Duho Kim

The origins and mechanism of phase transformation in bulk Li2MnO3: first-principles calculations and experimental studies

Lithium-rich oxide materials are promising candidates for high-energy lithium ion batteries, but currently have critical challenges of poor cycle performance and voltage drop induced by undesirable phase transformation. To resolve these problems, it is necessary to identify the origins and mechanism of phase transformation in Li2MnO3, a key component of Li-rich oxides. In this work, the phase transformation of bulk Li2MnO3 is investigated by thermodynamic and kinetic approaches based on first-principles calculations and validated by experiments. Using the calculated thermodynamic energies, the most stable structure is determined as a function of Li extraction for Li2−xMnO3: monoclinic (x = 0.0–0.75), layered-like (x = 1.0–1.25), and spinel-like (x = 1.5–2.0) structures. The phase transformation becomes kinetically possible for Li2−xMnO3 (x > 1.0). Atomic scale origins and the mechanism of phase transformation are elucidated by the thermodynamically stable and kinetically movable tetrahedral coordination of Mn4+ in the transition state. These theoretical observations are validated by ex situ X-ray photoelectron spectroscopy combined with electrochemical experiments for Li2−xMnO3 with various Li contents upon cycling. The mechanistic understanding from theoretical calculations and experimental observations is expected to provide a fundamental solution and guidelines for improving the electrochemical performance of Li-rich oxides and, by extension, the battery performance.

Intrinsic Origins of Crack Generation in Ni-rich LiNi 0.8 Co 0.1 Mn 0.1 O 2 Layered Oxide Cathode Material

Ni-rich LiNi0.8Co0.1Mn0.1O2 layered oxide cathodes have been highlighted for large-scale energy applications due to their high energy density. Although its specific capacity is enhanced at higher voltages as Ni ratio increases, its structural degradation due to phase transformations and lattice distortions during cycling becomes severe. For these reasons, we focused on the origins of crack generation from phase transformations and structural distortions in Ni-rich LiNi0.8Co0.1Mn0.1O2 using multiscale approaches, from first-principles to meso-scale phase-field model. Atomic-scale structure analysis demonstrated that opposite changes in the lattice parameters are observed until the inverse Li content x = 0.75; then, structure collapses due to complete extraction of Li from between transition metal layers. Combined-phase investigations represent the highest phase barrier and steepest chemical potential after x = 0.75, leading to phase transformations to highly Li-deficient phases with an inactive character. Abrupt phase transformations with heterogeneous structural collapse after x = 0.81 (~220 mAh g−1) were identified in the nanodomain. Further, meso-scale strain distributions show around 5% of anisotropic contraction with lower critical energy release rates, which cause not only micro-crack generations of secondary particles on the interfaces between the contracted primary particles, but also mechanical instability of primary particles from heterogeneous strain changes.

Anti-fluorite Li6CoO4 as an alternative lithium source for lithium ion capacitors: an experimental and first principles study

As a promising hybrid energy storage system, lithium ion capacitors (LICs) have been intensively investigated regarding their practical use in various applications, ranging from portable electronics to grid support. The asymmetric LIC offers high-energy and high-power densities compared with conventional energy storage systems such as electrochemical double-layer capacitors (EDLCs) and lithium ion batteries (LIBs). To enable suitable operation of the LIC, the negative electrode should be pre-lithiated prior to cell operation, which is regarded as a key technology for developing self-sustainable LICs. In this work, we have demonstrated the potential use of Li6CoO4 as an alternative lithium source to metallic lithium. A large amount of Li+ can be electrochemically extracted from the structure incorporated into the positive electrode via a highly irreversible process. Most of the extracted Li+ is available for pre-lithiation of the negative electrode during the first charge. This intriguing electrochemical behaviour of Li6CoO4 is suitable for providing sufficient Li+ to the negative electrode. To obtain a fundamental understanding of this system, the electrochemical behaviour and structural stability of Li6CoO4 is thoroughly investigated by means of electrochemical experiments and theoretical validation based on first principles calculations.

Understanding of Surface Redox Behaviors of Li2MnO3 in Li‐Ion Batteries: First‐Principles Prediction and Experimental Validation

Critical degradation mechanism of many cathode materials for Li-ion batteries is closely related to phase transformations at the surface/interface. Li2MnO3 in x Li2MnO3 ⋅(1-x) LiMO2 (M=Ni, Co, Mn) provides high capacity, but the Li2MnO3 phase is known to degrade during cycling through phase transformation and O2 evolution. To resolve such degradation problems, it is critical to develop a fundamental understanding of the underlying mechanism. Using first-principles calculations, we identified the surface delithiation potential (<4.5 V vs. Li/Li(+) ) of Li2MnO3, which is significantly lower than the bulk redox potential. A lower Mn oxidation state at the surface would reduce the delithiation potential compared with the fully oxidized Mn(4+) in the bulk. As a result, the delithiation would be initiated from the surface, which induces a phase transformation of Li2MnO3 into a spinel-like structure from the surface. These theoretical findings have been confirmed by experimental analyses. Based on these detailed mechanistic understanding, it would be possible to develop rational approaches to modify and coat the surface to suppress degradation mechanisms.

Mn-Based Cathode with Synergetic Layered-Tunnel Hybrid Structures and Their Enhanced Electrochemical Performance in Sodium Ion Batteries

A synergistic approach for advanced cathode materials is proposed. Sodium manganese oxide with a layered-tunnel hybrid structure was designed, synthesized, and subsequently investigated. The layered-tunnel hybrid structure provides fast Na ion diffusivity and high structural stability thanks to the tunnel phase, enabling high rate capability and greatly improved cycling stability compared to that of the pure P2 layered phase while retaining the high specific capacity of the P2 layered phase. The hybrid structure provided a decent discharge capacity of 133.4 mAh g-1 even at 8 C, which exceeds the reported best rate capability for Mn-based cathodes. It also displayed an impressive cycling stability, maintaining 83.3 mAh g-1 after 700 cycles at 10 C. Theoretical calculation and the potentiostatic intermittent titration technique (PITT) demonstrated that this hybrid structure helps enhance Na ion diffusivity during charge and discharge, attaining, as a result, an unprecendented electrochemical performance.

