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Study of lithium ion conducting glasses with Li2SO4 addition
Journal of Non-Crystalline Solids 527 (2020) 119737
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Study of lithium ion conducting glasses with Li2SO4 addition V.K. Deshpande , Megha A. Salorkar, Nalini Nagpure ⁎
T
Department of Physics, Visvesvaraya National Institute of Technology, Nagpur 440010, India
ARTICLE INFO
ABSTRACT
Keywords: Lithium Borosilicate glass Ionic Conductivity Density Tg FTIR
Glasses of series 40Li2O: (40−x)B2O3: 20SiO2: xLi2SO4 have been synthesized. The Tg, density, ionic conductivity, ionic transference number and FTIR of these glasses were studied. It was observed that with Li2SO4 addition, the Tg and density of these glasses decreased, while the ionic conductivity increased. There is a good correlation among Tg, density and electrical conductivity results. Ionic transference number suggest that glass samples studied in the present work are predominantly ion conducting in nature. The FTIR results also support the observed results of ionic conductivity with Li2SO4 addition. Among the glass samples studied, the maximum conductivity of 1.46 × 10-2 S/cm at 523 K is exhibited by the glass sample 40Li2O: 32.5B2O3: 20SiO2: 7.5Li2SO4. This glass composition has the potential for being used as a solid electrolyte.
1. Introduction The higher energy and power density are required in energy storage systems. The problem can be partly solved by the batteries. The suitable solid electrolyte material is essential for the solid-state batteries. There are some merits of glassy solid electrolytes viz. the isotropic nature, lack of grain boundaries, ease in thin film preparation, synthesis in various size and shapes over a wide range of composition [1,2]. Glassy solid electrolytes are more important for electrochemical applications [1,3–7]. Kulkarni et al have reviewed fast ion conducting lithium glasses [8]. An equivalent weight of lithium is lower than other alkali atoms. Since it is most electropositive, it gives higher cell voltage. Hence with a view to develop glass with high lithium ion conductivity several researchers are working in this field. Lithium borate (LB) glasses consists of a boron network in BO3 and BO4 forms along with non-bridging oxygen (NBO) [9]. It has been reported [10] that an increase in the alkali ion concentration gives rise to enhancement in the electrical conductivity. However, there is a limit to increase in alkali concentration in LB glasses. The addition of lithium salts viz halides, sulphates, phosphates, have been reported [11–17] to give enhancement in the conductivity of LB glasses. The incorporation of Li2SO4 in the glass gives rise to increase in ionic conductivity of LB glasses [2]. These glasses are hygroscopic and hence their stability is an issue. This problem can be partially addressed by the SiO2 addition in LB glasses. 40Li2O: 40B2O3: 20SiO2 (LBS) is the eutectic composition in the ternary glass system. Eutectic is the lowest melting composition and hence it has a possibility of higher conductivity. The addition of Li2SO4 in the above eutectic glass system at the cost of both glass formers keeping Li2O content fixed has been reported [7] to give increase in the
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ionic conductivity. The reasons for studying the LBS eutectic glass with Li2SO4 addition are as follows: i. Li2SO4 is chemically durable. ii. The Li2SO4 added glass presents extensive redox stability. iii. If SO42- is not incorporated in the macromolecular network, it's mobility will be negligible due to its size and charge. If it is incorporated then its alkali transport number will be zero, in either case, lithium cations will exclusively contribute to the conductivity of the glass system. Hence the study of LBS eutectic glass with Li2SO4 addition has been undertaken. In particular the density, Tg, electrical conductivity (σ) and FTIR have been studied. 2. Experimental details The glasses prepared in the present work can be represented by 40Li2O: (40−x)B2O3: 20SiO2: xLi2SO4, where x = 0, 2.5, 5, 7.5, 10, 12.5 mol% and the glass samples can be denoted by G0, G1, G2, G3, G4, G5 respectively. The glasses of this system were synthesized by using starting materials Li2CO3 (Merck, >99 %), B2O3 (Merck, >99.9 %), SiO2 (Sigma- Aldrich, >99.9 %), Li2SO4.H2O (Loba Chemie, >99 %). All the raw materials were mixed properly in a mortar-pestle and then heated in a platinum crucible. The melting temperatures of all the glass samples were in the range from 1073 K to 1223 K. Melt was quenched in a cylindrical aluminium mould at room temperature and glasses were annealed at 623 K for 2 h. The Tg of each glass sample was measured with a heating rate of 5 °C/min by DTA (DTG-60, Shimadzu). The
Corresponding author.
https://doi.org/10.1016/j.jnoncrysol.2019.119737 Received 22 July 2019; Received in revised form 23 September 2019; Accepted 6 October 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.
