Interfacial Stability and Design Strategies for Halide Solid Electrolytes in High‐Voltage All‐Solid‐State Sodium‐Ion Batteries

ABSTRACT

All‐solid‐state sodium‐ion batteries (ASSSIBs) based on halide solid electrolytes (HSEs) are emerging as promising systems for high energy density and stable energy storage. Although HSEs are generally regarded as compatible with high‐voltage oxide cathodes, their interfacial stability remains insufficiently understood. Here, we evaluate the interfacial chemical compatibility between representative HSEs and high‐voltage sodium cathode materials through mutual decomposition reaction energy calculations. The analysis reveals interfacial instability of HSEs against high voltage cathodes, challenging the prevailing assumption of their intrinsic stability and highlighting the need for targeted interface design. To address this issue, a high‐throughput computational screening of 12 800 sodium‐containing compounds was performed, identifying several coating materials that effectively suppress interfacial reaction driving forces. These coatings promote stable SE–cathode interfaces, ensuring chemical compatibility under high voltage operation. This study establishes a strategic framework for interfacial design that deepens the understanding of HSE stability and advances the development of durable, high‐energy ASSSIBs.

All‐solid‐state sodium‐ion batteries using halide solid electrolytes are evaluated for interfacial stability with high‐voltage cathodes. Decomposition energy calculations reveal unexpected interfacial instability. High‐throughput screening of 12 800 sodium compounds identifies effective coating materials that suppress interfacial reactions, enabling stable electrolyte–cathode interfaces and advancing durable, high‐energy solid‐state sodium‐ion batteries.

1. Introduction

Lithium‐ion batteries (LIBs) are widely used in portable electronics, electric vehicles, and stationary energy storage systems (ESSs). However, the increasing cost and limited availability of lithium resources have driven the development of alternative battery technologies beyond LIBs [1]. Among these, sodium‐ion batteries (SIBs) are considered promising next‐generation candidates owing to the low cost and natural abundance of sodium [2]. Recently, all‐solid‐state sodium‐ion batteries (ASSSIBs) incorporating inorganic sodium solid electrolytes (SEs) have attracted growing attention because of their inherent safety and potential for high energy density. With their combined advantages in safety, energy density, and cost, ASSSIBs are being investigated as viable options for large‐scale stationary ESS applications [3, 4].

Various inorganic SEs, including sulfides and oxides, have been investigated for their high ionic conductivities [4, 5]. Sulfide‐based SEs exhibit excellent ionic conductivities of 1–10 mS cm⁻¹ but generally suffer from chemical instability when in contact with high‐voltage cathode materials [6, 7]. In contrast, oxide SEs provide enhanced chemical stability but often require high‐temperature sintering to achieve intimate interfacial contact [8, 9]. Recently, halide solid electrolytes (HSEs) have emerged as promising alternatives owing to their favorable combination of high ionic conductivity, mechanical deformability, and chemical stability [10, 11, 12]. HSEs have been investigated for high‐energy ASSSIBs because they enable the use of high‐voltage layered oxide cathodes.

Following the development of lithium‐based HSEs with high ionic conductivities [13, 14, 15, 16], analogous sodium HSEs have recently been reported. These include Zr‐based chlorides (Na_0.7La_0.7Zr_0.3Cl₄), a Ta‐based chloride (NaTaCl₆), and oxychlorides (NaAlCl_2.5O_0.75 and NaTaOCl₄), all exhibiting high ionic conductivities of ∼1 mS cm⁻¹ [17, 18, 19, 20, 21, 22]. These materials offer high oxidative stability and good chemical compatibility with 4 V class high voltage cathodes [20, 21, 22]. In addition, fluorine substitution, as in Na_0.5ZrCl₄F_0.5, has proven to be an effective strategy for further enhancing oxidative stability, thereby enabling the use of uncoated high voltage cathodes [23, 24]. HSEs demonstrate promising compatibility with high voltage cathodes, including polyanionic compounds and layered oxides, enabling the realization of high energy density ASSSIBs.

