Bit Na Choi†
,
Jin Hoon Yang†,
Yong Seok Kim and
Chan-Hwa Chung*
School of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea. E-mail: chchung@skku.edu; Tel: +82-31-290-7275
First published on 15th July 2019
Solid polymer electrolytes (SPEs) for Li-metal polymer batteries are prepared, in which poly(ethylene oxide) (PEO), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), and copper-oxide fillers are formulated. Their structural and electrochemical properties are analyzed when the morphology of the copper-oxide fillers has been modulated to spherical or dendritic structure. The ionic conductivity obtained by electrochemical impedance spectroscopy (EIS) has been increased to 1.007 × 10−4 S cm−1 at 30 °C and 1.368 × 10−3 S cm−1 at 60 °C, as the 5 wt% dendritic fillers have been added to the SPEs. This ionic conductivity value is 1.3 times higher than that of 5 wt% spherical filler-contained SPEs. The analyses of differential scanning calorimetry (DSC) and X-ray diffraction (XRD) indicate that the increase of ionic conductivity is due to the remarkable decrease of crystallinity upon the addition of copper-oxide filler into PEO matrix of SPEs. The fabricated SPEs with the dendritic copper-oxide fillers present a total ionic transference number of 0.99 and a lithium-ion transference number of 0.38. More importantly, it presents a stable potential window of 2.0–4.8 V at 25 °C and high thermal stability up to 300 °C. The specific discharge capacity of the prepared cell with the dendritic filler-contained SPEs is measured to be 51 mA h g−1 and 125 mA h g−1 under 0.1 current-rate (C-rate) at 25 °C and 60 °C, respectively. In this study, the ionic conductivity and the electrochemical performance of the PEO-based polymer electrolyte have been evaluated when morphologically different copper-oxide fillers have been incorporated into the PEO matrix. We have also confirmed the safety and the flexibility of the prepared solid polymer electrolytes when they are used in flexible lithium-metal polymer batteries (LMPBs).
Recently, some solid polymer electrolytes have been developed with several types of polar polymers such as polyethylene oxide (PEO), polyvinyl acetate (PVA), polycarbonate (PC), polyvinylidene fluoride (PVdF), and so on.17–21 So far, polyethylene oxide (PEO) is the most widely used polymer matrix in the SPEs for the Li-metal batteries, due to its high chain flexibility.22,23 In addition, the PEO can effectively dissolve the various lithium salts in it by organizing lithium ion with ether oxygen of PEO through the coupling effect.24
In the PEO, the migration of lithium ion generally occurs in the amorphous region, which facilitates higher segment motion of PEO matrix, because of low barriers to rotating ions in the main matrix.25 At ambient temperatures, however, the PEO has low ionic conductivity due to the limited segment motion of the matrix. It represents that the intrinsic PEO contains the high crystalline region, which heavily hinders lithium ion mobility, leading to low ionic conductivity.26 Therefore, the crystalline region in the PEO should be reduced to improve ionic conductivity of the SPEs.27–29
To enhance ionic conductivity of the solid polymer electrolytes, one of the approaches is the addition of the organic or ionic liquid into the SPEs as a plasticizer.30–33 With these plasticizers, however, the SPE itself becomes weak in mechanical strengths and flammable. Another approach is the addition of inorganic materials as a filler, which seems to be better method to enhance ionic conductivity and interfacial stability because the mechanical strength of SPE has been maintained. The inorganic fillers, such as metal oxides, clays, metal–organic frameworks (MOFs), and carbon nanotubes (CNTs), have been proposed and reported that they present the improvement of ionic conductivity and electrochemical performance in Li-battery application, due to the several beneficial effects as an inorganic filler.34–38 One is that the inorganic fillers physically prevent the PEO from recrystallization of its chain, resulting more amorphous region produced.39 The inorganic filler also accelerates the dissociation of PEO segment and makes more free anions, which increases lithium ion mobility, according to the Lewis acid–base interactions between the inorganic fillers and the lithium salt.40 The other effect is that the inorganic filler forms the Li+ ion conducting pathways on the inorganic filler itself.41 Because of these beneficial effects, the use of inorganic fillers in the SPEs has been considered for the application in Li-metal polymer batteries (LMPBs).42–46
In this work, we use the copper oxides of different morphologies as an inorganic filler and identify the morphological effect of the inorganic filler on the electrochemical performance of the solid polymer electrolytes. The SPEs are prepared with a blending method using polyethylene oxide (PEO) as a polymer matrix, bis(trifluoromethanesulfonyl)imide (LiTFSI) as a lithium salt, and inorganic fillers of either spherical or dendritic copper-oxide powders, respectively. The dendritic copper-oxide powders used in this work have been prepared with the spontaneous galvanic displacement reaction, of which details are noted in our previous work.47 These high-surface-area powders of dendritic morphologies are supposed to interact more with the PEO matrix than the spherical particles, which results the reduction of crystalline phase in PEO. Hence, the structural effect of the inorganic fillers on the ion conductivity and the electrochemical performance of the fabricated polymer electrolyte have been analyzed and evaluated for the use in Li-metal batteries.
