2.1. Evaluation of CPEs
The electrical conductivities of PEO
18LiTFSI-
xG4 (TEGDME abbreviated as G4 hereafter) were measured as a function of
x (the molar ratio of G4 to ethylene oxide).
Figure 1 presents Arrhenius plots for the conductivity of PEO
18LiTFSI-
xG4. The conductivity increases significantly with increasing
x up to 2 and there is little difference in conductivity between the samples with
x = 2 and 3. The electrical conductivity of PEO
18LiTFSI-2G4 at 25 °C is almost one order of magnitude higher and that at 60 °C is three times higher than that of PEO
18LiTFSI. TEGDME loosens the coordination of lithium ions with EO units in the PEO matrix and thus enhances the mobility of ions, and could also enable lithium ions to decouple from ion pairs, as shown by Kriz
et al. [
15]. The activation energies for conduction in PEO
18LiTFSI-
xG4 at high temperature decrease with increasing
x. That of PEO
18LiTFSI-2G4 was calculated to be 25.3 kJ·mol
−1, in the range of 55 to 80 °C, which is lower than that for PEO
18LiTFSI (38.8 kJ·mol
−1) and that for PEO
18LiTFSI-2.0
N-methyl-
N-propylpiperdinium bis(fluorosulfonyl) imide (PP13FSI) (33.8 kJ·mol
−1) [
16].
Figure 1.
Temperature dependence of the electrical conductivity for PEO18LiTFSI-xG4 as a function of x.
Figure 1.
Temperature dependence of the electrical conductivity for PEO18LiTFSI-xG4 as a function of x.
The overpotentials between lithium metal and polymer electrolytes for lithium deposition and stripping generally increase with current density and undergo a sudden increase with the polarization period at a current density (limiting current density). When the ionic concentration in the vicinity of cathode drops to zero, the limiting current density is reached and dendrite growth is initiated (Sand time) [
17]. To operate the Li/PEO
18LiTFSI/LTAP/Pt, air cell at high current density, it is necessary to enhance the limiting current density of lithium-ion conducting PEO-based polymer electrolytes (
Il), which is proportional to the salt diffusion coefficient and lithium ion transference number, as shown by Equation (1):
where
Co is the initial concentration,
e is the elemental charge,
D is the salt diffusion coefficient,
ta is the anion transport number, and
L is the thickness of the electrolyte. The salt diffusion coefficient of PEO
18LiTFSI-
xG4 was estimated using the method proposed by Ma
et al. [
18] as a function of
x. Typical curves for the natural logarithm of potential
versus time for the Li/PEO
18LiTFSI-
xG4/Li cells at 60 °C are shown in
Figure 2, where the cells were polarized at 50 mV prior to the potential being interrupted. A distinct linear relation, which corresponds to the linear diffusion region as the concentration gradient of the cell relaxes, is observed for all cells after sufficient time. The salt diffusion coefficient (
D) of PEO
18LiTFSI-
xG4 was calculated from the slope of the linear curves using Equation (2):
The calculated
D are summarized in
Table 1 along with the electrical conductivity results. A maximum salt diffusion coefficient of 3.37 × 10
−7 cm
2·s
−1 was determined for
x = 2.0, which is almost one order of magnitude higher than that for PEO
18LiTFSI (3.60 × 10
−8 cm
2·s
−1) and PEO
18LiTFSI-1.44PP13FSI (2.25 × 10
−8 cm
2 s
−1) [
16], and four times higher than that for PEO
18LiTFSI-18 wt % PEGDME (8.38 × 10
−8 cm
−1 s
−1) [
14].
Figure 2.
Natural logarithm of potential vs. time curves for the Li/PEO18LiTFSI-xG4/Li cells at 60 °C.
Figure 2.
Natural logarithm of potential vs. time curves for the Li/PEO18LiTFSI-xG4/Li cells at 60 °C.
Lithium ion transference numbers for PEO18LiTFSI-
xG4 were measured using the method reported by Evans and Vincent [
19].
