Towards a fully printable battery : robocast deposition of separators. (2010)

The development of thin batteries has presented several interesting problems which are not seen in traditional battery sizes. As the size of a battery reaches a minimum, the usable capacity of the battery decreases due to the fact that the major constituent of the battery becomes the package and separator. As the size decreases, the volumetric contribution from the package and separator increases. This can result in a reduction of capacity from these types of batteries of nearly all of the available power. The development of a method for directly printing the battery layers, including the package, in place would help to alleviate this problem. The technology used in this paper to directly print battery components is known as robocasting and is capable of direct writing of slurries in complex geometries. This method is also capable of conformally printing on three dimensional surfaces, opening up the possibility of novel batteries based on tailoring battery footprints to conform to the available substrate geometry. Interfacial resistance can also be reduced by using the direct write method. Each layer is printed in place on the battery stack instead of being stacked one at a time. This ensures an intimate contact and seal atmore » every interface within the cell. By limiting the resistance at these interfaces, we effectively help increase the useable capacity of our battery through increase transport capability. We have developed methodology for printing several different separator materials for use in a lithium cell. When combined with a printable cathode comprised of LiFePO{sub 4} (as seen in Figure 1) and a lithium anode, our battery is capable of delivering a theoretical capacity of 170 mAh g{sup -1}. This capacity is diminished by transport phenomena within the cell which limit the transport rate of the lithium ions during the discharge cycle. The material set chosen for the printable separator closely resemble those used in commercially available separators in order to keep the transport rates high within the cell during charge and discharge. In order to evaluate the effect of each layer being printed using the robocasting technique, coin cells using printed separator materials were assembled and cycled vs. Li/Li{sup +}. This allows for the standardization of a test procedure in order to evaluate each layer of a printed cell one layer at a time. A typical charge/discharge curve can be seen in Figure 2 using a printed LiFePO{sub 4} cathode and a printed separator with a commercial Celgard separator. This experiment was run to evaluate the loss in capacity and slowdown of transport within the cell due to the addition of the printed separator. This cell was cycled multiple times and showed a capacity of 75 mAh/g. The ability for this cell to cycle with good capacity indicates that a fully printable separator material is viable for use in a full lithium cell due to the retention of capacity. Most of the fully printed cathode and separator cells exhibit working capacities between 65 and 95 mAh/g up to this point. This capacity should increase as the efficiency of the printed separator increases. The ability to deposit each layer within the cell allows for intimate contact of each layer and ensures for a reduction of interfacial impedance of each layer within the cell. The overall effect of printing multiple layers within the cell will be an overall increase in the ionic conductivity during charge and discharge cycles. Several different polymer membranes have been investigated for use as a printed separator. The disadvantage of using polymer separators or solid electrolyte batteries is that they have relatively low conductivities at room temperature (10{sup -6} - 10{sup -8} S cm{sup -1}). This is orders of magnitude lower than the typically accepted 10{sup -3} S cm{sup -1} needed for proper ionic transport during battery discharge Because of their low conductivity, typical polymer separators such as polyethylene oxide (PEO) have a normal operational temperature well above ambient. At elevated temperature the conductivity of these polymers increases. These polymer membranes are, however, ideal for printable applications due to their ease of fabrication using the robocasting process and their ability to conform to surfaces uniformly. While the ability to print cathodes and separators is advantageous as a technology, several of the components still need to be fully optimized. The overall design for the full printed lithium cell can be seen in Figure 3. The printed cathode and separator will interface with a printed anode and current collector, using the LiFePO{sub 4} cycling to plate out a metallic lithium anode on the current collector during cycling. The ability to print every layer of the cell conformally using the robocasting technique will allow for ultimate flexibility in the application of a printed battery.« less