High Lithium Content and Site Disorder in the Transition Metal Oxide Argyrodites Li₇TiO₅X (X = Cl^–, Br^–)

Abstract

Sulfide lithium argyrodites are a key materials family that are studied as solid electrolytes in commercial all-solid-state batteries (ASSBs), while their oxide analogues remain relatively unexplored. This study presents the discovery of Li₇TiO₅X (X = Cl^–, Br^–), the first lithium argyrodite materials in which a transition metal is used as the framework-forming cation, expanding the chemical space that is accessible for oxide argyrodites. Incorporation of Ti⁴⁺ enables the lithium content to be maximized to 7 Li⁺ per formula unit. Interestingly, even with the high lithium content, Li₇TiO₅Cl retains a Li⁺ site disordered cubic F4̅3m structure at room temperature with Li⁺ occupancy of the T5, T5a, and T3 positions, and exhibits an ionic conductivity of 2.2(2) × 10^–6 S cm^–1 with the lowest reported activation energy (0.36(2) eV) for bulk Li⁺ ion transport in an oxide argyrodite. Conversely, Li₇TiO₅Br adopts the same F4̅3m symmetry at room temperature but with an ordered arrangement of Li⁺ positions via full occupancy of the T5a and T3 positions, and thus has an ionic conductivity that is 3 orders of magnitude lower (∼10^–9 S cm^–1) and a much higher activation energy (0.58(2) eV) than Li₇TiO₅Cl. Order–disorder behavior is observed below 250 K in Li₇TiO₅Cl, where a Li⁺ site ordering pattern is observed that is distinct from Li₇TiO₅Br and all sulfide argyrodites, yielding a tetragonal symmetry (I4̅) for only the second time to date in the argyrodite structure type. This unique order–disorder behavior, alongside the ability to incorporate transition metal cations within this material family emphasizes the potential to access much greater structural diversity via the expansive chemical space that is available for exploration in oxide argyrodites.

1. Introduction

Lithium-ion conductors are used in a range of electrochemical technologies, including in thin-film batteries, , as protective coatings in lithium-ion batteries (LIBs), , and as solid electrolytes (SEs) in all-solid-state batteries (ASSBs). , For example, lithium phosphorus oxynitride (LiPON) is widely employed in commercial thin-film batteries, exhibiting room-temperature ionic conductivities of approximately 10^–6 S cm^–1 and good electrochemical stability. Several oxide SEs are used as protective coatings to stabilize electrode–electrolyte interfaces, including Li₂SiO₃, Li₄Ti₅O₁₂, LiTaO₃, and LiNbO₃ with ionic conductivities ranging from 10^–5 S cm^–1 (amorphous LiNbO₃) to 10^–8 S cm^–1 (Li₅Ti₅O₁₂). Lithium-conducting SEs with room-temperature conductivities that surpass those of organic liquid electrolytes (>10^–3 S cm^–1), such as lithium argyrodites, e.g., Li_6.6Si_0.6Sb_0.4S₅I (2.4 × 10^–2 S cm^–1), can be used to develop ASSBs, which offer the prospect of enhanced safety alongside increased power and energy densities. , Almost all reported lithium argyrodites utilize sulfide as the main anion, benefiting from the polarizability of the S^2– anion that enhances Li⁺ ion mobility.

Argyrodites, characterized by the general formula A_{(12–n–y)/m} ^m+M_6–y ⁿ⁺Ch_6–y ^2–X _y ^– (A = Ag⁺, Cu⁺, Cd²⁺, Li⁺, etc.; M = tetrahedral forming cations; Ch = chalcogenide; X = halides), exist in a variety of compositions. Most argyrodites are based on nonmetallic cations (M), such as P⁵⁺, Si⁴⁺, and Ge⁴⁺, that form tetrahedral units with Ch anions (Ch = O^2–, S^2–, Se^2–, Te^2–) (Figure S1). A limited number of argyrodites which contain transition metal cations have been explored, e.g., Ag₇TaS₆, Ag₇NbS₆, and Ag₈TiS₆, all with F4̅3m symmetry. Notably, no transition-metal-containing lithium argyrodites have been reported to date. This is likely due to the tendency of transition-metal sites to undergo redox reactions, which can generate electronically conductive interfacial layers, increase interfacial impedance, and accelerate degradation, rendering the materials unsuitable as separators in ASSBs. However, replacing oxygen with sulfur may help to suppress redox activity. Significant progress has been made recently with the discovery of LiMOCl₄ (M = Nb⁵⁺, Ta⁵⁺) , and Li₃Ta₃O₄Cl₁₀ that incorporate transition metals alongside oxide and halide anions while exhibiting greater resistance to the conditions experienced within electrochemical cells. Transition metals feature in other oxide structure types that have been studied as solid electrolytes, including the highly conducting NaSICON-type Li_1+x Al _x Ti_2–x (PO₄)₃, perovskite-type Li_3x La_2/3‑x □_1/3–2x TiO₃ (LLTO), and Li₅La₃ M ₂O₁₂ (M = Nb⁵⁺, Ta⁵⁺) garnets, but have not yet featured in oxide argyrodite solid electrolyte chemistry. Argyrodites, which combine O^2– and halide X^– anions, are underexplored as potential solid-state electrolytes relative to sulfide argyrodites and may offer enhanced electrochemical stability. The first reported oxide argyrodite Li₆PO₅X (X = Cl^–, Br^–) exhibited low ionic conductivities of ∼10^–9 S cm^–1. Recent optimization of composition through silicon substitution in Li_6+x P_1–x Si _x O₅Cl led to a 1000-fold improvement in ionic conductivity to 1.82(1) × 10^–6 S cm^–1, demonstrating that substantial improvements to ionic conductivity are possible for oxide argyrodite materials. Typically, a lithium content greater than 6 Li⁺ per formula unit leads to high Li⁺ ion diffusion, as demonstrated in many argyrodite solid solutions, e.g., Li_6+x P_1–x M _x S₅I (M = Si⁴⁺ and Sn⁴⁺); however, argyrodites with a lithium content of 7 Li⁺ per formula unit are comparatively rare, for which Li⁺ site ordering is commonplace, negatively impacting the ionic conductivity. ,

