The effects of lithium doping level on the structural, electrical properties of Li+-doped BPO4 solid electrolyte

Materials Research Bulletin 48 (2013) 2896–2900

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The effects of lithium doping level on the structural, electrical properties of Li+-doped BPO4 solid electrolyte Shan Gao, Miao Shui *, Weidong Zheng, Tianci Yang, Jie Shu, Liangliang Cheng, Lin Feng, Yuanlong Ren The State Key Laboratory Base of Novel Functional Materials and Preparation Science, The Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 6 July 2012 Received in revised form 15 March 2013 Accepted 4 April 2013 Available online 11 April 2013

A series of lithium ion conducting solid electrolytes LixB1x/3PO4(x = 0.01, 0.05, 0.09, 0.13, 0.17, 0.20) is synthesized by a soft-chemistry route. FTIR and XRD measurements reveal that the electrolyte is pure phase of tetragonal structure. AC-impedance spectroscopy (AC-IS) at room temperature shows that LixB1x/3PO4 exhibits higher ionic conductivities in the range 0.05  x  0.13, beyond which, the ionic conductivities decrease quickly. Maximum ionic conductivity of the LixB1x/3PO4 reaches 3.35  105 S cm1 at room temperature for x = 0.05. Direct current polarizing (DCP) measurement indicates that the decomposition voltage for the solid electrolyte reaches up to 3.7 V. Micro-structure parameters of synthesized LixB1x/3PO4 samples are calculated by Rietveld refinement of X-ray diffraction spectra. The unit-cell parameters, lattice strain, crystal grain size and ionic conductivities of the samples are correlated with the lithium ion doping level x. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Inorganic compounds B. Chemical synthesis C. Impedance spectroscopy D. Electrochemical properties

1. Introduction Lithium-ion battery has been widely applied in portable electrical apparatus due to its high voltage, high energy density, low self-discharge, long cycling life, easy to maintenance, etc. However, the safety issues associated with conventional lithiumion systems prevent further development toward large scale and higher-energy-density batteries. Now days, significant efforts have been devoted to develop all-solid-state Li-ion batteries, which have the advantages of high security (no leakage of liquid electrolyte), compact space, easy to miniaturization, volume energy density higher than 600–900 wh L1, weight energy density higher than 330–450 wh Kg1, higher Li+ transport number which reaches up to 1.0 compared with 0.3–0.6 for liquid or gel polymer electrolyte. Furthermore, it will be possible to use lithium as negative electrode to get higher energy density and stack directly to form high voltage single battery. Above all, Li+ conductivity of solid electrolyte is a key factor in determining the performance of all-solid-state batteries. However, other elements like decomposition voltage, stability are also important. For instance, perovskite (ABO3)-type Li0.35La0.55TiO3 (LLTO) possesses a high ionic conductivity up to 103–104 S cm1 at room temperature, but its decomposition voltage is very low.

* Corresponding author. Tel.: +86 574 87600787. E-mail addresses: [email protected], [email protected] (M. Shui). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.04.006

Li1+xAlxTi2x(PO4)3 (LATP) is unstable with lithium metal due to Ti4+ reduction. The ionic conductivity of thio-LISICON reaches 2.2  103 S cm1 at room temperature, yet sulfur-containing solid electrolyte is unstable in air. Therefore, new solid-state electrolyte that is stable in air, able to use Li metal as negative electrolyte, is highly desirable for the fabrication of high performance all-solid-state batteries. In the year 1996, Kelder et al. [1] demonstrated the application of Li+-doped BPO4 as a potential solid electrolyte for Li-ion rechargeable batteries for the first time. Li+-doped BPO4 solid electrolyte was steady in air and stable toward metallic lithium. As a kind of mineral glue, Li+-doped BPO4 showed close contact with electrodes in a solid-state battery, resulting in a lower interfacial impedance. It seemed to be a very promising inorganic solid electrolyte. The defect structure of Li+-doped BPO4 was studied by Jak et al. [2] and two possible defect models were proposed, Li00B þ 2Lii and V000 B þ 3Lii . In these two kinds of defect models, the ionic conductivity originated from interstitial Li ions. Whereas, from reports of papers [3,4], the Li+-ion conductivities of Li+-doped BPO4 showed quite a difference. At present, there are few reports concerning comparative studies on the effect of Li-doping level on the Li+ ionic conductivity. In order to study the influence of different Li-doping levels on its crystal structure and ionic conductivity, in this paper LixB1x/ 3PO4 samples with doping levels of 1, 5, 9, 13, 17, and 20 mol% Li were synthesized. Possible defect models were discussed.

