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Studies on electrical properties of composites based on lithium titanium phosphate with lithium iodide
SOSI-14146; No of Pages 5 Solid State Ionics xxx (2016) xxx–xxx
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Studies on electrical properties of composites based on lithium titanium phosphate with lithium iodide K. Kwatek ⁎, J.L. Nowiński Warsaw University of Technology, Faculty of Physics, 00–662 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 29 July 2016 Received in revised form 5 December 2016 Accepted 7 December 2016 Available online xxxx Keywords: Solid electrolyte Grain boundary conductivity enhancement Composite Lithium iodide
a b s t r a c t While grain conductivity of lithium titanium phosphate materials (LTP) is high, the total conductivity is relatively low due to high resistant grain boundary phase. The work demonstrates that this resistance can be substantially reduced by helps of LiI. For this purpose a series of the LTP–LiI composites were formed employing mechanical ball–milling. X–ray diffractometry, thermogravimetry, impedance spectroscopy, scanning electron microscopy and density methods were used to characterize obtained materials. A maximum enhancement of total ionic conductivity at 30 °C was recorded for the LTP containing 8 wt.% of LiI (7.3 × 10−6 S·cm−1 comparing to 1.5 × 10−8 S·cm−1 for pure LTP). © 2016 Elsevier B.V. All rights reserved.
1. Introduction A solid electrolyte suitable for lithium ion batteries (LIBs) is a challenging issue stimulating intensive searching for new Li–ion conductors. Commonly used LIBs composed of liquid electrolytes are not appropriate for further researches due to their high flammability, poor mechanical properties and limited width of electrochemical window. Intensive works are focused on replacing these liquid electrolytes by another, in particular, solid ones. It is expected that the batteries with solid electrolytes (all–solid batteries) could offer, in respect to the traditional, liquid electrolyte based ones, improved mechanical properties and wider electrochemical window. Proposed all solid–state battery system requires fast Li–ion conductors, which will act as solid electrolyte. For practical use, materials with lithium–ion conductivity higher than 10−4 S·cm−1 are required [1]. Various solid compounds were investigated, but only few of them might be characterized as good lithium ion conductors [2–5]. Among them, the best conductivity demonstrate Li3N single–crystal and glasses based on Li2S. Their conductivity at room temperature approaches to 10−3 S·cm−1. However, they do not fulfill all high standards required for commercially oriented, mass production. Poor chemical stability of these materials excludes them for practical purposes as solid electrolytes [6–8]. One of the favorites as a solid electrolyte is LiTi2(PO4)3 (LTP), which is intensively studied in the recent years. The LTP possesses NASICON–type structure with rhombohedral symmetry and belongs to R–3c space group. It forms three dimensional network structure, composed of TiO6 octahedra and PO4 tetrahedra, which are linked together by their corners [9]. Lithium ⁎ Corresponding author. E-mail address: [email protected] (K. Kwatek).
titanium phosphate is of great interest due to its high structural and chemical stability. Unfortunately, ceramic LTP samples show too low total ionic conductivity from 10−8 S·cm− 1 to 10− 6 S·cm−1 at room temperature, as it has been reported in the literature [6,9,11,25–27]. This major issue is caused by high grain boundaries resistance [9–12]. H. Aono showed that total conductivity of LTP derived materials greatly depends on density of sintered pellet. Materials with high porosity are worse conductors because lithium ions have to travel bigger distances between grains. Modifying LTP by doping it with trivalent atoms tends to lower porosity of ceramics [9]. Most of the interest in Li1+xMxTi2–x(PO4)3 system was found when titanium atom is substituted by aluminum (x = 0.3). It is reported that Li1.3Al0.3Ti1.7(PO4)3 (LATP) has the highest ionic conductivity among all LTP derived materials [9, 13]. Several researchers attempted to achieve a further enhance of conductivity of lithium titanium phosphate by the glass–ceramic route [6, 14–16]. Despite of reported high values of conductivity for all LTP derived materials, this method is problematic. First of all, it requires preheated stainless–steel plates on which glass formation proceeds. Unfortunately, that process may cause numerous undemanded cracks due to resulting stresses. Melting of substrates occurs at high temperatures reaching 1450 °C [14]. Development of glass–ceramic conductor for large scale might be troublesome. In comparison, the described by Aono, ceramic route is easier to apply. Other alternative way to enhance of grain boundary conductivity of the lithium titanium phosphate derivatives one can search via adding some foreign compounds to the parent LATP. The resulting heterogeneous materials could possess, new, interesting properties due to space charge effects [17–19]. Such attempt undertook Kumar et al. [20] studying ion properties of LATP glass ceramics with Al2O3 and LATP glass ceramics with Ba0.6Sr0.4TiO3 (0.6BST) composites. In both
http://dx.doi.org/10.1016/j.ssi.2016.12.007 0167-2738/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: K. Kwatek, J.L. Nowiński, Studies on electrical properties of composites based on lithium titanium phosphate with lithium iodide, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.007
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K. Kwatek, J.L. Nowiński / Solid State Ionics xxx (2016) xxx–xxx
Fig. 1. XRD pattern for LTP material and composite with 8 wt.% addition of LiI.
cases, the additive lowered ion conductivity. The author observed the space charge effect enhancing conductivity, but concluded that the competing, blocking effect of the insulating phase prevailed. Inspiration of our work was the seminal publication by Liang [21], in which high ion conductivity of LiI–Al2O3 composites was reported. In the later works [17–19] the phenomenon was explained by enormously high space charge effect occurring, in particular, in this composite. Our approach aimed to examine LTP–LiI system. There is known, that properties of the lithium iodide based composites depends on specimen preparation technique [20]. We wanted to check if simplified technique employing powdered components, processed only by high pressure could produce a material demonstrating high total ion conductivity. This work presents that grain boundary resistance can be reduced by insertion of the other foreign phases to the material. In our investigations, we focused attention on composites and found that NASICON– type structure LiTi2(PO4)3 could be a promising base for formation such materials. The work presents a new approach, showing LiTi2(PO4)3–LiI composite family obtained in result of mixing LTP with lithium iodide. As far as we are aware, the LTP–LiI system has not yet been reported in the literature.