Our aim is to expand the chemical space accessible to argyrodites beyond the small number of typical framework-forming tetrahedral cations that dominate sulfide argyrodite chemistry (i.e., P⁵⁺, Si⁴⁺, Ge⁴⁺, As⁵⁺, Sb⁵⁺, Sn⁴⁺). Thus, we target the compositions of Li₇TiO₅X (X = Cl^–, Br^–) through solid-state reaction utilizing the Ti⁴⁺ valence to maximize the Li⁺ content. Both Li₇TiO₅Cl and Li₇TiO₅Br adopt a cubic argyrodite structure (F4̅3m) at room temperature. Li₇TiO₅Br exhibits an ordered structure with Li⁺ ions fully occupying T5a and T3 sites, while Li₇TiO₅Cl displays Li⁺ site disorder with partial occupancy of T5 and T5a and full occupancy of T3 sites at room temperature. This difference in Li⁺ site distribution has a profound impact on the transport properties of the two materials, with an increase in ionic conductivity of 3 orders of magnitude from Li₇TiO₅Br to Li₇TiO₅Cl, highlighting the importance of disorder to achieve increased ionic conductivities. Furthermore, Li₇TiO₅Cl undergoes a phase transition to a Li⁺ site-ordered structure with tetragonal I4̅ symmetry below 250 K, a symmetry previously observed only once for all argyrodites. The ability to incorporate transition metal cations into oxide argyrodites emphasizes the extensive chemical space that is available for exploration and subsequent optimization in the underexplored field of oxide argyrodites.

2. Experimental Section

2.1. Synthesis

2.1.1. Materials

TiO₂ (anatase, nanoparticles, >99.0%), LiBr (>99.0%), and LiCl (>99.0%) were purchased from Sigma-Aldrich. Li₂O (greater than 99.0%) was purchased from Alfa Aesar.

2.1.2. Synthesis of Li₇TiO₅X (X = Cl^–, Br^–)

Li₂O, TiO₂, LiBr, and LiCl were dried under dynamic vacuum (<10^–4 mbar) at 473 K (LiCl, LiBr), 773 K (TiO₂), and 1223 K (Li₂O) overnight before placing them inside an Ar-filled drybox (O₂ < 0.1 ppm, H₂O < 0.1 ppm). Drying the starting materials was critical to avoid the formation of antiperovskite Li₂OHX (X = Cl^–, Br^–) phases and other impurity phases; thus, all reagents and samples were handled under dry inert atmospheres.

Li₂O, TiO₂, and LiCl or LiBr were mixed in the stoichiometric ratios using ball milling under an Ar atmosphere. The precursors were ball-milled in 1 g batches for a total time of 6 h (12 intervals of 10 min on followed by 20 min off) in 45 mL zirconia jars using seven zirconia balls (diameter: 10 mm) at a rate of 300 rpm. The resulting powder was then pressed into 5 mm diameter pellets using 300 MPa pressure. The pellets were placed into alumina crucibles, which were inserted into quartz tubes sealed with Swagelok end-caps through which dried argon gas was flown at a rate of 50 mL min^–1. The samples were heated to 898 K for Li₇TiO₅Cl and 823 K for Li₇TiO₅Br at a ramp rate of 5 K min^–1, held at the reaction temperature for 10 h, and cooled at a rate of 5 K min^–1. The quartz tubes were closed airtight using the Swagelok end-caps and returned to the Ar-filled glovebox, where they were opened, and powders were ground in an agate mortar for further characterization.

2.2. Powder X-ray Diffraction (PXRD)

Routine assessment of sample purity was carried out using a Bruker D8 Discover diffractometer with monochromatic Cu radiation (Kα₁, λ = 1.54056 Å) in Debye–Scherrer transmission geometry with sample powders loaded into 0.7 mm or 0.3 mm diameter borosilicate glass capillaries for Li₇TiO₅Cl and Li₇TiO₅Br, respectively.

Synchrotron X-ray diffraction (SXRD) was performed at Diamond Light Source, U.K., on high-resolution beamline I11 for all samples. The data were recorded in transmission mode using a position sensitive detector (PSD, [0° < 2θ < 100°], λ = 0.82660 Å). The experiments were carried out at room temperature and at 100 K on samples introduced into 1 mm (Li₇TiO₅Cl) and 0.3 mm (Li₇TiO₅Br) diameter borosilicate glass capillaries.

Synchrotron variable-temperature X-ray diffraction (VT-XRD) experiments were performed on powder samples of Li₇TiO₅Cl that were introduced into 1 mm diameter silica capillaries in the temperature range of 100–298 K in 25 K steps on heating. To identify the phases present, databases (ICSD, PCD) were searched for known materials containing Li, Ti, O, Cl, and Br alongside possible contaminants originating from the synthesis procedure, such as Al, Zr, C, and H.

Rietveld refinements were carried out using TOPAS Academic. Initially, Pawley fits were performed on SXRD data, refining the lattice parameters and the background terms using a Chebyshev function with 12 parameters. The parameters refined from final Pawley fits were then used as starting points for Rietveld refinements, where the following parameters were refined: (1) Scale factor, (2) atomic coordinates, (3) isotropic displacement parameters, and (4) atomic occupancies: the occupancies of Ti⁴⁺ were set to unity, while Cl^–, Br^–, and O^2– were refined initially. Since all occupancies were refined to values close to unity, they were set to 1 in the refinement. Additional Li⁺ sites were identified in the Fourier difference map (FDM) through the presence of positive electron density, introduced into the model and refined (atomic coordinates, atomic occupancies, isotropic displacement parameters). The overall Li⁺ content for each composition was fixed to charge-balance the refined anion content.

2.3. Alternating Current (AC) Impedance Spectroscopy and Direct Current (DC) Polarization

Pellets of Li₇TiO₅X (X = Cl^–, Br^–) were prepared through reactive sintering by uniaxially pressing ∼30 mg of starting material in a 5 mm cylindrical steel die at a pressure of 300 MPa, followed by annealing under argon flow at the respective reaction conditions. Using this method, relative pellet densities up to 81% were achieved.

AC impedance measurements were conducted using an impedance analyzer (Keysight impedance analyzer E4990A). A sputtered gold coating of ∼300 nm thickness was used as the ion-blocking electrode. Sputtering was achieved in a glovebox using a Q150R sputter coater at a pressure of 10^–1 mBar and a sputtering current of 40 mA. Temperature-dependent conductivity measurements were performed under a dry argon atmosphere in a frequency range of 2 MHz to 20 Hz (with an amplitude of 100 mV). Measurements were performed at room temperature and in the temperature range 323–573 K in 25 K steps. The ZView2 program was used to fit the impedance spectra with an equivalent circuit.

