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Enthalpy of formation of the Li22Si5 intermetallic compound
Thermochimica Acta 551 (2013) 53–56
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Short Communication
Enthalpy of formation of the Li22 Si5 intermetallic compound ∗ ˛ ˛ A. Debski , W. Zakulski, Ł. Major, A. Góral, W. Gasior Institute of Metallurgy and Materials Science, Polish Academy of Sciences, 25, Reymonta Street, 30-059 Kraków, Poland
a r t i c l e
i n f o
Article history: Received 24 July 2012 Received in revised form 1 October 2012 Accepted 8 October 2012 Available online 2 November 2012 Keywords: Lithium–silicon system Li–Si Calorimetry Thermochemistry
a b s t r a c t The solution-reaction calorimetric method was used for the determination of the formation enthalpy of the Li22 Si5 intermetallic compound and the heat of the reaction of the Li-component with an acetic acid bath. The phase was prepared in a glove box with high purity argon protective atmosphere. The phase was homogenized and then analyzed by means of the X-ray diffraction technique. Also, the topography analysis was performed with the use of a scanning electron microscopy (SEM), whereas the microstructure analysis was performed by means of a transmission electron microscopy (TEM). The phase analysis was also run by the selected area electron diffraction pattern (SAEDP). The experiments in the calorimeter were carried out at room temperature with acetic acid as the calorimetric solvent. The enthalpy of the lithium’s reaction with the acetic acid was measured and it equaled −261.1 ± 1.6 kJ/g·atom. The formation enthalpy value f H for the Li22 Si5 compound was equal to −24.4 ± 3.0 kJ/g·atom. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The Li–Si alloys belong to the group of materials which have an applicable character. The role of these alloys should be emphasized mostly regarding the use of the lithium-ion type batteries, which are one of the most popular types of rechargeable batteries for portable electronic devices. They characterize in one of the best energy densities, no memory effect and a slow loss of charge when not in use. They are in common use not only in consumer electronics but also in the military, electric vehicle, implantable medical device and aerospace applications [1]. For years, the traditional lithium-ion (sometimes named LIB) technology has been improved in regard to the higher energy capacity, the durability, the longer cycle life, the cost, and the intrinsic safety. The anode is the critical component for storing energy in lithium-ion batteries. In the recent years, silicon has been seen as a potential substitute for graphite, traditionally used as the material for the negative electrode. A silicon electrode stores ten times more lithium than graphite and allows for a far greater energy density on the anode, thus reducing the mass of the battery [2]. Lithium storage alloys with Si as the active element can alloy with Li to form Lix Si, with the maximum uptake of Li22 Si5 and the theoretical capacity equaling 4200 mAh/g [3]. The solid lithium–silicon alloy is also used as the anode for a lithium–metal sulfide high energy density battery [4], as well as in thermally activated batteries [5].
∗ Corresponding author. Tel.: +48 12 295 2814; fax: +48 12 295 2804. ˛ E-mail address: [email protected] (A. Debski). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.10.015
According to the experimental results of Wen and Huggins [6], as well as the assessments published by Okamoto [7] and Braga [8], the stable intermediate compounds in the Li–Si system are: Li12 Si7 , Li7 Si3 , Li13 Si4 , and Li22 Si5 (Fig. 1). The Li22 Si5 compound is the most Li-rich compound in the Li–Si system and has the cubic structure of the “Li22 Pb5 ” prototype, with the a parameter being equal to 1.875 [nm] [9]. This intermediate compound is formed as a result of the peritectic reaction between the liquid phase and the Li13 Si4 phase: L + Li13 Si4 = Li22 Si5 . In the past, instead of the Li22 Si5 phase, the “Li4 Si” or “Lil5 Si4 ” phases were considered in literature as the most Li-rich compound. As it was shown in the assessments [7,8] and is visible in Fig. 1, there is a conspicuous disagreement between the experimental liquidus points and the calculated liquidus line in the region near the Li22 Si5 phase. It is highly probable that these inconsistencies are strictly related to the very limited information available in the literature concerning the thermodynamics of the Li–Si system and particularly the Li22 Si5 phase. There is no available literature experimental thermodynamic data for the liquid phase. There are only those [7,8], derived from the assessed eutectic point of L = Li12 Si7 + (Si), as well as from the Gibbs energies of formation of the Li22 Si5 phase and the other intermediate compounds existing in the system, measured by [4,6,10,11]. The value of the Gibbs free energy of formation of the Li22 Si5 compound modeled at 688 K by Okamoto [7] is equal to −17.3 [kJ/g·atom], whereas Wen and Huggins [6], in their measurements, obtained the value of −16.5 [kJ/g·atom]. From the EMF relations, Demidov et al. [12] calculated the enthalpy of formation of the Li22 Si5 phase.