Journal of Non-Crystalline Solids 527 (2020) 119737
V.K. Deshpande, et al.
density of glass samples was measured by the Archimedes principle with toluene as an immersion liquid. The electrical conductivity of glasses was measured with respect to temperature and frequency using Alpha Analyzer in the range from 373 K to 573 K and from 0.1 Hz to 20 MHz. The ionic transference number was measured by DC polarization technique using KEITHLEY 6512 programmable electrometer. The IR spectroscopy study was done by using IRAffinity-1, Shimadzu.
3. Result and discussion 3.1. DTA The results of DTA have been shown in Fig. 1. Fig. 2 shows the variation of Tg with respect to mol% of Li2SO4. The values of Tg and Tc for the glasses are tabulated in Table 1. It shows that the Tg decreases with increase in Li2SO4 content. This decreasing nature of Tg is due to
Fig. 1. DTA for all glass samples.
Fig. 2. Variation of Tg with Li2SO4 content. 2
Journal of Non-Crystalline Solids 527 (2020) 119737
V.K. Deshpande, et al.
Table 1 Values of Tg, Tc and ∆T for the system of glass.
Table 2 The values of ρ and Vm for all glass samples.
Sample no.
Glass transition Temp (Tg) (K)
Crystallization Temp (Tc) (K)
*∆T = Tc−Tg (K)
G0 G1 G2 G3 G4 G5
720 707 683 680 671 653
990 983 980 979 983 809
270 276 297 299 312 156
The values of ∆T (∆T = Tc−Tg) indicates the stability of the glass. It is evident from the table that the stability of glass increases with increase in mol% of Li2SO4.
Sample no.
ρ (g/cm3)
Vm (cm3)
G0 G1 G2 G3 G4 G5
2.40 2.28 2.27 2.26 2.31 2.34
28.92 31.08 31.86 32.65 32.57 32.52
up to 7.5 mol% and beyond this, it increases for 10 mol% Li2SO4 containing glass sample. The variation of electrical conductivity as a function of Li2SO4 content is exactly opposite to that of activation energy. The results for the glass sample containing 12.5 mol% Li2SO4 could not be included due to the low softening temperature of this sample. The electrical conductivity at 523 K, Ea and lithium mole fraction (fLi) for all compositions are given in Table 3. The observed increase in σ with Li2SO4 addition may be understood in the light of (a) increase in lithium mole fraction (fLi), (b) increase in interstices which leads to an increase in mobility of Li+ and (c) increase in non-bridging oxygens (NBOs) due to Li2SO4 addition [18]. The addition of Li2SO4 beyond 7.5 mol% causes filling of vacant sites or interstices which may lead to a decrease in lithium-ion mobility which in turn decreases the σ. The electrical conductivity results are supported by the density results discussed earlier. The LBS glass sample 40Li2O: 32.5B2O3: 20SiO2: 7.5Li2SO4 exhibited the highest ionic conductivity 1.46 × 10-2 S/cm at 523 K in the entire work. This is two orders of magnitude higher than the LBS glass containing Li2SO4 reported earlier [7]. Thus, even though the base system is the same, the addition of Li2SO4 at the cost of B2O3 has given higher ionic conductivity compared to the glass system wherein Li2SO4 added at the cost of both formers.
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the incorporation of SO4 tetrahedra which causes expansion of the glass network and hence decreases the rigidity [7,18]. This suggests that Tg can be attributed to changes in the structural groupings such as SO4 tetrahedra which weaken the glass network. Similar results have been reported earlier [19] which supports the present findings. 3.2. Density The varying nature of density (ρ) and molar volume (Vm) for the system of glass is depicted in Fig. 3. The values of ρ and Vm are given in Table 2. It is found that the ρ decreases with increase in mol% of Li2SO4 up to 7.5 and then increases. This decrease in density is due to the expansion of the glassy network. Increase in Vm is related with an increase in the interstices in the glass. This may lead to an increase in Li+ mobility giving rise to an enhancement in electrical conductivity of the glass samples. Further, the addition of Li2SO4 (10 and 12.5 mol%) causes an increase in density. This is because of the occupancy of vacant sites in the glass matrix and conversion of BO3 units to BO4 units which results in an increase in network linkage [20]. Similar results have been reported [21] earlier which supports the present work.
3.4. DC polarization
3.3. Electrical conductivity
To determine the ionic transference number of glass samples, DC polarization technique has been used. Fig. 6 depicts the variation of current versus time for sample G3 observed by using KEITHLEY programmable electrometer. Similar results have been obtained for all other samples. Ionic transference number has been calculated by using
The Arrhenius plots for all the glass samples are depicted in Fig. 4. The variation of activation energy (Ea) and electrical conductivity (σ) at 523 K with mol% of Li2SO4 is displayed in Fig. 5. It shows that the activation energy for the glass samples decreases with Li2SO4 addition
Fig. 3. Nature of ρ and Vm for all glass samples. 3
Journal of Non-Crystalline Solids 527 (2020) 119737
V.K. Deshpande, et al.
