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In this study, we present improved power characteristics and suppressed phase transition by incorporating elemental doping into a P2-type cathode of sodium ion batteries. A Cu-doped Fe–Mn based P2-type Na0.67Cu0.125Fe0.375Mn0.5O2 cathode was designed based on the calculations of the electronic structure and then examined experimentally. Using first principles, we introduced instrinsic p-type conductivity by elemental doping with Cu. Introduction of Cu generated electron holes above the Fermi level in the electronic structure, which is typical of p-type semiconductors. Charge analyses suggested that the hole generation was driven primarily by the greater reduced characteristics of Cu as compared with those of Fe and Mn. In addition, introduction of Cu retaining high reduced property also suppressed phase transition from the P2 to Z phase by Fe migration to empty Na layers mainly. Electrochemical experiments revealed improved power characteristics upon the introduction of p-type conductivity. This could be attributed to the increase in the electronic conductivity by hole generation in the valence band. This study suggests that the introduction of p-type conductivity could be a rational tactic for the development of promising cathode materials for high performance sodium ion batteries.

Summary The realization of lithium-ion battery (LIB) anodes with high volumetric energy densities and minimal Li plating at high rates remains a key challenge for emerging technologies, including electric vehicles and grid-level energy storage. Here, we present graphene-functionalized Li2TiSiO5 (G-LTSO) as a high volumetric energy and power density anode for LIBs. G-LTSO forms a dense electrode structure with electronically and ionically conductive networks that deliver superior electrochemical performance. Upon lithiation, in situ transmission electron microscopy reveals that graphene functionalization yields minimal structural changes compared with pristine LTSO, resulting in high cycling stability. Furthermore, G-LTSO exhibits not only high charge and discharge capacities but also low overpotentials at high rates with minimal voltage fading due to reduced formation of a solid-electrolyte interphase. The combination of highly compacted electrode morphology, stable high-rate electrochemistry, and low operating potential enables G-LTSO to achieve exceptional volumetric energy and power densities that overcome incumbent challenges for LIBs.

Through a simple gelation-solvothermal method with graphene oxide as the additive, a Cu4SnS4-rich composite of nanoparticles and nanotubes is synthesized and applied for thin and flexible Li-metal batteries. Unlike the Cu2SnS3-rich electrode, the Cu4SnS4-rich electrode cycles stably with an enhanced conversion capacity of ∼416 mAh g-1 (∼52% of total capacity) after 200 cycles. The lithiation/delithiation mechanisms of Cu-Sn-S electrodes and the voltage ranges of conversion and alloying reactions are informed by in situ X-ray diffraction tests. The conversion process of three Cu-Sn-S compounds is compared by density functional theory (DFT) calculations based on three algorithms, elucidating the enhanced conversion stability and superior diffusion kinetics of Cu4SnS4 electrodes. The reaction pathway of Cu-Sn-S electrodes and the root cause for the unstable capacity are revealed by in situ/ex situ characterizations, DFT calculations, and various electrochemical tests. This work provides insight into developing energy materials and power devices based on multiple lithiation mechanisms.

In this study, we present improved power characteristics and suppressed phase transition by incorporating elemental doping into a P2-type cathode of sodium ion batteries. A Cu-doped Fe-Mn based P2-type Na0.67Cu0.125Fe0.375Mn0.5O2 cathode was designed based on the calculations of the electronic structure and then examined experimentally. Using first principles, we introduced instrinsic p-type conductivity by elemental doping with Cu. Introduction of Cu generated electron holes above the Fermi level in the electronic structure, which is typical of p-type semiconductors. Charge analyses suggested that the hole generation was driven primarily by the greater reduced characteristics of Cu as compared with those of Fe and Mn. In addition, introduction of Cu retaining high reduced property also suppressed phase transition from the P2 to Z phase by Fe migration to empty Na layers mainly. Electrochemical experiments revealed improved power characteristics upon the introduction of p-type conductivity. This could be attributed to the increase in the electronic conductivity by hole generation in the valence band. This study suggests that the introduction of p-type conductivity could be a rational tactic for the development of promising cathode materials for high performance sodium ion batteries.