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XLi Li + XSi Si + C2 H4 O2 = XLi C2 H3 O2 Li + XSi Si +
1 X H2 2 Li
+ (1 − XLi )C2 H4 O2
(5)
The thermo-chemical equations suitable for equations 4 and 5 are as it is written below: HLi22 Si5 + HC2 H4 O2 = XLi HC2 H3 Li + XSi HSi +
1 X HH 2 Li 2
+ (1 − XLi )HC2 H4 O2 − HI
XLi HLi + XSi HSi + HC2 H4 O2 = XLi HC2 H5 Li + XSi HSi +
(6)
1 X HH 2 Li 2
+ (1 − XLi )HC2 H4 O2 − HII
Fig. 1. The Li–Si phase diagram proposed by Okamoto [7].
Beside the information given above, there is no other experimentally determined thermodynamic literature data concerning the Li22 Si5 solid phase and there is no calorimetrically measured heat of formation of that compound. The lack of relevant information of the thermodynamic properties of the compounds in the Li–Si system in the literature data basis is not the effect of accidental negligence, but is related to the extreme experimental difficulties, mainly to the high reactivity of Li with O2 , N2 and the moisture. 2. Experimental The measurements of the enthalpy of formation of the Li22 Si5 intermetallic compound were performed by means of the solution-reaction calorimetry method with acetic acid solution (C2 H4 O2 , 99.75% pure p.a., CHEMPUR) as the calorimetric bath. The method is based on the determination of the heat effects accompanying the dissolution of the components (Li, Si) and the intermetallic phase in the suitable solvent. The formation enthalpy of the Li22 Si5 , compound is defined by the following equation: 0 0 f HLi22 Si5 = HLi22 Si5 − (XLi HLi + XSi HSi )
(1)
and the enthalpy of the compound is as follows: p
p
0 0 HLi22 Si5 = XLi (HLi + HLi ) + XSi (HSi + HSi ) p
(2)
p
0 and H 0 , and H and H are the standard and partial where: HLi Si Li Si enthalpies of Li and Si, respectively, while XLi and XSi are their mole fractions. Substituting Eq. (2) by Eq. (1), one can obtain the following equation connecting the partial enthalpy of the components with the formation enthalpy of the compound: p
p
f HLi22 Si5 = XLi HLi + XSi HSi
(3)
The measurement of the formation enthalpy of the Li22 Si5 compound by the reaction with the acetic acid is possible on the basis of the following deduction: The same products of the dissolution of Li and Si in acetic acid can be obtained by the dissolution of the Li22 Si5 compound, which is shown in following chemical reactions: Li22 Si5 + C2 H4 O2 = XLi C2 H3 O2 Li + XSi Si + + (1 − XLi )C2 H4 O2
1 X H2 2 Li (4)
(7)
In Eqs. (6) and (7), the symbols HI and HII denote the heat effects of the suitable reaction. Each of the reactions (4) and (5) possesses a different heat effect (HI and HII ) and their difference is equal to the formation enthalpy of the Li22 Si5 compound, according to the following equation: p
p
f HLi22 Si5 = XLi HLi + XSi HSi = HII − HI
(8)
where HII = XLi ·Qr ; XLi is the mole fraction of lithium; Qr is the heat of the lithium reaction in C2 H4 O2 at 25 ◦ C (298 K). As the silicon did not dissolve in the acetic acid, the only unknown quantities are the heat effects of the reaction of Li and the compound with the acetic acid. The reactions of dissolution, both in the case of Li and the Li22 Si5 compound, were conducted in the same volume of acetic acid bath and the concentration of C2 H3 Li, after the reaction, was lower than 0.5 mass percent. Because of the high reactivity of lithium, the Li22 Si5 intermetallic compound was prepared inside an M. Braun glove box, in high purity argon atmosphere, with trace amounts of moisture and with O2 being continuously removed by the purification system to the level of less than 1 ppm, through the use of a molecular sieve and catalytic Cu. To eliminate the N2 , circulating Ar was passed over a Ti sponge kept at 1300 K. The weighted amounts of high purity metals: Li 99.9% (Alfa Aesar), Si 99.9999% (Alfa Aesar) were melted in a molybdenum crucible protected inside by a tantalum foil. The samples were heated up to the temperature about 50 K higher than the liquidus temperature, and finally the phase was homogenized at 748 K (475 ◦ C) for 1 day. After the homogenization, the Li22 Si5 phase composition was analyzed by means of the X-ray diffraction technique. Also, the topography analysis was conducted with the use of a scanning electron microscopy (SEM), whereas the microstructure analysis was performed by means of a transmission electron microscopy (TEM). The phase analysis was run by the selected area electron diffraction pattern (SAEDP) and confirmed by High Resolution Transmission Electron Microscopy (HRTEM) images. The calorimetric experiments were carried out at room temperature in a solution-reaction calorimeter, described in detail in [13]. The experiments were conducted in a glass container. The calibration coefficient was assumed to be the same as that determined in the earlier work [14] and equal to: k = 2.5E−05 kJ/a.u. The Li22 Si5 phase obtained inside the glove box was granulated in an agate mortar to the form of lumps. Next, the sample was weighted, placed in a tightly closed glass tube and finally removed from the glove box. At the beginning of the experimental run, the samples were placed in the calorimetric chamber through a delivery device (glass tube) and dropped into the solvent. During the solution reaction of the Li22 Si5 phase, the solvent was continuously stirred. The voltage
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Table 1 Heats of formation f H and heat effects measured for the Li22 Si5 intermetallic compound at 298 K. Phase
Li22 Si5
Fig. 2. X-ray diffraction pattern of the Li22 Si5 phase.
