Limitations of the flammable liquid electrolyte of the Li-ion battery have motivated reconsideration of all-solid-state batteries.^[1–7] Aluminum (Al) is an abundant, light-weight metal that offers three electrons per atom Al. Therefore, a theoretical specific capacity of 2980 mAh·g^–1 and a volume capacity of 8046 Ah·L^–1 can be achieved.^[8,9] Recently, Dai et al.^[10] reported a rechargeable Al battery with a three-dimensional graphitic-foam cathode that showed high-rate capability and excellent cycling performance up to 7500 cycles at room temperature. However, their cells used an ionic liquid electrolyte, AlCl₃/[EMIm]Cl, that is costly and not easy to prepare. Development of an Al battery with a solid electrolyte is plagued by poor Al³⁺ conductivity, σ_Al, in the electrolyte. However, Imanaka et al. have reported a high σ_Al at high temperatures in (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃.^[11]

In this work, we identify the Al diffusion mechanism in (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ through high-temperature neutron powder diffraction (NPD) experiments and atomic-resolution scanning transmission electron microscopy (STEM) analyses. Temperature-driven Al displacements indicate that Al³⁺ ions diffuse across the structure by a vacancy mechanism. Furthermore, negative thermal expansion (NTE) behavior was also observed in (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ over a wide temperature range by NPD experiments. Importantly, we have demonstrated a rechargeable solid-state Al battery with V₂O₅ nanorods/rGO (reduced graphene oxide) as cathode, a dense (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ pellet as electrolyte and Al as anode. Preliminary results show that (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ is an electrolyte material for a solid-state Al battery operating at high temperatures. High-temperature batteries are important power sources for aeronautics, astronautics, and military applications.^[12–14]

(Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ was prepared by solid-state reaction. The phase and crystal structure of the product were examined by powder x-ray diffraction (XRD); the pattern is shown in Supplementary Material, Fig. S1. All of the peaks of the well-crystallized structure correspond to the rhombohedral NASICON phase isostructural to NaZr₂(PO₄)₃.^[19,20] The octahedrally coordinated Zr⁴⁺ are partially replaced by Nb⁵⁺, which reduce the volume of the unit cell sufficiently to accommodate the small Al³⁺ (0.535 Å in octahedral coordination) in the interstitial space of the M(II) site of the structure.^[21] In NaZr₂(PO₄)₃, the Na⁺-ions occupy the interstitial M(I) sites, and in (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃, the M(I) sites are occupied by Nb and/or Zr. Our original strategy was to also increase the oxygen bonding of the framework to reduce the strength if the Al³⁺ is bonded to the framework.^[11]

Electron-diffraction pattern (EDP) and aberration-corrected high-resolution transmission electron microscopy (HRTEM) taken at 25 °C were also performed to characterize the crystal structure. All of the EDPs and HRTEM images can also be well-indexed with a NASICON-type model(Supplementary Material, Fig. S2, Table S1). Atomic-resolution high-angle annular dark field (HAADF) imaging was performed with an aberration-corrected scanning transmission electron microscopy (STEM) to observe the Al³⁺ distribution. Figure 1(a) shows a Z-contrast (Z: atomic number) HAADF image of (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ taken along the [010] zone-axis. Atomic columns of Zr/Nb, P and Al in the HAADF image can be identified by the intensity of projected atomic columns; they correspond to a NASICON structural model of a unit cell viewed along the [010] zone-axis, as shown in the inset. Al³⁺-ion columns are shown by the yellow arrows; Al³⁺-ionic channels can be identified in the {10-2} crystallographic planes. Figure 1(b) shows the intensity profile of the Al³⁺ ion columns along a dash–dot line AB in Fig 1(a). The variation of the intensity of the Al³⁺-site suggests a random distribution of Al³⁺ and vacancies, which promotes transportation of Al³⁺ in the ionic channel.

Fig. 1.

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Fig. 1. (color online) Room temperature STEM characterizations of (Al0.2Zr0.8)20/19Nb(PO4)3. (a) Atomic resolution HAADF image overlapped with a structural model of a unit cell viewed along [010] zone-axis. The magenta, red and yellow spheres represent Nb/Zr, P, and Al site respectively. Oxygen was omitted for clarity. Yellow arrows illustrate the Al3+ ion columns. (b) Intensity profile measured from the dash–dot line AB in panel (a), yellow arrows illustrating the position of Al3+-site. The weak peaks shown by the magenta arrows come from the O2– ion columns along the dash–dot line AB.

