Lithium ions transport number

A battery is a transducer that converts chemical energy into electrical energy and vice versa. It contains an anode, a cathode, and an electrolyte. The anode, in the case of a lithium battery, is the source of lithium ions. The cathode is the sink for the lithium ions and is chosen to optimize a number of parameters, discussed below. The electrolyte provides for the separation of ionic transport and electronic transport, and in a perfect battery the lithium ion transport number will be unity in the electrolyte. The cell potential is determined by the difference between the chemical potential of the lithium in the anode and cathode, AG = —EF. 

The lithium polymer battery (LPB), shown schematically in Fig. 7.21, is an all-solid-state system which in its most common form combines a lithium ion conducting polymer separator with two lithium-reversible electrodes. The key component of these LPBs is the polymer electrolyte and extensive work has been devoted to its development. A polymer electrolyte should have (1) a high ionic conductivity (2) a lithium ion transport number approaching unity (to avoid concentration polarization) (3) negligible electronic conductivity (4) high chemical and electrochemical stability with respect to the electrode materials (5) good mechanical stability (6) low cost and (7) a benign chemical composition. 

Polymer Electrolytes. An alternative to the liquid electrolytes is a solid polymer electrolyte (SPE) formed by incorporating lithium salts into polymer matrices and casting into thin films. These can be used as both the electrolyte and separator. These electrolytes have lower ionic conductivities and low lithium-ion transport numbers compared to the liquid electrolytes, but they are less reactive with lithium, which should enhance the safety of the battery. The use of thin polymer films or operation at higher temperatures (60-100°C) compensate in part for the lower conductivity of the polymer film. The solid polymers also offer the advantages of a nonliquid battery and the flexibility of designing thin batteries in a variety of configurations. 

The increasing need for novel electrolytes that have high conductivity, high lithium ion transport number, a wide electrochemical stability window, and are suitable for lithium batteries with safe operation (Armand, 1994) is stimulating the third generation materials to be amorphous to temperatures as low as -40 °C and have conductivity at room temperature of the order of 10 Scm approaching that of Uquid electrolytes. 

Figure 2.3 Basis of a lithium iodide cell (schematic) (a) electrons are liberated at the lithium metal anode and re-enter the cell via the I2/P2PV cathode (b) lithium ion transport across the electrolyte via Li vacancies, to form Lil at the anode. The number of vacancies (Schottky defects) has been grossly exaggerated.

A lithium ion transference number significantly less than 1 is certainly an undesired property, because the resultant overwhelming anion movement and enrichment near electrode surfaces would cause concentration polarization during battery operation, especially when the local viscosity is high (such as in polymer electrolytes), and extra impedance to the ion transport would occur as a consequence at the interfaces. Fortunately, in liquid electrolytes, this polarization factor is not seriously pronounced. 

The mobility of lithium ions in cells based on cation intercalation reactions in clearly a crucial factor in terms of fast and/or deep discharge, energy density, and cycle number. This is especially true for polymer electrolytes. There are numerous techniques available to measure transport... 

Therefore, the temperature dependence of the conductivity of complexes (LiX)o, igy/MEEP (X=CF3C00, SCN, SO3CF3, BF4) were also compared. The highest conductivity was obtained with BF4, and the activation energies for ion transport were found to be similar, suggesting that the mechanism for ion motion is independent on the salt. The lithium transport number, which varies from 0.3 to 0.6, depending on the complexed salt, does not change with concentration. 

It is clear from these relationships that the transport number of an ion is different in different electrolytes. Thus when solutions of, say, potassium chloride and lithium chloride are electrolyzed, the fraction of current carried by the chloride ions is not the same in the two cases. 

In densely packed solids without obvious open channels, the transport number depends upon the defects present, a feature well illustrated by the mostly ionic halides. Lithium halides are characterized by small mobile Li+ ions that usually migrate via vacancies due to Schottky defects and have tc for Li+ close to 1. Similarly, silver halides with Frenkel defects on the cation sublattice have lc for Ag+ close to 1. Barium and lead halides, with very large cations and that contain... 

A number of researchers, ° ° particularly in Japan, have been pursuing the oxides of iron as potential cathode materials for lithium cells. However, materials of the type LiFeOz have shown little ability for lithium removal. A number of other iron compounds have been studied over the years, including FeOCl, ° FePS3, ° KFeSz, and FeSz, but none showed much reversibility. Although metal phosphates have been studied for more than 20 years since the discovery of fast ion transport in NASICON, it is only recently that they have been considered as cathodes or anodes " of lithium batteries. 

New lithium salts used in electrochemistry (e.g., LiPFg, LiCF3S03, LiAsFg, and so on) have much lower melting points and ion transport properties than conventional lithium salts, and they could be considered as ILs. The viscosity values for lithium salts, LiTFA-n, depending on the number of oxyethylene groups in the oligo-ether substituents, are from 370 to 790 cP (303.15 K) [46]. 

According to F. Kohlrausch,28 these anomalous results are not due to the friction between the elementary ions and the water mols., but rather to the friction between the water mols. and complexes—hydrated ions. The absolute velocity of transport of the ions is calculated by dividing the transport numbers by 96540, the electric charge carried by the transported ions. Lithium salts are also strongly ionized in many non-aqueous solns.—e.g. methyl or ethyl alcohol. P. Lenard and co-workers calculate the number, n, of molecules of water combined with the ions in aq. soln., and, r, the radius of the ions ... 

The electrolyte must be a pure ionic conductor, preferably with a high transport number for lithium ions, as an electronic conductivity of the electrolyte would create short-circuit ( leakage ) currents between the electrodes. Both electrodes must have a high electronic conductivity and a sufficient ionic conductivity for lithium. The metal current collectors foils (current collectors) are pure electron conductors that allow only electrons to migrate to the external electric leads to the consumer or charger unit. 

The most conductive material of this class is lithium nitride [1-3, 7,10,13, 22, 242]. Its hexagonal structure (Figure 7.24) contains two types of lithium ion, vhth coordination numbers 2 and 3 only the latter contributes to the transport process and, thus, conductivity perpendicular to the principal axis is two orders of magnitude higher than that parallel to c. Stoichiometric LijN does not contain lithium defects. 

The last few years have witnessed a high level of activity pertaining to the research and development of all-solid, thin-film polymer electrolyte batteries most of these use lithium as the active anode material, polymer-based matrices as solid electrolytes, and insertion compounds as active cathode materials. High-performance prototypes of such batteries stand currently under research, whose trends are expected to include the development of amorphous polymers with very low glass-transition temperatures, mixed polymer electrolytes, and fast-ion conductors in which the cationic transport number approaches unity. 

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