A GeP 3 compound is introduced here for the first time as a promising anode for sodium-ion batteries (SIBs). The compound shows a high capacity and good cyclic stability, which well agree with the first-principle calculation results regarding its soft bonding nature and related innovative mechanical endurance. The binding energies of Ge–P and P–P verify that Ge–P has softer bonding feature with smaller energy variations compared to P–P when the bonding length is changed. In order to confirm the bonding natures and their effect on the mechanical and electrochemical properties, two layered GeP n compounds with different Ge–P content (i.e., GeP 3 and GeP 5 ) have been synthesized using a high energy mechanical ball-milling (HEMM) method. GeP 3 maintains high discharge and charge capacities of 1.274 and 1.269 Ah g −1 even after 150 cycles, respectively, which correspond to capacity retentions of 87.6% and 88.0% from the 5th cycle, respectively. A comparative study on the elastic moduli of GeP 3 and GeP 5 demonstrates how the superior electrochemical performance of GeP 3 is correlated with its more softened bonding feature compared to GeP 5 both in the pristine state and during charge/discharge. The elastic modulus of GeP 3 shows significantly softer features than that of GeP 5 , implying that mechanical stress or strain can be more easily alleviated in GeP 3 than in GeP 5 during charge/discharge. A morphological comparison between GeP 3 and GeP 5 electrodes reveals that GeP 5 electrode undergoes very serious morphology and volume changes, whereas GeP 3 electrode does not show any significant change in good accordance with the elastic modulus comparison from the first-principle calculations.

Ni-rich layered oxides have been identified as promising candidates for commercial cathodes in Li-ion batteries. However, the commercialization has been hindered by severe cyclic degradation and mechanical failure induced by severe phase transformations and fractures. To resolve these challenges by understanding their fundamental mechanisms, we present mechanics-based multiscale investigations to elucidate the fundamental mechanisms of mechanical failure including deformations and fractures. We have also suggested a practical solution to the failure, which involves enhancing electronic interactions between transition metal layers. The methodological framework for our investigations was developed from first-principles atomic calculations, electronic structure, thermodynamics and kinetics for combined phase transformation, phase field modeling, finite element methodology for mechanical deformation, and phase field crack modeling. Our practical solution addresses the electronic interactions that can be strengthened when O ions are reduced by substituting strongly oxidizing elements such as Ti. Our multiscale framework shows that the reduced O ions are responsible for higher fracture toughness, reduced volume changes, stable deformation, mitigated stress generation, and suppressed fractures. Thus, this study proposes a practical solution for the improvement and design of Ni-rich layered oxide cathode materials. Furthermore, the mechanics-based multiscale methodology employed herein could be applied to a number of other solid-state energy materials suffering from mechanical failures.

Phase transformations in most electrodes used for electrochemical energy storages follow the conserved dynamics of combined one- and two-phase reactions, which leads to complicated charge–discharge processes with various voltage plateaus; this could affect an electrochemical performance as a generic phenomenon in electrochemical system. In order to fully describe the combined-phase reactions from the atomic scale to the mesoscale, we propose a multiscale-based phase transformation model that also considers electrochemical states and mechanical deformations. This model predicts the miscibility gap, spinodal region, voltage profile, phase transformation, and stress generations of the combined-phase electrodes in the electrochemical reactions. We apply this multiscale model to high-rate cathode material Li x FePO 4 to fundamentally understand the experimental phase transformation behaviors (Yamada et al., 2006). This model is applicable to various electrodes for phase behaviors too complex to be detected experimentally due to combined-phase reactions.

