Hyungjun Kim

Ceria species migrated onto Ni surfaces by strong metal–support interactions exhibit different Ce redox states through redox-state engineering, which modulate the reverse Boudouard reaction at the interface and thereby tune carbon coking tolerance.

Problematic issues with electrically inert binders have been less serious in the conventional lithium-ion batteries by virtue of permeable liquid electrolytes (LEs) for ionic connection and/or carbonaceous additives for electronic connection in the electrodes. Contrary to electron-conductive binders used to maximize an active loading level, the development of ion-conductive binders has been lacking owing to the LE-filled electrode configuration. Herein, we represent a tactical strategy for improving the interfacial Li⁺ conduction in all-solid-state electrolyte-free graphite (EFG) electrodes where the solid electrolytes are entirely excluded, using lithium-substitution-modulated (LSM) binders. Finely tuning a lithium substitution ratio, a conductive LSM-carboxymethyl cellulose (CMC) binder is prepared from a controlled direct Na⁺/Li⁺ exchange reaction without a hazardous acid involvement. The EFG electrode employing LSM with a maximum degree of substitution of lithium (DS_Li) of ∼68% in our study shows a considerably higher rate capability of 1.05 mA h cm^-2 at 1 C and a capacity retention of ∼61.9% after 200 cycles at 0.5 C than those using sodium-CMC (Na-CMC) (0.78 mA h cm^-2, ∼49.5%) and LSM with ∼35% lithium substitution (0.93 mA h cm^-2, ∼55.4%). More importantly, the correlation between the phase transition near the bottom region of the EFG electrode and the state of charge (SOC) is systematically investigated, clarifying that the improvement of the interfacial conduction is proportional to the DS_Li of the CMC binders. Theoretical calculations combined with experimental results further verify that creating the continuous interface through abundant pathways for mobile ions using the Li⁺-conductive binder is the enhancement mechanism of the interfacial conduction in the EFG electrode, mitigating serious charge transfer resistance.

Solid electrolytes (SEs) are critical for next-generation all solid-state batteries with high energy density and fire safety. However, recent studies observed that the Li metallic phase nucleates at the electrode interfaces as well as the interfaces between crystalline grains of SEs. Many studies have revealed the origins and control methods for Li metallic phase formation at the anode interface, but a thorough understanding of metallic Li formation at intergranular regions in SEs has not been developed yet. Through systematic DFT simulations, we present a thorough atomistic study that reveals the impact of intergranular regions on Li-metallic phase formation in SEs using the perovskite Li_3xLa_(2/3)−x□_(1/3)−2xTiO₃ (0 < x < 0.167) (LLTO) as a model SE. We investigated the three representative model structures for intergranular regions, which are experimentally observed with various microstructure configurations: (i) stoichiometric grain boundary (GB), (ii) A-site deficient GB, and (iii) intergranular pore space. In the stoichiometric GB, the GB region has an electron insulating feature regardless of A-site compositions (0 < x < 0.167). In the A-site deficient GB, however, the GB region has electronic conductivity, but it has a high repulsive force against Li-ions moving into the GB region. However, in the intergranular pore structure, Li-ions prefer to move with a neutral charge state into the pore space which shows a p-type conductive property. Accordingly, Li metallic phase nucleation starts in the intergranular pore space of the SE. These results elucidate the critical role of pore space in SEs for Li metallic phase nucleation and provide an insight into the design of Li metallic phase-free SEs and further studies on SE materials.

Exploiting oxygen redox reactions (ORRs) in sodium layered oxides is a breakthrough for overcoming the intrinsic low energy density of sodium-ion batteries (SIBs), where Li-excess transition metal (TM) layers are considered requisite for the ORRs during (de)sodiation. However, non-Li-excess Mn–Fe binary oxides have emerged as viable OR-based cathode materials, although stabilizing the reversible oxygen capacity to harness the full OR potential remains challenging. Considering the ORR mechanisms in NaFeO₂, those in Na_1−x[Mn_1/2Fe_1/2]O₂ were elucidated by using the “selective and successive ORRs” mechanism to unlock the origin of cycle retention degradation. The thermodynamic formation energies revealed that the oxygen stability in the Mn–Fe oxides with x = 0.75 and above varies with the coordination number of the TM neighboring the oxygen ions; that is, the oxygen stability dominantly declines at Fe-rich oxygen ions upon charging. The electronic structures of the Fe- and Mn-rich O(2p) ions reconfirmed the selective OR in Mn–Fe oxide with 0.5 ≤ x ≤ 0.75 and confirmed successive anion redox processes after the breakpoint (x = 0.75). The two-type OR mechanism mainly originates from the Fe-rich oxygen ions over the crystal framework. Analysis of the crystal orbital overlap populations showed that reorientation of the Fe³⁺(3d)–O(2p) bonds comprising Fe-rich oxygen ions was an intriguing trigger of the latter ORR upon deep desodiation. This unified concept of the Mn–Fe model over the full ORR reveals the origin of the oxygen (in)stability and consequent unstable cycle retention, and is expected to be universal for Mn-based binary oxide cathodes for advanced SIBs.