Figure 3 shows a typical cell current decay curve upon application of a DC bias of 20 mV and impedance profiles for the Li/PEO18LiTFSI-2G4/Li cell. From these results,
ta was calculated using the Evans and Vincent equation, and the results are summarized in
Table 1. Significant increase of the lithium ion transport number (
i.e., decrease of the anion transport number) was observed by addition of G4 into PEO
18LiTFSI. The lithium ion transport number for PEO
18LiTFSI-2G4 is slightly higher than that for LiTFSI-2G4. Based on these results for
D and
ta, the limiting current density (
Il) for PEO
18LiTFSI-
xG4 was calculated and the results are summarized in
Table 1, where the thickness (
L) was 100 μm. PEO
18LiTFSI-2G4 exhibits the highest
Il of 15.8 mA·cm
−2, which is higher than that for PEO
18LiTFSI by a factor of approximately 15. The high limiting current density of PEO
18LiTFSI-2G4 suggests that a WSLE with PEO
18LiTFSI-2G4 could be operated at a high current density.
Table 1.
Ionic transport properties of PEO18LiTFSI-xTEGDME.
Table 1.
Ionic transport properties of PEO18LiTFSI-xTEGDME.
PEO18LiTFSI-
xTEGDME | 25 °C(×106 S·cm−1) | 60 °C(×104 S·cm−1) | Ea (kJ·mol−1) | D (×107 cm2·s−1) | tLi+ | Il (mA cm−2) L = 100 μm |
---|
σ | σLi+ | σ | σLi+ | Low temp. region | High temp. region |
---|
x = 0 | 5.64 | 1.35 | 5.29 | 1.27 | 115.7 | 38.8 | 0.36 | 0.24 | 1.02 |
x = 1.0 | 28.0 | 12.0 | 10.7 | 4.60 | 99.8 | 30.7 | 1.05 | 0.43 | 3.97 |
x = 1.5 | 40.7 | 18.3 | 12.5 | 5.62 | 82.4 | 27.1 | 1.50 | 0.45 | 5.87 |
x = 2.0 | 60.8 | 32.8 | 16.5 | 8.91 | 78.2 | 25.3 | 3.37 | 0.54 | 15.8 |
x = 3.0 | 68.3 | 39.6 | 16.4 | 9.51 | 72.9 | 25.0 | 2.27 | 0.58 | 11.6 |
Figure 3.
Steady-state method for lithium ion transference number determination. The symmetric Li/PEO18LiTFSI-2G4/Li cell current decays with time upon application of a DC bias of 20 mV until steady-state is reached. Corresponding impedance measurements performed at the initial state and steady-state, and the fitting results are shown in the inset.
Figure 3.
Steady-state method for lithium ion transference number determination. The symmetric Li/PEO18LiTFSI-2G4/Li cell current decays with time upon application of a DC bias of 20 mV until steady-state is reached. Corresponding impedance measurements performed at the initial state and steady-state, and the fitting results are shown in the inset.
Low and stable interface resistance between lithium metal and polymer electrolytes is essential for the electrochemical performance of WSLEs and to reduce lithium dendrite formation [
7].
Figure 4 shows the impedance profile changes for the Li/PEO
18LiTFSI-
xG4/Li cells at 60 °C as a function of storage time. PEO
18LiTFSI-G4 and PEO
18LiTFSI-1.5G4 show a significant increase of interface resistance with storage time; the initial cell resistance of 69 Ω·cm
2 increased to 141 Ω·cm
2 for PEO
18LiTFSI-G4 after 28 days. However, low interface resistance was obtained for PEO
18LiTFSI-2G4 and PEO
18LiTFSI-3G4. After 28 days, PEO
18LiTFSI-2G4 and PEO
18LiTFSI-3G4 showed interface resistances of 34 and 74 Ω·cm
2, respectively, which are much lower than 253 Ω·cm
2 for PEO
18LiTFSI after storage for 28 days [
16]. The interface resistance behavior for PEO
18LiTFSI-2G4 was unusual, where the interfacial resistance increased during the initial seven days and then decreased continuously to a value lower than the original. A similar change in the interface resistance was reported for the Li/PEGDME-LiTFSI/Li cell by Bernhard
et al. [
20], which could be attributed to the solid electrolyte interphase (SEI) formation-re-dissolution process [
21]. These impedance profiles show a diminished semicircle, which is associated with the resistance of a passivation film (SEI) formed on the lithium electrode surface by the reaction of lithium with the polymer electrolyte and the charge transfer resistance.
