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Valve regulated lead acid (VRLA) battery constitutes towards the largest part of the worldwide secondary battery market share. Indisputably, absorptive glass mat (AGM) is a key component in a VRLA battery that is often engineered utilizing the synergy that exists between fiber and structural parameters. Over the past few decades, a considerable progress has been made in designing and tailoring the AGM separators but often limited by ‘trial-and-error’ approach. Here in, we present a comprehensive review that specifically targets the design of AGM separators by bridging a gap between fiber-structure-property relationships. The critical review on various experimental and theoretical studies aims to present a template for ‘designer’ AGM separator to fulfill the demands of any application of VRLA batteries. The review surveys important functions and various properties of AGM separators, which have been related with a key set of fiber and structural parameters. Several challenges presented in the form of failure modes of the VRLA battery can be overcome by leveraging the synergy between key set of fiber and structural parameters. This review also highlights the structural commonalities between AGM separators and the other nonwoven materials. This opens up an opportunity to utilize the existing theoretical models of nonwovens to the AGM separators.

Valve regulated lead acid (VRLA) batteries are traditionally classified on the basis of gel and absorptive glass mat (AGM) separators. To fulfill the desired functions of AGM batteries, a key design feature of the separator relies on the uptake of the electrolyte in shortest transport time. Herein, we present a three-dimensional (3D) analytical model to predict the fastest electrolyte uptake in AGM separators based upon the optimal set of fiber and structural parameters. The predictive model has utilized 3D data of fiber orientation in AGM separators, obtained via X-ray micro-computed tomography analysis. Such realistic structural information of AGM has assisted in simulating the separators made up of cheaper coarser glass fibers, which was subsequently benchmarked with the experimental samples consisting of finer fibers for attaining the fastest electrolyte uptake. Through theoretical modeling, a design criterion has successfully evolved for the fastest electrolyte uptake by mapping the key effects of the fiber diameter, 3D fiber orientation distribution and porosity of AGM separators. In general, high-density AGM separators comprising of preferentially aligned coarser fibers tend to attain the fastest electrolyte uptake.

Needlepunched nonwovens represent a special class of three-dimensional (3D) fiber networks that successfully generated a host of applications but the rationale of their design involving the intricate microstructure and understanding of the mechanics of the relevant deformation is still lacking. Herein, the micromechanical behavior of needlepunched nonwoven materials possessing out-of-plane auxetic characteristics has been investigated by unraveling the anisotropic structure. The evolution of the 3D intricate architecture of anisotropic nonwoven materials under prescribed levels of tensile strain has been determined using a 3D optical profiler. Most importantly, the analytical models expressed in terms of the initial modulus and in-plane Poisson's ratio of the nonwoven materials are presented. A comparison between the theory and experiments revealed a satisfactory agreement for the modulus and in-plane Poisson's ratio. Further, a comparative analysis of well-known analytical models of in-plane Poisson's ratio with the experimental data clearly demonstrated that the plausible mechanism of deformation of fibers is bending and not stretching.

Self-similar materials can assist in the transition of hydrophobic to hydrophilic state in a well-controlled manner. Herein, we report an ensemble of carboxylic functionalized carbon nanotubes (CNTs) decorated on the self-similar nonwoven surface using a facile, scalable and inexpensive vacuum filtration process for tunable surface wettability. Increasing the amount of CNTs combined with the vacuum assisted nonwoven material reduced the apparent equilibrium contact angle systematically. A simple analytical model has been proposed to predict the apparent equilibrium contact angle by formulating a direct relationship with the structural parameters of CNTs and nonwoven materials. In general, a satisfactory agreement was observed between the theoretical and experimental apparent equilibrium contact angles of CNT decorated nonwoven materials.

Absorptive glass mat (AGM) separator is a vital technical component in valve regulated lead acid (VRLA) batteries that can be tailored for a desired application. To selectively design and tailor the AGM separator, the intricate three-dimensional (3D) structure needs to be unraveled. Herein, a toolkit of 3D analytical models of pore size distribution and electrolyte uptake expressed via wicking characteristics of AGM separators under unconfined and confined states is presented. 3D data of fiber orientation distributions obtained previously through X-ray micro-computed tomography (microCT) analysis are used as key set of input parameters. The predictive ability of pore size distribution model is assessed through the commonly used experimental set-up that usually apply high level of compressive stresses. Further, the existing analytical model of wicking characteristics of AGM separators has been extended to account for 3D characteristics, and subsequently, compared with the experimental results. A good agreement between the theory and experiments pave the way to simulate the realistic charge-discharge modes of the battery by applying cyclic loading condition. A threshold criterion describing the invariant behavior of pore size and wicking characteristics in terms of maximum permissible limit of key structural parameters during charge-discharge mode of the battery has also been proposed.

Absorptive glass mat (AGM) separators are widely used for valve regulated lead acid (VRLA) batteries due to their remarkable fiber and structural characteristics. Discharge performance and recharge ...

The valve regulated lead acid (VRLA) battery is a predominant electrochemical storage system that stores energy in a cheap, reliable and recyclable manner for innumerable applications. The absorptive glass mat (AGM) separator is a key component, which is pivotal for the successful functioning of the VRLA battery. Herein, the intricate three-dimensional (3D) porous structure of AGM separators has been unveiled using X-ray micro-computed tomography (microCT) analysis. X-ray microCT has quantified a variety of fiber and structural parameters including fiber orientation, porosity, tortuosity, pore size distribution, pore interconnectivity and pore volume distribution. A predictive model of hydraulic tortuosity has been developed based upon some of these fiber and structural parameters. Moreover, the pore size distribution extracted via X-ray microCT analysis has served as a benchmark for making a comparison with the existing analytical model of the pore size distribution of AGM separators. Pore size distributions obtained via the existing analytical model and through X-ray microCT analysis are in close agreement.

Fiber orientation is a key parameter affecting the geometrical, hydraulic and mechanical properties of nonwoven materials. The effect of fiber orientation on the pore size has been experimentally investigated based on air-laid, parallel-laid, and cross-laid structures following through-air bonding. It was evident that there is a discernible difference between the mean flow and maximum pore sizes of these nonwoven materials. The influence on pore size was further elucidated by evaluating experimental and theoretical models based on sieving-percolation pore network theory including a model that incorporates directional parameter to account for the effect of fiber orientation. It was established that good agreement with experimental data can be obtained using such a model. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010