S. Gao et al. / Materials Research Bulletin 48 (2013) 2896–2900

2. Experimental

2 FWHMðG  sizeÞ2 ¼ Hgz ¼

2.1. The preparation of LixB1x/3PO4 solid electrolyte ¼ Stoichiometric amount of H2NH4PO4 (AR) and H3BO3 (99.99%) were mixed and dissolved gradually in distilled water at 90 8C under strong stirring. Then certain amount of LiOHH2O (GR) was added slowly into the solution. Continue to heat the mixture until the water was slowly evaporated and viscous slurry was left. Six LixB1x/3PO4 samples with doping levels of 1, 5, 9, 13, 17, and 20 mol% Li were synthesized by changing the amount of LiOHH2O. As was shown in following discussion, the ionic conductivity of 9 mol% Li doping level sample was crucial to explain the relationship between the lithium ion doping level and structural, electro-chemical properties. To ensure the accuracy of the data, we have prepared 2 Li0.09B0.97PO4 samples. The precursor powders were heated in air using a tube furnace at 110 8C for 2 h and 500 8C for 6 h. Pellets of solid electrode with 1–2 mm in thickness and 13 mm in diameter were prepared by applying a pressure of 250 MPa for 3 min on the powders in the stainless steel mold. The pellets were sintered at 500 8C for 4 h for later use. 2.2. Characterization Fourier transform infrared (FT-IR) spectra were recorded on a Shimadzu FTIR-8900 infrared spectrophotometer in the range of 4000–400 cm1 using the KBr pellet method. Silver paste was coated on the two opposite faces of the pellet as ion blocking electrodes. Silver coated pellets were thermally treated at 200 8C for 2 h for AC-IS measurements. AC-impedance spectroscopy was measured with a CHI660D (Shanghai Chenhua, China) electrochemical working station in a frequency range from 1 Hz to 100 KHz with a voltage amplitude of 5 mV The decomposition voltage was studied using direct current polarizing (DCP) technique (scanning velocity 0.0417 V s1, potential range 0–5 V). The X-ray diffraction (XRD) patterns of the sample powders were recorded with Bruker AXS D8 FOCUS X-ray diffractometer using Cu Ka radiation. The patterns were obtained in the scanning velocity of 0.0298 s1 over a angle range from 20 to 708. The whole x-ray pattern was refined by Fullprof2000, a free software developed by Juan Rodrı´guez-Carvajal of Laboratoire Le´on Brillouin (CEA-CNRS) in France for the Rietveld refinement of structural models to both X-ray and neutron diffraction data. The grain size and lattice strain were calculated according to Eq. (1) Size ðAngstromsÞ ¼

1

bsize

; Max  strain ð%Þ ¼

1 2bstrain dðhklÞ

(1)

where bsize means the peak broadening ascribed to particle size and bstrain represents the peak broadening ascribed to lattice strain. Both bsize and bstrain are the convolutions of Gaussian component Hg and HL. The relationships of, b, Hg, HL, H and eta are illustrated according to the following equations.

b¼

0:5H eta= pi þ ð1:0  etaÞ=sqrtð pi=Inð2ÞÞ

H¼ eta ¼

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IG ; FWHMðL  sizeÞ ¼ HLz cos2 ðuÞ

