II. Synthesis and characterization of a family of oligoethers end-capped by lithium aryl
II.2. Preparation of salts and their characterizations
II.2.3. Physical characterizations of salts
Chapter II: Synthesis and characterization of a family of oligoethers end-capped by lithium aryl perfluorosulfonates
61 Elemental analysis
The purity of the salts were checked also by the elemental analysis, the results are gathered in the Table 2.4. The obtained values seem to be very close from the calculated ones and confirm the high purity of the synthetized salts as also shown by 1H and 19F NMR.
Table 2.4 Elemental analyses of salt 2n1, salt 2n7, salt 2n16
Salts % C H S F Li
Salt 2n1 Calc. 35.68 2.99 17.32 20.52 1.87
Found. 35.37 2.9 17.64 20.84 1.8
Salt 2n7 Calc. 43.67 5.57 9.95 11.82 1.09
Found. 43.57 5.44 9.13 11.54 1.07
Salt 2n16 Calc. 47.86 6.92 6.14 7.29 0.67
Found. 47.75 7.06 5.93 7.33 0.57
Chapter II: Synthesis and characterization of a family of oligoethers end-capped by lithium aryl perfluorosulfonates
62 results, it can be pointed out that the bonding of short mPEG chains on the anion favors the salt organization and reinforce the intermolecular interactions, due probably to the interaction of Li with the oxygen from oligoether. However, for n7 and n16 the salts are completely amorphous at room temperature. It is known that five O atoms are necessary for the solvation of Li+. In the salt 2 n7 and n16, 2 and 11 O atoms respectively are in excess; therefore we assume the mPEG chain can organize differently and lead to amorphous materials.
Table 2.5 Glass transition temperature (Tg) and melting temperature (Tm) of mPEG, Salt 1, salt 2 and PhSCF2CF2SO3Li
Samples Tg (°C) Tm (°C) H (J/g)
PhSCF2CF2SO3Lia 63 118 20
Salt 1 - 162 58
Salt 2n1 - 197 8
mPEG (n=3)b -73 - -
Salt 2n3 29 130 76
mPEG (n=7.2)b -64 -7 54
Salt 2n7 -11 - -
mPEG (n=16.3)b -60 22 118
Salt 2n16 -39 - -
a: data from thesis of E. Paillard, INPG (2008) [17]
b: data from thesis of C. Chauvin, INPG (20005) [18]
The thermal stability was studied by TGA (Figure 2.15). The percentage of weight loss of samples is followed as a function of temperature; the measurements were scanned from 25°C to 500°C under nitrogen with a heat rate of 5°C/min.
Figure 2.15 shows TGA curves of salt 2 with different chain lengths of mPEG. The onset of weight loss varying from 210 °C to 250 °C and depends on the lengths of mPEG. The decomposition of salt 2n3 starts at 210°C, similar to that of an aryl lithium perfluorosulfonte salt (PhSCF2CF2SO3-Li+) [19] while the thermal stability is enhanced for the salts 2n7 and 2n16. Taking into account the similar structure of the salts and the degradation temperature of mPEG we suppose that the degradation starts at the same temperature for all the salts but with the increase of mPEG molecular weight, the salts are more dissociated and interact with mPEG chains and probably its resulted degradation fragments, decreasing their volatility.
Chapter II: Synthesis and characterization of a family of oligoethers end-capped by lithium aryl perfluorosulfonates
63 Figure 2.15. Thermogravimetric analysis of aromatic lithium salts.
II.2.3.2. Intrinsic conductivity of salts
The conductivities of the salts were measured by electrochemical impedance spectroscopy in the frequency range of 5 Hz – 13 MHz and temperature range from 20° to 110°C. In this range of temperature only salt 2 n7 and salt 2 n16 are in their amorphous state while the others are crystalline and don’t present any measurable conductivity. Figure 2.16 shows the conductivity curves of salt 2n7 and salt 2n16 as a function of 1000/T. It can be seen that, the conductivity of salt 2n16 is higher than that of salt 2n7 with 2.91×10-4 S/cm at 110°C and 7.22×10-6 S/cm at 30 °C respectively, despite a higher ionic concentration of the salt 2n7. However, to solvate the Li and to dissociate the lithium salt at least five oxyethylene units are necessary. The interaction of Li+ with several oxygen leads to inter-chains physical cross-linking that could stiffen the system. The increase of molar ratio O/Li contributes both (i) to the solvation of Li+ and thus the increase of the dissociation, and (ii) to the increase in ions mobility, proved also by the lower Tg in the case of salt 2n16. However by analogy with the Li+ migration, in the case of poly(oxyethylene) (POE), the Li+ migration from one coordinating site to another is controlled by the segmental motion, that might be higher is the case of salt 2n7 (lower Tg values).