By preventing electrical contact between anode and cathode electrodes while promoting ionic transport, separators are critical components in the safe operation of rechargeable battery technologies. However, traditional polymer-based separators have limited thermal stability, which has contributed to catastrophic thermal runaway failure modes that have conspicuously plagued lithium-ion batteries. Here, we describe the development of phase-inversion composite separators based on carbon-coated hexagonal boron nitride (hBN) nanosheets and poly(vinylidene fluoride) (PVDF) polymers that possess high porosity, electrolyte wettability, and thermal stability. The carbon-coated hBN nanosheets are obtained through a scalable liquid-phase shear exfoliation method using ethyl cellulose as a polymer stabilizer and source of the carbon coating following thermal pyrolysis. When incorporated within the PVDF matrix, the carbon-coated hBN nanosheets promote favorable interfacial interactions during the phase-inversion process, resulting in porous, flexible, free-standing composite separators. The unique chemical composition of these carbon-coated hBN separators implies high wettability for a wide range of liquid electrolytes. This combination of high porosity and electrolyte wettability enables enhanced ionic conductivity and lithium-ion battery electrochemical performance that exceeds incumbent polyolefin separators over a wide range of operating conditions. The hBN nanosheets also impart high thermal stability, providing safe lithium-ion battery operation up to 120 °C.

Irreversible phase transformation of layered structure into spinel structure is considered detrimental for most of the layered structure cathode materials. Here we report that this presumably irreversible phase transformation can be rendered to be reversible in sodium birnessite (NaxMnO2·yH2O) as a basic structural unit. This layered structure contains crystal water, which facilitates the formation of a metastable spinel-like phase and the unusual reversal back to layered structure. The mechanism of this phase reversibility was elucidated by combined soft and hard X-ray absorption spectroscopy with X-ray diffraction, corroborated by first-principle calculations and kinetics investigation. These results show that the reversibility, modulated by the crystal water content between the layered and spinel-like phases during the electrochemical reaction, could activate new cation sites, enhance ion diffusion kinetics and improve its structural stability. This work thus provides in-depth insights into the intercalating materials capable of reversible framework changes, thereby setting the precedent for alternative approaches to the development of cathode materials for next-generation rechargeable batteries., The irreversible layered-to-spinel phase transformation is detrimental for many cathode materials. Here, the authors show that reversibility can be realized in crystal water containing sodium birnessite by controlled dehydration, leading to enhanced ion diffusion kinetics and improved structural stability.

Through a simple gelation-solvothermal method with graphene oxide as the additive, a Cu

Solid-state electrolytes based on ionic liquids and a gelling matrix are promising for rechargeable lithium-ion batteries due to their safety under diverse operating conditions, favorable electrochemical and thermal properties, and wide processing compatibility. However, gel electrolytes also suffer from low mechanical moduli, which imply poor structural integrity and thus an enhanced probability of electrical shorting, particularly under conditions that are favorable for lithium dendrite growth. Here, we realize high-modulus, ion-conductive gel electrolytes based on imidazolium ionic liquids and exfoliated hexagonal boron nitride (hBN) nanoplatelets. Compared to conventional bulk hBN microparticles, exfoliated hBN nanoplatelets improve the mechanical properties of gel electrolytes by 2 orders of magnitude (shear storage modulus ∼5 MPa), while retaining high ionic conductivity at room temperature (1 mS cm

The power characteristics of lithium-ion batteries (LIBs) are crucial for the advent of commercialized, high-power applications, such as electric vehicles. Through both first-principles multiscale simulations and experiments, here, we present fundamental understanding on the power characteristics of the high-voltage spinel cathode correlated with its elastic softness and phase transformation in nanodomains for high-power LIBs. Atomic models of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5−xTixO4 are developed for multiscale phase field modeling based on structural information for the as-prepared nanopowders. The combined computational and experimental investigations suggest that the thermodynamic phase stability of LiNi0.5Mn1.5O4 can be effectively enhanced by the incorporation of Ti into the structure without any change to the redox mechanism. Ti incorporation provides a faster ionic mobility and the improved phase stability because of the reinforced Ti4+–O bonds. Based on the multiscale phase transformation kinetics, LiNi0.5Mn1.5−xTixO4 exhibits an enhanced elastic softness and slower phase separation than LiNi0.5Mn1.5O4 in the nanodomain during Li+ insertion and extraction. Such characteristics are mainly responsible for the improved electrochemical performance at higher current rates, as confirmed by electrochemical experiments. This fundamental understanding of the power characteristics with respect to the correlations with elastic softness and phase transformation will provide a guideline to develop and design advanced materials for high-power LIBs.