Fig. 2. SEM images for the powder compressed pellets of: LTP–2%LiI (A), LTP–8%LiI (B).
The X–ray diffraction method employing Philips X'Pert Pro (Cu Kα) was used for examination of the quality of both, the as–synthesized and composite products. Data were collected in the range of 10°–90° with 0.05° per step and a count rate of 0.5 s at each step.
2. Experimental The LiTi2(PO4)3 material was synthesized by conventional solid– state reaction method. Stoichiometric amounts of Li2CO3 (POCh), NH4H2PO4 (POCh) and TiO2 (Sigma Aldrich) reagents were weighted, mixed with a mortar and pestle. Then, the mixture was annealed at 900 °C for 10 h and the final product was formed. Further process of preparation involved ball–milling of the LiTi2(PO4)3 product with LiI·2H2O (Sigma Aldrich) employing a planetary ball mill Pulverisette 7. The materials immersed in ethanol were ball–milled with rotation speed of 600 rpm for 1 h. The composites containing lithium iodide were formed in different weight ratio varying from 2 to 8%. Table 1 Results of apparent, theoretical and relative density for relevant composites with lithium iodide. Composite
Apparent density [g·cm−3]
Theoretical density [g·cm−3]
Relative density [%]
LTP–2%LiI LTP–4%LiI LTP–6%LiI LTP–8%LiI
2.459 2.481 2.492 2.572
2.957 2.966 2.974 2.983
83.2 83.6 83.8 86.2
Fig. 3. The TGA traces for composites formed with LiI.
Please cite this article as: K. Kwatek, J.L. Nowiński, Studies on electrical properties of composites based on lithium titanium phosphate with lithium iodide, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.007
K. Kwatek, J.L. Nowiński / Solid State Ionics xxx (2016) xxx–xxx
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Fig. 5. Arrhenius plots for composites with different amount of lithium iodide.
thermal stability of the composites, in particular, their lithium iodide component. For density, microstructure and electrical investigations the pelletized samples of ca. 8 mm in diameter and 1.5–2 mm thick were formed under uniaxial 14 MPa pressure. Apparent density of the pelletized materials was determined using Archimedes method with isobutanol as an immerse liquid. The microstructure was investigated by means of scanning electron microscopy (SEM) employing Raith eLINE plus. Surfaces of freshly fractured pellets were scanned. For electric measurements, both bases of the as–form pellets were, at first, polished and then graphite electrodes were put on both sides of the each pellet. Impedance investigations were performed employing Solatron 1260 frequency analyzer in a frequency range of 10−2–3·107 Hz. Impedance data were collected in temperature range from 30 °C to 100 °C, both during heating and cooling runs. 3. Results and discussion
Fig. 4. Nyquist plots for LTP (A) and composite LTP–8%LiI (B) at 30 °C.
Thermal gravimetric analysis (TGA) was carried out on the powdered composites using TA Instruments Q600 to observe mass loss during heating under air flow in temperature range of 30 °C to 150 °C. The measurements were performed with heating rate of 10 °C·min−1 on samples, ca. 15 mg each. The investigation aims to determine
Table 2 Values of total conductivity at 30 °C, activation energy and pre–exponential factor for composites with various amount of lithium iodide. Composite LTP–2%LiI LTP–4%LiI LTP–6%LiI LTP–8%LiI
σ(30 °C) [S·cm−1] 1.24 1.77 4.23 7.30
× × × ×
−6
10 10−6 10−6 10−6
Ea [eV]
log σ0 [S·cm−1]
0.47 0.47 0.46 0.48
4.29 4.55 4.71 5.35
Fig. 1 presents a collection of the XRD patterns for the as–prepared LTP powder, and for the relevant composite prepared from that LTP and containing 8 wt.% of the lithium iodide. The positions and relative intensity of the diffraction lines recorded for the LTP agreed very well with database (ICDD 00–035–0754). Some very weak X–ray reflexions are visible on the pattern. These additional X–ray lines were assigned to Ti2P2O7 (ICDD 00–038–1468) and TiO2 (rutile – ICDD 01–082– 0514, anatase – ICDD 00–001–0562). According to a rough estimation, concentration of the foreign phases should not exceed ca. 5%. Similar X–ray diffraction investigation was performed for the composite (Fig. 1). Also, in this case, the positions of the observed X–ray diffraction lines showed no difference comparing to the positions of the relevant reflexes for the pure LTP. Unfortunately, the X–ray investigation did not detect lines attributed to LiI. That is because the positions of lithium iodide diffraction peaks coincide with those of LTP. Nevertheless, no additional lines were recorded suggesting that the lithium iodide added to the polycrystalline LTP did not impel formation of some additional foreign crystalline phases in the composite. Apparent, theoretical and relative densities are presented in Table 1. Relative density was calculated as a ratio between apparent and theoretical densities. Predicted theoretical density should increase with LiI contents in the composite. In fact, such trend is observed for the apparent densities experimentally determined. However, the experimental values are much lower (ca. 16%) than theoretical ones. The difference is caused by high porosity of the pressed material.
