Lithium electrode passivation

Other Models. In addition to Besenhard s model, the other models were mainly modifications developed from the original Peled s concept for lithium electrode passivation, with surface reaction as the major process, and emphasis was placed upon the composition and structure of the precipitated film or the interaction between the precipitated products and the bulk electrolyte components. 

On the basis of the results from XPS studies by Kanamura and co-workers that the SEI has a multilayered structure,Peled and co-workers modified their lithium electrode passivation model to include carbonaceous anodes and proposed a so-called mosaic model to describe the SEI structure on the anode, as Figure 15a shows.According to this model, multiple reductive decompositions occur between the negatively charged anode surface and the various electrolyte components simultaneously, depositing a mixture of insoluble products on the anode. This heteropolymicrophase SEI consists of many microregions that are of entirely different chemical... 

The corrosion resistance of lithium electrodes in contact with aprotic organic solvents is due to a particular protective film forming on the electrode surface when it first comes in contact witfi tfie solvent, preventing further interaction of the metal with the solvent. This film thus leads to a certain passivation of lithium, which, however, has the special feature of being efiective only while no current passes through the external circuit. The passive film does not prevent any of the current flow associated with the basic current-generating electrode reaction. The film contains insoluble lithium compounds (oxide, chloride) and products of solvent degradation. Its detailed chemical composition and physicochemical properties depend on the composition of the electrolyte solution and on the various impurity levels in this solution. 

Electrochemical noise studies have also been beneficial in lithium battery research. The lithium electrode sitting in the aprotic electrolyte is covered by a passivating film... 

On the fundamental front, Dahn et al. successfully accounted for the irreversible capacity that accompanies all carbonaceous anodes in the first cycling. They observed that the irreversible capacity around 1.2 V follows an almost linear relation with the surface area of the carbonaceous anodes and that this irreversible process is essentially absent in the following cycles. Therefore, they speculated that a passivation film that resembles the one formed on lithium electrode in nonaqueous electrolyte must also be formed on a carbonaceous electrode via similar electrolyte decompositions, and only because... 

Solvents were initially selected primarily on the basis of the conductivity of their salt solutions, the classical example being propylene carbonate (PC). However, solutions based on PC on its own were soon found to cause poor cyclability of the lithium electrode, due to uncontrolled passivation phenomena. Solvent mixtures or blends were therefore developed and selection currently focuses on a combination of high dielectric solvents (e.g. ethylene carbonate, EC) with an alkyl carbonate (e.g. dimethy(carbonate, DMC), to stabilize the protective passivation film on the lithium electrode, and/or with a low viscosity solvent [e.g. 1,2-dimethoxyethane (DME) or methyl formate (MF)], to ensure adequate conductivity. 

A key technical problem in developing practical lithium batteries has been poor cycle life attributable to the lithium electrode. The highly reactive nature of freshly plated lithium leads to reactions with electrolyte and impurities to form passivating films that electrically isolate the lithium metal. 

The last electrolyte system to be mentioned in connection with lithium electrodes is the room temperature chloroaluminate molten salt. (AlCl3 LiCl l-/ -3/ "-imidazolium chloride. R and R" are alkyl groups, usually methyl and ethyl, respectively.) These ionic liquids were examined by Carlin et al. [227-229] as electrolyte systems for Li batteries. They studied the reversibility of Li deposition-dissolution processes. It appears that lithium electrodes may be stable in these systems, depending on their acidity [227], It is suggested that Li stability in these systems relates to passivation phenomena. However, the surface chemistry of lithium in these systems has not yet been studied. 

The rate of this process in aprotic electrolytes is rather high the exchange current density is fractions to several mA/cm. As pointed out already, the first contact of metallic lithium with electrolyte results in practically the instantaneous formation of a passive film on its surface conventionally denoted as solid electrolyte interphase (SEI). The SEI concept was formulated yet in 1979 and this film still forms the subject of intensive research. The SEI composition and structure depend on the composition of electrolyte, prehistory of the lithium electrode (presence of a passive film formed on it even before contact with electrode), time of contact between lithium and electrolyte. On the whole, SEI consists of the products of reduction of the components of electrolyte. In lithium thionyl chloride cells, the major part of SEI consists of lithium chloride. In cells with organic electrolyte, SEI represents a heterogeneous (mosaic) composition of polymer and salt components lithium carbonates and alkyl carbonates. It is essential that SEI features conductivity by lithium ions, that is, it is solid electrolyte. The SEI thickness is several to tens of nanometers and its composition is often nonuniform a relatively thin compact primary film consisting of mineral material is directly adjacent to the lithium surface and a thicker loose secondary film containing organic components is turned to electrolyte. It is the ohmic resistance of SEI that often determines polarization of the lithium electrode. 

One may then conclude that, the gel-type electrolytes, and the PAN-based ones in particular, have electrochemical properties that in principle make them suitable for application in versatile, high-energy lithium batteries. In practice, their use may be limited by the reactivity towards the lithium electrodes induced by the high content of the liquid component. Indeed, severe passivation phenomenon occurs when the lithium metal electrode is kept in contact with the gel electrolytes [60, 69]. This confirms the general rule that if from one side the wet-like configuration is essential to confer high conductivity to a given polymer electrolyte, from the other it unavoidably affects its interfacial stability with the lithium metal electrode. 

Y. Zhu, Y. Li, M. Bettge, D. P. Abraham, J. Electrochem. Soc. 2012, 159, A2109-A2118. Positive electrode passivation by LiDFOB electrolyte additive in high-capacity lithium-ion cells batteries and energy storage. 

Le Granvalet-Mandni M., Hanrath T, Teeters D. Characterization of the passivation layer at the polymer electrolyte/lithium electrode interface, Sohd State Ionics 2000, 135, 283-290. 

The lithium passivation in liquid electrolyte cells has been extensively investigated and its characteristics - in terms of nature and of growth rate of the passivation film, as well as its effect on the cyclability of the lithium electrode - have been well established. As a result of these studies, it is now clear that the efficiency of the lithium plating-stripping process greatly depends on the nature of the selected electrolyte solution. 

On the contrary, investigations of the lithium interface in polymer electrolyte cells are still scarce and the mechanism of the passivation process has not yet been clarified. Some impedance studies on the reaction occurring at the lithium electrode/PEO-LiX polymer electrolyte interface, as a function of... 

J. Thevenin [1985] Passivating Films on Lithium Electrodes. An Approach by Means of Electrode Impedance Spectroscopy, Journal of Power Sources 14, 45-52. 

Metal lithium is known to be extremely reactive with organic electrolytes and their impurities. The electrolyte becomes passivized immediately when the battery is activated. The SEI is then composed of the products of degradation of the organic solvents, the lithium salt and the impurities that are present. If this passivation layer formed in situ is not a good ionic conductor, the electrode will be blocked and ihe battery will be faulty. Similarly, if this layer is continuously formed, the lithium electrode will gradually be consumed, and will therefore no longer be available to participate in the electrochemical processes. 

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