ðY þ FðszÞÞ cosðuÞ

2 FWHMðG  strainÞ2 ¼ Hgs

¼ ðU þ ½ð1  zÞDST2 Þt 2 gðuÞ; FWHMðL  strainÞ ¼ HL ¼ ðx þ z:DSTÞtgðuÞ Here we only focus on isotropic broadening effects. Therefore only U, Y, x and IG are refined in the Rietveld program. 3. Results and discussion 3.1. FTIR spectra analysis The IR spectra of LixB1x/3PO4 (x = 0.01, 0.05, 0.09, 0.13, 0.17, 0.20) show main absorption bands over the range from 1600 to 500 cm1, which are demonstrated in Fig. 1. Two peaks located at 1097 and 633 cm1 are due to the asymmetric stretching vibrations and the bending vibrations of the [PO4]3 tetrahedron. The bands at 931 and 557 cm1 are attributed to the pseudo-lattice vibrations of boron lattice [5]. The peak intensities located at 1097 cm1 of the P–O bond show gradual weakening and broadening with the increasing Li-doping levels. This indicates that the coherence of [PO4]3 tetrahedron structure is reduced for the formation of more boron vacancies with increasing Li-doping levels. Whereas, the peak intensities of B–O bond show no obvious change with increasing boron vacancies, perhaps because of the increasingly stronger superimposing Li–O vibration absorption located at 500–600 cm1. Interestingly, especially apparent in the case of x = 0.05, a small and sharp shoulder peak appears at 1050 cm1 for x = 0.05, 0.09 and 0.13, meanwhile the Li+-ion conductivities of Li+-doped BPO4 happen to be higher in this range. It is assumed that the broad peak at 1097 cm1 can be described as the superposition of four P–O asymmetric stretching vibrations at 1145, 1093, 1045 and 984 cm1 [6]. With the increasing Li+ doping levels, Li+ will occupy the interstitial sites around [PO4]3 tetrahedron, therefore affecting the peak position and intensities of four vibration modes of [PO4]3 tetrahedron. The intensified vibration modes are more likely to present itself as a split peak or shoulder peak. However, the further introduction of lithium ions into interstitial sites will possibly equalize this effect, therefore causes the gradual weakening of the shoulder peak at 1050 cm1. Therefore, the shoulder peak at 1050 cm1, to some extent, reflects the partially occupied interstitial sites. 3.2. AC-IS analysis and decomposition voltage measurement The AC impedance spectrum of LixB1x/3PO4 (x = 0.05) at room temperature is shown in Fig. 2. The Nyquist complex plane

0:2

ðHG2:5 þ 2:69269HG2 HL þ 2:42843HG1:5 HL2 þ 4:47163HG HL3 þ 0:07842HG0:5 HL4 þ HL5 Þ 1:36603HL =H  0:47719ðHL =HÞ2 þ 0:11116ðHL =HÞ3

To calculate Hg and HL, we shall use the following equations with parameters Ig, Y (isotropic size broadening) U, x, (isotropic strain broadening) F(sz), (anisotropic size broadening) DST, (anisotropic strain broadening) obtained from the results of the Rietveld refinements.

diagram of the sample for x = 0.05 consists of a portion of arc and an inclined line. The inset on the top left shows its equivalent circuit model (Qd[RbQ]), where Rb means the total resistance of the solid electrolyte (bulk resistance plus grain boundary resistance), Q the constant-phase element which reflects the Li+ diffusion across

S. Gao et al. / Materials Research Bulletin 48 (2013) 2896–2900

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Fig. 3. The ionic conductivity of LixB1x/3PO4 (x = 0.01, 0.05, 0.09, 0.13, 0.17, 0.20) as function of temperature.

Fig. 1. FT-IR spectra of LixB1x/3PO4 (x = 0.01, 0.05, 0.09, 0.13, 0.17, 0.20).

