Effect of Percolation on the Capacitance of Supercapacitor Electrodes Prepared from Composites of Manganese Dioxide Nanoplatelets and Carbon Nanotubes

Abstract

Here we demonstrate significant improvements in the performance of supercapacitor electrodes based on 2D MnO2 nano-platelets by the addition of carbon nanotubes. Electrodes based on MnO2 nano-platelets do not display high areal capacitance because the electrical properties of such films are poor, limiting the transport of charge between redox sites and the external circuit.In addition, the mechanical strength is low, limiting the achievable electrode thickness, even in the presence of binders. By adding carbon nanotubes to the MnO2-based electrodes, we have increased the conductivity by up to eight orders of magnitude, in line with percolation theory.The nanotube network facilitates charge transport, resulting in large increases in capacitance, especially at high rates, around 1 V/s. The increase in MnO2 specific capacitance scaled with nanotube content in a manner fully consistent with percolation theory. Importantly, the mechanical robustness was significantly enhanced, allowing the fabrication of electrodes that were 10 times thicker than could be achieved in MnO2-only films. This resulted in composite films with areal capacitances up to 40 times higher than could be achieved with MnO2-only electrodes.

Electrical, Mechanical, and Capacity Percolation Leads to High-Performance MoS2/Nanotube Composite Lithium Ion Battery Electrodes

“…This value of 0 is consistent with other percolation studies, 35,48 but also with measurements on nanotubes films showing conductivities of ~10 5 S/m are generally achieved. 55 The percolation threshold is consistent with theory which predicts ϕc,e to be approximately given by the ratio of mean nanotube length to diameter.…”

“…Recently, it has been shown that when nanotubes are added to low conductivity MnO2 supercapacitor electrodes, the capacitance increases in a manner which is consistent with percolation theory. 35 By analogy with this result and in line with previous observations for mechanical percolation, 58 we expect the excess capacity (i.e. the capacity increase relative to the MoS2-only anodes) to scale with nanotube volume fraction according to percolation theory:…”

“…Once this has been achieved, further capacity increases will be marginal and the MoS2 will be able to store lithium at its maximum capacity (see ref 35 for more detailed discussion).…”

“…For the MoS2-only film, we find an-plane conductivity of ~10 -6 S/m, close to that reported for similar systems. 35,46,48 On addition of nanotubes, the conductivity increases sharply, reaching 1 S/m for 1wt% nanotubes before further increasing to ~10 4 S/m for 20wt% nanotubes.The increase in conductivity when adding conductors to an insulating matrix is usually described via percolation theory. 53 In this framework, at low nanotube loading levels (usually expressed as volume fractions ), the conductors form no conducting path through the composite.…”

“…This allows electrons to be supplied throughout the electrode via the network with only a small portion of the journey through the MoS2. As the nanotube content increases, the fraction of MoS2 in close proximity of the network increases steadily to the point where electrons can be effectively transported from the current collector to every lithium storage site.Once this has been achieved, further capacity increases will be marginal and the MoS2 will be able to store lithium at its maximum capacity (see ref 35 for more detailed discussion). …”