ), high electrical conductivity (1532 S m(-1) ), and low oxygen content (1.5 wt %) for electrical double-layer capacitor applications. The specific capacitance of N-RGO was 291 F g(-1) at a current density of 1 A g(-1) , and a capacitance of 261 F g(-1) was retained at 50 A g(-1) , which indicated a very good rate capability. N-RGO also showed excellent cycling stability and preserved 96 % of the initial specific capacitance after 100 000 cycles. Near-edge X-ray absorption fine-structure spectroscopy results provided evidenced for the recovery of π conjugation in the carbon networks with the removal of oxygenated groups and revealed chemical bonding of the nitrogen atoms in N-RGO. The good electrochemical performance of N-RGO is attributed to its high surface area, high electrical conductivity, and low oxygen content.

O3-type layered transition metal oxide Na(NiCoFeTi)1/4O2 as a high rate and long cycle life cathode material for sodium ion batteries

High rate capability and long cycle life are challenging goals for the development of room temperature sodium-ion batteries. Here we report a new single phase quaternary O3-type layer-structured transition metal oxide Na(NiCoFeTi)1/4O2 synthesized by a simple solid-state reaction as a new cathode material for sodium-ion batteries. It can deliver a reversible capacity of 90.6 mA h g−1 at a rate as high as 20C. At 5C, 75.0% of the initial specific capacity can be retained after 400 cycles with a capacity-decay rate of 0.07% per cycle, demonstrating a superior long-term cyclability at high current density. X-ray diffraction and absorption characterization revealed reversible phase transformations and electronic structural changes during the Na+ deintercalation/intercalation process. Ni, Co and Fe ions contribute to charge compensation during charge and discharge. Although Ti ions do not contribute to the charge transfer, they play a very important role in stabilizing the structure during charge and discharge by suppressing the Fe migration. In addition, Ti substitution can also smooth the charge–discharge plateaus effectively, which provides a potential advantage for the commercialization of this material for room temperature sodium-ion batteries.

Utilizing Co2+/Co3+ Redox Couple in P2-Layered Na0.66Co0.22Mn0.44Ti0.34O2 Cathode for Sodium-Ion Batteries

Abstract Developing sodium‐ion batteries for large‐scale energy storage applications is facing big challenges of the lack of high‐performance cathode materials. Here, a series of new cathode materials Na0.66CoxMn0.66– xTi0.34O2 for sodium‐ion batteries are designed and synthesized aiming to reduce transition metal‐ion ordering, charge ordering, as well as Na+ and vacancy ordering. An interesting structure change of Na0.66CoxMn0.66– xTi0.34O2 from orthorhombic to hexagonal is revealed when Co content increases from x = 0 to 0.33. In particular, Na0.66Co0.22Mn0.44Ti0.34O2 with a P2‐type layered structure delivers a reversible capacity of 120 mAh g−1 at 0.1 C. When the current density increases to 10 C, a reversible capacity of 63.2 mAh g−1 can still be obtained, indicating a promising rate capability. The low valence Co2+ substitution results in the formation of average Mn3.7+ valence state in Na0.66Co0.22Mn0.44Ti0.34O2, effectively suppressing the Mn3+‐induced Jahn–Teller distortion, and in turn stabilizing the layered structure. X‐ray absorption spectroscopy results suggest that the charge compensation of Na0.66Co0.22Mn0.44Ti0.34O2 during charge/discharge is contributed by Co2.2+/Co3+ and Mn3.3+/Mn4+ redox couples. This is the first time that the highly reversible Co2+/Co3+ redox couple is observed in P2‐layered cathodes for sodium‐ion batteries. This finding may open new approaches to design advanced intercalation‐type cathode materials.

Soft templated mesoporous manganese oxide/carbon nanotube composites via interfacial surfactant assembly

A mesoporous manganese oxide/carbon nanotube (CNT) composite was successfully synthesized using cetyltrimethylammonium bromide as a protection layer to prevent the direct contact between CNT and MnO4− ions, a linking agent to interact with MnO4− ions and a structure-directing agent for the formation of mesoporous oxide. The observed improvement in electrochemical utilization of MnO2 was due to the synergetic effect of the mesoporous structure and CNTs, which enhanced the overall electronic and ionic conductivities.

One-step preparation of reduced graphene oxide/carbon nanotube hybrid thin film by electrostatic spray deposition for supercapacitor applications

In this paper, we describe a binder-free reduced graphene oxide/carbon nanotube hybrid thin film electrode prepared using a one-step electrostatic spray deposition method. Though we introduce a novel method, we suspect that the greater potential impact is more related to the fact that this technique is able to accomplish producing an electrode with a single process and allows a degree of control over the film properties not yet found in other fabrication methods that require multiple steps (that include post processing). In order to investigate the effect of carbon nanotube as a nano-spacer on the electrochemical properties of the reduced graphene oxide/carbon nanotube hybrid thin film electrodes, the various content of carbon nanotube was incorporated between the 2 dimensional layered reduced graphene oxide sheets to prevent restacking among reduced graphene oxide sheets and their electrochemical properties were systemically investigated using cyclic voltammetry, galvanostatic charge/discharge test and electrochemical impedance spectroscopy. The hybrid thin film electrode delivered a higher reversible specific capacitance of 187 F·g−1 at 0.5 A·g−1 and showed a better rate capability by maintaining 73% of the specific capacitance at 16 A·g−1 (vs. 0.5 A·g−1), which exhibit remarkable electrochemical performances than a RGO thin film electrodes for supercapacitor applications.