High-capacity Positive Electrode Materials Research Articles

A methodology to synthesize easily oxidized materials containing Li ions in an inert atmosphere

A simple methodology to synthesize easily oxidized materials with Mn3+ and V3+ ions, which are used as emerging high-capacity positive electrode materials for lithium-ion batteries, is proposed.

A Near Dimensionally Invariable Electrode Material: Li8/7Ti2/7V4/7O2

Our group has reported that Li-excess V-based oxide with Nb ions, Li1.25Nb0.25V0.25O2, delivers a large reversible capacity of over 250 mA g-1 with V3+/V5+ two-electron redox reaction.[1] Recently, Li8/7Ti2/7V4/7O2 has been also synthesized and examined as high-capacity positive electrode materials with V cationic redox.[2] Nanosized Li8/7Ti2/7V4/7O2 was also synthesized by high-energy mechanical milling. Nanosized Li8/7Ti2/7V4/7O2 delivers a high reversible capacity of ~300 mA h g-1 with no capacity deterioration for 100 cycles, which originates from a near dimensionally invariable nature as high-capacity electrode materials. Moreover, Li8/7Ti2/7V4/7O2 is used for as positive electrode materials for all-solid-state batteries with argyrodite-type electrolyte, Li6PS5Cl. An all-solid-state cell with Li8/7Ti2/7V4/7O2 and Li6PS5Cl delivers a large reversible capacity of 300 mA h g-1 with excellent retention for 700 cycles. From these results, the importance of a near dimensionally invariable character is discussed in detail.[1] M. Nakajima and N. Yabuuchi, Chem. Mater., 29, 6927 (2017)[2] I. Konuma et al. and N. Yabuuchi, Nat. Mater., 22, 225 (2023)

Durable Manganese-Based Li-Excess Electrode Material without Voltage Decay: Metastable and Nanosized Li2MnO1.5F1.5

Li-excess manganese-based oxides have been proposed as high-capacity positive electrode materials, but voltage decay associated with gradual oxygen loss hinders its use for practical applications. Herein, Li-excess manganese oxides with different fluorine contents are synthesized by high-energy mechanical milling. Although Li2MnOF2 with only divalent manganese ions cannot be synthesized, Li2MnO2F and Li2MnO1.5F1.5 are successfully synthesized. When the samples are charged to 5.0 V, both oxyfluorides deliver large reversible capacities, ∼350 mA h g–1 and ∼1000 mWh g–1. However, insufficient capacity retention is also observed because of the instability of anionic redox. In contrast, better capacity retention and higher energy density (730 mWh g–1) are obtained for Li2MnO1.5F1.5 with a 4.4 V cutoff because of the enrichment of fluoride ions and activation of Mn2+/Mn4+ cationic redox. Moreover, electrode durability is significantly improved by using a highly concentrated electrolyte, and good capacity retention without voltage decay is achieved for >180 cycles.

Improved Electrode Reversibility of Nanosized Li<sub>1.15</sub>Nb<sub>0.15</sub>Mn<sub>0.7</sub>O<sub>2</sub> through Li<sub>3</sub>PO<sub>4</sub> Integration

A lithium-excess cation-disordered rocksalt oxide, Li1.15Nb0.15Mn0.7O2, is synthesized and tested as positive electrode materials for battery applications. Although nanosized Li1.15Nb0.15Mn0.7O2 delivers a large reversible capacity using cationic/anionic redox reaction, the inferior capacity retention hinders its use for practical applications. Such degradation of electrode reversibility, including electrochemical and structural reversibility, is anticipated to originate from the gradual oxygen loss for the electrode materials with anionic redox. Herein, Li3PO4 is integrated into Li1.15Nb0.15Mn0.7O2 by high-energy mechanical milling, and 7 mol% Li3PO4 integrated Li1.15Nb0.15Mn0.7O2, Li1.2P0.06Nb0.13Mn0.61O2, shows much improved cyclability when compared with the sample without Li3PO4. Approximately 80 % of reversible capacity is retained after 100-cycle test at a rate of 200 mA g−1. Moreover, electrode kinetics are significantly improved by Li3PO4 integration, and Li1.2P0.06Nb0.13Mn0.61O2 delivers a discharge capacity of 200 mA h g−1 at a rate of 640 mA g−1. Li1.2P0.06Nb0.13Mn0.61O2 also shows improved thermal stability at elevated temperatures. From these results, the effectiveness of Li3PO4 integration into nanosized disordered rocksalt oxides with anionic redox is discussed, and this finding leads to the development of metastable high-capacity positive electrode materials for advanced Li-ion batteries.

