Ryoung-Hee Kim

Is Li4Ti5O12 a solid-electrolyte-interphase-free electrode material in Li-ion batteries? Reactivity between the Li4Ti5O12 electrode and electrolyte

Does the Li4Ti5O12 electrode need a carbon additive in lithium-ion batteries? We answered this question in our previous work by showing that the partially reduced Ti4+ on the surface of Li4Ti5O12 could provide enough electronic conductivity to initiate the electrochemical process. In this work, we discuss the generally accepted fact that a solid electrolyte interphase (SEI) is hardly formed on the surface of the Li4Ti5O12 electrode owing to its high redox potential by using the carbon-free Li4Ti5O12 electrode to exclude the influences of carbon. In contrast to the previous argument, Li4Ti5O12 was found to have certain reactivity towards electrolytes at room and high temperature (60 °C). Moreover, in the presence of carbon in the electrode (i.e. the conventional electrode formulation), the reactivity of the electrode towards an electrolyte was significantly increased at high temperatures. These results were discussed based on surface analyses and electrode morphology observation before and after cycling, and correlated with the electrochemical performance.

Effects of Ni Doping on the Initial Electrochemical Performance of Vanadium Oxide Nanotubes for Na-Ion Batteries

In this study, we demonstrated the intercalation of Na in hydrothermally synthesized VOx nanotubes (NTs) and Ni-doped VOx NTs. The changes induced in the structures of the two nanomaterials during the Na intercalation process were investigated through X-ray diffraction (XRD) analyses. It was observed that the initial capacity and rate performance of the Ni-doped VOx NTs were improved. The results of X-ray photoelectron spectroscopy (XPS) and conductance measurement confirmed that higher initial capacity and rate performance were attributed to an increase in the valence states of vanadium and increased conductivity after the Ni exchange process.

Highly reduced VOx nanotube cathode materials with ultra-high capacity for magnesium ion batteries

Here, we describe novel VOx nanotubes with vanadium at various oxidation states (V3+/V4+/V5+) as cathode materials for magnesium ion batteries. The VOx nanotubes synthesized by a microwave-assisted hydrothermal process using an amine as an organic template show a high initial discharge capacity (∼218 mA h g−1) of more than 200 mA h g−1 and an outstanding cycling performance, which have not been previously reported for magnesium ion batteries. These improvements in the electrochemical performance of our VOx nanotubes originate from the trivalent vanadium ions generated in the highly reduced VOx nanotubes. The VOx nanotubes with trivalent vanadium ions exhibit a lower charge transfer resistance at the electrode/electrolyte interfaces and superior cycling performance than the VOx nanotubes containing vanadium ions of a higher oxidation state. We first suggest that the pristine oxidation state of the vanadium ions and the maintenance of a bonding structure on the surface of the VOx nanotubes are the most important factors determining the magnesium insertion/extraction kinetics into/out of the VOx nanotubes. Our findings offer a breakthrough strategy for achieving high-energy-density magnesium rechargeable batteries using VOx nanotube cathode materials in combination with nanoarchitecture tailoring.

Aluminum Manganese Oxides with Mixed Crystal Structure: High‐Energy‐Density Cathodes for Rechargeable Sodium Batteries

We report a new discovery for enhancing the energy density of manganese oxide (Nax MnO2 ) cathode materials for sodium rechargeable batteries by incorporation of aluminum. The Al incorporation results in NaAl(0.1) Mn(0.9) O2 with a mixture of tunnel and layered crystal structures. NaAl(0.1) Mn(0.9) O2 shows a much higher initial discharge capacity and superior cycling performance compared to pristine Na(0.65) MnO2 . We ascribe this enhancement in performance to the formation of a new orthorhombic layered NaMnO2 phase merged with a small amount of tunnel Na(0.44) MnO2 phase in NaAl(0.1) Mn(0.9) O2 , and to improvements in the surface stability of the NaAl(0.1) Mn(0.9) O2 particles caused by the formation of Al-O bonds on their surfaces. Our findings regarding the phase transformation and structure stabilization induced by incorporation of aluminum, closely related to the structural analogy between orthorhombic Na(0.44) MnO2 and NaAl(0.1) Mn(0.9) O2 , suggest a strategy for achieving sodium rechargeable batteries with high energy density and stability.

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.