HSEs are generally regarded as compatible with high‐voltage oxide cathodes; however, their interfacial stability with these cathodes remains insufficiently understood. A systematic evaluation of the electrochemical stability and interfacial chemical compatibility of HSEs is therefore essential for their practical implementation in ASSSIBs. In this study, we comprehensively investigate the interfacial chemical compatibility between representative HSEs and high‐voltage cathode materials. Mutual decomposition reaction energy calculations between HSEs and cathode materials were performed to determine the thermodynamic driving forces for interfacial reactions. The results revealed considerable interfacial reactivity between them, contradicting the general assumption that HSEs are chemically compatible with high‐voltage cathodes.

Protective coating layers formed at the interface between SEs and cathodes represent an effective approach to suppress interfacial decomposition in solid‐state batteries [25, 26, 27, 28]. In this study, a high‐throughput computational screening was carried out to identify coating materials capable of mitigating such decomposition reactions. A dataset of 12 800 sodium‐containing compounds was systematically screened, leading to the discovery of several promising coating candidates that significantly reduce interfacial reactivity. These coating materials can promote the formation of stable interfaces between HSEs and high voltage cathodes, thereby advancing the development of high energy density ASSSIBs. Overall, this study offers fundamental insights into the interfacial stability of sodium‐based HSEs and provides practical guidelines for designing chemically stable interfaces in ASSSIBs.

2. Results

The electrochemical stability windows of sodium HSEs were analyzed using grand potential phase diagrams, as presented in Figure 1. For reference, sulfide‐based SEs exhibit relatively narrow stability ranges, mainly constrained by their low oxidation potentials, which fall below the typical operating voltage of cathode materials. The oxidation potentials of representative sulfide SEs are approximately 2 V (1.66 V for Na₃SbS₄, 2.05 V for Na₃PS₄, and 1.91 V for Na₁₁Sn₂PS₁₂), suggesting that oxidative decomposition is likely to occur within the cathode operating range. In contrast, HSEs exhibit considerably broader stability windows, with oxidation potentials around 3.76 V, indicating enhanced oxidative stability. The oxidation potentials of the seven investigated HSEs are nearly identical, as their oxidative decomposition consistently involves the formation of NaCl₃. The corresponding decomposition products include ZrCl₄ for Zr‐based chlorides, binary metal chlorides for other chloride electrolytes, NaAlCl₄ and Al₂O₃ for NaAlCl_2.5O_0.75, TaCl₃O for NaTaOCl₄, and ZrF₄ for Na_0.5ZrCl₄F_0.5. Although the intrinsic oxidation potential does not fully cover the operating voltage range of 4 V‐class cathodes, kinetic overpotentials during oxidation can effectively extend the practical oxidative stability to higher voltages. The reduction potentials of HSEs vary depending on the central metal element. Zr‐based HSEs exhibit reduction potentials of approximately 1.69 V, corresponding to the formation of ZrCl₃ and NaCl. Ta‐based HSEs show slightly higher reduction potentials near 2.18 V, whereas Al‐based HSE displays lower reduction potentials (∼1.5 V), indicating greater reductive stability. Overall, the reduction potentials of all HSEs remain below 2.2 V, suggesting sufficient thermodynamic stability against reduction. The calculated oxidation and reduction potentials of the HSEs, together with their equilibrium decomposition phases, are summarized in Table S2.

FIGURE 1.

Electrochemical stability windows of sodium HSEs—including chlorides (blue), oxychlorides (orange), and fluorine‐substituted chloride (green)—in comparison with sodium sulfide SEs (red). Gray bars represent the operating voltage ranges of cathode materials. The dashed box indicates the kinetically extended stability region (up to −25 meV/atom).