Fig. 4. Arrhenius plots for all glass samples.
Fig. 5. Variation of σ at 523 K and Ea.
equation ti = (σt−σe)/σt, where σt is total electrical conductivity and σe is electronic conductivity. For all the glass samples the ionic transference number ti was observed to be > 0.986 which indicates that the present glass system exhibits predominantly ionic conductivity.
Table 3 Values of σ at 523 K, activation energy (Ea), lithium mole fraction (fLi) for all glass samples. Sample no.
σ (S/cm)
G0 G1 G2 G3 G4
6.23 2.91 5.22 1.46 1.53
× × × × ×
10−4 10−3 10−3 10−2 10−3
Ea (eV)
*fLi
0.86 0.85 0.84 0.81 0.98
0.8 0.85 0.9 0.95 1
3.5. Modulus study and scaling The modulus study has been carried out for all glass samples. The variation of the imaginary part of electric modulus (M″) with respect to applied frequency for 7.5 mol% Li2SO4 containing glass sample (G3) is as shown in Fig. 7. It is found that, as the temperature increases, the peak height of M″
⁎ fLi = (2nLi2O+2nLi2SO4)/(nLi2O+nB2O3+nSiO2+nLi2SO4), Where n is mol% of that constituent.
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Journal of Non-Crystalline Solids 527 (2020) 119737
V.K. Deshpande, et al.
Fig. 6. Variation of current (μA) versus time (min) for G3 glass sample.
Fig. 7. Variation of M″ versus frequency at different temperature for sample G3.
goes on increasing and the graph is shifted towards higher frequency side. By using relaxation frequency (fmax) which corresponds to the peak value of M″max, relaxation time (τ) for each glass sample has been calculated. The frequency region below fmax indicates the motion of charge carriers over long distances whereas frequency region beyond the fmax determines the localized motion of charge carriers inside the potential well [22,23]. The relaxation time plot with respect to temperature for all the compositions is shown in Fig. 8. It is found that the sample G3 has less relaxation time as compared to other glass samples. Here, G3 sample possesses minimum relaxation time which indicates that short time interval for the propagation of Li+. This explains the observed
maximum conductivity for this sample [21,24]. The variation of M″/M″max versus log(f/fmax) for G3 sample is displayed in Fig. 9 which is used to study the relaxation mechanism scaling. It is evident from this figure that, all the isotherms overlap into a single master curve which means the relaxation mechanism is independent of temperature for the glass sample [25–27]. All other samples exhibited similar results. To study the composition dependence of the conduction mechanism, compositional scaling has been done. The plot of log(σ/σdc) versus log (f.x/σdc.T) is shown in Fig. 10. Here, each isotherm retains their unique behaviour for the glass sample which indicates that the conduction mechanism is dependent on 5
Journal of Non-Crystalline Solids 527 (2020) 119737
V.K. Deshpande, et al.
Fig. 8. Plot of Relaxation time with respect to temperature.
Fig. 9. Normalised modulus plots of M″/M″max versus log(f/fmax) for G3 sample.
composition. This conclusion is based on the Scaling model proposed by B. Roling [28,29]. Addition of Li2SO4 at the cost of B2O3 causes a change in the glass structure which involves the creation of NBOs, enhancement in the concentration of interstices or vacant sites, etc. It also shows reverse effects after a certain concentration of Li2SO4 (in the present work for glasses beyond 7.5 mol% Li2SO4). As the vacant sites and NBOs govern the mobility of Li ion [30,31], the conduction mechanism is composition dependent (Fig. 10).
3.6. FTIR The FTIR results for all the glass samples shown in Fig. 11. Various peaks and the structural groups assigned with them are tabulated in Table 4. For the samples containing Li2SO4, the bands around 1136 cm−1 were assigned to vibrations of BO3 units with large number of NBOs [32,37]. This band is not present in the base sample which supports the increase in the NBOs and in turn an increase in conductivity with the 6
Journal of Non-Crystalline Solids 527 (2020) 119737
V.K. Deshpande, et al.
Fig. 10. Compositional Scaling for Li2SO4 added glass samples at 523 K.
Fig. 11. FTIR spectra for all glass samples.
addition of Li2SO4. The figure also indicates that there is shifting of bands towards higher wave number side with the addition of Li2SO4 which is assigned to the formation of BO3 units with NBOs at the cost of BO4 units [7,43]. The intensity of these bands increases with the addition of Li2SO4 which also supports the observed conductivity results.