(1) |
The ionic transference number (tion), lithium-ion transference number (tLi+), and the linear sweep voltammetry (LSV) studies are figured out using WMPG 1000S analyzer managed by WMPG 3.3 software (Wonatech). The ionic transference number (tion) is analyzed using DC polarization technique at 25 °C. The two stainless-steel blocking electrodes are used as same as in ionic conductivity measurement, and the current is monitored as a figure of time at 10 mV for 1 h. The ionic transference number (tion) is calculated with the following equation:
(2) |
On the other hands, the lithium-ion transference number (tLi+) is calculated using AC/DC polarization technique at 25 °C. At this time, two lithium metal electrodes are used, and the prepared SPEs are placed between these two Li electrodes. The currents are monitored when the 10 mV has been applied for 4 h. The resistances are studied before and after polarization by the AC impedance technique. The lithium-ion transference number (tLi+) of prepared SPE is calculated with the Bruce–Vincent's equation:
(3) |
The electrochemical stability window of the prepared SPEs is determined using linear sweep voltammetry (LSV) from 2 V to 6 V at the voltage rate of 1 mV s−1 at 25 °C. Prepared SPEs are again sandwiched in between a working electrode of stainless steel and a counter electrode of lithium metal.51 The galvanostatic charge–discharge (GCD) cycling test in CR2032 cells (Li|solid polymer electrolyte|LiFePO4) is also carried using WMPG 1000S analyzer. After the 24 h heat treatment at 80 °C for the solid contact between electrodes and the SPE film, the GCD cycles are performed from 2.8 V to 4.2 V at various current rates (1C = 150 mA h g−1) at various temperatures.
Fig. 1 Characterization of the prepared dendritic copper-oxide powders; (a) FE-SEM image, (b) the result of particle size analysis, and (c) XRD patterns. |
The oxidation states and crystallinity of the dendritic powders are also characterized a little more in detail using the X-ray diffraction (XRD) patterns shown in Fig. 1c. After the oxidation step, the prepared dendritic powders show XRD peaks at several 2θ values, such as 35.5°, 37.1°, 38.8°, and 63.2°. Compared with Cu (no. 04-0836), Cu2O (no. 05-0667), and CuO (no. 45-0937) of JCPDS indexes, the peaks identified with Cu2O (β) and CuO (γ) are evident without any apparent peaks of metallic Cu (α), which confirms that the prepared powders are mostly oxidized into Cu2O and CuO, while their dendritic structures are maintained.
Fig. 2 Top-view FE-SEM images for (a) intrinsic P(EO)15LiTFSI, (b) 5 wt% spherical filler-contained P(EO)15LiTFSI, and (c) 5 wt% dendritic filler-contained P(EO)15LiTFSI. |
As shown in Fig. 2b, these spherulitic features are reduced and get smaller on the surface of the spherical filler-contained P(EO)15LiTFSI than that on the intrinsic P(EO)15LiTFSI, due to the suppression of recrystallization by the inorganic filler particles in the PEO matrix.54 Notably, for the dendritic filler-contained P(EO)15LiTFSI, the spherulitic morphology significantly disappears on the surface (see Fig. 2c), which represents that dendritic particles are highly impeding the recrystallization of PEO by the sufficient interaction with PEO matrix at their hierarchical structures of high surface area. Such recrystallization features in PEO matrix are also confirmed by XRD and DSC analyses (vide infra). To elucidate the degree of recrystallization of the PEO matrix, the XRD patterns are carefully analyzed, based on the addition of lithium salt (LiTFSI) and copper-oxide fillers.
Fig. 3 presents the XRD patterns of pure PEO, recrystallized intrinsic P(EO)15LiTFSI, 5 wt% spherical filler-contained P(EO)15LiTFSI, and 5 wt% dendritic filler-contained P(EO)15LiTFSI.