signal from the thermopile, situated directly under the calorimetric crucible with the solvent, was registered in the digital form in the computer memory, and the heat effects as well as the enthalpies of formation were calculated with the use of computer software.
3. Results and discussion The phase composition of the samples after homogenization was analyzed by means of the X-ray diffraction technique (Philips PW 1710 Diffractometer with Co-K␣ filtered radiation). For the identification of the phase, the EVA (Bruker) software was used. The analysis confirmed the proper structure of the investigated intermetallic Li22 Si5 phase. The X-ray diffraction pattern of the Li22 Si5 is shown in Fig. 2. No peak characteristics for other phases were observed. That means – according to the accuracy of the X-ray diffraction technique – that the purity of the Li22 Si5 samples can be estimated as close to 100%. The topography analysis of the Li22 Si5 phase was performed with the use of the Quanta 200 3D microscope. The phase analysis was conducted by the selected area electron diffraction pattern (SAEDP) and next, it was confirmed by the High Resolution Transmission Electron Microscopy (HRTEM) images. All the results of the above analysis confirmed the proper preparation of the sample with the Li22 Si5 phase. The topography analysis of the Li22 Si5 phase was performed with the use of the Quanta 200 3D microscope. The microstructure analysis was performed with the application of a transmission electron microscopy (TEM) (TECNAI G2 F20 (200 kV) FEG microscope). The thin foil for the TEM investigations was prepared by means of the Focused Ion Beam (FIB) technique. The bright field TEM image confirmed the crystalline character of the alloy. From the X-ray diffraction analysis, the lattice parameter a of the Li22 Si5 intermetallic compound was calculated and was equal to 1.866 [nm]. The same parameter obtained by Gladyshevskii et al. [9] was equal to 1.875 [nm]. As it was successfully shown in the earlier work [14], the enthalpy of formation of intermetallic compounds can be calorimetrically determined not only with use of Al, Sn or Cu as the typical metallic bath [15], but also with the use of water as the solvent. It is a very useful method when one or both components of the intermetallic phase are water soluble. The water-reaction-roomtemperature calorimeter was proposed in [14] and experimentally used for the determination of the intermetallic phases in the B–Li system. In this study, as the preliminary step, water was also used as the calorimetric solvent for the Li22 Si5 compound. Unfortunately, the dissolution reaction of the Li22 Si5 compound with water proceeded very rapidly. The explosive kind of dissolution of even very small pieces of samples with water made it impossible to measure the
T [K]
298
Measurement no.
Heat effect, HI [kJ/g·atom]
Enthalpy of formation, f H [kJ/g·atom]
1 2 3 4 5 Average Standard Dev.
−190.2 −187.5 −189.5 −188.4 −186.1 −188.4 ±1.7
−24.4 ± 3.0
Table 2 Enthalpy of formation of the Li22 Si5 intermetallic compound obtained by various authors. Author
Method
Enthalpy of formation f H [kJ/g·atom]
This work Nikolaev et al. [11] Sharma et al. [10] Lai [4] Okamoto [7] Braga et al. [8] Braga et al. [8]
Calorimetric EMF measurements EMF measurements EMF measurements Calculations Calculations I Calculations II
−24.4 ± 3.0 −26.1 −25.0 −24.2 −27.2 −21.4 −14.5
heat effect of that reaction. Therefore, instead of water, acetic acid was used as the calorimetric solvent. As the first stage of the study, the heat effect related to the lithium dissolution in the acetic acid was experimentally determined for four samples and the range of the measured values was from −259.1 to −262.9 [kJ/g·atom], with the average of −261.1 ± 1.6 [kJ/g·atom]. No systematic change (increase or decrease) in the subsequent measurements was observed, and so, the average value was taken for the calculation of f H of the Li22 Si5 intermetallic compound. The heat effect of the reaction of the silicon with the acetic acid is zero, as Si did not react with C2 H4 O2 and, after the reaction, was cumulated at the bottom of the crucible in the form of powder. The EDS (Energy Dispersive X-Ray Spectroscopy) analysis of the powder performed after the reaction of Li22 Si5 with the acetic acid showed only silicon [13]. In the next stage, the heat effects of the reaction of