Influence of Pressure on Structure, Stability and Ionic Conductivity of Lithium Argyrodite Solid Electrolytes

Influence of Pressure on Structure, Stability and Ionic Conductivity of Lithium Argyrodite Solid Electrolytes | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Influence of Pressure on Structure, Stability and Ionic Conductivity of Lithium Argyrodite Solid Electrolytes James Dawson, Jemma Cox, Jack Hemingway, James Quirk, Erli Lu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8404265/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 12 You are reading this latest preprint version Abstract The atomistic understanding of solid electrolytes is essential for advancing solid-state batteries and enabling their large-scale commercialisation. Whilst the importance of external pressure in the manufacturing and operation of practical solid-state batteries has been clearly recognised, its impact on the fundamental properties of solid electrolytes remains underexplored. Here, we combine density functional theory and ab initio molecular dynamics simulations to investigate the effect of pressure (0−10 GPa) on the structural, electronic and ion transport properties of lithium argyrodite solid electrolytes Li 6 PS 5 X (X = Cl and/or Br) with different levels of halide mixing and anion site (S 2 ⁻/X⁻) disorder. We show that increased pressure induces systematic decreases in lattice volume and stability for both ordered and disordered systems. Anion disorder narrows the bandgap, which can change dramatically as a function of pressure. The Li-ion conductivities (~ 0.54−1.05 S cm⁻ 1 at 600 K) and activation energies ( 2 GPa), most compositions show a clear increase in activation energy (0.15−0.42 eV) accompanied by reduced conductivity (~ 0.12−0.59 S cm⁻ 1 at 600 K). These findings establish external pressure, site disorder and halide mixing as interdependent parameters that can be used to tune argyrodite solid electrolytes for high-performance solid-state batteries. Physical sciences/Chemistry Physical sciences/Materials science Physical sciences/Physics Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 INTRODUCTION A key component of the transition to sustainable energy solutions is the development of safer and more efficient energy storage technologies. 1 – 5 One particularly promising advancement is the replacement of conventional organic liquid electrolytes in Li-ion batteries with solid electrolytes, resulting in the creation of solid-state batteries. These next-generation batteries could address the safety concerns associated with flammable liquid electrolytes and offer improvements in performance and a wider range of operating conditions. 6 – 11 Nevertheless, achieving high ionic conductivity at both the material and device scales remains a significant challenge for the widespread application of solid-state batteries. To date, only a limited number of solid electrolytes have demonstrated the superionic conductivity (~ 10 mS cm⁻ 1 ) required at room temperature for practical devices, most of which are sulfide based. 12 – 15 With Li-ion conductivities comparable to those of conventional liquid electrolytes, lithium argyrodites with the general formula Li 6 PS 5 X (X = Cl and/or Br) have gained considerable attention as promising solid electrolytes for solid-state batteries. 16 – 21 In addition to their highly promising Li-ion conductivity, Li 6 PS 5 X argyrodites are also notable for their degree of mechanical softness, which facilitates reduced interfacial resistance and improves processability. 22 , 23 To further enhance the Li-ion conductivity of argyrodite solid electrolytes, research has focused on controlling the degree of S 2− /X − site disorder, which has been shown to directly influence Li-ion diffusivity and, consequently, the material’s ionic conductivity. 24 – 28 Both experimental and computational studies indicate that increasing this site disorder lowers the activation energy barrier for Li-ion migration by altering the local anionic charge distribution and introducing charge inhomogeneity. 29 – 32 As a result, the Li-ion jump distances between adjacent cages decrease, leading to a more diffuse Li-ion distribution throughout the three-dimensional lattice, ultimately improving the Li-ion conductivity. 30 , 33 , 34 Additional studies have explored the impact of halide (Cl/Br) mixing on Li-ion transport in Li 6 PS 5 X. 35 – 38 These studies have revealed substantial enhancements in Li-ion conductivity compared to single-halide (Cl- or Br-only) argyrodites. For example, Kraft et al. reported that Li 6 PS 5 Cl 0.5 Br 0.5 presented an improved Li-ion conductivity due to its softer lattice. 36 However, the softer bonds resulting from increased halide polarisability were also found to decrease the prefactor of the moving ion, thereby resulting in the requirement for a delicate balance to achieve optimised Li-ion conductivity in these materials. The existence of these often intertwined mechanisms has led to significant debate over the factors that dominate Li-ion conductivity in argyrodite solid electrolytes. Furthermore, the vast number of possible atomic configurations arising from the complex distribution of Li cations and S, Cl and Br anions within their respective sublattices result in an increase in configurational entropy, which has been proposed as another cause of high Li-ion diffusivity. 39 – 42 The application of external pressure is another important consideration for solid electrolyte and solid-state battery design and optimisation. 43 As a key thermodynamic parameter, pressure can modify the crystal lattice, defect concentrations and phase stability, thereby altering ion dynamics. These structural changes, in turn, impact energy barriers for ion migration and the overall ionic conductivity. 44 – 46 However, despite the fact the real-world applications expose solid electrolytes and solid-state batteries to significant pressures and mechanical stresses during both assembly and operation, 46 their critical properties, including structure, stability and ionic conductivity, are often only assessed under ambient pressure conditions. Two important exceptions to this are the works of Faka et al., who explored the pressure dependence of Li-ion conductivity 46 and pressure-induced dislocations 45 in Li 6 PS 5 Br using a range of experimental techniques. Nevertheless, our understanding of pressure effects in argyrodite solid electrolytes (and solid electrolytes more broadly) remains limited, particularly at the atomistic scale. To rectify this omission, in this study, we investigate the impact of a range of external pressures (0–10 GPa) on the structural, electronic and ion transport properties of Li 6 PS 5 X (X = Cl and/or Br) with varying degrees of halide mixing and S 2 ⁻/X⁻ site disorder. Using density functional theory (DFT) and ab initio molecular dynamics (AIMD), we show that increasing pressure leads to systematic lattice compression, reduced thermodynamic stability, bandgap narrowing, lower Li-ion conductivities and higher activation energies. Nevertheless, anion disorder and Br substitution can be used to enhance lattice flexibility and preserve reasonable ionic conductivity under pressures of \(\:<\) 10 GPa, identifying them as key design parameters for pressure-resilient argyrodite solid electrolytes. By developing a comprehensive understanding of these interlinking factors, this work helps to guide the design of next-generation argyrodite solid electrolytes with enhanced performance and reliability across a diverse range of operating conditions. METHODS DFT calculations were performed using the projector-augmented wave (PAW) method, as implemented in the Vienna Ab Initio Simulation Package (VASP). 47 , 48 The Perdew-Burke-Ernzerhof for solids (PBEsol) generalized gradient approximation (GGA) was used to describe exchange-correlation effects. Given that the Li ions partially occupy the 48h site in Li 6 PS 5 X, a screening over the possible configurations was performed using pymatgen. 49 After enumerating the possible structures, 50 were selected through minimisation of their electrostatic energies and then fully relaxed using DFT. The configuration with the lowest energy was identified and used for subsequent simulations. A similar process was also applied for the generation of the mixed halide and anion site-disordered structures. All structural optimisations and energy calculations employed a plane-wave cutoff energy of 600 eV and Brillouin-zone integrations were performed using a single k -point mesh (Γ-point). Energy and force convergence criteria of 10⁻ 5 eV and 0.01 eV Å⁻ 1 , respectively, were used throughout. Structural optimisations were completed with the complete relaxation of atomic position, cell shape and cell volume, under applied pressures of 0, 1, 2, 3, 4 and 10 GPa. Density of states (DOS) calculations were performed using the HSE06 hybrid functional for a more accurate electronic structure. 50 The sumo Python toolkit was used for postprocessing of the DOS data. 51 AIMD simulations were performed in the canonical (NVT) ensemble, using the Nose-Hoover thermostat, to analyse a range of optimised structures incorporating varying degrees of anion site disorder and halide mixing as a function of pressure. The plane-wave energy cutoff was reduced to 400 eV. Given the substantial number of AIMD simulations required to consider the effects of pressure, halide mixing and anion site disorder, only a single Li 6 PS 5 X unit cell (52 atoms) was used. The simulations were conducted with a 2 fs timestep over a total duration of 200 ps at four temperatures (600−1050 K at 150 K intervals). Li-ion diffusion coefficients ( D Li ) were obtained from the mean squared displacement (MSD) of Li ions during the simulations: $$\:⟨{r}_{i}^{2}\left(t\right)⟩=6{D}_{\text{L}\text{i}}t$$ where \(\:⟨{r}_{i}^{2}\left(t\right)⟩\) represents the MSD and \(\:t\) is time. The Li-ion diffusion coefficients were subsequently converted into Li-ion conductivities ( σ ) using the Nernst−Einstein equation. Harmonic phonon calculations were conducted on the structures of both Li 6 PS 5 X (X = Cl or Br) with no S 2 ⁻/X⁻ site disorder at 0 and 10 GPa to explore the influence of pressure on the vibrational characteristics of the materials. The optimised geometries were further optimised for these calculations to an ionic force convergence of 0.001 eV Å ⁻ 1 with a 2×2×2 k-point mesh. The lattice dynamics calculations were completed using the finite differences approach, as implemented in PHONOPY. 52 The optimised geometries were expanded to 2×2×2 supercells, which were sampled on a proportionally smaller k-point mesh (Γ-point). Long range electrostatic interactions were accounted for using the non-analytical term correction, as implemented in PHONOPY, using the static dielectric constant tensor and born effective charges calculated using VASP. The phonon density of states ( \(\:\text{g}\) ( \(\:{\omega\:}\) )) was calculated on a 10×10×10 mesh of points. To explore the influence of pressure on the ionic \(\:\text{g}\) ( \(\:{\omega\:}\) ) spectra, the phonon band centre (PBC) was calculated using the following equation: $$\:PBC=\:\frac{\int\:\omega\:\times\:g\left(\omega\:\right).d\omega\:}{\int\:g\left(\omega\:\right).d\omega\:}$$ where ω represents a vibrational frequency and g(ω) represents either the total (all ion) or ion projected phonon density of states for the all ion or projected PBC, respectively. RESULTS AND DISCUSSION Local Structure and Stability The crystal structure of Li 6 PS 5 X (X = Cl and/or Br) adopts a face-centred cubic lattice, belonging to the F 4̅3 m space group. 