The reduction and oxidation reactions of HSEs as a function of the sodium chemical potential were further analyzed, as shown in Figure 2 and Figure S1 and summarized in Table S3. The decomposition energy profiles in Figure 2 and Figure S1 illustrate the thermodynamic driving forces for reduction and oxidation reactions as a function of potential. Within the electrochemical stability window, the decomposition energy remains zero but increases upon reaching the reduction and oxidation potentials. At the onset of oxidation, beginning around 3.8 V, the decomposition energy rises, leading to oxidative decomposition of the HSEs. The decomposition energies remain relatively low (<25 meV/atom) up to approximately 4.0 V, suggesting that oxidation can be kinetically suppressed, thereby extending the stability window, as indicated by the dashed bars in Figure 1. The oxidation potentials are extended to 3.92–4.18 V across the examined HSEs: 3.92 V for Na₂ZrCl₆, 4.06 V for Na_0.7La_0.7Zr_0.3Cl₄, 4.12 V for Na_0.625Y_0.25Zr_0.75Cl_4.375, 4.06 V for NaTaCl₆, 4.15 V for NaAlCl_2.5O_0.75, 4.03 V for NaTaOCl₄, and 4.18 V for Na_0.5ZrCl₄F_0.5. Among these, the Al‐based HSE exhibits relatively high oxidative stability, while fluorination of Na₂ZrCl₆ further enhances oxidation stability, increasing the potential from 3.92 to 4.18 V in Na_0.5ZrCl₄F_0.5. Overall, HSEs exhibit broad electrochemical stability windows that fully cover the cathode operating potentials against reduction, while kinetic suppression of oxidative decomposition extends the effective oxidation limit to approximately 4.1 V. The detailed reduction and oxidation reactions as a function of potential, together with the corresponding decomposition products, are summarized in Table S3.

FIGURE 2.

(a–d) Decomposition energy profiles of sodium HSEs as a function of sodium chemical potential and (e–h) corresponding electrochemical potential curves under oxidation (desodiation) and reduction (sodiation) conditions for: (a,e) Na₂ZrCl₆, (b,f) NaTaCl₆, (c,g) NaAlCl_2.5O_0.75, and (d,h) Na_0.5ZrCl₄F_0.5.

Although HSEs exhibit intrinsically wide electrochemical stability windows that broadly cover the operating potential range of cathodes, their chemical compatibility with cathode materials remains a critical factor governing interfacial stability. Interfacial reactions between HSEs and cathodes can lead to the formation of decomposition interphases, which degrade the electrochemical performance of ASSSIBs. To assess potential interfacial reactions, Δ𝐸_{D, mutual} calculations were conducted. The maximum mutual reaction energies between HSEs and cathodes were determined to quantify the thermodynamic driving forces for interfacial decomposition. The calculation results are presented as a heatmap in Figure 3, providing a comparative overview of interfacial stability across various HSE–cathode pairs. More negative reaction energies reflect stronger thermodynamic tendencies toward interfacial reactions. Detailed reaction and corresponding decomposition products for each HSE–cathode pair are summarized in Table S4.

FIGURE 3.

Heatmap of Δ𝐸_{D, mutual} between sodium SEs and sodium oxide cathodes. More negative values indicate a stronger thermodynamic driving force for interfacial decomposition. The color bar denotes the mutual decomposition reaction energy scale, with blue indicating lower reaction energies (greater interfacial stability) and red indicating higher reaction energies (greater interfacial reactivity).

The reaction energies between seven HSEs and fourteen cathode materials—including two polyanionic compounds, six P2‐type, and six O3‐type layered oxides—are presented as a heatmap in Figure 3. For reference, the bottom three rows show the reaction energies between three representative sulfide SEs and the same cathode materials. Overall, HSEs exhibit negative reaction energies, indicating thermodynamic driving forces for interfacial decomposition. Among the cathodes, polyanionic compounds display the highest stability, with reaction energies of approximately −80 meV/atom against HSEs. In contrast, P2‐ and O3‐type layered oxides exhibit more negative reaction energies, reflecting stronger driving forces for interfacial reactions. The HSEs exhibit average reaction energies of approximately −140 meV/atom with P2‐type cathodes and further instability of around −220 meV/atom with O3‐type cathodes, whereas variations in Δ𝐸_{D, mutual} among layered oxides with different metal compositions are minimal. Although these reaction energies are less negative than those calculated for sulfide SEs, which generally exhibit chemical instability (−230 and −250 meV/atom for P2‐ and O3‐type oxides, respectively), HSEs still demonstrate considerable interfacial reactivity toward both P2‐ and O3‐type layered cathodes. Despite their strong intrinsic oxidative stability, HSEs exhibit pronounced chemical reactivity with high‐voltage oxides. These findings highlight the importance of incorporating protective coating layers to mitigate interfacial decomposition, particularly for P2‐ and O3‐type cathodes.