We report the introduction of p-type conductivity in high-Mn, Li-rich oxides (HMLOs) by the introduction of Cu doping to improve intrinsic electronic conduction. The study is based on experimental observations and a fundamental understanding through first-principles electronic structure analysis. Although the Cu-doped HMLO (CuHMLO) has a crystal structure identical to the original HMLO, the electrochemical performance of CuHMLO is superior in terms of specific energy and power characteristics. Specifically, CuHMLO exhibits a larger specific capacity with enhanced rate capability, and could be charged at lower voltages and discharged at higher voltages. For the first-principles calculations, HMLO and CuHMLO structures are modeled based on Rietveld refinement of the powder X-ray diffraction data of the powders synthesized herein. The electronic structure of CuHMLO reveals the generation of an electron hole in the valence band, above the Fermi level, indicating p-type conductivity and improving the electroni...

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 (LiNi0.5Mn1.5O4) nanoparticles to find a practical method for its mitigation. Atomic structures of the LiNi0.5Mn1.5O4 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 LiNi0.5Mn1.5O4. Experimentally synthesized LiNi0.5Mn1.5O4 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 LiNi0.5Mn1.5O4.

Layered, nickel-rich lithium transition metal oxides have emerged as leading candidates for lithium-ion battery (LIB) cathode materials. High-performance applications for nickel-rich cathodes, such as electric vehicles and grid-level energy storage, demand electrodes that deliver high power without compromising cell lifetimes or impedance. Nanoparticle-based nickel-rich cathodes seemingly present a solution to this challenge due to shorter lithium-ion diffusion lengths compared to incumbent micrometer-scale active material particles. However, since smaller particle sizes imply that surface effects become increasingly important, particle surface chemistry must be well characterized and controlled to achieve robust electrochemical properties. Moreover, residual surface impurities can disrupt commonly used carbon coating schemes, which result in compromised cell performance. Using x-ray photoelectron spectroscopy, here we present a detailed characterization of the surface chemistry of LiNi0.8Al0.15Co0.05O2 (NCA) nanoparticles, ultimately identifying surface impurities that limit LIB performance. With this chemical insight, annealing procedures are developed that minimize these surface impurities, thus improving electrochemical properties and enabling conformal graphene coatings that reduce cell impedance, maximize electrode packing density, and enhance cell lifetime fourfold. Overall, this work demonstrates that controlling and stabilizing surface chemistry enables the full potential of nanostructured nickel-rich cathodes to be realized in high-performance LIB technology.

2D materials have shown great promise to advance next-generation lithium-ion battery technology. Specifically, tin-based chalcogenides have attracted widespread attention because lithium insertion can introduce phase transformations via three types of reactions-intercalation, conversion, and alloying-but the corresponding structural changes throughout these processes, and whether they are reversible, are not fully understood. Here, the first real-time and atomic-scale observation of reversible phase transformations is reported during the lithiation and delithiation of SnSe