DC polarization data were collected on Au|Li₇TiO₅X|Au (X = Cl^–, Br^–) symmetric cells at 298 K. Constant voltages of 0.1–1 V were applied for ∼2 h, and the current variation with time was recorded. The DC polarization curves were recorded until a steady current was obtained at any applied voltage.

For low-temperature impedance measurements, pellets were mounted and sealed in a spring-loaded sample jig inside the glovebox, which was then placed into a Delta 9023 environmental test chamber. Argon gas was passed from the inner tube to the outer tube to protect samples from moisture/air. Data points were collected in the temperature range 303–233 K in 10 K steps with an equilibration time of 30 min for each step.

2.4. Compositional Analysis

The Li and Ti content was determined through inductively coupled plasma optical emission spectroscopy (ICP-OES). 10 mg of Li₇TiO₅X (X = Cl^–, Br^–) was dissolved in 100 mL of 1 M nitric acid. Data were collected using an Agilent Technologies 5110 ICP-OES instrument, and three measurements were collected per sample, and the average was used as the reported value. Calibrations were performed using appropriate elemental standards. The Cl and Br contents were measured via ion chromatography (IC) using a Metrohm 881 compact IC Pro with an 858 professional sample processor. 5 mg of Li₇TiO₅X (X = Cl^–, Br^–) was dissolved in 50 mL of ultrapure distilled water and measured in triplicate. Solutions were diluted by a factor of 10 for measurement, and data were corrected using appropriate lithium halide salts as standards.

2.5. Nuclear Magnetic Resonance (NMR) Spectroscopy

All NMR experiments were performed under static conditions on a 9.4 T Bruker Avance III HD spectrometer. Samples were flame-sealed in a 3.2 mm borosilicate glass insert placed inside a 4 mm ZrO₂ rotor. A 4 mm high-temperature HX solid-state NMR probe was used to acquire spectra above room temperature, while experiments below room temperature were recorded with a 4 mm HXY solid-state NMR probe in double resonance mode. All ⁷Li solid-state NMR spectra were recorded at a Larmor frequency ν₀ of 156 MHz and are referenced to the ⁷Li chemical shift of LiCl in D₂O at 0 ppm. Pulse-acquire spectra were recorded using optimized 90° pulses at a radio frequency field amplitude of 83 kHz and recycle delays longer than five times the ⁷Li spin–lattice relaxation time constant in the laboratory frame, T₁, determined from ⁷Li saturation recovery experiments. Temperature calibrations of the probes were performed with Pb­(NO₃)₂ as ²⁰⁷Pb NMR chemical shift thermometers (below room temperature and above room temperature up to 400 K) and with the CuI and CuBr phase transitions using ⁶³Cu NMR. , The largest errors associated with this method arise from temperature gradients in the sample, which were calculated using the isotropic peak line broadening and range from 5 to 20 K.

The fitting of Boltzmann sigmoid functions to the Δν­(T) vs T line narrowing curves enables access to the full width at half-maximum of the completely narrowed ⁷Li central transition in the fast motional regime (Δν_∞), the full width at half-maximum of the ⁷Li central transition governed by ⁷Li–⁷Li homonuclear dipole interactions in the rigid lattice regime at temperatures below T _onset (Δν_R), and the temperature corresponding to the inflection point of the Δν­(T) vs T line narrowing curve (T _inflection). From this, it is possible to obtain mean Li-ion jump rates based on τ^–1 = 2πΔν_R.

2.6. Normal Mode Calculations

The normal modes of Li₇TiO₅Cl were calculated in the conventional cubic cell of the F4̅3m structure using periodic density functional theory (DFT) as implemented in VASP. Calculations were performed using the PBEsol functional, a plane-wave cutoff energy of 700 eV, and a 5 × 5 × 5 k-point grid. The projector augmented-wave method was used to treat core electrons. The geometry of the F4̅3m structure was initially optimized until all forces fell below 0.001 eV Å^–1, and then normal modes were calculated using finite differences with a displacement of 0.01 Å. Following the identification of unstable modes with imaginary frequencies, new structures were generated by perturbing the F4̅3m structure along the direction of the unstable modes. These new structures were then optimized using the same parameters as those given above.

2.7. Diffuse Reflectance Measurements

Diffuse reflectance data of Li₇TiO₅X (X = Cl, Br) powders were measured using an Agilent Cary 5000 instrument between 200 and 2500 nm with a step size of 1 nm. Calibration to 100 and 0% reflectance was performed prior to measurement using a poly­(tetrafluoroethylene) (PTFE) standard and a light trap, respectively. Samples were loaded into holders inside a glovebox (O₂ < 0.1 ppm, H₂O < 0.1 ppm), and the threads of the holders were wrapped in PTFE tape to avoid atmospheric exposure of the material during measurement. The band gap was determined from a Tauc plot using a method described by Makuła et al.

2.8. Bond Valence Sum Energy (BVSE) Calculations

The BVSE calculations were performed for Li₇TiO₅Cl −300 K and Li₇TiO₅Cl −100 K of Li-argyrodite using softBV(v1.3.1). − The crystallographic information file (CIF), obtained through the Rietveld refinement, was assessed in softBV, and the global instability index (GII) was found to be no larger than 0.18 in both refined crystal structures; hence, the validity of the CIFs was confirmed. The input file (.cube) was generated in softBV, and the BVSE landscape was plotted in VESTA to visualize the Li-ion migration pathways.

