Ella Zinigrad

A short review of failure mechanisms of lithium metal and lithiated graphite anodes in liquid electrolyte solutions

Abstract Li electrodes in any relevant electrolyte solution (i.e., polar aprotic) are covered by surface films of a very complicated structure. It was found that even in cases where the surface films formed on lithium contain elastomers, or where the lithium metal reactivity is reduced by doping with elements such as N, As, Al, Mg, Ca, etc., it is impossible to achieve sufficient passivation with lithium electrodes and liquid solutions. Passivation is considerably worsened when Li electrodes are operated at high rates (especially at high charging, Li deposition rates). Thus, there is no way that rechargeable Li batteries can compete with Li-ion batteries in any application that requires high charging rates (e.g., in powering portable electronic devices). The electrochemical behavior of lithiated graphite electrodes also depends on passivation phenomena. The surface films formed on lithiated graphite are similar to those formed on Li metal in the same solutions. The volume changes of graphite electrodes during Li insertion–deinsertion are small enough to enable their reasonable passivation in a variety of electrolyte solutions. A critical factor that determines the stability of graphite electrodes is their morphology. It was found that the shape of graphite particles plays a key role in their application as active mass in anodes for Li-ion batteries.

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 Study of Electrolyte Solutions Based on Ethylene and Diethyl Carbonates for Rechargeable Li Batteries I . Li Metal Anodes

The behavior of Li electrodes was studied in ethylene and diethyl carbonates (EC-DEC) solutions of LiAsF{sub 6}, LiClO{sub 4}, LiBF{sub 4}, and LiPF{sub 6}. The correlation of the surface chemistry to the interfacial properties, morphology, and Li cycling efficiency was investigated using surface sensitive Fourier transform infrared spectroscopy and impedance spectroscopy, scanning electron microscopy, X-ray energy dispersive microanalysis, and standard electrochemical techniques. The Li surface chemistry is initially dominated by EC reduction to an insoluble species, probably (CH{sub 2}OCO{sub 2}Li){sub 2}. Upon storage, several aging processes may take place, depending on the salt used. Their mechanisms are discussed. Although EC-DEC solutions were found to be adequate for Li ion rechargeable batteries, this work indicates that they are not suitable as electrolyte solutions for batteries with Li metal electrodes. This is mostly because Li electrodes cannot be considered stable in these systems and Li deposition is highly dendritic.

LiMn0.8Fe0.2PO4: An Advanced Cathode Material for Rechargeable Lithium Batteries†

Keywords: cathode materials ; lithium batteries ; nanoparticles ; surface chemistry ; thermal stability ; Performance ; Electrodes Reference EPFL-ARTICLE-159236doi:10.1002/anie.200903587View record in Web of Science Record created on 2010-11-30, modified on 2017-05-12

Electrolyte Solutions with a Wide Electrochemical Window for Rechargeable Magnesium Batteries

Electrolyte solutions for rechargeable Mg batteries were developed, based on reaction products of phenyl magnesium chloride (PhMgCl) Lewis base and Alcl 3 Lewis acid in ethers. The transmetallation of these ligands forms solutions with Mg x Cl + y and AlCl 4-n Ph n - ions as the major ionic species, as analyzed by multinuclei nuclear magnetic resonance spectroscopy. Tetrahydrofuran (THF) solutions of (PhMgCl) 2 -Alcl 3 exhibit optimal properties: highly reversible Mg deposition (100% cycling efficiency) with low overvoltage: <0.2 V and electrochemical windows wider than 3 V. A specific conductivity of 2-5 X 10 -3 Ω -1 cm -1 could be measured between -10 and 30°C for these solutions, similar to that of standard electrolyte solutions for Li batteries. Mg ions intercalate reversibly with Chevrel phase (Mg x Mo 6 S 8 ) cathodes in these solutions. These systems exhibit high thermal stability. The solutions may enable the use of high voltage, high-capacity Mg insertion materials as cathodes and hence open the door for research and development of high-energy density, rechargeable Mg batteries.

Factors Which Limit the Cycle Life of Rechargeable Lithium (Metal) Batteries

Failure mechanisms due to high charging rates of rechargeable lithium batteries comprised of Li metal anodes, cathodes (tunneled structure), and electrolyte solutions based on the combination of 1,3‐dioxolane (DN), , and tributylamine (antipolymerization stabilizer) were explored with the aid of postmortem analysis. It was found that at high charging rates, lithium deposition produces small grains, which are too reactive toward the electrolyte solution, in spite of the excellent passivation of lithium in this solution. In practical batteries such as AA cells with spirally wound configurations, the amount of solution is relatively small, and the solution is spread throughout the battery in a thin layer. Therefore, upon cycling, the Li‐solution reactions deplete the amount of the solution below a critical value, so that only part of the active materials continues to function. This leads to a pronounced increase in the internal resistance of these batteries, which fail as a result of their high impedance and the decrease in the effective working electrodes area. Another failure mechanism relates to the extremely high charge‐discharge current densities developed as the active electrode area decreases. These high currents, developed after prolonged cycling, lead to the formation of dendrites that short‐circuit the battery, thus terminating its life.