A high‐throughput computational screening was carried out to identify coating materials capable of stabilizing the interface between HSEs and cathodes, as illustrated in the overall workflow shown in Figure 4. A total of 12 800 sodium‐containing compounds were obtained from the Materials Project database and systematically filtered according to several criteria. In the first step, compounds containing heavy elements (atomic number > 86), alkali metals other than sodium (e.g., Li, K, Rb, Cs), or exhibiting electronic conductivity with band gaps below 0.5 eV were excluded, resulting in 7,995 remaining compounds. From this subset, thermodynamically unstable phases were removed. In the subsequent step, materials with narrow electrochemical stability windows were screened out, retaining those with reduction potentials below 2.5 V and oxidation potentials above 3.5 V, yielding 289 remaining compounds.

FIGURE 4.

Schematic diagram of the high‐throughput computational workflow used to identify coating materials capable of stabilizing interfaces between sodium HSEs and cathode materials.

For these 289 compounds, a data‐driven unsupervised learning approach integrating principal component analysis (PCA) with K‐means clustering was employed to classify coating materials according to their reaction‐energy profiles, as shown in Figure 5. The analysis distinctly separated the dataset into four chemically meaningful groups (A–D, Figure 5a), each representing a unique interfacial‐stability regime between the coating, solid electrolyte, and cathode. The clustering was performed in the full mutual decomposition–energy descriptor space to group materials with similar interfacial reactivity patterns, while principal component analysis was employed for low‐dimensional visualization of these high‐dimensional relationships (Figure 5a). Group A exhibited slightly negative reaction energies toward solid electrolyte interfaces while maintaining low reactivity with cathodes, indicating a stable group with good interfacial compatibility. Group B, showing even lower overall reaction energies than Group A, demonstrated good chemical compatibility with both SEs and cathodes and was therefore identified as the most stable group. Group C maintained intermediate reaction energies across both SEs and cathodes, suggesting a moderately stable group that is less stable than Groups A and B. In contrast, Group D remained relatively stable with SEs but displayed pronounced reactivity toward cathodes and was thus classified as the reactive group. The complete list of materials belonging to Groups A–D is provided in Table S5.

FIGURE 5.

(a) PCA–K‐means clustering of coating materials based on Δ𝐸_{D, mutual} profiles, defining four stability groups (A–D). (b) Corresponding heatmap showing average Δ𝐸_{D, mutual} of each group with sodium HSEs and cathodes, highlighting their interfacial reactivity trends.

The chemical compatibility of the remaining 289 candidates was further evaluated using their mutual reaction energies. Compounds exhibiting average reaction energies below 50 meV/atom and maximum values below 60 meV/atom were identified as promising coating candidates for interfacial stabilization. Through the screening process, eleven promising coating materials—NaAlH₂CO₅, Na₆CdCl₈, Na₂Ca(SO₄)₂, Na₂Al₂Si₃(HO₃)₄, NaB₃O₅, NaAlSi₃O₈, NaB₅(H₂O₅)₂, Na₂B₈O₁₃, BaNa(B₃O₅)₃, Na₆Mg(SO₄)₄, and Na₃B₆PO₁₃—were identified as potential candidates for stabilizing the HSE/cathode interface, as summarized in Figure 6 and Table S6. These compounds exhibited low mutual reaction energies, demonstrating their potential to mitigate interfacial degradation.

FIGURE 6.

Heatmap of Δ𝐸_{D, mutual} of (a) sodium HSEs and (b) cathodes against energetically stable coating materials. The bottom two rows correspond to cathodes and HSEs for comparison.