For large-scale energy storage applications requiring high energy density, the development of Li-rich oxides with enhanced cyclic stabilities during high-voltage operations and large specific capacities is required. In this regard, high-Mn, Li-rich oxides (HMLOs; xLi2MnO3 (1 − x)LiNi1/3Co1/3Mn1/3O2 at x > 0.5) warrant an in-depth study because of their good cyclic performance at high operating voltages and potentially large specific capacities. Here, to understand the synergistic effects and enhanced cyclic stability of HMLOs, mechanically blended HMLO (m-HMLO) and chemically bonded HMLO (c-HMLO) were prepared and investigated. c-HMLO exhibits relatively high reaction voltages, large specific capacities, and enhanced cyclic stabilities (∼99%) at a high operating voltage (∼4.8 V vs. Li/Li+) compared with m-HMLO. First-principles calculations with electronic structure analysis were performed using an atomic model developed by Rietveld refinement using as-synthesised c-HMLO. The redox mechanisms of Ni, Co, and Mn ions were determined via the partial density of states of the ground states predicted using the cluster expansion method, which elucidates that LiNi1/3Co1/3Mn1/3O2 stabilises the transition metal (TM) layer of Li2MnO3 and separates Li delithiation potentials in Li2MnO3 in the HMLO. Kinetic analyses including electronic structures revealed that the interlayer migration of TMs from the TM layer to the Li layer depends on the crystal field stabilisation. Thus, TMs with reduced character in the tetrahedral sites than the octahedral sites owing to the effects of crystal field stabilisation, such as Ni ions, in HMLOs would face a higher interlayer migration barrier, impeding phase transformation into spinel phases. Furthermore, Cu ions could constitute a doping source for HMLOs to improve the material’s cyclic stability through this mechanism. These characteristics may be widely applied to explain experimental phenomena and improve the properties of cathode materials for Li-ion batteries.

Through first-principles calculations and experimental observations, we first present the correlation between the Ni and Mn ratio and the redox behaviors of the layered NCM cathodes. The equilibrium potentials based on redox reactions of Ni2+/Ni3+ are highly dependent on the Mn ratio (NCM523 and NCM721: ∼3.7 and 3.5 V) because of a donor electron, in the eg band, transferred from Mn to Ni owing to their crystal field splitting (CFS) with different electronegativities, leading to oxidation states of Ni2+-like and Mn4+. Considering the electronic donor (Mn) based on CFS with electronegativity of transition metals (TMs), we finally expect V as a promising doping source to provide donor electrons for Ni redox reactions in Ni-rich layered oxides, leading to be higher delithiation potentials (NCV523: 3.8 V). From our theoretical calculations in the NCV oxide, the oxidation states of Ni and V are stable Ni2+-like and V5+, respectively, and the fractional d-band fillings of Ni are the highest value as compared wi...

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.

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.

Through a gelation–solvothermal method without heteroadditives, Cu–Sn–S composites self-assemble to form nanotubes, sub-nanotubes, and nanoparticles. The nanotubes with a Cu3–4SnS4 core and Cu2SnS3 shell can tolerate long cycles of expansion/contraction upon lithiation/delithiation, retaining a charge capacity of 774 mAh g–1 after 200 cycles with a high initial Coulombic efficiency of 82.5%. The importance of the Cu component for mitigation of the volume expansion and structural evolution upon lithiation is informed by density functional theory calculations. The self-generated template and calculated results can inspire the design of analogous Cu–M–S (M = metal) nanotubes for lithium batteries or other energy storage systems.

A fundamental understanding of anomalous redox mechanisms in hexacyanometallate compounds, compared with conventional NaMO2 systems (M: transition metals), is presented based on first-principles calculations and experimental validations. From theoretical calculations, we identified low-spin and high-spin states of Fe ions coordinated by the cyanide group (-CN) with the same oxidation state (Fe2+) in Na2Fe2(CN)6. Considering the site dependency of d electronic spin configurations based on the crystal field theory (CFT) of transition metals (TMs), we calculated the thermodynamic mixing energy using Na2Fe2(CN)6 and Na2Mn2(CN)6 for obtaining a thermodynamically stable phase of Na2FeMn(CN)6. The phase stabilities of Na2Fe2-xMnx(CN)6 among many atomic configurations and lattice parameters originating from octahedral structures (i.e., Fe(CN)6 and Mn(NC)6) are highly dependent on the electronic structures of TMs with spin states. From partial density of states (PDOS) and spatial electron distributions, it was observed that Fe2+ in the low-spin state (t) and Mn2+ in the high-spin states (t and e) in the stable phase lead to higher redox potentials (∼3.55 V vs. Na/Na+) with the removal of Na+ as compared to that of Na2Fe2(CN)6. In addition, lattice parameters from x = 0 to x = 1 in Na2Fe2-xMnx(CN)6 are increased due to the larger ionic radius of Mn2+ in the high-spin states. On the other hand, Fe2+ in the high-spin states (t and e) and Mn2+ in the low-spin state (t) in the most unstable phase of Na2FeMn(CN)6 would have lower redox potentials. Based on the fundamental correlation between redox potentials and CFT with spin configurations of TMs, we suggest a material design concept for intercalation compounds with higher energy densities for rechargeable battery systems.