4. Conclusion It can be concluded from the present work that with the addition of Li2SO4 in the LBS eutectic glass, the Tg, ρ decreases. The ionic conductivity of glass increases with Li2SO4 content up to 7.5 mol%. This is due to the increase in the fLi and an increase in NBOs caused by Li2SO4
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Journal of Non-Crystalline Solids 527 (2020) 119737
V.K. Deshpande, et al.
Table 4 The IR spectra for all the compositions with their particulates. Peak position (cm−1)
Assignment
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addition. FTIR result supports the increase in NBOs with Li2SO4 addition. The study of modulus spectra and scaling behaviour suggest that the conduction mechanism is dependent on the composition. The LBS glass sample 40Li2O: 32.5B2O3: 20SiO2: 7.5Li2SO4 exhibited the highest ionic conductivity 1.46 × 10-2 S/cm at 523 K in the entire work. Declaration of Competing Interest In this article, the effect of Li2SO4 has been studied on the physical and electrical properties of lithium borosilicate glasses with Li2SO4 addition. There exists a good correlation among Tg, density, electrical conductivity and FTIR. There is no conflict of interest to declare. References [1] D. Ravaine, Glasses as solid electrolytes, J. Non Cryst. Solids 38–39 (1980) 353–358, https://doi.org/10.1016/0022-3093(80)90444-5. [2] P.R. Gandhi, V.K. Deshpande, K. Singh, Conductivity enhancement in Li2SO4 incorporated Li2O:B2O3 glass system, Solid State Ion. 36 (1989) 97–102, https://doi. org/10.1016/0167-2738(89)90066-0. [3] Solid electrolyte batteries, in: B.B. Owens, C.W. Tobias (Eds.), Advances in Electrochemistry and Electrochemical Engineering, 8 Wiley, New York, 1971, pp. 1–62 chap 1. [4] W.V. Gool, Fast Ion Transport in Solids: Solid State Batteries and Devices, Elsevier Science Publishing Co Inc., U.S., 1973 ISBN 10: 0720402239, ISBN 13: 9780720402230. [5] J. Akridge, H. Vourlis, Solid state batteries using vitreous solid electrolytes, Solid State Ion. 18–19 (1986) 1082–1087, https://doi.org/10.1016/0167-2738(86) 90313-9. [6] C. Julien, Technological applications of solid state ionics, Mater. Sci. Eng. B 6 (1990) 9–28, https://doi.org/10.1016/0921-5107(90)90109-O. [7] S.S. Gundale, V.V. Behare, A.V. Deshpande, Study of electrical conductivity of Li2O–B2O3–SiO2–Li2SO4 glasses and glass-ceramics, Solid State Ion. 298 (2016) 57–62, https://doi.org/10.1016/j.ssi.2016.11.002. [8] A.R. Kulkarni, H.S. Maiti, A. Paul, Fast ion conducting lithium glasses-review, Bull. Mater. Sci. 6 (1984) 201–221, https://doi.org/10.1007/BF02743897. [9] P.J. Bray, J.G. O'Keefe, Nuclear magnetic resonance investigations of the structure of alkali borate glasses, Phys. Chem. Glasses 4 (1963) 37–46. [10] H.L. Tuller, D.P. Button, Transport- structure relations in fast ion and mixed conductors, Proc. 6th Int. Symp. on Metallurgy and Material Science, RISØ, Denmark, 1985, p. 119 ISBN 87-550-1137-3. [11] B. Calès, A. Levasseur, C. Fouassier, J.M. Réau, P. Hagenmuller, Conductivite ionique du lithium dans les solutions solides de structure boracite Li4+xB7O12+x/2X (X = Cl, Br) (0 ≤ x ≤ 1), Solid State Commun. 24 (1977) 323–325, https://doi. org/10.1016/0038-1098(77)90219-8. [12] A. Levasseur, M. Kbala, J.C. Brethous, J.M. Réau P. Hagenmuller, M. Couzi, Etudes electrique et raman des verres du systeme, B2O3–Li2O–Li2SO4, Solid State Commun. 32 (1979) 839–844, https://doi.org/10.1016/0038-1098(79)90482-4. [13] M. Irion, M. Couzi, A. Levasseur, J.M. Reau, J.C. Brethous, An infrared and Raman study of new lonic-conductor lithium glasses, J. Solid State Chem. 31 (1980) 285–294, https://doi.org/10.1016/0022-4596(80)90090-0. [14] A. Levasseur, J.C. Brethous, J.M. Reau, P. Hagenmuller, M. Couzi, Synthesis and characterization of new solid electrolyte conductors of lithium ions, Solid State Ion. 1 (1980) 177–186, https://doi.org/10.1016/0167-2738(80)90002-8. [15] V.K. Deshpande, S. Rokade, K. Singh, Transport- structure relations in fast ion and mixed conductors, Proc. 6th Int. Symp. on Metallurgy and Material Science, RISØ, Denmark, 1985, p. 227 ISBN 87-550-1137-3.