Because the strict Δ𝐸_{D, mutual} criterion may exclude some practically viable coating materials, the final screening step was refined by relaxing the Δ𝐸_{D, mutual} threshold to a maximum value below −110 meV/atom. For practical considerations, only oxides and polyanionic compounds containing fewer than four elements were selected, while compounds composed solely of sodium and anions were excluded. This modified screening approach yielded a broader set of twenty‐five candidate materials, including Na₂Ca(SO₄)₂, NaAlSi₃O₈, Na₆Mg(SO₄)₄, NaB₃O₅, BaNa(B₃O₅)₃, Na₂B₈O₁₃, Na₃B₆PO₁₃, Na₂Ta₄O₁₁, Na₃Sc₂(PO₄)₃, NaNb₃O₈, NaBe₄SbO₇, NaTiPO₅, NaAlSiO₄, NaHf₂(PO₄)₃, NaNb₂PO₈, NaZr₂(PO₄)₃, NaScP₂O₇, NaTi₂(PO₄)₃, NaNb₁₃O₃₃, Na₄Mg₃P₄O₁₅, NaTaO₃, NaBePO₄, NaSn₂(PO₄)₃, Na₃Mg₂P₅O₁₆, and NaNbO₃, as shown in Figure 7 and Table S7. The identified coating materials can be broadly classified into several chemical families, including phosphates (Na–M–PO₄, M = Zr, Hf, Ti, Sn, Sc, B), borates (Na–B–O), aluminosilicates (Na–Al–SiO₄), sulfates (Na–M–SO₄, M = Ca, Mg), high‐valent niobates (Na–Nb–O) and tantalates (Na–Ta–O), as well as several oxides with mixed cation compositions. From a practical perspective, synthesizability and coating‐process compatibility should also be considered for the screened materials. Compounds with high compositional complexity, particularly those containing more than four elements, may present challenges in synthesis, phase purity control, or conformal coating deposition. By contrast, simpler oxide‐ or ternary‐based materials are generally more amenable to experimental coating processes. The identified candidates, therefore, represent thermodynamically viable materials that require further experimental evaluation to assess practical feasibility.

FIGURE 7.

Heatmap of Δ𝐸_{D, mutual} of sodium HSEs and cathodes against practically relevant coating materials. The bottom two rows represent cathodes and HSEs for comparison.

The chemical reactivity of the coating materials with HSEs and cathodes is illustrated in Figures 6 and 7, which present the mutual reaction energies of the selected candidates for both energetically stable coating materials (Figure 6) and practically relevant coating materials (Figure 7). The bottom highlighted compounds in Figures 6 and 7 correspond to the HSEs and layered cathodes, which are included as references. Compared with the direct cathode–SE interfaces shown in the two bottom rows, the reaction energies at both interfaces (coating–SE and coating–cathode) are substantially reduced after introducing coating layers. The decreased reaction energies of the coated systems highlight the effectiveness of the coatings in mitigating interfacial reactivity. Whereas the reaction energies of SE–cathode pairs are typically around −140 meV/atom for P2‐type and −220 meV/atom for O3‐type cathodes, these values decrease to approximately −20 to −40 meV/atom upon the incorporation of coating layers, indicating a significant enhancement in interfacial stability.

The detailed interfacial reactions between HSEs and cathodes, along with the effects of the coating layer, are illustrated in Figure 8, which presents the mutual reaction energies as a function of composition ratio. As an example, the reaction energies between two representative HSEs (Na₂ZrCl₆ and NaTaOCl₄) and three layered oxide cathodes (Na_0.67Mn_0.65Co_0.2Ni_0.15O₂, NaNi_0.33Fe_0.33Mn_0.33O₂, and NaNi_0.61Co_0.12Mn_0.27O₂) are shown, together with the reduced reaction energies obtained after introducing a NaB₃O₅ coating layer. For the Na₂ZrCl₆‐based interfaces, the reaction energies with the three cathodes are −131, −213, and −224 meV/atom, respectively. Upon introducing the NaB₃O₅ coating, these cathode–coating reaction energies decrease to −35, −33, and −34 meV/atom, while the corresponding HSE–coating Δ𝐸_{D, mutual} is reduced to −19 meV/atom (Figure 8a–c). A similar trend is observed for the NaTaOCl₄‐based interfaces (Figure 8d–f): the HSE–cathode reaction energies of −137, −224, and −236 meV/atom are substantially lowered after applying the NaB₃O₅ coating, with the HSE–coating Δ𝐸_{D, mutual} reaching −24 meV/atom. These results clearly demonstrate the critical role of coating materials in stabilizing the HSE–cathode interface and underscore their significance in the design of durable, high‐energy ASSSIBs.

FIGURE 8.

