Spirally wound batteries, separators

This section reviews the state-of-the-art in battery separator technology for lithium-ion cells, with a focus on separators for spirally wound batteries in particular, button cells are not considered. 

To further reduce weight and improve energy density, several companies are developing thin lead film electrodes in a spiral-wound construction with glass fiber separators. Already on the market for cordless electric tools, this battery technology may eventually be used in electric vehicles. 

The requirements for a battery separator can best be understood in the context of how the separator is used. The conventional process (Fig. 1) for making spirally wound cells involves threading the separator (a) through a winding pin (b). 

The simulated short-circuit test was developed to characterize the response of the separator to a short circuit without the complications of battery electrodes. The separator was spirally wound between lithium foils and placed in an AA-size can. To avoid lithium dendrite formation, an alternating voltage was applied to the cell. The cell current and can temperature were monitored. Figure 6 shows the behavior of Celgard membranes. 

A novel microporous separator using polyolefins has been developed and used extensively in lithium-ion batteries since it is difficult for conventional separator materials to satisfy the characteristics required in lithium-ion batteries. In lithium-ion batteries two layers of separators are sandwiched between positive and negative electrodes and then spirally wound together in cylindrical and prismatic configurations. The pores of the separator are filled with ionically conductive liquid electrolyte. 

Suitable nanofiltration membranes based on the spiral-wound module technique are available from various suppliers they usually consist of a carrier film, a spacer, and the filtration membrane. After separation, the lower phase is heated preferably to ca. 25 C, adjusted to a predetermined pH by addition of acid, and pumped at ca. 30 bar through a battery of pressure tubes suitably arranged in parallel and fitted with spiral-wound modules. 

Such effects can be especially significant in many modem miniature electrochemical devices like sensors or batteries. For example, in a common Li battery, the spirally wound thin electrodes of metal foil are separated by an electrolyte layer of 10 cm order of thickness. The diffusion coefficient of Li in the electrode material is about 10 cm s the path of diffusion being of about 10 cm. Then the characteristic time is estimated as 10 s, that is, less than an hour. Since operation time of a battery (charge or discharge) is several hours, one can suspect that the coupling effects are important. Indeed, here we have the situation familiar to battery researchers— the behaviour of the electroactive materials in standard electrochemical cells with large volume of the electrolyte and in a real battery surrounding is often quite different. Thus, the reliable data oti the properties of electroactive battery materials can be obtained only in real batteries or mock-up cells with similar dimensions. That was established empirically and became a common research practice in the recent years. 

The second design is the spirally wound electrode construction, typically used in sealed portable rechargeable batteries and high-rate primary and rechargeable lithium batteries (Fig. 3.2lb). In this design, the electrodes are prepared as thin strips and then rolled, with a separator in between, into a jelly roll and placed into the cylindrical can. This design emphasizes surface area to enhance high-rate performance, but at the expense of active material and capacity. 

The very first functioning lead-acid battery was presented by Gaston Plante in 1860 spirally wound lead sheets served as electrodes, separated by a layer of felt - the first separator of a lead-acid battery [12]. This assembly in a cylindrical vessel in 10% sulfuric acid had only a low capacity, which prompted Plante to undertake a variety of experiments resulting in many improvements that are still connected with his name. 

The requirements for a battery separator can best be understood in the context of how the separator is used. Currently there are two major designs spirally wound or stacked plate. There are two types of stacked-plate designs, one relying on stack pressure to maintain good interfacial contact between the electrodes and separators, and a second which uses an adhesive to bond the electrodes and separators. The manufacturing processes for both spiral and stack cell designs are reviewed by Brodd and Tagawa [7]. Both processes put stress on the separator. 

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