3. Results and Discussion

3.1. Synthesis of Li₇TiO₅X (X = Cl^–, Br^–)

Syntheses of Li₇TiO₅X (X = Cl^–, Br^–) were attempted at several temperatures (723–923 K) and reaction times (3–24 h). Reactions executed at 898 and 823 K for 10 h under flowing Ar (50 mL min^–1) resulted in the formation of phase-pure cubic argyrodite powders for Li₇TiO₅Cl and Li₇TiO₅Br, respectively. Syntheses for Li₇TiO₅Cl undertaken at 823 K resulted in samples of reduced purity (Figure S2a,b). The reflections observed in XRD patterns of both materials were consistent with the systematic absences characteristic of the F4̅3m space group often adopted by cubic argyrodites (hkl: h + k,h + l,k + l = 2n, 0kl: k,l = 2n, hhl: h + l = 2n, h00: h = 2n) (Figure a). The observed reflections are shifted to lower Q for Li₇TiO₅Br due to the increased ionic radii (r_Cl: 1.81 Å vs r_Br: 1.96 Å), with an associated increase in unit cell parameter from a = 8.527050(9) Å to a = 8.576847(11) Å, respectively. (Figure b). Additionally, differences in relative reflection intensities are observed between Li₇TiO₅Cl and Li₇TiO₅Br, most notably for the (11–1) reflection at ∼1.3 Å^–1.

1.

(a) Room-temperature XRD patterns for Li₇TiO₅Cl (orange) and Li₇TiO₅Br (blue) powder samples can be indexed to cubic F4̅3m symmetry typical of argyrodite materials. (b) Reflections shift to lower Q as the larger Br^– anion (blue) is substituted in place of Cl^– (orange).

3.2. Structure Determination

3.2.1. Li₇TiO₅X: Room-Temperature Structure Determination

The crystal structures of Li₆PO₅X (X = Cl^–, Br^–), which adopt F4̅3m symmetry, were used as starting points for Rietveld refinements of Li₇TiO₅X (X = Cl^–, Br^–) by replacing the P⁵⁺ for Ti⁴⁺ and placing the one additional Li⁺ onto the tetrahedral T3 (4d) site, alongside the fully occupied trigonal bipyramidal T5a (24g) site. The total Li⁺ content in each model was set to charge-balance the refined anion content.

For Li₇TiO₅Br, refinement of the positions and displacement parameters of the Br^–, O^2–, Ti⁴⁺, and Li⁺ sites as well as the occupancies of the Li⁺ sites gave an R _wp = 1.20%, GOF = 4.10, and a good visual fit to the experimental data (Figure a, and Table ). The isotropic displacement value of the T5a positions is comparatively high at 2.27(6) Ų, which may be indicative of local displacements of the Li⁺ ions potentially toward the T5 position. However, inclusion and refinement of the T5 site as well as other additional Li⁺ sites (T4, T2) in the model for Li₇TiO₅Br led to negligible occupancies, with no evidence for electron density on those sites in the Fourier difference map (FDM). A composition of Li_6.96(5)Ti_1.00(5)O₅Br_1.09(1) is measured via analytical methods for Li₇TiO₅Br (Table S1).

2.

(a) Rietveld refinement of Li₇TiO₅Br against high-resolution synchrotron XRD data at 300 K. (b) Fourier difference map obtained from Rietveld refinement against 300 K PXRD data of Li₇TiO₅Cl with Li⁺ modeled on the T5a and T3 positions, highlighting additional electron density (yellow) observed on the T5 position. (c) Rietveld refinement of Li₇TiO₅Cl against high-resolution synchrotron XRD data at 300 K. (d) Rietveld refinement of Li₇TiO₅Cl against high-resolution synchrotron XRD data at 100 K. Y _obs (black circles), Y _calc (red line), Y _obs–Y _calc (gray line), and Bragg reflections (orange tick marks for Li₇TiO₅X (X = Cl^–, Br^–) and blue tick marks for 0.99 wt % Li₂O impurity).

1. Atomic Positions, Isotropic Atomic Displacement Parameters (B _iso, Ų), and Site Occupancy Factors for Li₇TiO₅Cl and Li₇TiO₅Br Obtained from Rietveld Refinement against X-ray Powder Diffraction Data at 300 and 100 K.

site	Wyckoff position	x	y	z	s.o.f.	B iso (Å2)
Li7TiO5Br-300 K-F4̅3m–a = 8.576847(11) Å-R wp = 1.20%
Ti1	4a	0	0	0	1.0000	0.34(2)
Br1	4b	0.5	0.5	0.5	1.0000	1.051(18)
O1	16e	0.88106(9)	0.88106(9)	0.88106(9)	1.0000	0.91(3)
O2	4c	0.25	0.25	0.25	1.0000	1.02(5)
Li1 (T5a)	24g	0.4748(4)	0.25	0.25	1.0000	2.27(6)
Li2 (T3)	4d	0.75	0.75	0.75	1.0000	0.17(10)
Li7TiO5Br-100 K-F4̅3m −a = 8.562056(11) Å-R wp = 1.16%
Ti1	4a	0	0	0	1.0000	0.293(15)
Br1	4b	0.5	0.5	0.5	1.0000	0.20(1)
O1	16e	0.88043(8)	0.88043(8)	0.88043(8)	1.0000	0.55(3)
O2	4c	0.25	0.25	0.25	1.0000	0.19(4)
Li1 (T5a)	24g	0.4770(4)	0.25	0.25	1.0000	1.31(8)
Li2 (T3)	4d	0.75	0.75	0.75	1.0000	0.20(18)
Li7TiO5Cl-300 K-F4̅3m −a = 8.527040(9) Å-R wp = 4.55%
Ti1	4a	0	0	0	1.0000	0.264(4)
Cl1	4b	0.5	0.5	0.5	1.0000	1.592(7)
O1	16e	0.87933(4)	0.87933(4)	0.87933(4)	1.0000	0.782(11)
O2	4c	0.25	0.25	0.25	1.0000	0.685(10)
Li1 (T5a)	24g	0.4753(4)	0.25	0.25	0.6013(14)	1.881(8)
Li2 (T5)	48h	0.71141(2)	0.71141(2)	0.4658(5)	0.199(7)	0.43(12)
Li3 (T3)	4d	0.75	0.75	0.75	1.0000	0.56(5)
Li7TiO5Cl-100 K-I4̅ −a = 5.992190(14) Å; c = 8.550351(3)Å-R wp = 5.97%
Cl1	2a	0	0	0	1.0000	0.655(9)
Ti1	2b	0	0	0.5	1.0000	0.203(6)
O1	8g	0.5136(3)	0.25874(13)	0.12209(9)	1.0000	0.381(12)
O2	2c	0	0.5	0.25	1.0000	0.224(16)
Li1 (T5a)	4f	0	0.5	0.0240(3)	1.0000	0.84(8)
Li2 (T5)	8g	0.8168(4)	0.23816(4)	0.2232(3)	1.0000	0.94(5)
Li3 (T3)	2d	0	0.5	0.75	1.0000	1.52(5)