the Li22 Si5 intermetallic compound with C2 H4 O2 were measured. The time of the reaction and stabilization of the calorimeter’s baseline was about half an hour. The values of the heat effects and the formation enthalpy calculated according to Eq. (8) are shown in Table 1, together with the average values and the standard deviation. For five samples, the range of measured values was from −186.1 to −190.2 [kJ/g·atom], with the average of −188.4 ± 1.7 [kJ/g·atom]. As it is visible in the comparison presented in Table 2, the enthalpy of formation of the Li22 Si5 intermetallic compound measured in this study and that calculated by various authors from the EMF measurements are in good agreement. The f H values of the Li22 Si5 intermetallic compound from this work and those available from the literature [4,7,8,10,11] are presented in Table 2. 4. Summary The solution-reaction calorimetric method was used for the determination of the formation enthalpy of the Li22 Si5 intermetallic phase and the heat of the reaction of lithium with the calorimetric bath. The samples with Li and the Li22 Si5 phase were dissolved in an acetic acid bath, C2 H4 O2 , at room temperature. The phase was
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prepared in a glove box with high purity argon protective atmosphere. The lattice parameter a of the phase was determined by way of the X-ray diffraction technique and compared with the literature data. The enthalpy of the lithium reaction with the acetic acid was measured as equal to −261.1 ± 1.6 kJ/g·atom. As the silicon did not react with acetic acid, the heat effect of the reaction was assumed to be zero. The determined formation enthalpy value f H of the Li22 Si5 compound was equal to −24.4 ± 3.0 kJ/g·atom and was similar to the values available in the literature and those calculated from the EMF measurements. Acknowledgments The authors wish to express their gratitude to the Ministry of Science and Higher Education of Poland for funding Project No. IP2010007170 “Thermodynamic studies of Li–Si alloys as a material for safe storage of hydrogen”, financed from the budget for science in the years 2010–2011. References [1] J. Li, L. Christensen, M.N. Obrovac, K.C. Hewitt, J.R. Dahn, Effect of heat treatment on Si electrodes using polyvinylidene fluoride binder, J. Electrochem. Soc. 155 (2008) A234–A238.
[2] C.K. Chan, H. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Highperformance lithium battery anodes using silicon nanowires, Nat. Nanotechnol. 3 (2008) 31–35. [3] J. Hassoun, S. Panero, B. Scrosati, Metal alloy electrode configurations for advanced lithium-ion batteries, Fuel Cells 9 (2009) 277–283. [4] S.C. Lai, Solid lithium–silicon electrode, J. Electrochem. Soc. 123 (1976) 1196–1197. [5] R.A. Guidotti, P.J. Masset, Thermally activated (thermal) battery technology. Part IV. Anode for thermal batteries, J. Power Sources 183 (2008) 388–398. [6] C.J. Wen, R.A. Huggins, Chemical diffusion in intermediate phases in the lithium–silicon system, J. Solid State Chem. 37 (1981) 271–278. [7] H. Okamoto, The Li–Si system, Bull. Alloy Phase Diagrams 11 (1990) 306–312. [8] M.H. Braga, L.F. Malheiros, I. Ansara, Thermodynamic assessment of the Li–Si system, J. Phase Equilib. 16 (1995) 324–329. [9] E.I. Gladyshevskii, G.I. Oleksiv, P.I. Kripyakevich, New examples of the structural type Li22 Pb5 , Kristallografiya 9 (1964) 338–341 (Sov. Phys. Crystallogr. (Engl. Transl.) 9 (1964) 269–271). [10] R.A. Sharma, R.N. Seefurth, Thermodynamic properties of the lithium–silicon system, J. Electrochem. Soc. 123 (1976) 1763–1768. [11] V.P. Nikolaev, A.G. Morachevskii, A.I. Demidov, E.V. Bairachnyi, Thermodynamic properties of the Li–Si solid alloys, Zh. Prikl. Khim. (Leningrad) 53 (1980) 2088–2090. [12] A.I. Demidov, A.G. Morachevskii, V.P. Nikolaev, N.V. Berenda, Thermodynamic characteristics of lithium–silicon compounds, J. Appl. Chem. (USSR) 61 (1988) 1254–1256. ˛ ˛ W. Gasior, A. Góral, Enthalpy of formation of intermetallic com[13] A. Debski, pounds from the Li–Si system, Intermetallics 26 (2012) 157–161. ˛ ˛ [14] W. Gasior, A. Debski, R. Major, Ł. Major, A. Góral, Enthalpy of formation of B13 Li, B14 Li3 and B3 Li (B6 Li2 ) compounds measured with reaction calorimetric method, Intermetallics 24 (2012) 120–127. ˛ ˛ W. Gasior, Enthalpy of formation of the CaLi2 phase, [15] W. Zakulski, A. Debski, Intermetallics 23 (2012) 76–79.
