The development of printed electronics has brought with it a demand for printed energy storage devices in order to enable fully printed devices [75]. Printing itself is an attractive manufacturing method for electronic devices due to its cost-effectiveness, high throughput, reduced material wastage, and simple patterning techniques [46]. Furthermore, printing allows for manufacturing on flexible substrates, which enables the manufacturing of flexible electronics and their myriad applications.
A number of printed primary and secondary batteries devices have been reported which use screen printing and gravure printing [98, 10, 64]. In addition, previous work has manufac- tured secondary Zn-MnO2 cells via dispenser printing, flexographic printing, stencil casting,
and doctor blade coating [39, 100, 47, 48]. While dispenser printing allows for control of fine feature geometries through dropwise ink deposition, it is unsuited for scaled up manufactur- ing due to its low throughput and high variability between samples. Flexographic, gravure, and screen printing are easily scalable printing methods but require stricter ink rheologies in order to be compatible with the demands of rollers and screen meshes. While doctor blade coating offers the lowest amount of feature control, its tolerance to a wide range of ink rheologies and high throughput make it an attractive option for printed batteries where deposition of large print areas is necessary.
An ideal printing process would minimize the number of layers and steps involved to re- duce possibilities for contamination or errors in the printing process. Individual inks should therefore be optimized not only for device performance but also for manufacturing. For inks for printed batteries in particular, optimization of ink rheology and surface profile is neces- sary, in addition to performance metrics such as electrical conductivity and electrochemical performance.
Use of the solution cast ionic liquid gel polymer electrolyte is critical to enabling printed cells. Because the low vapor pressure of the ionic liquid enables its use as a plasticizer in the formation of the polymer membrane structure, it can be easily integrated into the printing process, removing the need for separate processing and flushing as required other processing methods.
Chapter 3
Nickel Current Collector and Ball
Milling
This chapter presents findings from ball milling on commercially available nickel powder and the results of adjusting ball milling parameters on particle size and morphology. The goals of this work are
1. to qualitatively identify the effect of each parameter has on the resulting particle size and morphology,
2. to use these findings to develop a printable nickel current collector, and
3. to translate these findings on a metal powder to powders of other compositions Specifically, these results are used to inform process parameters in optimizing electrode compositions, as seen in Chapter 4.
Section 3.1 presents the motivation and the reasoning behind the material selection of nickel for this work. Section 3.2 discusses the theory behind the conductivity of polymer com- posites, principles of planetary ball milling, and methods of sheet resistance for printed films. Section 3.3 discusses the experimental methods and procedures used for this work. Section 3.4 presents the findings from ball milling nickel powder and conductivity measurements of the printed current collectors made from the ball milled powder. Section 3.5 summarizes these findings and suggests future work.
3.1 Motivation
The goal of this work is to characterize the relationship between various ball milling param- eters on the resulting particle morphology as a result of ball milling. This is in order to develop a printable nickel current collector as well as to apply the findings to powders of other materials, ultimately to aid in the optimization of the printed MnO2 cathode.
Nickel powder was investigated as an initial target for ball milling for number of reasons. Being a metal powder, nickel is able to plastically deform prior to fracture and thus allows for significant and observable changes in morphology as a result of ball milling. Compared to stainless steel, nickel has a lower yield stress as well as lower resistivity. While zinc and copper have even lower resistivities, their yield stresses are lower, resulting in excessive deformation and mechanical alloying of the powder to the grinding balls [14]. Similarly, silver and gold offer the same benefits with the same low yield stress disadvantage, but with the added disadvantage of high cost. Aluminum powder was investigated as a possible alternative due to its low resistivity and low cost, but it was removed from consideration due to the fact that aluminum powder is extremely pyrophoric in ambient environments. Table 3.1 summarizes the differences in material properties of these metals.
Table 3.1: Electrophysical properties of candidate metals for a printed current collector [66] Electrical Yield Stress Ultimate Tensile Metal Resistivity [⌦-cm] [MPa] Stress [MPa]
Nickel 6.4e-6 59 317 1199 Aluminum 2.7e-6 10.0 45.0 316 Stainless Steel 7.4e-5 240 550 Zinc 5.92e-6 – 37.0 Copper 1.7e-6 33.3 210 Silver 1.55e-6 – 140 Gold 2.20e-6 – 120
One requirement for current collectors for electrochemical cells is that they be nonreac- tive with the materials in the electrodes and electrolyte in order to remove the presense of any parasitic side reactions that may degrade the cell [73]. While it has not been investi- gated whether nickel is a suitable choice for the ionic liquid system studied in this work, its electrophysical properties make it an excellent candidate to investigate changes in particle morphology via ball milling.