An investigation of the crystal structure by NPD was performed to determine structural details, since neutrons allow us to explore a much wider range of the reciprocal space and the lack of form factor permits the precise determination of the anisotropic displacement factors. The neutron pattern confirms the rhombohedral symmetry with a = b = 8.7509(2) Å and c = 22.4368(7) Å in the hexagonal setting at 25 °C; the structure was defined in the R-3c space group (No. 167) with Al located at 6b Wyckoff site (0,0,0); Nb and Zr distributed at random at 12c (0,0,z) site; P at 18e (x, 0, 1/4) positions and the two types of oxygen O at 36f (x,y,z) sites. The Nb and Zr occupancy factors were defined according to the nominal stoichiometry; the Al occupancy factor was also fixed to the nominal composition since it was found to be strongly correlated with its thermal displacement factors. The final structural parameters and agreement factors are gathered in Supplementary Material, Table S1.

Conductivity measurements, as described later, demonstrate that (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ is an exceptionally good solid-state conductor for Al³⁺ ions. A temperature-dependent NPD study appears to be critical to understand the crystal structure evolution with the functional temperatures of this material. NPD diagrams collected at 200, 400, 600, and 800 °C with λ = 1.594 Å show the same crystal symmetry but dramatic changes in the magnitude and anisotropy of the displacement parameters. All the peaks can be indexed in the rhombohedral R-3c unit cell, which demonstrates the structure is stable at these working conditions. The access to a wide region of the reciprocal space allowed us a successful refinement of the anisotropic displacement factors of the aluminum atoms. The good agreement between observed and calculated profiles is displayed in the patterns tested at 25 °C and 800 °C, respectively (Supplementary Material, Figs. S3(a) and S3(b)). Figures 2(a) and 2(b) illustrate the evolution of the rhombohedral crystal structure (hexagonal setting) displaying a remarkable anisotropy in the cigar-shaped (prolate) displacement ellipsoids for aluminum. Table S1 contains the main structural parameters of the crystal structures at 200, 400, 600, and 800 °C, and Table S2 contains the main bond distances and angles. Figures 2(c)–2(e) display the thermal variation of the unit-cell parameters and volume, figure 2(f) shows the evolution of the B_eq thermal displacement for Al across the measured temperature range.

Fig. 2.

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Fig. 2. (color online) (a) and (b) Crystal structure of (Al0.2Zr0.8)20/19Nb(PO4)3 consisting of a framework of (Nb,Zr)O6 octahedra sharing corners with PO4 tetrahedra. The strongly anisotropic thermal displacements of the Al3+ ions with the ellipsoids directed along the [001] directions are enhanced at (a) 200 °C, and (b) 800 °C. Thermal variation of [panels (c) and (d)] unit-cell parameters and (e) volume, and (f) thermal displacement (Beq) of Al.

It is remarkable that whereas the c parameter expands upon heating, the a parameter contracts in all the studied temperature range (25 °C–800 °C), yielding an overall, anisotropic volume contraction as temperature is increased. The thermal expansion coefficient based upon the volume variation may be defined as α_V = (V_T – V₀)/V₀(T – T₀), giving α_V = –4.49×10^–6 K⁻¹. If we estimate the contraction experienced along the a or b direction, according to α_l = (l_T – l₀)/l₀(T – T₀), we obtain α_a = –3.92×10^–6 K⁻¹. The expansion along c is α_c = 3.37×10^–6 K⁻¹. This anisotropic thermal expansion has been observed before in other NASICON-type structures.^[22,23] The origin of the observed NTE has been previously described as a folding of the structure on warming up. The microscopic origin of the NTE is identified by our NPD data.

The magnitude of the thermal displacement increases monotonically with temperature, as is shown in Table S1 and Fig. S3. The anisotropy of the Al thermal ellipsoids has the largest thermal displacement along the c axis. At 800 °C, the root mean square (r.m.s.) displacements of Al are 1.56 Å along the [001] direction, and 0.1 Å and 0.01 Å perpendicular to it, in a flattened and extraordinarily elongated prolate configuration (Fig. S3). As discussed later, this configuration shows a strong displacement of the Al atoms, even penetrating into the adjacent (Nb,Zr)O₆ octahedra that contain some vacancies at 800 °C for the nominal composition (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃, implying that 8% of the octahedral M(I) positions are empty. Diffusion by a vacancy diffusion mechanism is possible because only 41% of the 6b positions are occupied by Al atoms. This result is consistent with the HRTEM results. Given the rombohedral symmetry, a largely anisotropic Al diffusion would be expected for this compound, mainly along the c axis.

The feasibility of a dense (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ pellet as a solid electrolyte material was investigated by impedance spectra measurements. The cross sectional SEM images of the prepared (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ pellet are shown in Fig. S4. Swagelok cells were prepared with V₂O₅ nanorods/graphene as cathode,^[24] aluminum as anode and the dense (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ solid pellet (∼ 300 μm in thickness) as electrolyte. Figure 3(a) shows the schematic diagram of the swagelok cell used for electrochemical tests. Figure 3(b) illustrates the preparation process of the anode. The temperature dependence of the Al³⁺ conductivity of (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ is shown in Fig. 3(c). By fitting the slope of the Arrhenius plots, the activation energy (E_a) of the electrolyte was determined using the Arrhenius equation:

Fig. 3.