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Short Communication
Enthalpy of formation of the Li22 Si5 intermetallic compound ∗ ˛ ˛ A. Debski , W. Zakulski, Ł. Major, A. Góral, W. Gasior Institute of Metallurgy and Materials Science, Polish Academy of Sciences, 25, Reymonta Street, 30-059 Kraków, Poland
a r t i c l e
i n f o
Article history: Received 24 July 2012 Received in revised form 1 October 2012 Accepted 8 October 2012 Available online 2 November 2012 Keywords: Lithium–silicon system Li–Si Calorimetry Thermochemistry
a b s t r a c t The solution-reaction calorimetric method was used for the determination of the formation enthalpy of the Li22 Si5 intermetallic compound and the heat of the reaction of the Li-component with an acetic acid bath. The phase was prepared in a glove box with high purity argon protective atmosphere. The phase was homogenized and then analyzed by means of the X-ray diffraction technique. Also, the topography analysis was performed with the use of a scanning electron microscopy (SEM), whereas the microstructure analysis was performed by means of a transmission electron microscopy (TEM). The phase analysis was also run by the selected area electron diffraction pattern (SAEDP). The experiments in the calorimeter were carried out at room temperature with acetic acid as the calorimetric solvent. The enthalpy of the lithium’s reaction with the acetic acid was measured and it equaled −261.1 ± 1.6 kJ/g·atom. The formation enthalpy value f H for the Li22 Si5 compound was equal to −24.4 ± 3.0 kJ/g·atom. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The Li–Si alloys belong to the group of materials which have an applicable character. The role of these alloys should be emphasized mostly regarding the use of the lithium-ion type batteries, which are one of the most popular types of rechargeable batteries for portable electronic devices. They characterize in one of the best energy densities, no memory effect and a slow loss of charge when not in use. They are in common use not only in consumer electronics but also in the military, electric vehicle, implantable medical device and aerospace applications [1]. For years, the traditional lithium-ion (sometimes named LIB) technology has been improved in regard to the higher energy capacity, the durability, the longer cycle life, the cost, and the intrinsic safety. The anode is the critical component for storing energy in lithium-ion batteries. In the recent years, silicon has been seen as a potential substitute for graphite, traditionally used as the material for the negative electrode. A silicon electrode stores ten times more lithium than graphite and allows for a far greater energy density on the anode, thus reducing the mass of the battery [2]. Lithium storage alloys with Si as the active element can alloy with Li to form Lix Si, with the maximum uptake of Li22 Si5 and the theoretical capacity equaling 4200 mAh/g [3]. The solid lithium–silicon alloy is also used as the anode for a lithium–metal sulfide high energy density battery [4], as well as in thermally activated batteries [5].
∗ Corresponding author. Tel.: +48 12 295 2814; fax: +48 12 295 2804. ˛ E-mail address: [email protected] (A. Debski). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.10.015
According to the experimental results of Wen and Huggins [6], as well as the assessments published by Okamoto [7] and Braga [8], the stable intermediate compounds in the Li–Si system are: Li12 Si7 , Li7 Si3 , Li13 Si4 , and Li22 Si5 (Fig. 1). The Li22 Si5 compound is the most Li-rich compound in the Li–Si system and has the cubic structure of the “Li22 Pb5 ” prototype, with the a parameter being equal to 1.875 [nm] [9]. This intermediate compound is formed as a result of the peritectic reaction between the liquid phase and the Li13 Si4 phase: L + Li13 Si4 = Li22 Si5 . In the past, instead of the Li22 Si5 phase, the “Li4 Si” or “Lil5 Si4 ” phases were considered in literature as the most Li-rich compound. As it was shown in the assessments [7,8] and is visible in Fig. 1, there is a conspicuous disagreement between the experimental liquidus points and the calculated liquidus line in the region near the Li22 Si5 phase. It is highly probable that these inconsistencies are strictly related to the very limited information available in the literature concerning the thermodynamics of the Li–Si system and particularly the Li22 Si5 phase. There is no available literature experimental thermodynamic data for the liquid phase. There are only those [7,8], derived from the assessed eutectic point of L = Li12 Si7 + (Si), as well as from the Gibbs energies of formation of the Li22 Si5 phase and the other intermediate compounds existing in the system, measured by [4,6,10,11]. The value of the Gibbs free energy of formation of the Li22 Si5 compound modeled at 688 K by Okamoto [7] is equal to −17.3 [kJ/g·atom], whereas Wen and Huggins [6], in their measurements, obtained the value of −16.5 [kJ/g·atom]. From the EMF relations, Demidov et al. [12] calculated the enthalpy of formation of the Li22 Si5 phase.