Fig. 2. The AC impedance spectrum of LixB1x/3PO4 (x = 0.05) at room temperature.

the electrode/electrolyte boundary and Qd the constant phase element associated with double layer capacitive reactance [7]. That portion of arc can be attributed to the impedance character of parallel CPEd and series [RbCPE], while the inclined line is due to the impedance character of CPE. The span of the arc shows the ionic conductivity of this solid electrolyte. As is seen from the figure that

only minor deviations are observed between the measured and the fitted values. The ionic conductivities of LixB1x/3PO4 calculated by fitting the electrochemical impedance spectroscopy (EIS) data with ZSimpWin software are listed in Table 1. Higher ionic conductivities are observed when 0.05  x  0.13 with the maximum of 3.35  105 S cm1 at x = 0.05. The calculated ionic conductivities of two Li0.09B0.97PO4 samples are approached to each other. This validates the convex-shaped relationships between the lithium ion doping level and conductivity. Fig. 3 shows the ionic conductivity as function of temperature. Apparently, Arrhenius equation sT = kexp(Ea/RT) is still satisfied. Plotting ln(sT) against 1/T allows the calculation of activation energy for lithium ion transportation, which is also listed in Table 1. Sample with higher lithium ion conductivity exhibits lower activation energy for ion transportation, as is usually considered to be the measure of ion migration capability. Previous reports about the ionic conductivity of solid electrolyte LixB1x/3PO4 are also summarized in Table 2. The results vary from 3.5  104 S cm1 in case of pellet prepared by MPC (magnetic pulse compression) method to 3.74  106 S cm1 reported by T. J. Kim. The ionic conductivities of the solid electrolyte pellet are probably affected greatly by many factors, like the purity, composition, doping element, molding method, pressing pressure, calcinating temperature, stability in air, etc. Here, in this article, we focus on the effect of micro-structure on the ionic mobility when the lithium concentration in the BPO4 lattice varies while other factors remain constant. As an important element that may influence the ionic conductivity, the density of the pellet listed in Table 1 is around 2.11 g cm3. Fig. 4 illustrates the direct current polarizing curve of LixB1x/ 3PO4 sample for x = 0.05. It can be seen that the current is rather small at the early stage of voltage elevating. A sharp rise occurs when the voltage goes up to a certain value. The estimated decomposition voltage of LixB1x/3PO4 with x = 0.05 is 3.7 V. 3.3. Rietveld whole pattern fitting structure analysis As is well known, micro-parameters such as cell parameters, lattice strains, crystal grain sizes and so on can be achieved from Rietveld whole pattern refinement accurately. Rietveld refinement

Table 1 Pellet densities, conductivities and activation energies of LixB1x/3PO4 (x = 0.01, 0.05, 0.09, 0.13, 0.17, 0.20) at 298.15 K. x/mol%

0.01

0.05

0.09

0.13

0.17

0.20

s/S cm1

2.17  106

3.35  105

3.05  105

4.44  106

4.55  106

Ea/kJ mol1 Density/g cm3

33.5  0.8 2.11

27.1  0.5 2.06

2.15  105a 2.63  105a 28.9  0.3 2.10a 2.08a

27.7  0.3 2.05

31.8  0.7 2.13

30.9  0.5 2.07

a

The ionic conductivities of two prepared Li0.09B0.97PO4 samples.

S. Gao et al. / Materials Research Bulletin 48 (2013) 2896–2900 Table 2 Conductivities and activation energies of LixB1x/3PO4 in previous reports. Authors

Conductivity/S cm1and activation energy/ev

Kim [7] Dodd [8]

Maximum 3.74  106/0.3039 eV(x = 0.1) at 298 K Maximum Li+ mobility at 10 mol% doping level by 7Li NMR at 298 K Maximum 2.1  106 (x = 0.07)/0.3 eV at 298 K Maximum 3.5  104 (x = 0.1) by MPC method at 298 K Maximum 9.0  105 (x = 0.1) by MPC method at 298 K

