Li₃Y(PS₄)₂ and Li₅PS₄Cl₂: New Lithium Superionic Conductors Predicted from Silver Thiophosphates using Efficiently Tiered Ab Initio Molecular Dynamics Simulations

We report two novel, earth-abundant lithium superionic conductors, Li₃Y­(PS₄)₂ and Li₅PS₄Cl₂, that are predicted to satisfy the necessary combination of good phase stability, high Li⁺ conductivity, wide band gap and good electrochemical stability for solid electrolyte applications in all-solid-state rechargeable lithium-ion batteries. These candidates were identified from a high-throughput first-principles screening of the Li–P–S ternary and Li–M–P–S (where M is a non-redox-active element) quaternary chemical spaces, including candidates obtained by replacing Ag with Li in the Ag–P–S and Ag–M–P–S chemical spaces. An efficient tiered screening strategy was developed that combines topological analysis with <i>ab initio</i> molecular dynamics simulations to exclude rapidly candidates unlikely to satisfy the stringent conductivity requirements of lithium superionic conductors. In particular, we find Li₃Y­(PS₄)₂ to be an extremely promising candidate exhibiting a room-temperature Li⁺ conductivity of 2.16 mS/cm, which can be increased multifold to 7.14 and 5.25 mS/cm via aliovalent doping with Ca²⁺ and Zr⁴⁺, respectively. More critically, we show that the phase and electrochemical stability of Li₃Y­(PS₄)₂ is expected to be better than current state-of-the-art lithium superionic conductors

Elucidating Structure–Composition–Property Relationships of the β‑SiAlON:Eu²⁺ Phosphor

In this work, we performed a systematic investigation of structure–composition–property relationships in Eu2+-activated β-SiAlON, one of the most promising narrow-band green phosphors for high-power light-emitting diodes and liquid crystal display backlighting with wide color gamut. Using first-principles calculations, we identified and confirmed various chemical rules for Si–Al, O–N, and Eu activator ordering within the β-SiAlON structure. Through the construction of energetically favorable models based on these chemical rules, we studied the effect of oxygen content and Eu2+ activator concentrations on the local EuN9 activator environment, and its impact on important photoluminescence properties such as emission peak position (using the band gap as a proxy), bandwidth, and thermal quenching resistance. Increasing oxygen content is shown to lead to an increase in Eu–N bond lengths and distortion of the EuN9 coordination polyhedron, modifying the crystal field environment of the Eu2+ activator, and resulting in red-shifting and broadening of the emission. We also show that the calculated excited band structure of β-SiAlON exhibits a large gap between the 5d levels and the conduction band of the host, indicating a large barrier toward thermal ionization (>0.5 eV) and, hence, excellent thermal quenching stability. Based on these insights, we discuss potential strategies for further composition optimization of β-SiAlON

Elucidating Structure–Composition–Property Relationships of the β‑SiAlON:Eu²⁺ Phosphor

In this work, we performed a systematic investigation of structure–composition–property relationships in Eu²⁺-activated β-SiAlON, one of the most promising narrow-band green phosphors for high-power light-emitting diodes and liquid crystal display backlighting with wide color gamut. Using first-principles calculations, we identified and confirmed various chemical rules for Si–Al, O–N, and Eu activator ordering within the β-SiAlON structure. Through the construction of energetically favorable models based on these chemical rules, we studied the effect of oxygen content and Eu²⁺ activator concentrations on the local EuN₉ activator environment, and its impact on important photoluminescence properties such as emission peak position (using the band gap as a proxy), bandwidth, and thermal quenching resistance. Increasing oxygen content is shown to lead to an increase in Eu–N bond lengths and distortion of the EuN₉ coordination polyhedron, modifying the crystal field environment of the Eu²⁺ activator, and resulting in red-shifting and broadening of the emission. We also show that the calculated excited band structure of β-SiAlON exhibits a large gap between the 5d levels and the conduction band of the host, indicating a large barrier toward thermal ionization (>0.5 eV) and, hence, excellent thermal quenching stability. Based on these insights, we discuss potential strategies for further composition optimization of β-SiAlON

Data-Driven First-Principles Methods for the Study and Design of Alkali Superionic Conductors

We present a detailed exposition of how first-principles methods can be used to guide alkali superionic conductor (ASIC) study and design. Using the argyrodite Li₆PS₅Cl as a case study, we demonstrate how modern information technology (IT) infrastructure and software tools can facilitate the assessment of alkali superionic conductors in terms of various critical properties of interest such as phase and electrochemical stability and ionic conductivity. The emphasis is on well-documented, reproducible analysis code that can be readily generalized to other material systems and design problems. For our chosen Li₆PS₅Cl case study material, we show that Li excess is crucial to enhancing its conductivity by increasing the occupancy of interstitial sites that promote long-range Li⁺ diffusion between cage-like frameworks. The predicted room-temperature conductivities and activation barriers are in reasonably good agreement with experimental values

Role of Na⁺ Interstitials and Dopants in Enhancing the Na⁺ Conductivity of the Cubic Na₃PS₄ Superionic Conductor

