The Influential Properties of Polymer Electrolytes on the Performance and Longevity of Solids-State Flexible Supercapacitors

Abstract

Polymer electrolytes are the key enablers for solid-state energy storage including batteries and supercapacitors. Not only they can provide leakage-free thin and flexible form factors for solid supercapacitors, but also they can reduce and simplify the packaging of the devices to increase the actual energy and power densities. An ideal polymer electrolytes for supercapacitor should have: 1) high ionic conductivity; 2) wide electrochemical stability window and chemical stability without undesirable side reactions; 3) good environmental stability and mechanical properties; and 4) low material and processing cost. While most of the research are focusing on attributes 1) and 2) as well as the enabled solid supercapacitor devices in pristine condition, there are very few studies on the long-term service life and shelf life or the failure mechanisms of the polymer electrolytes. Although many polymer electrolytes have excellent performance at the beginning, they deteriorated rapidly over time. These can be related to the material properties (e.g. the crystallinity of the polymer host), the compatibility of ionic conductors and polymer host, sensitivity of the electrolytes to the environment etc. Fig. 1 compares the conductivities of two pairs of polymer electrolytes over a period of 4 weeks. One pair is a OH- ion conducting polymer electrolytes using the same polymer host poly(vinyl-alcohol) (PVA) but different ionic conductors, KOH or tetraethylammonium hydroxide (TEA-OH). The other set is a neutral pH salt polymer electrolytes that have the same Li2SO4 ionic conductor but different polymer hosts, PVA or polyacrylamide (PAM). PVA-KOH has been the most widely used electrolyte in all solid supercapacitors that need alkaline media. However, despite of its initial good performance, it tends to crystalize and dry out [1]. TEAOH-PVA, on the other hand, has a much longer shelf life. The solid capacitor it enabled was functional for AC-line filtering for over 18 month [2]. The neutral polymer electrolytes using Li2SO4 can reach to 2.0 V in solid symmetric supercapacitor devices [3] , which is very promising to increase energy density. Nonetheless, the same ionic conductor with different polymer hosts led to very different trends in the longevity and device shelf life. These differences in long term stability are the results of the intrinsic properties, degree of hydration within the materials and the conduction and failure mechanisms. In this talk, we will address these issues through the examples to show the influential factors on the performance of the polymer electrolytes at different time and temperature and the impact on the longevity of the solid supercapacitor devices. References Gao, J. Li, and K. Lian, "Alkaline quaternary ammonium hydroxides and their polymer electrolytes for electrochemical capacitors", RSC Advances, 4 (41), 21332-21339, 2014.Gao, J. Li, J. Miller, R. Outlaw, S. Butler, and K. Lian, “Solid-state electric double layer capacitors for ac line-filtering", Energy Storage Materials, 4, 66–70, 2016.Virya and K.Lian, “Li2SO4-Polyacrylamide Polymer Electrolytes for 2.0 V Solid Symmetric Supercapacitors”, Electrochemistry Communications, 81, 52–55， 2017. Figure 1