# Synthesis, structure, and ionic transport properties of lithium monothiophosphate pentahydrate Li₃PO₃S×5H₂O and its anhydrous form Li₃PO₃S

Jamal Bouaouina, Guillaume Laurent, Maxim Avdeev, Gwenaëlle Rousse, Jaafar Ghanbaja, Gwladys Steciuk, Christel Laberty-Robert, Arnaud Perez

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## To cite this version:

Jamal Bouaouina, Guillaume Laurent, Maxim Avdeev, Gwenaëlle Rousse, Jaafar Ghanbaja, et al. Synthesis, structure, and ionic transport properties of lithium monothiophosphate pentahydrate Li₃PO₃S×5H₂O and its anhydrous form Li₃PO₃S. Journal of Materials Chemistry A, 2025, 13 (34), pp.28496-28507. 10.1039/D5TA03395K. hal-05318558

HAL Id: hal-05318558

https://hal.sorbonne-universite.fr/hal-05318558v1

Submitted on 21 Nov 2025

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Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
Synthesis, structure, and ionic transport properties of lithium
monothiophosphate pentahydrate Li₃PO₃S·5H₂O and its anhydrous form
Li₃PO₃S

Jamal Bouaouina¹, Guillaume P. Laurent¹, Maxim Avdeev²,³, Gwenaëlle Rousse⁴,⁵, Jaafar Ghanbaja⁶,
Gwladys Steciuk⁶, Christel Laberty-Robert¹,⁵, Arnaud Perez¹,⁵,*

¹ Sorbonne Université, CNRS, Laboratoire de Chimie de la Matière Condensée de Paris, LCMCP, F-75005, Paris, France

² Australian Nuclear Science and Technology Organisation, ANSTO, New Illawarra Road, Lucas Heights, NSW 2234, Australia

³ School of Chemistry, The University of Sydney, Sydney 2006 NSW, Australia

⁴ Sorbonne Université, Collège de France, CNRS UMR 8260, Laboratoire de Chimie du Solide et Energie, CSE, F-75005, Paris, 75005 Paris

⁵ CNRS, Réseau sur le Stockage Electrochimique de l'Energie (RS2E), F-80039, Amiens, France

⁶ Université de Lorraine, CNRS, IJL, F-54000 Nancy, France

⁷ Institute of Physics, Academy of Sciences of the Czech Republic (ASCR), v.v.i., Na Slovance 1999/2, 18221 Prague 8, Czech Republic

<footnote>* Corresponding author: arnaud.perez@sorbonne-universite.fr</footnote>
Abstract

Mixed anion materials are promising for application as solid electrolytes in all-solid-state batteries, offering enhanced chemical and electrochemical stability alongside balanced ionic conductivity. This work reports a novel lithium oxythiophosphate Li₃PO₃S·5H₂O, synthesized at a low temperature in an aqueous medium. The crystal structure and properties of Li₃PO₃S·5H₂O were investigated through 3-dimensional electron diffraction, combined with powder X-ray and neutron diffraction, thermogravimetric analysis, nuclear magnetic resonance spectroscopy, and electrochemical impedance spectroscopy. Li₃PO₃S·5H₂O crystallizes in the hexagonal space group P6₃cm with *a* = 16.48462(4), *c* = 5.36224(2) Å, *V* = 1261.928(6) Å³, and *Z* = 6. ³¹P solid-state MAS NMR spectroscopy revealed the presence of pure and ordered PO₃S tetrahedra. The dehydration of Li₃PO₃S·5H₂O was studied via TGA and temperature-dependent XRD, demonstrating the formation of the anhydrous phase Li₃PO₃S at 200 °C. The conductivity of anhydrous Li₃PO₃S (2.6 × 10⁻⁶ S cm⁻¹ at room temperature and *E*ₐ = 0.51 eV) is three orders of magnitude higher than that of the parent material Li₃PO₄ prepared by further heating the sample at 400°C, with a significantly lower activation energy. Our findings highlight the potential of mixed anion materials in improving solid electrolyte performance. This work brings new insight into the crystal chemistry of oxythiophosphate anions, which is key in developing fast ionic conductors for solid-state batteries.

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# 1. Introduction

Solid-state batteries (SSB) are a promising energy storage technology and an alternative to conventional lithium-ion batteries (LIB), owing to their potentially superior energy density and safety.¹ Intensive research on solid electrolytes (SE) over the past decades has resulted in the discovery of new lithium ionic conductors with conductivities exceeding that of liquid electrolytes (i.e., 1 M LiPF₆ in EC/DMC) used in LIB.² This is particularly the case of sulfide-based SE, such as Li₁₀GeP₂S₁₂ (LGPS), which exhibits an ionic conductivity above 10⁻² S cm⁻¹ at room temperature. Nevertheless, the electrochemical performances of SSB do not yet match those of LIB because of several remaining challenges, a major one being the poor interfacial stability of solid electrolytes.³⁻⁵ On the other hand, oxide-based SE exhibit higher stability, but they possess lower ionic conductivity and are brittle, making their assembly in SSB even more challenging.⁶,⁷ Therefore, developing new functional lithium SE using innovative chemistries remains crucial to enable the next generation of high-performance and safe SSB.