Please cite this article as: K. Kwatek, J.L. Nowiński, Studies on electrical properties of composites based on lithium titanium phosphate with lithium iodide, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.007
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Table 3 Values of resistances and capacitances obtained by fitting employing FIRDAC program for relevant composites with lithium iodide. Composite
R1 [Ω]
Q1 [F]
LTP–2%LiI LTP–4%LiI LTP–6%LiI LTP–8%LiI
6 × 103 5 × 103 4.1 × 103 6.8 × 103
1.2 4.1 1.7 2.7
× × × ×
R2 [Ω] 10−11 10−11 10−11 10−11
1.8 2.2 2.5 2.1
Fig. 2A, B present SEM images for LTP samples with addition of 2 and 8 wt.% of LiI respectively. Microstructure of each studied composites could be described as a mixture of large grains ca. 1 μm in diameter and very small crystallites ca. 200 nm in size decorating surfaces of the large ones. A number of the small crystallites per surface area increases with lithium iodide contents in the composite, as illustrates a comparison of the images related to 2 wt.% and 8 wt.% LiI composites. Also, it is worthy to note, that this observation correlates well with the density values, which also increase with LiI. These experimental evidences suggest, that the small crystallites should be attributed to LiI component. To examine thermal stability of the prepared composites, thermal gravimetric investigations (TGA) were carried out. Fig. 3 presents the mass loss traces for all studied composites. One can observe that mass decreases almost linearly with temperature. It is worthy to notice, that at 150 °C the mass loss was nearly the same as the mass of LiI component added initially to a composite. We think that such significant mass loss is caused not only by evaporation of moisture but also by lithium iodide mass loss. It is very likely that LiI decomposes in the composite during annealing. In result of iodine vapours are released. The electric investigations generally showed that addition of lithium iodide to the polycrystalline LTP resulted in substantial changes of ionic conductivity. To illustrate, one can compare two Nyquist plots for the impedance data collected for pure LTP (Fig. 4A) and LTP–8%LiI composite (Fig. 4B), when they were kept at 30 °C. Both compared samples had a similar geometry – the same electrode area and almost the same thickness. For the LTP, the impedance points form an arc which intersects ReZ axis at 4 × 107 Ω. This value expresses total resistance of the LTP sample. The impedance points for the LTP–8%LiI composite are arranged in Nyquist representation as two overlapped arcs ended with barely marked spur. For this case, the intersect point is estimated to be ca. 6 × 104 Ω, the value over two orders of magnitude lower than that for the LTP. Such enhancement of total ion conductivity, when LiI is added, is observed for all studied composites. Table 2 presents the determined values of total ion conductivity at 30 °C. One can notice, that the values increase slightly with LiI content in a composite. Temperature dependence of total ion conductivity, presented in the Arrhenius representation, revealed its more complex nature, as Fig. 5 shows. In the temperature range, RT – ca. 70 °C the dependence is linear, fulfilling the Arrhenius law. Whereas, for higher temperatures a significant departure from linearity is observed, even causing, the decrease of the total conductivity with temperature. In our opinion, this deterioration of ion properties is caused by some decomposition processes identified by TGA investigations. The values of activation energy determined from linear part of the log(σT) vs. 1000/T function are collected in Table 2. They seemed to be not dependent on LiI concentration in the composite. They also do not correlate with 0.51 eV reported for LiI [20], 0.54–0.69 eV presented for LiI–Al2O3 in [20] or 0.43 eV determined for the 0.6LiI–0.4Al2O3 composite [22]. The results of determination of the pre–exponential factor attributed to the linear section of the Arrhenius plots are listed in Table 2. The factor increases with LiI contents in the composite. To learn more about ion transport processes taking place in a composite material, the modeling of the electrical properties was performed by helps of FIRDAC fitting program [23]. Various equivalent electrical circuits were examined and analysed. The best fitting was achieved for the model coded as (R1Q1)(R2Q2)(R3Q3)Qdl according to Boukamp notation
× × × ×
Q2 [F]
104 104 104 104
1.1 1.7 1.2 1.8
× × × ×
R3 [Ω] 10−8 10−8 10−8 10−8
2.5 2.2 6.2 2.4
× × × ×
105 105 104 104
Q3 [F] 3 × 10−5 1.5 × 10−5 3.9 × 10−6 5 × 10−6
[24]. The adopted model suggests that three resistor–constant phase element (CPE) loops connected in series with Qdl element representing double–layer processes form the equivalent circuit. Table 3 presents the estimated fitting values of electric parameters for all studied composites for the impedance data collected at 30 °C. One can notice that R1 values are around 6 kΩ. Taking into account fitting uncertainties ca. 15%, there is evident that the R1 seem to be independent on LiI contents in the composite. Similar independence demonstrates R2. In contrary, the values of R3 do depend, decreasing with LiI contents. Also, the Q values demonstrate their own, characteristic trends. Those related to Q1 are 10−11 F rank of order, the Q2 adopts the values around 10−8 F, while Q3 respectively ca. 10−5 F. The Q1 values are typical for the grain capacity, the Q2 for grain boundary. The Q3 values are one order of magnitude higher than that expected for capacity of electrical double layer formed in an electrolyte–electrode interface. Trying to attribute the considered resistances to ion transport in grains and inter–grain phases, at first we took into consideration the literature indications. They pointed out, that for the LTP, a resistance of grains is always lower than grain boundary one. Also, the analysis of some literature data suggested that for samples like ours, taking into account a geometry, the resistance of grains should be an order of kΩ's [9,25–27]. Evaluation of conductivity value related to R1 resistance, using the formula σg = d/(Rg·A), where d and A represented sample thickness and electrode area respectively, gives the value ca. 0.7 × 10−4 S·cm−1. It is very close to conductivity of the LTP grains reported in the literature (1 × 10−4 S·cm−1) [9,26,27]. So, all these arguments advocate for the lowest resistance R1 to be attributed to the LTP grains, while the remaining two of higher values to the inter–grain region. Such interpretation implies a complex structure of the new grain boundary, formed in result of LiI insertion among LTP grains. We think that the nanocrystals of LiI decorating surface of LTP grains play the essential role. Further works are required for clarification of this issue.