Oxygen redox (OR) reactions in sodium layered oxide cathodes have been studied intensively to harness their full potential in achieving high energy density for sodium-ion batteries (SIBs). However, OR triggers a large hysteretic voltage during discharge after the first charge process for OR-based oxides, and its intrinsic origin is unclear. Therefore, in this study, an in-depth reinvestigation on the fundamentals of the reaction mechanism in Na[Li_1/3Mn_2/3]O₂ with a Mn/Li ratio (R) of 2 was performed to determine the factors that polarize the OR activity and to provide design rules leading to nonhysteretic oxygen capacity using first-principles calculations. Based on thermodynamic energies, the O^2-/O₂^2- and O^2-/O^n- conditions reveal the monophasic (0.0 ≤ x ≤ 4/6) and biphasic (4/6 ≤ x ≤ 1.0) reactions in Na_1-x[Li_2/6Mn_4/6]O₂, but each stability at x = 5/6 is observed differently. The O-O bond population elucidates that the formation of an interlayer O-O dimer is a critical factor in triggering hysteretic oxygen capacity, whereas that in a mixed layer provides nonhysteretic oxygen capacity after the first charge. In addition, the migration of Li into the 4h site in the Na metallic layer contributes less to the occurrence of voltage hysteresis because of the suppression of the interlayer O-O dimer. These results are clearly elucidated using the combined-phase mixing enthalpies and chemical potentials during the biphasic reaction. To compare the Mn oxide with R = 2, Na_1-x[Li_1/6Mn_5/6]O₂ tuned with R = 5 was investigated using the same procedure, and all the impeding factors in restraining the nonhysteretic OR were not observed. Herein, we suggest two strategies based on three types of OR models: (i) exploiting the migration of Li ions for the suppression of the interlayer O-O dimer and (ii) modulating the Mn/Li ratio for controlling the OR participation, which provides an exciting direction for nonhysteretic oxygen capacities for SIBs and lithium-ion batteries.

Nonhysteretic redox capacity is a critical factor in achieving high energy density without energy loss during cycling for rechargeable battery electrodes, which has been considered a major challenge in oxygen redox (OR) for Li-excess layered oxide cathodes for lithium-ion batteries (LIBs). Until recently, transition metal migration into the Li metal layer and the formation of O–O dimers have been considered major factors affecting hysteretic oxygen capacity. However, Li-excess layered oxides, particularly Ru oxides, exhibit peculiar voltage hysteresis that cannot be sufficiently described by only these factors. Therefore, this study aims to unlock the critical impeding factors in restraining the non-polarizing oxygen capacity of Li-excess layered oxides (herein, Li₂RuO₃) that exhibit reversible OR reactions. First, Li₂RuO₃ undergoes an increase in the chemical potential fluctuation as both the thermodynamic material instability and vacancy content increase. Second, the chemical compression of O–O bonds occurs at the early stage of the OR reaction (0.5 ≤ x ≤ 0.75) for Li_1–xRu_0.5O_1.5, leading to flexible voltage hysteresis. Finally, in the range of 0.75 ≤ x ≤ 1.0, for Li_1–xRu_0.5O_1.5, the formation of an O(2p)–O(2p)* antibonding state derived from the structural distortion of the RuO₆ octahedron leads to the irreversibility of the OR reaction and enhanced voltage hysteresis. Consequently, our study unlocks the new decisive factor, namely, the structural distortion inducing the O(2p)–O(2p)* antibonding state, of the hysteretic oxygen capacity and provides insights into enabling the full potential of the OR reaction for Li-excess layered oxides for advanced LIBs.

The cycle-life enhancing mechanism of Mg-doped LiNi_0.5Mn_1.5O₄ (LNMO) is investigated to unravel divalent pillar effects. The doped Mg atoms effectively mitigate lattice misfits and lower the phase transition barrier in the biphasic region.