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XLi Li + XSi Si + C2 H4 O2 = XLi C2 H3 O2 Li + XSi Si +
1 X H2 2 Li
+ (1 − XLi )C2 H4 O2
(5)
The thermo-chemical equations suitable for equations 4 and 5 are as it is written below: HLi22 Si5 + HC2 H4 O2 = XLi HC2 H3 Li + XSi HSi +
1 X HH 2 Li 2
+ (1 − XLi )HC2 H4 O2 − HI
XLi HLi + XSi HSi + HC2 H4 O2 = XLi HC2 H5 Li + XSi HSi +
(6)
1 X HH 2 Li 2
+ (1 − XLi )HC2 H4 O2 − HII
Fig. 1. The Li–Si phase diagram proposed by Okamoto [7].
Beside the information given above, there is no other experimentally determined thermodynamic literature data concerning the Li22 Si5 solid phase and there is no calorimetrically measured heat of formation of that compound. The lack of relevant information of the thermodynamic properties of the compounds in the Li–Si system in the literature data basis is not the effect of accidental negligence, but is related to the extreme experimental difficulties, mainly to the high reactivity of Li with O2 , N2 and the moisture. 2. Experimental The measurements of the enthalpy of formation of the Li22 Si5 intermetallic compound were performed by means of the solution-reaction calorimetry method with acetic acid solution (C2 H4 O2 , 99.75% pure p.a., CHEMPUR) as the calorimetric bath. The method is based on the determination of the heat effects accompanying the dissolution of the components (Li, Si) and the intermetallic phase in the suitable solvent. The formation enthalpy of the Li22 Si5 , compound is defined by the following equation: 0 0 f HLi22 Si5 = HLi22 Si5 − (XLi HLi + XSi HSi )
(1)
and the enthalpy of the compound is as follows: p
p
0 0 HLi22 Si5 = XLi (HLi + HLi ) + XSi (HSi + HSi ) p
(2)
p
0 and H 0 , and H and H are the standard and partial where: HLi Si Li Si enthalpies of Li and Si, respectively, while XLi and XSi are their mole fractions. Substituting Eq. (2) by Eq. (1), one can obtain the following equation connecting the partial enthalpy of the components with the formation enthalpy of the compound: p
p
f HLi22 Si5 = XLi HLi + XSi HSi
(3)
The measurement of the formation enthalpy of the Li22 Si5 compound by the reaction with the acetic acid is possible on the basis of the following deduction: The same products of the dissolution of Li and Si in acetic acid can be obtained by the dissolution of the Li22 Si5 compound, which is shown in following chemical reactions: Li22 Si5 + C2 H4 O2 = XLi C2 H3 O2 Li + XSi Si + + (1 − XLi )C2 H4 O2
1 X H2 2 Li (4)
(7)
In Eqs. (6) and (7), the symbols HI and HII denote the heat effects of the suitable reaction. Each of the reactions (4) and (5) possesses a different heat effect (HI and HII ) and their difference is equal to the formation enthalpy of the Li22 Si5 compound, according to the following equation: p
p
f HLi22 Si5 = XLi HLi + XSi HSi = HII − HI
(8)
where HII = XLi ·Qr ; XLi is the mole fraction of lithium; Qr is the heat of the lithium reaction in C2 H4 O2 at 25 ◦ C (298 K). As the silicon did not dissolve in the acetic acid, the only unknown quantities are the heat effects of the reaction of Li and the compound with the acetic acid. The reactions of dissolution, both in the case of Li and the Li22 Si5 compound, were conducted in the same volume of acetic acid bath and the concentration of C2 H3 Li, after the reaction, was lower than 0.5 mass percent. Because of the high reactivity of lithium, the Li22 Si5 intermetallic compound was prepared inside an M. Braun glove box, in high purity argon atmosphere, with trace amounts of moisture and with O2 being continuously removed by the purification system to the level of less than 1 ppm, through the use of a molecular sieve and catalytic Cu. To eliminate the N2 , circulating Ar was passed over a Ti sponge kept at 1300 K. The weighted amounts of high purity metals: Li 99.9% (Alfa Aesar), Si 99.9999% (Alfa Aesar) were melted in a molybdenum crucible protected inside by a tantalum foil. The samples were heated up to the temperature about 50 K higher than the liquidus temperature, and finally the phase was homogenized at 748 K (475 ◦ C) for 1 day. After the homogenization, the Li22 Si5 phase composition was analyzed by means of the X-ray diffraction technique. Also, the topography analysis was conducted with the use of a scanning electron microscopy (SEM), whereas the microstructure analysis was performed by means of a transmission electron microscopy (TEM). The phase analysis was run by the selected area electron diffraction pattern (SAEDP) and confirmed by High Resolution Transmission Electron Microscopy (HRTEM) images. The calorimetric experiments were carried out at room temperature in a solution-reaction calorimeter, described in detail in [13]. The experiments were conducted in a glass container. The calibration coefficient was assumed to be the same as that determined in the earlier work [14] and equal to: k = 2.5E−05 kJ/a.u. The Li22 Si5 phase obtained inside the glove box was granulated in an agate mortar to the form of lumps. Next, the sample was weighted, placed in a tightly closed glass tube and finally removed from the glove box. At the beginning of the experimental run, the samples were placed in the calorimetric chamber through a delivery device (glass tube) and dropped into the solvent. During the solution reaction of the Li22 Si5 phase, the solvent was continuously stirred. The voltage
A. D˛ebski et al. / Thermochimica Acta 551 (2013) 53–56
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Table 1 Heats of formation f H and heat effects measured for the Li22 Si5 intermetallic compound at 298 K. Phase
Li22 Si5
Fig. 2. X-ray diffraction pattern of the Li22 Si5 phase.
