Understanding the Effect of Polymer Surface Layers on Magnesium Insertion in PEDOT/MnO2 Coaxial Nanowires
Authors: Sahadeo Emily , Henry Hakeem Kimani , Lee Sang Bok 
Published: February 2020

Rechargeable magnesium batteries are a possible alternative technology to lithium-ion batteries. However, there are challenges associated with all parts of magnesium battery systems – anode, cathode, and electrolyte. Regarding cathodes, the divalent Mg ion has slow insertion kinetics in many common metal oxide materials due to strong electrostatic interactions with the host ionic lattice. These materials are desirable due to their high voltage and thus the potential to contribute to a high energy density battery, therefore strategies to aid Mg2+ insertion in metal oxides are an important research focus.

Two strategies to improve Mg2+ insertion include utilizing nanostructured cathode materials as well as introducing water, either in the electrolyte or crystal water within the metal oxide structure. Previous work demonstrated that manganese dioxide (MnO2) nanowires deposited via anodic electrodeposition effectively intercalated Mg ions when water was present in a ratio of 6:1 (H2O:Mg2+) in Mg(ClO4)2/propylene carbonate electrolyte (1). Without water in the electrolyte to solvate the Mg2+ and shield the charge, Mg2+ does not insert into the MnO2. Further, an investigation of the charge storage mechanism for this process determined that a conversion reaction on the MnO2 surface in addition to Mg insertion into the bulk both were dependent on the presence of water (2).

Following this work, there were two additional characteristics of interest regarding MnO2 cathodes cycled in water-containing organic electrolyte: manganese dissolution and electronic conductivity. A previously published one-step co-electrodeposition process of MnO2 and poly(3,4-ethylenedioxythiophene)(PEDOT) provided a tunable structure to test both effects (3). With the PEDOT surface layer, there are two possible advantages: first is confining the MnO2 layer to minimize Mn dissolution, and second is addition of the more electronically conductive PEDOT layer for fast electron transport along the comparatively resistive MnO2 material.

Here, coaxial PEDOT/MnO2 nanowires served as a test structure to investigate how a PEDOT surface layer affected Mg ion insertion into the MnO2 at the core and to develop additional understanding of the properties of MnO2 and how structural alterations could influence the electrochemical reactions. By altering the voltage of electrodeposition, the thickness of the PEDOT layer was varied. Cyclic voltammetry and galvanostatic charge/discharge tests were done to evaluate the Mg insertion capabilities, power performance, and cyclability of coaxial nanowires with PEDOT thicknesses ranging from 10-90 nm. Transmission electron microscopy and scanning electron microscopy were used to monitor structural properties and changes. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to monitor changes in manganese content.

This work will address how PEDOT thickness influences the ability of Mg2+ to insert into MnO2, how it affects Mn dissolution, and how the increased electronic conductivity of PEDOT impacts performance when cycling at higher current densities. These results could help inform better design of future cathode materials for rechargeable Mg battery systems.

1. J. Song, M. Noked, E. Gillette, J. Duay, G. Rubloff and S. B. Lee, Physical Chemistry Chemical Physics, 17, 5256 (2015).

2. E. Sahadeo, J. Song, K. Gaskell, N. Kim, G. Rubloff and S. B. Lee, Physical Chemistry Chemical Physics, 20, 2517 (2018).

3. R. Liu and S. B. Lee, Journal of the American Chemical Society, 130, 2942 (2008).