For Li₇TiO₅Cl, refinement of the Cl^–, O^2–, and Ti⁴⁺ positions and displacement parameters gave a reasonable visual fit to the experimental data but with clear intensity discrepancies and an R _wp of 9.25% (Figure S2a). In contrast to the model refined for Li₇TiO₅Br, the displacement parameter for the Li⁺ T5a site (24g) in Li₇TiO₅Cl was refined to a large value of 5.31 Ų compared to 0.87 Ų for the T3 site. Additional electron density centered around the T5 site in proximity to the T5a site was visible upon inspection of the FDM (Figure b). Lithium was introduced onto the T5 site with the combined maximum occupancies of the T5 and T5a sites set to unity. The refined site occupancy factor (s.o.f.) for T5a and T5 sites were 0.6013(14) and 0.199(7), respectively, and both sites then displayed satisfactory displacement parameters (1.881(8) Ų and 0.43(12) Ų, respectively). The final refinement gave an R _wp of 4.55% and GOF of 3.64 (Table ) and a good visual fit (Figure c) to the data. Analytical methods return a composition of Li_6.86(7)Ti_1.00(3)O₅Cl_0.968(9) for Li₇TiO₅Cl (Table S1). It is noted that these refinements against high-resolution synchrotron X-ray data extract the maximum amount of information available from the data, and that collection of neutron diffraction data could benefit the further study of Li⁺ ion migration in such disordered materials.

3.2.2. Order–Disorder Behavior at Low Temperature in Li₇TiO₅Cl

Li₇TiO₅Br retains the same cubic F4̅3m symmetry and ordered distribution of Li⁺ sites (T5a and T3 sites) at 100 K as is seen at 300 K (Figure S2b). In contrast, the data collected at 100 K for Li₇TiO₅Cl revealed splitting of some reflections, indicating a transition to a lower structural symmetry (Figure a). The reflections observed in the 100 K data could be indexed to the tetragonal I4̅ space group (hkl: h + k + l = 2n, hk0: h + k = 2, 0kl: k + l = 2n, hhl: l = 2n, 00l: l = 2n, h00: h = 2n), a possible subgroup of F4̅3m, with lattice parameters of a = 5.992190(14) Å and c = 8.550351(3) Å. The resulting I4̅ unit cell is related to the high-symmetry F4̅3m cell via: $a_t=\sqrt{2}\times \frac{a_c}{2},c_t=a_c$ (where a _t , c _t , and a _c denote the lattice parameters for the tetragonal (t) and cubic cell (c)). Variable-temperature XRD data collected from 100 to 300 K in 25 K steps revealed the onset of reflection splitting below 250 K (Figure b). The observed reflection splitting is most prominent at 100 K with a c _t /a _c lattice parameter ratio of 1.009. Convergence of the a and c parameters (at c/a = 1) is observed as the temperature is increased to 250 K (Figure c), as well as a discontinuity in the change in unit cell volume (Figure d).

3.

(a, b) Reflection splitting consistent with I4̅ symmetry is observed for Li₇TiO₅Cl below room temperature with a transition between F4̅3m and I4̅ at 250 K. (a) The (002) reflection at 300 K (orange) is split into (002) and (110) reflection at 100 K (blue), with (b) the convergence of the (002) and (110) reflections in the temperature range 100–225 K. (c) Variation in lattice parameters as a function of temperature for Li₇TiO₅Cl, lattice parameter a in the tetragonal unit cell is plotted using the pseudo-cubic unit cell with the conversion: $a_c=2\times \frac{a_t}{\sqrt{2}}$ and (d) variation in unit cell volume as a function of temperature for Li₇TiO₅Cl.

The starting model for refinement of the Li₇TiO₅Cl I4̅ structure at 100 K was obtained from normal mode calculations, which suggested a set of stable lower-symmetry structures arising from Li⁺ displacement from the high-symmetry positions within the room-temperature F4̅3m structure (Tables S2–S3). The most energetically favorable among these was a tetragonal structure with I4̅ symmetry, in excellent agreement with the observations from experiment. The Cl^–, O^2–, and Ti⁴⁺ positions and displacement parameters as well as the Cl^– occupancy were refined prior to careful assessment of the Li⁺ sites analogous to the room-temperature structures, with the overall Li⁺ content set to charge-balance the refined anion content. Refinement of Li⁺ positions (T5a, T5), occupancies (T5a, T5, T3) and displacement parameters (T5a, T5, T3) produced the final Rietveld fit with R _wp = 5.97% and GOF = 4.64 (Figure d and Table ).

3.3. Room- and Low-Temperature Structure Descriptions

3.3.1. Li₇TiO₅X (X = Cl^–, Br^–)Room-Temperature Structures

Li₇TiO₅Br and Li₇TiO₅Cl adopt the cubic argyrodite structure (F4̅3m) at room temperature (Figure a,b) comparable to the crystal structures of other oxide and sulfide argyrodites (Figure S1). The X^– and O^2– anions form a tetrahedrally close-packed structure in which the Ti⁴⁺ and Li⁺ cations occupy a proportion of the tetrahedral holes, e.g., Ti⁴⁺ ions (4a) are coordinated by four O^2– ions (16e), defining isolated TiO₄ ^4– tetrahedra (Figure a–c).

4.

(a–c) Unit cells for (a) Li₇TiO₅Br-300 K, (b) Li₇TiO₅Cl-300 K, and (c) Li₇TiO₅Cl-100 K. (d–f) Lithium environments highlighting the different (T5, T5a, and T3) sites which are either fully or partially occupied giving rise to order/disorder and symmetry changes in (d) Li₇TiO₅Br-300 K, (e) Li₇TiO₅Cl-300 K, and (f) Li₇TiO₅Cl-100 K. (g–i) Octahedral cages highlighting occupied Li⁺ sites as well as triangular bottle necks in (g) Li₇TiO₅Br-300 K, (h) Li₇TiO₅Cl-300 K, and (i) Li₇TiO₅Cl-100 K.