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.

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.

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 (LiNi

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.

2D materials have shown great promise to advance next-generation lithium-ion battery technology. Specifically, tin-based chalcogenides have attracted widespread attention because lithium insertion can introduce phase transformations via three types of reactions-intercalation, conversion, and alloying-but the corresponding structural changes throughout these processes, and whether they are reversible, are not fully understood. Here, the first real-time and atomic-scale observation of reversible phase transformations is reported during the lithiation and delithiation of SnSe2 single crystals, using in situ high-resolution transmission electron microscopy complemented by first-principles calculations. Lithiation proceeds sequentially through intercalation, conversion, and alloying reactions (SnSe2 → Lix SnSe2 → Li2 Se + Sn → Li2 Se + Li17 Sn4 ) in a manner that maintains structural and crystallographic integrity, whereas delithiation forms numerous well-aligned SnSe2 nanodomains via a homogeneous deconversion process, but gradually loses the coherent orientation in subsequent cycling. Furthermore, alloying and dealloying reactions cause dramatic structural reorganization and thereby consequently reduce structural stability and electrochemical cyclability, which implies that deep discharge for Sn chalcogenide electrodes should be avoided. Overall, the findings elucidate atomistic lithiation and delithiation mechanisms in SnSe2 with potential implications for the broader class of 2D metal chalcogenides.

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.

In past experimental studies, piezoelectric nanostructures have exhibited extraordinary behavior and unusual deformations. In order to establish the corresponding theoretical framework, a scale-bridging model, which takes into account surface piezoelectricity and the wave nature of electrons in ultra-narrow media by reflecting lattice distortions of atomic structures, has been proposed in this work. After applying this model to ZnO nanofilms with thicknesses ranging between 0.3 nm and 2.8 nm, asymmetric lattice distortions of the Zn- and O-terminated surfaces were observed and subsequently quantified using a lattice distortion factor. The material characteristics evaluated by using this model were found to be in good agreement with the results of first-principles calculations and corresponding experiments, and thus can be used for predicting the properties of thicker nanofilms. After bridging to the continuum scale, the data produced via finite element analysis significantly differed from the results obta...

