Yossi Gofer

LiMnPO4 as an Advanced Cathode Material for Rechargeable Lithium Batteries

LiMnPO4 nanoparticles synthesized by the polyol method were examined as a cathode material for advanced Li-ion batteries. The structure, surface morphology, and performance were characterized by X-ray diffraction, high resolution scanning electron microscopy, high resolution transmission electron microscopy, Raman, Fourier transform IR, and photoelectron spectroscopies, and standard electrochemical techniques. A stable reversible capacity up to 145 mAh g(-1) could be measured at discharge potentials > 4 V vs Li/Li+, with a reasonable capacity retention during prolonged charge/discharge cycling. The rate capability of the LiMnPO4 electrodes studied herein was higher than that of LiNi0.5Mn0.5O2 and LiNi0.8Co0.15Al0.05O2 (NCA) in similar experiments and measurements. The active mass studied herein seems to be the least surface reactive in alkyl carbonate/LiPF6 solutions. We attribute the low surface activity of this material, compared to the lithiated transition-metal oxides that are examined and used as cathode materials for Li-ion batteries, to the relatively low basicity and nucleophilicity of the oxygen atoms in the olivine compounds. The thermal stability of the LiMnPO4 material in solutions (measured by differential scanning calorimetry) is much higher compared to that of transition-metal oxide cathodes. This is demonstrated herein by a comparison with NCA electrodes

The High Performance of Crystal Water Containing Manganese Birnessite Cathodes for Magnesium Batteries.

Rechargeable magnesium batteries have lately received great attention for large-scale energy storage systems due to their high volumetric capacities, low materials cost, and safe characteristic. However, the bivalency of Mg(2+) ions has made it challenging to find cathode materials operating at high voltages with decent (de)intercalation kinetics. In an effort to overcome this challenge, we adopt an unconventional approach of engaging crystal water in the layered structure of Birnessite MnO2 because the crystal water can effectively screen electrostatic interactions between Mg(2+) ions and the host anions. The crucial role of the crystal water was revealed by directly visualizing its presence and dynamic rearrangement using scanning transmission electron microscopy (STEM). Moreover, the importance of lowering desolvation energy penalty at the cathode-electrolyte interface was elucidated by working with water containing nonaqueous electrolytes. In aqueous electrolytes, the decreased interfacial energy penalty by hydration of Mg(2+) allows Birnessite MnO2 to achieve a large reversible capacity (231.1 mAh g(-1)) at high operating voltage (2.8 V vs Mg/Mg(2+)) with excellent cycle life (62.5% retention after 10000 cycles), unveiling the importance of effective charge shielding in the host and facile Mg(2+) ions transfer through the cathodes interface.

Structural Analysis of Electrolyte Solutions for Rechargeable Mg Batteries by Stereoscopic Means and DFT Calculations

We present a rigorous analysis of unique, wide electrochemical window solutions for rechargeable magnesium batteries, based on aromatic ligands containing organometallic complexes. These solutions are comprised of the transmetalation reaction products of Ph(x)MgCl(2-x) and Ph(y)AlCl(3-y) in different proportions, in THF. In principle, these reactions involve the exchange of ligands between the magnesium and the aluminum based compounds, forming ionic species and neutral molecules, such as Mg(2)Cl(3)(+)·6THF, MgCl(2)·4THF, and Ph(y)AlCl(4-y)(-) (y = 0-4). The identification of the equilibrium species in the solutions is carried out by a combination of Raman spectroscopy, multinuclear NMR, and single-crystal XRD analyses. The association of the spectroscopic results with explicit identifiable species is supported by spectral analyses of specially synthesized reference compounds and DFT quantum-mechanical calculations. The correlation between the identified solution equilibrium species and the electrochemical anodic stability window is investigated. This study advances both development of new nonaqueous solution chemistry and possible development of high-energy density rechargeable Mg batteries.