Fig. 3 XRD patterns of (a) pure PEO, (b) intrinsic P(EO)15LiTFSI, (c) 5 wt% spherical filler-contained P(EO)15LiTFSI, and (d) 5 wt% dendritic filler-contained P(EO)15LiTFSI. |
In Fig. 3a, two strong XRD peaks at 2θ = 19.2° and 23.4° are appeared due to its semi-crystalline nature of PEO, which are assigned to the (1 2 0) and (1 1 2), respectively.55 As the reduction of peak intensities noticed in Fig. 3, this semi-crystalline feature has been suppressed during the recrystallization step, once it has been dissolved and recrystallized after the addition of LiTFSI and inorganic fillers. During the recrystallization, the Li+, TFSI−, and inorganic fillers are incorporated into the PEO matrix, which causes the reduction of crystalline phase and the increase of amorphous region in the recrystallized PEO.
In the cases of filler-contained SPE film, the patterns of copper oxide additionally appear at 2θ = 35.7° and 37.5°. In addition, the morphological difference of inorganic fillers makes the degree of re-crystallinity different, as noticed in Fig. 3c and d. The high-surface-area dendritic fillers reduce the crystalline peak intensities at 2θ = 19.1° and 23.2° more than the spherical fillers, even when the same amount has been added in weight. It is possibly because the inorganic fillers hinder the dissolved PEO from recrystallization at the interface of organic and inorganic materials surface. Therefore, the sufficient interaction between the high-surface-area dendritic fillers and the dissolved PEO results less crystalline phase and more amorphous region in the recrystallized matrix, which is consistent with the microscopic images (vide ante).
The differential scanning calorimetry (DSC) is also performed to investigate the phase change behavior of SPEs, such as glass transition temperature (Tg), melting temperature (Tm), and degree of crystallinity (χc). All of the measurement data are summarized in Table 1, which are evaluated from the DSC data obtained during the 2nd heating to remove any thermal history. The DSC thermos grams of pure PEO, intrinsic P(EO)15LiTFSI, 5 wt% spherical filler-contained P(EO)15LiTFSI, and 5 wt% dendritic filler-contained P(EO)15LiTFSI are presented in Fig. 4.
Tg | Tm | Hm (J g−1) | χc (%) | |
---|---|---|---|---|
Pure PEO | −54.2 | 66.48 | 123 | 57.55 |
Intrinsic P(EO)15LiTFSI | −47.61 | 51.15 | 51.3 | 24.00 |
5 wt% spherical filler-contained P(EO)15LiTFSI | −48.23 | 40.84 | 29.7 | 13.89 |
5 wt% dendritic filler-contained P(EO)15LiTFSI | −49.43 | 38.52 | 28.7 | 13.43 |
Fig. 4 DSC thermos grams of (a) pure PEO, (b) intrinsic P(EO)15LiTFSI, (c) 5 wt% spherical filler-contained P(EO)15LiTFSI, and (d) 5 wt% dendritic filler-contained P(EO)15LiTFSI. |
An endothermic peak of melting temperature (Tm) of the crystalline state of pristine PEO is obtained at the temperature of 66.48 °C, while the transition from rigid to flexible phase of PEO (Tg) is observed at the temperature of −54.2 °C (see Fig. 4a). When the LiTFSI has been added into PEO matrix, the glass transition temperature (Tg) rises to −47.61 °C, because the mobility of EO segment is lowered. As also shown in Fig. 5b, the Tm of this intrinsic P(EO)15LiTFSI is decreased to 51.15 °C due to the strong electron-withdrawing group (–SO2CF3) and the highly dissociated ions from LiTFSI in the PEO segment caused by highly flexible segment (–N(SO2)2).22
Fig. 5 TGA thermos grams for (a) pure PEO, (b) intrinsic P(EO)15LiTFSI, (c) 5 wt% spherical filler-contained P(EO)15LiTFSI, and (d) 5 wt% dendritic filler-contained P(EO)15LiTFSI. |
When two different fillers are incorporated into the intrinsic P(EO)15LiTFSI, the Tg and the Tm are moved towards lower temperatures as well, due to the interaction between PEO segment and copper-oxide fillers (cf. Fig. 4c, d and Table 1). Decrease in Tg and Tm of copper-oxide filer-contained PEO highlights the flexible matrix of PEO. It attains the flexible partial movement of ions in PEO matrix resulting to higher ionic conductivity of SPEs. The degree of crystallinity (χc) of SPEs is analyzed using the values of the melting enthalpy (ΔHm) of prepared SPEs and the melting enthalpy (ΔHPEO = 213.7 J g−1) of 100% crystalline PEO, as noted in the following equation:56
(4) |
As listed in Table 1, the degree of crystallinity (χc) was decreased from 57.55% to 24.00% when the LiTFSI was added into pure PEO. It was drastically reduced down to 13.89% and 13.43% when the spherical and the dendritic copper-oxide fillers were dispersed into the intrinsic P(EO)15LiTFSI, respectively. On the comparison between the spherical and the dendritic morphologies of copper-oxide fillers, it has been found that the dendritic fillers interact more with the PEO matrix and form more amorphous structures in the SPEs than the spherical fillers, resulting in the decrease of the crystallinity of the SPEs.