A near dimensionally invariable high-capacity positive electrode material.

Delivering inherently stable lithium-ion batteries is a key challenge. Electrochemical lithium insertion and extraction often severely alters the electrode crystal chemistry, and this contributes to degradation with electrochemical cycling. Moreover, electrodes do not act in isolation, and this can be difficult to manage, especially in all-solid-state batteries. Therefore, discovering materials that can reversibly insert and extract large quantities of the charge carrier (Li+), that is, high capacity, with inherent stability during electrochemical cycles is necessary. Here lithium-excess vanadium oxides with a disordered rocksalt structure are examined as high-capacity and long-life positive electrode materials. Nanosized Li8/7Ti2/7V4/7O2 in optimized liquid electrolytes deliver a large reversible capacity of over 300 mAh g-1 with two-electron V3+/V5+ cationic redox, reaching 750 Wh kg-1 versus metallic lithium. Critically, highly reversible Li storage and no capacity fading for 400 cycles were observed in all-solid-state batteries with a sulfide-based solid electrolyte. Operando synchrotron X-ray diffraction combined with high-precision dilatometry reveals excellent reversibility and a near dimensionally invariable character during electrochemical cycling, which is associated with reversible vanadium migration on lithiation and delithiation. This work demonstrates an example of an electrode/electrolyte couple that produces high-capacity and long-life batteries enabled by multi-electron transition metal redox with a structure that is near invariant during cycling.

Enhancing Cycle Performance for Li-Rich Layered Oxides By the Stabilization of Crystal Structure

Li-rich layered oxides (LLO) have been studied as a next generation low-cost and high-capacity positive electrode materials. In LLO, the structural stability must be ensured to reversible Li intercalation. Blocking the tetrahedral site, the path of ions moving from transition metal sites to the lithium layer, may reduce the cation mxing which casuses the structural instability. In addition, introducing electrochemically inactive metal ions in the transition metal layer would stabilize the O-Me-O stacking by binding energy with oxygen which is higher than the transition metals.In this study, LLOs were obtained with and without doping at varous conditions. Doping elements are B and Ge of x mol% with repect of transition metals (Mn, Ni, Co). The effect of doping of is evaluated through the evaluation of the physical and electrochemical characteristics of the synthesized LLOs. Especially, the elelctrochemical cycle stability is throughly studied at various charge cuf-off voltages. The details of evaluation and analysis results will be presented.

A higher redox potential of solid state oxygen redox in Li4SiO4–LiCoO2 nano composite cathode

Modern and futuristic products, such as electric and hybrid electric vehicles, require high-capacity batteries. Solid state oxygen redox-type active materials are candidate as high-capacity positive electrode materials for Li-ion batteries. A promising high-capacity cathode material for Li-ion batteries is Li4SiO4 that can have higher redox potential compared with other Li2O-based materials. In this study, anion redox of Li4SiO4 was activated by nano-compositing with LiCoO2 via mechanical alloying. The nano composite cathode exhibits a specific capacity of ∼220 mAh g−1 with ∼0.5 V higher redox potential than that of Li2O-based cathode materials. X-ray absorption/photoelectron spectroscopy analyses show that the redox reaction of oxygen, including the formation and decomposition of superoxide and peroxide, is responsible for the charge compensation in the nano-composite cathode.