Exceptional Electrochemical Performance of Si-Nanowires in 1,3-Dioxolane Solutions: A Surface Chemical Investigation

The effect of 1,3-dioxolane (DOL) based electrolyte solutions (DOL/LiTFSI and DOL/LiTFSI-LiNO(3)) on the electrochemical performance and surface chemistry of silicon nanowire (SiNW) anodes was systematically investigated. SiNWs exhibited an exceptional electrochemical performance in DOL solutions in contrast to standard alkyl carbonate solutions (EC-DMC/LiPF(6)). Reduced irreversible capacity losses, enhanced and stable reversible capacities over prolonged cycling, and lower impedance were identified with DOL solutions. After 1000 charge-discharge cycles (at 60 °C and a 6 C rate), SiNWs in DOL/LiTFSI-LiNO(3) solution exhibited a reversible capacity of 1275 mAh/g, whereas only 575 and 20 mAh/g were identified in DOL/LiTFSI and EC-DMC solutions, respectively. Transmission electron microscopy (TEM) studies demonstrated the complete and uniform lithiation of SiNWs in DOL-based electrolyte solutions and incomplete, nonuniform lithiation in EC-DMC solutions. In addition, the formation of compact and uniform surface films on SiNWs cycled in DOL-based electrolyte solutions was identified by scanning electron microscopic (SEM) imaging, while the surface films formed in EC-DMC based solutions were thick and nonuniform. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy were employed to analyze the surface chemistry of SiNWs cycled in EC-DMC and DOL based electrolyte solutions. The distinctive surface chemistry of SiNWs cycled in DOL based electrolyte solutions was found to be responsible for their enhanced electrochemical performances.

LiPF3 ( CF 2 CF 3 ) 3 : A Salt for Rechargeable Lithium Ion Batteries

LiPF 3 (CF 2 CF 3 ), from Merck KGaA (LiFAP) was tested as a new electrolyte for Li-ion batteries that can replace the commonly used LiPF 6 . The latter salt is known to be unstable, to decompose thermally to LiF and PF 5 , and to readily undergo hydrolysis with protic species to form HF contamination in solutions. The latter contamination may have a detrimental impact on the performance of both anodes and cathodes for Li-ion batteries. Solutions comprising LiFAP, LiPF 6 , and LiN(SO 2 CF 5 CF 3 ) 2 (LiBETI) in mixtures of ethylene, dimethyl, and diethyl carbonates were tested with composite graphite and LiMn 2 O 4 electrodes. The tools for this study included voltammetry (fast and slow scan rates), chronopotentiometry, impedance spectroscopy, Fourier transform infrared, and X-ray and photoelectron spectroscopies. It was found that LiFAP is superior to LiPF 6 as an electrolyte for both graphite anodes and LiMn 5 O 4 cathodes. This should be attributed to the different surface chemistry developed on these electrodes when LiPF 6 is replaced by LiFAP. An important impact of such a replacement is probably the absence of possible pronounced HF contamination in LiFAP solutions.

Solutions of LiAsF6 in 1,3-dioxolane for secondary lithium batteries

Abstract Highly stable solutions of 1,3-dioxolane (DN) with LiClO 4 or LiAsF 6 may be prepared by the use of tertiary amine additives. Very high Li-cycling efficiency is obtained with stabilized LiAsF 6 /DN solutions. These electrolytes can be further improved by addition of alkyl carbonates as cosolvents. The correlation between Li-cycling efficiency and Li-surface chemistry in these systems was investigated using surface sensitive Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray microanalysis techniques.

Basic electroanalytical characterization of lithium insertion into thin, well-crystallized V2O5 films

Slow-scan rate cyclic voltammetry (SSCV), potentiostatic intermittent titration (PITT) and electrochemical impedance spectroscopy (EIS) have been applied simultaneously to study Li ion intercalation into V2O5 films prepared by evaporative vacuum-deposition on Pt foils. Two different films, 1600 and 3600 A thick, were used to study the influence of the films thickness on the major electroanalytical characteristics of these intercalation electrodes. Modeling of the impedance spectrum related to the thin V2O5 film was performed using an equivalent circuit analog including the following elements: three R ∣∣ C semicircles (covering the high-frequency domain) and finite-length Warburg in sequence with the intercalation capacity (a straight line of unit slope at intermediate frequencies, and a sloping capacitive line at the very low frequencies). Sharp minima on D versus E plots, which are observed in the vicinity of the cyclic voltammetric peaks, present further evidence of very high, attractive electron–ion interactions during Li ion intercalation into the V2O5 electrode, as was already described for similar processes in graphite and some transition metal oxides: LixCoO2, LixNiO2, LixCoyNi1−yO2 and LixMn2O4. The diffusion length in these electrodes related to the V2O5 films thickness.

