Controlling Gel Morphology for Enhancing the Cation Mobility of Poly(vinylidene difluoride)-Based Gel Electrolytes for Lithium Secondary Batteries

Abstract

We evaluated the relaxation times (T 2 ), mobilities, and microviscosities of ionic species in terms of the crystallinity, X c , of the constituent poly(vinylidene difluoride) (PVDF) polymer of the PVDFbased gel electrolytes through NMR studies to confirm the anomalous features of the location and mobility of ions. Two T 2 components of the anion (T 2 (F1) > T 2 (F2)) and a significant increase in the longer T 2 (F1) of the major component with increasing X c were detected in annealed gels, contrary to the single component of T 2 (F) that was short and independent of the X c of quenched gels. This indicated that the anions of annealed gels had two different locations, and their contributions to the anion migration changed with X c . As the PVDF-based gel consists of a mixture of crystalline and amorphous phases, and the crystal domain size depends on X c , T 2 (F1) would reflect the sizes of the crystal domains that selectively influence the anion mobility. Furthermore, mobilities and microviscosities of the carrier ions were evaluated to confirm the ion-migration mechanism associated with the interaction between the ion and crystalline polymer in the gel. This revealed the basis for designing polymer gel electrolytes for lithium secondary batteries by controlling the two-phase morphology of the PVDF-based gels.

Ion Transport in Solid Medium—Evaluation of Ionic Mobility for Design of Ion Transport Pathways in Separator and Gel Electrolyte

“…With increasing the crystalline domain size, the sizes of the amorphous domains, including the solution, and the puddles of the solution that could not be incorporated into the amorphous phase would increase to fill the space among the large crystal domains. As a result, the annealed gels are composed of a mixture of larger-sized crystalline and amorphous domains, as shown in Figure 14 b [ 14 ]. On the contrary, there is no time for the precipitated crystallites to grow by the incorporation of surrounding polymer chains for the gels prepared by the quenching process.…”

“…The difference in PVDF gel morphology attributes not only to the cooling condition in gel formation but to the crystallinity of the constituent PVDF polymer before gelation. T 2 (F) of the annealed gels significantly increased with increasing the crystallinity of the polymer ( X c ), contrary to the X c independent features of T 2 (Li) of the annealed gels and T 2 (Li) and T 2 (F) of the quenched gels as shown in Figure 16 [ 14 ]. This implies that the gel morphology obtained by annealing depends sensitively on the crystallinity of the constituent polymer.…”

“…Therefore, the gels with larger crystal and amorphous domains have longer T 2 (F) due to larger amount of free anion species apart from the influence of the crystalline phase, and the gels with smaller crystalline and amorphous domains have shorter T 2 (F) due to short distance and a wider interface between the crystal and amorphous domains, which promotes the interaction leading to the restriction of anionic motion. This assumption of the crystallinity effect on the gel morphology was reflected in the change in β values with X c of the constituent polymer as shown in Figure 17 [ 14 ]. In the case of annealed gels, β an appeared in lower X c , decreased, and changed to β ca with increasing X c ( Figure 17 d).…”

“…The ionic conductivity of the gel electrolyte or the solution in a separator membrane was measured by the complex impedance method using a frequency analyzer (Model 1250, Solartron Analytical, Wokingham, U.K.) combined with a potentiostat (Model 1287) or the impedance analyzer (HZ-Pro S4, Hokuto Denko Corp. Tokyo, Japan). A constant alternating current (ac) voltage in the range of 20–100 mV was applied in the frequency range of 1 mHz to 65 kHz or to 1 MHz to detect the impedance data [ 14 , 15 ]. By fitting the data to the equivalent circuit model, the impedance values of gel electrolytes and an electrolyte solution within a separator membrane were estimated.…”

“… Schematic features of PVDF gels composed of crystalline and amorphous phases with ( a ) smaller crystals formed via the quenching process or via the annealing process and ( b ) larger crystals formed via the annealing process. Reprinted with permission from [ 14 ]. Copyright 2020 ACS Publications.…”