In this work, we performed a first-principles investigation of the phase stability, dopant formation energy and Na⁺ conductivity of pristine and doped cubic Na₃PS₄ (c-Na₃PS₄). We show that pristine c-Na₃PS₄ is an extremely poor Na ionic conductor, and the introduction of Na⁺ excess is the key to achieving reasonable Na⁺ conductivities. We studied the effect of aliovalent doping of M⁴⁺ for P⁵⁺ in c-Na₃PS₄, yielding Na_3+<i>x</i>M_<i>x</i>P_1–<i>x</i>S₄ (M = Si, Ge, and Sn with <i>x</i> = 0.0625; M = Si with <i>x</i> = 0.125). The formation energies in all the doped structures with dopant concentration of <i>x</i> = 0.0625 are found to be relatively low. Using <i>ab initio</i> molecular dynamics simulations, we predict that 6.25% Si-doped c-Na₃PS₄ has a Na⁺ conductivity of 1.66 mS/cm, in excellent agreement with previous experimental results. Remarkably, we find that Sn⁴⁺ doping at the same concentration yields a much higher predicted Na⁺ conductivity of 10.7 mS/cm, though with a higher dopant formation energy. A higher Si⁴⁺ doping concentration of <i>x</i> = 0.125 also yields a significant increase in Na⁺ conductivity with an even higher dopant formation energy. Finally, topological and van Hove correlation function analyses suggest that the channel volume and correlation in Na⁺ motions may play important roles in enhancing Na⁺ conductivity in this structure

CrystalDNN

Python scripts, models and data used to predict the formation energies(Ef) and to calculate the energies above hull(Ehull) of garnet and perovskite crystals accompanying the above publication

Charge Transport in a Quantum Dot Supercrystal

Colloidal semiconductor quantum dots connected by organic or inorganic molecules can form periodic supercrystals. These supercrystals can be used for various types of electronic and optical applications with properties superior to those of random quantum dots and organic polymer mixtures. We have used ab initio calculations to study the charge transport and carrier mobility in such supercrystals. Among the different possible charge transport mechanisms, we found that the phonon-assisted hopping is the most likely mechanism. The calculated carrier mobility agrees well with the experimentally measured results. Our predictions of the size and temperature dependences on the mobility are awaiting experimental confirmation

Insights into the Performance Limits of the Li₇P₃S₁₁ Superionic Conductor: A Combined First-Principles and Experimental Study

The Li7P3S11 glass-ceramic is a promising superionic conductor electrolyte (SCE) with an extremely high Li+ conductivity that exceeds that of even traditional organic electrolytes. In this work, we present a combined computational and experimental investigation of the material performance limitations in terms of its phase and electrochemical stability, and Li+ conductivity. We find that Li7P3S11 is metastable at 0 K but becomes stable at above 630 K (∼360 °C) when vibrational entropy contributions are accounted for, in agreement with differential scanning calorimetry measurements. Both scanning electron microscopy and the calculated Wulff shape show that Li7P3S11 tends to form relatively isotropic crystals. In terms of electrochemical stability, first-principles calculations predict that, unlike the LiCoO2 cathode, the olivine LiFePO4 and spinel LiMn2O4 cathodes are likely to form stable passivation interfaces with the Li7P3S11 SCE. This finding underscores the importance of considering multicomponent integration in developing an all-solid-state architecture. To probe the fundamental limit of its bulk Li+ conductivity, a comparison of conventional cold-press sintered versus spark-plasma sintering (SPS) Li7P3S11 was done in conjunction with ab initio molecular dynamics (AIMD) simulations. Though the measured diffusion activation barriers are in excellent agreement, the AIMD-predicted room-temperature Li+ conductivity of 57 mS cm–1 is much higher than the experimental values. The optimized SPS sample exhibits a room-temperature Li+ conductivity of 11.6 mS cm–1, significantly higher than that of the cold-pressed sample (1.3 mS cm–1) due to the reduction of grain boundary resistance by densification. We conclude that grain boundary conductivity is limiting the overall Li+ conductivity in Li7P3S11, and further optimization of overall conductivities should be possible. Finally, we show that Li+ motions in this material are highly collective, and the flexing of the P2S7 ditetrahedra facilitates fast Li+ diffusion

New Insights into the Interphase between the Na Metal Anode and Sulfide Solid-State Electrolytes: A Joint Experimental and Computational Study

In this work, we investigated the interface between the sodium anode and the sulfide-based solid electrolytes Na₃SbS₄ (NAS), Na₃PS₄ (NPS), and Cl-doped NPS (NPSC) in all-solid-state-batteries (ASSBs). Even though these electrolytes have demonstrated high ionic conductivities in the range of 1 mS cm^–1 at ambient temperatures, sulfide sold-state electrolytes (SSEs) are known to be unstable with Na metal, though the exact reaction mechanism and kinetics of the reaction remain unclear. We demonstrate that the primary cause of capacity fade and cell failure is a chemical reaction spurred on by electrochemical cycling that takes place at the interface between the Na anode and the SSEs. To investigate the properties of the Na-solid electrolyte interphase (SSEI) and its effect on cell performance, the SSEI was predicted computationally to be composed of Na₂S and Na₃Sb for NAS and identified experimentally via X-ray photoelectron spectroscopy (XPS). These two compounds give the SSEI mixed ionic- and electronic-conducting properties, which promotes continued SSEI growth, which increases the cell impedance at the expense of cell performance and cycle life. The SSEI for NPS was similarly found to be comprised of Na₂S and Na₃P, but XPS analysis of Cl-doped NPS (NPSC) showed the presence of an additional compound at the SSEI, NaCl, which was found to mitigate the decomposition of NPS. The methodologies presented in this work can be used to predict and optimize the electrochemical behavior of an all-solid-state cell. Such joint computational and experimental efforts can inform strategies for engineering a stable electrolyte and SSEI to avoid such reactions. Through this work, we call for more emphasis on SSE compatibility with both anodes and cathodes, essential for improving the electrochemical properties, longevity, and practicality of Na-based ASSBs