Atsuo Yamada

Optimized LiFePO4 for Lithium Battery Cathodes

LiFePO 4 powders were synthesized under various conditions and the performance of the cathodes was evaluated using coin cells, The samples were characterized by X-ray diffraction, scanning electron microscope observations, Brunauer, Emmett, and Teller surface area measurements, particle-size distribution measurements, and Mossbauer spectroscopy. Ab initio calculation was used to confirm the experimental redox potentials and Mossbauer parameters. The choice of a moderate sintering temperature (500°C 95% of the 170 mAh/g theoretical capacity at room temperature. There are two main obstacles to achieving optimum charge/discharge performance of LiFePO 4 : (i) undesirable particle growth at T > 600°C and (ii) the presence of a noncrystalline residual Fe 3+ phase at T < 500°C.

A 3.8-V earth-abundant sodium battery electrode.

Rechargeable lithium batteries have ushered the wireless revolution over last two decades and are now matured to enable green automobiles. However, the growing concern on scarcity and large-scale applications of lithium resources have steered effort to realize sustainable sodium-ion batteries, Na and Fe being abundant and low-cost charge carrier and redox centre, respectively. However, their performance is limited owing to low operating voltage and sluggish kinetics. Here we report a hitherto-unknown material with entirely new composition and structure with the first alluaudite-type sulphate framework, Na2Fe2(SO4)3, registering the highest-ever Fe3+/Fe2+ redox potential at 3.8 V (versus Na, and hence 4.1 V versus Li) along with fast rate kinetics. Rare-metal-free Na-ion rechargeable battery system compatible with the present Li-ion battery is now in realistic scope without sacrificing high energy density and high power, and paves way for discovery of new earth-abundant sustainable cathodes for large-scale batteries.

CRYSTAL CHEMISTRY OF THE OLIVINE-TYPE LI(MNYFE1-Y) PO4 AND (MNYFE1-Y)PO4 AS POSSIBLE 4V CATHODE MATERIALS FOR LITHIUM BATTERIES

A potential 4 V cathode material for lithium batteries was investigated. The crystal chemistry of the olivine-type of Li(Mn y 2+ Fe 1-y 2+ )PO 4 (discharged state) and its delithiated form (Mn y 3+ Fe 1-y 3+ )PO 4 (charged state) were comparatively studied using X-ray diffraction, Mossbauer spectroscopy, and ab initio calculations. A strong oxidizer, nitronium tetrafluoroborate, NO 2 BF 4 , was used for chemical delithiation of Li(Mn y 2+ Fe 1-y 2+ )PO 4 to obtain (Mn y 3+ Fe 1-y 3+ )PO 4 . The strong electron/lattice interaction induced by the trivalent manganese (3d 4 ) in (Mn y 3+ Fe 1-y 3+ )PO 4 (charged state) is highlighted as the intrinsic obstacle to generating the full theoretical capacity (ca. 170 mAh/g) of the Mn-rich phase (y >0.8), followed by an efficient cathode performance of the optimized Li(Mn 0.6 Fe 0.4 )PO 4 .

Comparative Kinetic Study of Olivine Li x MPO 4 ( M = Fe , Mn)

A huge kinetic difference in olivine Li x MPO 4 (M = Fe,Mn) is demonstrated in a quantitative manner. Galvanostatic discharge profiles and the current relaxation to the stepwise anodic overvoltage (chronoamperometry) are comparatively measured for the Li x FePO 4 and Li x MnPO 4 under identical extrinsic conditions, which are carefully controlled and confirmed using Rietveld refinement for the X-ray diffraction profiles, direct texture observation by scanning electron microscope, Brunauer-Emmett-Teller surface area measurements, and tap density measurements. The current durability for Li x MnPO 4 is orders-of-magnitude inferior to that of Li x FePO 4 , the origin of which is clearly attributed to their intrinsic crystallographic and transport property differences. Heavy polaronic holes localized on the Mn 3+ sites are suggested as an important rate-limiting factor. In spite of the higher open-circuit voltage of Mn 3+ /Mn 2+ (4.05 V) compared to that of Fe 3+ /Fe 2+ (3.45 V) in the olivine framework, the abnormally large polarization may eliminate pure LiMnP0 4 as a practical lithium battery cathode due to much lower effective energy density than LiFePO 4 .

Pseudocapacitance of MXene nanosheets for high-power sodium-ion hybrid capacitors.

High-power Na-ion batteries have tremendous potential in various large-scale applications. However, conventional charge storage through ion intercalation or double-layer formation cannot satisfy the requirements of such applications owing to the slow kinetics of ion intercalation and the small capacitance of the double layer. The present work demonstrates that the pseudocapacitance of the nanosheet compound MXene Ti2C achieves a higher specific capacity relative to double-layer capacitor electrodes and a higher rate capability relative to ion intercalation electrodes. By utilizing the pseudocapacitance as a negative electrode, the prototype Na-ion full cell consisting of an alluaudite Na2Fe2(SO4)3 positive electrode and an MXene Ti2C negative electrode operates at a relatively high voltage of 2.4 V and delivers 90 and 40 mAh g−1 at 1.0 and 5.0 A g−1 (based on the weight of the negative electrode), respectively, which are not attainable by conventional electrochemical energy storage systems.

