Wonyoung Chang

Carbon-free cobalt oxide cathodes with tunable nanoarchitectures for rechargeable lithium–oxygen batteries

Carbon-free cobalt oxide cathodes for lithium-oxygen batteries are fabricated via an electrodeposition-conversion process. The Co3O4-only cathodes show a remarkably reduced voltage gap (by ca. 550 mV compared to the carbon-only cathode) as well as excellent long-term cyclability.

Manganese oxide/carbon composite nanofibers: electrospinning preparation and application as a bi-functional cathode for rechargeable lithium–oxygen batteries

Manganese oxide/carbon composite nanofibers are fabricated via an electrospinning technique. The electrospun composite nanofibers show an excellent catalytic performance for oxygen reduction and evolution, leading to reduced discharge–charge overpotentials and improved cycling properties of a Li–O2 battery with a hybrid electrolyte.

Carbon-, binder-, and precious metal-free cathodes for non-aqueous lithium-oxygen batteries: nanoflake-decorated nanoneedle oxide arrays.

Rechargeable lithium-oxygen (Li-O2) batteries have higher theoretical energy densities than todays lithium-ion batteries and are consequently considered to be an attractive energy storage technology to enable long-range electric vehicles. The main constituents comprising a cathode of a lithium-oxygen (Li-O2) battery, such as carbon and binders, suffer from irreversible decomposition, leading to significant performance degradation. Here, carbon- and binder-free cathodes based on nonprecious metal oxides are designed and fabricated for Li-O2 batteries. A novel structure of the oxide-only cathode having a high porosity and a large surface area is proposed that consists of numerous one-dimensional nanoneedle arrays decorated with thin nanoflakes. These oxide-only cathodes with the tailored architecture show high specific capacities and remarkably reduced charge potentials (in comparison with a carbon-only cathode) as well as excellent cyclability (250 cycles).

Superior high rate performance of core–shell Li4Ti5O12/carbon nanocomposite synthesized by a supercritical alcohol approach

A facile, fast, and effective method for the preparation of a core–shell LTO/C nanocomposite via a supercritical alcohol route is presented. Organic-modified LTO nanocrystals were first synthesized in supercritical methanol at a short reaction time of 15 min using oleylamine as a surface-modifier and as a carbon precursor. Subsequent calcination under inert conditions resulted in a highly crystalline LTO core with a size of 5–15 nm and highly graphitic carbon shell with a thickness of 0.7–2.3 nm. The obtained core–shell LTO/C nanocomposite exhibited superior high rate performance, long-term cyclability, and low-temperature discharge capacity.

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.

Mechanochemical Synthesis of Li2MnO3 Shell/LiMO2 (M = Ni, Co, Mn) Core-Structured Nanocomposites for Lithium-Ion Batteries

Core/shell-like nanostructured xLi2MnO3·(1−x)LiMO2 (M = Ni, Co, Mn) composite cathode materials are successfully synthesized through a simple solid-state reaction using a mechanochemical ball-milling process. The LiMO2 core is designed to have a high-content of Ni, which increases the specific capacity. The detrimental surface effects arising from the high Ni-content are countered by the Li2MnO3 shell, which stabilizes the nanoparticles. The electrochemical performances and thermal stabilities of the synthesized nanocomposites are compared with those of bare LiMO2. In particular, the results of time-resolved X-ray diffraction (TR-XRD) analyses of xLi2MnO3·(1−x)LiMO2 nanocomposites as well as their differential scanning calorimetry (DSC) profiles demonstrate that the Li2MnO3 shell is effective in stabilizing the LiMO2 core at high temperatures, making the nanocomposites highly suitable from a safety viewpoint.

Understanding local degradation of cycled Ni-rich cathode materials at high operating temperature for Li-ion batteries

We utilize transmission electron microscopy in conjunction with electron energy loss spectroscopy to investigate local degradation that occurs in LixNi0.8Co0.15Al0.05O2 cathode materials (NCA) after 30 cycles with cutoff voltages of 4.3 V and 4.8 V at 55 °C. NCA has a homogeneous crystallographic structure before electrochemical reactions; however, we observed that 30 cycles of charge/discharge reactions induced inhomogeneity in the crystallographic and electronic structures and also introduced porosity particularly at surface area. These changes were more noticeable in samples cycled with higher cutoff voltage of 4.8 V. Effect of operating temperature was further examined by comparing electronic structures of oxygen of the NCA particles cycled at both room temperature and 55 °C. The working temperature has a greater impact on the NCA cathode materials at a cutoff voltage of 4.3 V that is the practical the upper limit voltage in most applications, while a cutoff voltage of 4.8 V is high enough to cause su...