2 6 Fig. 1 (a) illustrates the fully ordered unit cell of Li 6 PS 5 X, in which halide ions are located at the Wyckoff 4a positions, while S 2 ⁻ anions occupy the Wyckoff 4d positions. As discussed above, S 2 ⁻ and X⁻ anions can exhibit site disorder by exchanging positions and occupying both 4a and 4d sites. The unit cell of Li 6 PS 5 X contains four distinct Li + ‘cages’, with Li ions distributed across 48h sites, each centred around an S 2 ⁻ anion at the 4d site, as shown in Fig. 1 (b). 2 6 , 3 3 To capture the role of anionic disorder, we compare fully ordered configurations (0% S 2 ⁻/X⁻ site exchanged) with partially disordered ones in which three of the four Li + cages per unit cell have their central S 2 ⁻ exchanged with X⁻ (75% S 2 ⁻/X⁻ site exchanged). Previous experimental studies and simulations have shown that such a degree of anion disorder directly influences Li-ion mobility, by expanding Li + cages, reducing inter-cage jump distances, and enhances conductivity by creating more connected diffusion pathways. 29 , 33 We have therefore focused on these two configurations, fully ordered (0%) and partially disordered (75%), as structural limits for assessing how pressure influences the fundamental properties of argyrodites as solid electrolytes. We first examined the structural response of Li 6 PS 5 Cl, Li 6 PS 5 Br, Li 6 PS 5 Cl 0.25 Br 0.75 , Li 6 PS 5 Cl 0.50 Br 0.50 and Li 6 PS 5 Cl 0.75 Br 0.25 to external pressure for both the 0% and 75% site exchanged structures, as shown in Fig. 2 (a). Across all pressures, the lattice parameters follow a clear compositional trend governed by the ionic radius of the halide anion. As Br⁻ (1.96 Å) is larger than Cl⁻ (1.81 Å), 53 Li 6 PS 5 Br exhibits the largest lattice parameter at all pressures, while Li 6 PS 5 Cl shows the smallest. The mixed halide compositions lie between these two endmembers, with their lattice parameters scaling smoothly with Br/Cl ratio. As previously reported for Li 6 PS 5 Br, increasing anion disorder results in reduced unit cell volumes/lattice parameters as a result of the more homogeneous charge distribution of Li ions, leading to contraction of the unit cell due to electrostatic interactions. 46 Examination of the pressure-dependent lattice parameters of Li 6 PS 5 X (X = Cl and/or Br) reveals a clear contraction with increasing pressure, in agreement with the in-situ X-ray diffraction data of Faka et al. for Li 6 PS 5 Br, who observed reversible compression without phase transitions up to 10 GPa. 45 For the fully ordered structures, the cubic lattice parameters decrease from 9.87 Å (X = Cl), 9.96 Å (X = Br) and 9.90 Å (X = Cl 0.5 Br 0.5 ) at 0 GPa to 9.12 Å (X = Cl), 9.21 Å (X = Br) and 9.19 Å (X = Cl 0.5 Br 0.5 ) at 10 GPa. The partially disordered (75% site exchanged) structures show a comparable continuous contraction, with lattice parameters decreasing from 9.80 Å (X = Cl), 9.89 Å (X = Br) and 9.84 Å (X = Cl 0.5 Br 0.5 ) at 0 GPa to 9.06 Å (X = Cl), 9.13 Å (X = Br) and 9.10 Å (X = Cl 0.5 Br 0.5 ) at 10 GPa. The magnitudes of these pressure-induced reductions are similar regardless of the halide nature, which reflects the similar bulk moduli of 28.7 and 29.0 for Li 6 PS 5 Cl and Li 6 PS 5 Br, respectively, calculated from DFT simulations. 54 Such reductions in lattice parameters are expected to have a significant impact on the migration volume for Li ions and therefore the overall Li-ion conductivity. 46 Figure 2 (b) presents the calculated formation energies of Li 6 PS 5 X (X = Cl and/or Br) from their respective binary precursors (i.e., LiX (X = Cl or Br), Li 2 S and P 2 S 5 ) as a function of pressure for both ordered (0%) and partially disordered (75%) structures. All compositions exhibit negative formation energies below 5 GPa, indicating reasonable stability under moderate compression. Although the formation energy of Li 6 PS 5 Br is more exothermic at 0 GPa, it increases slightly more rapidly with pressure compared to the Cl-rich argyrodites, i.e., an average increase 0.520 eV/GPa for Br compared to 0.505 eV/GPa for Cl in the fully ordered structures. This trend becomes more pronounced in the disordered systems (75%), where the rates rise to 0.568 eV/GPa for Br and 0.545 eV/GPa for Cl. These results show that the Cl-rich systems exhibit a smaller pressure-induced destabilisation compared to the Br-rich compositions, consistent with their stiffer bonding environment, stronger electrostatic interactions and lower polarisability. 55 , 56 Radial distribution functions (RDFs) are plotted in Fig. 3 to provide complementary local structural insights into the effect of pressure on Li 6 PS 5 X (X = Cl and/or Br). At ambient pressure (0 GPa), our calculated RDFs for both the 0% and 75% site exchanged systems are in excellent agreement with previous AIMD studies. 34 For the Li–Li RDFs, a broad distribution of Li–Li separations are observed above 3 Å. The primary peaks in the Li–Li RDFs are smaller and broader in anion site exchanged systems, indicative of a greater range of site–site separations and a more disordered lithium sublattice. In contrast, the Li–anion RDFs present single sharp primary peaks at 2.2–2.9 Å depending on the halide identity and level of site disorder. With increasing pressure, the peaks in each RDF all shift to shorter distances; however, the magnitudes of these shifts differ considerably depending on the element pair and composition. For the Li–Li pair, the RDFs for each system at 0, 1 and 3 GPa are remarkably commensurate with each other. In contrast, at 10 GPa, there is a clear change with a substantial reduction in the Li–Li separation distance of the main peak. This reduction is also more significant for Li 6 PS 5 Br compared to Li 6 PS 5 Cl. This change at 10 GPa reflects both the significant lattice compression and more compact Li sublattice. Li–Li separation is well known to have a strong influence on fast Li-ion transport in Li-ion conductors. 57 , 58 In previous molecular dynamics simulations of - and \(\:{\beta\:}\) -Li 3 PS 4 , similar reductions in the Li–Li separation of the main RDF peaks were found to directly contribute to inferior Li-ion transport, in agreement with our current findings for Li 6 PS 5 X at 10 GPa. A similar phenomenon is also observed for the Li–X RDFs, where there is a significant shift in the peaks to shorter distances with increasing pressure, particularly for the 0% site exchanged systems. At 10 GPa, this shift results in the distance of the main Li–X peak becoming similar or even shorter than for the main Li–S peaks. Such behaviour is also typically observed with a reduction in temperature and the resulting increase in local ordering and decrease in Li-ion conductivity. The Li–X RDFs allow for direct comparison between chloride and bromide coordination environments. Quantitatively, the first peak positions confirm that Li–Br distances are consistently longer than Li–Cl under ambient conditions (2.67 Å for Br and 2.49 Å for Cl at 0 GPa). Under maximum compression (10 GPa), both coordination shells contract, but the Li–Br environment exhibits a larger magnitude of contraction (2.67 to 2.43 Å, Δ = − 0.24 Å) compared to Li–Cl (2.49 to 2.34 Å, Δ = − 0.15 Å), reflecting a softer and more polarisable Br – coordination shell. This softer environment compresses more readily, illustrative of a larger response of Br-rich systems to applied pressure. Interestingly, and as discussed above, this increased rate of contraction with pressure for Li–Br compared to Li–Cl distance is not reflected in the calculated lattice parameters in Fig. 2 (a). This suggests that the influence of pressure is not uniform for both average and local structure in lithium argyrodites. In contrast to Li–Li and Li–X, the effect of pressure on Li–S distances is more subtle, with only a minor shift to shorter distances, even at 10 GPa. Overall, the above analysis demonstrates that halide chemistry and anion disorder jointly control the pressure stability of Li 6 PS 5 X. Cl-rich compositions are more rigid and cohesive, whereas Br-rich systems are softer and more responsive to external pressure. These distinctions underpin the pressure-dependent electronic and Li-ion transport behaviour discussed in the following sections. Electronic Structure Even though electronic structure is directly connected to electrochemical stability, electronic conduction and resistance to dendrite formation, the influence of external pressure on the electronic structure and bandgaps of solid electrolytes for solid-state batteries has rarely been considered in literature. To understand how pressure affects the electronic structures of Li 6 PS 5 X (X = Cl or Br), we calculated the projected electronic density of states (DOS) at the 0% and 75% site exchanged systems as function of pressure, as shown in Figures S1 and 4, respectively. The DOS plots for both Li 6 PS 5 Cl and Li 6 PS 5 Br reveal that the valence band maximum is primarily composed of S 3p orbitals, while the conduction band minimum is dominated by P 3s and 3p states across the entire pressure range. 59 The calculated bandgaps for Li 6 PS 5 X (X = Cl and/or Br) with 0% and 75% site exchange as a function of pressure are presented in Fig. 5 . For the anion site ordered compositions (0% site exchanged), the bandgaps occupy a narrow range between 3.19 eV (Li 6 PS 5 Cl 0.25 Br 0.75 ) and 3.27 eV (Li 6 PS 5 Br) at 0 GPa, in agreement with previous DFT studies. 59 , 60 For the anion site disordered compositions (75% site exchanged), the bandgaps are remarkably similar (2.85−2.87 eV) at 0 GPa but somewhat smaller than the values obtained for the ordered compositions (3.19−3.27 eV). Given that bandgaps can be used as upper limits of the electrochemical stability window, it is noteworthy that anion site disordering may reduce the stability of these argyrodite solid electrolytes. 56 , 61 These values show that the halide composition in these argyrodites has minimal impact on the bandgap and therefore their electrochemical stability window. 62 As shown in Fig. 5 (a), with increasing pressure, the bandgaps of anion site ordered Li 6 PS 5 X (X = Cl and/or Br) show a clear decreasing trend. Nevertheless, these decreases are relatively modest (~ 0.2−0.5 eV) and are therefore not expected to substantially impact the stability or electronic conductivity of the materials. In contrast, the disordered compositions do not show a clear single trend with increasing external pressure (Fig. 5 (b)). For the Br-containing systems, a notable reduction in bandgap of ~ 0.4 eV occurs at pressures of 4 GPa. The pressure at which this reduction occurs is different depending on the Br content. Furthermore, it is important to bear in mind that the electronic structure will depend on the particular cation and anion ordering in each composition, which, to some extent, increases variability in the calculated band gaps. This aligns with the experimental findings of Minafra et al., 33 which show that anion disorder broadens Li + distributions and introduces charge inhomogeneity. At the maximum calculated pressure of 10 GPa, the bandgaps of the Br-containing systems increase by 0.2−0.7 eV, possibly suggestive of a stabilising effect at high pressures. In the case of Li 6 PS 5 Cl, the bandgap remains relatively constant as a function of pressure, with only a small ~ 0.2 eV at 10 GPa. This result highlights the excellent performance of Li 6 PS 5 Cl as a solid electrolyte for practical solid-state batteries. The narrowing of band gaps is an important consideration in the development of solid electrolytes as these fast ionic conductors must also have poor electronic conductivity, i.e., wide band gaps. Whilst the bandgaps calculated here are still too large to any significant levels of electronic conduction, this may not be the case for the grain boundaries of argyrodites, especially when under external pressure. It has been shown that grain boundaries in solid electrolytes can lead to reductions in the bandgap, sometimes with the appearance of highly localised trap states for electrons or holes on ions which are coordinated differently than in the bulk. 63 This is an important consideration for solid electrolyte design as the combined effect of pressure and grain boundaries may lead to significantly narrowed bandgaps with the potential for detrimental electronic conductivity and dendrite initiation. Li-Ion Transport Figure 7. (a) Pressure dependence of Li-ion conductivity at 600 K for Li 6 PS 5 X (X = Cl and/or Br) with 75% S 2 ⁻/X⁻ site exchange. Insufficient Li-ion diffusion events were observed for the 0% S 2 ⁻/X⁻ site exchanged systems at 600 K so the corresponding conductivities could not be calculated. (b) Pressure dependence of Li-ion activation energy ( E a ) for Li 6 PS 5 X (X = Cl or Br) with 0% and 75% S 2 ⁻/X⁻ site exchange. The equivalent activation energies ( E a ) for Li 6 PS 5 Cl 0.5 Br 0.5 , Li 6 PS 5 Cl 0.75 Br 0.25 and Li 6 PS 5 Cl 0.25 Br 0.75 are given in Figure S2. To elucidate the effects of pressure, halide chemistry and anion disorder on Li-ion transport, temperature-dependent Li-ion conductivities were derived from the AIMD simulations. Figure 6 presents Arrhenius plots of the Li-ion conductivity for Li 6 PS 5 X (X = Cl and/or Br) with 75% S 2 ⁻/X⁻ site exchange under pressures of 0–10 GPa. At ambient pressure (0 GPa), all compositions exhibit high Li-ion conductivities (~ 0.54−1.05 S cm⁻ 1 at 600 K) and low activation energies in the range of 0.14–0.16 eV, consistent with the superionic behaviour observed experimentally for argyrodites. 