Mixed-anion compounds, which contain multiple anions in a single phase, emerge as an interesting chemical field to prepare materials with new functions not found in single-anion phases, such as oxides and sulfides.⁸,⁹ Whereas cation substitution strategies have been widely studied as a means to optimize ionic conductivity properties, anion substitution is less systematic, but has been shown to improve both ionic conductivity and chemical/electrochemical stability.¹⁰,¹¹ Thus, making solid electrolytes with mixed sulfide and oxide anions could result in an optimal balance of ionic conductivity and chemical/electrochemical stability. Partial oxygen substitution for sulfur has been shown to improve the chemical stability of sulfides, as the bond dissociation energy is much higher for the P-O bond (597 kJ mol⁻¹) than for the P-S bond (346 kJ mol⁻¹).¹² For instance, Kanno and his group demonstrated that oxygen substitution in LGPS-type SE can lead to the formation of Li₁₀GeP₂S₁₂-xOx (0.3 ≤ x ≤ 0.6) and Li₉P₃S₁₂-xOx (0 < x ≤ 3.6) phases, both exhibiting enhanced chemical and electrochemical stability.¹³,¹⁴ Li₉P₃S₉O₃ phase demonstrated improved air stability compared to the original LGPS, while SSB using these SE retained high ionic conductivity (≥ 10⁻³ S cm⁻¹) and stable charge-discharge cycling performance with lithium metal anode. Furthermore, computational calculations have suggested that a critical amount of x = 0.25 within Li₃PS₄-xOx with β-Li₃PS₄ type structure could achieve a high ionic conductivity (≥ 10⁻² S cm⁻¹) and a minimum activation energy (0.28 eV).¹⁵
However, high-temperature synthetic methods, used classically in solid-state chemistry, promote the formation of the thermodynamically stable phases and result in a limited range of O/S substitutions within $Li_9P_3S_{12-x}O_x$ ($x \le 3.6$) with the LGPS structure and $\gamma-Li_3PS_{4-x}O_x$ ($x \le 1$) with $\gamma-Li_3PS_4$ structure.<sup>14,15</sup> Additionally, solid-state nuclear magnetic resonance (NMR) spectroscopy revealed that materials prepared at high temperatures are composed of multiple $(PS_{4-x}O_x)^{3-}$ anionic units with all possible values of x, without any control on their distribution and stoichiometry, similar to what have been observed for the $Li_9PS_9O_3$ phase.<sup>14</sup> To control the S/O substitution ratio, achieve a better distribution over $(PS_{4-x}O_x)$ units, and obtain pure oxythiophosphate anions such as $(POS_3)^{3-}$, $(PO_2S_2)^{3-}$, and $(PS_3O)^{3-}$, an interesting alternative is to use liquid synthesis approaches at low temperature, as demonstrated in the Na-P-O-S system.<sup>16</sup>

In this work, we were inspired by previous research on sodium oxythiophosphate $Na_3PO_3S$ to explore the Li-P-S-O system. Herein, we report the successful synthesis of a new lithium oxythiophosphate analog phase $Li_3PO_3S \cdot 5H_2O$ through a simple wet chemistry route. NMR spectroscopy revealed the formation of pure $(PO_3S)$ units and X-ray diffraction (XRD) showed that $Li_3PO_3S \cdot 5H_2O$ is a new phase. Its crystal structure was solved, and the symmetry tests were carried out through continuous rotation 3D electron diffraction (3D ED) in a transmission electron microscopy (TEM), revealing that $Li_3PO_3S \cdot 5H_2O$ crystallizes in the hexagonal space group $P6_3cm$. The structural model was refined against 3D ED using the dynamical approach, and XRD and neutron powder diffraction (NPD) multipattern. The $Li_3PO_3S$ phase, obtained from the dehydration of $Li_3PO_3S \cdot 5H_2O$, exhibits an ionic conductivity three orders of magnitude higher than that of $\gamma-Li_3PO_4$ prepared from the same precursor, and has a lower activation energy. This highlights the strategic role of wet chemistry approaches for the synthesis of pure mixed anion compounds and their potential in finding a compromise between their properties. However, further optimization is needed to meet the requirements for practical application as a solid electrolyte.
## 2. Methods

### 2.1. Synthesis

Li₃PO₃S·5H₂O phase was synthesized based on the protocol reported by Pompetzki and Jansen to synthesize the sodium analog Na₃PO₃S.¹⁶ Thiophosphoryl chloride PSCl₃ (Sigma-Aldrich, 98%) and lithium hydroxide monohydrate LiOH·H₂O (Sigma-Aldrich, >99%) were used as starting materials. PSCl₃ was added to a solution of LiOH·H₂O 3M with a molar ratio of 1:6 at room temperature and then heated at 50 °C for 30 minutes. The mixture was then cooled down to room temperature, then dropped slowly in an excess volume of ethanol cooled to 0°C to precipitate Li₃PO₃S·5H₂O. The precipitated product was isolated with suction filtration and dried overnight in ambient atmosphere to obtain a powder of Li₃PO₃S·5H₂O with a yield of about 48% based on the amount of phosphorus converted from PSCl₃ to Li₃PO₃S.

Samples for ex situ ssNMR and ionic conductivity measurements were prepared by annealing Li₃PO₃S·5H₂O at different temperatures (150, 200, 250, 350, and 400 °C) under an Ar atmosphere for six hours.

### 2.2. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX)

Li₃PO₃S·5H₂O and Li₃PO₃S-200 °C samples were analyzed in a Hitachi S-3400 N microscope at an accelerating voltage of 10 kV for SEM and EDX, after the deposition of a carbon layer of 20 nm on the samples.

### 2.3. Nuclear magnetic resonance (NMR) spectroscopy

³¹P solution and solid-state NMR spectroscopy were employed to investigate the local environment of phosphorus in the synthesized compound. NMR experiments were performed on two 7.0 T Bruker Avance-III spectrometer. 5 mm solution NMR tubes were used and loaded with roughly 1 mL of Li₃PO₃S·5H₂O with a concentration of 5 mg/mL in a H₂O: D₂O solution with 10 wt.% of D₂O (Sigma-Aldrich, 99.9%) for ³¹P solution NMR experiments. The ³¹P chemical shifts were determined relative to phosphoric acid (85%). Only ³¹P NMR spectra recorded with spin-spin decoupling {¹H}–³¹P have been considered in this study. Solid state NMR (ssNMR) spectra of ¹H, ⁷Li, and ³¹P were measured using magic angle spinning (MAS). Li₃PO₃S·5H₂O powder was loaded into a 4 mm zirconia rotor, closed using a Kel-F cap, and spun at a MAS frequency of 14 kHz.
Spectra were calibrated to TMS at 1 % in CDCl₃ at 25°C using the ¹³C adamantane peak difference as a secondary reference.¹⁷ ⁷Li and ³¹P MAS ssNMR measurements were recorded using high-power decoupling (HPDEC) with a pulse width of 2 µs (90° pulse) and 1.7 µs (30° pulse), respectively, and a relaxation delay of 10 s and 60 s, respectively. MAS ssNMR data were processed using Bruker TopSpin 3.7.0 and fitted using DMfit software.¹⁸