Molybdenum polysulfide electrode with high capacity for all-solid-state sodium battery

All-solid-state sodium secondary batteries have attracted significant attention as an alternative for post‑lithium-ion batteries. However, to obtain all-solid-state sodium secondary batteries with high-energy density, it is necessary to develop a high-capacity positive electrode material. In this study, two types of amorphous (a-)MoSx (x ≃ 6) active electrode materials were prepared using a mechanochemical (MC) process and through the thermal decomposition (TD) of (NH4)2Mo2S12·2H2O. Herein, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) analyses were conducted, and the results revealed significantly different local structures of a-MoSx (MC) and a-MoSx (TD). The all-solid-state sodium cells with a-MoSx (MC) and a-MoSx (TD) possessed high reversible capacities of 510 and 690 mAh g−1, as well as high-capacity retention rates of 80% and 92% for the 2nd to 30th cycles, respectively. XPS analysis of the discharge–charge products further suggested that dissociation and formation of disulfide bonds occurred reversibly during the discharge–charge reaction. This study shows that a-MoSx (MC) and a-MoSx (TD) are promising active electrode materials for all-solid-state sodium batteries.

Facile Material Design Concept for Co-Free Lithium Excess Nickel-Manganese Oxide as High-Capacity Positive Electrode Material

High-capacity Li1+x(Ni0.3Mn0.7)1-xO2, (0 < x < 1/3) samples were synthesized by the coprecipitation–calcination method. Both electrochemical cycle and high-rate performances were drastically improved by selecting an N2 atmosphere as final calcination. Scanning transmission electron microscopy—energy dispersive X-ray spectroscopy analysis showed that the sample calcined in an N2 atmosphere had a more homogeneous transition metal distribution into primary particles than that calcined in air. The solid-state 7Li nuclear magnetic resonance data showed that electrochemically inactive domains were only diminished for the sample calcined in an N2 atmosphere after electrochemical activation. X-ray Rietveld analysis revealed that the suitable transition metal distribution and content of the samples were different from those of typical layered rock-salt materials. Only that calcined in an N2 atmosphere had no spinel formation during charging and no oxide ion insertion reaction during discharging. No positive Co substitution effect was observed under the optimized preparation conditions. At the 100th cycle, the discharge capacity was 216 mAh g−1, which corresponds to 87% of the initial capacity (251 mAh g−1) at optimizing synthetic condition.

Operando XRD study on the Effect of Boron Doping on the Failure Mechanisms of Na-, Ni- and Mn-based Positive Electrodes in Sodium-Ion Batteries

Layered oxides have emerged as one of the most promising candidates for high capacity positive electrode materials in sodium-ion batteries (SIB). It has been demonstrated that Na, Ni and Mn-based layered oxide materials are relatively sustainable in terms of their composition, and high capacities have been achieved. However, it has been noted that at higher voltages, the crystal structure of these materials tends to degrade during cycling1,2. Doping the electrode with different transition metals such as Fe, Mg, Ti etc., has been an immensely popular strategy to avoid this degradation3–5. However, there has been little attention on doping with lighter electrochemically inactive elements such as boron that do not lower the materials' theoretical capacity.In this study, layered oxide cathode materials with varying Na, Ni and Mn contents were synthesised. Electrodes were made from synthesised materials (using 80% active material, 10% conductive additive and 10% binder) and measured in sodium-ion battery half-cells using the galvanostatic charge-discharge method (GCD). The best performing material was doped with 10% boron to investigate its influence on the materials' capacity during galvanostatic cycling (Figure 1a). Operando XRD was used to evaluate the changes in the crystal structure of synthesised layer oxides during galvanostatic cycling (Figure 1b). XRD diffractograms were measured on different cell potentials, which corresponded to voltage plateaus from previous galvanostatic charge-discharge measurements. Crystallite size, lattice parameters, and atom occupancies were calculated from measured diffractograms to evaluate changes in crystal structure during cycling. Operando XRD measurements were coupled with electrochemical impedance spectroscopy to tie structural degradation with contact impedance. Figure 1

(Invited) Nanostructured High-Capacity Positive Electrode Materials for Li-Ion Batteries

