Masanobu Nakayama

High-capacity electrode materials for rechargeable lithium batteries: Li3NbO4-based system with cation-disordered rocksalt structure.

Significance This study describes new and promising electrode materials, Li3NbO4-based electrode materials, which are used for high-energy rechargeable lithium batteries. Although its crystal structure is classified as a cation-disordered rocksalt-type structure, lithium ions quickly migrate in percolative network in bulk without a sacrifice in kinetics. Moreover, the large reversible capacity originates from the participation of oxide ions for a charge compensation process, which has been confirmed by first-principles calculations combined with X-ray absorption spectroscopy. This finding can be further expanded to the design of innovative positive electrode materials beyond the restriction of the solid-state redox reaction based on the transition metals used for the past three decades. Rechargeable lithium batteries have rapidly risen to prominence as fundamental devices for green and sustainable energy development. Lithium batteries are now used as power sources for electric vehicles. However, materials innovations are still needed to satisfy the growing demand for increasing energy density of lithium batteries. In the past decade, lithium-excess compounds, Li2MeO3 (Me = Mn4+, Ru4+, etc.), have been extensively studied as high-capacity positive electrode materials. Although the origin as the high reversible capacity has been a debatable subject for a long time, recently it has been confirmed that charge compensation is partly achieved by solid-state redox of nonmetal anions (i.e., oxide ions), coupled with solid-state redox of transition metals, which is the basic theory used for classic lithium insertion materials, such as LiMeO2 (Me = Co3+, Ni3+, etc.). Herein, as a compound with further excess lithium contents, a cation-ordered rocksalt phase with lithium and pentavalent niobium ions, Li3NbO4, is first examined as the host structure of a new series of high-capacity positive electrode materials for rechargeable lithium batteries. Approximately 300 mAh⋅g−1 of high-reversible capacity at 50 °C is experimentally observed, which partly originates from charge compensation by solid-state redox of oxide ions. It is proposed that such a charge compensation process by oxide ions is effectively stabilized by the presence of electrochemically inactive niobium ions. These results will contribute to the development of a new class of high-capacity electrode materials, potentially with further lithium enrichment (and fewer transition metals) in the close-packed framework structure with oxide ions.

Improvement of Degradation at Elevated Temperature and at High State-of-Charge Storage by ZrO2 Coating on LiCoO2

A uniform ZrO 2 coating on LiCoO 2 cathode materials for rechargeable lithium batteries was applied by a spray coating technique. The cells showed improved cycle performance and better durability of storing the cell (calendar life) under a high-voltage charging condition (4.2 V-313 K). X-ray diffraction and calorimetric study revealed that no marked change was observed in the bulk properties, such as crystal structure and phase transition, in the cathode during charge and discharge. The suppression of the increase of cathode/electrolyte interfacial impedance was observed by ZrO 2 coating. Thus, the improved electrochemical performance in the higher voltage region (>4.2 V) is ascribed to the stabilization of the interface between the cathode and electrolyte materials.

High-pressure synthesis, crystal structure, and phase stability relations of a LiNbO3-type polar titanate ZnTiO3 and its reinforced polarity by the second-order Jahn-Teller effect.

A polar LiNbO3-type (LN-type) titanate ZnTiO3 has been successfully synthesized using ilmenite-type (IL-type) ZnTiO3 under high pressure and high temperature. The first principles calculation indicates that LN-type ZnTiO3 is a metastable phase obtained by the transformation in the decompression process from the perovskite-type phase, which is stable at high pressure and high temperature. The Rietveld structural refinement using synchrotron powder X-ray diffraction data reveals that LN-type ZnTiO3 crystallizes into a hexagonal structure with a polar space group R3c and exhibits greater intradistortion of the TiO6 octahedron in LN-type ZnTiO3 than that of the SnO6 octahedron in LN-type ZnSnO3. The estimated spontaneous polarization (75 μC/cm(2), 88 μC/cm(2)) using the nominal charge and the Born effective charge (BEC) derived from density functional perturbation theory, respectively, are greater than those of ZnSnO3 (59 μC/cm(2), 65 μC/cm(2)), which is strongly attributed to the great displacement of Ti from the centrosymmetric position along the c-axis and the fact that the BEC of Ti (+6.1) is greater than that of Sn (+4.1). Furthermore, the spontaneous polarization of LN-type ZnTiO3 is greater than that of LiNbO3 (62 μC/cm(2), 76 μC/cm(2)), indicating that LN-type ZnTiO3, like LiNbO3, is a candidate ferroelectric material with high performance. The second harmonic generation (SHG) response of LN-type ZnTiO3 is 24 times greater than that of LN-type ZnSnO3. The findings indicate that the intraoctahedral distortion, spontaneous polarization, and the accompanying SHG response are caused by the stabilization of the polar LiNbO3-type structure and reinforced by the second-order Jahn-Teller effect attributable to the orbital interaction between oxygen ions and d(0) ions such as Ti(4+).