Jahn-Teller structural phase transition around 280K in LiMn2O4

The structural phase transition of spinel LiMn2O4 (Fd3m at room temperature) was investigated using thermal analysis and powder x-ray diffraction. The phase transition was observed around Tt=280K with a temperature hysteresis of about 10K. It was revealed that the low temperature phase (T < Tt) is a mixture of Fd3m (c/a = 1) and I41/amd (c/a = 1.011). The phase transformation from Fd3m to I41/amd proceeds with the decrease in temperature until the volume fraction of the I41/amd phase saturates at 65% at around 260K, just below Tt. The single phase of I41/amd never appeared even at 220K, far below Tt. This phase transition is of a first-order character and is owing to the cooperative Jahn-Teller effect of Mn3+. Cell performance utilizing the LiMn2O4 cathode is also discussed.

Unusual Stability of Acetonitrile-Based Superconcentrated Electrolytes for Fast-Charging Lithium-Ion Batteries

The development of a stable, functional electrolyte is urgently required for fast-charging and high-voltage lithium-ion batteries as well as next-generation advanced batteries (e.g., Li-O2 systems). Acetonitrile (AN) solutions are one of the most promising electrolytes with remarkably high chemical and oxidative stability as well as high ionic conductivity, but its low stability against reduction is a critical problem that hinders its extensive applications. Herein, we report enhanced reductive stability of a superconcentrated AN solution (>4 mol dm(-3)). Applying it to a battery electrolyte, we demonstrate, for the first time, reversible lithium intercalation into a graphite electrode in a reduction-vulnerable AN solvent. Moreover, the reaction kinetics is much faster than in a currently used commercial electrolyte. First-principle calculations combined with spectroscopic analyses reveal that the peculiar reductive stability arises from modified frontier orbital characters unique to such superconcentrated solutions, in which all solvents and anions coordinate to Li(+) cations to form a fluid polymeric network of anions and Li(+) cations.

Reaction Mechanism of the Olivine-Type Li x ( Mn0.6Fe0.4 ) PO 4 ( 0 ⩽ x ⩽ 1 )

The charge-discharge reaction mechanism of the olivine-type Li x (Mn 0.6 Fe 0.4 )PO 4 (0 ≤ x ≤ 1.0), a possible 4 V class cathode material for lithium batteries, was investigated using equilibrium voltage measurements, X-ray diffraction. Mossbauer spectroscopy, and X-ray absorption spectroscopy. The flat two-phase region with an open-circuit voltage (OCV) of ca. 4.1 V (region 1: 0 ≤ x ≤ 0.6, Mn 3+ /Mn 2+ ) and the S-curved single-phase region with OCV 3.5 V (region 11: 0.6 ≤ x ≤ 1.0, Fe 3+ /Fe 2+ ) were clearly identified together with the corresponding change in the unit cell dimensions of the orthorhombic lattice. These features show significant differences from the reaction mechanism of Li x FePO 4 (0 ≤ x ≤ 1) ), in which the whole Fe 3+ /Fe 2+ reaction proceeds in a two-phase manner (LiFePO 4 -FePO 4 ) with a flat voltage profile at 3.4 V.

Structure of Li2FeSiO4

A large-scale lithium-ion battery is the key technology toward a greener society. A lithium iron silicate system is rapidly attracting much attention as the new important developmental platform of cathode material with abundant elements and possible multielectron reactions. The hitherto unsolved crystal structure of the typical composition Li2FeSiO4 has now been determined using high-resolution synchrotron X-ray diffraction and electron diffraction experiments. The structure has a 2 times larger superlattice compared to the previous beta-Li3PO4-based model, and its origin is the periodic modulation of coordination tetrahedra.

New Lithium Iron Pyrophosphate as 3.5 V Class Cathode Material for Lithium Ion Battery

A new pyrophosphate compound Li(2)FeP(2)O(7) was synthesized by a conventional solid-state reaction, and its crystal structure was determined. Its reversible electrode operation at ca. 3.5 V vs Li was identified with the capacity of a one-electron theoretical value of 110 mAh g(-1) even for ca. 1 μm particles without any special efforts such as nanosizing or carbon coating. Li(2)FeP(2)O(7) and its derivatives should provide a new platform for related lithium battery electrode research and could be potential competitors to commercial olivine LiFePO(4), which has been recognized as the most promising positive cathode for a lithium-ion battery system for large-scale applications, such as plug-in hybrid electric vehicles.