(Invited) Smart Separator Integrating Self-Healing, Non-Flammable and Detecting Functions with the Objectives of Making Safer and Durable Battery
Authors: Coustan Laura , Laberty-Robert Christel 
Published: February 2020

Most batteries are composed of two solid, electrochemically active layers called electrodes, separated by a polymer membrane infused with a liquid or gel electrolyte. Improving safety, durability while maintaining performances of batteries are highly desirable for consumer electronics, electrical vehicles and grid energy. Whereas the energy density, power density and cycling life of batteries have been significantly improved in the past two decades, battery safety remains an important and unresolved issue. A high battery specific energy density generally increases the energetic response when the batteries are subjected to abuse. Safety issues have become a major obstacle impeding the large-scale application of high-energy-density batteries.

To ensure good performance, batteries generally operate within a limited range of current density, voltage and temperature.1 However, at an abnormal temperature, typically caused by shorting, overcharging or other abuse conditions, a series of exothermic reactions can be initiated and rapidly propagate to further increase the internal cell temperature and pressure, which results in catastrophic battery explosion and fire.2,3 Commercial batteries are equipped with external pressure release vents and positive temperature coefficient (PTC) resistors on their cases to prevent overpressure and overheating. However, temperature and pressure increases inside cells can occur at much higher speeds than can be detected by these external devices.4 Thus, internal safety strategies are more effective in preventing thermal runaway.

There have been several efforts to design internal functional components to address battery safety issues, including novel separators, electrolyte additives and positive temperature coefficient-modified current collector. Several novel separator approaches, including bilayer or trilayer separators,5,6 thermoresponsive microspheres7 and ceramic particle coating8, are effective in shutting down batteries or improving their thermal tolerance. But, the process is irreversible and the battery is no longer functional afterwards. On the other hand, electroactive polymer separator coatings for overcharge protection have limited operating voltages.9 Additives (for example, flame retardants,10 redox shuttles11) to electrolytes have been also explored. They are called for improvements with the hope of designing one that include wide operating voltage range and cycling stability. Non-flammable or solid-state electrolytes have been proposed with the aim of avoiding volatile solvents, but the battery performance is generally decreased owing to their low ionic conductivity.12,13 PTC-modified current collectors are a promising approach owing to their simplicity and reversible operation.14 But, their application has been limited by low temperature conductivity and a considerable leakage current. Clearly, in spite of efforts made thus far, battery safety remains an important concern, thus calling for new approaches.

In this presentation, we will discuss the different strategies explored in the literature to propose smart separator while keeping good power densities, cycle life and safety. We will also report a strategy based on electrospinning and sol-gel chemistry to design separators exhibiting high performances and low flammable properties, compared to celgard.15 This separator integrates organic/inorganic fibers are organized in a 3-dimensional array, delineating a highly porous network. Because of the chemistry of the fibers, it exhibits high wettability with a large family of solvent, good electrolyte up-take while keeping is initial dimension. Because of the processing flexibility, various functions will be added to the fibers bringing it i) good self-healing capability through quadruple hydrogen bonding, ii) complexing capability and iii) restriction in term of intermediate diffusion through the grafting of quaternary ammonium group.

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