4. Conclusions Our study demonstrated that addition of lithium iodide to the LTP resulted in a significant enhancement of total ion conductivity. The discovered phenomenon is due to modification of the inter–grain region in a microstructure of the LTP–LiI composite. These facilitate lithium ion transport among the grains. This new group of lithium ion conducting composites could be considered as interesting, potential candidates for applications as solid electrolytes in the LIBs technology. However, there is necessary to note, that the studied composites maintain its high ion properties only up to ca. 70 °C. Above, some deterioration processes cause the decrease of ion conductivity.
References [1] K. Dokko, K. Hoshina, H. Nakano, K. Kanamura, J. Power Sources 174 (2007) 1100–1103. [2] S.S. Das, P.K. Srivastava, N.B. Singh, J. Non-Cryst. Solids 358 (2012) 2841–2846. [3] P. Knauth, Solid State Ionics 180 (2009) 911–916. [4] A. Chandra, A. Bhatt, A. Chandra, J. Mater. Sci. Technol. 29 (2013) 193–208. [5] J.F.M. Oudenhoven, L. Baggetto, P.H.L. Notten, Adv. Energy Mater. 1 (2011) 10–33. [6] I. Abrahams, E. Hadzifejzovic, Solid State Ionics 134 (2000) 249–257. [7] E.C. Bucharsky, K.G. Schell, A. Hintennach, M.J. Hoffmann, Solid State Ionics 274 (2015) 77–82.
Please cite this article as: K. Kwatek, J.L. Nowiński, Studies on electrical properties of composites based on lithium titanium phosphate with lithium iodide, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.007
K. Kwatek, J.L. Nowiński / Solid State Ionics xxx (2016) xxx–xxx [8] S. Duluard, A. Paillassa, L. Puech, P. Vinatier, V. Turq, P. Rozier, P. Lenormand, P.–.L. Taberna, P. Simon, F. Ansart, J. Eur. Ceram. Soc. 33 (2013) 1145–1153. [9] H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka, G. Adachi, J. Electrochem. Soc. 137 (1990) 1023–1027. [10] M. Schroeder, S. Glatthaar, J.R. Binder, Solid State Ionics 201 (2011) 49–53. [11] Y. Kobayashi, T. Takeuchi, M. Tabuchi, K. Ado, H. Kageyama, J. Power Sources 81–82 (1999) 853–858. [12] C.R. Mariappan, M. Gellert, C. Yada, F. Rosciano, B. Roling, Electrochem. Commun. 14 (2012) 25–28. [13] J. Fu, J. Mater. Sci. 33 (1998) 1549–1553. [14] J. Fu, Solid State Ionics 96 (1997) 195–200. [15] H. Hosono, Y. Abe, J. Non-Cryst. Solids 190 (1995) 185–197. [16] S. Soman, Y. Iwai, J. Kawamura, A. Kulkarni, J. Solid State Electrochem. 16 (2012) 1761–1766.
[17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
5
J. Jamnik, Solid State Ionics 177 (2006) 2543–2547. J. Maier, J. Electrochem. Soc. 134 (1987) 1524–1535. J. Maier, Z. Phys. Chem. 217 (2003) 415–436. B. Kumar, S. Nellutla, J.S. Thokchom, C. Chen, J. Power Sources 160 (2006) 1329–1335. C.C. Liang, J. Electrochem. Soc. 120 (1973) 1289–1292. C.C. Liang, A.V. Joshi, N.E. Hamilton, J. Appl. Electrochem. 8 (1978) 445–454. J.R. Dygas, Ph.D. Thesis, Northwestern University, Evanston, USA, 1986. B.A. Boukamp, Solid State Ionics 20 (1986) 31s–344. K. Arbi, S. Mandal, J.M. Rojo, J. Sanz, Chem. Mater. 14 (2002) 1091–1097. S. Wang, L. Ben, H. Li, L. Chen, Solid State Ionics 268 (2014) 110–116. L. Vijayan, G. Govindaraj, Physica B 404 (2009) 3539–3543.