33 , 64 – 67 Among the halides, Li 6 PS 5 Cl shows the lowest E a (~ 0.14 eV), marginally below that of Li 6 PS 5 Br (~ 0.15 eV), reflecting the more compact Cl-framework that facilitates Li + hopping through interconnected 48h sites via transient occupation of 24g positions. 16 , 19 , 20 , 33 Upon compression, however, conductivity decreases monotonically across all systems, accompanied by a corresponding increase in E a to 0.22–0.26 eV at 10 GPa. The steeper rise of E a in Li 6 PS 5 Cl relative to Li 6 PS 5 Br indicates that Li + migration in Cl-rich compositions is more sensitive to lattice densification. Bromide substitution mitigates this effect through enhanced lattice softness, which allows local structural relaxation and partial preservation of the percolating Li + conduction network under pressure. 36 , 68 The mixed-halide systems display intermediate behaviour, following the same qualitative trend of conductivity suppression and barrier growth with increasing pressure. However, compositions containing higher Br fractions (Li 6 PS 5 Cl 0.5 Br 0.5 and Li 6 PS 5 Cl 0.25 Br 0.75 ) exhibit the highest pressure resilience, suggesting a synergistic balance between the strong electrostatic framework of Cl⁻ and the softer, more compliant bonding introduced by Br⁻. This trend mirrors the mechanical compliance discussed earlier, where Br-rich frameworks accommodated strain more effectively than Cl-rich analogues. Figure 7 summarises the pressure dependence of conductivity (σ) and activation energy ( E a ) for Li 6 PS 5 X (X = Cl and Br) with 0% and 75% site exchange. For both Li 6 PS 5 Cl and Li 6 PS 5 Br, introducing 75% S 2 ⁻/X⁻ site exchange significantly lowers E a relative to the fully ordered (0%) structures, resulting in higher conductivities over the entire pressure range. This behaviour reflects the established role of anion disorder in broadening the distribution of accessible Li sites and enabling more three-dimensional, energetically favourable migration pathways. 29 , 31 , 33 , 34 , 46 At the atomistic level, Li-ion diffusion in argyrodites proceeds through hops between 48h cages via transient occupations of 24g sites, forming a three-dimensional percolation network. 16 , 20 , 27 , 29 , 34 Anion disorder (75% S 2 ⁻/X⁻ site exchanged) enhances this connectivity by perturbing local charge distributions and creating multiple low-energy pathways, while applied pressure acts oppositely by narrowing the bottlenecks between cages. The overall transport response therefore arises from a balance between disorder-induced pathway diversification and pressure-driven lattice densification. Exploration of the lattice dynamics properties for Li 6 PS 5 X (X = Cl or Br) at 0 and 10 GPa revealed entirely real vibrational frequencies for each structure suggesting the pressurised optimised structures are stable states (phonon dispersion plots are shown in Figure S3). The application of pressure leads to general vibrational hardening of modes, reflected in both the g(ω) spectra, as well as the total (all ion) and ion projected PBC values. The g(ω) for each material are shown in Fig. 8 , with the PBC values shown by dashed vertical lines, which are connected across the two pressure values for each Li 6 PS 5 X composition. It is clear from the PBC values that there is a general shift upwards in vibrational frequency (hardening) for each ionic species (and therefore also the all-ion PBC), with the most significantly upshifted species being the Li ions for both Li 6 PS 5 X (X = Cl or Br). Vibrational hardening of phonons under pressure is most likely a result of a decrease in interatomic distances leading a stiffer lattice and an increase in restoring force of vibrations, and has been widely reported for a number of different materials and molecular solids. 69 – 71 The Li projected PBC has been correlated to some success with ionic conductivity for a number of Li-ion conductors, with lower PBC (soft) lattices typically exhibiting increased Li conductivity as the vibrations are more easily excited and assist with ionic migration. 72 – 74 The increase in Li-ion PBC as a result of the application of pressure for the Li 6 PS 5 X (X = Cl or Br) materials considered in this work is in agreement with the calculated decrease in σ, further consolidating the negative influence of pressure on Li-ion conductivity in argyrodites. Our results demonstrate that Li-ion transport in Li 6 PS 5 X is highly sensitive to external pressure, with both halide chemistry and anion disorder governing the extent of this response. Cl-rich compositions show lower activation barriers under ambient conditions but a greater loss of conductivity under higher pressures (3 GPa), indicating a stronger sensitivity to densification. In contrast, Br-analogues, particularly those with substantial disorder (75%), exhibit a more resilient transport response, consistent with the greater chemical softness and structural compliance of the Br-substituted lattice. In addition to the negative influence of pressure on Li-ion conductivity observed in this work, it has been previously shown that external pressure can be used to induce dislocations in Li 6 PS 5 Br, which can actually enhance its Li-ion conductivity. 45 These findings therefore emphasise that pressure, halide substitution and anion disorder are interdependent design parameters for optimising argyrodite solid electrolytes capable of sustaining high ionic conductivity under mechanical stress and internal strain. CONCLUSIONS The understanding of pressure effects in solid electrolytes is essential for their large-scale utilisation in solid-state batteries. Despite this fact, the role of pressure in these materials has been so far underexplored, particularly using atomistic modelling. In this work, we have investigated how pressure influences the structure, electronic properties and Li-ion conductivity in the lithium argyrodite family Li 6 PS 5 X (X = Cl and/or Br) and considered its intricate interplay with halide mixing and S 2 ⁻/X⁻ anion disorder at the atomic scale. The key results are summarised as follows: All Li 6 PS 5 X (X = Cl and/or Br) compositions remain structurally coherent under compression up to 10 GPa, with continuous lattice contraction and no evidence of phase transitions. The impact of pressure on the average and local structure is not uniform. For example, while the magnitude in the reduction of the lattice parameter with pressure is similar for all Li 6 PS 5 X compositions, the primary Li–Br distance decreases more substantially with increasing pressure than the primary Li–Cl distance. Electronic structure analysis indicates wide bandgaps (2.8–3.3 eV) that are largely insensitive to halide composition. Anion disorder slightly narrows the bandgap and pressure induces modest, composition-dependent variations that are not expected to significantly affect electrochemical stability. Li-ion conductivities remain high (0.54−1.05 S cm⁻ 1 at 600 K) and activation energies low (~ 0.14–0.16 eV) up to \(\:\sim\) 2 GPa, confirming robust Li-ion transport pathways under moderate compression. At higher pressures, Li-ion conductivities decrease and activation energies increase, with Br-rich and partially disordered structures showing the strongest resistance to pressure-induced transport degradation owing to their greater lattice flexibility and polarisability. From a design perspective, our results demonstrate that the interplay of pressure, halide chemistry and anion disorder critically governs the key properties of argyrodites as solid electrolytes, including Li-ion conductivity and electrochemical stability. These insights help to rationalise experimental observations of pressure-sensitive Li-ion transport in Li 6 PS 5 X and establish practical design rules for engineering solid electrolytes that maintain high performance in solid-state batteries under external pressure constraints. Declarations Funding Declaration This work was supported by the Engineering and Physical Sciences Research Council (EPSRC, EP/S023836/1) as part of the ReNU CDT. J.M.H, E.L. and J.A.D. acknowledge the Leverhulme Trust for financial support (RPG-2022-231. J.A.Q. and J.A.D. acknowledge the Faraday Institution (FIRG026) for funding. J.A.D. also acknowledges UK Research and Innovation (UKRI) under the UK government’s Horizon Europe funding guarantee (EP/z000254/1). Via membership of the U.K.’s HEC Materials Chemistry Consortium, which is funded by the EPSRC (EP/X035859/1), this work used the A2RCHER2 UK National Supercomputing Service. Author Contribution J.L.C. and J.M.H. performed all simulations with supervision from J.A.D. J.A.D. designed the research programme. All authors contributed to the writing and revision of the manuscript. Acknowledgement This work was supported by the Engineering and Physical Sciences Research Council (EPSRC, EP/S023836/1) as part of the ReNU CDT. J.M.H, E.L. and J.A.D. acknowledge the Leverhulme Trust for financial support (RPG-2022-231. J.A.Q. and J.A.D. acknowledge the Faraday Institution (FIRG026) for funding. J.A.D. also acknowledges UK Research and Innovation (UKRI) under the UK government’s Horizon Europe funding guarantee (EP/z000254/1). Via membership of the U.K.’s HEC Materials Chemistry Consortium, which is funded by the EPSRC (EP/X035859/1), this work used the A2RCHER2 UK National Supercomputing Service. Data Availability All data supporting the findings of this study are available within the paper and its Supplementary Information. References G. De Carne, S. M. Maroufi, H. 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Ohno, O. Delaire, Y. Shao-Horn and W. G. Zeier, J Am Chem Soc , 2018, 140, 14464–14473. Additional Declarations No competing interests reported. Supplementary Files AgroSI.