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	Fig. 3. (color online) (a) Schematic diagram of the swagelok cell used for electrochemical tests. (b) The preparation process of Al anode. (c) Temperature dependence of the Al3+ ion conductivity of (Al0.2Zr0.8)20/19Nb(PO4)3. (d) Initial discharge–charge profiles of the solid-state V2O5 nanorods/rGO|Al battery at a current density of 2 mA·g–1 at 120 °C and 150 °C, respectively.

The (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ crystal structure is a three-dimensional (3D) network containing vertex-linked (Nb,Zr)O₆ octahedra and PO₄ tetrahedra, with the Al-ions partially occupying interstitial trigonal antiprism spaced at intervals along the c axis. There is only one crystallographically distinct Al site. Figure 4 displays the coordination of the Al site, the size of which is basically determined by the distance between O₂ atoms along the c direction. The NPD results clearly show that the rhombohedral structure, defined in the space group R-3c, is stable across the temperature range 25 °C to 800 °C because no superstructure reflections (Fig. S3(a)) justify a departure of this symmetry to describe the crystal structure.

Fig. 4.

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Fig. 4. (color online) (a) Close-up of the crystal structure highlighting the Al coordination polyhedra, defined as a trigonal antiprism, where the O2–O2 distances determine the size of the polyhedron and the length of the c unit-cell axis; (b) and (c) thermal variation of the (Nb,Zr)–O–P angles, determining the tilting angle between octahedral and tetrahedral units in the framework of (Al0.2Zr0.8)20/19Nb(PO4)3.

The thermal expansion of the c axis observed in this temperature range may be explained by the expansion of the c-axis O₂–O₂ separation of the Al site. This governs the ‘size’ of the trigonal prismatic site along the c axis. Between 25 °C and 800 °C the O₂–O₂ distances along the c axis (d_O2–O₂ in Fig. 4(a)) increase from 2.07 Å to 2.08 Å, accounting for the overall expansivity α_c. Regarding the contraction of the a parameter, figures 4(b) and 4(c) demonstrate that there is a folding of the crystal structure in such a way that (Nb,Zr)–O–P angles become smaller upon heating, which is ascribed to transverse vibration modes called “guitar string” vibrations that tend to bend the chemical bonds and lead to NTE.^[23] NTE behavior was observed in (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ over a wide temperature range.

This NTE behavior is a general trend in many frameworks containing octahedral and tetrahedral units, such as ZrW₂O₈,^[24,25] ZrMo₂O₈,^[26] and other NASICON-type structures including the related Al-free NbZr(PO₄)₃ with (αl = 2.3×10₆)^[27] and (HfSc)_0.83W_2.25P_0.83O_12δ.^[28] This behavior is a consequence of the structural constraint of atomic bridging motifs.^[26] The ability to bend these linkages at the O atoms means that the dominant thermal response is associated with flexing bonds rather than stretching them, and the geometrical flexibility explains why the crystallites deform readily in response to thermal and/or mechanical stress in NTE materials.

The high Al³⁺conductivity is primarily linked to the weak chemical bonds established between Al and O₂ in an over-sized cage exhibiting Al–O₂ distances in the 2.64 Å–2.65 Å range. The ionic radii sums for Al³⁺ (i.r.(VI) = 0.535 Å) (16) and O^2– (i.r.(IV) = 1.38 Å) is just 1.915 Å, meaning that Al ions are rattling in the cages, as is visualized by the large displacement factors observed even at room temperature. It is noteworthy that the orientation of the anisotropic thermal displacements is along the axial directions of the trigonal antiprism; i.e., approximately along the bonding direction within the octahedra. This arrangement of the thermal ellipsoids is not the one usually found in oxides; normally the thermal vibrations are perpendicular to the chemical bonds, displaying oblate spheroids. Taking the cubic perovskite Ba_0.87K_0.13BiO₃^[27] as an example, the oblate vibrations lock the oxygen atoms at mid-bismuth positions with enhanced vibrations perpendicular to the Bi–Bi distance. A prolate ellipsoid for the oxygen anisotropic thermal displacement parameters indicates a softening of “breathing” modes at the bismuth atoms. In the present (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ phosphate, the prolate thermal ellipsoids of the Al³⁺ ions are oriented along the chemical bonds and are clearly correlated with the Al diffusivity.

In the present investigations, the NASICON-type oxide (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ is a good solid-state conductor for Al³⁺ ions. Temperature-dependent neutron powder diffraction (NPD) and aberration-corrected scanning transmission electron microscopy (STEM) experiments identify a strong displacement of Al ions along the c axis of the unit cell as they diffuse across the structure by a vacancy mechanism. The Al³⁺ conductivity and NTE of the (Al_0.2Zr_0.8)_20/19Nb(PO₄)₃ may be applicable in wide applications, such as solid-state Al batteries and precision optical devices used on satellites.^[29]