Please cite this article as: K. Kwatek, J.L. Nowiński, Studies on electrical properties of composites based on lithium titanium phosphate with lithium iodide, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.007
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Studies on electrical properties of composites based on lithium titanium phosphate with lithium iodide K. Kwatek ⁎, J.L. Nowiński Warsaw University of Technology, Faculty of Physics, 00–662 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 29 July 2016 Received in revised form 5 December 2016 Accepted 7 December 2016 Available online xxxx Keywords: Solid electrolyte Grain boundary conductivity enhancement Composite Lithium iodide
a b s t r a c t While grain conductivity of lithium titanium phosphate materials (LTP) is high, the total conductivity is relatively low due to high resistant grain boundary phase. The work demonstrates that this resistance can be substantially reduced by helps of LiI. For this purpose a series of the LTP–LiI composites were formed employing mechanical ball–milling. X–ray diffractometry, thermogravimetry, impedance spectroscopy, scanning electron microscopy and density methods were used to characterize obtained materials. A maximum enhancement of total ionic conductivity at 30 °C was recorded for the LTP containing 8 wt.% of LiI (7.3 × 10−6 S·cm−1 comparing to 1.5 × 10−8 S·cm−1 for pure LTP). © 2016 Elsevier B.V. All rights reserved.
1. Introduction A solid electrolyte suitable for lithium ion batteries (LIBs) is a challenging issue stimulating intensive searching for new Li–ion conductors. Commonly used LIBs composed of liquid electrolytes are not appropriate for further researches due to their high flammability, poor mechanical properties and limited width of electrochemical window. Intensive works are focused on replacing these liquid electrolytes by another, in particular, solid ones. It is expected that the batteries with solid electrolytes (all–solid batteries) could offer, in respect to the traditional, liquid electrolyte based ones, improved mechanical properties and wider electrochemical window. Proposed all solid–state battery system requires fast Li–ion conductors, which will act as solid electrolyte. For practical use, materials with lithium–ion conductivity higher than 10−4 S·cm−1 are required [1]. Various solid compounds were investigated, but only few of them might be characterized as good lithium ion conductors [2–5]. Among them, the best conductivity demonstrate Li3N single–crystal and glasses based on Li2S. Their conductivity at room temperature approaches to 10−3 S·cm−1. However, they do not fulfill all high standards required for commercially oriented, mass production. Poor chemical stability of these materials excludes them for practical purposes as solid electrolytes [6–8]. One of the favorites as a solid electrolyte is LiTi2(PO4)3 (LTP), which is intensively studied in the recent years. The LTP possesses NASICON–type structure with rhombohedral symmetry and belongs to R–3c space group. It forms three dimensional network structure, composed of TiO6 octahedra and PO4 tetrahedra, which are linked together by their corners [9]. Lithium ⁎ Corresponding author. E-mail address: [email protected] (K. Kwatek).
titanium phosphate is of great interest due to its high structural and chemical stability. Unfortunately, ceramic LTP samples show too low total ionic conductivity from 10−8 S·cm− 1 to 10− 6 S·cm−1 at room temperature, as it has been reported in the literature [6,9,11,25–27]. This major issue is caused by high grain boundaries resistance [9–12]. H. Aono showed that total conductivity of LTP derived materials greatly depends on density of sintered pellet. Materials with high porosity are worse conductors because lithium ions have to travel bigger distances between grains. Modifying LTP by doping it with trivalent atoms tends to lower porosity of ceramics [9]. Most of the interest in Li1+xMxTi2–x(PO4)3 system was found when titanium atom is substituted by aluminum (x = 0.3). It is reported that Li1.3Al0.3Ti1.7(PO4)3 (LATP) has the highest ionic conductivity among all LTP derived materials [9, 13]. Several researchers attempted to achieve a further enhance of conductivity of lithium titanium phosphate by the glass–ceramic route [6, 14–16]. Despite of reported high values of conductivity for all LTP derived materials, this method is problematic. First of all, it requires preheated stainless–steel plates on which glass formation proceeds. Unfortunately, that process may cause numerous undemanded cracks due to resulting stresses. Melting of substrates occurs at high temperatures reaching 1450 °C [14]. Development of glass–ceramic conductor for large scale might be troublesome. In comparison, the described by Aono, ceramic route is easier to apply. Other alternative way to enhance of grain boundary conductivity of the lithium titanium phosphate derivatives one can search via adding some foreign compounds to the parent LATP. The resulting heterogeneous materials could possess, new, interesting properties due to space charge effects [17–19]. Such attempt undertook Kumar et al. [20] studying ion properties of LATP glass ceramics with Al2O3 and LATP glass ceramics with Ba0.6Sr0.4TiO3 (0.6BST) composites. In both
http://dx.doi.org/10.1016/j.ssi.2016.12.007 0167-2738/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: K. Kwatek, J.L. Nowiński, Studies on electrical properties of composites based on lithium titanium phosphate with lithium iodide, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.007
2
K. Kwatek, J.L. Nowiński / Solid State Ionics xxx (2016) xxx–xxx
Fig. 1. XRD pattern for LTP material and composite with 8 wt.% addition of LiI.
cases, the additive lowered ion conductivity. The author observed the space charge effect enhancing conductivity, but concluded that the competing, blocking effect of the insulating phase prevailed. Inspiration of our work was the seminal publication by Liang [21], in which high ion conductivity of LiI–Al2O3 composites was reported. In the later works [17–19] the phenomenon was explained by enormously high space charge effect occurring, in particular, in this composite. Our approach aimed to examine LTP–LiI system. There is known, that properties of the lithium iodide based composites depends on specimen preparation technique [20]. We wanted to check if simplified technique employing powdered components, processed only by high pressure could produce a material demonstrating high total ion conductivity. This work presents that grain boundary resistance can be reduced by insertion of the other foreign phases to the material. In our investigations, we focused attention on composites and found that NASICON– type structure LiTi2(PO4)3 could be a promising base for formation such materials. The work presents a new approach, showing LiTi2(PO4)3–LiI composite family obtained in result of mixing LTP with lithium iodide. As far as we are aware, the LTP–LiI system has not yet been reported in the literature.