signal from the thermopile, situated directly under the calorimetric crucible with the solvent, was registered in the digital form in the computer memory, and the heat effects as well as the enthalpies of formation were calculated with the use of computer software.
3. Results and discussion The phase composition of the samples after homogenization was analyzed by means of the X-ray diffraction technique (Philips PW 1710 Diffractometer with Co-K␣ filtered radiation). For the identification of the phase, the EVA (Bruker) software was used. The analysis confirmed the proper structure of the investigated intermetallic Li22 Si5 phase. The X-ray diffraction pattern of the Li22 Si5 is shown in Fig. 2. No peak characteristics for other phases were observed. That means – according to the accuracy of the X-ray diffraction technique – that the purity of the Li22 Si5 samples can be estimated as close to 100%. The topography analysis of the Li22 Si5 phase was performed with the use of the Quanta 200 3D microscope. The phase analysis was conducted by the selected area electron diffraction pattern (SAEDP) and next, it was confirmed by the High Resolution Transmission Electron Microscopy (HRTEM) images. All the results of the above analysis confirmed the proper preparation of the sample with the Li22 Si5 phase. The topography analysis of the Li22 Si5 phase was performed with the use of the Quanta 200 3D microscope. The microstructure analysis was performed with the application of a transmission electron microscopy (TEM) (TECNAI G2 F20 (200 kV) FEG microscope). The thin foil for the TEM investigations was prepared by means of the Focused Ion Beam (FIB) technique. The bright field TEM image confirmed the crystalline character of the alloy. From the X-ray diffraction analysis, the lattice parameter a of the Li22 Si5 intermetallic compound was calculated and was equal to 1.866 [nm]. The same parameter obtained by Gladyshevskii et al. [9] was equal to 1.875 [nm]. As it was successfully shown in the earlier work [14], the enthalpy of formation of intermetallic compounds can be calorimetrically determined not only with use of Al, Sn or Cu as the typical metallic bath [15], but also with the use of water as the solvent. It is a very useful method when one or both components of the intermetallic phase are water soluble. The water-reaction-roomtemperature calorimeter was proposed in [14] and experimentally used for the determination of the intermetallic phases in the B–Li system. In this study, as the preliminary step, water was also used as the calorimetric solvent for the Li22 Si5 compound. Unfortunately, the dissolution reaction of the Li22 Si5 compound with water proceeded very rapidly. The explosive kind of dissolution of even very small pieces of samples with water made it impossible to measure the
T [K]
298
Measurement no.
Heat effect, HI [kJ/g·atom]
Enthalpy of formation, f H [kJ/g·atom]
1 2 3 4 5 Average Standard Dev.
−190.2 −187.5 −189.5 −188.4 −186.1 −188.4 ±1.7
−24.4 ± 3.0
Table 2 Enthalpy of formation of the Li22 Si5 intermetallic compound obtained by various authors. Author
Method
Enthalpy of formation f H [kJ/g·atom]
This work Nikolaev et al. [11] Sharma et al. [10] Lai [4] Okamoto [7] Braga et al. [8] Braga et al. [8]
Calorimetric EMF measurements EMF measurements EMF measurements Calculations Calculations I Calculations II
−24.4 ± 3.0 −26.1 −25.0 −24.2 −27.2 −21.4 −14.5
heat effect of that reaction. Therefore, instead of water, acetic acid was used as the calorimetric solvent. As the first stage of the study, the heat effect related to the lithium dissolution in the acetic acid was experimentally determined for four samples and the range of the measured values was from −259.1 to −262.9 [kJ/g·atom], with the average of −261.1 ± 1.6 [kJ/g·atom]. No systematic change (increase or decrease) in the subsequent measurements was observed, and so, the average value was taken for the calculation of f H of the Li22 Si5 intermetallic compound. The heat effect of the reaction of the silicon with the acetic acid is zero, as Si did not react with C2 H4 O2 and, after the reaction, was cumulated at the bottom of the crucible in the form of powder. The EDS (Energy Dispersive X-Ray Spectroscopy) analysis of the powder performed after the reaction of Li22 Si5 with the acetic acid showed only silicon [13]. In the next stage, the heat effects of the reaction of