Manipulating the Nanophase Separation of a Polymer–Salt Microfluid Generates an Advanced In Situ Separator for Component-Integrated Energy Storage Devices

A polymer separator plays a pivotal role in battery safety, overall electrochemical performance, and cell assembly process. Traditional separators are separately produced from the electrodes and dominated by porous polyolefin thin films. In spite of their commercial success, today's separators are facing growing challenges with the increasing demand on the device safety and performance. As an attempt to address this urgent need, here, we propose a concept of in situ separator technology by manipulating the two-dimensional (2D) microfluid nanophase separation (2D-MFPS) of a poly(vinylidene difluoride)/lithium salt solution during drying. Particularly, nanophase separation is effectively regulated by low humidity, salt type, and compositions. For application studies, this 2D-MFPS is directly performed onto commercial electrodes under drying conditions with low humidity to fabricate a high-performance in situ separator with thickness and porous structures comparable to those of commercial Celgard separators. This in situ separator shows superior performance in high-temperature stability and wetting capability to a variety of liquid electrolytes. Finally, pouch cells with this in situ separator technology are successfully assembled with an extremely simplified separator-stacking-free process and demonstrate stable cycle performance due to the well-controlled porous structures and electrode-separator interface.