Fig. 2. SEM images for the powder compressed pellets of: LTP–2%LiI (A), LTP–8%LiI (B).
The X–ray diffraction method employing Philips X'Pert Pro (Cu Kα) was used for examination of the quality of both, the as–synthesized and composite products. Data were collected in the range of 10°–90° with 0.05° per step and a count rate of 0.5 s at each step.
2. Experimental The LiTi2(PO4)3 material was synthesized by conventional solid– state reaction method. Stoichiometric amounts of Li2CO3 (POCh), NH4H2PO4 (POCh) and TiO2 (Sigma Aldrich) reagents were weighted, mixed with a mortar and pestle. Then, the mixture was annealed at 900 °C for 10 h and the final product was formed. Further process of preparation involved ball–milling of the LiTi2(PO4)3 product with LiI·2H2O (Sigma Aldrich) employing a planetary ball mill Pulverisette 7. The materials immersed in ethanol were ball–milled with rotation speed of 600 rpm for 1 h. The composites containing lithium iodide were formed in different weight ratio varying from 2 to 8%. Table 1 Results of apparent, theoretical and relative density for relevant composites with lithium iodide. Composite
Apparent density [g·cm−3]
Theoretical density [g·cm−3]
Relative density [%]
LTP–2%LiI LTP–4%LiI LTP–6%LiI LTP–8%LiI
2.459 2.481 2.492 2.572
2.957 2.966 2.974 2.983
83.2 83.6 83.8 86.2
Fig. 3. The TGA traces for composites formed with LiI.
Please cite this article as: K. Kwatek, J.L. Nowiński, Studies on electrical properties of composites based on lithium titanium phosphate with lithium iodide, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.007
K. Kwatek, J.L. Nowiński / Solid State Ionics xxx (2016) xxx–xxx
3
Fig. 5. Arrhenius plots for composites with different amount of lithium iodide.
thermal stability of the composites, in particular, their lithium iodide component. For density, microstructure and electrical investigations the pelletized samples of ca. 8 mm in diameter and 1.5–2 mm thick were formed under uniaxial 14 MPa pressure. Apparent density of the pelletized materials was determined using Archimedes method with isobutanol as an immerse liquid. The microstructure was investigated by means of scanning electron microscopy (SEM) employing Raith eLINE plus. Surfaces of freshly fractured pellets were scanned. For electric measurements, both bases of the as–form pellets were, at first, polished and then graphite electrodes were put on both sides of the each pellet. Impedance investigations were performed employing Solatron 1260 frequency analyzer in a frequency range of 10−2–3·107 Hz. Impedance data were collected in temperature range from 30 °C to 100 °C, both during heating and cooling runs. 3. Results and discussion
Fig. 4. Nyquist plots for LTP (A) and composite LTP–8%LiI (B) at 30 °C.
Thermal gravimetric analysis (TGA) was carried out on the powdered composites using TA Instruments Q600 to observe mass loss during heating under air flow in temperature range of 30 °C to 150 °C. The measurements were performed with heating rate of 10 °C·min−1 on samples, ca. 15 mg each. The investigation aims to determine
Table 2 Values of total conductivity at 30 °C, activation energy and pre–exponential factor for composites with various amount of lithium iodide. Composite LTP–2%LiI LTP–4%LiI LTP–6%LiI LTP–8%LiI
σ(30 °C) [S·cm−1] 1.24 1.77 4.23 7.30
× × × ×
−6
10 10−6 10−6 10−6
Ea [eV]
log σ0 [S·cm−1]
0.47 0.47 0.46 0.48
4.29 4.55 4.71 5.35
Fig. 1 presents a collection of the XRD patterns for the as–prepared LTP powder, and for the relevant composite prepared from that LTP and containing 8 wt.% of the lithium iodide. The positions and relative intensity of the diffraction lines recorded for the LTP agreed very well with database (ICDD 00–035–0754). Some very weak X–ray reflexions are visible on the pattern. These additional X–ray lines were assigned to Ti2P2O7 (ICDD 00–038–1468) and TiO2 (rutile – ICDD 01–082– 0514, anatase – ICDD 00–001–0562). According to a rough estimation, concentration of the foreign phases should not exceed ca. 5%. Similar X–ray diffraction investigation was performed for the composite (Fig. 1). Also, in this case, the positions of the observed X–ray diffraction lines showed no difference comparing to the positions of the relevant reflexes for the pure LTP. Unfortunately, the X–ray investigation did not detect lines attributed to LiI. That is because the positions of lithium iodide diffraction peaks coincide with those of LTP. Nevertheless, no additional lines were recorded suggesting that the lithium iodide added to the polycrystalline LTP did not impel formation of some additional foreign crystalline phases in the composite. Apparent, theoretical and relative densities are presented in Table 1. Relative density was calculated as a ratio between apparent and theoretical densities. Predicted theoretical density should increase with LiI contents in the composite. In fact, such trend is observed for the apparent densities experimentally determined. However, the experimental values are much lower (ca. 16%) than theoretical ones. The difference is caused by high porosity of the pressed material.