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 06 Mar, 2026 Reviews received at journal 25 Jan, 2026 Reviews received at journal 21 Jan, 2026 Reviews received at journal 15 Jan, 2026 Reviewers agreed at journal 14 Jan, 2026 Reviewers agreed at journal 12 Jan, 2026 Reviewers agreed at journal 12 Jan, 2026 Reviewers agreed at journal 12 Jan, 2026 Reviewers invited by journal 12 Jan, 2026 Editor assigned by journal 10 Jan, 2026 Submission checks completed at journal 23 Dec, 2025 First submitted to journal 19 Dec, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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08:05:13","extension":"png","order_by":27,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":3976,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8404265/v1/a5371d81ac1e3fb6b63bdf9e.png"},{"id":100218277,"identity":"37bf330a-65dc-4d48-9d19-c7e02eceb87c","added_by":"auto","created_at":"2026-01-14 09:00:53","extension":"png","order_by":28,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":4317,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8404265/v1/d05363155e2959c46fa71f28.png"},{"id":100370149,"identity":"f8780731-689b-428f-bcda-b8e6df1cbbb6","added_by":"auto","created_at":"2026-01-16 08:00:08","extension":"png","order_by":29,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":935,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8404265/v1/dbc41f53ee41048d77b1a6dd.png"},{"id":100218275,"identity":"126fa687-fdfa-4d47-9e6e-a410ca0def2a","added_by":"auto","created_at":"2026-01-14 09:00:53","extension":"xml","order_by":30,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":153932,"visible":true,"origin":"","legend":"","description":"","filename":"b8984baad575468982c96027d316a8e71structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8404265/v1/64d1b66a8a3e7d9e7b80aaa4.xml"},{"id":100218281,"identity":"5fd3b5be-2ba6-4f33-b5d5-d457598c5c54","added_by":"auto","created_at":"2026-01-14 09:00:53","extension":"html","order_by":31,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":165680,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8404265/v1/f10b0095c947c1df714bdcbd.html"},{"id":100218241,"identity":"c3bd1e62-ee7c-441b-86d6-c414c5f512da","added_by":"auto","created_at":"2026-01-14 09:00:52","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":63843,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Fully ordered unit cell of Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X = Cl or Br). (b) Crystallographic description of the lithium-ion cages around the centrally positioned S\u003csup\u003e2\u003c/sup\u003e⁻.\u003c/p\u003e","description":"","filename":"Picture1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8404265/v1/e56c628016eebb5e026075ee.jpg"},{"id":100369994,"identity":"cfbc9533-b15b-44eb-ab2f-da719f016e1d","added_by":"auto","created_at":"2026-01-16 07:59:43","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":76209,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Unit cell volume and (b) formation energy for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X = Cl and/or Br) with 0% and 75% S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ site exchange as a function of pressure. The formation energies were calculated with respective to the relevant binary precursors, i.e., LiX (X = Cl or Br), Li\u003csub\u003e2\u003c/sub\u003eS and P\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e5\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"Picture2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8404265/v1/199019250af4476733d1e543.jpg"},{"id":100218243,"identity":"d91c1472-2e4d-40bb-913a-9dfb90016870","added_by":"auto","created_at":"2026-01-14 09:00:52","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":65672,"visible":true,"origin":"","legend":"\u003cp\u003eRadial distribution functions, \u003cem\u003eg(r),\u003c/em\u003e for Li–Li, Li–S and Li–X (X = Cl or Br) in Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X = Cl or Br) at 600 K under pressures of 0, 1, 3 and 10 GPa. Data are time averaged from production AIMD trajectories.\u003c/p\u003e","description":"","filename":"Picture3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8404265/v1/8e6947d5ff86a8abfe3c5eb6.jpg"},{"id":100369796,"identity":"4725cf61-f1cf-4e01-a6e6-1329f6900cac","added_by":"auto","created_at":"2026-01-16 07:59:29","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":87018,"visible":true,"origin":"","legend":"\u003cp\u003eProjected electronic density of states (DOS) for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X = Cl or Br) with 75% S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ site exchange as a function of pressure. Rows show the DOS at increasing pressures of 0, 1, 2, 3, 4 and 10 GPa (from top to bottom). The valence band maximum is set to 0 eV.\u003c/p\u003e","description":"","filename":"Picture4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8404265/v1/cab250c3c3fd6b235f4e378f.jpg"},{"id":100218250,"identity":"ffe0260e-7f5b-4a80-9a54-d93cc7325dad","added_by":"auto","created_at":"2026-01-14 09:00:52","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":59037,"visible":true,"origin":"","legend":"\u003cp\u003eBandgaps for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X = Cl and/or Br) with (a) 0% and (b) 75% S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ site exchange as a function of pressure.\u003c/p\u003e","description":"","filename":"Picture5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8404265/v1/b2c908f64e9c1ffbc53508a9.jpg"},{"id":100218253,"identity":"41f6962e-05fd-4118-88c0-d76c51c9da28","added_by":"auto","created_at":"2026-01-14 09:00:52","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":52870,"visible":true,"origin":"","legend":"\u003cp\u003eArrhenius plots of Li-ion conductivity for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX with 75% S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ site exchange as a function of pressure. \u0026nbsp;\u003c/p\u003e","description":"","filename":"Picture6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8404265/v1/f293e975e00f3f43ea160ddf.jpg"},{"id":100371343,"identity":"94c5fa12-801a-46d8-a2ca-2ad64ab72f30","added_by":"auto","created_at":"2026-01-16 08:09:53","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":45860,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Pressure dependence of Li-ion conductivity at 600 K for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X = Cl and/or Br) with 75% S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻\u003csup\u003e \u003c/sup\u003esite exchange. Insufficient Li-ion diffusion events were observed for the 0% S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ site exchanged systems at 600 K so the corresponding conductivities could not be calculated. (b) Pressure dependence of Li-ion activation energy (\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e) for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X = Cl or Br) with 0% and 75% S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻\u003csup\u003e \u003c/sup\u003esite exchange. The equivalent activation energies (\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e) for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl\u003csub\u003e0.5\u003c/sub\u003eBr\u003csub\u003e0.5\u003c/sub\u003e, Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl\u003csub\u003e0.75\u003c/sub\u003eBr\u003csub\u003e0.25\u003c/sub\u003e and Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl\u003csub\u003e0.25\u003c/sub\u003eBr\u003csub\u003e0.75 \u003c/sub\u003eare given in Figure S2.\u003c/p\u003e","description":"","filename":"Picture7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8404265/v1/092259729f03c6b282bb19c6.jpg"},{"id":100218247,"identity":"7f702441-097a-4870-8d6f-a72f8a712032","added_by":"auto","created_at":"2026-01-14 09:00:52","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":244577,"visible":true,"origin":"","legend":"\u003cp\u003eg(w) spectra for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X = Cl or Br) with 0% S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ site exchange at 0 and 10 GPa. PBC values are denoted by dashed vertical lines.\u003c/p\u003e","description":"","filename":"Picture8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8404265/v1/bc3752d3f0249772569603fd.jpg"},{"id":100383630,"identity":"b3e72be6-5d7c-4cda-a8ea-c7cd0326d284","added_by":"auto","created_at":"2026-01-16 10:47:45","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1374430,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8404265/v1/15d0d0aa-afb0-429e-a7e9-d09dd5674260.pdf"},{"id":100369792,"identity":"53d70774-75c8-48da-b825-29a2a849919c","added_by":"auto","created_at":"2026-01-16 07:59:29","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":2418027,"visible":true,"origin":"","legend":"","description":"","filename":"AgroSI.docx","url":"https://assets-eu.researchsquare.com/files/rs-8404265/v1/48e347962c3029e447bc246a.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Influence of Pressure on Structure, Stability and Ionic Conductivity of Lithium Argyrodite Solid Electrolytes","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eA key component of the transition to sustainable energy solutions is the development of safer and more efficient energy storage technologies.\u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3 CR4\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e One particularly promising advancement is the replacement of conventional organic liquid electrolytes in Li-ion batteries with solid electrolytes, resulting in the creation of solid-state batteries. These next-generation batteries could address the safety concerns associated with flammable liquid electrolytes and offer improvements in performance and a wider range of operating conditions.\u003csup\u003e\u003cspan additionalcitationids=\"CR7 CR8 CR9 CR10\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e Nevertheless, achieving high ionic conductivity at both the material and device scales remains a significant challenge for the widespread application of solid-state batteries. To date, only a limited number of solid electrolytes have demonstrated the superionic conductivity (~\u0026thinsp;10 mS cm⁻\u003csup\u003e1\u003c/sup\u003e) required at room temperature for practical devices, most of which are sulfide based.\u003csup\u003e\u003cspan additionalcitationids=\"CR13 CR14\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eWith Li-ion conductivities comparable to those of conventional liquid electrolytes, lithium argyrodites with the general formula Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl and/or Br) have gained considerable attention as promising solid electrolytes for solid-state batteries.\u003csup\u003e\u003cspan additionalcitationids=\"CR17 CR18 CR19 CR20\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e In addition to their highly promising Li-ion conductivity, Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX argyrodites are also notable for their degree of mechanical softness, which facilitates reduced interfacial resistance and improves processability.\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e To further enhance the Li-ion conductivity of argyrodite solid electrolytes, research has focused on controlling the degree of S\u003csup\u003e2\u0026minus;\u003c/sup\u003e/X\u003csup\u003e\u0026minus;\u003c/sup\u003e site disorder, which has been shown to directly influence Li-ion diffusivity and, consequently, the material\u0026rsquo;s ionic conductivity.\u003csup\u003e\u003cspan additionalcitationids=\"CR25 CR26 CR27\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e Both experimental and computational studies indicate that increasing this site disorder lowers the activation energy barrier for Li-ion migration by altering the local anionic charge distribution and introducing charge inhomogeneity.\u003csup\u003e\u003cspan additionalcitationids=\"CR30 CR31\" citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e As a result, the Li-ion jump distances between adjacent cages decrease, leading to a more diffuse Li-ion distribution throughout the three-dimensional lattice, ultimately improving the Li-ion conductivity.\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eAdditional studies have explored the impact of halide (Cl/Br) mixing on Li-ion transport in Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX.\u003csup\u003e\u003cspan additionalcitationids=\"CR36 CR37\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e These studies have revealed substantial enhancements in Li-ion conductivity compared to single-halide (Cl- or Br-only) argyrodites. For example, Kraft et al. reported that Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl\u003csub\u003e0.5\u003c/sub\u003eBr\u003csub\u003e0.5\u003c/sub\u003e presented an improved Li-ion conductivity due to its softer lattice.\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e However, the softer bonds resulting from increased halide polarisability were also found to decrease the prefactor of the moving ion, thereby resulting in the requirement for a delicate balance to achieve optimised Li-ion conductivity in these materials. The existence of these often intertwined mechanisms has led to significant debate over the factors that dominate Li-ion conductivity in argyrodite solid electrolytes. Furthermore, the vast number of possible atomic configurations arising from the complex distribution of Li cations and S, Cl and Br anions within their respective sublattices result in an increase in configurational entropy, which has been proposed as another cause of high Li-ion diffusivity.\u003csup\u003e\u003cspan additionalcitationids=\"CR40 CR41\" citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eThe application of external pressure is another important consideration for solid electrolyte and solid-state battery design and optimisation.\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e As a key thermodynamic parameter, pressure can modify the crystal lattice, defect concentrations and phase stability, thereby altering ion dynamics. These structural changes, in turn, impact energy barriers for ion migration and the overall ionic conductivity.\u003csup\u003e\u003cspan additionalcitationids=\"CR45\" citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e However, despite the fact the real-world applications expose solid electrolytes and solid-state batteries to significant pressures and mechanical stresses during both assembly and operation,\u003csup\u003e46\u003c/sup\u003e their critical properties, including structure, stability and ionic conductivity, are often only assessed under ambient pressure conditions. Two important exceptions to this are the works of Faka et al., who explored the pressure dependence of Li-ion conductivity\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e and pressure-induced dislocations\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e in Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eBr using a range of experimental techniques. Nevertheless, our understanding of pressure effects in argyrodite solid electrolytes (and solid electrolytes more broadly) remains limited, particularly at the atomistic scale.\u003c/p\u003e \u003cp\u003eTo rectify this omission, in this study, we investigate the impact of a range of external pressures (0\u0026ndash;10 GPa) on the structural, electronic and ion transport properties of Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl and/or Br) with varying degrees of halide mixing and S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ site disorder. Using density functional theory (DFT) and \u003cem\u003eab initio\u003c/em\u003e molecular dynamics (AIMD), we show that increasing pressure leads to systematic lattice compression, reduced thermodynamic stability, bandgap narrowing, lower Li-ion conductivities and higher activation energies. Nevertheless, anion disorder and Br substitution can be used to enhance lattice flexibility and preserve reasonable ionic conductivity under pressures of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\u0026lt;\\)\u003c/span\u003e\u003c/span\u003e10 GPa, identifying them as key design parameters for pressure-resilient argyrodite solid electrolytes. By developing a comprehensive understanding of these interlinking factors, this work helps to guide the design of next-generation argyrodite solid electrolytes with enhanced performance and reliability across a diverse range of operating conditions.\u003c/p\u003e"},{"header":"METHODS","content":"\u003cp\u003eDFT calculations were performed using the projector-augmented wave (PAW) method, as implemented in the Vienna \u003cem\u003eAb Initio\u003c/em\u003e Simulation Package (VASP).\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e,\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e The Perdew-Burke-Ernzerhof for solids (PBEsol) generalized gradient approximation (GGA) was used to describe exchange-correlation effects. Given that the Li ions partially occupy the 48h site in Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX, a screening over the possible configurations was performed using pymatgen.\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e After enumerating the possible structures, 50 were selected through minimisation of their electrostatic energies and then fully relaxed using DFT. The configuration with the lowest energy was identified and used for subsequent simulations. A similar process was also applied for the generation of the mixed halide and anion site-disordered structures. All structural optimisations and energy calculations employed a plane-wave cutoff energy of 600 eV and Brillouin-zone integrations were performed using a single \u003cem\u003ek\u003c/em\u003e-point mesh (Γ-point). Energy and force convergence criteria of 10⁻\u003csup\u003e5\u003c/sup\u003e eV and 0.01 eV \u0026Aring;⁻\u003csup\u003e1\u003c/sup\u003e, respectively, were used throughout. Structural optimisations were completed with the complete relaxation of atomic position, cell shape and cell volume, under applied pressures of 0, 1, 2, 3, 4 and 10 GPa. Density of states (DOS) calculations were performed using the HSE06 hybrid functional for a more accurate electronic structure.\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e The sumo Python toolkit was used for postprocessing of the DOS data.\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eAIMD simulations were performed in the canonical (NVT) ensemble, using the Nose-Hoover thermostat, to analyse a range of optimised structures incorporating varying degrees of anion site disorder and halide mixing as a function of pressure. The plane-wave energy cutoff was reduced to 400 eV. Given the substantial number of AIMD simulations required to consider the effects of pressure, halide mixing and anion site disorder, only a single Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX unit cell (52 atoms) was used. The simulations were conducted with a 2 fs timestep over a total duration of 200 ps at four temperatures (600\u0026minus;1050 K at 150 K intervals). Li-ion diffusion coefficients (\u003cem\u003eD\u003c/em\u003e\u003csub\u003eLi\u003c/sub\u003e) were obtained from the mean squared displacement (MSD) of Li ions during the simulations:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:⟨{r}_{i}^{2}\\left(t\\right)⟩=6{D}_{\\text{L}\\text{i}}t$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:⟨{r}_{i}^{2}\\left(t\\right)⟩\\)\u003c/span\u003e\u003c/span\u003e represents the MSD and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:t\\)\u003c/span\u003e\u003c/span\u003e is time. The Li-ion diffusion coefficients were subsequently converted into Li-ion conductivities (\u003cem\u003eσ\u003c/em\u003e) using the Nernst\u0026minus;Einstein equation.\u003c/p\u003e \u003cp\u003eHarmonic phonon calculations were conducted on the structures of both Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl or Br) with no S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ site disorder at 0 and 10 GPa to explore the influence of pressure on the vibrational characteristics of the materials. The optimised geometries were further optimised for these calculations to an ionic force convergence of 0.001 eV \u0026Aring;\u003csup\u003e⁻\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e with a 2\u0026times;2\u0026times;2 k-point mesh. The lattice dynamics calculations were completed using the finite differences approach, as implemented in PHONOPY.\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e The optimised geometries were expanded to 2\u0026times;2\u0026times;2 supercells, which were sampled on a proportionally smaller k-point mesh (Γ-point). Long range electrostatic interactions were accounted for using the non-analytical term correction, as implemented in PHONOPY, using the static dielectric constant tensor and born effective charges calculated using VASP. The phonon density of states (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{g}\\)\u003c/span\u003e\u003c/span\u003e(\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\omega\\:}\\)\u003c/span\u003e\u003c/span\u003e)) was calculated on a 10\u0026times;10\u0026times;10 mesh of points. To explore the influence of pressure on the ionic \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{g}\\)\u003c/span\u003e\u003c/span\u003e(\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\omega\\:}\\)\u003c/span\u003e\u003c/span\u003e) spectra, the phonon band centre (PBC) was calculated using the following equation:\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:PBC=\\:\\frac{\\int\\:\\omega\\:\\times\\:g\\left(\\omega\\:\\right).d\\omega\\:}{\\int\\:g\\left(\\omega\\:\\right).d\\omega\\:}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere ω represents a vibrational frequency and g(ω) represents either the total (all ion) or ion projected phonon density of states for the all ion or projected PBC, respectively.\u003c/p\u003e"},{"header":"RESULTS AND DISCUSSION","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eLocal Structure and Stability\u003c/h2\u003e \u003cp\u003eThe crystal structure of Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl and/or Br) adopts a face-centred cubic lattice, belonging to the \u003cem\u003eF\u003c/em\u003e4̅3\u003cem\u003em\u003c/em\u003e space group.\u003csup\u003e2\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a) illustrates the fully ordered unit cell of Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX, in which halide ions are located at the Wyckoff 4a positions, while S\u003csup\u003e2\u003c/sup\u003e⁻ anions occupy the Wyckoff 4d positions. As discussed above, S\u003csup\u003e2\u003c/sup\u003e⁻ and X⁻ anions can exhibit site disorder by exchanging positions and occupying both 4a and 4d sites. The unit cell of Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX contains four distinct Li\u003csup\u003e+\u003c/sup\u003e \u0026lsquo;cages\u0026rsquo;, with Li ions distributed across 48h sites, each centred around an S\u003csup\u003e2\u003c/sup\u003e⁻ anion at the 4d site, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b).\u003csup\u003e2\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, 3\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo capture the role of anionic disorder, we compare fully ordered configurations (0% S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ site exchanged) with partially disordered ones in which three of the four Li\u003csup\u003e+\u003c/sup\u003e cages per unit cell have their central S\u003csup\u003e2\u003c/sup\u003e⁻ exchanged with X⁻ (75% S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ site exchanged). Previous experimental studies and simulations have shown that such a degree of anion disorder directly influences Li-ion mobility, by expanding Li\u003csup\u003e+\u003c/sup\u003e cages, reducing inter-cage jump distances, and enhances conductivity by creating more connected diffusion pathways.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e We have therefore focused on these two configurations, fully ordered (0%) and partially disordered (75%), as structural limits for assessing how pressure influences the fundamental properties of argyrodites as solid electrolytes.