Kelder [1] Jak [4] Jak [3]

pattern in the case of LixB1x/3PO4 (x = 0.05) is shown in Fig. 5. The diffraction peaks at 24.4, 26.8, 28.9, 39.8, 45.9, 48.6, 50.0, 59.9 and 63.58 are characteristic reflections of (1 0 1), (0 0 2), (1 1 0), (1 1 2), (1 0 3), (1 2 1), (2 0 2), (2 2 0) and (2 1 3) crystal faces for I-4 symmetry BPO4, indicating that solid solution of single phase forms in the whole range of 0.01  x  0.20. As is seen form the figure that the calculated pattern fits the recorded data well. Unit cell parameters, crystal grain sizes and lattice strains of LixB1x/3PO4 (x = 0.01, 0.05, 0.09, 0.13, 0.17, 0.20) calculated according to the method described in 2.2 are listed in Table 3. The relationship between those parameters mentioned above and Li+ doping level are also illustrated in Fig. 6. As is seen that LixB1x/ 3PO4 shows high conductivities in the range of x = 0.05–0.13. The sample with x = 0.05 possesses the best ionic conductivity of 3.35  105 S cm1 at room temperature. According to jump-diffusion model, continuous Li+ ions flow forms in such a way that they jump from one interstitial Li+ ion site to adjacent interstitial vacancy and then to another interstitial vacancy, and so on. Suppose that the ionic conductivity of the solid electrolyte can be written as the multiplication of the

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Table 3 Unit cell parameters, crystal grain sizes and lattice strains of LixB1x/3PO4 (x = 0.01, 0.05, 0.09, 0.13, 0.17, 0.20). x/mol%

a/(A˚)

c/(A˚)

Grain size/nm

Lattice strain/H

0.01 0.05 0.09 0.13 0.17 0.20

4.335(0) 4.343(2) 4.343(2) 4.342(2) 4.334(6) 4.332(4)

6.636(2) 6.635(0) 6.633(6) 6.635(9) 6.633(9) 6.633(9)

38.1 24.1 25.8 25.6 36.6 36.0

17.0 28.4 26.5 30.0 18.4 19.5

concentration of the occupied interstitial sites by the jumping frequency of those interstitial Li+ ions, which is in verse proportion to the amount of occupied site, Li+ ion diffusion rate in matrix of BPO4, or Li+ conductivity, depends on two factors at least, that is the amount of occupied interstitial Li+ ions and the amount of unoccupied interstitial vacancies. Therefore, only when the unoccupied interstitial vacancies are sufficient for the jump diffusion, would the solid state electrolyte present better ionic conductivity. When Li+ is first introduced into the BPO4 lattice, the ionic conductivity increases rapidly, as in the case of x = 0.05, and it remains high with the further increase of doping level in that still enough unoccupied interstitial vacancies are left for jumping migration. If lithium doping level continues to grow, the beneficial effects on lithium-ion migration of increasing interstitial Li+ ions is outweighed by decreasing interstitial Li+-ion vacancies, the ionic conductivity decreases. According to the Ref. [2], Li00B þ 2Lii and V000 B þ 3Lii are two possible defect models of Li+-doped BPO4. The former is that one Li+ replaces the boron ion of BO4 tetrahedron and is surrounded by two interstitial Li+ ions. The latter is that one boron vacancy is surrounded by three interstitial Li+ ions. Taking into consideration the average B–O bond length 1.48 A˚, the average O2 ion radius 1.40 A˚, the maximum ionic radius that is allowed to enter the BO4 tetrahedrons would be 0.346 A˚ according to the following simple relationship.

Rmax ¼

  pffiffiffi 3 3LBO pffiffiffi  RO2  3 6

Fig. 4. Direct current polarizing curve of LixB1x/3PO4 (x = 0.05).