In the past decade, lithium-enriched compounds, Li2 MO3 (M = Mn4+, Ru4+ etc.), have been extensively studied for high-capacity positive electrode materials of Li-ion batteries. Large reversible capacities originate from charge compensation by anions species (anionic redox) coupled with partial cationic redox of transition metal ions. Recently, many cation-disordered rocksalt oxides have been proposed as a new series of electrode materials, which utilize reversible anionic redox. Nevertheless, insufficient electrode kinetics for the cation-disordered rocksalt system limits its use for practical applications. One simple strategy is synthesizing nanosized materials to overcome a problem of electrode kinetics (for electrons, holes and ions), and electrode kinetics are significantly improved through nanosizing even for a non-lithium-excess and stoichiometric system.1) Moreover, Nanosized Li/Na-excess oxides also deliver large reversible capacities at room temperature,2-4) which shows much better electrode kinetics compared with as-prepared samples.From these findings, we discuss the advantages/disadvantages of “nanostructured” electrode materialsto develop high energy advanced Li-ion batteries.References1) Sato et al., and N. Yabuuchi., Journal of Materials Chemistry A, 6, 13943 (2018).2) Kobayashi et al., and N. Yabuuchi, Small, 15, 1902462 (2019).3) Kobayashi et al., and N. Yabuuchi, Materials Today, 37, 43 (2020).4) Sawamura et al., and N. Yabuuchi, ACS Central Science, 6, 2326 (2020).

Nanostructured LiMnO2 with Li3PO4 Integrated at the Atomic Scale for High-Energy Electrode Materials with Reversible Anionic Redox.

Nanostructured LiMnO2 integrated with Li3PO4 was successfully synthesized by the mechanical milling route and examined as a new series of positive electrode materials for rechargeable lithium batteries. Although uniform mixing at the atomic scale between LiMnO2 and Li3PO4 was not anticipated because of the noncompatibility of crystal structures for both phases, our study reveals that phosphorus ions with excess lithium ions dissolve into nanosize crystalline LiMnO2 as first evidenced by elemental mapping using STEM-EELS combined with total X-ray scattering, solid-state NMR spectroscopy, and a theoretical ab initio study. The integrated phase features a low-crystallinity metastable phase with a unique nanostructure; the phosphorus ion located at the tetrahedral site shares faces with adjacent lithium ions at slightly distorted octahedral sites. This phase delivers a large reversible capacity of ∼320 mA h g–1 as a high-energy positive electrode material in Li cells. The large reversible capacity originated from the contribution from the anionic redox of oxygen coupled with the cationic redox of Mn ions, as evidenced by operando soft XAS spectroscopy, and the superior reversibility of the anionic redox and the suppression of oxygen loss were also found by online electrochemical mass spectroscopy. The improved reversibility of the anionic redox originates from the presence of phosphorus ions associated with the suppression of oxygen dimerization, as supported by a theoretical study. From these results, the mechanistic foundations of nanostructured high-capacity positive electrode materials were established, and further chemical and physical optimization may lead to the development of next-generation electrochemical devices.

High-capacity Li-excess lithium nickel manganese oxide as a Co-free positive electrode material

For the design of high-capacity and Co-free positive electrode material, stoichiometric LiNi1/2Mn1/2O2 and Li-excess Li1+x(NiyMn1-y)1-xO2 (0<x<1/3, y = 0.4 and 0.5) samples were prepared using coprecipitation–calcination. The basic LiNi1/2Mn1/2O2 structure is hexagonal layered rock-salt structure (R3¯m), but the sample has a biphasic composite of main Li-poor Li1-xTM1+xO2 (TM, transition metal) and minor Li-rich Li1+xTM1-xO2 phases after calcination below 950 °C. The fraction of the minor Li-rich phase decreased concomitantly with increasing calcination temperature. Only at 1000 °C was single-phase LiNi1/2Mn1/2O2 obtainable. The biphasic samples exhibited better electrochemical performance than that for the single-phase sample. Based on this result, two Li-excess Li1+x(NiyMn1-y)1-xO2 (0<x<1/3, y = 0.4 and 0.5) samples were synthesized by setting an excess amount of Li2CO3 (nominal Li/(Ni + Mn) ratio = 2) and calcination in air. Both samples were usefulness as a high-capacity positive electrode material exceeding 180 mA h g−1, with good cycling properties at 2.0–4.6 V.