Factors affecting cyclic durability of all-solid-state lithium polymer batteries using poly(ethylene oxide)-based solid polymer electrolytes

In this paper, the electrochemical properties and performances of all-solid-state lithium polymer batteries (LPBs) using standard PEO-based solid-state polymer electrolytes (SPEs) are reported and discussed. The assembled cell showed stable charge–discharge cycles (>150 cycles) at 30 °C. This is due to desirable solid electrolyte interface (SEI) film formation at the SPE | cathode interface at the first cycle indicated by activation energy measurements for interfacial Li ion exchange reaction. However, sudden capacity fading for prolonged electrochemical cycles was indicated by an accelerated aging test at higher current density (1 C) and temperature conditions (60 °C), accompanied by an increase of electrochemical polarization. This degradation phenomenon may be fatal for practical usage of large-scale batteries which requires extremely long-time durability. Two sequential factors affecting the capacity fading are proposed through the studies of in situ19F-NMR imaging, real-time monitoring of the total cell thickness, and electrochemical measurements such as AC impedance. One factor is degradation of the cathode sheet or cathode composite assembly, owing to cyclic volumetric change from the two-phase LiFePO4–FePO4 reaction. Such degradation leads to uneven electric contact at the electrode | electrolyte interface, thereby enhancing local electrochemical polarization. The second factor, namely, Li salt decomposition, is triggered by this local polarization, giving rise to the continuous capacity fading and the increase of polarization. This degradation scenario can be general enough to include the full range of all-solid-state LPB devices, since the trigger of degradation owes to non-fluidity of solid | solid contact, or solid electrolytes cannot immerse into the cavities caused by pulverization of cathode particles unlike liquid electrolytes. On the basis of these results, we attempted to improve the mechanical properties of the binder materials of cathode sheets, and demonstrated improved cyclic durability.

Origin of stabilization and destabilization in solid-state redox reaction of oxide ions for lithium-ion batteries

Further increase in energy density of lithium batteries is needed for zero emission vehicles. However, energy density is restricted by unavoidable theoretical limits for positive electrodes used in commercial applications. One possibility towards energy densities exceeding these limits is to utilize anion (oxide ion) redox, instead of classical transition metal redox. Nevertheless, origin of activation of the oxide ion and its stabilization mechanism are not fully understood. Here we demonstrate that the suppression of formation of superoxide-like species on lithium extraction results in reversible redox for oxide ions, which is stabilized by the presence of relatively less covalent character of Mn4+ with oxide ions without the sacrifice of electronic conductivity. On the basis of these findings, we report an electrode material, whose metallic constituents consist only of 3d transition metal elements. The material delivers a reversible capacity of 300 mAh g−1 based on solid-state redox reaction of oxide ions.

First‐Principles Studies on Novel Polar Oxide ZnSnO3; Pressure‐Induced Phase Transition and Electric Properties

Adv. Mater. 2010, 22, 2579–2582 2010 WILEY-VCH Verlag G An increased interest has developed around noncentrosymmetric (NCS) oxides because of their symmetry dependent properties, such as ferroelectricity, piezoelectricity, and second-order non-linear optical behavior. Among these NCS oxides, special attention has been paid to the R3c structure owing to a fascination of fundamental science and technical applications. For instance, LiNbO3 and LiTaO3 are typical representatives of non-linear optical materials, and BiFeO3 is known as a multiferroic material. We recently synthesized novel NCS oxides of ZnSnO3 with a LiNbO3 (LN)-type structure (R3c) under high-pressure (HP) conditions ( 7GPa). The refined crystal structure determined by Rietveld analyses confirmed the non-cetrosymmetry, primarily because of a large displacement of Zn2þ. Later, Son et al. succeeded in synthesizing a LN-type ZnSnO3 thin film with a high ferroelectric polarization of 47 8C cm 2 by pulsed laser deposition (PLD). Since thin film formation enables one to utilize the ferroelectric material in electric circuits, a LN-type ZnSnO3 can potentially replace existing materials in electronic devices. Another aspect of the general interest in LN-type ZnSnO3 materials with high ferroelectric polarization lies in the fact that this compound consists only of main-group cations, Zn2þ and Sn4þ. Until now, Pb-based ferroelectric oxides such as Pb(Zr,Ti)O3 (PZT) have been widely utilized because of their high ferroelectric polarization. Pb, however, causes environmental pollution, so that many attempts have been devoted to find novel lead-free ferroelectric compounds. In addition, most of the recent studies to develop ferroelectric materials have focused on the oxides containing second-order Jahn–Teller (SOJT) distorted cations with d transition metal ions (such as, Nb5þ and Ta5þ) and/or cations with lone pair electrons of ns (such as Bi3þ).[13–22] On the other hand, the LN-type ZnSnO3 is composed of two cations with the electronic configuration of (n 1)dns. Thus, the previous discovery of LN-type ZnSnO3 by the HP technique suggested a new strategy to explore new NCS crystals with R3c symmetry. Nevertheless, searching for new compounds using HP techniques could demand numerous experiments on a general trial-and-error basis. Knowledge of the pressure-dependent phase stability and the relationship between physical properties and crystal/electronic structures may offer the way to systematically search for new polar oxides. In this study, we demonstrate a first-principles approach to predict the pressure dependence of the phase stability for Zn2þ Sn4þ O ternary systems. In addition, Born effective charge tensors and spontaneous polarization of LN-type ZnSnO3 were calculated and compared with those of LiNbO3 reported in the literature. [24]

