Synthesis and lithium ion conductivities of zirconium phosphate-based solid electrolytes

Kazuto Ide, Shinya Suzuki and Masaru Miyayama Research Center for Advanced Science and Technology, The University of Tokyo

4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan

e-mail: miyayama@rcast.u-tokyo.ac.jp

Keywords: lithium ion conductivity, solid electrolyte, zirconium phosphate

ABSTRACT

The lithium ion conducting properties of lithium- and MⅣ (M = Al, In, Y)-doped zirconium pyrophosphates synthesized via solid state reaction were investigated. The ionic conductivity of the compounds increased with increasing Li content. The activation energy of LixMxZr1-xP2O7 decreased as the lattice parameter increased owing to the enlargement of the size of bottleneck between the cavities. Li0.55(Li0.15Y0.1Zr0.75)P2O7 with a high Li content and a large lattice parameter exhibited a conductivity of 1.7×10-3 S cm-1 at 350 ºC, which is sufficient for its application as a solid electrolyte for sensors.

ITRODUCTIO

Solid lithium ion conductors have been widely investigated owing to their expected use as a solid electrolyte used in gas sensors [1,2] and all-solid-state lithium ion batteries [3]. The required properties for these electrolytes are as follows: high conductivity (~10-6 S cm-1 for sensors, >10-3 S cm-1 for batteries), chemical stability, and low cost. Many Li+ conductors, such as Li+-conducting perovskites [4], NASICON-type materials [5], and Li2S-based glass [6,7] have been investigated, and considerably higher conductivities at room temperature have been reported in these materials. However, they have insufficient stabilities because they contain easily reducible elements such as Ti, or they are unstable in air. Transitional metal pyrophosphates of the form MIVP2O7 (MIV is tetravalent) have high chemical stability because of the presence of (PO4)3- with a strong covalent bonding between P and O [8]. Among them, ZrP2O7 is resistant to oxidation and reduction because zirconium ion is stable in its tetravalent state. Figure 1 shows the crystal structure of ZrP2O7. ZrO6 octahedra link to three of the four corners of PO4 tetrahedra; the fourth corner of each tetrahedron links to another tetrahedron to form P2O7 pyrophosphate groups. The structure can be likened to that of NaCl with ZrO6 octahedra occupying the Na+ positions and P2O7 groups occupying the Cl- positions [9]. When Zr4+ ions are replaced by M3+ and Li+ pairs, M3+ ions are located in the Zr site and the Li+ ions are located in the cavities of the framework [10,11]. These cavities are connected through distorted pentagonal bottlenecks. Therefore,

ZrO6

PO4

Fig. 1 Crystal structure of ZrP2O7.

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three-dimensional Li+ conductivity is expected by Li+ hopping from one cavity to another [11]. This makes the synthesis of stable ZrP2O7-based Li+ conducting solid electrolytes feasible. The conductivity of 2.0×10-5 S cm-1 at 330 °C has been reported for lithium- and indium-doped ZrP2O7, i.e., Li0.2In0.2Zr0.8P2O7 [11]. However, higher conductivities are expected for Li+ and M3+ doped ZrP2O7 with larger amounts of lithium contents or other substituents. In this study, the synthesis and Li+ conducting properties of Li+- and M3+- doped ZrP2O7 were investigated.

EXPERIMETAL

Zirconium phosphate-based solid electrolytes of the form LixMxZr1-xP2O7 (M = Al, In, Y) and Li3x+y(LixYyZr1-x-y)P2O7 were prepared by solid state reaction. ZrCl2O·8H2O, LiOH·H2O, H3PO4, and a substitute metal compound (i.e., AlCl3·6H2O or In2O3 or Y2O3) were mixed in a mortar. The resultant suspension was evaporated to dryness and calcined at 600 ºC for 4 h. The calcined materials were ground by ball milling for 1 h and pressed into pellets with a one-axis press and a cold isostatic press. Then, these pellets were calcined at 1200 ºC for 6 h. The products were identified and the lattice parameters were determined by X-ray powder diffraction (XRD) analysis. For impedance measurements, Au electrodes were sputtered onto the both surfaces of each pellet. The conductivity of these pellets was measured in the temperature range of 150 ºC - 400 ºC in a N2 atmosphere by the AC impedance method in the frequency range of 10M Hz - 0.1 Hz.