## 2.4. X-ray Powder diffraction

Phase identification of the Li₃PO₃S·5H₂O phase was performed on PXRD data obtained on a Bruker D8-Discover diffractometer operating at 40 kV and 30 mA and with CuKα radiation (λ₁ = 1.54056 Å, λ₂ = 1.54439 Å) in the range 2θ = 5-80° and a step size of 0.01°. Synchrotron X-ray powder diffraction (SPXRD) measurements were collected using the Debye-Scherrer transmission geometry (0.5 mm borosilicate capillary, λ = 0.67153 Å) at the SOLEIL Cristal beamline (Paris, France). The usable data range was from 1.5 to 55° 2θ, and the step size was 0.009° 2θ. Temperature-dependent XRD was performed on a Bruker D8-Discover diffractometer, and Li₃PO₃S·5H₂O powder was placed on a ceramic support. A nitrogen flux was flushed through the sample chamber at a rate of 50 mL/min during the experiment. Acquisitions were taken on the sample heated from room temperature to 600 °C with a heating rate of 2 °C/min. The XRD patterns were acquired in the range 2θ = 5-80° with 0.02° steps and a time of 2 s/step every 20 °C from 20 to 500 °C. Fox (Free Objects for crystallography) software was used for ab initio structure determination from XRD data obtained at 200 °C from the temperature-dependent XRD by a parallel tempering algorithm.¹⁹

## 2.5. Three-dimensional electron diffraction (3D ED)

3D ED experiments were carried out using a transmission electron microscope (TEM). The sample was dispersed in ethanol and deposited on a Cu-grid coated with a thin film of holey amorphous carbon.²⁰,²¹ To prevent dehydration caused by the TEM high vacuum, the grid was plunged into liquid nitrogen and then transferred to the TEM using a Gatan cryo-plunging holder. The 3D ED data were collected at 95 K with a continuous rotation mode in an FEI Tecnai G2 TEM (acceleration voltage of 200 kV, LaB₆) equipped with a side-mounted hybrid single-electron detector ASI Cheetah M3, 512 x 512 pixels with high sensitivity and fast readout. For each selected needle-like crystal, a series of non-oriented patterns was continuously collected by steps of 0.3°, on all the accessible tilt ranges of the goniometer. The data collections are automated by the in-
house software RATS, including the crystal tracking, following the procedure described by Plana-Ruiz.²² In addition to the low-temperature experiment, low-illumination settings were used to limit the beam-induced damage on the crystal. Continuous-rotation 3D ED data (cRED) reduction was performed using the computer program PETS2.²³⁻²⁵, including correcting geometrical and optical distortions to get accurate lattice parameters and better integrating the reflections.²⁵ The specific data processing for cRED data was used in the structure solution and the refinement (with and without the dynamical approaches).²⁴ Overlapping virtual frames (*OVFs*) are produced to model experimental intensities from continuous rotation data by summing consecutive experimental diffraction patterns into a set of virtual frames. Each *OVF* is characterized by its angular range $\Delta\alpha_v$ covered by the virtual frame and the angular step between two virtual frames. The quality of the dataset is visible on the Camel plot with the values of the *Rocking curve width* = 0.001 Å⁻¹ and the *apparent mosaicity* = 0.1 deg (Figure S1). The result of the data reduction is a *hkl*-type obtained from the best crystal ($R_{int}(obs/all) = 0.2280/0.2928$) for a maximum resolution of $sin\theta/\lambda = 0.6$ Å⁻¹. This file is used in the structural solution and the kinematical refinement. The best data set was then used for the dynamical approach where each *OVF* is considered independent.²³,²⁴,²⁶,²⁷ The structure was solved using Superflip in Jana2020 and refined using the dynamical approach through DYNGO and Jana2020.²⁸,²⁹,³⁰ The details of the data collection and refinement from 3D ED are given in Table S1.

Additional microscopy observations were carried out using a JEM-ARM 200F Cold FEG TEM/STEM operating at 200 kV and equipped with a spherical aberration (Cs) probe and image correctors (point resolution 0.12 nm in TEM mode and 0.078 nm in STEM mode).

## 2.6. Thermogravimetric analysis (TGA)

TGA was performed on the Li₃PO₃S·5H₂O sample using a NETZSCH STA 409 PC/PG instrument between 30 and 800 °C at a heating rate of 5 °C/min. The experiments were carried out in an alumina crucible under an Argon atmosphere with a flow rate of 50 mL/min.

## 2.7. Neutron powder diffraction

Hydrogenated Li₃PO₃S·5H₂O and deuterated Li₃PO₃S·5D₂O phases were prepared similarly as discussed above and by replacing LiOH·H₂O with LiOD 3N in D₂O (Cambridge Isotope, 99.5%). NPD of both samples was carried out using the high-resolution neutron powder diffractometer
ECHIDNA ($\lambda$ = 1.62206 Å) at the Open Pool Australian Light Water Reactor (OPAL) operated by the Australian Nuclear Science and Technology Organisation (ANSTO).³¹

## 2.8. Electrochemical impedance spectroscopy (EIS)

The ionic conductivity was evaluated through alternating current (AC) impedance spectroscopy on pelletized samples at room temperature using a 1260 Solartron FRA device between 10 MHz and 10 Hz, with an amplitude of 100 mV at the open circuit voltage. The pelletized samples were prepared by pressing under 500 MPa for 5 min with a 10 mm diameter die and were assembled in a Swagelok-type cell with two stainless-steel blocking electrodes (BEs) under an Argon atmosphere in a glove box. AC impedance measurements were conducted over a range of temperatures between 20-100 °C on the $Li_3PO_3S$ sample obtained by annealing at 200 °C and 400 °C under a vacuum atmosphere for 6h, denoted $Li_3PO_3S-200$ and $Li_3PO_3S-400$, respectively. The samples were held for 1 hour at each temperature to reach thermal equilibrium. The pellet relative density was calculated to be 93%, 79%, and 72% for $Li_3PO_3S \cdot 5H_2O$, $Li_3PO_3S-200$, and $Li_3PO_3S-400$, respectively, from the crystallographic density: 1.75, 2.10, and 2.30 g cm⁻³, respectively. The impedance spectra were fitted with an equivalent circuit using the ZView4 software.