-100 -80 -60 -40 -20 0
0 100 200 300 400 500
Weight Loss (%)
Temperature (°C)
salt 2n3 salt 2n7 salt 2n16 mPEG n=16
Chapter II: Synthesis and characterization of a family of oligoethers end-capped by lithium aryl perfluorosulfonates
64 Figure 2.16. Conductivity of aromatic lithium salt in a family of salt 2.
II.2.3.3. Diffusion coefficient of salts
The self-diffusion coefficient of anion (D19F) and Li+ (D7Li) of salts 2n7 and n16, measured by multinuclear Pulse Field Gradient NMR (7Li and 19F) are gathered in the Table 2.6. It is observed that the diffusion coefficients of anions are lower than that of lithium for both salts. As for comparison of self-diffusion coefficient of both salts, the values of anions (F) are very close while a factor more than 2 is obtained for the diffusion coefficient values of lithium cations. The diffusion coefficient is directly related to the viscosity of the salt and the bulkiness of species that are diffusing. Taking into account the differences in size of mPEG and the close values of diffusion coefficients of anions of both salts it can be supposed that the viscosity of salt 2n16 is lower than of that of salt 2n7. The higher value of diffusion coefficient of Li in the salt 2n16 is due to higher flexibility and segmental motion of mPEG chain as compared to those of mPEG from salt 2n7.
1.00E-06 1.00E-05 1.00E-04 1.00E-03
2.5 2.7 2.9 3.1 3.3 3.5
Conductivity (S/cm)
1000/T (K-1)
salt 2n7
salt 2n16
Chapter II: Synthesis and characterization of a family of oligoethers end-capped by lithium aryl perfluorosulfonates
65 Table 2.6 Comparison of diffusion coefficients determined by PFG-NMR at 383K
Salt 2n7 Salt 2n16
D19F(cm2s-1) 0.69×10-7 0.67×10-7
D7Li(cm2s-1) 1.22×10-7 1.52×10-7
Nernst (Scm-1) 4.95×10-4 4.07×10-4
exp (Scm-1) 2.52×10-4 2.91×10-4
% dissociation 51 71
With the help of self-diffusion coefficients the conductivities can be calculated (calc ) by using Nernst-Einstein equation.
� � = 2 ++ −
�
(4) Where F Faraday constant (9.6485×106 Coulomb/mol), R gas constant (8.31446 J/(K mol), C ionic molar concentration mol.
However, this equation is valid for the infinitely dilute solution. For our salts, it is sure the calculated values are not correct but it permits the comparison between the two salts.
In order to calculate the ionic molar concentration (C), we first calculate the salt density by using the next formula: [18]
LiPST LiPST
PEG LIPST PEG
LiPST PEG
PEG
salt M M
M M
M
M
* *
(5)
where MPEG et PEG the molecular weight and density of mPEG used in the synthesis of salt 2, MLiPST is the molecular weight of LiPST et LiPST is the density of LIPST measured with a pycnometer (2.4915 g/cm3). The values of density of different salts are gathered in Table 2.7 and the calculated values of conductivity of salt 2n7 and n16 are presented in Table 2.6.
Chapter II: Synthesis and characterization of a family of oligoethers end-capped by lithium aryl perfluorosulfonates
66
Density (g/cm3) Concentration (mol/cm3)
Salt 2 n1 2.20 5.94×10-3
Salt 2 n3 1.99 4.35×10-3
Salt 2 n7 1.75 2.73×10-3
Salt 2 n16 1.51 1.45×10-3
Table 2.7 Density and molar concentration of salt 2
The NMR provides the diffusion coefficient of a specie but it does not discriminate between the charged and neutral species (dissociated anion-cation or ion pairs), while the ionic conductivity measure the diffusion of only the charged species. Hence, by the ratio between exp and calc (equation (5)),, the dissociation degrees can be inferred (Table 2.5).
