The Forgotten Concept of the Three-Phase Boundary

The safety and overall operation of lithium metal batteries depends largely on the prevention of lithium dendrite build. Numerous methods have been proposed for dendrite suppression such as use of external pressure, modification of the nucleation sites for dendrite initiation and the use of solid electrolytes. Each of these approaches shows promise, but neither has led so far to a practical solution. A conceptually different design approach involves the use of high-surface area, 3D structures. Electrodes built on the principle of porous or three-dimensional substrate enable low current densities, while still offering high overall current. The low current density in turn enables long Sands time and prolonged lithium deposition without dendrite build. The entire concept relies on the creation of a high-surface area current collector in direct contact with both the electrolyte and the active material. This is the, often forgotten, principle of the three-phase boundary. The principles of creating large three-phase boundary are used to design the pore structure and to balance the distribution of the electrode components. While the effective design of the three-phase boundary is critical for both charge and discharge reactions in the lithium metal battery, it’s principles are more obvious during the discharge reaction. It is demonstrated how the effective three-phase boundary affects the cell performance. A high-surface area porous silicon current collector is used for the metal silicon anode as well as for the air cathode. The study depicts the fundamental behavior and dependence of lithium dendrite build on the current density at the anode. Cycling of a symmetrical cell demonstrates stable voltage and current, with no capacity fade, i.e., no dendrite build, for C rates below a critical value. Figure 1 depicts stable capacity during cycling, while figure 2 shows reduction in impedance as the cycling progresses. Further suppression in dendrite creation is demonstrated through the geometrical effects of the structure as well as by modifying the nature of the current collector. The application of this 3D structure for the air cathode is also evaluated and modeled.