## 2.9. Symmetric cell

Symmetrical cell $Li/Li_3PO_3S/Li$ cycling was performed on a pellet made from 80 mg of $Li_3PO_3S$ powder pressed under 400 MPa for 5 minutes with a diameter of 10 mm and a thickness of 0.7 mm. Li metal foils of 0.1 mm thickness were placed on each side of the pellet as electrodes. Stainless-steel rods were used to stack the $Li/Li_3PO_3S/Li$ in a Swagelok cell. The experiment was conducted on a VSP biologic Potentiostat at a temperature of 60 °C. The cell was held at OCV for 2 hours, then the galvanostatic cycling tests started with current densities of 20, 40, 100, and 200 nA cm⁻² for 10, 5, 2, and 1 minute, respectively.
**3. Results and discussion**

**3.1. Synthesis and structure determination**

Lithium oxythiophosphate hydrate Li₃PO₃S·5H₂O phase was synthesized after several optimizations of the previous method reported for sodium analogs by decreasing the temperature from 110 °C to 50 °C.¹⁶,³² **Figure 1a** shows the SEM image of Li₃PO₃S·xH₂O powder, which is composed of aggregated needles of lengths between 1 and 10 µm. The sulfur-to-phosphorus ratio (S/P) was estimated to be 0.97 through SEM-based energy dispersive X-ray spectroscopy (EDS), which is very close to the stoichiometry in Li₃PO₃S·5H₂O. EDS mapping revealed a homogeneous elemental distribution, confirming the single-phase nature of the Li₃PO₃S·5H₂O sample (**Figure S2**). The substitution of one oxygen with a sulfur atom in (PO₄) decreases the electronic shield around phosphorus, which leads to a significant downfield shift (~ 30 ppm) in the NMR ³¹P spectrum.³³ The formation of (PO₃S) units within Li₃PO₃S·5H₂O was easily confirmed through ³¹P solution NMR, which showed a chemical shift around 31.78 ppm and agreed with the one found for Na₃PO₃S (31.51 ppm, **Figure 1b**).¹⁶
a

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b

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c

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c

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**Figure 1:** (a) SEM image collected from Li₃PO₃S·5H₂O. (b) ³¹P solution NMR spectra of Li₃PO₃S·5H₂O (red) and Na₃PO₃S (black). (c) TGA curve of Li₃PO₃S·5H₂O under Ar atmosphere. (d) powder XRD of the obtained Li₃PO₃S·5H₂O and its anhydrous phase after annealing at 200 °C under Ar atmosphere.

To determine the water content within the crystal structure of Li₃PO₃S·xH₂O, the sample was studied by TGA as shown in **Figure 1c**. The TGA curve indicated a weight loss of 42% between 30 °C and 200 °C, corresponding to 5 molecules of water, which results in the overall formula Li₃PO₃S·5H₂O. The XRD pattern of Li₃PO₃S·5H₂O showed a well-crystallized phase and could not be assigned to any of the known structures within the ICDD database (PDF+5), suggesting that Li₃PO₃S·5H₂O represents a new phase with an unknown crystal structure (**Figure 1d**). The anhydrous phase Li₃PO₃S was obtained after drying Li₃PO₃S·5H₂O under an argon atmosphere at 200 °C according to TGA, and XRD revealed that a phase transformation occurred during the thermal treatment (**Figure 1d**). The peaks broadening in the XRD pattern of the anhydrous sample
Li₃PO₃S suggests the formation of small crystallites, consistent with the SEM image of the sample annealed at 200 °C, which shows an aggregation of needle-like particles (Figure S3).

The most straightforward way to solve such a crystal structure would be by performing single-crystal XRD, however, our attempts to grow sufficiently large single crystals have failed because of the formation of small and thin needle-like crystals. Instead, we combined 3D ED data, to determine the unit cell and to solve the structure from single crystal data, with SPXRD and NPD to check the structural model obtained at the nanoscale on a larger portion of the Li₃PO₃S·5H₂O sample. 3D ED has become a powerful technique for solving and refining crystal structures, particularly when the growth of large single crystals is difficult.²¹ Notably, 3D ED has proven capable of determining the position of water molecules and even hydrogen in the lattice due to the stronger Coulombic interaction than X-rays.³⁴,³⁵ Bragg peaks within the SPXRD pattern of Li₃PO₃S·5H₂O were initially indexed using the DICVOL program³⁶ in two possible unit cells. One with orthorhombic symmetry (a = 14.27345(4) Å, b = 8.24080(2) Å, c = 5.36123(1) Å, V = 630.613(3) Å³) and the other with hexagonal symmetry (a = 16.48172(3) Å, c = 5.36128(1) Å, V = 1261.260(5) Å³). 3D ED was employed on data collected at 95 K to resolve the ambiguity between these two indexing solutions. The reciprocal space was indexed unambiguously in a hexagonal unit cell, a = 16.6397(3) Å, c = 5.2673(2) Å, and V = 1263.0(1) Å³. The orthorhombic unit cell could be viewed as double-distorted to form the hexagonal unit cell (V<sub>hexagonal</sub> = 2*V<sub>orthorhombic</sub>).

The sections of the reciprocal space clearly show the extinction conditions *l* = 2*n* on *0kl h0l* and *hhl* and *00l*, compatible with the space groups *P*6₃/*mcm* and *P*6₃/*cm* (**Figure 2**). The space group *P*6₃/*mcm* was initially chosen to solve the structure, allowing a partial orientational disorder of PO₃S tetrahedra. Eventually, comparison of refinements showed that the most likely space group is the non-centrosymmetric *P*6₃/*cm*, which can allow a total ordering of the main ring composed of connected PO₃S and LiO₃(H₂O) tetrahedra units. The dynamical refinement leads to *R*<sub>obs</sub>/w*R*<sub>obs</sub> = 0.1082/0.1005, *R*<sub>all</sub>/w*R*<sub>all</sub> = 0.1678/0.1076 for 1425/3015 observed/all reflections and 97. Restrictions were applied on O-H distances (1 Å), H-O-H angles, and the displacement parameters of hydrogen atoms that were set as riding on their bonded oxygen with a factor of 1.5. Some constraints were also added to ensure the occupancy sum is equal to one for the two disordered oxygen positions O3 and O4, and a similar displacement parameter. The *R*-values are reasonable for 3D ED of average quality due to the presence of ice molecules and the impossibility
of isolating a single needle during data collection. Nevertheless, the refinement quality is proven
by detecting lithium in the circular channel and all hydrogen positions (Figure S4a). The 3D ED
analysis led to the general formula Li₃PO₃S·5H₂O (for Z = 6), with a clean and unambiguous
structural model with reasonable atomic environments. The tetrahedra around Li and P of the main
ring are disordered with two possible configurations described with two O3 (O3-1, O3-2) and two
O4 (O4-1, O4-2) positions (Figure S4b). One configuration is dominant (80%:20%), and the
structure is represented in this paper with only one configuration for clarity purposes. This disorder
is only visible at low temperatures. The details of the refinement and the resulting positional
parameters of the refined model against 3D ED data are given in Tables S1 and S2. Bright field
micrographs and the related selected area diffraction pattern show the needle-like morphology of
the sample and the growth direction along [001] (Figure 3).