% � �� � = �����
� × (6)
For both salts values of 51 % and 71 % were calculated for salt 2n7 and salt 2n16 respectively. These results indicate a higher dissociation of salt 2n16 as compared with salt 2n7. The higher dissociation degree can explain the higher conductivity of salt 2n16 despite a lower lithium concentration.
II.2.3.4. Cationic transference number
One of the factors affecting the performances of lithium salt is the transference number. The ideal transference number is T+ = 1, that means the conductivity is done only by the migration of Li+. But for most of the salts the migration of the anion is much higher than that of cation and the existence of anionic mobility in the electrolyte leads to the detrimental effect of large salt concentration gradient causing the concentration polarization and limiting the power output of the cell.
The intrinsic cationic transference numbers were determined here starting from diffusion coefficients using the next equation (7), where D+ is D7Li and D- is D19F.
�+ = ++ + − (7)
Chapter II: Synthesis and characterization of a family of oligoethers end-capped by lithium aryl perfluorosulfonates
67 The cationic conductivities are calculated with the equation: + = T+ where is experimental conductivity. (Table 2.8)
Table 2.8 Comparison of intrinsic cationic transference numbers and cationic conductivities of salt 2n7 and salt 2n16 investigated by PFG-NMR at 383K
Salt O/Li
(S/cm)
T+
+ (S/cm)
Salt 2n7 7.2 2.52×10-4 0.64 1.61×10-4
Salt 2n16 16.3 2.91×10-4 0.72 2.09×10-4
From the result in Table 2.8, the transference number of salt 2n16 (T+ = 0.72) exhibits higher value than that of salt 2n7 (T+ = 0.64). The cationic conductivities reached 2.09×10-4 S/cm for salt 2n16, higher than for salt 2n7 i.e. 1.61×10-4 S/cm. These results are different from those reported by C. Chauvin [18] for POE oligomer mono-end capped by sulfate group (CH3-O (-CH2-CH2-O)n-OSO3Li for which they reported a higher T+ for the salts with shorter mPEG (n= 7 T+ = 0.58, n=16 T+=0.46).
II.2.3.5. Electrochemical stability of salts
The cyclic voltammetry measurements (figure 2.17) were performed in ACN + 0.1 M TEABF4 as support electrolyte, scan rate of 10 mVs-1 at 20°C. The salt was dissolved in the electrolyte with a concentration of 5 mmol/L. The oxidation wall of the salt 2 starts at 4.25 V versus Li/Li+ and seems to be independent of the chain length of mPEG and very close to that of PhSCF2CF2SO3Li (4.3 V) [17]. Moreover, the stability seems to be slightly higher than that of POE that is known to be close to 3.9 V versus Li/Li+. This result is a good surprise and is in line with theoretical calculations performed by Chauvin et al. [18] Concerning the salt 1 its stability seems to be slightly lower, a first shoulder appear to 3.9 V versus Li/Li+ probably due to the presence of the double bond. However, all synthesized salts seem to be enough stable for an utilization in lithium polymer batteries.
Chapter II: Synthesis and characterization of a family of oligoethers end-capped by lithium aryl perfluorosulfonates
68 Figure 2.17. Cyclic voltamperometry of salt 1, salt 2n3 and salt 2n16 at 5 mmol/l in ACN + TEABF4 (0.1 M), scan rate: 10 mVs-1 at 20°C on platinum as working electrode.
Table 2.9 Oxidation potential of different salts in the solution of TEABF4
Salt Oxidation potential
(V vs Li/Li+) Supporting electrolyte (CH3CN + TEABF4) 5.3
Salt 1 3.9
Salt 2n1 4.25
Salt 2n3 4.25
Salt 2n7 4.25
Salt 2n16 4.25
0 1 2 3 4 5 6 7
-4,0 -3,5 -3,0 -2,5 -2,0 -1,5 -1,0 -0,5 0,0 0,5 1,0 1,5 2,0
salt 1 salt 2n3 salt 2n16 Blank solution Current density (mA/cm2 )
Potential (V vs Li/Li+)
Chapter II: Synthesis and characterization of a family of oligoethers end-capped by lithium aryl perfluorosulfonates
69