Please cite this article as: K. Kwatek, J.L. Nowiński, Studies on electrical properties of composites based on lithium titanium phosphate with lithium iodide, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.007
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K. Kwatek, J.L. Nowiński / Solid State Ionics xxx (2016) xxx–xxx
Table 3 Values of resistances and capacitances obtained by fitting employing FIRDAC program for relevant composites with lithium iodide. Composite
R1 [Ω]
Q1 [F]
LTP–2%LiI LTP–4%LiI LTP–6%LiI LTP–8%LiI
6 × 103 5 × 103 4.1 × 103 6.8 × 103
1.2 4.1 1.7 2.7
× × × ×
R2 [Ω] 10−11 10−11 10−11 10−11
1.8 2.2 2.5 2.1
Fig. 2A, B present SEM images for LTP samples with addition of 2 and 8 wt.% of LiI respectively. Microstructure of each studied composites could be described as a mixture of large grains ca. 1 μm in diameter and very small crystallites ca. 200 nm in size decorating surfaces of the large ones. A number of the small crystallites per surface area increases with lithium iodide contents in the composite, as illustrates a comparison of the images related to 2 wt.% and 8 wt.% LiI composites. Also, it is worthy to note, that this observation correlates well with the density values, which also increase with LiI. These experimental evidences suggest, that the small crystallites should be attributed to LiI component. To examine thermal stability of the prepared composites, thermal gravimetric investigations (TGA) were carried out. Fig. 3 presents the mass loss traces for all studied composites. One can observe that mass decreases almost linearly with temperature. It is worthy to notice, that at 150 °C the mass loss was nearly the same as the mass of LiI component added initially to a composite. We think that such significant mass loss is caused not only by evaporation of moisture but also by lithium iodide mass loss. It is very likely that LiI decomposes in the composite during annealing. In result of iodine vapours are released. The electric investigations generally showed that addition of lithium iodide to the polycrystalline LTP resulted in substantial changes of ionic conductivity. To illustrate, one can compare two Nyquist plots for the impedance data collected for pure LTP (Fig. 4A) and LTP–8%LiI composite (Fig. 4B), when they were kept at 30 °C. Both compared samples had a similar geometry – the same electrode area and almost the same thickness. For the LTP, the impedance points form an arc which intersects ReZ axis at 4 × 107 Ω. This value expresses total resistance of the LTP sample. The impedance points for the LTP–8%LiI composite are arranged in Nyquist representation as two overlapped arcs ended with barely marked spur. For this case, the intersect point is estimated to be ca. 6 × 104 Ω, the value over two orders of magnitude lower than that for the LTP. Such enhancement of total ion conductivity, when LiI is added, is observed for all studied composites. Table 2 presents the determined values of total ion conductivity at 30 °C. One can notice, that the values increase slightly with LiI content in a composite. Temperature dependence of total ion conductivity, presented in the Arrhenius representation, revealed its more complex nature, as Fig. 5 shows. In the temperature range, RT – ca. 70 °C the dependence is linear, fulfilling the Arrhenius law. Whereas, for higher temperatures a significant departure from linearity is observed, even causing, the decrease of the total conductivity with temperature. In our opinion, this deterioration of ion properties is caused by some decomposition processes identified by TGA investigations. The values of activation energy determined from linear part of the log(σT) vs. 1000/T function are collected in Table 2. They seemed to be not dependent on LiI concentration in the composite. They also do not correlate with 0.51 eV reported for LiI [20], 0.54–0.69 eV presented for LiI–Al2O3 in [20] or 0.43 eV determined for the 0.6LiI–0.4Al2O3 composite [22]. The results of determination of the pre–exponential factor attributed to the linear section of the Arrhenius plots are listed in Table 2. The factor increases with LiI contents in the composite. To learn more about ion transport processes taking place in a composite material, the modeling of the electrical properties was performed by helps of FIRDAC fitting program [23]. Various equivalent electrical circuits were examined and analysed. The best fitting was achieved for the model coded as (R1Q1)(R2Q2)(R3Q3)Qdl according to Boukamp notation
× × × ×
Q2 [F]
104 104 104 104
1.1 1.7 1.2 1.8
× × × ×
R3 [Ω] 10−8 10−8 10−8 10−8
2.5 2.2 6.2 2.4
× × × ×
105 105 104 104
Q3 [F] 3 × 10−5 1.5 × 10−5 3.9 × 10−6 5 × 10−6
[24]. The adopted model suggests that three resistor–constant phase element (CPE) loops connected in series with Qdl element representing double–layer processes form the equivalent circuit. Table 3 presents the estimated fitting values of electric parameters for all studied composites for the impedance data collected at 30 °C. One can notice that R1 values are around 6 kΩ. Taking into account fitting uncertainties ca. 15%, there is evident that the R1 seem to be independent on LiI contents in the composite. Similar independence demonstrates R2. In contrary, the values of R3 do depend, decreasing with LiI contents. Also, the Q values demonstrate their own, characteristic trends. Those related to Q1 are 10−11 F rank of order, the Q2 adopts the values around 10−8 F, while Q3 respectively ca. 10−5 F. The Q1 values are typical for the grain capacity, the Q2 for grain boundary. The Q3 values are one order of magnitude higher than that expected for capacity of electrical double layer formed in an electrolyte–electrode interface. Trying to attribute the considered resistances to ion transport in grains and inter–grain phases, at first we took into consideration the literature indications. They pointed out, that for the LTP, a resistance of grains is always lower than grain boundary one. Also, the analysis of some literature data suggested that for samples like ours, taking into account a geometry, the resistance of grains should be an order of kΩ's [9,25–27]. Evaluation of conductivity value related to R1 resistance, using the formula σg = d/(Rg·A), where d and A represented sample thickness and electrode area respectively, gives the value ca. 0.7 × 10−4 S·cm−1. It is very close to conductivity of the LTP grains reported in the literature (1 × 10−4 S·cm−1) [9,26,27]. So, all these arguments advocate for the lowest resistance R1 to be attributed to the LTP grains, while the remaining two of higher values to the inter–grain region. Such interpretation implies a complex structure of the new grain boundary, formed in result of LiI insertion among LTP grains. We think that the nanocrystals of LiI decorating surface of LTP grains play the essential role. Further works are required for clarification of this issue.