<figure><img src="image_9.png" /><figcaption>**Figure 2:** Reciprocal space sections reconstructed from 3D ED (T = 95 K) by PETS2 showing systematic extinctions compatible with the *P*6<sub>3</sub>cm or *P*6<sub>3</sub>/mcm space groups. The sections are produced with a resolution up to about 1 Å<sup>-1</sup>.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 3:** Bright field TEM micrographs of the needle-like crystals and corresponding selected area diffraction (SAED) pattern along [-110] zone axis showing growth direction [001].</figcaption></figure>

To further study the structural models obtained from 3D ED with two possible configurations of oxygen tetrahedra around P and Li1 atoms, a combined multi-pattern Rietveld refinement³⁷ for Li₃PO₃S·5H₂O was performed against SPXRD and NPD data of both hydrogenated and deuterated samples using FullProf.³⁸ Deuterium atoms were assumed to occupy the same sites as hydrogen in the crystal structure. Rietveld analysis indicates that the ordered model, which assumes a single configuration of oxygen positions in P and Li1 tetrahedra, yielded better reliability factors and better overall fits than the disordered model. To further check the structure, difference Fourier maps were calculated from the combined Rietveld refinement of the ordered model, and no unassigned electron density was observed, confirming that the average crystal structure of Li₃PO₃S·5H₂O at room temperature does not indicate any disorder of oxygen atoms of P and Li1 tetrahedra. The discrepancy between the best structural model obtained from 3D ED and the final model derived from SPXRD combined with NPD refinement could be attributed to a temperature-dependent structural effect. Specifically, oxygen disorder around the P and Li1 atoms is observed only at low temperature (95 K), whereas at room temperature, their tetrahedral coordination adopts a more ordered configuration. Thus, the analysis of SPXRD and NPD data was continued with the ordered structural model, as it provided the most accurate configuration of the crystal structure of Li₃PO₃S·5H₂O. The cell parameters, atomic position, and isotropic atomic displacements were constrained during the Rietveld refinement of SPXRD and NPD. Combined Rietveld refinement of SXRD and NPD data is presented in **Figure 4**, demonstrating excellent agreement between experimental and calculated patterns. The final refined structural parameters are listed in **Table 1**.
Note that the strain broadening in SPXRD data was refined anisotropically.
<figure><img src="image_11.png" /><figcaption>**Figure 4:** Combined Rietveld refinement of: (a) NPD ($R_{Bragg}$ = 9.44%, $R_p$ = 25.5%, $\chi^2$ = 1.23) of Li₃PO₃S·5H₂O. (b) NPD ($R_{Bragg}$ = 8.43%, $R_p$ = 21%, $\chi^2$ = 3.17) of Li₃PO₃S·5D₂O. (c) SPXRD ($R_{Bragg}$ = 20.3%, $R_p$ = 16.7%, $\chi^2$ = 2.1) of Li₃PO₃S·5H₂O. The red circles, black continuous lines,</figcaption></figure>

Intensity (a.u.)

Intensity (a.u.)

Intensity (a.u.)
and bottom blue lines represent the observed, calculated, and the difference between patterns. Vertical green tick bars are the Bragg positions allowed in the hexagonal *P*6<sub>3</sub>cm space group.

<table><caption>**Table 1:** Cell parameters, fractional atomic positions, isotropic displacement parameters, and BVS obtained from the combined Rietveld refinement of the synchrotron X-ray and Neutron Diffraction data of the crystal structure of Li<sub>3</sub>PO<sub>3</sub>S·5H<sub>2</sub>O phase.</caption><thead><tr><th>Space group</th><th>P6<sub>3</sub>cm</th></tr></thead><tbody><tr><td><i>a</i> (Å)</td><td>16.48482(5)</td></tr><tr><td><i>c</i> (Å)</td><td>5.36228(2)</td></tr><tr><td><i>V</i> (Å<sup>3</sup>)</td><td>1261.968(7)</td></tr><tr><td><i>Z</i></td><td>6</td></tr><tr><td>Density (g/cm<sup>3</sup>)</td><td>1.752</td></tr></tbody></table><table><thead><tr><th>Atom</th><th>Wyckoff position</th><th>x</th><th>y</th><th>z</th><th>Occupancy</th><th><i>B</i> (Å<sup>2</sup>)</th><th>BVS</th></tr></thead><tbody><tr><td>P</td><td>6c</td><td>0.2797(1)</td><td>0.2797(1)</td><td>3/4</td><td>1</td><td>0.68(4)</td><td>4.88(2)</td></tr><tr><td>S</td><td>6c</td><td>0.4069(1)</td><td>0.4069(1)</td><td>0.7477(9)</td><td>1</td><td>1.25(4)</td><td>1.15(6)</td></tr><tr><td>O1</td><td>6c</td><td>0.2510(2)</td><td>0.2510(2)</td><td>0.0311(8)</td><td>1</td><td>1.49(3)</td><td>2.18(2)</td></tr><tr><td>O2</td><td>12d</td><td>0.2056(2)</td><td>0.2943(2)</td><td>0.6247(8)</td><td>1</td><td>1.49(3)</td><td>2.07(2)</td></tr><tr><td>Ow1</td><td>12d</td><td>0.4450(1)</td><td>0.3166(1)</td><td>0.2435(14)</td><td>1</td><td>1.49(3)</td><td>1.88(2)</td></tr><tr><td>Ow2</td><td>12d</td><td>0.5635(1)</td><td>0.3235(1)</td><td>0.7476(11)</td><td>1</td><td>1.49(3)</td><td>1.81(2)</td></tr><tr><td>Ow3</td><td>12d</td><td>0.1104(2)</td><td>0.0471(4)</td><td>0.5446(15)</td><td>0.5</td><td>1.49(3)</td><td>1.64(3)</td></tr><tr><td>Li1</td><td>12d</td><td>0.3109(5)</td><td>0.2040(5)</td><td>0.2383(27)</td><td>1</td><td>2.25(16)</td><td>1.03(1)</td></tr><tr><td>Li2</td><td>4b</td><td>1/3</td><td>2/3</td><td>0.002(57)</td><td>1</td><td>2.25(16)</td><td>1.06(2)</td></tr><tr><td>Li3</td><td>2a</td><td>0</td><td>0</td><td>0.26171(70)</td><td>1</td><td>2.25(16)</td><td>1.20(2)</td></tr><tr><td>H1w1</td><td>12d</td><td>0.4497(13)</td><td>0.3518(14)</td><td>0.3788(38)</td><td>1</td><td>3.79(14)</td><td>0.79(1)</td></tr><tr><td>H2w1</td><td>12d</td><td>0.4449(14)</td><td>0.3544(14)</td><td>0.0969(36)</td><td>1</td><td>3.79(14)</td><td>0.73(1)</td></tr><tr><td>H1w2</td><td>12d</td><td>0.5304(10)</td><td>0.3598(10)</td><td>0.7715(45)</td><td>1</td><td>3.79(14)</td><td>0.73(1)</td></tr><tr><td>H2w2</td><td>12d</td><td>0.7715(91)</td><td>0.2564(89)</td><td>0.7535(56)</td><td>1</td><td>3.79(14)</td><td>1.08(9)</td></tr><tr><td>H1w3</td><td>12d</td><td>0</td><td>-0.1383(16)</td><td>0.0361(53)</td><td>0.5</td><td>3.79(14)</td><td>0.83(1)</td></tr><tr><td>H2w3</td><td>12d</td><td>0.1090(27)</td><td>-0.0456(25)</td><td>0.0481(75)</td><td>0.5</td><td>3.79(14)</td><td>0.96(2)</td></tr></tbody></table>
## 3.2. Structure description

