Grossular and andradite are garnet end-members stable under upper mantle conditions. We perform ab initio simulations to investigate the dependence of the bulk modulus on chemical composition of the grossular-andradite solid solution, Ca3Fe(2-2x)Al(2x)(SiO4)3. All-electron local basis sets of Gaussian-type orbitals and the hybrid B3LYP density functional are used. Our calculations predict a linear modulus-composition trend, in contrast to previous conjectures based on "heterogeneous" experimental measurements. We estimate the largest deviation from linearity to be about 0.5 GPa under ambient conditions, and to progressively reduce to less than 0.2 GPa at pressure P = 20 GPa. The bulk modulus is computed over the whole composition range 0 ≤x≤ 1 following two independent approaches: fitting energy-volume data to an equation-of-state and calculating elastic tensors. Results from the two methods are in perfect agreement, assuring consistency and high numerical accuracy of the adopted algorithms.
Despite ongoing efforts aimed at increasing energy density in all-solid-state batteries (ASSBs), the optimal composite cathode morphology, which requires minimal volume change, small void development, and good interfacial contact, remains a significant concern within the community. In this work, we focus on the theoretical investigation of the aforementioned mechanical defects in the composite cathode during electrochemical cycling. It is demonstrated that these mechanical defects are highly dependent on the solid electrolyte (SE) material properties, the external stack pressure, and the cathode active material (CAM) loading. The following conclusions are highlighted in this study: (1) Higher CAM loading (>50 vol %) causes an increase in mechanical defects, including large cathode volume change (>5%), contact loss (50%), and porosity (>1%). (2) High external stack pressure up to 7 MPa reduces mechanical defects while preventing internal fracture in the cathode. (3) Soft SE materials with small Young's modulus (<10 GPa) and low hardness (<2 GPa) can significantly minimize these mechanical defects during cycling. (4) A design strategy is proposed for high CAM loading with minimal mechanical defects when different SE materials are utilized in the composite cathode, including an oxide-type SE, a sulfide-type SE, and a halide-type SE. The research provides specific guidelines to optimize the composite cathode in terms of its mechanical properties. These guidelines broaden the design approach toward improving the performance of ASSBs, by highlighting the importance of considering the mechanical properties of battery materials.
Na Super Ionic Conductor (NASICON) materials are an important class of solid-state electrolytes owing to their high ionic conductivity and superior chemical and electrochemical stability. In this paper, we combine first-principles calculations, experimental synthesis and testing, and natural language-driven text-mined historical data on NASICON ionic conductivity to achieve clear insights into how chemical composition influences the Na-ion conductivity. These insights, together with a high-throughput first-principles analysis of the compositional space over which NASICONs are expected to be stable, lead to the successful synthesis and electrochemical investigation of several new NASICONs solid-state conductors. Among these, a high ionic conductivity of 1.2 mS cm-1 could be achieved at 25 °C. We find that the ionic conductivity increases with average metal size up to a certain value and that the substitution of PO4 polyanions by SiO4 also enhances the ionic conductivity. While optimal ionic conductivity is found near a Na content of 3 per formula unit, the exact optimum depends on other compositional variables. Surprisingly, the Na content enhances the ionic conductivity mostly through its effect on the activation barrier, rather than through the carrier concentration. These deconvoluted design criteria may provide guidelines for the design of optimized NASICON conductors.
2LiX-GaF3 (X = Cl, Br, I) electrolytes offer favorable features for solid-state batteries: mechanical pliability and high conductivities. However, understanding the origin of fast ion transport in 2LiX-GaF3 has been challenging. The ionic conductivity order of 2LiCl-GaF3 (3.20 mS/cm) > 2LiBr-GaF3 (0.84 mS/cm) > 2LiI-GaF3 (0.03 mS/cm) contradicts binary LiCl (10-12 S/cm) < LiBr (10-10 S/cm) < LiI (10-7 S/cm). Using multinuclear 7Li, 71Ga, 19F solid-state nuclear magnetic resonance and density functional theory simulations, we found that Ga(F,X)n polyanions boost Li+-ion transport by weakening Li+-X- interactions via charge clustering. In 2LiBr-GaF3 and 2LiI-GaF3, Ga-X coordination is reduced with decreased F participation, compared to 2LiCl-GaF3. These insights will inform electrolyte design based on charge clustering, applicable to various ion conductors. This strategy could prove effective for producing highly conductive multivalent cation conductors such as Ca2+ and Mg2+, as charge clustering of carboxylates in proteins is found to decrease their binding to Ca2+ and Mg2+.
Structural and energetic properties of the grossular-katoite solid solution are studied with a full ab initio quantum chemical approach. An all-electron basis set and the hybrid B3LYP functional are used. Calculations are performed within the primitive cell of cubic garnets. The hydrogarnet substitution, SiO4 ↔ H4O4, yields 136 symmetry-independent configurations ranging from triclinic to cubic symmetry. All of them have been structurally optimized, the relaxed geometries being characterized by pseudo-cubic conventional cells. At the present level of approximation, the most stable configurations constitute by far the largest contributions to the system properties. Considering only the most stable configurations, average geometrical features of the actual solid solution are closely approximated. The excess volume displays a highly non-ideal behavior that is favorably compared with carefully analyzed and selected experimental data. The excess enthalpy deviates from the regular model; it draws an asymmetric function of composition with two minima that can be associated to structures or compositions observed in nature. Geometrical variations and distribution of the tetrahedra are analyzed. Calculations provide independent support to the use of a split-atom model for experimental refinements on these compounds. The asymmetry of the enthalpy of mixing can be associated with two distinct distribution patterns of the tetrahedra. Hydrogen interactions also contribute to the asymmetry of the excess enthalpy, as it turns out by comparison between compositions close to fully hydrated katoite and those close to grossular. Hydrogen interactions in Si-free katoite are found to be weak as suggested by dramatic changes in the H environment associated with the introduction of SiO4 tetrahedra.
Abstract In this paper we develop the stability rules for NASICON-structured materials, as an example of compounds with complex bond topology and composition. By first-principles high-throughput computation of 3881 potential NASICON phases, we have developed guiding stability rules of NASICON and validated the ab initio predictive capability through the synthesis of six attempted materials, five of which were successful. A simple two-dimensional descriptor for predicting NASICON stability was extracted with sure independence screening and machine learned ranking, which classifies NASICON phases in terms of their synthetic accessibility. This machine-learned tolerance factor is based on the Na content, elemental radii and electronegativities, and the Madelung energy and can offer reasonable accuracy for separating stable and unstable NASICONs. This work will not only provide tools to understand the synthetic accessibility of NASICON-type materials, but also demonstrates an efficient paradigm for discovering new materials with complicated composition and atomic structure.
The key challenges in all-solid-state batteries (ASSBs) are establishing and maintaining perfect physical contact between rigid components for facile interfacial charge transfer, particularly between the solid electrolyte and cathode, during repeated electrochemical cycling. Here, we introduce inorganic-based pliable solid electrolytes that exhibit extraordinary clay-like mechanical properties (storage and loss moduli <1 MPa) at room temperature, high lithium-ion conductivity (3.6 mS cm–1), and a glass transition below −50 °C. The unique mechanical features enabled the solid electrolyte to penetrate into the high-loading cathode like liquid, thereby providing complete ionic conduction paths for all cathode particles as well as maintaining the pathway even during cell operation. We propose a design principle in which the complex anion formation including Ga, F, and a different halogen can induce the clay-like features. Our findings provide new opportunities in the search for solid electrolytes and suggest a new approach for resolving the issues caused by the solid electrolyte–cathode interface in ASSBs.
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