High Capacity Li-Excess Vanadium Oxides for Positive Electrode Materials

Further demand for higher energy density of lithium-ion batteries (LIBs) is growing, especially for the development of electric vehicles to reduce dependence on fossil fuel. Although Co/Ni ions are used as positive electrode materials, its depletion of material resources is an emerging problem. Among electrode materials with 3d transition metal ions, LiVO2 with a layered rocksalt structure (s.g. R-3m) is known to be electrochemical inactive, associated with phase transition during charge. Nevertheless, our group has reported Li-excess Li3NbO4–LiVO2 binary oxides, and Li1.25Nb0.25V0.5O2 on this binary system with a cation disordered rocksalt structure delivers a large reversible capacity of 250 mA h g-1 with two-electron redox of V3+/V5+ at room temperature. (1) In this study. Instead of Li3NbO4, Li2TiO3–LiVO2 binary oxides are targeted as potential high capacity positive electrode materials. We also discuss the possibility of high capacity and long cycle life batteries without Co/Ni ions in the future. Li2TiO3–LiVO2 binary oxides were prepared by conventional calcination method from stoichiometric amounts of Li2CO3, anatase type TiO2, and V2O3. The precursors were mixed by wet ball milling and dried in air, and then calcined at 900 oC for 12 h in argon atmosphere. Thus obtained oxides were mechanically milled at 600 rpm for 36 h to prepare nanosized oxides. All of synthesized samples were stored in an argon filled glovebox to prevent the contact with oxygen and water. Electrode performance of the oxides was examined after reducing particle sizes by ball milling with 10 wt% acetylene black. Crystal structures and electrochemical properties of the oxides were studied by X-ray/neutron diffraction and galvanostatic charge/discharge measurement in two-electrode cells. XRD patterns and SEM images of LiVO2 (x = 0) and Li8/7Ti2/7V4/7O2 (x = 0.33) in the binary system x Li2TiO3–(1 – x) LiVO2 before and after mechanical milling are shown in Fig. 1. As-prepared LiVO2 and Li8/7Ti2/7V4/7O2 crystallized into the layered rocksalt structure (with partial cation disordering for Li8/7Ti2/7V4/7O2) change into nanosized and cation-disordered rocksalt structure (s.g. Fm-3m) after mechanical milling. The mechanical milled samples consist of nanosized, less than 10 nm, and low crystallinity oxides, which are agglomerated for each other, forming (sub-)micrometer-sized secondary particles. Discharge capacities of LiVO2 and Li8/7Ti2/7V4/7O2 obtained by galvanostatic charge/discharge are plotted in Fig. 2. LiVO2 after mixing with carbon by milling delivers a reversible capacity of 150 mA h g-1 in the Li cell, and a much higher reversible capacity of 270 mA h g-1 is observed for Li8/7Ti2/7V4/7O2. XAS study reveals reversible two-electron vanadium redox reaction (V3+/V5+) is activated for Li8/7Ti2/7V4/7O2. Moreover, the nanosized and rocksalt sample prepared by mechanical milling delivers over 300 mA h g-1 even though capacity deterioration is non-negligible in the Li cell with 1 M LiPF6 used as electrolyte. Nevertheless, electrode reversibility is significantly improved for the Li cell with the concentrated electrolyte, LiFSA:DMC = 1:1.1 in a molar ratio,(2) and excellent capacity retention is achieved for continuous 100 cycle test. From these results, we further discuss the origin of excellent capacity retention as electrode materials and possibility of high capacity LIBs with V3+/V5+ two-electron redox in the future.Reference(1) M. Nakajima and N. Yabuuchi, Chem. Mater., 29, 6927 (2017).(2) A. Yamada, et al, Nat. Commun, 7, 12032 (2016). Figure 1

