Defence of doctoral thesis: Patrik Isacsson – Material Design for Paper Electrodes: A Papermaking Perspective on Electrode Fabrication

The electrification and digitalization of our society has propelled the demand for energy storage solutions. High-end technologies have been developed to satisfy the requirements of demanding applications, such as electromobility and portable consumer electronics, which also increasingly find markets for less demanding applications. These markets include grid and domestic energy storage, as well as Internet of Things (IoT). However, using high-end technologies for low-end applications is a waste of resources that puts unnecessary stress on the supply lines. Thus, more low-cost cost and environmentally friendly alternative technologies are sought, among which renewable biobased materials derived from agriculture and forestry play a prominent role.

The dominant chemical constituents in plants, cellulose and lignin, exhibit some intriguing electrochemical and colloidal properties. Cellulose has been found to efficiently stabilize various electronic materials, whereas lignin can be used as an electronic material itself. Lignocellulosic materials also open for papermaking as an alternative manufacturing approach. Taking the step to using papermaking methods is, however, a bit far from the technology readiness level, as the vast majority of the research on paper electrodes is based on nanocellulose. The material properties of such nanopapers are indeed extraordinary, but the lack of large-scale production methods for nanopapers is a serious challenge.

To circumvent this obstacle and find a shortcut to the realization of paper electrodes, this thesis has turned to conventional papermaking techniques. Fibres are essentially different to nanofibrils by their difference in size, and the papermaking process requires careful composition of the formulations. Thus, as the research on nanopaper electrodes cannot be directly translated into conventional papermaking techniques, this calls for separate studies on fibre-based systems.

This thesis is based on four separate works carried out by an explorative approach, where different kinds of paper electrodes have been investigated with touchdowns in example applications. Based on these studies, general knowledge has been concluded. This has been summarized by four important aspects for materials design of paper electrodes:

1. Colloidal Systems. The paper electrode formulations exhibit both familiar and unfamiliar colloidal interactions. Established wet-end chemistry including charge balance control and electrostatic interactions remain important in parallel with unconventional behaviours. Exfoliated graphite forms water-stable coatings around pulp fibres and exhibit auto-retention mechanism(s). The conducting polymer system PEDOT:PSS, which can adsorb to chemical pulp fibres, does not exhibit affinity to chemi-thermomechanical pulp.
2. Percolating Networks. Cellulosic fibres constitute an insulative matrix, in which efficient percolating conductive networks must be formed. The way a conducting additive is introduced, as well as the morphology of the additive, is important. Combining conducting polymers with nanocarbons is a promising concept for material-efficient networks. For a filler used as an electrode active material, it is important to acknowledge whether it is electronically conductive or not. A higher amount of conductive additives is required for insulative electrode active materials than for those with internal conductivity.
3. Lignin Electrochemistry. Residual lignin present in softwood pulps, in both mechanical and chemical pulps, is electrochemically active. This can either be wanted or unwanted depending on application. Fines differ from fibres in terms of electrochemical stability and oxidative activity. Substantial competing electrochemical reactions occur, which might be related to the electrochemical stability.
4. Mechanical Properties. Percolating conductive networks require high interconnectivity, which entails a cross-linked structure. This brings increased stiffness to the papers, which can be observed both for exfoliated graphite as a filler as well as for papers impregnated with PEDOT:PSS.

Based on the four aspects described above, prospects for a few paper electrode applications have been reviewed. The prospects are mixed, each with their own challenges and opportunities which requires further research and development. While this thesis can conclude that we have not yet reached the point where paper electrodes can be realized, it certainly paves the way to get there.