Mixed conduction for the spinel type (1−x)Li4/3Ti5/3O4–xLiCrTiO4 system

Abstract Structure refinement and electrical conductivity measurement were performed for a series of spinel oxides, (1− x )Li 4/3 Ti 5/3 O 4 – x LiCrTiO 4 . The AC conductivity with composition x was abruptly increased for x >0.6, but no structural phase transition was obtained from structure refinement data by Rietveld analysis. Since the shape of complex impedance plots changes at x =0.6, it is assumed that mechanism of electrical conduction changes at the composition. From the site-percolation theory conduction pathways by d electrons of Cr 3+ in octahedral site are connected at x =0.623. Accordingly it may be concluded that there is only ionic conduction in x x ≥0.623.

First-principles density functional calculation of electrochemical stability of fast Li ion conducting garnet-type oxides

The research and development of rechargeable all-ceramic lithium batteries are vital to realize their considerable advantages over existing commercial lithium ion batteries in terms of size, energy density, and safety. A key part of such effort is the development of solid-state electrolyte materials with high Li(+) conductivity and good electrochemical stability; lithium-containing oxides with a garnet-type structure are known to satisfy the requirements to achieve both features. Using first-principles density functional theory (DFT), we investigated the electrochemical stability of garnet-type Li(x)La(3)M(2)O(12) (M = Ti, Zr, Nb, Ta, Sb, Bi; x = 5 or 7) materials against Li metal. We found that the electrochemical stability of such materials depends on their composition and structure. The electrochemical stability against Li metal was improved when a cation M was chosen with a low effective nuclear charge, that is, with a high screening constant for an unoccupied orbital. In fact, both our computational and experimental results show that Li(7)La(3)Zr(2)O(12) and Li(5)La(3)Ta(2)O(12) are inert to Li metal. In addition, the linkage of MO(6) octahedra in the crystal structure affects the electrochemical stability. For example, perovskite-type La(1/3)TaO(3) was found, both experimentally and computationally, to react with Li metal owing to the corner-sharing MO(6) octahedral network of La(1/3)TaO(3), even though it has the same constituent elements as garnet-type Li(5)La(3)Ta(2)O(12) (which is inert to Li metal and features isolated TaO(6) octahedra).

First-principles study of lithium ion migration in lithium transition metal oxides with spinel structure

The migration of lithium (Li) ions in electrode materials is an important factor affecting the rate performance of rechargeable Li ion batteries. We have examined Li migration in spinels LiMn(2)O(4), LiCo(2)O(4), and LiCo(1/16)Mn(15/16)O(4) by means of first-principles calculations based on density functional theory (DFT). The results showed that the trajectory of the Li jump was straight between the two adjacent Li ions for all of the three spinel compounds. However, there were significant differences in the energy profiles and the Li jump path for LiMn(2)O(4) and LiCo(2)O(4). For LiMn(2)O(4) the highest energy barrier was in the middle of the two tetrahedral sites, or in the octahedral vacancy (16c). For LiCo(2)O(4) the lowest energy was around the octahedral 16c site and the energy barrier was located at the bottleneck sites. The difference in the energy profile for LiCo(2)O(4) stemmed from the charge disproportion of Co(3.5+) to Co(3+)/Co(4+) caused by a Li vacancy forming and jumping, which was not observed for LiMn(2)O(4). Charge disproportion successfully accounted for the faster Li migration mechanism observed in LiCo(1/16)Mn(15/16)O(4). Our computational results demonstrate the importance of the effect of charge distribution on the ion jump.

A concerted migration mechanism of mixed oxide ion and electron conduction in reduced ceria studied by first-principles density functional theory

Ceria based oxides are regarded as key oxide materials for energy and environmental applications, such as solid oxide fuel cells, oxygen permeation membranes, fuel cell electrodes, oxygen storage, or heterogeneous catalysis. This great versatility in applications is rendered possible by the fact that rare earth-doped ceria is a pure oxygen ion conductor while undoped ceria, CeO(2-δ), is a mixed oxygen ion-electron conductor. To get deeper insight into the mixed conduction mechanism of oxygen ions and electrons from atomistic and electronic level viewpoints we have applied first-principles density functional theory (DFT + U method). The calculation results show that oxygen vacancies strongly attract localized electrons, forming associates between them. The migration energy of an oxygen vacancy in such an associate is substantially lowered compared to the unassociated case due to the simultaneous positional rearrangement of localized electrons during the ionic jump process. Accordingly, we propose a concerted migration mechanism of oxygen vacancies and localized electrons in reduced ceria; this mechanism results in an increased diffusivity of oxygen vacancies supported by localized electrons compared with that in pure oxide ion conductors.