Li-rich oxide cathode is one of the most promising high capacity cathode materials for the next commercialized Li-ion battery application, and its composition can be described as a mixture of Li2MnO3 and Li(TM)O2(TM: Transition metal). Li-rich cathode material has large capacity based on relatively cheap Mn oxide, and it is easy to synthesize. However, the Li-rich cathode material has critical material challenges such as capacity degradation and voltage drop. It has been reported that the main source of the material challenge is related to phase transformation inside the bulk cathode material during charge/discharge processes. In this work, detailed phase transformation mechanism of Li2MnO3 is investigated by combining the experimental and computational approaches to develop fundamental understanding on the atomic scale processes. The structure of Li-rich oxide is a composite of Li2MnO3 and layered Li(TM)O2, and the monoclinic layered structure of Li2MnO3 is known to be the source of the degradation problems. We have observed the evidence of phase transformation in Li2MnO3experimentally, and a theoretical analysis based on density functional theory calculations is combined with experimental data for a systematic comparative study. For experimental study, we have synthesized Li2MnO3 powder by solid state method at low temperature following the literature procedures. Using this powder, we made a coin cell with standard Li reference electrode, and the CV measurement shows the phase transformation during charge/discharge processes. First, basic powder characterization was conducted through XRD, and SEM analyses. Second, coin cell was assembled for cyclic performance test, and charge/discharge profile and cyclic voltammogram were obtained. From the experimental investigation, synthesized Li2MnO3is identified as monoclinic C2/m space group structure and their particle size is around 200~300 nm. The evidences of phase transformation are found from cyclic charge/discharge profile as well as cyclic voltammogram. As reaction cycle is progressed, charge/discharge capacity operated under 4.6 V has a steady increase indicating that, the initial active material is getting transformed to another phase with lower reaction voltage than 4.6 V. As described in Fig. 1, we could observe the changes of charge/discharge profile and reaction voltage fundamentally caused by phase transformation consistent with similar previous experiment studies. As observed before, the first charge voltage is around 4.6 V, but the charge voltage changes to around 3.2 V as cycle goes on. Remarkably, discharge voltage reveals at three different values around 2.8 V, 3.3 V, and 4.0 V, suggesting that there are three different transformed phases inside the active material as the cycle goes on. To understand this and similar experimental observations of phase transformation, we have examined how and why it could happen in terms of thermodynamics and kinetics based on density functional theory investigation. In case of thermodynamic study, phase stability, intercalation voltage, electronic charge, and electronic structures are studied. From phase stability and intercalation voltage analyses, we would estimate when initial structure could be transformed. The electronic charge and partial density of state for both initial and transformed structure (whose Mn ion is migrated) are investigated for structure stability and physical/chemical characters. We found that thermodynamic stability of structures and the changes in bonding characters between Mn and O ions are to the main cause of phase transformation. For kinetic analysis, we investigated the migration barriers of Li and Mn ions in the Li2MnO3framework with controlled delithiation. Based on the kinetic calculation results, we could show the possibility of Li and Mn ion migrations with different Li contents in active material. As shown in Fig 2, not only the possibility and the delithiation effect of phase transformation, but also the detailed Mn migration mechanism could be predicted providing atomic scale explanation of phase transformation. As a result, detailed phase transformation mechanism could be quantitative understood, and it would be possible to suppress such phase transformation based on theoretical studies on material design such as doping on effect the electronic structure analyses. By understanding detailed phase transformation mechanism of Li2MnO3, we are developing material modification strategy to solve capacity degradation and voltage drop problems. The developed strategy will be critically validated by experimental implementation of the designed material modification. Such combined material design and experimental validation approaches will accelerated the high capacity cathode material development based on atomic scale understanding. This work was supported by the Industrial Strategic technology development program(10041589) funded by the Ministry of Knowledge Economy(MKE, Korea)

