Redox Voltage Research Articles

Improvement of High Voltage Cyclic Performance By Novel Binder for High Energy Lithium Ion Battery Application

Introduction Lithium-ion batteries (LIB) have been used as high density storage devices for several kinds of applications. Higher energy density and higher durability are required for use of high-performance energy devices. Among various attempts to improve energy density of LIB, technologies which increase battery voltage have emerged as a viable solution.Binder affects electrochemical performance reaction directly because it exists near the active materials. In a previous report, Co-ion dissolution and Co3O4 formation were observed in the electrode with PVdF (Polyvinylidene fluoride) [1].In this study, we clarified the degradation mechanism of cathode electrode that contain PVdF binder. Experimental The cathode electrodes consisted of LiCoO2, acetylene black, and PVdF. The performance tests were investigated in 40mAh laminate type lithium ion cells. The cycle life test was performed at 4.2V or 4.4V. Solid F-NMR of electrode was used for the analysis of binder degradation after cycle test.Thin-film electrode of LiCoO2 was fabricated on Au substrate by rf-sputtering method [2]. X-ray diffraction pattern indicated that high crystalline LiCoO2 thin-film electrode was prepared. Binders were spin-coated on the thin-film electrode. Three-electrode cell was used for electrochemical measurements of LiCoO2 thin-film electrode. The electrochemical properties of LiCoO2 thin films were investigated by cyclic voltammetry (CV) Results Figure 1 shows cycle performance at each cut-off voltage. Cycle performance was dramatically decreased at high cut-off voltage. Figure 2 shows the cyclic voltammograms of LiCoO2 thin-film electrode coated with PVdF in LiClO4/EC/DEC at 60°C. Around 3.9 V, peaks of lithium-ion de-intercalation /intercalation were observed. With cycling, redox currentsgradually decreased and redox voltage shifted. This result suggested the degradation of LiCoO2.We evaluated the structure change of LiCoO2 after cycle test by TEM spectroscopy. From the TEM spectroscopy, degradation of surface structure was observed at the electrode after cycle test. From the F-NMR, structure of PVdF was slightly changed after the cycle test. It is suggested that PVdF increased the reactivity of active material. From ICP measurements, the amount of metal precipitation on anode was also increased after cycle test. And adhesion strengths wassignificantly decreased after cycle test.In our presentation, we will show a new type of water-based binder developed by Zeon which can contribute to improvement of cell performance, such as improved cycling performance.

Single Crystal Nuclear Magnetic Resonance Studies of LiMnxFe1-XPO4

LiMPO4 are orthorhombic crystals, belonging to the olivine family and exhibit exceptionally high applicability for electrochemical energy storage in lithium-ion secondary batteries due to their charge storage capacity, chemical and thermal stability, and cost. Furthermore, mixed transition metal ion compounds Li(M,M’)PO4 are particularly promising cathode materials because the redox voltage increases as from Fe to Mn, Ni, Co 1and the mixed compounds can be tailored for specific applications.The aim of these studies is to better understand the key role played by the transition metals such as Fe, Mn and particularly their mixtures in determining the properties of these materials. Single crystal 7Li and 31P NMR studies are used to obtain precise information on the magnetic environments of the lithium ions and phosphate groups which may ultimately be correlated with processes and structural features that determine the electrochemical behavior of these olivine phosphates.By measurements of 7Li and 31P NMR frequencies, i.e. ν = B0∙σ∙Ι, where B0 is the applied field and Ι is the nuclear spin, with crystal orientation, it is possible to determine interaction tensor components {σij } giving rise to the lithium and phosphorus shifts. The series of the single crystals of LiMPO4 were prepared by a reciprocal salt flux growth technique, which allows the facile preparation of single crystals up to 1 cm across with well-developed facets at relatively low temperatures. Crystals for the NMR measurements present their natural faceting2.X-ray diffraction measurements were performed in order to identify the principal crystallographic axes [100], [010] and [001] of the samples. Subsequently the crystals were fixed to a rigid insert in the NMR probe, which was mounted onto a goniometer and inserted between the gap of a permanent magnet (B0=2.1T). The use of a relatively low field strength allowed for acquisition of the entire rotation pattern within the spectrometer bandwidth. Rotations were made with the goniometer shaft perpendicular to applied magnetic field, in this way it was possible to generate 24 spectra, one every 15o, for each crystallographic axis [100],[010] and [001].Previous studies3 showed that in the case of 31P, crystals exhibit large (paramagnetic) shifts on the order of 3000ppm, primarily due to the hyperfine interactions between the unpaired electron spins of the paramagnetic ion and the phosphorus nuclei.The 7Li resonances are also strongly affected by the paramagnetic susceptibility though not to the same extent as 31P. However in addition, there is a nuclear quadrupole interaction which splits the resonance and provides information concerning the electric field gradientIn addition to the endpoints (Fe and Mn analogues), we present an NMR analysis of the mixed transition metal ion compounds LiFexMn1-xPO4 , where x = 0.25, 0.5, and 0.75 in order to shed some light on the degree of cation disorder between the lithium, phosphorus, and transition metal sites that can result in substantial degradation of cathode performance particularly with regard to reduced rate capability.This research was supported by the U.S. Department of Energy