Synthesis and Electrochemical Properties of Mg-Substituted Ni/Mn-Based High-Voltage Spinel Oxides

1. Introduction Currently, research on layered and rocksalt Li-rich materials is being actively conducted as a high-capacity positive electrode material for lithium ion batteries, and now it is widely accepted that large capacity originates from oxygen redox reaction.[1] Oxygen redox is proposed to be activated for oxygen ions with a Li-O-Li configuration.[2] It was also reported that oxygen redox is activated for Li-rich manganese spinel-type oxides, i.e., Li[Li x Mn2–x ]O4.[3] Nevertheless, its origin of activation of anionic redox for spinel-type oxides is not known. In this study, LiMg y Ni0.5-y Mn1.5O4, which is solid solution samples between LiNi0.5Mn1.5O4 and LiMg0.5Mn1.5O4, is targeted as positive electrode materials with anionic redox for spinel-type oxides. Mg ions have a high ionic bonding nature with oxygen, and thus better reversibility for anionic redox is anticipated.[1] We report the impact of Mg substitution on reversibility of Ni cationic and O anionic redox in the spinel framework structure, and the possibility of 5 V-class high-voltage electrode materials with excellent capacity retention is discussed. 2. Experimental LiMg y Ni0.5-y Mn1.5O4 (0 ≤ y ≤ 0.5) were synthesized from mixtures of Li2CO3, Mg(OH)2, Ni(OH)2 and Mn2O3 by two-step solid-state reaction at 950 oC for 6 h to 700 °C for 48 h in air atmosphere. Characterization of LiMg y Ni0.5-y Mn1.5O4 were conducted by X-ray diffraction (XRD) and scanning electron microscopy (SEM). For the evaluation of electrochemical properties, composite electrodes consisting of 80 wt% active material, 10 wt% AB, and 10 wt% PVdF, and were casted on aluminum foil. 1.0 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) (3/7 by volume) solution was used as electrolyte. 3. Results and Discussion XRD patterns of LiMg y Ni0.5-y Mn1.5O4 (0 ≤ y ≤ 0.5) are assigned into the spinel structure with space group of Fd-3m. From SEM images, particle sizes of LiMg y Ni0.5-y Mn1.5O4 are estimated to be ~4 μm. No difference in particle sizes regardless of chemical compositions is observed. Figure 1 compares charge/discharge curves of LiMg y Ni0.5-y Mn1.5O4. After 5 cycles, the non Mg-substituted sample, LiNi0.5Mn1.5O4, delivers 135 mAh g−1 of the discharge capacity in a lithium cell when the cut-off voltage is set to 5.3 V. After 50 cycles test, discharge capacity of LiNi0.5Mn1.5O4 is decreased to 125 mAh g-1. On the other hand, significant improvement of cyclability is evidenced for the partially Mg-substituted samples. No capacity fading and no increase in polarization is observed for LiMg0.06Ni0.44Mn1.5O4 even charge to 5.3 V vs. Li for 50 cycles at a rate of 10 mA g-1. On the basis of these findings, Mg substitution is proposed to be effective to reduce electrolyte decomposition and improve structural stability upon high-voltage exposure. Nevertheless, the further enrichment of Mg ions in the structure results in loss of reversible capacity, and anionic redox seems to be inactive for LiMg0.5Mn1.5O4. From these results, factors affecting electrode performance of high voltage spinel with cationic/anionic redox will be discussed in detail.References(1) N. Yabuuchi, Chem. Rec., 19, 703 (2019).(2) D.-H. Seo, Nature Chem., 8, 692 (2016).(3) E. Iwata et al., and T. Ohzuku, Electrochemistry, 71,1187 (2003). Figure 1