Investigation of Thermal Stability of P2–NaxCoO2 Cathode Materials for Sodium Ion Batteries Using Real-Time Electron Microscopy

Here, we take advantage of in situ transmission electron microscopy (TEM) to investigate the thermal stability of P2-type NaxCoO2 cathode materials for sodium ion batteries, which are promising candidates for next-generation lithium ion batteries. A double-tilt TEM heating holder was used to directly characterize the changes in the morphology and the crystallographic and electronic structures of the materials with increase in temperature. The electron diffraction patterns and the electron energy loss spectra demonstrated the presence of cobalt oxides (Co3O4, CoO) and even metallic cobalt (Co) at higher temperatures as a result of reduction of Co ions and loss of oxygen. The bright-field TEM images revealed that the surface of NaxCoO2 becomes porous at high temperatures. Higher cutoff voltages result in degrading thermal stability of NaxCoO2. The observations herein provide a valuable insight that thermal stability is one of the important factors to be considered in addition to the electrochemical properties when developing new electrode materials for novel battery systems.

A simple, one-pot synthesis of molybdenum oxide-reduced graphene oxide composites in supercritical methanol and their electrochemical performance

A simple and green supercritical methanol (scMeOH) route is developed to tightly anchor molybdenum oxide (MoO2) nanoparticles on reduced graphene oxide (RGO). In scMeOH, graphene oxide is reduced, and MoO2 nanoparticles with sizes of 10–20 nm are simultaneously deposited on the basal plane of RGO in a short time without using any reducing agents or additives. When tested as an anode in lithium ion batteries, the MoO2–RGO composites show enhanced electrochemical performance compared to bare MoO2. The composite with a MoO2 loading of 37.0 wt% delivers a high reversible discharge capacity of 793 mA h g−1 at 50 mA g−1 and an excellent rate performance of 205 mA h g−1 at 2.5 A g−1. After 100 cycles of high rate testing of up to 50 A g−1, the MoO2–RGO composite recovers most of its initial capacity. The improved electrochemical performance of MoO2–RGO can be attributed to the tight anchoring of nanosized MoO2 on RGO and the mesoporous structure of the composite. Consequently, the transport length of Li diffusion into the MoO2 phase is shortened, charge transfer kinetics at the electrode–electrolyte interface is facilitated, and the volume expansion associated with the conversion reaction can be accommodated.

Lithium intercalation mechanism into FeF 3 ·0.5H 2 O as a highly stable composite cathode material

The growing demand for lithium-ion batteries (LIBs) requires investigation of high-performance electrode materials with the advantages of being environmentally friendly and cost-effective. In this study, a nanocomposite of open-pyrochlore-structured FeF3·0.5H2O and reduced graphene oxide (RGO) is synthesized for use as a high-performance cathode in LIBs, where RGO provides high electrical conductivity to the composite material. The morphology of the composite shows that FeF3·0.5H2O spheres are embedded into RGO layers and high-resolution TEM image shows that those spheres are composed of primary nanoparticles with a size of ~5 nm. The cycling performance indicates that the composite electrode delivers an initial high discharge capacity of 223 mAh g−1 at 0.05 C, a rate capability up to a high C-rate of 10 C (47 mAh g−1) and stable cycle performance at 0.05 C (145 mAh g−1 after 100 cycles) and 0.2 C (93 mAh g−1 after 100 cycles) while maintaining high electrochemical reversibility. Furthermore, the responsible electrochemical reaction is investigated using in-situ XRD and synchrotron-based X-ray absorption spectroscopy (XAS), and the XRD results show that FeF3·0.5H2O transitions to an amorphous-like phase through a lithiation process. However, a reversible oxidation change of Fe3+ ↔ Fe2+ is identified by the XAS results.