Photoelectrochemical properties of MWCNT–TiO2 hybrid materials as a counter electrode for dye-sensitized solar cells

A B S T R A C T The MWCNT–TiO2 hybrid materials were prepared by a simply mixing method and used as a counter electrode (CE) for dye-sensitized solar cells. Compared with the platinum CE, MWCNT–TiO2 CE has the similar redox voltage and current response in the cyclic voltammetry. The electrochemical catalytic activity was characterized by the electrochemical impedance spectroscopy and Tafel curve, including the equivalent circuit, the exchange current density, the limiting diffusion current density, and the diffusion coefficient of triiodide/iodide redox species. The results indicate that the reduction process from triiodide to iodide is determined by the kinetic-controlled and diffusion-limited processes. The device performance is optimal based on the MWCNT–TiO2 (mass ratio of 2:1) CE, such as the open-circuit voltage of 0.72 V, the short-circuit photocurrent density of 15.71 mA/cm 2 , the fill factor of 0.68, and the photon-to-electron conversion efficiency of 7.69%.

A method of separating the capacities of layer and spinel compounds in blended cathode

Some mixtures of two active materials are utilized for the blended cathodes in lithium-ion batteries. The blended cathode and unitary cathodes were prepared by a layer compound, LiNi1/3Mn1/3Co1/3O2, and a spinel compound, LiAl0.1Mn1.9O4 as simple active materials in order to develop the method to estimate the capacities of the both active materials in the blended cathode. The differential curves (dQ/dV) clearly provided the information on the redox voltage region of the layer and the spinel compounds in the unitary and blended cathodes. A regression curve of the layer compound was partially calculated using the dQ/dV of blended cathode in the layered redox voltage region, and it was extrapolated until the mixed redox voltage region of the layer and spinel compounds. The capacity in the blended cathode was divided into the capacities of the active materials by this curve. This method was also successfully applied to the degraded blended cathode. This method proposed in this paper is a significant technique for degradation analysis of both active materials in the blended cathode without the need for the same active material lots as those of blended cathodes, using information on electrochemically active voltage ranges of each active material such as references.

Effect of Both Mn and Zn Partial Substitution on the Electrochemical Performance of LiFeSO4F

The tavorite structured LiFeSO4F forms a potential cathode with a competent mix of high redox voltage (ca 3.6 V) and reversible capacity (~140 mAh/g). It exists as a metastable phase, capable of showing polymorphism that can be triggered thermodynamically or chemically. Fluorosulphate (LiMSO4F) is a structurally diverse family of compounds. Here, we demonstrate the effect of Mn and Zn substitution into the structural and electrochemical properties of parent LiFeSO4F. Mn and Zn substitution into LiFeSO4F stabilizes triplite and sillimanite phases respectively, tuning the Fe2+/Fe3+ redox activity in the voltage range of 3.6~3.9 V. Notably, the polymorphism in LiFeSO4F leads to the realization of cathode material with the highest Fe2+/Fe3+ redox potential close to 4 V.

LiMn0.8Fe0.2PO4/Li4Ti5O12, a Possible Li-Ion Battery System for Load-Leveling Application

We examined a new Li-ion battery system based on the combination of a high voltage LiMn0.8Fe0.2PO4 (LMFP) cathode with a Li4Ti5O12 (LTO) anode. Due to the relatively high red-ox voltage of LTO (1.5 V vs. Li) and the excellent stability of its spinel structure, as well as the fact that the red-ox potential of a Li[MnFe]PO4 cathode, up to 4.1 V, does not endanger the anodic stability of a standard electrolyte solutions, it is assumed that such cells can be safe, stable, highly reversible, and suitable for load-leveling applications. The LMFP/LTO cells exhibited excellent rate capability and cycle life at 30°C, delivering discharge capacity of 153, 152, 146, and 118 mAhg−1 (cathode) at 0.1 C, C, 2 C, and 5 C rates. These cells demonstrated excellent high temperature performance when the LTO anodes were pre-passivated before cell operation. By XRD and ICP analyzes of C-LiMn0.8Fe0.2PO4 electrodes before and after charge/discharge cycles, the capacity fading of these systems at high temperatures was attributed to depletion of active Li from the electrodes, due to side reactions on the anode side. These were avoided by pre-passivation of the LTO electrodes. The surface chemistry of Li4Ti5O12 anodes was investigated with the anodic surface reactions arising mainly from the salt (LiPF6).

