Secondary micropore filling

From the limits of the two stages of pore filling in Table 9.2 we are able to arrive at the tentative ranges of pore size summarized in Table 9.3. However, caution must be exercised in the acceptance of these limits in view of the the complexity and nonrigidity of the pores in activated carbons. Since there appears to be no sharp boundary between the completion of co-operative micropore filling and the beginning of reversible capillary condensation, we cannot at present define the upper limit of secondary micropore filling. 

The limiting dimensions of micropores are difficult to specify exactly, but the concept of micropore filling is especially useful when it is applied to the primary filling of pore space as distinct from the secondary process of capillary condensation in mesopores. 

The filling of these molecular-sized pores, which is associated with the isotherm distortion at very low pip0, has been called primary micropore filling . Wider micro-pores are filled by a secondary , or co-operative, process over a range of higher p/p0 (Sing, 1979). These processes are discussed in some detail in subsequent chapters. 

Lithium secondary batteries can be classified into three types, a liquid type battery using liquid electrolytes, a gel type battery using gel electrolytes mixed with polymer and liquid, and a solid type battery using polymer electrolytes. The types of separators used in different types of secondary lithium batteries are shown in Table 1. The liquid lithium-ion cell uses microporous polyolefin separators while the gel polymer lithium-ion cells either use a PVdF separator (e.g. PLION cells) or PVdF coated microporous polyolefin separators. The PLION cells use PVdF loaded with silica and plasticizer as separator. The microporous structure is formed by removing the plasticizer and then filling with liquid electrolyte. They are also characterized as plasticized electrolyte. In solid polymer lithium-ion cells, the solid electrolyte acts as both electrolyte and separator. 

The experimental data obtained are given in graphs as functions of the relative elongation of the pellets (A l/l) on the adsorption value (ml/ gram) or on degree of filling of the micropores (0), where 0=1 represents the adsorption of water at a relative equilibrium pressure p/p = 0.4—i.e., when the micropores are completely filled and when the capillary condensation in the transitional pores and macropores of the secondary porous structure of the pellets has not yet begun. 

For relatively poorly adsorbed vapors and at small equilibrium relative pressures, the values of the filling factor F are low, differing by orders of magnitude. These values strongly depend on the value of constant B so that in snch cases the value of B is the determining factor for adsorption of these vapors, the development of micropore volume playing a secondary role. 

In order to further elucidate the complex pore filling process in microporous carbons, the empirical Dubinin-Serpinsky (DS) equation (refs. 14-15) was used to assess the influence of polar sites on the shape of the isotherm. This equation was developed from the concept of adsorption of water molecules at uniform high energy primary adsorption centres. Molecules adsorbed on these sites act as secondary adsorption centres via a hydrogen-bonding mechanism. Thus, this model does not refer explicitly to the role played by pore size. The DS equation may be written in its modified form (ref. 15) as ... 

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