Lithium Storage Properties of Rocksalt-Type Li-Excess Titanium Sulfides

In recent years, the demands for lithium-ion batteries has increased rapidly all over the world. To further increase the energy density, Li-excess electrode materials which deliver higher reversible capacities have been actively studied as advanced positive electrode materials. A Li-excess sulfide, Li2TiS3, delivers a large reversible capacity on the basis of reversible anion redox.1) Li2TiS3 has the advantage of high electronic conductivity even though it is composed of Ti4+ ions without valence electrons. This is a characteristic feature of sulfides that is different from an oxide counterpart, Li2TiO3. In this study, we study factors affecting on electrode performance of Li2TiS3 and discuss reaction mechanisms as the potential high-capacity positive electrode material for lithium-ion batteries.Li2TiS3 was prepared by mechanical milling. Li2S and TiS2 were used as precursors, and the mixture of the precursors were mixed by using a zirconia pot and zirconia balls. Crystal structures of the samples were examined by using an X-ray diffractometer. Composite electrodes consisted of 80 wt% active materials, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride, pasted on aluminum foil as a current collector. Metallic lithium was used a negative electrode. The electrolyte solutions used were 1.0 M LiPF6 dissolved in ethylene carbonate:dimethyl carbonate (3:7 by volume) and 5.3 M LiFSA dissolved in trimethylphosphine as a concentrated electrolyte solution.2) Two-electrode cells were used and cycled at a rate of 20 mA g-1 at room temperature.Figure 1 shows the X-ray diffraction patterns of the precursors and the sample after mechanical milling. From Fig. 1, diffraction lines of the starting materials with high crystallinity disappear after mechanical milling and broad diffraction lines after milling are assigned into a cation-disordered rocksalt-type structure with low crystallinity. Although Li2TiO3 is electrochemically inactive, Li2TiS3 delivers a large reversible capacity of over 300 mA h g-1 in Li cells. However, the reversible capacity of Li2TiS3 is reduced to approximately 150 mA h g-1 after the 30 cycles test with the aprotic electrolyte solution with 1 M LiPF6 as shown in Fig. 2. On the other hand, Li2TiS3 delivers a reversible capacity of 230 mA h g-1 even after 50 cycles with the concentrated electrolyte solution (5.3 M LiFSA) because dissolution of the sulfide could be suppressed for the concentrated electrolyte solution with less free solvents.3) From these results, we discuss the factors affecting electrode performance of Li2TiS3 for advanced positive electrode materials with sulfide ions.References(1) Atsushi Sakuda et al., Scientific Reports. 8, 15086 (2018).(2) Jianhui Wang et al., Nature Energy. 3, 22–29 (2018).(3) Takayuki Doi et al., ChemistrySelect. 2, 8824-8827 (2017). Figure 1

(Invited) Nanostructured Rocksalt-Based Positive Electrode Materials for Rechargeable Li/Na Batteries

In the past decade, lithium-enriched compounds, Li2MO3 (M = Mn4+, Ru4+ etc.), have been extensively studied for high-capacity positive electrode materials of lithium batteries. Although the origin of high reversible capacities was a debatable subject for a long time, recently it has been evidenced that charge compensation is partly achieved by solid-state redox of non-metal anions, i.e., oxide ions (anionic redox), coupled with redox reaction of transition metal ions (cationic redox), which is the basic theory used for classical lithium/sodium insertion materials. Recently, many cation-disordered rocksalt oxides have been proposed as a new series of electrode materials, which utilize reversible anionic redox. Nevertheless, insufficient electrode kinetics for the cation-disordered rocksalt system limits its use for practical applications. One simple strategy is to synthesize nanosized materials, which overcomes a problem of electrode kinetics (for electrons, holes and ions), and indeed electrode kinetics is significantly improved through nanosizing even for a non-lithium-excess and stoichiometric system.1) Moreover, Nanosized Li/Na-excess oxides also deliver large reversible capacities even at room temperature,2, 3) which shows much better electrode kinetics compared with as-prepared submicrometer-sized samples.From these findings, we discuss the advantages/disadvantages of “nanostructured” rocksalt-based electrode materialsto develop high energy Li/Na-ion batteries in the future.References Sato et al., and N. Yabuuchi., Journal of Materials Chemistry A, 6, 13943 (2018).Kobayashi et al., and N. Yabuuchi, Small, 15, 1902462 (6 pages) (2019).Kobayashi et al., and N. Yabuuchi, Materials Today, in-press, DOI: 10.1016/j.mattod.2020.03.002