Preparation and electrochemical study of Mn-doped Li9V3(P2O7)3(PO4)2 cathode material for lithium ion batteries

A series of Mn-doped Li9V3−xMnx(P2O7)3(PO4)2/C (x=0.00, 0.02, 0.05, 0.08, and 0.10) compounds have been successfully prepared by using sol–gel method. ICP, XRD, and Rietveld refinement results analyses indicate that Mn ions were sufficiently doped in Li9V3(P2O7)3(PO4)2 without changing its trigonal structure, and the cell parameters (including a, c, and V) of Li9V3−xMnx(P2O7)3(PO4)2/C (x=0.00, 0.05, and 0.10) increase with the increasing of x. The X-ray photoelectron spectroscopy (XPS) analysis reveals that the manganese oxidation state was +2 and both V4+ and V3+ ions were present in the Li9V2.9Mn0.1(P2O7)3(PO4)2 compound. The electronic conductivity increases monotonically with Mn-doped content x. The electrochemical insertion/extraction properties of Li9V3−xMnx(P2O7)3(PO4)2/C (x=0.00, 0.02, 0.05, 0.08, and 0.10) phases are also presented. The Mn-doped phases exhibit smaller redox voltage polarization, higher cycle performance and better rate capability than that of the pristine one. Particularly, Li9V2.9Mn0.1(P2O7)3(PO4)2/C shows both good rate capability and best capacity recovered ability among all the samples. The enhancement of rate performance and cyclic capability may be attributed to the optimizing particle size, enhancement of electronic conductivity, lithium ion mobility and structural stability owing to the proper amount of Mn-doping in V sites.

Observation of the highest Mn3+/Mn2+ redox potential of 4.45 V in a Li2MnP2O7 pyrophosphate cathode

By exploring the pyrophosphate chemistry for rechargeable Li-ion batteries, we report the synthesis and electrochemical characterization of a Li2MnP2O7 cathode. Easily prepared by conventional solid-state synthesis (at 600 °C), the Li2MnP2O7 displays an Mn3+/Mn2+ redox potential centered at 4.45 V versus lithium. It has registered the highest voltage ever obtained for the Mn3+/Mn2+ redox couple in any Mn-based cathode material. Following the pristine and partially (Mn) substituted Li2FeP2O7 (3.5–3.9 V vs. Li) and Li2CoP2O7 (4.9 V vs. Li), which show the highest Fe3+/Fe2+ and Co3+/Co2+ redox potentials among the known cathode compounds, the highest Mn3+/Mn2+ redox voltage (ca. 4.45 V) in Li2MnP2O7 establishes ‘pyrophosphates’ as a novel family displaying the highest M3+/M2+ redox potentials among all polyanionic compounds. Fundamental study of these pyrophosphates can provide useful insights into design of high-voltage cathode materials.

Highly reversible conversion-capacity of MnOx-loaded ordered mesoporous carbon nanorods for lithium-ion battery anodes

An ordered mesoporous carbon (OMC) with a nanorod-shaped morphology and enhanced graphitic character was employed as an ideal support for MnOx (major phase of Mn3O4 with a small portion of MnO) nanocrystals which possess a high theoretical conversion capacity as a Li-ion battery anode. The MnOx/OMC nanocomposite was prepared by a simple wet-impregnation of Mn(NO3)2 aqueous solution onto OMC nanorods followed by thermal treatment at 450 °C in an Ar flow. The electrochemical properties of MnOx/OMC were investigated in comparison to those of bare OMC and a commercial graphite as an anode for Li-ion batteries. Transmission electron microscopy, scanning electron microscopy, X-ray diffraction, N2 adsorption–desorption analysis, X-ray photoelectron spectroscopy, and thermogravimetric analysis revealed that 3–30 nm MnOx nanocrystals at a high loading of 68.4 wt% were formed and well dispersed in the pore structure of OMC nanorods. The MnOx/OMC exhibited a high reversible capacity (>950 mAh g−1) after 50 deep charge–discharge cycles with excellent cycling stability, Coulombic efficiency and rate capability. As an anode for Li-ion batteries, the incorporation of insulating high density MnOx nanocrystals into OMC nanorods showed synergistic benefits of high volumetric capacity as well as specific capacity, and small redox voltage hysteresis compared to OMC nanorods.

