Mechanism of Zn Insertion into Nanostructured δ‐MnO2: A Nonaqueous Rechargeable Zn Metal Battery

Unlike the more established lithium-ion based energy storage chemistries, the complex intercalation chemistry of multivalent cations in a host lattice is not well understood, especially the relationship between the intercalating species solution chemistry and the prevalence and type of side reactions. Among multivalent metals, a promising model system can be based on nonaqueous Zn ion chemistry. Several examples of these systems support the use of a Zn metal anode, and reversible intercalation cathodes have been reported. This study utilizes a combination of analytical tools to probe the chemistry of a nanostructured δ-MnO2 cathode in association with a nonaqueous acetonitrile−Zn(TFSI)2 electrolyte and a Zn metal anode. As many of the issues related to understanding a multivalent battery relate to the electrolyte−electrode interface, the high surface area of a nanostructured cathode provides a significant interface between the electrolyte and cathode host that maximizes the spectroscopic signal of any side reactions or minor mechanistic pathways. Numerous factors affecting capacity fade and issues associated with the second phase formation including Mn dissolution in heavily cycled Zn/δ-MnO2 cells are presented including dramatic mechanistic differences in the storage mechanism of this couple when compared to similar aqueous electrolytes are noted. T quest for new energy storage technologies for transportation applications is rapidly moving toward high performance, safe, and low cost technologies. Recently, nonaqueous multivalent metal (e.g., Mg, Zn, Ca, and Al) based cells have drawn attention from numerous research groups as a promising advanced energy storage technology due to their higher theoretical volumetric capacity, limited dendrite formation, and lower materials cost. A major obstacle for multivalent systems, however, is the identification of electrolytes that show reversible deposition/dissolution on a metal anode and support reversible intercalation of multivalent ions into a cathode. In the case of nonaqueous Mg or Ca ion systems the electrolyte compatibility issues (e.g., low Coulombic efficiency, a high overpotential, and corrosion) with electrodes and current collectors have held back the development of these systems. Recently developed noncorrosive halogen-free Mg electrolyte is compatible with Mg metal and possesses high anodic stability, which possibly accelerates the research and development of high-voltage cathodes. However, nonaqueous Zn ion chemistry with a reversible intercalation cathode is an exception among multivalent metals with several promising features including highly efficient reversible Zn deposition behavior on a Zn metal anode with a wide electrochemical window, relatively lower activation barrier energy for diffusion in several cathode materials (e.g., NiO2, V2O5, and FePO4), 5 a similar ionic radius to Li and Mg ions, and high volumetric capacity. Considering these intrinsic properties, a nonaqueous Zn system provides an opportunity to delve into the mechanisms of a multivalent-ion based electrochemical cell and develop a better understanding of the interactions between the electrode, solvated species, and electrolyte. Recently developed nonaqueous Zn(II) electrolytes show reversible electrodeposition on a Zn metal anode (≥99% of Coulombic efficiency) and provide wide electrochemical window (up to ∼3.8 V vs Zn/Zn vs a Pt electrode). At the same time, oxide cathode materials that have intrinsically higher voltages (with respect to more commonly studied Received: February 28, 2017 Revised: May 5, 2017 Published: May 8, 2017 Article