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Study of lithium ion conducting glasses with Li2SO4 addition V.K. Deshpande , Megha A. Salorkar, Nalini Nagpure ⁎
T
Department of Physics, Visvesvaraya National Institute of Technology, Nagpur 440010, India
ARTICLE INFO
ABSTRACT
Keywords: Lithium Borosilicate glass Ionic Conductivity Density Tg FTIR
Glasses of series 40Li2O: (40−x)B2O3: 20SiO2: xLi2SO4 have been synthesized. The Tg, density, ionic conductivity, ionic transference number and FTIR of these glasses were studied. It was observed that with Li2SO4 addition, the Tg and density of these glasses decreased, while the ionic conductivity increased. There is a good correlation among Tg, density and electrical conductivity results. Ionic transference number suggest that glass samples studied in the present work are predominantly ion conducting in nature. The FTIR results also support the observed results of ionic conductivity with Li2SO4 addition. Among the glass samples studied, the maximum conductivity of 1.46 × 10-2 S/cm at 523 K is exhibited by the glass sample 40Li2O: 32.5B2O3: 20SiO2: 7.5Li2SO4. This glass composition has the potential for being used as a solid electrolyte.
1. Introduction The higher energy and power density are required in energy storage systems. The problem can be partly solved by the batteries. The suitable solid electrolyte material is essential for the solid-state batteries. There are some merits of glassy solid electrolytes viz. the isotropic nature, lack of grain boundaries, ease in thin film preparation, synthesis in various size and shapes over a wide range of composition [1,2]. Glassy solid electrolytes are more important for electrochemical applications [1,3–7]. Kulkarni et al have reviewed fast ion conducting lithium glasses [8]. An equivalent weight of lithium is lower than other alkali atoms. Since it is most electropositive, it gives higher cell voltage. Hence with a view to develop glass with high lithium ion conductivity several researchers are working in this field. Lithium borate (LB) glasses consists of a boron network in BO3 and BO4 forms along with non-bridging oxygen (NBO) [9]. It has been reported [10] that an increase in the alkali ion concentration gives rise to enhancement in the electrical conductivity. However, there is a limit to increase in alkali concentration in LB glasses. The addition of lithium salts viz halides, sulphates, phosphates, have been reported [11–17] to give enhancement in the conductivity of LB glasses. The incorporation of Li2SO4 in the glass gives rise to increase in ionic conductivity of LB glasses [2]. These glasses are hygroscopic and hence their stability is an issue. This problem can be partially addressed by the SiO2 addition in LB glasses. 40Li2O: 40B2O3: 20SiO2 (LBS) is the eutectic composition in the ternary glass system. Eutectic is the lowest melting composition and hence it has a possibility of higher conductivity. The addition of Li2SO4 in the above eutectic glass system at the cost of both glass formers keeping Li2O content fixed has been reported [7] to give increase in the
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ionic conductivity. The reasons for studying the LBS eutectic glass with Li2SO4 addition are as follows: i. Li2SO4 is chemically durable. ii. The Li2SO4 added glass presents extensive redox stability. iii. If SO42- is not incorporated in the macromolecular network, it's mobility will be negligible due to its size and charge. If it is incorporated then its alkali transport number will be zero, in either case, lithium cations will exclusively contribute to the conductivity of the glass system. Hence the study of LBS eutectic glass with Li2SO4 addition has been undertaken. In particular the density, Tg, electrical conductivity (σ) and FTIR have been studied. 2. Experimental details The glasses prepared in the present work can be represented by 40Li2O: (40−x)B2O3: 20SiO2: xLi2SO4, where x = 0, 2.5, 5, 7.5, 10, 12.5 mol% and the glass samples can be denoted by G0, G1, G2, G3, G4, G5 respectively. The glasses of this system were synthesized by using starting materials Li2CO3 (Merck, >99 %), B2O3 (Merck, >99.9 %), SiO2 (Sigma- Aldrich, >99.9 %), Li2SO4.H2O (Loba Chemie, >99 %). All the raw materials were mixed properly in a mortar-pestle and then heated in a platinum crucible. The melting temperatures of all the glass samples were in the range from 1073 K to 1223 K. Melt was quenched in a cylindrical aluminium mould at room temperature and glasses were annealed at 623 K for 2 h. The Tg of each glass sample was measured with a heating rate of 5 °C/min by DTA (DTG-60, Shimadzu). The
Corresponding author.
https://doi.org/10.1016/j.jnoncrysol.2019.119737 Received 22 July 2019; Received in revised form 23 September 2019; Accepted 6 October 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.