**Figure 5a** illustrates the crystal structure of Li₃PO₃S·5H₂O, where P, S, and O1 atoms are in the Wyckoff position 6c, while O2 atoms are located in the Wyckoff position 12d, forming [PO₃S] tetrahedra. Li atoms occupy three different crystallographic sites. Li1 is located in the Wyckoff position 12d and is coordinated by O1, O2, and Ow1 (a water molecule), forming tetrahedral units [LiO₃(H₂O)]. Li2 and Li3 atoms occupy 4b and 2a sites, respectively, and are each surrounded by six water molecules, respectively, Ow2 and Ow3, forming octahedral units [Li(H₂O)₆]. Water molecules Ow1 and Ow2 fully occupy the 12d Wyckoff site, whereas Ow3 partially occupies the same crystallographic site. These water molecules are oriented so that hydrogen bonding is formed between H and O/S atoms (H-O: 1.83 Å and H-S: 2.42 Å). The structural model is consistent with the expected stoichiometry of Li₃PO₃S·5H₂O and is visualized along different crystallographic directions in **Figure 5**.
<figure><img src="image_12.png" /><figcaption>**Figure 5:** Projection view along the c axis of the unit cell content of Li₃PO₃S·5H₂O: P is gray, S is yellow, Li is green, O is red, and H is pink. (a) Visualization of the arrangement of [PO₃S] tetrahedral anionic sub-lattice. (b) Illustration of the tubular chain formed by corner-sharing of [PO₃S] and [LiO₃(H₂O)] running along [001] with H-bonding (dotted red line). (c) face sharing chain of [Li(H₂O)₆] within the tubular chains running along [001] (left), and face sharing chain of [Li(H₂O)₆] located between the tubular chains running along [001] (right).</figcaption></figure>

The centers of [PO₃S] tetrahedra units in the crystal structure of Li₃PO₃S·5H₂O are arranged in a (ABAB) stacking sequence along [001], with the P-S bonds aligned to the (110) plane in reverse orientations, forming a hexagon as shown in **Figure 5b**. [PO₃S] tetrahedra are connected one to another through two [LiO₃(H₂O)] tetrahedra, which spiral around 6₃ symmetry axes, forming circular chains along the [001] axis (**Figure 5c**). Moreover, a chain of face-sharing [Li(H₂O)₆] octahedral units coordinated by water molecules extends along the [001] axis within the tubular chains and forms H-bonds with the inner wall of these chains. These octahedra exhibit a rotational disorder, occupying one of two possible orientations related by a pseudo twofold rotation axis along the [001] axis, resulting in a disorder in the water molecules (**Figure 5d left**). In contrast, another chain of face-sharing [Li(H₂O)₆] octahedral units is located between the tubular chains and
maintains a single and well-defined orientation (Figure 5d right). The 3D framework of Li₃PO₃S·5H₂O is ensured through hydrogen bonding between the Li octahedral units, the inner and outer walls of the tubular chains formed by [PO₃S] and [LiO₃(H₂O)] units. This tendency of rotational disorder in the octahedra was observed before in the sodium oxoselenoantimonates Na₆(Sb₁₂O₁₈)(SbSe₃)₂(Na₁.₈₆Sb₁.₈₄)(OH)₂.₂₈(H₂O)₄.₀₂.³⁹ In this compound, the disorder of hydroxyl and water molecules coordinating Na ions within the [Sb₁₂O₁₈] circular chains was observed due to the pseudo twofold axis as discussed previously. Despite the discrepancies in their space groups (P6₃cm and P6₃/m, respectively) and chemistries, lithium oxythiophosphate and sodium oxoselenoantimonates share a similar concept of the tubular framework and orientationally disordered cationic octahedra.

The bond-valence sums (BVS) calculated from Rietveld analysis, using BONDSTR software, confirm that the P, Li, and H atoms adopt their expected oxidation states +5, +1, and +1, respectively (Table 1). The [PO₃S] tetrahedra are quite rigid units in which the distance of P-O ranges from 1.515(5) Å to 1.580(5) Å and P-S = 2.097(2) Å, in good agreement with bond distances and angles found in the case of Na₃PO₃S (P-O = 1.545(3) Å, P-S = 2.166(2) Å, S-P-O = 106.78(9)°, O-P-O = 121.02(8)°) (Table S3).

The P, Li, and H local structural environments within Li₃PO₃S·5H₂O were investigated using ³¹P, ⁷Li, and ¹H MAS ssNMR spectroscopy, respectively. The ³¹P MAS ssNMR spectrum shown in Figure 6a presents a well-defined peak at 32.35 ppm, indicating that phosphorus atoms occupy a single crystallographic site, consistent with the determined crystal structure. The minor peaks around 38.19 and 9.73 ppm, which amount to 6% and 3% of the signal, could be attributed to an amorphous dehydrated phase formed due to a partial water loss during sample drying in air and to the lithium phosphate Li₃PO₄ formed due to decomposition, respectively. In Figure 6b, the ⁷Li MAS ssNMR reveals a broad peak centered around 0.22 ppm, suggesting contributions from different Li environments in the Li₃PO₃S·5H₂O structure. Peak deconvolution showed a prominent large peak at -0.11 ppm and a secondary narrow peak at 0.24 ppm, attributed to the Li octahedral and tetrahedral sites, respectively.⁴⁰ An additional small peak around 3.90 ppm is detected, potentially associated with an amorphous phase of Li₃PO₃S·5H₂O. The ¹H MAS ssNMR spectrum of Li₃PO₃S·5H₂O is shown in Figure 6c, with a major peak at 4.70 ppm assigned to the bound water protons in Li₃PO₃S·5H₂O.⁴¹ These findings align with the previously discussed structural
model, as it contains coordinated water molecules. A small peak at 1.38 ppm is also observed and
could be attributed to residual ethanol from the precipitation process.
<figure><img src="image_13.png" /><figcaption>**Figure 6:** (a) <sup>31</sup>P MAS ssNMR spectrum of Li<sub>3</sub>PO<sub>3</sub>S·5H<sub>2</sub>O. (b) <sup>7</sup>Li MAS ssNMR spectrum of Li<sub>3</sub>PO<sub>3</sub>S·5H<sub>2</sub>O. The experimental (red line) and spectral deconvolution (blue, green, and brown lines) are also shown. (c) <sup>1</sup>H MAS ssNMR spectrum of Li<sub>3</sub>PO<sub>3</sub>S·5H<sub>2</sub>O.</figcaption></figure>