Local structure and electrochemical performances of sulfurized polyethylene glycol after heat treatment

Designing a high-capacity positive electrode material is critical for the advancement of lithium-ion batteries. Sulfurized polyethylene glycol (SPEG), containing ca. 61 wt% of sulfur, is a promising positive electrode material that exhibits a large initial discharge capacity of more than 800 mAh g−1. In this study, we present the local structure and electrochemical performances of SPEG. A high-energy X-ray total scattering experiment revealed that sulfur in SPEG is predominantly fragmented and bound to carbon atoms. The changes in the physicochemical properties of SPEG due to heat treatment with nitrogen gas at various temperatures were investigated using thermogravimetric analysis, Raman spectroscopy, X-ray absorption near edge structure, and extended X-ray absorption fine structure. Comparing the electrochemical performances of SPEG after heat treatment at various temperatures, it was found that S–S and C=S bonds contribute to the overall electrochemical performance of SPEG.

Electrochemical Characteristics of Co-Substituted α- and β-Li5AlO4 as High-Specific Capacity Positive Electrode Materials

Electric vehicles and hybrid electric vehicles require batteries with higher energy densities than conventional batteries. Anion redox-type active materials have been proposed as new high-capacity positive electrode materials for Li-ion batteries with high-energy densities. Co-substituted Li5AlO4 is a novel and promising high-capacity positive electrode material for Li-ion batteries. In this study, we investigated the influence of different synthesis conditions on the enhancement of the specific capacity. The material prepared via mechanical alloying of β-Li5AlO4 with LiCoO2 at 300 rpm for 24 h exhibited a higher specific capacity than that prepared from α-Li5AlO4 and LiCoO2. Co-substituted β-Li5AlO4 demonstrated a specific capacity of approximately 250 mA h g–1. The specific capacity of Co-substituted α and β-Li5AlO4 increased with increasing Co content in the samples. According to X-ray absorption near edge structure measurements, the irreversible oxygen redox reaction and a reversible reaction involving the formation and consumption of peroxide were responsible for the charge compensation of Co-substituted β-Li5AlO4 and α-Li5AlO4, respectively.

Electrochemical characteristics and charge-discharge mechanisms of Co-substituted Li5AlO4 as a novel positive electrode material

Modern and future products such as electric vehicles and hybrid electric vehicles require batteries with higher energy densities than that of conventional batteries. Anion redox-type active materials are proposed as a new high-capacity positive electrode material for Li-ion batteries with high energy density. We developed Li5AlO4 as an anion redox-type active material with a higher stability than that of Co-substituted Li2O. Li5AlO4 was substituted with Co by mechanical alloying with LiCoO2 to enhance its conductivity and its reactivity as an oxide anion. Co-substituted Li5AlO4 showed a slightly higher electron conductivity and a remarkably higher oxide anion reactivity than the as-prepared Li5AlO4. From electrochemical analysis, it was shown that Co-substituted Li5AlO4 had a reversible capacity of approximately 140 mAh g−1, while the as-prepared Li5AlO4 had no reversible capacity. According to the atomic valence of Co and Al and the partial structure for Co-substituted Li5AlO4 during the first charge-discharge cycle, we found that the charge capacity of Co-substituted Li5AlO4 is derived from the oxidation of oxide anions, the formation of peroxide and superoxide, and the fact that the partial structure of Co-substituted Li5AlO4 remains unchanged. These results indicate that the charge-discharge reaction of Co-substituted Li5AlO4 proceeds reversibly. Co-substituted Li5AlO4 demonstrates a relatively high specific capacity and good reversibility during the charge-discharge process.