Journal of Non-Crystalline Solids 527 (2020) 119737
V.K. Deshpande, et al.
density of glass samples was measured by the Archimedes principle with toluene as an immersion liquid. The electrical conductivity of glasses was measured with respect to temperature and frequency using Alpha Analyzer in the range from 373 K to 573 K and from 0.1 Hz to 20 MHz. The ionic transference number was measured by DC polarization technique using KEITHLEY 6512 programmable electrometer. The IR spectroscopy study was done by using IRAffinity-1, Shimadzu.
3. Result and discussion 3.1. DTA The results of DTA have been shown in Fig. 1. Fig. 2 shows the variation of Tg with respect to mol% of Li2SO4. The values of Tg and Tc for the glasses are tabulated in Table 1. It shows that the Tg decreases with increase in Li2SO4 content. This decreasing nature of Tg is due to
Fig. 1. DTA for all glass samples.
Fig. 2. Variation of Tg with Li2SO4 content. 2
Journal of Non-Crystalline Solids 527 (2020) 119737
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Table 1 Values of Tg, Tc and ∆T for the system of glass.
Table 2 The values of ρ and Vm for all glass samples.
Sample no.
Glass transition Temp (Tg) (K)
Crystallization Temp (Tc) (K)
*∆T = Tc−Tg (K)
G0 G1 G2 G3 G4 G5
720 707 683 680 671 653
990 983 980 979 983 809
270 276 297 299 312 156
The values of ∆T (∆T = Tc−Tg) indicates the stability of the glass. It is evident from the table that the stability of glass increases with increase in mol% of Li2SO4.
Sample no.
ρ (g/cm3)
Vm (cm3)
G0 G1 G2 G3 G4 G5
2.40 2.28 2.27 2.26 2.31 2.34
28.92 31.08 31.86 32.65 32.57 32.52
up to 7.5 mol% and beyond this, it increases for 10 mol% Li2SO4 containing glass sample. The variation of electrical conductivity as a function of Li2SO4 content is exactly opposite to that of activation energy. The results for the glass sample containing 12.5 mol% Li2SO4 could not be included due to the low softening temperature of this sample. The electrical conductivity at 523 K, Ea and lithium mole fraction (fLi) for all compositions are given in Table 3. The observed increase in σ with Li2SO4 addition may be understood in the light of (a) increase in lithium mole fraction (fLi), (b) increase in interstices which leads to an increase in mobility of Li+ and (c) increase in non-bridging oxygens (NBOs) due to Li2SO4 addition [18]. The addition of Li2SO4 beyond 7.5 mol% causes filling of vacant sites or interstices which may lead to a decrease in lithium-ion mobility which in turn decreases the σ. The electrical conductivity results are supported by the density results discussed earlier. The LBS glass sample 40Li2O: 32.5B2O3: 20SiO2: 7.5Li2SO4 exhibited the highest ionic conductivity 1.46 × 10-2 S/cm at 523 K in the entire work. This is two orders of magnitude higher than the LBS glass containing Li2SO4 reported earlier [7]. Thus, even though the base system is the same, the addition of Li2SO4 at the cost of B2O3 has given higher ionic conductivity compared to the glass system wherein Li2SO4 added at the cost of both formers.
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the incorporation of SO4 tetrahedra which causes expansion of the glass network and hence decreases the rigidity [7,18]. This suggests that Tg can be attributed to changes in the structural groupings such as SO4 tetrahedra which weaken the glass network. Similar results have been reported earlier [19] which supports the present findings. 3.2. Density The varying nature of density (ρ) and molar volume (Vm) for the system of glass is depicted in Fig. 3. The values of ρ and Vm are given in Table 2. It is found that the ρ decreases with increase in mol% of Li2SO4 up to 7.5 and then increases. This decrease in density is due to the expansion of the glassy network. Increase in Vm is related with an increase in the interstices in the glass. This may lead to an increase in Li+ mobility giving rise to an enhancement in electrical conductivity of the glass samples. Further, the addition of Li2SO4 (10 and 12.5 mol%) causes an increase in density. This is because of the occupancy of vacant sites in the glass matrix and conversion of BO3 units to BO4 units which results in an increase in network linkage [20]. Similar results have been reported [21] earlier which supports the present work.
3.4. DC polarization
3.3. Electrical conductivity
To determine the ionic transference number of glass samples, DC polarization technique has been used. Fig. 6 depicts the variation of current versus time for sample G3 observed by using KEITHLEY programmable electrometer. Similar results have been obtained for all other samples. Ionic transference number has been calculated by using
The Arrhenius plots for all the glass samples are depicted in Fig. 4. The variation of activation energy (Ea) and electrical conductivity (σ) at 523 K with mol% of Li2SO4 is displayed in Fig. 5. It shows that the activation energy for the glass samples decreases with Li2SO4 addition
Fig. 3. Nature of ρ and Vm for all glass samples. 3
Journal of Non-Crystalline Solids 527 (2020) 119737
V.K. Deshpande, et al.
