Electrolyte - lithium anode interface

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. 

Almost during the entire 1990s. the main interest of the lithium ion research community was focused on electrolyte/anode interfaces while its cathode counterpart was overlooked until various lithium ion systems, especially those based on manganese spinel cathodes, were found to suffer power loss and capacity fade upon prolonged cycling or storage at elevated temperatures. Preliminary diagnostic studies... 

This sharp decline in cell output at subzero temperatures is the combined consequence of the decreased capacity utilization and depressed cell potential at a given drain rate, and the possible causes have been attributed so far, under various conditions, to the retarded ion transport in bulk electrolyte solutions, ° ° - ° ° the increased resistance of the surface films at either the cathode/electrolyte inter-face506,507 Qj. anode/electrolyte interface, the resistance associated with charge-transfer processes at both cathode and anode interfaces, and the retarded diffusion coefficients of lithium ion in lithiated graphite anodes. - The efforts by different research teams have targeted those individual electrolyte-related properties to widen the temperature range of service for lithium ion cells. 

There is by far less information available on the cathode interface than the anode interface. However, the reports appeared recently, which insisted that there is a film on the cathode, which may be called a SFI as well as the anode." Since the oxidative reactions on the cathode cannot immobilize reaction products like the reductive reactions on the anode, the amount of SFI on the cathode is much smaller than that on the anode, as demonstrated in Fig. 4.11. Due to the analytical difficulties, a very few data in the literature report the effects of electrolyte additives on the cathode. It was reported that the addition of VC reduced the interfacial impedance and improved a bit the rate capability. It was speculated that this effect is caused by the polymer formation by VC on the cathode, which suppresses the deposition of lithium fluoride, since this effect disappeared when VC contained polymerization inhibitors such as BHT. This is reasonable because the oxidation potential of VC is lower than those of other carbonate solvents. ... 

Solid Electrolytes. A protected Lithium anode is under development for both primary and secondary batteries that promise much larger capacities. This strategy is illustrated by the Li/seawater primary battery in which a Lithium anode is immersed in a nonaqueous electrolyte, the anolyte, that is separated from seawater contacting a cathode current collector by a Li -ion solid-electrolyte separator. The seawater acts as a liquid cathode. Except for contact with a negative post, the Lithium anode and its anolyte are sealed in a compartment containing a Li -ion solid-electrolyte wall that interfaces the seawater. The anolyte is chemically stable to both the Lithium and the solid electrolyte the solid electrolyte must not be reduced on contact with the Li anode. Moreover, eiflier the seal or the compartment must be compliant to allow for the change in volume of the Lithium on discharge. The seawater is not ccmtained in an open cell, it is contained within a battery in a closed cell. The LF ions from the anode react with water at the cathode current collector ... 

Electrolytes which have been employed with this system are 1 molar LiBp4 in propylene carbonate (PC) and 1 Molar LiAsFg in 1 1 by volume PC-DME. The latter appears to be the electrolyte of choice at present. The addition of CO2 or substances such as dibenzyl carbonate (DBC) or benzyl succinimidyl carbonate have been reported to reduce anode passivation which causes voltage delay and increased DC resistance between 40 and 70% depth of discharge." These materials are believed to operate by lowering the impedance of the solid electrolyte interface (SEI) layer on the surface of the lithium anode. 

The major differences between PEs and liquid electrolytes result from the physical stiffness of the PE. PEs are either hard-to-soft solids, or a combination of solid and molten in phases equilibrium. As a result, wetting and contact problems are to be expected at the Li/PE interface. In addition, the replacement of the native oxide layer covering the lithium, under the OCV conditions, by a newly formed SEI is expected to be a slow process. The SEI is necessary in PE systems in order to prevent the entry of solvated electrons to the electrolyte and to minimize the direct reaction between the lithium anode and the electrolyte. SEI-free Li/PE batteries are not practical. The SEI cannot be a pure polymer, but must consist of thermodynamically stable inorganic reduction products of PE and its impurities. 

The early literature (until 1982) is summarized in Refs. [1] and [2], Hundreds of papers have been published since then (most of them in since 1994) and it is impossible to summarize all of them here. The Proceedings of the conferences mentioned above are good, sources of recent developments though sometimes incomplete. Since the early 1980s new systems have been introduced. The most important of these are lithium-ion batteries (which have lithiated carbonaceous anodes) and polymer-electrolyte batteries. Until 1991 very little was published on the Li/polymer-electrolyte interface [3, 4], The application of the SEI model to Li-PE batteries is ad-... 

Peled s Model Anode/Electrolyte Interface Film. In their proposal of SEI formation on a carbonaceous electrode in nonaqueous electrolytes, Dahn actually adopted Peled s model for lithium s surface and extended it to carbonaceous electrodes. By this model, a two-dimensional passivation film is established via a surface reaction. 

The latter authors used anode and cathode symmetrical cells in EIS analysis in order to simplify the complication that often arises from asymmetrical half-cells so that the contributions from anode/ electrolyte and cathode/electrolyte interfaces could be isolated, and consequently, the temperature-dependences of these components could be established. This is an extension of their earlier work, in which the overall impedances of full lithium ion cells were studied and Ret was identified as the controlling factor. As Figure 68 shows, for each of the two interfaces, Ra dominates the overall impedance in the symmetrical cells as in a full lithium ion cell, indicating that, even at room temperature, the electrodic reaction kinetics at both the cathode and anode surfaces dictate the overall lithium ion chemistry. At lower temperature, this determining role of Ra becomes more pronounced, as Figure 69c shows, in which relative resistance , defined as the ratio of a certain resistance at a specific temperature to that at 20 °C, is used to compare the temperature-dependences of bulk resistance (i b), surface layer resistance Rsi), and i ct- For the convenience of comparison, the temperature-dependence of the ion conductivity measured for the bulk electrolyte is also included in Figure 69 as a benchmark. Apparently, both and Rsi vary with temperature at a similar pace to what ion conductivity adopts, as expected, but a significant deviation was observed in the temperature dependence of R below —10 °C. Thus, one... 

As has been shown in Eigure 68, since the time constants for these two electrochemical components, Rsei and Ra, are comparable at anode/electrolyte and cathode/electrolyte interfaces, respectively, the impedance spectra of a full lithium ion could have similar features in which the higher frequency semicircle corresponds to the surface films on both the anode and the cathode, and the other at lower frequency corresponds to the charge-transfer processes occurring at both the anode and the cathode. ... 

Ionically conducting polymers and their relevance to lithium batteries were mentioned in a previous section. However, there are several developments which contain both ionically conducting materials and other supporting agents which improve both the bulk conductivity of these materials and the properties of the anode (Li)/electrolyte interface in terms of resistivity, passivity, reversibility, and corrosion protection. A typical example is a composite electrolyte system comprised of polyethylene oxide, lithium salt, and A1203 particles dispersed in the polymeric matrices, as demonstrated by Peled et al. [182], By adding alumina particles, a new conduction mechanism is available, which involved surface conductivity of ions on and among the particles. This enhances considerably the overall conductivity of the composite electrolyte system. There are also a number of other reports that demonstrate the potential of these solid electrolyte systems [183],... 

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