**3.3. Structural transitions upon dehydration**
**Figure 7a** and S5 illustrate temperature-dependent XRD patterns demonstrating the structural evolution of Li₃PO₃S·5H₂O upon heating between 20 °C and 600 °C, revealing a sequential phase transformation associated with dehydration and decomposition. Phase (I) is assigned to the room temperature XRD pattern of Li₃PO₃S·5H₂O. Upon heating, a phase transformation to phase (II) occurs at an early temperature of 60 °C as the peaks in the XRD pattern of phase (I) completely disappear and the formation of new broad peaks, mainly at 2θ values of 8.15°, 20.11°, and 35.20°. This phase transformation is likely attributed to the loss of water molecules, leading to the collapse of the initial structure of Li₃PO₃S·5H₂O, and is complete at 200°C. As the temperature increases, the XRD revealed that the newly formed phase remains stable up to 350 °C with a slight increase in the intensity of the broad peaks around 13.93° and 22.28°. Moreover, it should be noted that a detectable fraction of γ-Li₃PO₄ was already observed at 200 °C and remained constant up to around 320 °C. Beyond this temperature, the fraction of Li₃PO₄ began to increase significantly, until a complete transformation to the γ-Li₃PO₄ structure (III) at 350 °C, as indicated by its characteristic diffraction peaks, which could be explained by the decomposition of lithium monothiophosphate Li₃PO₃S into lithium phosphate γ-Li₃PO₄.
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

**Figure 7:** (a) Temperature-dependent XRD patterns of Li₃PO₃S·5H₂O acquired under nitrogen atmosphere, temperature range 20-600 °C. (b) Temperature *ex situ* <sup>31</sup>P MAS NMR of Li₃PO₃S·5H₂O, and samples annealed at 150 °C, 200 °C, 250 °C 350, and 400 °C.

**Figure 7b** shows a series of *ex situ* <sup>31</sup>P ssNMR spectra collected at different annealing temperatures (Pristine, 150, 200, 250, 350, and 400 °C) under an Ar atmosphere for six hours. The single <sup>31</sup>P chemical shift observed on the spectrum of the pristine sample broadens and splits upon annealing, accompanied by a small downfield shift of about 5-6 ppm. This signal confirms the presence of the (PO₃S)³⁻ anion. For monothiophosphates, chemical shifts are expected between 30-50 ppm, as reported by Lebedev et al.<sup>33</sup> The downfield shift and splitting of the single peak could be explained by the reorganization of phosphorus environments in the anhydrous Li₃PO₃S phase. A small signal corresponding to (PO₄)³⁻ species is also observed around 10 ppm, consistent with the small fraction of γ-Li₃PO₄ observed by XRD, that may begin forming prior to complete decomposition above 350 °C.
distribution of NMR signals measured between 150-350 °C (Figure 7b) and their evolution with
temperature.

Upon further heating beyond 400°C, Li₃PO₃S undergoes disproportionation, leading to the formation of Li₃PO₄, Li₃PS₄, and Li₄P₂S₆, consistent with the ³¹P ssNMR spectrum at 400°C. This decomposition is likely accompanied by sulfur evaporation at higher temperatures, as TGA indicated more weight loss between 400 and 600 °C. Figure S9 presents the Rietveld refinement of the XRD pattern obtained at 400 °C using the γ-Li₃PO₄ structural model, showing good agreement with the experimental data.

**3.4. Ionic transport properties**

AC impedance spectroscopy was used to measure the ionic conductivity of Li₃PO₃S·5H₂O and two
samples annealed at 200°C and 400°C, noted Li₃PO₃S-200 and Li₃PO₃S-400, respectively. The
samples were prepared as pellets by pressing powder at room temperature without any sintering.

**Figure 8a** and **b** show the Nyquist plot of impedance measured at room temperature for Li₃PO₃S·5H₂O and Li₃PO₃S-200, whereas the Nyquist plot for Li₃PO₃S-400 samples is shown in **Figure S10** in SI. All the samples exhibit a semicircle characteristic of a typical ionic conductor.

It is impossible to distinguish between the bulk and grain boundary impedance contributions to the ionic conductivity; therefore, the total resistance of the sample was used to determine the ionic conductivity. Except for Li₃PO₃S-400, the impedance data showed a significant contribution of the grain boundary impedance to the overall impedance, which was modeled by adding a second component composed of a resistance parallel to a constant-phase element. The EIS data were fitted using modified Debye equivalent circuits shown in the insets of the Nyquist plot for all samples.

Ion diffusion within the material is measured at high frequency, while the ion-blocking electrode contributes at low frequency, which allows for obtaining ionic conductivity. The room temperature ionic conductivity for Li₃PO₃S·5H₂O, Li₃PO₃S-200 and Li₃PO₃S-400 (γ-Li₃PO₄) determined from the total resistance were found to be: σ = 5.4 × 10⁻⁴ S cm⁻¹, σ = 2.6 × 10⁻⁶ S cm⁻¹ and σ = 5.6 × 10⁻⁹ S cm⁻¹, respectively. The AC impedance was performed over a range of temperatures between 20-100 °C for Li₃PO₃S-200 and Li₃PO₃S-400, and the ionic conductivity was extracted at each temperature point to determine activation energies: Eₐ = 0.51 eV and Eₐ = 0.91 eV for Li₃PO₃S-200 and Li₃PO₃S-400, respectively (Figure 8c).
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /><figcaption>**Figure 8:** Nyquist plots of the EIS measured at room temperature for: (a) $Li_3PO_3S \cdot 5H_2O$ (empty red circles), and (b) $Li_3PO_3S-200$ (empty blue circles). The fits of the impedance data using the equivalent electrical circuits (black line) are shown in the insets. CPE presents a Constant Phase Element, R is a resistance, and BE stands for blocking electrode. (c) Arrhenius plots of the two samples measured by AC impedance and the linear fit using the Arrhenius law (black line). Red, blue, and green filled circles correspond to $Li_3PO_3S \cdot 5H_2O$, $Li_3PO_3S-200$, and $Li_3PO_3S-400$ ($\gamma$-$Li_3PO_4$), respectively. (d) Voltage/current density vs time curves for the $Li/Li_3PO_3S/Li$ symmetric cell tests at 60 °C. Voltage (black) and current (red).</figcaption></figure>

