Letter Cite This: J. Phys. Chem. Lett. 2017, 8, 6106−6112
pubs.acs.org/JPCL
Bidirectional Correlation between Mechanics and Electrochemistry of Poly(vinyl alcohol)-Based Gel Polymer Electrolytes Jingzhi Hu,†,§ Tong Li,‡,§ and Bingqing Wei*,†,‡ †
State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Laboratory of Graphene (NPU), Xi’an 710072, China ‡ Department of Mechanical Engineering, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *
ABSTRACT: The electrochemical−mechanical coupling property of solid electrolyte membranes is critical to improving the performance of solid-state energy storage devices. A new phenomenon was observed in which the electrochemical charge−discharge process induced aligned wrinkles on the edge of poly(vinyl alcohol)-H2SO4 gel polymer electrolytes (GPEs), which is attributed to the deformation of polymer chains under electrochemical stimulation according to multiscale simulations. In the reverse direction, by means of modeling and testing, it was proved that the ionic conductivity of GPEs can be tuned by mediating the mechanical properties of GPEs via tailoring the polymer at the nanoscale. This bidirectional correlation reveals the coupling mechanisms between mechanical and electrochemical properties of GPEs and provides an insightful understanding of the origin and regulation of the ionic conductivity of GPEs, which is fundamental to improving the performance of GPEs.
A
methacrylate) (PMMA), and poly(ether ether ketone) (PEEK). Among these polymers, PVA is the most extensively employed GPE because of its excellent mechanical, chemical, and thermal stability, in particular as an excellent ionic conductor for the development of solid-state and flexible/stretchable supercapacitors.14 Wada et al. developed an acidic gel electrolyte of PVA-H2SO4 whose ionic conductivity is close to that of an aqueous sulfuric acid.15 This strategy became one of the most popular methods for preparation of GPEs in the assembly of solid-state supercapacitors.16,17 As the essential characteristic of and electrolyte, many contributions have been made to improve the ionic conductivity of GPEs, including the selection of proper aqueous solutions18 and the addition of nanofillers19 or redox couples.20 However, at the same time as massive material and strategy screening, the fundamental understanding of ion-conducting mechanisms in GPEs is still immature. A proton-conducting theory has been introduced to explain the ionic conductivity mechanisms,21 in which the protons (cation) transfer in GPEs through the Grotthuss mechanism,22 vehicle mechanism, or segmental motions23 under different conditions, while the anions are attached to the polymer chains and immobilized.24 This theory has difficulty explaining the performance of GPEs in supercapacitors, especially for electric double-layer capacitors, in which the capacitance is contributed by both cations and anions.25 Compared to the fast diffusion of protons, the slow transport of anions is the dominating factor of the ionic
dvanced energy storage devices (e.g., Li-ion batteries and supercapacitors) are composed of electrodes, an electrolyte, and a separator and play critical roles in the rapid development of cordless electronics.1,2 In parallel to electrode materials that are greatly sought-after, electrolytes have been attracting more attention in recent years because of the development of advanced solid-state energy storage devices for printable, flexible, and stretchable electronics. Electrolytes in supercapacitors are mainly made of aqueous or nonaqueous solutions (e.g., organic and ionic liquids).3 Aqueous electrolytes limit the cell voltage (