Figure 5 shows the temperature dependence of the inverse of the passivation film resistance and the charge transfer resistance for the Li/PEO
18LiTFSI-
xG4/Li cells as a function of
x. The activation energy for the inverse of the passivation film resistance (
Rp) decreased from 76.2 kJ·mol
−1 for PEO
18LiTFSI to 54.7 kJ·mol
−1 for PEO
18LiTFSI-2G4. A decrease was also observed for PEO
18LiTFSI-G4 and PEO
18LiTFSI-3G4, but it was not as significant as that for PEO
18LiTFSI-2G4. The activation energies for the inverse of the charge transfer resistance (
Rc) decreased with increasing
x. The lowest value of 63.4 kJ·mol
−1 was calculated for PEO
18LiTFSI-3G4, in comparison with 81.7 kJ·mol
−1 for PEO
18LiTFSI and 68.1 kJ·mol
−1 for PEO
18LiTFSI-2G4. These results suggest that the low-molecular weight oligomer ether G4 could reduce the resistance of the SEI and facilitate the charge transfer reaction at the interface. The former role is similar to other additives, such as nanofillers and ionic liquids, whereas the latter role is only performed by G4.
Figure 4.
Impedance spectra of PEO18LiTFSI-xG4 as a function of the storage time at 60 °C.
Figure 4.
Impedance spectra of PEO18LiTFSI-xG4 as a function of the storage time at 60 °C.
Figure 5.
Temperature dependence of the inverse of (a) the passivation film resistance; and (b) the charge transfer resistance for Li/PEO18LiTFSI-xG4/Li.
Figure 5.
Temperature dependence of the inverse of (a) the passivation film resistance; and (b) the charge transfer resistance for Li/PEO18LiTFSI-xG4/Li.
2.2. Electrochemical Performance of WSLEs
The high lithium ionic diffusion coefficient and low interface resistance with lithium metal for PEO
18LiTFSI-2G4 motivated us to examine its role as the protective layer in a WSLE. The impedance of the Li/PEO
18LiTFSI-2G4/LTAP/sat. LiCl aqueous solution/Pt, air cell was measured at 60 °C, using a platinized platinum reference electrode.
Figure 6 presents the impedance spectra of this cell for various storage times. The open circuit voltage (OCV) was stabilized at 3.48 V after one week, which is comparable with that reported previously (3.43 V) [
8] and slightly lower than that of the calculated OCV (3.59 V). After 28 days, a stable cell resistance of 84 Ω·cm
2 was obtained, which is comparable with those reported previously; the cell resistance for the Li/PEO
18LiTFSI/LTAP/1 M LiCl/Pt, air cell was 539 Ω·cm
2 [
1], 118 Ω·cm
2 for the Li/PEO
18LiTFSI-40 nm BaTiO
3/LTAP/1 M LiCl aqueous solution/Pt, air cell [
22] and 130 Ω·cm
2 for the Li/PEO
18LiTFSI-1.44
N-methyl-
N-propylpiperdinium-bis(trifluromethanesulfonyl)imide/LTAP/1 M LiCl aqueous solution/Pt, air cell [
8]. An equivalent circuit proposed in our previous study was utilized to analyze the impedance spectra, which consists of the total resistances of the polymer electrolyte and the LTAP plate (
Rb), the interfacial resistance between the polymer electrolyte and lithium metal electrode (
Rf1), the interfacial resistance between the polymer electrolyte and the LTAP plate (
Rf2), the charge-transfer resistance (
Rc), and the Warburg impedance (
W1) [
14].
Rb was quite stable at around 38 Ω·cm
2 during the storage period. The fitting results suggest that
Rf2 increased from 19.4 to 29.3 Ω·cm
2,
Rc decreased from 18.0 to 7.4 Ω·cm
2, and
Rf1 increased from 3.8 to 5.8 Ω·cm
2 over the 28-day storage period.
Figure 6.
Impedance spectra of the Li/PEO18LiTFSI-2G4/LTAP/saturated LiCl aqueous solution/Pt, air cell as a function of the storage time at 60 °C.
Figure 6.
Impedance spectra of the Li/PEO18LiTFSI-2G4/LTAP/saturated LiCl aqueous solution/Pt, air cell as a function of the storage time at 60 °C.
The electrochemical performance of a WSLE protected by PEO
18LiTFSI-2G4 and LTAP was investigated in an aqueous electrolyte using platinized platinum electrodes as the counter and reference electrodes.