A large ionic conductivity value is obtained for the hydrated $Li_3PO_3S \cdot 5H_2O$ phase. At this point, it is not possible to ascribe this conductivity to lithium or other charge carriers like protons from the water molecules. More advanced characterization and simulation would be needed to shed light on the exact diffusion mechanism. In any case, the presence of water molecules is not suitable for high-energy-density applications such as solid-state lithium batteries.
The decomposition of Li₃PO₃S into γ-Li₃PO₄ at 400 °C explains the low ionic conductivity of Li₃PO₃S-400. It should be noted that the room temperature conductivity value of γ-Li₃PO₄ obtained by this method (5.6 × 10⁻⁹ S cm⁻¹, 0.91 eV) is higher than that reported in the literature for γ-Li₃PO₄ (2 × 10⁻¹³ S cm⁻¹ at 30 °C, 1.1 eV) prepared by solid state reaction between (NH₄)₂HPO₄ and Li₂CO₃, and the activation energy is lower.⁴² Furthermore, the sample Li₃PO₃S-200, which corresponds to anhydrous Li₃PO₃S, exhibits an ionic conductivity (σ = 2.6 × 10⁻⁶ S cm⁻¹ at RT) which is three orders of magnitude higher than that of γ-Li₃PO₄ (Li₃PO₃S-400). Thus, substituting O by S in the Li₃PO₄ phase significantly improves the lithium ionic transport properties. This is consistent with the recent report by El-Shinawi et al. of an improvement in the ionic conductivity and activation energy of sulfur-doped γ-Li₃PO₄.⁴³

Finally, symmetric cell tests were performed for the lithium monothiophosphate Li₃PO₃S to check its ability to reversibly plate and strip Li and to evaluate the reductive stability against Li metal. The symmetric cell Li/Li₃PO₃S/Li was cycled at 60 °C due to the low ionic conductivity of Li₃PO₃S at room temperature. **Figure 8d** shows the expected behavior of increasing voltages and a stable voltage profile before reaching a current density of 0.2 µA cm⁻², where the polarization became too large. Using the polarization values of the symmetric cell, we calculated an effective conductivity of 2.7 × 10⁻⁸ S cm⁻¹, which is significantly lower than that measured by EIS at 60°C (1.8 × 10⁻⁵ S cm⁻¹). This suggests an additional contribution to the resistance of the symmetric cell which could come from the formation of a resistive interface with lithium. The stability of the voltage at a given current density further suggests that this interface is passivating. We did not observe the formation of dendrites at these current densities.

Several factors can contribute to the significant ionic conductivity difference between γ-Li₃PO₄ and Li₃PO₃S: the different crystal structure, the more polarizable P-S bond compared with P-O, and the volume per unit formula modification.⁴⁴ Despite these enhancements, the ionic conductivity of the newly synthesized phase Li₃PO₃S remains too low to be used in bulk solid-state batteries. This is expected from the full occupation of all Li sites within the structure, which hinders the migration of Li ions. Further improvement of the ionic conductivity of Li₃PO₃S could be achieved by aliovalent doping strategies to create vacancies or interstitial sites and facilitate lithium diffusion or amorphization of the bulk to increase lithium mobility within the material. Furthermore, Li₃PO₃S material could be used as an artificial protective layer on the cathode or the anode side with a highly conductive solid electrolyte in solid-state batteries.
4. Conclusion

In this work, a novel lithium oxythiophosphate ionic conductor Li₃PO₃S·5H₂O was prepared by a low-temperature route in an aqueous medium. Combining 3D ED, SPXRD, and NPD techniques, a complete structural model of the Li₃PO₃S·5H₂O phase was obtained. ³¹P NMR spectroscopy revealed that the phase is made exclusively of PO₃S units, in contrast with results from high-temperature synthesis. High temperature-dependent XRD showed a phase transition to the anhydrous Li₃PO₃S phase upon heating due to the loss of crystalline water as early as 60 °C and decomposition into Li₃PO₄ beyond 350 °C. Anhydrous Li₃PO₃S, annealed at 200 °C, exhibits a room temperature ionic conductivity of 2.6 × 10⁻⁶ S cm⁻¹ (Eₐ = 0.51 eV), which is three orders of magnitude higher than that of γ-Li₃PO₄. This work provides new insights for advancing the exploration and understanding of oxythiophosphate ionic conductors. This new family of materials with S/O mixed anions might lead to a good balance between ionic conductivity and chemical stability, both crucial properties for developing next-generation solid-state batteries. Further investigations are underway to complete the structural model of the anhydrous Li₃PO₃S phase and to understand Li diffusion within its structure. Furthermore, improving the ionic conductivity properties of Li₃PO₃S could benefit from synthetic strategies, such as aliovalent doping and amorphization to introduce disorder into the Li⁺ sub-lattice.
**Data availability**

The data supporting this article have been included as part of the Supplementary Information.

CCDC 2434639 and CCDC 2434640 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

**Conflict of interest**

There are no conflicts to declare.

**Acknowledgements**

The authors acknowledge Erik Elkaïm and the SOLEIL synchrotron facility for beamtime at the CRISTAL beamline (proposal number 20230995) and Emmanuelle Suard at the Institut Laue Langevin neutron scattering facility for preliminary experiments. This work has been partially funded by the CNRS Energy unit (Cellule Energie) through the project THIOBAT. J. B. acknowledges ED397 for PhD funding. GS thanks the CzechNanoLab Research Infrastructure supported by MEYS CR (LM2023051).

A CC-BY public copyright license has been applied by the authors to the present document and
will be applied to all subsequent versions up to the Author Accepted Manuscript arising from this
submission, in accordance with the grant's open access conditions.

**Keywords**

Oxythiophosphate, solid electrolyte, mixed anion, lithium-ion conductor, structure determination

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