In Li₇TiO₅Br, Li⁺ ions occupy the trigonal bipyramidal T5a (24g) and tetrahedral T3 (4d) sites to give an ordered Li⁺ site distribution (Figure d), similar to that of Li₆PO₅Br. Occupancy of the T5a sites produces octahedral cages of Li⁺ ions that surround central oxide anions (4d) (Figure g).

In Li₇TiO₅Cl, Li⁺ ions occupy the trigonal bipyramidal T5a (24g), and tetrahedral T5­(48h) and T3 (4d) sites (Figure e), which is one additional site beyond Li₇TiO₅Br. The T5 and T5a sites are partially occupied with s.o.f. of 0.6013(14) and 0.199(7), respectively, while the T3 site is fully occupied, giving a disordered Li⁺ site distribution (Figure h). Such Li⁺ site disorder on the T5a and T5 sites within the octahedral cages has been linked to high ionic conductivities in oxide and sulfide argyrodites such as Li_6+x P_1–x Si _x O₅Cl and Li_6+x Si _x Sb_1–x S₅I.

3.3.2. Li₇TiO₅ClLow-Temperature Structure

Below 250 K, the Li₇TiO₅Cl structure transitions from cubic F4̅3m to tetragonal I4̅ symmetry (Figure , ). The unit cells of Li₇TiO₅Cl at 300 and 100 K are compared in Figure , with the cubic unit cell depicted as dashed lines in Figure d–f to facilitate the comparison between the two symmetries. The poly­(anion) structure remains largely unchanged through the transition, and the reduction in symmetry originates from changes to the Li⁺ sites, similar to order–disorder transitions known for other argyrodites. , A subset of the T5a and T5 sites that are partially occupied at 300 K in Li₇TiO₅Cl (Figure ) becomes fully occupied at 100 K; 25% of the available T5 (8g) sites and 50% of the available T5a (4f) sites are fully occupied, while the T3 (2c) position remains fully occupied at both temperatures (Figure f). This results in fully ordered Li⁺ sites at 100 K, distorting the Li⁺ octahedral cages (Figures i and d–f) and breaking symmetry elements such as the 3-fold rotational axis along the body diagonal (Figure b,e) and mirror plane along the face-diagonal (Figure c,f) of the unit cell, reducing the symmetry to tetragonal I4̅.

5.

Comparison between the unit cell of Li₇TiO₅Cl at (a) 300 K and (d) 100 K. Ordering of Li⁺ ions onto 25% of the available T5 sites leads to distorted Li⁺ octahedra, which breaks the 3-fold rotational axis (b, e) and mirror plane (c, f) symmetry operators of F4̅3m symmetry. The higher-symmetry cubic unit cell is drawn with dashed lines in (d–f) to aid comparison, and symmetry elements are drawn in red. Atom colors: TiO₄ tetrahedra (light blue), OLi₆ octahedra (light gray), Cl^– (turquoise), Li⁺ (T3) (purple):.

Order–disorder transitions of this kind are well-documented for many argyrodite materials, even those beyond Li⁺. Numerous ordered cubic, orthorhombic, and monoclinic structures have been reported for argyrodites, such as Li₇SiO₅Cl (P2₁3), Li₇PS₆ (Pna2₁), and Li₆PS₅I (Cc). However, tetragonal symmetry has been observed only for one other argyrodite: Li₇Zn_0.5SiS₆. This sulfide argyrodite crystallizes in the same I4̅ space group at room temperature, albeit with a significantly larger unit cell, which results from a complex ordering pattern and undergoes a phase transition at 411 K to F4̅3m symmetry. This highlights the potential structural diversity in lithium argyrodites that remains unexplored, which is further emphasized by noting that the ordering behavior observed for Li₇TiO₅Cl accesses a structure that is distinct from the ordered cubic structure of Li₇TiO₅Br, which likely results from lattice pressure induced by the different ionic radii of the halide anions. Further exploration of these systems may include the substitution of F^– at the Cl^–/Br^– or even O^2– sites, with similar ionic radius (r _O: 1.38 Å vs r_F: 1.31 Å), to access compositions with reduced Li content and to induce anion site disorder that may enable enhanced control over the ionic conductivity of these materials.

3.4. Transport Properties

The transport properties of Li₇TiO₅X (X = Cl^–, Br^–) were investigated using AC impedance spectroscopy at and above room temperature, coupled with DC polarization measurements and NMR spectroscopy. Pellets utilized in AC Impedance measurements were prepared via reactive sintering, resulting in densities of 80% and 81% of the theoretical densities for Li₇TiO₅Cl and Li₇TiO₅Br, respectively. A typical Nyquist plot collected for Li₇TiO₅Cl at room temperature is shown in Figure a. The plot contains a single semicircle and a low-frequency spike, a characteristic feature of an ion conductor arising from the buildup of charge at the ion-blocking electrode caused by the movement of Li⁺ ions. The capacitance at the frequency of maximum loss was calculated to be 2.11 pF, indicating that the signal corresponded to a bulk response (Figure S4). The Nyquist data were therefore modeled using a resistor in parallel with a constant phase element (CPE), integrated in series with another CPE to account for the low-frequency spike. The ionic conductivity of Li₇TiO₅Cl, calculated from the low-frequency intercept of the semicircle assigned to the bulk contribution of the material, was measured to be 2.2(2) × 10^–6 S cm^–1 at room temperature. Nyquist plots collected for Li₇TiO₅Br (Figure S3a) reveal a very low room-temperature ionic conductivity of ∼10^–9 S cm^–1, 3 orders of magnitude lower than that of Li₇TiO₅Cl.

6.

(a) Typical Nyquist plot for Li₇TiO₅Cl at room temperature, with the equivalent circuit used to fit the data inset. (b) Arrhenius plot for Li₇TiO₅Cl (orange) and Li₇TiO₅Br (blue) with extracted activation energies. (c) ⁷Li variable-temperature NMR spectra of Li₇TiO₅Cl recorded at 9.4 T under static conditions. (d) Temperature-dependent full width at half-maximum of the static ⁷Li NMR central transition, Δν­(T), measured for Li₇TiO₅Cl at 9.4 T, highlighting the onset temperature, T _onset, of line narrowing (vertical, dashed line).