Design of Surface Doping for Mitigating Transition Metal Dissolution in LiNi0.5Mn1.5O4 Nanoparticles

In lithium-ion batteries (LIBs) comprising spinel cathode materials, the dissolution of transition metals (TMs) in the cathodes causes severe cyclic degradation. We investigate the origin and mechanism of surface TM dissolution in high-voltage spinel oxide (LiNi0.5 Mn1.5 O4 ) nanoparticles to find a practical method for its mitigation. Atomic structures of the LiNi0.5 Mn1.5 O4 surfaces are developed, and the electronic structures are investigated by first-principles calculations. The results indicate that titanium is a promising dopant for forming a more stable surface structure by reinforcing metal-oxygen bonds in LiNi0.5 Mn1.5 O4 . Experimentally synthesized LiNi0.5 Mn1.5 O4 with titanium surface doping exhibits improved electrochemical performance by suppressing undesirable TM dissolution during cycles. The theoretical prediction and experimental validation presented here suggest a viable method to suppress TM dissolution in LiNi0.5 Mn1.5 O4 .

Phase Separation and d Electronic Orbitals on Cyclic Degradation in Li–Mn–O Compounds: First-Principles Multiscale Modeling and Experimental Observations

A combined study involving experiments and multiscale computational approaches is conducted to propose a theoretical solution for the suppression of the Jahn-Teller distortion which causes severe cyclic degradation. As-synthesized pristine and Al-doped Mn spinel compounds are the focus to understand the mechanism of the cyclic degradation in terms of the Jahn-Teller distortion, and the electrochemical performance of the Al-doped sample shows enhanced cyclic performance compared with that of the pristine one. Considering the electronic structures of the two systems using first-principles calculations, the pristine spinel suffers entirely from the Jahn-Teller distortion by Mn(3+), indicating an anisotropic electronic structure, but the Al-doped spinel exhibits an isotropic electronic structure, which means the suppressed Jahn-Teller distortion. A multiscale phase field model in nanodomain shows that the phase separation of the pristine spinel occurs to inactive Li0Mn2O4 (i.e., fully delithiated) gradually during cycles. In contrast, the Al-doped spinel does not show phase separation to an inactive phase. This explains why the Al-doped spinel maintains the capacity of the first charge during the subsequent cycles. On the basis of the mechanistic understanding of the origins and mechanism of the suppression of the Jahn-Teller distortion, fundamental insight for making tremendous cuts in the cyclic degradation could be provided for the Li-Mn-O compounds of Li-ion batteries.

Rational Design of Na(Li1/3Mn2/3)O2 Operated by Anionic Redox Reactions for Advanced Sodium-Ion Batteries

In an effort to develop high-energy-density cathodes for sodium-ion batteries (SIBs), low-cost, high capacity Na(Li1/3 Mn2/3 )O2 is discovered, which utilizes the labile O 2p-electron for charge compensation during the intercalation process, inspired by Li2 MnO3 redox reactions. Na(Li1/3 Mn2/3 )O2 is systematically designed by first-principles calculations considering the Li/Na mixing enthalpy based on the site preference of Na in the Li sites of Li2 MnO3 . Using the anionic redox reaction (O2- /O- ), this Mn-oxide is predicted to show high redox potentials (≈4.2 V vs Na/Na+ ) with high charge capacity (190 mAh g-1 ). Predicted cathode performance is validated by experimental synthesis, characterization, and cyclic performance studies. Through a fundamental understanding of the redox reaction mechanism in Li2 MnO3 , Na(Li1/3 Mn2/3 )O2 is designed as an example of a new class of promising cathode materials, Na(Li1/3 M2/3 )O2 (M: transition metals featuring stabilized M4+ ), for further advances in SIBs.

Rational design of Na(Li1/3Mn1/2Cr1/6)O2 exhibiting cation–anion-coupled redox reactions with superior electrochemical, thermodynamic, atomic, and chemomechanical properties for advanced sodium-ion batteries

Anionic redox reactions (O2−/O−), an alternative to conventional cationic redox reactions (Mn+/M(n+1)+; M: transition metal), have recently been identified as essential to achieve high energy density cathodes for sodium-ion batteries (SIBs). To overcome the drawbacks of anionic redox reactions leading to phase change and separation in the newly discovered Na(Li1/3Mn2/3)O2 material (NLMO, ∼4.2 V vs. Na/Na+ with a high charge capacity of 190 mAh g−1), we have rationally designed high energy density Na(Li1/3Mn1/2Cr1/6)O2 (NLMCO) in which the Cr 3d-electron is coupled with the labile O 2p-electron coordinated with Mn4+ for charge compensation during desodiation processes. NLMCO exhibits reduced phase change and separation, and chemomechanical strain and stress compared to NLMO and is thus expected to show high electrochemical performance, where the formation of short O–O bonds is not observed. By correlating the thermodynamic energy behavior with the redox mechanism in NLMO, it is concluded that our systematically designed cation–anion-coupled NLMCO is an excellent cathode material, introducing advanced materials of formula Na(Li1/3M2/3(1−y)Mcy)O2 (M and Mc: transition metals with stabilized M4+ species and cationic redox active Mc4+ species) for next-generation SIBs.