RESULTS AD DISCUSSIO

1. Li+ conducting properties of LixMxZr1-xP2O7 The limits of solubility, x, in LixMxZr1-xP2O7 (M = Al, In, Y) determined by XRD analysis were 0.5, 0.1, and 0.18, respectively. The relative densities of the sample pellets were over 93 %. A typical impedance plot of LixMxZr1-xP2O7 is shown in Figure 2. One semicircle and a spike were observed. The appearance of the spike on the low-frequency side, which is due to the response of the electrolyte-Au blocking electrode interface, is evidence that LixMxZr1-xP2O7 are ionic conductors. Grain and grain boundary contributions could not be separated clearly. The total (grain + grain boundary) conductivity was estimated from the resistance of the semicircle. The component capacitance C of the semicircle was calculated using 2πfRC = 1, where f is the frequency at the top of the semicircle and R is the diameter of the semicircle. Capacitance is on the pF and nF orders of magnitude for the grain interior and grain boundary, respectively [12]. The estimated capacitance was on the pF order: therefore, the semicircle seems to be attributed to the grain response. The chemical composision dependence of the conductivity of LixMxZr1-xP2O7 was evalutated. Figure 3 shows the

M=Y

M=In

M=Al

Figure 2 Impedance plot of Li0.5Al0.5Zr0.5P2O7 at 250 ºC.

Fig. 3 Conductivities of LixMxZr1-xP2O7 at 300 ºC as a function of x value.

106 Electroceramics in Japan XIII

conductivities of LixMxZr1-xP2O7 (M = Al, In, Y) at 300 ºC. The conductivity of undoped ZrP2O7 was 2.7×10-7 S/cm at 300 ºC. The conductivities of M = Al materials were 1.9×10-5 S cm-1 (x = 0.1) and 6.8×10-5 S cm-1 (x = 0.5) at 300 ºC. As x in LixAlxZr1-xP2O7 increased, the conductivity increased. The DC conductivities of LixMxZr1-xP2O7 were less than 10-6 S cm-1 at 300 ºC. These results indicate that Li+ conduction appears with Li++M3+ codoping and that Li+ conductivity increases with increasing Li content. Li0.18Y0.18Zr0.82P2O7 showed the highest conductivity of 2.5×10-4 S cm-1 at 300 ºC among the obtained LixMxZr1-xP2O7. The conductivities of Li0.1In0.1Zr0.9P2O7 and Li0.1Y0.1Zr0.9P2O7 were 4.6×10-5 S cm-1 and 1.4×10-4 S cm-1, respectively. Li0.1Y0.1Zr0.9P2O7 exhibited the highest conductivity among the Li0.1M0.1Zr0.9P2O7 (M=Al, In, Y) systems. Figure 4 shows the temperature dependences of the conductivities for Li0.1M0.1Zr0.9P2O7 (M= Al, In, Y). The temperature dependences of the conductivities followed the Arrhenius-type equation

σT = A exp(-Ea/kT) (1)