4. Conclusions Our study demonstrated that addition of lithium iodide to the LTP resulted in a significant enhancement of total ion conductivity. The discovered phenomenon is due to modification of the inter–grain region in a microstructure of the LTP–LiI composite. These facilitate lithium ion transport among the grains. This new group of lithium ion conducting composites could be considered as interesting, potential candidates for applications as solid electrolytes in the LIBs technology. However, there is necessary to note, that the studied composites maintain its high ion properties only up to ca. 70 °C. Above, some deterioration processes cause the decrease of ion conductivity.
References [1] K. Dokko, K. Hoshina, H. Nakano, K. Kanamura, J. Power Sources 174 (2007) 1100–1103. [2] S.S. Das, P.K. Srivastava, N.B. Singh, J. Non-Cryst. Solids 358 (2012) 2841–2846. [3] P. Knauth, Solid State Ionics 180 (2009) 911–916. [4] A. Chandra, A. Bhatt, A. Chandra, J. Mater. Sci. Technol. 29 (2013) 193–208. [5] J.F.M. Oudenhoven, L. Baggetto, P.H.L. Notten, Adv. Energy Mater. 1 (2011) 10–33. [6] I. Abrahams, E. Hadzifejzovic, Solid State Ionics 134 (2000) 249–257. [7] E.C. Bucharsky, K.G. Schell, A. Hintennach, M.J. Hoffmann, Solid State Ionics 274 (2015) 77–82.
Please cite this article as: K. Kwatek, J.L. Nowiński, Studies on electrical properties of composites based on lithium titanium phosphate with lithium iodide, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.007
K. Kwatek, J.L. Nowiński / Solid State Ionics xxx (2016) xxx–xxx [8] S. Duluard, A. Paillassa, L. Puech, P. Vinatier, V. Turq, P. Rozier, P. Lenormand, P.–.L. Taberna, P. Simon, F. Ansart, J. Eur. Ceram. Soc. 33 (2013) 1145–1153. [9] H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka, G. Adachi, J. Electrochem. Soc. 137 (1990) 1023–1027. [10] M. Schroeder, S. Glatthaar, J.R. Binder, Solid State Ionics 201 (2011) 49–53. [11] Y. Kobayashi, T. Takeuchi, M. Tabuchi, K. Ado, H. Kageyama, J. Power Sources 81–82 (1999) 853–858. [12] C.R. Mariappan, M. Gellert, C. Yada, F. Rosciano, B. Roling, Electrochem. Commun. 14 (2012) 25–28. [13] J. Fu, J. Mater. Sci. 33 (1998) 1549–1553. [14] J. Fu, Solid State Ionics 96 (1997) 195–200. [15] H. Hosono, Y. Abe, J. Non-Cryst. Solids 190 (1995) 185–197. [16] S. Soman, Y. Iwai, J. Kawamura, A. Kulkarni, J. Solid State Electrochem. 16 (2012) 1761–1766.
[17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
5
J. Jamnik, Solid State Ionics 177 (2006) 2543–2547. J. Maier, J. Electrochem. Soc. 134 (1987) 1524–1535. J. Maier, Z. Phys. Chem. 217 (2003) 415–436. B. Kumar, S. Nellutla, J.S. Thokchom, C. Chen, J. Power Sources 160 (2006) 1329–1335. C.C. Liang, J. Electrochem. Soc. 120 (1973) 1289–1292. C.C. Liang, A.V. Joshi, N.E. Hamilton, J. Appl. Electrochem. 8 (1978) 445–454. J.R. Dygas, Ph.D. Thesis, Northwestern University, Evanston, USA, 1986. B.A. Boukamp, Solid State Ionics 20 (1986) 31s–344. K. Arbi, S. Mandal, J.M. Rojo, J. Sanz, Chem. Mater. 14 (2002) 1091–1097. S. Wang, L. Ben, H. Li, L. Chen, Solid State Ionics 268 (2014) 110–116. L. Vijayan, G. Govindaraj, Physica B 404 (2009) 3539–3543.
Please cite this article as: K. Kwatek, J.L. Nowiński, Studies on electrical properties of composites based on lithium titanium phosphate with lithium iodide, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.12.007