the Li22 Si5 intermetallic compound with C2 H4 O2 were measured. The time of the reaction and stabilization of the calorimeter’s baseline was about half an hour. The values of the heat effects and the formation enthalpy calculated according to Eq. (8) are shown in Table 1, together with the average values and the standard deviation. For five samples, the range of measured values was from −186.1 to −190.2 [kJ/g·atom], with the average of −188.4 ± 1.7 [kJ/g·atom]. As it is visible in the comparison presented in Table 2, the enthalpy of formation of the Li22 Si5 intermetallic compound measured in this study and that calculated by various authors from the EMF measurements are in good agreement. The f H values of the Li22 Si5 intermetallic compound from this work and those available from the literature [4,7,8,10,11] are presented in Table 2. 4. Summary The solution-reaction calorimetric method was used for the determination of the formation enthalpy of the Li22 Si5 intermetallic phase and the heat of the reaction of lithium with the calorimetric bath. The samples with Li and the Li22 Si5 phase were dissolved in an acetic acid bath, C2 H4 O2 , at room temperature. The phase was
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prepared in a glove box with high purity argon protective atmosphere. The lattice parameter a of the phase was determined by way of the X-ray diffraction technique and compared with the literature data. The enthalpy of the lithium reaction with the acetic acid was measured as equal to −261.1 ± 1.6 kJ/g·atom. As the silicon did not react with acetic acid, the heat effect of the reaction was assumed to be zero. The determined formation enthalpy value f H of the Li22 Si5 compound was equal to −24.4 ± 3.0 kJ/g·atom and was similar to the values available in the literature and those calculated from the EMF measurements. Acknowledgments The authors wish to express their gratitude to the Ministry of Science and Higher Education of Poland for funding Project No. IP2010007170 “Thermodynamic studies of Li–Si alloys as a material for safe storage of hydrogen”, financed from the budget for science in the years 2010–2011. References [1] J. Li, L. Christensen, M.N. Obrovac, K.C. Hewitt, J.R. Dahn, Effect of heat treatment on Si electrodes using polyvinylidene fluoride binder, J. Electrochem. Soc. 155 (2008) A234–A238.
[2] C.K. Chan, H. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Highperformance lithium battery anodes using silicon nanowires, Nat. Nanotechnol. 3 (2008) 31–35. [3] J. Hassoun, S. Panero, B. Scrosati, Metal alloy electrode configurations for advanced lithium-ion batteries, Fuel Cells 9 (2009) 277–283. [4] S.C. Lai, Solid lithium–silicon electrode, J. Electrochem. Soc. 123 (1976) 1196–1197. [5] R.A. Guidotti, P.J. Masset, Thermally activated (thermal) battery technology. Part IV. Anode for thermal batteries, J. Power Sources 183 (2008) 388–398. [6] C.J. Wen, R.A. Huggins, Chemical diffusion in intermediate phases in the lithium–silicon system, J. Solid State Chem. 37 (1981) 271–278. [7] H. Okamoto, The Li–Si system, Bull. Alloy Phase Diagrams 11 (1990) 306–312. [8] M.H. Braga, L.F. Malheiros, I. Ansara, Thermodynamic assessment of the Li–Si system, J. Phase Equilib. 16 (1995) 324–329. [9] E.I. Gladyshevskii, G.I. Oleksiv, P.I. Kripyakevich, New examples of the structural type Li22 Pb5 , Kristallografiya 9 (1964) 338–341 (Sov. Phys. Crystallogr. (Engl. Transl.) 9 (1964) 269–271). [10] R.A. Sharma, R.N. Seefurth, Thermodynamic properties of the lithium–silicon system, J. Electrochem. Soc. 123 (1976) 1763–1768. [11] V.P. Nikolaev, A.G. Morachevskii, A.I. Demidov, E.V. Bairachnyi, Thermodynamic properties of the Li–Si solid alloys, Zh. Prikl. Khim. (Leningrad) 53 (1980) 2088–2090. [12] A.I. Demidov, A.G. Morachevskii, V.P. Nikolaev, N.V. Berenda, Thermodynamic characteristics of lithium–silicon compounds, J. Appl. Chem. (USSR) 61 (1988) 1254–1256. ˛ ˛ W. Gasior, A. Góral, Enthalpy of formation of intermetallic com[13] A. Debski, pounds from the Li–Si system, Intermetallics 26 (2012) 157–161. ˛ ˛ [14] W. Gasior, A. Debski, R. Major, Ł. Major, A. Góral, Enthalpy of formation of B13 Li, B14 Li3 and B3 Li (B6 Li2 ) compounds measured with reaction calorimetric method, Intermetallics 24 (2012) 120–127. ˛ ˛ W. Gasior, Enthalpy of formation of the CaLi2 phase, [15] W. Zakulski, A. Debski, Intermetallics 23 (2012) 76–79.