Fig. 4. Arrhenius plots for all glass samples.
Fig. 5. Variation of σ at 523 K and Ea.
equation ti = (σt−σe)/σt, where σt is total electrical conductivity and σe is electronic conductivity. For all the glass samples the ionic transference number ti was observed to be > 0.986 which indicates that the present glass system exhibits predominantly ionic conductivity.
Table 3 Values of σ at 523 K, activation energy (Ea), lithium mole fraction (fLi) for all glass samples. Sample no.
σ (S/cm)
G0 G1 G2 G3 G4
6.23 2.91 5.22 1.46 1.53
× × × × ×
10−4 10−3 10−3 10−2 10−3
Ea (eV)
*fLi
0.86 0.85 0.84 0.81 0.98
0.8 0.85 0.9 0.95 1
3.5. Modulus study and scaling The modulus study has been carried out for all glass samples. The variation of the imaginary part of electric modulus (M″) with respect to applied frequency for 7.5 mol% Li2SO4 containing glass sample (G3) is as shown in Fig. 7. It is found that, as the temperature increases, the peak height of M″
⁎ fLi = (2nLi2O+2nLi2SO4)/(nLi2O+nB2O3+nSiO2+nLi2SO4), Where n is mol% of that constituent.
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Journal of Non-Crystalline Solids 527 (2020) 119737
V.K. Deshpande, et al.
Fig. 6. Variation of current (μA) versus time (min) for G3 glass sample.
Fig. 7. Variation of M″ versus frequency at different temperature for sample G3.
goes on increasing and the graph is shifted towards higher frequency side. By using relaxation frequency (fmax) which corresponds to the peak value of M″max, relaxation time (τ) for each glass sample has been calculated. The frequency region below fmax indicates the motion of charge carriers over long distances whereas frequency region beyond the fmax determines the localized motion of charge carriers inside the potential well [22,23]. The relaxation time plot with respect to temperature for all the compositions is shown in Fig. 8. It is found that the sample G3 has less relaxation time as compared to other glass samples. Here, G3 sample possesses minimum relaxation time which indicates that short time interval for the propagation of Li+. This explains the observed
maximum conductivity for this sample [21,24]. The variation of M″/M″max versus log(f/fmax) for G3 sample is displayed in Fig. 9 which is used to study the relaxation mechanism scaling. It is evident from this figure that, all the isotherms overlap into a single master curve which means the relaxation mechanism is independent of temperature for the glass sample [25–27]. All other samples exhibited similar results. To study the composition dependence of the conduction mechanism, compositional scaling has been done. The plot of log(σ/σdc) versus log (f.x/σdc.T) is shown in Fig. 10. Here, each isotherm retains their unique behaviour for the glass sample which indicates that the conduction mechanism is dependent on 5
Journal of Non-Crystalline Solids 527 (2020) 119737
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Fig. 8. Plot of Relaxation time with respect to temperature.
Fig. 9. Normalised modulus plots of M″/M″max versus log(f/fmax) for G3 sample.
composition. This conclusion is based on the Scaling model proposed by B. Roling [28,29]. Addition of Li2SO4 at the cost of B2O3 causes a change in the glass structure which involves the creation of NBOs, enhancement in the concentration of interstices or vacant sites, etc. It also shows reverse effects after a certain concentration of Li2SO4 (in the present work for glasses beyond 7.5 mol% Li2SO4). As the vacant sites and NBOs govern the mobility of Li ion [30,31], the conduction mechanism is composition dependent (Fig. 10).
3.6. FTIR The FTIR results for all the glass samples shown in Fig. 11. Various peaks and the structural groups assigned with them are tabulated in Table 4. For the samples containing Li2SO4, the bands around 1136 cm−1 were assigned to vibrations of BO3 units with large number of NBOs [32,37]. This band is not present in the base sample which supports the increase in the NBOs and in turn an increase in conductivity with the 6
Journal of Non-Crystalline Solids 527 (2020) 119737
V.K. Deshpande, et al.
Fig. 10. Compositional Scaling for Li2SO4 added glass samples at 523 K.
Fig. 11. FTIR spectra for all glass samples.
addition of Li2SO4. The figure also indicates that there is shifting of bands towards higher wave number side with the addition of Li2SO4 which is assigned to the formation of BO3 units with NBOs at the cost of BO4 units [7,43]. The intensity of these bands increases with the addition of Li2SO4 which also supports the observed conductivity results.