On the Electrochemical Behavior of Aluminum Electrodes in Nonaqueous Electrolyte Solutions of Lithium Salts

We studied the electrochemical behavior of aluminum electrodes in solutions comprising ethylene carbonate (EC)-dimethyl carbonate (DMC) and lithium salts: lithium hexafluorophospate (LiPF 6 ), lithium perchlorate (LiClO 4 ), or lithium bis(oxalato)borate (LiBOB). Under anodic polarization within the potential range of 3.00-4.00 V in these solutions, aluminum electrodes demonstrate a stable behavior due to their passivation by surface films. Aluminum electrodes passivate in EC-DMC/LiPF 6 and EC-DMC/LiBOB solutions both at 30 and 60°C, whereas these electrodes remain active and corrode in EC-DMC/LiClO 4 solutions. LiBOB may decompose at anodic potentials, thus forming passive films comprising B 2 O 3 and metal-oxalate species on the aluminum electrodes polarized to 4.50-5.30 V. Li 2 CO 3 , LiF and AlPO 4 , and LiCl species were also detected on the electrodes anodically polarized in LiBOB-, LiPF 6 -, and LiClO 4 -containing solutions, respectively. At some conditions, current oscillations can be developed on aluminum electrodes upon their polarization at constant potentials. These oscillations may relate to the successive formation and dissolution of the passivating surface films formed on electrodes. The development of F 2 P(=O)O - species due to the polarization of aluminum electrodes in EC-DMC/LiPF 6 solutions was confirmed by solution NMR studies.

Kinetic and Thermodynamic Studies of Mg2 + and Li + Ion Insertion into the Mo6 S 8 Chevrel Phase

Slow-scan rate cyclic voltammetry (SSCV) and chronopotentiometry were used for a quantitative comparison of the thermodynamic and kinetic characteristics of Li + and Mg 2+ -ion insertion into the Mo 6 S 8 chevrel phase compound. The Li-insertion process consists mainly of three stages with the relative stoichiometries 1:2:1, corresponding to the formation of Li 1 Mo 6 S 8 , Li 3 Mo 6 S 8 , and Li 4 Mo 6 S 8 , respectively. The kinetics of the intercalation is relatively fast. Mg-ion insertion was found to have the stoichiometry 2:2, i.e., Mg 1 Mo 6 S 8 and Mg 2 Mo 6 S 8 are formed. The initial magnesiation and the final demagnesiation of the chevrel phase (Mo 6 S 8 ↔ Mg 1 Mo 6 S 8 ) reveal intrinsically slow kinetics, accompanied by a substantial decrease in the intercalation level. This probably results from a low ionic conductivity of the electrode bulk caused by both small concentration and low mobility of the Mg ion in this potential region, related to the sites that the Mg intercalants occupy in the Mg x Mo 6 S 8 phase. A moderate increase in temperature results in a drastic increase of ion mobility. In Mg(AlCl (4-n) R n ) 2 solution, the difference of the two sequential insertions of Mg ion into the chevrel phase was found to be 0.26 V, i.e., by 0.08 V lower than that for the insertion of Li ion.

On the Study of Electrolyte Solutions for Li-Ion Batteries That Can Work Over a Wide Temperature Range

Based on previous data and an understanding of possible reactions with electrodes, we selected five electrolyte solutions as promising components for Li-ion batteries that can operate down to -40°C, consisting of solutions of LiPF 6 or LiTFSI electrolytes in optimized ternary carbonate solvent mixtures and quaternary solvent mixtures containing esters, both with and without vinylene carbonate and LiBOB salt as additives. The main criteria for selecting these solutions were a specific conductivity ≥1 mS/cm at -40°C and the ability to work well with a wide variety of electrode materials (for example, transition metal oxides and phosphoolivine cathodes, lithiated titanium oxide and carbon anodes) over a temperature range of -40 to + 60°C. As a first step in the selection of battery materials for operation at low temperatures, we focused our work on the negative electrodes and tested three types of graphite electrodes with these electrolyte solutions. In general, practical graphite electrodes can work reasonably well only at temperatures above -20°C. A limited improvement of their low temperature performance can be achieved by increasing the surface area (i.e., decreasing the particle size) of the active material at the expense of high initial irreversible capacity.