On the other hands, two TGA curves for 5 wt% spherical filler-contained P(EO)15LiTFSI and 5 wt% dendritic filler-contained P(EO)15LiTFSI have three-step decomposition, as noticed in Fig. 5c and d. First decomposition observed at 300 °C is related to the complex of copper oxide and PEO matrix. Second step at 380 °C and third step at 430 °C imply the non-complexed PEO and the complex of LiTFSI salt and PEO matrix, respectively, as same as those of intrinsic P(EO)15LiTFSI. It represents that the prepared SPEs with copper-oxide fillers are thermally stable up to 300 °C and fairly good for the battery application.
Fig. 6 (a) Linear sweep voltammetry curve and (b) cyclic voltammetry curves obtained from the cell of (Li|5 wt% dendritic filler-contained P(EO)15LiTFSI|stainless steel). |
Ionic conductivity of the SPEs is one of the most important factor for the use in the battery application, because it is directly associated with the major performance of LMPBs. Fig. 7a and b show the Nyquist plots of intrinsic P(EO)15LiTFSI, 5 wt% spherical filler-contained P(EO)15LiTFSI, and 5 wt% dendritic filler-contained P(EO)15LiTFSI monitored at 30 °C and 60 °C, respectively. Using the values in Nyquist plots, the calculated ionic conductivity (σ) at 30 °C was 0.7258 × 10−4 S cm−1 and 1.007 × 10−4 S cm−1 for 5 wt% spherical filler-contained P(EO)15LiTFSI and 5 wt% dendritic filler-contained P(EO)15LiTFSI, respectively. These values are more than twice as high as that of intrinsic P(EO)15LiTFSI without the fillers (σ = 0.3623 × 10−4 S cm−1). The increase of ionic conductivity in the filler-contained P(EO)15LiTFSI arises from the increase in the amorphous region of the PEO matrix by adding the copper-oxide fillers, as explained in the discussions of XRD and DSC data. It is known that more amorphous region of SPEs provides the higher ionic conductivity, as also reported in the literature.39 Furthermore, the ionic conductivity of 5 wt% dendritic filler-contained P(EO)15LiTFSI is about 1.3 times higher than that of 5 wt% spherical filler-contained P(EO)15LiTFSI at 30 °C. It is related to sufficient interaction of PEO matrix with the high-surface-area dendritic fillers than the relatively smaller surface-area spherical fillers. At 60 °C, the ionic conductivity of SPEs was increased to 0.9615 × 10−3 S cm−1 and 1.368 × 10−3 S cm−1 for 5 wt% spherical filler-contained P(EO)15LiTFSI and 5 wt% dendritic filler-contained P(EO)15LiTFSI, respectively, whereas the ionic conductivity of intrinsic P(EO)15LiTFSI was less increased to 0.457 × 10−3 S cm−1. It is because the ion mobility has been enhanced in the amorphous region of SPEs at higher temperatures.
Fig. 7 Nyquist plots obtained (a) at 30 °C and (b) 60 °C, and (c) temperature-dependent ion conductivity of the prepared SPEs. |
The Fig. 7c presents temperature dependency of the ionic conductivities of the prepared SPEs in this work. Overall, it shows that ionic conductivity (σ) of SPEs has been improved as temperature increases, following the Arrhenius-type activated process. As the temperature approaches up to phase-transition temperature (Ttrans) of PEO, the sharp increase in ionic conductivity of SPEs has been monitored, while the ionic conductivity increases slowly as the temperature goes over the Ttrans of PEO as interpreted by logσ vs. 1/T plots of Fig. 7c. The activation energy of temperature-dependent ionic conductivity is obtained according to Arrhenius-type behavior given in the following equation:41
(5) |
As shown in Fig. 8, the DC polarization curve has been also obtained using the cell of (stainless steel|5 wt% dendritic filler-contained P(EO)15LiTFSI|stainless steel) at 25 °C. The total ionic transference number (tion) of dendritic filler-contained P(EO)15LiTFSI was then evaluated by the eqn (2). The tion value was observed to be 0.99, which means that the ionic conductivity has been contributed by almost all the present ions in the polymer. To estimate the specific distribution of Li+ ion in LMPBs, the lithium-ion transference number (tLi+) in the filler-contained P(EO)15LiTFSI are also examined by the AC/DC polarization using the cell of (Li|filler-contained P(EO)15LiTFSI|Li) at 25 °C. The lithium-ion transference numbers (tLi+) were calculated by using the eqn (3). The tLi+ value was 0.29 and 0.38 for 5 wt% spherical filler-contained P(EO)15LiTFSI and 5 wt% dendritic filler-contained P(EO)15LiTFSI, respectively (cf. Fig. 9a and b), whereas the tLi+ value of intrinsic P(EO)15LiTFSI was only 0.17.58 The higher tLi+ value of the polymer provides the better electrochemical performance as the SPEs,59,60 which is explained by the increased ion mobility due to the Lewis acid–base effect. Furthermore, the value of tLi+ for dendritic filler-contained P(EO)15LiTFSI is higher than that of spherical filler-contained P(EO)15LiTFSI, which represents that dendritic filler-contained polymer presents better ion mobility in it. It is consistent with all the results of SEM, XRD, and DSC.
