Impact of Interconnections, Dynamic Conductivity, Pore Size on the Performance of V2O5 Cathode for Lithium Ion Batteries

Abstract

It is well known that a nanostructured electrodes have superior performance over their bulk counter parts. It has been pursued by the battery community to explore different nanostructures to discover critical parameters for designing next generation batteries. Here, we describe a systematic study from 1D array electrode to 3D electrode enabled by nanostructured scaffolds and atomic layer deposition. Nanostructured scaffolds have been used as templates for the fabrication of nanostructured materials for wide range of applications. One of the commonly used scaffolding material is porous anodized aluminum oxide films (AAO). AAO are used for their hexagonally ordered nanoporous structures and the ease of controlling the structural parameters such as the pore diameter and the pore length by altering the anodization conditions. Here we use voltage control to fabricate not only the traditional straight porous AAO templates, for 1D nanotube array, but also interconnected porous AAO templates, 3D interconnected nanotube array. From previous work, by deviating from a constant voltage, which results in straight pores, to lower voltage and back to the initial voltage cause the pores to deviate from their straight structure to interconnecting layer and back to their straight structure allowing controllable insertion of the interconnecting layers throughout the film. Using atomic layer deposition, V2O5 was conformally coated along the walls of these high aspect ratio AAO with sputtering of gold current collector at the top. Using AAO as a template which can be tuned systematically, the resulting nanostructured V2O5 electrodes were examined for Li ion batteries as it is one of the well characterized cathode material. V2O5 can reversibly accommodate up to two Li ions. From the previous study, when V2O5 was cycled within the one Li insertion window, the 3D interconnected nanotubular structure showed superior power capabilities over the 1D nanotubular array. This was due to the interconnections allowed more of the materials closer to the top current collector. As faradaic reactions are much more sensitive to the ohmic loss, this close proximity to the current collector allowed more efficient utilization of the overall electrode. With the two Li ion insertion window, an opposite trend was observed, with interconnected electrode performing worse. As the structure between the one and two Li insertion windows were kept constant, we hypothesized two possibilities. One, because we have more material within the interconnected electrodes, it will require more Li ions than the straight electrodes. However the pore opening is the same which could lead to ion starvation in the interconnected electrodes because the diffusion of the Li ions into these interconnected electrodes could not keep up with the demand. Two, V2O5 with the second lithiation causes a dramatic change the electrochemical properties more severely in the interconnected electrode. To test these hypothesis, different pore diameter electrodes were used. With the larger pores, the electrodes overall performed better but the difference between the interconnected and the straight electrodes could not be explained. Literatures show the electronic conductivity of V2O5 varies dramatically depending on the degree of lithiation. Up to one lithiation, the conductivity of the material slightly increases whereas with the second lithiation the conductivity dramatically drops by two orders of magnitude due to the disruption in the electron conduction mechanism. This could explain the performance difference between the straight and the interconnected electrodes. When the current density is low, the electrons have enough time to travel through the electrode regardless of the structure. When the current density is high, now the conductivity matters. In the interconnected electrode, with one Li insertion, the conductivity of the electrode near the current collector increases with the degree of lithiation allowing better usage of the material farther away from the current collector due to lower ohmic loss of the overpotential needed. However, with two Li insertion the conductivity drops near the current collector which prevents the utilization of the material farther away resulting in lower power performance. Comsol simulations are in progress to better understand the overpotentials and ion depletion in these straight and interconnected electrodes. This work is able to show the need to match the properties of the electroactive material with the properties of the nanostructure for high performance electrodes.