Phase Separating Inks for the Additive Manufacturing of Porous Polymer Composites in Battery Applications

The additive manufacturing of batteries offers great potential for the future of power integration in electronics, freeing up design space and enabling new device capabilities. This is especially true for thin film and flexible devices where current packaging (ex. coin cells, cylindrical cells, and pouches) prevents intimate incorporation of the battery into the device and must be designed around to maintain a device’s flexibility. The ability to fabricate a cell directly onto the device itself can remove this restriction, as the shape and structure of the cell can be tailored to the device, fitting into unique form factors or even being designed to enable deformation. To achieve this, additive manufacturing approaches must be developed to deposit high performance battery component materials and to combine these materials to create high functioning galvanic cells.<br/><br/>One of the great challenges in the additive manufacturing of batteries is developing process compatible materials with the suitable composition and microstructure to enable appropriate performance, particularly porosity and pore structure. Electrodes must be porous to allow for electrolyte penetration and ion conductivity, highly loaded with active material to maximize energy density, and electrically conductive to transport energy to and from the active material. Likewise, separator membranes must be porous for electrolyte penetration and ion conductivity, but these features must be small enough and mechanically robust enough to prevent shorting and dendrite growth. In addition, the processing requirements for these materials must be straight forward to make techniques like 3D printing and coating possible.<br/><br/>We have developed an evaporation induced phase separation technique that utilizes a mixed solvent system to fabricate polymer materials and polymer-particle composites with controlled porous microstructures. The polymer is dissolved in a solvent solution composed of a good and poor solvent for the polymer, chosen such that the good solvent evaporates preferentially to the poor solvent. This leads to an instability in the ink upon drying, thereby causing phase separation into polymer poor and polymer rich phases and creating porosity upon solidification. We have used this process to develop a PVDF and Al₂O₃ composite that functions as a separator membrane for lithium ion batteries and can be deposited by multiple printing and casting techniques. Additionally, we have demonstrated the ability to control the microstructures that form in these polymer-particle composites through the selection of particle size and loading, including creating both foam-like and granular morphologies. When combined with similar composite electrode materials, such as graphite or LiFePO₄ for lithium ion batteries, printed full cells can be achieved. Here, we explore approaches to how we can create fully printed batteries from these materials, as well as how to fabricate them on complex and non-planar substrates to enhance energy storage integration. Furthermore, we examine the challenges faced in controlling microstructure in multi-material and multi-layer additive manufacturing, particularly at the interfaces between different materials and how this can impact performance.