Therefore, compared with the average Li+ ion radius 0.59 A˚, we suppose that instead of occupying the boron vacancies inside the BO4 tetrahedron, Li+ ions are more likely to take interstitial sites. With the increasing Li+ doping levels, they enter the crystal lattice of matrix BPO4, preferentially oriented in [0 k 0] direction, migrate along a or b axis through the narrowest bottleneck, 0.55 A˚ [3] in radius, in their traveling passageway. As a result, the squeezing

Fig. 5. Rietveld refinement of XRD pattern for LixB1x/3PO4 (x = 0.05).

Fig. 6. Grain size, lattice strain and ionic conductivity of LixB1x/3PO4 (x = 0.01, 0.05, 0.09, 0.13, 0.17, 0.20) vs. Li+ doping level x.

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S. Gao et al. / Materials Research Bulletin 48 (2013) 2896–2900

between the migrating Li+ ions and the lattice backbone causes the lattice expansion and lattice strain in [1 0 0] or [0 1 0] direction. In a certain sense, the augmentation of unit cell parameter a and lattice strain in LixB1x/3PO4 suggest the more vigorous Li+ migration, that is, higher ionic conductivity, in BPO4 matrix. From Table 3, both unit cell parameter a and lattice strain are higher in the range of 0.05  x  0.13 where the solid electrolytes show better Li+ ionic conductivities. Understandably, the unit cell parameter c changes little over the whole Li+ doping level. Most interestingly, the grain sizes of LixB1x/3PO4 from Table 3 change in an opposite way with that of lattice strains and unit cell parameter a. The finest crystal grain sizes are observed in the range of 0.05  x  0.13. The minimum grain size of 24.1 nm is observed at x = 0.05. However, up until now, there are not yet enough evidences to explain the reason for the variation of grain sizes.

lattice strain in the composition range 0.05  x  0.13 indicate more vigorous Li+ migration, that is, higher Li+ conductivities. The shoulder peak at 1050 cm1 reflects the partially filled interstitial sites, where higher Li+ conductivities are anticipated.

4. Conclusions

References

LixB1x/3PO4 (x = 0.01, 0.05, 0.09, 0.13, 0.17, 0.20) samples are synthesized by a simple wet chemistry route. AC-impedance measurements indicate that the samples with 0.05  x  0.13 have better ionic conductivities, the maximum of which reaches 3.35  105 S cm1 when x = 0.05. Direct current polarization measurements show that the sample with x = 0.05 possesses a high decomposition voltage up to 3.7 V. Micro-structure parameters of LixB1x/3PO4 are calculated according to Rietveld whole pattern refinement. The expansion of unit cell parameter a and

[1] E.M. Kelder, M.J.G. Jak, F. De Lange, J. Schoonman, Solid State Ionics 85 (1996) 285–291. [2] M.J.G. Jak, E.M. Kelder, J. Schoonman, J. Solid State Chem. 142 (1) (1999) 74–79. [3] M.J.G. Jak, E.M. Kelder, Z.A. Kaszkur, J. Pielaszek, J. Schoonman, Solid State Ionics 119 (1) (1999) 159–164. [4] M.J.G. Jak, E.M. Kelder, S.J. Everstein, J. Schoonman, J. Power Sources 81–82 (1999) 808–812. [5] Abdelmaula Aboulaich, Manfred Womes, Josette Olivier-Fourcade, Patrick Willmann, Jean-Claude Jumas, Solid State Sci. 12 (2010) 65–72. [6] A. Adamczyk, M. Handke, J. Mol. Struct. 555 (2000) 159–164. [7] T.J. Kim, Moon H-, S.W. Lee, J-. Park, J. Power Sources 123 (1) (2003) 65–68. [8] A.J. Dodd, E.R.H. van Eck, J. Solid State Chem. 153 (2) (2000) 282–286.

Acknowledgements We gratefully acknowledge the support for this work from 973 Fundamental Research Program from the Ministry of Science and Technology of China (grant number 2010CB635116), NSFC project 21173190, Educational Commission of Zhejiang Province (grant number Y201017390), Ningbo Science & Technology Bureau Project 2011A610086 and K.C. Wong Magna Fund in Ningbo University.