Both materials follow Arrhenius-type behavior (Figure b) at elevated temperatures, characterized by activation energies of 0.36(2) eV and 0.58(2) eV for Li₇TiO₅Cl and Li₇TiO₅Br, respectively. A change in slope is clearly observed in the Arrhenius plot of Li₇TiO₅Cl below room temperature (Figure S5), which can be attributed to the Li⁺ site ordering observed in the I4̅ structure. An increased activation energy of 0.509(3) eV is obtained in the temperature range of 276–233 K, which is more closely comparable with the activation energy of 0.58(2) eV extracted above room temperature from the Li⁺ site-ordered F4̅3m structure of Li₇TiO₅Br, highlighting the impact of Li⁺ site order vs disorder within these materials. XRD patterns collected from a ground pellet after the AC impedance measurements show no changes, confirming the stability of the Li₇TiO₅Cl under the measurement conditions (Figure S6). Measurement of DC polarization data at room temperature indicates that the electronic contribution to the total conductivity is negligible at 1%. for Li₇TiO₅Cl. (Figure S3c,d). This low electronic contribution is further supported by the measurement of a wide band gap of 4.29(7) eV for Li₇TiO₅Cl (Figure S7).

Static ⁷Li solid-state NMR spectra for Li₇TiO₅Cl (Figure c) and Li₇TiO₅Br (Figure S3b) in the 190–588 K temperature range enable detection of Li-ion dynamics on the kHz time scale and are complementary to AC impedance measurement. The central transition detected in the spectrum acquired for Li₇TiO₅Cl at 190 K is characterized by a full width at half-maximum in the rigid lattice regime, Δν_R, of ∼9.9 kHz and exhibits a predominantly Gaussian line shape dominated by ⁷Li–⁷Li homonuclear dipole interactions (Figure c). As the temperature is increased above ∼247 K (onset temperature T _onset in Figure d), line narrowing of the ⁷Li central transition for Li₇TiO₅Cl is observed, which reveals an increase in Li⁺ motion at this temperature. Above ∼425 K, a predominantly Lorentzian line shape and line width in the fast motional regime Δν_∞ lower than 570 Hz are observed, indicating that the ⁷Li–⁷Li homonuclear dipole interactions are fully averaged by Li⁺ ion motion. Fitting a Boltzmann sigmoid function to the Δν­(T) vs T line narrowing curve (Figure d) enables extraction of a mean Li-ion jump rate of τ^–1 = 2πΔν_R = 6.24 × 10⁴ s^–1 for Li₇TiO₅Cl at the observed inflection point of 247 K.

The ⁷Li central transition of Li₇TiO₅Br at 205 K has a predominantly Gaussian line shape with Δν_R of ∼10.5 kHz at this temperature (Figure S3b), which narrows significantly as the temperature is increased, revealing a two-component line shape in spectra acquired in the 329–449 K temperature range (Figure S3b), attributed to two distinct Li⁺ environments that each undergo motional processes in different temperature ranges. The narrow component observed in Li₇TiO₅Br has a constant Δν within this temperature range and is tentatively assigned to the small percentage of Li⁺ cations in the fast motional regime occupying the T3 position that are likely involved in localized Li-ion motional processes. The dominant broader component is observed to undergo line narrowing at higher temperatures and is assigned to the larger percentage of Li⁺ cations in the T5a position, with a Lorentzian line shape and line width Δν_∞ lower than 725 Hz observed above 569 K. Despite the multicomponent contribution in Li₇TiO₅Br, comparison of the observed temperature dependence is indicative of overall enhanced Li-ion dynamics in Li₇TiO₅Cl, in accordance with the measured impedance data.

3.5. Structure–Property Relationship

Three dominant local Li⁺ ion jumps have been identified in lithium argyrodites: doublet jumps between T5 sites within one trigonal bipyramidal unit via a trigonal window defined by three anions, intracage jumps within one octahedral cage, and intercage jumps connecting neighboring octahedral cages, all of which are important for long-range lithium mobility (Figure a). Enhanced lithium diffusion can be facilitated by the shortening of these inter- and intracage distances, increased Li⁺ site disorder, enhanced lithium content, and improved mobility between neighboring sites via the optimization of anionic windows that the Li⁺ ions traverse. ,−

7.

(a) Octahedral Li⁺ cages in argyrodites highlighting possible Li⁺ ion sites, intra- and intercage jumps, and the trigonal window through which Li⁺ ions traverse. (b) Octahedral cage in the Li-site-ordered Li₇TiO₅Br and its intra- and intercage distances and the anion window size. (c) Jump distances are significantly smaller in the Li-site disordered Li₇TiO₅Cl resulting in a 1000-fold increase in ionic conductivity compared to Li₇TiO₅Br.

Li₇TiO₅Br has a low ionic conductivity (∼10^–9 S cm^–1) and high activation energy (0.58(2) eV), comparable to the values reported for Li₆PO₅Br (∼10^–9 S cm^–1, ∼0.60 eV) (Table ). This results from large intra- and intercage jump distances of 2.712(1) Å (T5a-T5a) and 2.370(1) Å (T5a-T3), respectively (Figure b, and Table ). These are driven by the unit cell expansion due to the larger ionic radius of Br ions as well as the ordered Li⁺ site distribution with fully occupied T5a and T3. The occupancy of Li­(T5a) sites in Li₇TiO₅Br is similar to that of Li₆PO₅Br; however, occupancy of the additional T3 site in Li₇TiO₅Br offers a shorter Li–Li distance of 2.370(1) Å that connects separate cages via T5a–T3–T5a jumps (Figure b) compared to the intercage distance of 3.155(4) Å in Li₆PO₅Br (Table ). The trigonal oxide anion window defining the doublet jump is comparable in Li₆PO₅Br (5.014 Ų) to Li₇TiO₅Br (5.03(2) Ų).