\u003c/p\u003e \u003cp\u003eWe first examined the structural response of Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl, Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eBr, Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl\u003csub\u003e0.25\u003c/sub\u003eBr\u003csub\u003e0.75\u003c/sub\u003e, Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl\u003csub\u003e0.50\u003c/sub\u003eBr\u003csub\u003e0.50\u003c/sub\u003e and Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl\u003csub\u003e0.75\u003c/sub\u003eBr\u003csub\u003e0.25\u003c/sub\u003e to external pressure for both the 0% and 75% site exchanged structures, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a). Across all pressures, the lattice parameters follow a clear compositional trend governed by the ionic radius of the halide anion. As Br⁻ (1.96 \u0026Aring;) is larger than Cl⁻ (1.81 \u0026Aring;),\u003csup\u003e53\u003c/sup\u003e Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eBr exhibits the largest lattice parameter at all pressures, while Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl shows the smallest. The mixed halide compositions lie between these two endmembers, with their lattice parameters scaling smoothly with Br/Cl ratio. As previously reported for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eBr, increasing anion disorder results in reduced unit cell volumes/lattice parameters as a result of the more homogeneous charge distribution of Li ions, leading to contraction of the unit cell due to electrostatic interactions.\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eExamination of the pressure-dependent lattice parameters of Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl and/or Br) reveals a clear contraction with increasing pressure, in agreement with the in-situ X-ray diffraction data of Faka et al. for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eBr, who observed reversible compression without phase transitions up to 10 GPa.\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e For the fully ordered structures, the cubic lattice parameters decrease from 9.87 \u0026Aring; (X\u0026thinsp;=\u0026thinsp;Cl), 9.96 \u0026Aring; (X\u0026thinsp;=\u0026thinsp;Br) and 9.90 \u0026Aring; (X\u0026thinsp;=\u0026thinsp;Cl\u003csub\u003e0.5\u003c/sub\u003eBr\u003csub\u003e0.5\u003c/sub\u003e) at 0 GPa to 9.12 \u0026Aring; (X\u0026thinsp;=\u0026thinsp;Cl), 9.21 \u0026Aring; (X\u0026thinsp;=\u0026thinsp;Br) and 9.19 \u0026Aring; (X\u0026thinsp;=\u0026thinsp;Cl\u003csub\u003e0.5\u003c/sub\u003eBr\u003csub\u003e0.5\u003c/sub\u003e) at 10 GPa. The partially disordered (75% site exchanged) structures show a comparable continuous contraction, with lattice parameters decreasing from 9.80 \u0026Aring; (X\u0026thinsp;=\u0026thinsp;Cl), 9.89 \u0026Aring; (X\u0026thinsp;=\u0026thinsp;Br) and 9.84 \u0026Aring; (X\u0026thinsp;=\u0026thinsp;Cl\u003csub\u003e0.5\u003c/sub\u003eBr\u003csub\u003e0.5\u003c/sub\u003e) at 0 GPa to 9.06 \u0026Aring; (X\u0026thinsp;=\u0026thinsp;Cl), 9.13 \u0026Aring; (X\u0026thinsp;=\u0026thinsp;Br) and 9.10 \u0026Aring; (X\u0026thinsp;=\u0026thinsp;Cl\u003csub\u003e0.5\u003c/sub\u003eBr\u003csub\u003e0.5\u003c/sub\u003e) at 10 GPa. The magnitudes of these pressure-induced reductions are similar regardless of the halide nature, which reflects the similar bulk moduli of 28.7 and 29.0 for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl and Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eBr, respectively, calculated from DFT simulations.\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e Such reductions in lattice parameters are expected to have a significant impact on the migration volume for Li ions and therefore the overall Li-ion conductivity.\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b) presents the calculated formation energies of Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl and/or Br) from their respective binary precursors (i.e., LiX (X\u0026thinsp;=\u0026thinsp;Cl or Br), Li\u003csub\u003e2\u003c/sub\u003eS and P\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e5\u003c/sub\u003e) as a function of pressure for both ordered (0%) and partially disordered (75%) structures. All compositions exhibit negative formation energies below 5 GPa, indicating reasonable stability under moderate compression. Although the formation energy of Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eBr is more exothermic at 0 GPa, it increases slightly more rapidly with pressure compared to the Cl-rich argyrodites, i.e., an average increase 0.520 eV/GPa for Br compared to 0.505 eV/GPa for Cl in the fully ordered structures. This trend becomes more pronounced in the disordered systems (75%), where the rates rise to 0.568 eV/GPa for Br and 0.545 eV/GPa for Cl. These results show that the Cl-rich systems exhibit a smaller pressure-induced destabilisation compared to the Br-rich compositions, consistent with their stiffer bonding environment, stronger electrostatic interactions and lower polarisability.\u003csup\u003e\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e,\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eRadial distribution functions (RDFs) are plotted in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e to provide complementary local structural insights into the effect of pressure on Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl and/or Br). At ambient pressure (0 GPa), our calculated RDFs for both the 0% and 75% site exchanged systems are in excellent agreement with previous AIMD studies.\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e For the Li\u0026ndash;Li RDFs, a broad distribution of Li\u0026ndash;Li separations are observed above 3 \u0026Aring;. The primary peaks in the Li\u0026ndash;Li RDFs are smaller and broader in anion site exchanged systems, indicative of a greater range of site\u0026ndash;site separations and a more disordered lithium sublattice. In contrast, the Li\u0026ndash;anion RDFs present single sharp primary peaks at 2.2\u0026ndash;2.9 \u0026Aring; depending on the halide identity and level of site disorder.\u003c/p\u003e \u003cp\u003eWith increasing pressure, the peaks in each RDF all shift to shorter distances; however, the magnitudes of these shifts differ considerably depending on the element pair and composition. For the Li\u0026ndash;Li pair, the RDFs for each system at 0, 1 and 3 GPa are remarkably commensurate with each other. In contrast, at 10 GPa, there is a clear change with a substantial reduction in the Li\u0026ndash;Li separation distance of the main peak. This reduction is also more significant for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eBr compared to Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl. This change at 10 GPa reflects both the significant lattice compression and more compact Li sublattice. Li\u0026ndash;Li separation is well known to have a strong influence on fast Li-ion transport in Li-ion conductors.\u003csup\u003e\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e,\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e\u003c/sup\u003e In previous molecular dynamics simulations of - and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\beta\\:}\\)\u003c/span\u003e\u003c/span\u003e-Li\u003csub\u003e3\u003c/sub\u003ePS\u003csub\u003e4\u003c/sub\u003e, similar reductions in the Li\u0026ndash;Li separation of the main RDF peaks were found to directly contribute to inferior Li-ion transport, in agreement with our current findings for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX at 10 GPa.\u003c/p\u003e \u003cp\u003eA similar phenomenon is also observed for the Li\u0026ndash;X RDFs, where there is a significant shift in the peaks to shorter distances with increasing pressure, particularly for the 0% site exchanged systems. At 10 GPa, this shift results in the distance of the main Li\u0026ndash;X peak becoming similar or even shorter than for the main Li\u0026ndash;S peaks. Such behaviour is also typically observed with a reduction in temperature and the resulting increase in local ordering and decrease in Li-ion conductivity. The Li\u0026ndash;X RDFs allow for direct comparison between chloride and bromide coordination environments. Quantitatively, the first peak positions confirm that Li\u0026ndash;Br distances are consistently longer than Li\u0026ndash;Cl under ambient conditions (2.67 \u0026Aring; for Br and 2.49 \u0026Aring; for Cl at 0 GPa). Under maximum compression (10 GPa), both coordination shells contract, but the Li\u0026ndash;Br environment exhibits a larger magnitude of contraction (2.67 to 2.43 \u0026Aring;, Δ = \u0026minus;\u0026thinsp;0.24 \u0026Aring;) compared to Li\u0026ndash;Cl (2.49 to 2.34 \u0026Aring;, Δ = \u0026minus;\u0026thinsp;0.15 \u0026Aring;), reflecting a softer and more polarisable Br\u003csup\u003e\u0026ndash;\u003c/sup\u003e coordination shell. This softer environment compresses more readily, illustrative of a larger response of Br-rich systems to applied pressure. Interestingly, and as discussed above, this increased rate of contraction with pressure for Li\u0026ndash;Br compared to Li\u0026ndash;Cl distance is not reflected in the calculated lattice parameters in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a). This suggests that the influence of pressure is not uniform for both average and local structure in lithium argyrodites. In contrast to Li\u0026ndash;Li and Li\u0026ndash;X, the effect of pressure on Li\u0026ndash;S distances is more subtle, with only a minor shift to shorter distances, even at 10 GPa.\u003c/p\u003e \u003cp\u003eOverall, the above analysis demonstrates that halide chemistry and anion disorder jointly control the pressure stability of Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX. Cl-rich compositions are more rigid and cohesive, whereas Br-rich systems are softer and more responsive to external pressure. These distinctions underpin the pressure-dependent electronic and Li-ion transport behaviour discussed in the following sections.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eElectronic Structure\u003c/h3\u003e\n\u003cp\u003e \u003c/p\u003e \u003cp\u003eEven though electronic structure is directly connected to electrochemical stability, electronic conduction and resistance to dendrite formation, the influence of external pressure on the electronic structure and bandgaps of solid electrolytes for solid-state batteries has rarely been considered in literature. To understand how pressure affects the electronic structures of Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl or Br), we calculated the projected electronic density of states (DOS) at the 0% and 75% site exchanged systems as function of pressure, as shown in Figures \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e and 4, respectively. The DOS plots for both Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl and Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eBr reveal that the valence band maximum is primarily composed of S 3p orbitals, while the conduction band minimum is dominated by P 3s and 3p states across the entire pressure range.\u003csup\u003e\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eThe calculated bandgaps for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl and/or Br) with 0% and 75% site exchange as a function of pressure are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. For the anion site ordered compositions (0% site exchanged), the bandgaps occupy a narrow range between 3.19 eV (Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl\u003csub\u003e0.25\u003c/sub\u003eBr\u003csub\u003e0.75\u003c/sub\u003e) and 3.27 eV (Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eBr) at 0 GPa, in agreement with previous DFT studies.