The development of rechargeable lithium-ion batteries (LIB) has progressed rapidly to meet the demand for consumer electronic devices such as cellular phones and laptop computers, and the recent demands for larger energy storage applications (xEV and grid storage) require further development of LIB or alternative energy storage technology. Among possible alternative cathode materials compared to the currently used material LiCoO2, Li-rich compounds xLi2MnO3·(1-x)LiMO2 (M = Ni, Co, Mn) are the most promising candidate with higher capacity, lower cost and absence of toxic element. Within the two-component composite compounds, Li2MnO3 plays a central role in increasing the capacity of the compounds and independently possess a high theoretical capacity of 460 mAh/g (if the entire lithium is utilized). Although the Li2MnO3 is not electrochemically active between 2.0V and 4.4V, it is active to extract lithium ions over 4.5V. During the initial delithiation process, Li2MnO3 is known to transform into layered LiMnO2structure, and the subsequent charge/discharge cycling induces a gradual phase transformation to the spinel structure, resulting in lowering voltage plateau and a large irreversible capacity accompanied by oxygen loss. To overcome these capacity loss problems, diverse approaches have been made to improve electrochemical properties of the cathode materials: adjusting the relative ratio of different transition metal ions, coating layers on the particles, and synthesizing at various temperature to control specific surface area. Even though the bulk properties of cathode materials have been studied by many research groups, the surface properties of cathode have seldom been researched so far. Specifically, theoretical study on cathode surface has been rarely performed even though major reactions and chemical transformations occur near the surface and interfaces between different phases. Recently, several research groups highlighted and reported experimental observations of a phase transformation from the layered structure to spinel phase at the particle surface upon the first charge cycle. However, it is still unclear and controversial for theoretical and fundamental understanding on the mechanism of phase transformations. Without the basic understanding based on detailed atomic scale theoretical analysis, many problems of LIB materials will take extreme efforts and time to resolve the problems. In the present paper, we established a fundamental understanding on the origin of phase transformation at the surface through first principles study of interface models. The theoretical findings suggest unusual redox activities of the surface layers of Li2MnO3which is confirmed by detailed experimental study. These theoretical and experimental findings provide basic understanding to improve performance degradation by phase transformation. For the experimental study, coin cells are made with Li2MnO3 cathode and Li reference electrode, and variation of voltage between 2.0 V and various limiting voltage is applied to the electrochemical test with a constant current at room temperature and under 0.05 C-rate during 10 cycles. There is a small amount of redox capacity (4 mAh/g) at lower voltage as shown in Fig. 1a inset, and Figs. 1d-f show 10 mAh/g capacity during subsequent 10 cycles with lower limiting voltage of 4.1-4.3 V. Fig. 1a shows voltage plateau at 4.6 V extracting Li in the bulk Li2MnO3 with accompanying phase change to layered LiMnO2with lower voltage during subsequent cycles. For the modeling study, in order to elucidate the low voltage redox mechanism observed in Fig. 1, we examined the Li intercalation potential at the surface using the model shown in Fig. 2. The model interface contains stoichiometric ratio of constituents (Li : Mn :O = 2 : 1 : 3) split into two regions of a semi-infinite bulk (SIB), which is inactive at low voltage (4-4.5 V), and a interface layer (IFL) exposed to vaccum, which is calculated to be active at low voltage (~4 V). Finally, detailed electronic structures of the interface model show different oxidation state of surface Mn atoms compared to the fully oxidized bulk Mn4+ within the bulk. Since the high activation potential of 4.6 V of bulk Mn is known to be related to electron extraction from oxygen atoms coordinating Mn4+, any lower Mn oxidation state on the surface would reduce the delithiation potential comparable to those of layered LiMnO2 and spinel (3-4 V). Furthermore, delithiation potential at the surface show strong dependence on the Li location in Mn-layer (4.6 V) and Li-layer (4V) at the surface of Li2MnO3. The preferential delithiation of Li from the surface Li-layer would facilitate the Mn atom migration to the Li-layer causing spinel phase transformation at the surface. These underlying understanding mechanisms of the surface will provide a conceptual basis to develop diverse approached to suppress phase transitions in the cathode materials in LIB, which is our current modeling and experimental research topics. This work was supported by the Industrial Strategic technology development program(10041589) funded by the Ministry of Knowledge Economy(MKE, Korea)

In order to exactly understand the curvature-induced secondary flow motion, the steady electro-osmotic flow (EOF) is investigated by applying the full Poisson-Boltzmann/Navier-Stokes equations in a whole domain of the rectangular microchannel. The momentum equation is solved with the continuity equation as the pressure-velocity coupling achieves convergence by employing the advanced algorithm, and generalized Navier’s slip boundary conditions are applied at the hydrophobic curved surface. Two kinds of channels widely used for lab-on-chips are explored with the glass channel and the heterogeneous channel consisting of glass and hydrophobic polydimethylsiloxane, spanning thin to thick electric double layer (EDL) problem. According to a sufficiently low Dean number, an inward skewness in the streamwise velocity profile is observed at the turn. With increasing EDL thickness, the electrokinetic effect gets higher contribution in the velocity profile. Simulation results regarding the variations of streamwise ve...