4. Conclusion It can be concluded from the present work that with the addition of Li2SO4 in the LBS eutectic glass, the Tg, ρ decreases. The ionic conductivity of glass increases with Li2SO4 content up to 7.5 mol%. This is due to the increase in the fLi and an increase in NBOs caused by Li2SO4
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Journal of Non-Crystalline Solids 527 (2020) 119737
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Table 4 The IR spectra for all the compositions with their particulates. Peak position (cm−1)
Assignment
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addition. FTIR result supports the increase in NBOs with Li2SO4 addition. The study of modulus spectra and scaling behaviour suggest that the conduction mechanism is dependent on the composition. The LBS glass sample 40Li2O: 32.5B2O3: 20SiO2: 7.5Li2SO4 exhibited the highest ionic conductivity 1.46 × 10-2 S/cm at 523 K in the entire work. Declaration of Competing Interest In this article, the effect of Li2SO4 has been studied on the physical and electrical properties of lithium borosilicate glasses with Li2SO4 addition. There exists a good correlation among Tg, density, electrical conductivity and FTIR. There is no conflict of interest to declare. References [1] D. Ravaine, Glasses as solid electrolytes, J. Non Cryst. Solids 38–39 (1980) 353–358, https://doi.org/10.1016/0022-3093(80)90444-5. [2] P.R. Gandhi, V.K. Deshpande, K. Singh, Conductivity enhancement in Li2SO4 incorporated Li2O:B2O3 glass system, Solid State Ion. 36 (1989) 97–102, https://doi. org/10.1016/0167-2738(89)90066-0. [3] Solid electrolyte batteries, in: B.B. Owens, C.W. Tobias (Eds.), Advances in Electrochemistry and Electrochemical Engineering, 8 Wiley, New York, 1971, pp. 1–62 chap 1. [4] W.V. Gool, Fast Ion Transport in Solids: Solid State Batteries and Devices, Elsevier Science Publishing Co Inc., U.S., 1973 ISBN 10: 0720402239, ISBN 13: 9780720402230. [5] J. Akridge, H. Vourlis, Solid state batteries using vitreous solid electrolytes, Solid State Ion. 18–19 (1986) 1082–1087, https://doi.org/10.1016/0167-2738(86) 90313-9. [6] C. Julien, Technological applications of solid state ionics, Mater. Sci. Eng. B 6 (1990) 9–28, https://doi.org/10.1016/0921-5107(90)90109-O. [7] S.S. Gundale, V.V. Behare, A.V. Deshpande, Study of electrical conductivity of Li2O–B2O3–SiO2–Li2SO4 glasses and glass-ceramics, Solid State Ion. 298 (2016) 57–62, https://doi.org/10.1016/j.ssi.2016.11.002. [8] A.R. Kulkarni, H.S. Maiti, A. Paul, Fast ion conducting lithium glasses-review, Bull. Mater. Sci. 6 (1984) 201–221, https://doi.org/10.1007/BF02743897. [9] P.J. Bray, J.G. O'Keefe, Nuclear magnetic resonance investigations of the structure of alkali borate glasses, Phys. Chem. Glasses 4 (1963) 37–46. [10] H.L. Tuller, D.P. Button, Transport- structure relations in fast ion and mixed conductors, Proc. 6th Int. Symp. on Metallurgy and Material Science, RISØ, Denmark, 1985, p. 119 ISBN 87-550-1137-3. [11] B. Calès, A. Levasseur, C. Fouassier, J.M. Réau, P. Hagenmuller, Conductivite ionique du lithium dans les solutions solides de structure boracite Li4+xB7O12+x/2X (X = Cl, Br) (0 ≤ x ≤ 1), Solid State Commun. 24 (1977) 323–325, https://doi. org/10.1016/0038-1098(77)90219-8. [12] A. Levasseur, M. Kbala, J.C. Brethous, J.M. Réau P. Hagenmuller, M. Couzi, Etudes electrique et raman des verres du systeme, B2O3–Li2O–Li2SO4, Solid State Commun. 32 (1979) 839–844, https://doi.org/10.1016/0038-1098(79)90482-4. [13] M. Irion, M. Couzi, A. Levasseur, J.M. Reau, J.C. Brethous, An infrared and Raman study of new lonic-conductor lithium glasses, J. Solid State Chem. 31 (1980) 285–294, https://doi.org/10.1016/0022-4596(80)90090-0. [14] A. Levasseur, J.C. Brethous, J.M. Reau, P. Hagenmuller, M. Couzi, Synthesis and characterization of new solid electrolyte conductors of lithium ions, Solid State Ion. 1 (1980) 177–186, https://doi.org/10.1016/0167-2738(80)90002-8. [15] V.K. Deshpande, S. Rokade, K. Singh, Transport- structure relations in fast ion and mixed conductors, Proc. 6th Int. Symp. on Metallurgy and Material Science, RISØ, Denmark, 1985, p. 227 ISBN 87-550-1137-3.
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