\u003csup\u003e\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e,\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e\u003c/sup\u003e For the anion site disordered compositions (75% site exchanged), the bandgaps are remarkably similar (2.85\u0026minus;2.87 eV) at 0 GPa but somewhat smaller than the values obtained for the ordered compositions (3.19\u0026minus;3.27 eV). Given that bandgaps can be used as upper limits of the electrochemical stability window, it is noteworthy that anion site disordering may reduce the stability of these argyrodite solid electrolytes.\u003csup\u003e\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e,\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e\u003c/sup\u003e These values show that the halide composition in these argyrodites has minimal impact on the bandgap and therefore their electrochemical stability window.\u003csup\u003e\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a), with increasing pressure, the bandgaps of anion site ordered Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl and/or Br) show a clear decreasing trend. Nevertheless, these decreases are relatively modest (~\u0026thinsp;0.2\u0026minus;0.5 eV) and are therefore not expected to substantially impact the stability or electronic conductivity of the materials. In contrast, the disordered compositions do not show a clear single trend with increasing external pressure (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b)). For the Br-containing systems, a notable reduction in bandgap of ~\u0026thinsp;0.4 eV occurs at pressures of 4 GPa. The pressure at which this reduction occurs is different depending on the Br content. Furthermore, it is important to bear in mind that the electronic structure will depend on the particular cation and anion ordering in each composition, which, to some extent, increases variability in the calculated band gaps. This aligns with the experimental findings of Minafra et al.,\u003csup\u003e33\u003c/sup\u003e which show that anion disorder broadens Li\u003csup\u003e+\u003c/sup\u003e distributions and introduces charge inhomogeneity. At the maximum calculated pressure of 10 GPa, the bandgaps of the Br-containing systems increase by 0.2\u0026minus;0.7 eV, possibly suggestive of a stabilising effect at high pressures. In the case of Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl, the bandgap remains relatively constant as a function of pressure, with only a small\u0026thinsp;~\u0026thinsp;0.2 eV at 10 GPa. This result highlights the excellent performance of Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl as a solid electrolyte for practical solid-state batteries.\u003c/p\u003e \u003cp\u003eThe narrowing of band gaps is an important consideration in the development of solid electrolytes as these fast ionic conductors must also have poor electronic conductivity, i.e., wide band gaps. Whilst the bandgaps calculated here are still too large to any significant levels of electronic conduction, this may not be the case for the grain boundaries of argyrodites, especially when under external pressure. It has been shown that grain boundaries in solid electrolytes can lead to reductions in the bandgap, sometimes with the appearance of highly localised trap states for electrons or holes on ions which are coordinated differently than in the bulk.\u003csup\u003e\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e\u003c/sup\u003e This is an important consideration for solid electrolyte design as the combined effect of pressure and grain boundaries may lead to significantly narrowed bandgaps with the potential for detrimental electronic conductivity and dendrite initiation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eLi-Ion Transport\u003c/b\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eFigure 7.\u003c/b\u003e (a) Pressure dependence of Li-ion conductivity at 600 K for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl and/or Br) with 75% S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ site exchange. Insufficient Li-ion diffusion events were observed for the 0% S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ site exchanged systems at 600 K so the corresponding conductivities could not be calculated. (b) Pressure dependence of Li-ion activation energy (\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e) for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl or Br) with 0% and 75% S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ site exchange. The equivalent activation energies (\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e) for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl\u003csub\u003e0.5\u003c/sub\u003eBr\u003csub\u003e0.5\u003c/sub\u003e, Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl\u003csub\u003e0.75\u003c/sub\u003eBr\u003csub\u003e0.25\u003c/sub\u003e and Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl\u003csub\u003e0.25\u003c/sub\u003eBr\u003csub\u003e0.75\u003c/sub\u003e are given in Figure S2.\u003c/p\u003e \u003cp\u003eTo elucidate the effects of pressure, halide chemistry and anion disorder on Li-ion transport, temperature-dependent Li-ion conductivities were derived from the AIMD simulations. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e presents Arrhenius plots of the Li-ion conductivity for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl and/or Br) with 75% S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ site exchange under pressures of 0\u0026ndash;10 GPa. At ambient pressure (0 GPa), all compositions exhibit high Li-ion conductivities (~\u0026thinsp;0.54\u0026minus;1.05 S cm⁻\u003csup\u003e1\u003c/sup\u003e at 600 K) and low activation energies in the range of 0.14\u0026ndash;0.16 eV, consistent with the superionic behaviour observed experimentally for argyrodites.\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan additionalcitationids=\"CR65 CR66\" citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e\u003c/sup\u003e Among the halides, Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl shows the lowest \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e (~\u0026thinsp;0.14 eV), marginally below that of Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eBr (~\u0026thinsp;0.15 eV), reflecting the more compact Cl-framework that facilitates Li\u003csup\u003e+\u003c/sup\u003e hopping through interconnected 48h sites via transient occupation of 24g positions.\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e Upon compression, however, conductivity decreases monotonically across all systems, accompanied by a corresponding increase in \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e to 0.22\u0026ndash;0.26 eV at 10 GPa. The steeper rise of \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e in Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl relative to Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eBr indicates that Li\u003csup\u003e+\u003c/sup\u003e migration in Cl-rich compositions is more sensitive to lattice densification. Bromide substitution mitigates this effect through enhanced lattice softness, which allows local structural relaxation and partial preservation of the percolating Li\u003csup\u003e+\u003c/sup\u003e conduction network under pressure.\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e,\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eThe mixed-halide systems display intermediate behaviour, following the same qualitative trend of conductivity suppression and barrier growth with increasing pressure. However, compositions containing higher Br fractions (Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl\u003csub\u003e0.5\u003c/sub\u003eBr\u003csub\u003e0.5\u003c/sub\u003e and Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl\u003csub\u003e0.25\u003c/sub\u003eBr\u003csub\u003e0.75\u003c/sub\u003e) exhibit the highest pressure resilience, suggesting a synergistic balance between the strong electrostatic framework of Cl⁻ and the softer, more compliant bonding introduced by Br⁻. This trend mirrors the mechanical compliance discussed earlier, where Br-rich frameworks accommodated strain more effectively than Cl-rich analogues.\u003c/p\u003e \u003cp\u003eFigure 7 summarises the pressure dependence of conductivity (σ) and activation energy (\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e) for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl and Br) with 0% and 75% site exchange. For both Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl and Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eBr, introducing 75% S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ site exchange significantly lowers \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e relative to the fully ordered (0%) structures, resulting in higher conductivities over the entire pressure range. This behaviour reflects the established role of anion disorder in broadening the distribution of accessible Li sites and enabling more three-dimensional, energetically favourable migration pathways.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e,\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eAt the atomistic level, Li-ion diffusion in argyrodites proceeds through hops between 48h cages via transient occupations of 24g sites, forming a three-dimensional percolation network.\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e Anion disorder (75% S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ site exchanged) enhances this connectivity by perturbing local charge distributions and creating multiple low-energy pathways, while applied pressure acts oppositely by narrowing the bottlenecks between cages. The overall transport response therefore arises from a balance between disorder-induced pathway diversification and pressure-driven lattice densification.\u003c/p\u003e \u003cp\u003eExploration of the lattice dynamics properties for Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl or Br) at 0 and 10 GPa revealed entirely real vibrational frequencies for each structure suggesting the pressurised optimised structures are stable states (phonon dispersion plots are shown in Figure S3). The application of pressure leads to general vibrational hardening of modes, reflected in both the g(ω) spectra, as well as the total (all ion) and ion projected PBC values. The g(ω) for each material are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003e, with the PBC values shown by dashed vertical lines, which are connected across the two pressure values for each Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX composition.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIt is clear from the PBC values that there is a general shift upwards in vibrational frequency (hardening) for each ionic species (and therefore also the all-ion PBC), with the most significantly upshifted species being the Li ions for both Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl or Br). Vibrational hardening of phonons under pressure is most likely a result of a decrease in interatomic distances leading a stiffer lattice and an increase in restoring force of vibrations, and has been widely reported for a number of different materials and molecular solids.\u003csup\u003e\u003cspan additionalcitationids=\"CR70\" citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e71\u003c/span\u003e\u003c/sup\u003e The Li projected PBC has been correlated to some success with ionic conductivity for a number of Li-ion conductors, with lower PBC (soft) lattices typically exhibiting increased Li conductivity as the vibrations are more easily excited and assist with ionic migration.\u003csup\u003e\u003cspan additionalcitationids=\"CR73\" citationid=\"CR72\" class=\"CitationRef\"\u003e72\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e74\u003c/span\u003e\u003c/sup\u003e The increase in Li-ion PBC as a result of the application of pressure for the Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl or Br) materials considered in this work is in agreement with the calculated decrease in σ, further consolidating the negative influence of pressure on Li-ion conductivity in argyrodites.