Ferredoxin Molecular Thin Film with Intrinsic Switching Mechanism for Biomemory Application

A biomemory device consisting of cysteine modified ferredoxin molecules which possess a memory effect via a charge transfer mechanism was developed. For achieving an efficient bioelectronic device, cysteine modified ferredoxin was developed by embodying cysteine residues in ferredoxin by site--directed mutagenesis method to directly coordinate with the gold (Au) surface without use of any additional linkers. The thin film formation of ferredoxin molecules on Au electrode is confirmed by surface plasmon resonance (SPR) spectroscopy and scanning tunneling microscope (STM). Cyclic voltammetry (CV) and open circuit potential amperometry (OCPA) methods were used to verify the memory switching characteristics of the fabricated device. The charge transfer between ferredoxin protein molecules and Au electrode enables a bi-stable electrical conductivity allowing the system to be used as a digital memory device. Data storage is achieved by applying redox voltages which are within the range of -500 mV. These results suggest that the proposed device has a function of memory and can be used for the construction of a nano-scale bioelectronic device.

Ionothermal Synthesis of Li-Based Fluorophosphates Electrodes

Fluoride-based materials have regained interest within the field of Li-ion batteries largely due to the advent of nanosizing which has transformed the insulating insertion compounds into attractive electrode materials. Herein we demonstrate the effectiveness of ionothermal synthesis in the preparation of nanometric LiFePO4F and LiTiPO4F phases with structures isotopic to tavorite LiFePO4(OH) at temperatures of only 260 °C, while temperatures of 600−700 °C are required to obtain coarse powders via the ceramic method. However, the redox-active phases, which are obtained in a high state of division, have lower redox voltages than LiFePO4 despite the presence of fluorine. Additionally, LiTiPO4F shows staircase charge/discharge profiles with Ti2/3+ and Ti3/4+ couples. Though quite unusual in lithium intercalation oxides, a hint of a Li-driven Fe3/4+ transition has been detected in LiFePO4F.

Valence band tunneling model for charge transfer of redox-active molecules attached to n- and p-silicon substrates

In this work, monolayers of the redox-active molecules, with cationic- accessible states, were incorporated on p- and n-type silicons of varying doping concentrations. The redox voltages and kinetics were found to be strongly dependent on the silicon doping concentrations, and ambient light in case of n-Si substrate, while there was no significant impact of substrate doping concentration or ambient light in case of p-Si substrate. These results suggest the redox energy states in the molecule align within the valence band of the silicon substrate. Based on this, a model for electronic coupling and charge transfer at the molecule-semiconductor interfaces is proposed.

Preparation of lithium manganese oxides containing iron

Preparation of LiFeO 2–Li 2MnO 3 solid solution has been attempted using hydrothermal and solid state reactions. Three 10% Fe-doped Li 2MnO 3 samples (Fe/(Fe+Mn)=0.1) with different average particle sizes could be obtained by both methods. The initial charge capacities for these cathodes were sensitive to the preparation method; the capacity up to 4.3 V is 45 mAh/g for the hydrothermally-obtained sample, 24 mAh/g for the post-annealed sample after hydrothermal-treatment at 673 K, and 6 mAh/g for the sample obtained by solid state reaction at 1173 K. 57 Fe Mössbauer spectra detect the oxidation of Fe 3+ to Fe 4+/reduction of Fe 4+ to Fe 3+ after initial charging/discharging for the hydrothermally obtained sample. The observed Fe 3+/4+ redox voltage is between 4.0 and 4.5 V which is lower than that predicted (5 V) for Fe-doped LiMn 2O 4. This suggests that hydrothermally synthesized Fe-doped Li 2MnO 3 is an environmentally–friendly candidate as cathode material, involving a Fe 3+/4+ redox reaction for rechargeable lithium batteries.

Li-insertion Behavior in NanocrystallineTiO2-(MoO3)z Core-Shell Materials

ABSTRACTA fundamental study of the Li insertion behavior of a series of materials consisting of a TiO2 core having MoO3 on the surface has been carried out in order to determine the influence of the shell. These TiO2-(MoO3)z materials, where (z) denotes the fraction of coverage from a partial to a double layer, range in diameter from 40-100 Å. Calculations have been done on their theoretical lithium capacity using a maximum of Li0.5TiO2 for the core, and Li1.5MoO3 at the TiO2/MoO3 interface, and they have been compared to that found experimentally. The reversible Li-insertion capacity was shown to increase from 0.34 per Ti for the pure TiO2 sample, to 0.91 Li per transition metal when the MoO3 coverage increased to one monolayer. There was only one plateau observed in the electrochemical scans for the samples showing that they function as a single-phase material making them interesting for electrodes. The redox voltage of the TiO2/Li0.5TiO2 biphasic transformation increased 60 mV from the pure TiO2 to the sample containing one monolayer of MoO3. This effect was interpreted as due to a change in TiO2 surface charge coming from an inductive effect of Ti-O-Mo bonds.