Passive layer formation, anodic

Most recently, Dedryvere and Edstrom et al. conducted the extensive research work on the interfacial mechanisms of Si anode, as shown in Fig. 5.35, including reaction of surface oxide, Li-Si alloying process, and passivation layer formation by XPS [102]. To reveal more depth information of SEI ingredients, they conducted a thorough nondestructive depth-resolved analysis by nsing both soft X-rays (100-800 eV) and hard X-rays (2,000-7,000 eV) from two different synchrotron facilities compared with in-house XPS (1,487 eV). The formation of SEI starts from 0.5 V vs. Li/LP. At the end of discharge (0.01 V vs. Li/LP), a thick SEI layer has formed, and carbon black (284 eV, black shadows) can be observed only at 2,3(K) and 6,900 eV. New carbonaceous species have been identified at the surface, C-0... 

R. N. Methekar, P. W. C. Northrop, K. J. Chen, R. D. Braatz, and V. R. Subramanian,/. Electrochem. Soc., 158, A363 (2011). Kinetic Monte Carlo Simulation of Surface Heterogeneity in Graphite Anodes for Lithium-Ion Batteries Passive Layer Formation. 

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid or polymer electrolytes, the anode is always covered by a passivating layer called the SEI. However, the chemical and electrochemical formation reactions and properties of this layer are as yet not well understood. In this section we discuss the electrode surface and SEI characterizations, film formation reactions (chemical and electrochemical), and other phenomena taking place at the lithium or lithium-alloy anode, and at the Li. C6 anode/electrolyte interface in both liquid and polymer-electrolyte batteries. We focus on the lithium anode but the theoretical considerations are common to all alkali-metal anodes. We address also the initial electrochemical formation steps of the SEI, the role of the solvated-electron rate constant in the selection of SEI-building materials (precursors), and the correlation between SEI properties and battery quality and performance. 

XPS investigations of the composition of the anodically grown passive layer on Ti electrodes were performed by Armstrong and Quinn [123, 124], The formation of a suboxide layer between the underlying Ti metal substrate and the stoichiometric Ti02 on top was demonstrated using XPS, AES and electrochemical techniques. 

Fig. 4 shows a simple phase diagram for a metal (1) covered with a passivating oxide layer (2) contacting the electrolyte (3) with the reactions at the interfaces and the transfer processes across the film. This model is oversimplified. Most passive layers have a multilayer structure, but usually at least one of these partial layers has barrier character for the transfer of cations and anions. Three main reactions have to be distinguished. The corrosion in the passive state involves the transfer of cations from the metal to the oxide, across the oxide and to the electrolyte (reaction 1). It is a matter of a detailed kinetic investigation as to which part of this sequence of reactions is the rate-determining step. The transfer of O2 or OH- from the electrolyte to the film corresponds to film growth or film dissolution if it occurs in the opposite direction (reaction 2). These anions will combine with cations to new oxide at the metal/oxide and the oxide/electrolyte interface. Finally, one has to discuss electron transfer across the layer which is involved especially when cathodic redox processes have to occur to compensate the anodic metal dissolution and film formation (reaction 3). In addition, one has to discuss the formation of complexes of cations at the surface of the passive layer, which may increase their transfer into the electrolyte and thus the corrosion current density (reaction 4). The scheme of Fig. 4 explains the interaction of the partial electrode processes that are linked to each other by the elec-... 

Fig. 25. Oxide thickness d, anodic charge of layer formation Q and inverse capacity 1/C of Cr passivated in 0.5 M H2SO4 for 300 s, do is the minimum layer thickness due to exposure of sputtered Cr to water [66],...

Thus, no passivating layer is formed due to the fact that silicon dissolves in such media. Therefore, the anodic I-V curve of silicon in HF is quite unique and differs from the I-V curves obtained in acidic (HF-free) and alkaline electrolytes. Less attention has been paid to the electrochemistry of silicon in alkaline electrolytes as compared to the study of the electrochemistry of silicon in acidic electrolytes, probably due to the fact that pore formation is observed only in acidic media. In contrast to acidic solutions, silicon dissolution occurs in alkaline solutions under - open circuit potential (OCP). 

The passivating effect is attributed to the formation of an insoluble oxide film on the silicon. The chemical attack by the hot alkaline solution on the silicon does not resume immediately after the anodic bias is removed. Several minutes are required before the passive layer breaks down and chemical attack resumes. A brief cathodic treatment, however, reactivates the silicon immediately. 

Chromates have been widely successful because most paint films are permeable to water. If the rate of moisture permeability is matched to the solubility of the chromate pigment used then enough chromate ions may migrate to the metal surface to initiate and sustain the formation of passive layers [5.51]. The effectiveness of chromates in both the cathodic and anodic areas is simplified and illustrated in Figure 5.11 [5.58]. 

A New Model. The results of the studies on anodic oxide films (see section 5.9 and chapter 3 on passive film and anodic oxides) show that anodic oxide properties (oxidation state, degree of hydration, 0/Si ratio, degree of crystallinity, electronic and ionic conductivities, and etch rate) are a function of the formation field (the applied potential). Also, they vary from the surface to the oxide/silicon interface, which means that they change with time as the layer of oxide near the oxide/silicon interface moves to the surface during the formation and dissolution process. The oxide near the silicon/oxide interface is more disordered in composition and structure than that in the bulk of the oxide film. Also, the degree of disorder depends on the formation field which is a function of thickness and potential. The range of disorder in the oxide stmcture is thus responsible for the variation in the etch rate of the oxide formed at different times during a period of the oscillation. The etch rate of silicon oxides is very sensitive to the stmcture and composition (see Chapter 4). 

The models based on surface passivation suggest that a passive layer, similar to the silicon oxide formed under an anodic potential, exists on (111) silicon at OCP but not on other planes [82, 126, 156]. Instead of oxide, formation of inactive hydration complexes of K+ and OH-has been proposed to block the (111) surface [49, 134]. The fact that the relative etch rates of the different planes vary with the type of solution was attributed to the orientation-dependent adsorption of solvation complexes on the surface. 

C. Chemical modification of the glued surfaces by the formation of passivating layers. The modification technique depends on the nature of the metal. The parts are most often subjected to acid pickling, e.g. aluminum alloys are anodized in sulfuric and chromic acids. It is preferable to anodize aluminum parts in sulfuric acid followed by treatment of the anodic film in a bichromate. There are several methods of pickling carbon and stainless steels, chemical oxidation of magnesium alloys as well as copper and titanium alloys before gluing [4]. 

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