\u003c/p\u003e \u003cp\u003eOur results demonstrate that Li-ion transport in Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX is highly sensitive to external pressure, with both halide chemistry and anion disorder governing the extent of this response. Cl-rich compositions show lower activation barriers under ambient conditions but a greater loss of conductivity under higher pressures (3 GPa), indicating a stronger sensitivity to densification. In contrast, Br-analogues, particularly those with substantial disorder (75%), exhibit a more resilient transport response, consistent with the greater chemical softness and structural compliance of the Br-substituted lattice. In addition to the negative influence of pressure on Li-ion conductivity observed in this work, it has been previously shown that external pressure can be used to induce dislocations in Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eBr, which can actually enhance its Li-ion conductivity.\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e These findings therefore emphasise that pressure, halide substitution and anion disorder are interdependent design parameters for optimising argyrodite solid electrolytes capable of sustaining high ionic conductivity under mechanical stress and internal strain.\u003c/p\u003e"},{"header":"CONCLUSIONS","content":"\u003cp\u003eThe understanding of pressure effects in solid electrolytes is essential for their large-scale utilisation in solid-state batteries. Despite this fact, the role of pressure in these materials has been so far underexplored, particularly using atomistic modelling. In this work, we have investigated how pressure influences the structure, electronic properties and Li-ion conductivity in the lithium argyrodite family Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl and/or Br) and considered its intricate interplay with halide mixing and S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻ anion disorder at the atomic scale. The key results are summarised as follows:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eAll Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl and/or Br) compositions remain structurally coherent under compression up to 10 GPa, with continuous lattice contraction and no evidence of phase transitions.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe impact of pressure on the average and local structure is not uniform. For example, while the magnitude in the reduction of the lattice parameter with pressure is similar for all Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX compositions, the primary Li\u0026ndash;Br distance decreases more substantially with increasing pressure than the primary Li\u0026ndash;Cl distance.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eElectronic structure analysis indicates wide bandgaps (2.8\u0026ndash;3.3 eV) that are largely insensitive to halide composition. Anion disorder slightly narrows the bandgap and pressure induces modest, composition-dependent variations that are not expected to significantly affect electrochemical stability.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eLi-ion conductivities remain high (0.54\u0026minus;1.05 S cm⁻\u003csup\u003e1\u003c/sup\u003e at 600 K) and activation energies low (~\u0026thinsp;0.14\u0026ndash;0.16 eV) up to \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sim\\)\u003c/span\u003e\u003c/span\u003e2 GPa, confirming robust Li-ion transport pathways under moderate compression.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eAt higher pressures, Li-ion conductivities decrease and activation energies increase, with Br-rich and partially disordered structures showing the strongest resistance to pressure-induced transport degradation owing to their greater lattice flexibility and polarisability.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003eFrom a design perspective, our results demonstrate that the interplay of pressure, halide chemistry and anion disorder critically governs the key properties of argyrodites as solid electrolytes, including Li-ion conductivity and electrochemical stability. These insights help to rationalise experimental observations of pressure-sensitive Li-ion transport in Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX and establish practical design rules for engineering solid electrolytes that maintain high performance in solid-state batteries under external pressure constraints.\u003c/p\u003e "},{"header":"Declarations","content":"\u003ch2\u003eFunding Declaration\u003c/h2\u003e\u003cp\u003eThis work was supported by the Engineering and Physical Sciences Research Council (EPSRC, EP/S023836/1) as part of the ReNU CDT. J.M.H, E.L. and J.A.D. acknowledge the Leverhulme Trust for financial support (RPG-2022-231. J.A.Q. and J.A.D. acknowledge the Faraday Institution (FIRG026) for funding. J.A.D. also acknowledges UK Research and Innovation (UKRI) under the UK government\u0026rsquo;s Horizon Europe funding guarantee (EP/z000254/1). Via membership of the U.K.\u0026rsquo;s HEC Materials Chemistry Consortium, which is funded by the EPSRC (EP/X035859/1), this work used the A2RCHER2 UK National Supercomputing Service.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eJ.L.C. and J.M.H. performed all simulations with supervision from J.A.D. J.A.D. designed the research programme. All authors contributed to the writing and revision of the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis work was supported by the Engineering and Physical Sciences Research Council (EPSRC, EP/S023836/1) as part of the ReNU CDT. J.M.H, E.L. and J.A.D. acknowledge the Leverhulme Trust for financial support (RPG-2022-231. J.A.Q. and J.A.D. acknowledge the Faraday Institution (FIRG026) for funding. J.A.D. also acknowledges UK Research and Innovation (UKRI) under the UK government\u0026rsquo;s Horizon Europe funding guarantee (EP/z000254/1). Via membership of the U.K.\u0026rsquo;s HEC Materials Chemistry Consortium, which is funded by the EPSRC (EP/X035859/1), this work used the A2RCHER2 UK National Supercomputing Service.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eAll data supporting the findings of this study are available within the paper and its Supplementary Information.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eG. De Carne, S. M. Maroufi, H. Beiranvand, V. De Angelis, S. D\u0026rsquo;Arco, V. Gevorgian, S. Waczowicz, B. Mather, M. Liserre and V. Hagenmeyer, \u003cem\u003eElectric Power Systems Research\u003c/em\u003e, 2024, 236, 110963.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eA. Manthiram, \u003cem\u003eACS Cent Sci\u003c/em\u003e, 2017, 3, 1063\u0026ndash;1069.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eC. P. Grey and D. S. 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Maglia, P. Lamp, W. G. Zeier and Y. Shao-Horn, \u003cem\u003eiScience\u003c/em\u003e, 2019, 16, 270\u0026ndash;282.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eT. Krauskopf, S. Muy, S. P. Culver, S. Ohno, O. Delaire, Y. Shao-Horn and W. G. Zeier, \u003cem\u003eJ Am Chem Soc\u003c/em\u003e, 2018, 140, 14464\u0026ndash;14473.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"npj-energy-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [npj Energy Materials](https://www.nature.com/npjenergymats/)","snPcode":"44456","submissionUrl":"https://submission.springernature.com/new-submission/44456/3","title":"npj Energy Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Unsupported Journal","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-8404265/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8404265/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe atomistic understanding of solid electrolytes is essential for advancing solid-state batteries and enabling their large-scale commercialisation. Whilst the importance of external pressure in the manufacturing and operation of practical solid-state batteries has been clearly recognised, its impact on the fundamental properties of solid electrolytes remains underexplored. Here, we combine density functional theory and \u003cem\u003eab initio\u003c/em\u003e molecular dynamics simulations to investigate the effect of pressure (0\u0026minus;10 GPa) on the structural, electronic and ion transport properties of lithium argyrodite solid electrolytes Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eX (X\u0026thinsp;=\u0026thinsp;Cl and/or Br) with different levels of halide mixing and anion site (S\u003csup\u003e2\u003c/sup\u003e⁻/X⁻) disorder. We show that increased pressure induces systematic decreases in lattice volume and stability for both ordered and disordered systems. Anion disorder narrows the bandgap, which can change dramatically as a function of pressure. The Li-ion conductivities (~\u0026thinsp;0.54\u0026minus;1.05 S cm⁻\u003csup\u003e1\u003c/sup\u003e at 600 K) and activation energies (\u0026lt;\u0026thinsp;0.16 eV), remain high and low, respectively, up to ~\u0026thinsp;2 GPa, with particularly pressure-resilient conduction networks observed in the mixed halide and disordered structures. At higher pressures (\u0026gt;\u0026thinsp;2 GPa), most compositions show a clear increase in activation energy (0.15\u0026minus;0.42 eV) accompanied by reduced conductivity (~\u0026thinsp;0.12\u0026minus;0.59 S cm⁻\u003csup\u003e1\u003c/sup\u003e at 600 K). These findings establish external pressure, site disorder and halide mixing as interdependent parameters that can be used to tune argyrodite solid electrolytes for high-performance solid-state batteries.\u003c/p\u003e","manuscriptTitle":"Influence of Pressure on Structure, Stability and Ionic Conductivity of Lithium Argyrodite Solid Electrolytes","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-14 09:00:47","doi":"10.21203/rs.3.rs-8404265/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-03-06T15:08:49+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-25T23:29:24+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-21T15:40:56+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-15T08:38:21+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"62374584555788939119351305212812071864","date":"2026-01-14T15:12:11+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"90579066288068240787053371396480815727","date":"2026-01-13T00:15:20+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"187713599491910583849131795150984501618","date":"2026-01-12T15:34:19+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"261996550871489350866548250278248056355","date":"2026-01-12T15:29:53+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-01-12T14:47:03+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-01-11T03:04:27+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-12-23T09:19:23+00:00","index":"","fulltext":""},{"type":"submitted","content":"npj Energy Materials","date":"2025-12-19T11:20:51+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-energy-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [npj Energy Materials](https://www.nature.com/npjenergymats/)","snPcode":"44456","submissionUrl":"https://submission.springernature.com/new-submission/44456/3","title":"npj Energy Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Unsupported Journal","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"b567eff1-2413-4f70-99f0-fff9bd0430a8","owner":[],"postedDate":"January 14th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":61028266,"name":"Physical sciences/Chemistry"},{"id":61028267,"name":"Physical sciences/Materials science"},{"id":61028268,"name":"Physical sciences/Physics"}],"tags":[],"updatedAt":"2026-05-04T14:39:35+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-14 09:00:47","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8404265","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8404265","identity":"rs-8404265","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}