where Ea is the activation energy. The values of the activation energies were 0.86 eV (M = Al), 0.72 eV (M = In), and 0.68 eV (M = Y), which are relatively high compared with the reported values of other Li+ conductors, that is, ~ 0.4 eV for Li+-conducting perovskites [4] and NASICON-type materials [5], and ~ 0.3 eV for Li2S-based glasses [6,7]. Figure 5 shows the activation energies and the conductivities measured at 300 ºC of Li0.1M0.1Zr0.9P2O7 as functions of lattice parameter. Activation energy decreased with increasing lattice parameter. This is due to the enlargement of the bottlenecks between the cavities of the ZrP2O7 framework, since the lattice parameter positively correlates with the size of the bottleneck. The correlation between activation energy and lattice parameter agrees with the results reported by Kato et al.[11]. The Li+ conductivity of LixMxZr1-xP2O7 is dependent on Li content and lattice parameter. The relatively high conductivity of Li0.18Y0.18Zr0.82P2O7 is caused by a large lattice parameter and a high Li content. 2. Li+ conducting properties of Li3x+y(LixYyZr1-x-y)P2O7 To obtain a solid electrolyte with a high Li+ conductivity, the synthesis of a material with a high Li content and a large lattice parameter was examined. To increase Li content in the cavities, Zr was replaced with Li and Y to give the composition Li3x+y(LixYyZr1-x-y)P2O7, in which the charge neutrality is maintained by the additional Li amount. The Y doping at the Zr site is effective for increasing the lattice parameter. Li3x+y(LixYyZr1-x-y)P2O7 was obtained by solid state reaction. In the XRD pattern of Li3x+y(LixYyZr1-x-y)P2O7, no diffraction peaks derived from second phases were

Al

InY

Fig. 4 Temperature dependences of conductivities of Li0.1M0.1Zr0.9P2O7.

Fig. 5 Lattice parameter (a) dependence of activation energy (Ea) and the conductivities of Li0.1M0.1Zr0.9P2O7 at 300 ºC.

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observed. Among them, Li0.55(Li0.15Y0.1Zr0.75)P2O7 had the highest Li+ content and the largest lattice parameter. The lattice parameter was 0.8262 nm, which was larger than that of Li0.1Y0.1Zr0.9P2O7 with the same Y3+ content. The cavities stuffed by Li+ would expand the ZrP2O7 framework. Figure 6 shows the complex impedance spectra of Li0.55(Li0.15Y0.1Zr0.75)P2O7.The activation energy of Li0.55(Li0.15Y0.1Zr0.75)P2O7 was 0.70 eV, which was slightly higher than that of Li0.1Y0.1Zr0.9P2O7. One of the reasons for this may be the formation of the glass phase. The glass phase is easily formed in compounds containing large amount of Li and P. The conductivities of Li0.55(Li0.15Y0.1Zr0.75)P2O7 were 5.0×10-4 S cm-1 at 300 ºC and 1.7×10-3 S cm-1 at 350 ºC. The pre-exponential term A in equation (1), which corresponds to carrier density, increased almost linearly with 4x+y in Li3x+y(LixYyZr1-x-y)P2O7. This suggests that Li+ in both cavities and Zr sites contribute to ionic conduction. The conductivities obtained were higher than those of LixMxZr1-xP2O7 systems. Since Li0.55(Li0.15Y0.1Zr0.75)P2O7 has sufficient Li+ conductivities for practical use at high temperatures (over 10-3 S cm-1 at 350 ºC), it should be useful for application as gas sensors.

COCLUSIO

The lithium ionic conducting properties of LixMxZr1-xP2O7 and Li3x+y(LixYyZr1-x-y)P2O7 were evaluated. The conductivities of the compounds increased with increasing Li content, owing to the increase in carrier density. The activation energy decreased as the lattice parameter increased, owing to the enlargement of the bottleneck for Li+ migration. Li0.55(Li0.15Y0.1Zr0.75)P2O7 with a high Li content and a large lattice parameter exhibited the highest conductivity of 1.7×10-3 S cm-1 at 350 ºC among Li+- and M3+-doped ZrP2O7 systems, which is sufficient for its application as a solid electrolyte of sensors.

REFERECES

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Fig. 6 Complex impedance spectra of Li0.55(Li0.15Y0.1Zr0.75)P2O7

350 ºC

300 ºC

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Electroceramics in Japan XIII 10.4028/www.scientific.net/KEM.445 Synthesis and Lithium Ion Conductivities of Zirconium Phosphate-Based Solid Electrolytes 10.4028/www.scientific.net/KEM.445.105