Battery separators tortuosity

Tye [38] explained that separator tortuosity is a key property determining the transient response of a separator (and batteries are used in a non steady-state mode) steady-state electrical measurements do not reflect the influence of tortuosity. He recommended that the distribution of tortuosity in separators be considered some pores may have less tortuous paths than others. He showed mathematically that separators with identical average tortuosities and porosities can be distinguished by their unsteady-state behavior if they have different distributions of tortuosity. 

One way to achieve some of these goals will be to develop mathematical models that reflect the effects of separator resistance, thickness, pore size, shrinkage, tortuosity, and mechanical strength on the final performance and safety of batteries. The battery separators for tomorrow will demand more than just good insulation and mechanical filtration they will require unique electrochemical properties. 

Battery separators are characterized by numerous properties, including material nature, membrane stractural and functional properties. Material nature includes chemical stability, crystalline structure, hydrophilicity, thermal shrinkage, melting point, M and Mv,/M of polyolefin materials. Structural properties include thickness, porosity, pore size, pore shape, pore tortuosity, and pore distribution. Functional properties include mechanical strength, electrical resistivity, air permeability, thermal shutdown, electrolyte wettability and retention. Many of the above properties are affected with each other and may be in a trade-off relationship. For example, the mechanical strength is affected in opposite manner by the thickness, porosity and permeability, as required by the battery performance. 

The measurement of separator resistance is very important to the art of battery manufacture because of the influence the separator has on electrical performance. Electrical resistance is a more comprehensive measure of permeability than the Gurley number in that the measurement is carried out in the actual electrolyte solution. The ionic resistivity of the porous membrane is essentially the resistivity of the electrolyte that is embedded in the pores of the separator. Typically, a micropo-rous separator, immersed in an electrolyte, has an electrical resistivity about six to seven times that of a comparable volume of electrolyte, which it displaces. It is a function of the membrane s porosity, tortuosity, the resistivity of the electrolyte, the thickness of the membrane, and the extent to which the electrolyte wets the pores of the membrane.The ER of the separator is the true performance indicator of the cell. It describes a predictable voltage loss within the ceU during discharge and allows one to estimate rate limitations. 

Battery performance is partially dependent on the separator and its integrity with other materials. Different battery designs will opt for different separators. Main separator parameters include pore diameter (d), porosity (e), pore tortuosity (q), and thickness Qi). The ionic conductivity of a separator is proportional to the velocity of the solute (v). In turn, the velocity of an electrolyte is related to the pore diameter, porosity, tortuosity, and thickness of the separator, as shown in Equation 12.1 ... 

As a result, larger porosity, smaller tortuosity, and lower air permeability will achieve lower real resistance. Out of safety considerations, t is generally between 100 and 1000 s/100 mL. Some required parameters for traditional separators for lithium-ion batteries are summarized in Table 12.5. 

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