2. Intra- and Intercage Distances, Trigonal Window Sizes, Activation Energies (E _a) and Room-Temperature Ionic Conductivities (σ_RT) in Selected Oxide Argyrodites.

compound	intracage jump/Å	intracage distance/Å	intercage jump/Å	intercage distance/Å	trigonal window size/Å2	Ea / eV	σRT / S cm–1
Li6PO5Br	T5a–T5a	2.737(9)	T5a–T5a	3.155(4)	5.014(3)	∼ 0.60	∼ 10–9
Li6PO5Cl	T5a–T5a	2.728(9)	T5a–T5a	3.087(9)	4.856(5)	∼ 0.60	∼ 10–9
Li7TiO5Br [this work]	T5a–T5a	2.712(1)	T5a–T3–T5a	2.370(1)	5.03(2)	0.58(2)	∼ 10–9
Li6.75P0.25Si0.75O5Cl	T5–T5	2.101(13)	T5–T4–T5	1.606(3)	4.827(4)	0.52(2)	1.82(1) × 10–6
Li7TiO5Cl [this work]	T5–T5	2.294(1)	T5a–T3–T5a	2.280(1)	4.915(4)	0.36(2)	2.2(2) × 10–6

Li₇TiO₅Cl exhibits an ionic conductivity of 2.2(2) × 10^–6 S cm^–1, three orders of magnitude higher than Li₇TiO₅Br, which results from its enhanced Li⁺ site disorder. This is also reflected in the 0.36(2) eV activation energy of Li₇TiO₅Cl, which is significantly lower than 0.58(2) eV for Li₇TiO₅Br and 0.52(2) eV for Li_6.75P_0.25Si_0.75O₅Cl. In fact the measured activation energy of 0.36(2) eV for Li₇TiO₅Cl is comparable to those measured for some sulfide Li₆PS₅X (X = Cl^–, Br^–) materials. , Li₇TiO₅Cl exhibits a high room-temperature ionic conductivity (2.2(2) × 10^–6 S cm^–1) and the lowest activation energy for bulk Li⁺ ion motion (0.36(2) eV) reported for all oxide argyrodites.

This performance is a direct consequence of the Li⁺ site disorder present in Li₇TiO₅Cl, and high lithium content (7 Li⁺ per formula unit). Occupancy of the T5a, T5, and T3 sites by Li⁺ in Li₇TiO₅Cl enables access to much shorter intra- and intercage distances compared to Li₇TiO₅Br, facilitating fast Li⁺ ion diffusion (Figure c and Table ) and underscoring the pivotal role of site disorder within oxide argyrodites. Through consideration of these Li–Li distances alone, it is clear that the disordered F4̅3m structure of Li₇TiO₅Cl facilitates extended Li⁺ ion diffusion through an interconnected network of Li⁺ sites in three dimensions (Figure S8). Bond valence site energy (BVSE) calculations confirm this, revealing a 3D percolation network with inter- and intracage jumps as often reported for Li-sulfide argyrodites (Figure S9). Specifically, the BVSE calculations indicate that the intracage jumps (T5–T5) are rate-limiting compared to lower-energy more favorable intercage jumps (T5–T3). Li_6.75P_0.25Si_0.75O₅Cl, which exhibits a comparable room-temperature ionic conductivity (1.82(1) × 10^–6 S cm^–1) to Li₇TiO₅Cl, also has extensive Li⁺ site disorder with occupancy of the T5, T5a, T3, and T4 sites. Occupancy of the T4 site in Li_6.75P_0.25Si_0.75O₅Cl, which is unoccupied in Li₇TiO₅Cl, enables access to alternative intercage pathways (T5–T4–T5) with a shorter distance of 1.606(3) Å compared to the shortest intercage (T5a–T3–T5a) pathway in Li₇TiO₅Cl with distances of 2.280(1) Å (Table ). Despite this, Li₇TiO₅Cl exhibits a slightly higher ionic conductivity and much lower activation energy, which likely results from the combination of increased Li⁺ carrier concentration (7 Li⁺ per formula unit) and the larger trigonal windows through which Li⁺ ions traverse. The trigonal window defined by oxide anions in Li₇TiO₅Cl (4.915(4) Ų) is larger than that of Li_6.75P_0.25Si_0.75O₅Cl (4.827(4) Ų) as a result of the increased unit cell size expanded by the incorporation of larger Ti⁴⁺, and likely facilitates higher Li⁺ ion mobility. This demonstrates that changes to the local Li⁺ motion within the T5/T5a octahedral cages via control of anionic window sizes alongside an increase lithium content, maximized to 7 Li⁺ per formula unit in this case, can influence long-range Li⁺ ion transport in these particular oxide argyrodites.

4. Conclusions

This study presents the synthesis, crystal structures, and transport properties of Li₇TiO₅X (X = Cl^–, Br^–), the first lithium argyrodites in which the framework-forming cation is a transition metal, highlighting the expansive chemical space that is accessible for oxide argyrodite chemistry. Incorporation of a large tetrahedron-forming cation with + IV oxidation state (Ti⁴⁺) allows us to maximize the lithium content to 7 Li⁺ per formula unit in the absence of Li⁺ site ordering in Li₇TiO₅Cl, which is common for high-lithium-content argyrodites. Li₇TiO₅Cl adopts a Li⁺ site disordered cubic F4̅3m structure at room temperature with partial occupancy of T5a and T5 sites alongside full occupancy of T3 sites. This disorder facilitates high Li⁺ ion mobility with a measured ionic conductivity of 2.2(2) × 10^–6 S cm^–1 at room temperature and the lowest reported activation energy of 0.36(2) eV for bulk lithium transport in any oxide argyrodite material. In contrast, Li₇TiO₅Br adopts a Li⁺ site-ordered cubic F4̅3m structure with fully occupied T5a and T3 sites and has a much lower ionic conductivity (∼10^–9 S cm^–1) and significantly higher activation energy (0.58(2) eV). Interestingly, Li₇TiO₅Cl exhibits an order–disorder phase transition below room temperature; however, adopting a more complex Li⁺ site ordering pattern than Li₇TiO₅Br that yields a tetragonal superstructure with I4̅ symmetry below 250 K. The low-temperature structure of Li₇TiO₅Cl represents only the second time that a tetragonal argyrodite structure has been observed, further emphasizing the potential for greater structural diversity in argyrodite materials that may be accessed by expanding the range of possible framework-forming cation chemistry.

The data that support the findings of this research will be made openly available via the University of Liverpool open research platform Liverpool Elements on acceptance of the publication.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.chemmater.6c00177.

• ICP-OES and IC results for Li₇TiO₅Cl and Li₇TiO₅Br, additional Rietveld refinement models tested during structure determination, details of normal mode calculations, and impedance and ⁷Li NMR data of Li₇TiO₅Br (PDF)

The authors declare no competing financial interest.

Published as part of Chemistry of Materials special issue “Honoring the Outstanding Scientific Contributions of Clare Grey”.