Fig. 8 Typical DC polarization curve for the cell of (stainless steel|5 wt% dendritic filler-contained P(EO)15LiTFSI|stainless steel) at 25 °C. |
The cyclic performances are measured with the coin cell of (Li|5 wt% dendritic filler-contained P(EO)15LiTFSI|LiFePO4), in which the capacity evaluation of first cycle is excluded due to the usual SEI formation process. Fig. 11a presents the galvanostatic charge/discharge curves at various current rates (C-rates) at 25 °C. The specific discharge capacities of the cell were 51.03, 35.19, 20.19, 15.94, and 0.8 mA h g−1 at 0.1, 0.2, 0.3, 0.5, and 1 C-rate at 25 °C, respectively. The specific discharge capacities are observed to decrease with increase in C-rate as usual, which is caused by a large polarization.61 In general, the LMPBs with the SPEs based on PEO matrix hardly operate at 25 °C. However, the prepared LMPB with the dendritic filler-contained SPEs in this work has the discharge capacity of 51.03 mA h g−1 at 0.1 C-rate, because of its high ionic conductivity even at 25 °C.
As the cell operating temperature increases to 60 °C, the specific discharge capacity has improved drastically to 125, 113, 97, 78, and 52 mA h g−1 at 0.1, 0.2, 0.3, 0.5, and 1 C-rate, respectively (see Fig. 11b). These values of discharge capacities are suitable enough for LMPB application. This improvement is due to the crystallinity changes of PEO matrix in SPEs at different temperatures as discussed in DSC results (cf. Fig. 4).
For further evaluation on the effect of filler morphology in SPEs on the cell performance, the rate capabilities of the cells with intrinsic P(EO)15LiTFSI, 5 wt% spherical filler-contained P(EO)15LiTFSI, and 5 wt% dendritic filler-contained P(EO)15LiTFSI are compared at different temperatures. The specific capacities at various current rates (C-rates) at 25 °C are shown in Fig. 11c. When the intrinsic P(EO)15LiTFSI was used in the cell, the specific capacity was 7 mA h g−1 at 0.1 C-rate at 25 °C, which means that it is hardly operated at 25 °C. On the other hands, the SPEs with the fillers were operated reasonably at low current rates even at 25 °C. Moreover, the cell performance of the cell with the 5 wt% dendritic filler-contained P(EO)15LiTFSI is higher than that with 5 wt% spherical filler-contained P(EO)15LiTFSI. Furthermore, the specific capacities of the cell increase drastically at 60 °C, as shown in Fig. 11(d). The 5 wt% dendritic filler-contained P(EO)15LiTFSI also shows higher performance than the other solid polymer electrolytes, due to its superior ionic conductivity through the amorphous phase of PEO in the SPEs.
To elucidate the merits of solid polymer electrolytes, we have assembled the pouch cell of (Li|5 wt% dendritic filler-contained P(EO)15LiTFSI|LiFePO4) operating the light-emitting diode (LED) after it has been cut and folded as shown in Fig. 12. The cell still works properly and lights the light-emitting diode, which confirms safety and flexibility of the Li-metal polymer battery with the prepared solid polymer electrolyte in this work.
Fig. 12 Photographs of the (Li|5 wt% dendritic filler-contained P(EO)15LiTFSI|LiFePO4) pouch cell operating the LED even (a) with being cut at corner and then (b) with being folded. |
Footnote |
† These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2019 |