Calculated mutual reaction energies among cathodes, HSEs, and coating materials. Each panel compares three interfaces—HSE–coating (red), cathode–coating (blue), and cathode–HSE (gray). Panels (a–c) correspond to Na₂ZrCl₆‐based systems, and (d–f) to NaTaOCl₄‐based systems, paired with cathodes (a,d) Na_0.67Mn_0.65Co_0.2Ni_0.15O₂, (b,e) NaNi_0.33Fe_0.33Mn_0.33O₂, and (c,f) NaNi_0.61Co_0.12Mn_0.27O₂.

The interfacial stability analysis presented in this study is based on mutual decomposition reaction energies derived from bulk thermodynamic data and does not explicitly capture atomic‐scale interface structures or kinetic barriers. By removing the intrinsic decomposition tendencies of the individual compounds, this metric isolates the thermodynamic driving force associated with interfacial contact and enables efficient high‐throughput screening. The resulting reaction energies should therefore be interpreted as relative indicators of thermodynamic compatibility, rather than as direct predictions of interfacial reaction pathways or decomposition products under realistic conditions. More detailed interface modeling and experimental validation are required to fully assess kinetic effects and interfacial reaction mechanisms.

Previous experimental and computational studies have shown that suppressing thermodynamically driven interfacial reactions, often through interfacial coatings or buffer layers, is critical for stabilizing solid electrolyte–cathode interfaces and improving electrochemical performance in solid‐state batteries. Good agreement has been reported between reaction‐energy‐based interfacial screening and experimental observations in solid‐state systems, including Na‐sulfide and Li‐chloride SEs [27, 28]. Although experimental studies on Na‐halide SEs remain limited, their interfacial instability is likewise governed by thermodynamic driving forces. Given the demonstrated predictive capability of reaction‐energy‐based screening even for chemically more reactive sulfide systems, the present framework is expected to provide a physically meaningful guideline for the design of stable interfaces in emerging Na‐halide solid‐state batteries.

The present thermodynamic analysis is based on bulk crystalline phases and equilibrium phase‐diagram calculations. In practice, halide electrolytes synthesized via non‐equilibrium routes may exhibit amorphous, nanocrystalline, or metastable phases, and such kinetic effects are not captured in the current framework. As a result, discrepancies between computational predictions and experimentally realized interfaces may arise, underscoring the need for future extensions that incorporate kinetic, synthesis‐aware, or finite‐temperature effects [29]. In addition, it should be noted that the present screening considers only intrinsic thermodynamic stability and first‐principles interfacial reaction energies, without accounting for processing‐related factors such as the moisture sensitivity of HSEs. Consequently, the identified coating candidates are thermodynamically compatible materials, and further evaluation of their moisture tolerance and practical processability will be required for experimental implementation.

3. Conclusions

HSEs are generally regarded as compatible with high‐voltage oxide cathodes, yet their interfacial stability remains insufficiently understood. In this study, the electrochemical stability of HSEs and their interfacial compatibility with high‐voltage cathode materials were systematically investigated. Although HSEs intrinsically exhibit wide electrochemical stability windows that broadly cover the operating potential range of cathodes, they show noticeable interfacial reactivity with P2‐ and O3‐type layered oxide cathodes, highlighting the need for targeted interface engineering. High‐throughput computational screening of 12 800 sodium‐containing compounds identified several coating materials capable of mitigating interfacial decomposition reactions. The selected coatings—including oxides, borates, sulfates, silicates, and phosphates—significantly reduce interfacial reaction energies, effectively stabilizing the HSE–cathode interface. Detailed interfacial analyses confirmed that oxygen‐based coatings, such as NaB₃O₅, markedly suppress interfacial reactivity, decreasing reaction energies from over −200 meV/atom to below −40 meV/atom. This work establishes a comprehensive interfacial design framework for achieving chemical compatibility in halide‐based high‐voltage ASSSIBs. The insights obtained here provide clear guidance for the development of durable, high‐energy, and intrinsically safe ASSSIBs.