Figure 7 shows the change in potential over time for the Li/PEO
18LiTFSI-2G4/LTAP/1 M LiCl-4 mM LiOH aqueous solution/Pt, air cell at current densities in the range of 0.5 to 4.0 mA·cm
−2 at 60 °C. This WSLE exhibited quite low lithium plating and stripping overpotentials at high current densities up to 4.0 mA·cm
−2. The lithium stripping and plating overpotentials at 1.5 mA·cm
−2 were 0.10 and 0.15 V, respectively, which are lower than the best results obtained for the Li/PEO
18LiTFSI-18 wt % PEGDME/LTAP/1 M LiCl-4 mM LiOH aqueous solution/Pt, air cell (0.29 V for plating and 0.21 V for stripping) in our previous studies [
14]. The low overpotentials of the cell at high current densities could be attributed to the high lithium ion diffusion coefficient, high lithium ion transport number, and low interfacial resistance of PEO
18LiTFSI-2G4.
Figure 7.
Discharge and charge profiles for the Li/PEO18LiTFSI-2G4/LTAP/1 M LiCl-4 mM LiOH aqueous solution/Pt, air cell at various current densities and 60 °C. The cell voltage was measured using a platinized platinum reference electrode.
Figure 7.
Discharge and charge profiles for the Li/PEO18LiTFSI-2G4/LTAP/1 M LiCl-4 mM LiOH aqueous solution/Pt, air cell at various current densities and 60 °C. The cell voltage was measured using a platinized platinum reference electrode.
To study the lithium dendrite formation at the interface of Li and PEO
18LiTFSI-2G4, a long period of lithium deposition at a constant current of 1 mA·cm
−2 was performed using the Li/PEO
18LiTFSI-2G4/LTAP/saturated LiCl aqueous solution/Pt, air cell, where the thickness of PEO
18LiTFSI-2G4 was around 100 μm and the impedance of the WSLE was measured at every 4 h polarization.
Figure 8a shows cell potential
versus time curves as a function of the polarization period. The lithium electrode potential increased suddenly after a 25 h polarization. This potential increase may be due to lithium deposition on LTAP by lithium dendrite formation, which would result in the formation of a high resistance layer by the reaction of lithium and LTAP [
4].
Figure 8b shows the impedance profiles as a function of the polarization period. The WSLE resistance increased gradually with the polarization period up to 13 h and then decreased up to 22 h polarization. The decrease of electrode resistance may be due to lithium dendrite formation. Finally, the WSLE resistance was increased to 220 Ω·cm
2 by the short circuit of the lithium metal electrode with LTAP. The short circuit period of 25 h is approximately 2.6 times longer than that measured for a Li/PEO
18LiTFSI/LTAP/10 M LiCl-4 mM LiOH aqueous solution/Pt, air cell [
14]. The long short circuit period for the Li/PEO
18LiTFSI-2G4/LTAP/saturated LiCl aqueous solution/Pt, air cell could be explained by the high lithium ion transport number, high salt diffusion coefficient, and low interface resistance [
17,
23].
Figure 8.
(a) Charge profiles for the Li/PEO18LiTFSI-2G4-100 μm/LTAP/sat. LiCl/Pt, air cell at 1 mA·cm−2 and 60 °C; and (b) impedance profiles after each polarization period. The cell voltage and impedance were measured using a platinized platinum air reference electrode.
Figure 8.
(a) Charge profiles for the Li/PEO18LiTFSI-2G4-100 μm/LTAP/sat. LiCl/Pt, air cell at 1 mA·cm−2 and 60 °C; and (b) impedance profiles after each polarization period. The cell voltage and impedance were measured using a platinized platinum air reference electrode.
The cyclability of the Li/PEO
18LiTFSI-2G4/LTAP/saturated LiCl aqueous solution/Pt, air cell for lithium deposition and stripping at a constant current density of 1.0 mA·cm
−2 was measured at 60 °C, where the current was passed for 2 h. The cell voltage
versus time profile is shown in
Figure 9. After 100 cycles, the overpotential for lithium deposition and stripping slightly increased from 0.10 to 0.15 V and from 0.08 to 0.13 V, respectively. This excellent cycling performance could be ascribed to the low and stable resistance for the SEI formed between lithium metal and PEO
18LiTFSI-2G4.
Figure 9.
Discharge and charge profiles for Li/PEO18LiTFSI-2G4-100 μm/LTAP/sat. LiCl/Pt, air cell at 1.0 mA·cm−2 and 60 °C.
Figure 9.
Discharge and charge profiles for Li/PEO18LiTFSI-2G4-100 μm/LTAP/sat. LiCl/Pt, air cell at 1.0 mA·cm−2 and 60 °C.