Solvation liquid electrolytes

The ion solvating polymers have found application mainly in power sources (all-solid lithium batteries, see Fig. 2.19), where polymer electrolytes offer various advantages over liquid electrolyte solutions. 

Reactions (5.5.30) and (5.5.31) proceed prevailingly during intercalation from solid or polymer electrolytes (cf. Section 2.6) or melts. When using common liquid electrolyte solutions, a co-insertion of solvent molecules (and/or intercalation of solvated ions) very often occurs. The usual products of electrochemical intercalation are therefore ternary compounds of a general composition ... 

The ability to conduct ions is the basic function of electrolytes, which would determine how fast the energy stored in electrodes can be delivered. In liquid electrolytes, the transport of ions is realized via a two-step process (1) the solvation and dissociation... 

The first section of this book covers liquids and. solutions at equilibrium. I he subjects discussed Include the thcrmodvnamics of solutions, the structure of liquids, electrolyte solutions, polar solvents, and the spectroscopy of solvation. The next section deals with non-equilibrium properties of solutions and the kinetics of reactions in solutions. In the final section emphasis is placed on fast reactions in solution and femtochemistry. The final three chapters involve important aspects of solutions at interfaces. Fhese include liquids and solutions at interfaces, electrochemical equilibria, and the electrical double layer. Author W. Ronald Fawcett offers sample problems at the end of every chapter. The book contains introductions to thermodynamics, statistical thermodynamics, and chemical kinetics, and the material is arranged in such a way that It may be presented at different levels. Liquids, Solutions, and Interfaces is suitable for senior undergr.iduates and graduate students and will be of interest to analytical chemists, physical chemists, biochemists, and chemical environmental engineers. 

The subject matter in this monograph falls into three general areas. The first of these involves liquids and solutions at equilibrium. These subjects are discussed in chapters 1-5, and include the thermodynamics of solutions, the structure of liquids, electrolyte solutions, polar solvents, and the spectroscopy of solvation. 

The similarity in the ionic transport mechanism in organic liquid electrolytes and solid polymer electrolytes is reflected in the ionic transport numbers measured in the two media. Table 3.5 lists the transport numbers for Li in LiC104 solutions in propylene carbonate (PC) and propylene carbonate/dimethoxy ethane (PC/DME) mixtures [26]. The t+ in PC/LiC104 is 0.28 which increases to between 0.40 and 0.50 with the addition of DME. This increase in t+ in PC/DME mixtures may reflect a change in the solvation characteristics of Li, and/or ionic species present, with the addition of DME. It is then possible that a range of cation transference numbers between 0.2 and 0.6 measured in polymer electrolytes is a reflection of the coordination properties of the particular polymer host with Li" and the nature of the ionic species present. 

In solid polymeric electrolytes, the polymers involved can be considered to be immobile nonaqueous solvents they have the chemical characteristics of typical nonaqueous solvents but are macroscopically immobile, unlike common nonaqueous solvents which are small molecules, free to move long distances. When considered from this viewpoint, one immediately sees that these systems are an exotic extension of nonaqueous solution chemistry, and that the classical chemical studies on the nature of solvation carried out on liquid electrolytes are extremely pertinent. 

Recently, a series of IL electrolytes were tested for their applications in Li-S cells. Traditionally, the TFSI anion dominates the anion part of the ILs for the Li-S electrolytes, while typical cation examples are including the l-butyl-3- methyl-imidazolium (BMIM), l-ethyl-3-methylimidazolium (EMIM), 1-butyl-1-methy Ipyrrolidinium (PYR14), and 1-butyl-1-methylpiperidinium (PiP14) in Fig. 11 [18]. As in the traditional liquid electrolyte systems, the physical properties determine the solubility power charge distribution, polarity, viscosity and so forth. In the IL systems, however, the permittivity is largely independent of the combination of cations and anions, while variation in cations and anions affects the molecular level interactions, type/strengtii, and solvation. Due to unique properties, the ILs were studied as effective liquid electrolytes for the Li-S cells. 

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid electrolytes or PEs, 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 yetnot 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 Q anode/electrolyte interface in both liquid electrolyte and PE 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. 

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. 

Future developments of ionically conductive polymers could certainly be accelerated by more fundamental research on the exact physical mechanisms involved in the transport of ions in those semi-liquid electrolytes. Improving the cationic conductivity of polymer-salt complexes is still a priority, especially at, and lower than, room temperature in this respect the development of new generations of solvating polymers has to be pursued. 

Since the initial discovery of Armand et al. [24], there has been a growing interest in ion-solvating polymers and there has been considerable effort to develop new types of polymer electrolytes. Most of the investigations have enhanced the ionic conductivity to about 10 S/cm. Further improvements could be made, such as decreasing the thickness of the membrane without decreasing its physical strength. However, the stabihty of this material is very low due to the presence of KOH within the matrix. Moreover, the ionic conductivity decreases continuously due to the leaking out of KOH. In reahty, this leads to the same problems as found with liquid electrolyte, such... 

Most of the solid polymer electrolytes used can be classified as follows (1) polymer or gel matrixes swollen with liquid electrolyte solutions (e.g. ethylene carbonate (EC)/PAN/sodium perchlorate (NaC104)) (2) singleion systems in which only one ionic species is mobile within a polymer matrix (e.g. perfiuorosulphonate ionomer Nafion ) (3) solvent-free ion-coupled systems consisting of ion-solvating polymers mixed with salts, so that cations and ions become mobile within the polymer network, e.g. PEO mixed with salts. 

In gel electrolytes, in which a liquid electrolyte is imbibed into a polymer matrix, calculation of the effeetive diffusivity and ionie eonductivity based on the apparent volume fraction of electrolyte in the polymer may be eomplieated by solvation of the polymer by the solvent, inereased tortuosity presented by the polymer, and possible interactions of the ions with solvating groups on the polymer. One way to handle these effects empirically is to treat the tortuosity... 

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