A Detailed Investigation of the Thermal Reactions of LiPF6 Solution in Organic Carbonates Using ARC and DSC

The thermal stability of 1 M LiPF 6 solutions in mixtures of ethylene carbonate, diethyl carbonate, and dimethyl carbonate in the temperature range of 40 to 350°C was studied by accelerating rate calorimeters (ARC) and differential scanning calorimeters (DSC). Nuclear magnetic resonance (NMR) was used to analyze the condensed reaction products at different reaction stages. Studies by DSC and pressure measurements during ARC experiments with LiPF 6 solutions detected a gas-releasing endothermic reaction starting at ∼170°C, involving diethyl carbonate which occurs before a number of exothermic reactions, which follow as the temperature increases. Fluoride ions are released and react with the alkyl carbonate molecules both as bases and as nucleophiles. The bulk analysis by NMR shows that HO-CH 2 -CH 2 -OH, FCH 2 CH 2 -OH, F-CH 2 CH 2 -F, and polymer are major products. Gas analysis by NMR and Fourier transform infrared spectroscopy shows PF 5 , CO 2 , CH 3 F, CH 3 CH 2 F, and H 2 O as major gaseous products of the thermal reaction of these solutions.

Attempts to Improve the Behavior of Li Electrodes in Rechargeable Lithium Batteries

In this work we studied properties of modified lithium electrodes in an attempt to improve the high rate performance of rechargeable Li (metal) batteries containing liquid electrolyte solutions. Li (metal)-Li 0.3 MnO 2 AA batteries with solutions containing 1,3-dioxolane (DN), LiAsF 6 , and a basic stabilizer became commercial several years ago but failed to compete with Li-ion battery technology because of a very limited cycle life at high charging rates. The problem relates to intensive reactions between Li deposited at high rates and the electrolyte solutions, which dry the batteries. The lithium-solution reactivity was modified through several approaches. Li anodes doped by Li 3 N, Al, and Mg were tested, as well as solutions containing derivatives of DN that are expected to be less reactive toward lithium than DN. It was concluded that reduction of the Li anode-solution reactivity by these approaches cannot solve the problem, because it is impossible to modify the rough morphology, high surface of lithium electrodes when charging (Li deposition) rates are high (>1 mA/cm 2 ). Since there is no hermetic passivation of any Li surface in liquid electrolyte solutions, the high-surface-area Li deposits react with solution components. Therefore, upon charge-discharge cycling of practical Li (metal) batteries, the electrolyte solution is consumed in these reactions. Hence, the future of Li (metal) rechargeable batteries lies either in the use of solid electrolyte matrices instead of the liquid solutions, or in applications where low charging rates are tolerable.

The use of accelerating rate calorimetry (ARC) for the study of the thermal reactions of Li-ion battery electrolyte solutions

The thermal stability of 1M LiPF6, LiClO4, LiN(SO2CF2CF3)2 (LiBETI) and LiPF3(CF2CF3)3 (LiFAP) solutions in mixtures of ethylene carbonate, diethyl carbonate and dimethyl carbonate in the temperature range 40–350 8C was studied by ARC and DSC. NMR was used to analyze the reaction products at different reaction stages. The least thermally stable are LiClO4 solutions. LiPF3(CF2CF3)3 solutions showed higher thermal stability than LiPF6 solutions. The highest thermal stability was found for LiN(SO2CF2CF3)2 solutions. Studies by DSC and pressure measurements during ARC experiments with LiPF6 and LiFAP solutions detected an endothermic reaction, which occurs before a number of exothermic reactions as the temperature increases. Fluoride ions are formed and react with the alkyl carbonate molecules both as bases and as nucleophiles. # 2003 Elsevier Science B.V. All rights reserved.

Structural and Electrochemical Studies of 3 V Li x MnO2 Cathodes for Rechargeable Li Batteries

X-ray diffraction studies (XRD) were carried out for the investigation of the synthesis and electrochemical reduction of lithiated MnO 2 . The optimal Li:Mn ratio for a heat-treated mixture of LiNO 3 + γ-MnO 2 at 370°C (20 h) with a minimum of impurities, such as γ-β-MnO 2 or spinel, was shown to be equal to 0.33. A combined application of the open-circuit voltage (OCV), slow-scan-rate cyclic voltammetry and XRD measurements was used for the investigation of the intercalation mechanism. The initial compound, Li 0.33 MnO 2 , was shown to undergo only one essential reversible transition during its electrochemical reduction to Li 0.75 MnO 2 , with a voltage plateau appearing around 3 V. It was conclusively demonstrated that both a thermal synthesis in a certain range of Li:Mn ratio and electrochemical reduction upon cycling result in the phase transition from Li 0.3 MnO 2 to Li 0.5 MnO 2 spinel. The characteristic feature of the latter reduction process is that it is essentially irreversible and occurs in a thin surface layer of the initial material. The formation of this thin layer seems to be responsible for a drop in the capacity of practical electrodes during their charge-discharge cycling. A plausible explanation for this effect is discussed.