Chemical Deposition of MnO2 Onto Pre-& Post- Processed Porous Substrates for Use in Zn-Based Secondary Battery Application
Authors: Negrete Karla , Poosapati Aswani , Madan Deepa 
Published: May 2020

The electrochemical performance of secondary batteries is heavily contingent upon the structural properties of the layers which compose it. In this work, we report a low energy and time efficient preparation process for development of conformal, nanoscale manganese oxide (MnO2) cathodes generated from a porous substrate to promote ion mobility and embedded with Ketjenblack to promote electrical conductivity of the electrode. MnO2 as an electrode material within secondary battery application has been increasingly investigated, as it serves as a low-cost replacement for other metal oxides. However due to the intrinsic poor electrical conductivity of MnO2, 10-8 S/cm at room temperature, it is necessary to produce this electrode material with critical architecture and enhanced conductivity (transport of electrons). These qualities are vital to cathode performance in battery applications.

For this reason, our work is based on the execution of an adapted energy efficient chemical deposition method in cathode preparation to generate nanoscale coatings of MnO2 on the exterior and interior of a porous substrate to obtain a three-dimensional material structure. In this process, chemical deposition is achieved through synthesis of resorcinol, formaldehyde, deionized water, and sodium permanganate onto Durapore membrane substrates. The latter results in a three-dimensional porous cathode material with high reactive surface area for higher capacity utilization. Extensive incorporation of vacuum infiltration is utilized in this preparation process to enforce proper integration of MnO2 within the three-dimensional pore structure of the material. In an effort to promote the electrical conductivity of the cathode material, this work heavily investigates the role of Ketjenblack in conjunction with the aforementioned chemical deposition process. A Ketjenblack study is thus explored with the porous substrates in five varied manual rubbing and soaking techniques, either at pre-processing or post-processing chemical deposition stages, to observe the best incorporation of the conductive additive. Where studied samples are organized as pre-rubbed (C1), pre-soaked (C2), control (C3), post-rubbed (C4), and post-soaked (C5).

From preliminary results, it was observed that the addition of Ketjenblack, either pre- or post- processed, reduced the resistance of the cathode material by two to three orders of magnitude. In order to test these cathodes, chitosan-based flexible electrolytes with 6M KOH (CPK), 1M ZnCl2 (CPZC), and 1M ZnSO4 (CPZS), with respective best ionic conductivities of 0.916 S/cm, 0.004913 S/cm, and 0.01918 S/cm, were utilized from a previous study. The oxidation/reduction peaks for each electrolyte and cathode variant configuration [Zn/ CPK or CPZS or CPZC/ C1 or C2 or...C5] were obtained from cyclic voltammetry tests and analyzed for performance measurements. Considerable amounts of oxidation and reduction peaks were observed for tested CPK and CPZS samples, however no significant peaks were detected among configurations which utilized CPZC electrolyte samples. Of the CPK samples tested against cathode variants (C1...C5), the best anodic and cathodic current densities of 0.01 mA/cm2 and 0.005 mA/cm2 were observed for [Zn/CPK/C1] configuration with a voltage window of 1.2V. Similarly, among the CPZS samples tested against cathode variants (C1...C5), it was obtained that [Zn/CPZS/C2] configuration had the best anodic and cathodic current densities of 0.001 mA/cm2 and 0.007 mA/cm2 with a voltage window of 2V. Other configurations either had lower current density peaks or working voltage window compared to the best obtained. Although currently we observe CPZS based configurations to have lower densities in comparison to CPK based ones, we expect CPZS configuration to have higher cyclability due to Zn2+ ions availability for intercalation. Further analysis of the best performed configurations will continue to be explored through full cell battery testing to examine the latter.