4. Methods

Crystal structures, formation energies, and convex‐hull stabilities were obtained from the Materials Project database, which provides a consistent first‐principles thermodynamic dataset [30, 31, 32]. These quantities served as inputs for all subsequent analyses conducted in this study, including (i) construction of sodium grand‐potential phase diagrams and determination of electrochemical stability windows for HSEs, (ii) calculation of mutual decomposition reaction energies for solid‐electrolyte/cathode and coating/cathode pairs, (iii) high‐throughput screening of sodium‐containing compounds based on thermodynamic stability, electrochemical limits, and interfacial decomposition reaction energetics, and (iv) statistical classification of coating candidates using unsupervised machine‐learning methods.

The calculations were conducted using data from the Materials Project database, which provides pre‐computed first‐principles thermodynamic information [30, 31, 32]. Structural and energetic data for all relevant compounds were retrieved and analyzed using the Python Materials Genomics (Pymatgen) package [30]. The HSEs and sodium cathode materials were assumed to be thermodynamically stable, corresponding to phases located on the convex hull. Electrochemical stability windows were evaluated using a grand‐potential phase‐diagram approach [26, 33, 34]. For each SE, phase diagrams were constructed, and the sodium chemical potential (corresponding to the voltage versus Na/Na⁺) was varied to identify the voltage range over which the SE remains thermodynamically stable without decomposition into competing phases. This voltage range is defined as the electrochemical stability window. The interfacial chemical compatibility between each HSE and cathode was assessed using mutual decomposition reaction energy calculations. For each pair, the maximum thermodynamic driving force was determined by varying the reaction ratio to identify the most energetically favorable decomposition reaction.

The decomposition reaction energy, denoted as Δ𝐸_D, was used to quantify the chemical compatibility between two compounds, A and B, across a range of molar fractions between zero and one. It was defined as the difference between the equilibrium energy of the lowest‐energy mixture of all possible competing phases and the energy of the ideal pseudo‐binary mixture of A and B, as given by:

$$\mathrm{\Delta }{E}_{\mathrm{D}}\left(\mathrm{A},\mathrm{B},x\right)=E_{\mathrm{eq}}\left(C_{\mathrm{bin}}\left(C_{\mathrm{A}},C_{\mathrm{B}},x\right)\right)-E_{\mathrm{bin}}\left(\mathrm{A},\mathrm{B},x\right)$$ (1)

where 𝐸_bin(A, B, 𝑥) represents the energy of the pseudo‐binary mixture, and 𝐸_eq (𝐶_bin(𝐶_A, 𝐶_B, 𝑥)) denotes the equilibrium energy of the system at the same composition, corresponding to the lowest energy among all competing phases. The pseudo‐binary energy was expressed as the weighted average of the total energies of A and B according to their molar fractions, while the corresponding composition was determined as the weighted sum of the atomic compositions of both compounds. A mutual decomposition reaction energy, denoted as Δ𝐸_{D, mutual}, was obtained by subtracting the intrinsic decomposition energies of each individual compound from the total decomposition energy, as defined by:

$$\begin{array}{cc}\hfill \mathrm{\Delta }{E}_{\mathrm{D},\mathrm{mutual}}\left(\mathrm{A},\mathrm{B},x\right)& =\mathrm{\Delta }{E}_{\mathrm{D}}\left(\mathrm{A},\mathrm{B},x\right)-x\mathrm{\Delta }{E}_{\mathrm{D}}\left(\mathrm{A}\right)\hfill \\ & -\left(1-x\right)\mathrm{\Delta }{E}_{\mathrm{D}}\left(\mathrm{B}\right)\hfill \end{array}$$ (2)

This definition can be interpreted within a convex‐hull‐based framework. The decomposition reaction energy defined in Equation (1) represents the total thermodynamic driving force for phase decomposition, including the intrinsic instability of the individual compounds. In contrast, the mutual decomposition reaction energy defined in Equation (2) subtracts these intrinsic contributions and isolates the additional thermodynamic driving force arising from interfacial contact between A and B, as schematically illustrated in Figure S2. A value of zero indicates complete chemical compatibility between the two materials, whereas nonzero value represents a thermodynamic driving force for interfacial reactions. Comprehensive descriptions of the Δ𝐸_{D, mutual} calculation methodology are provided in previous studies [25, 33, 34, 35].

A high‐throughput computational workflow was implemented to screen potential coating materials capable of reducing chemical instability at HSE–cathode interfaces ASSSIBs. A dataset comprising 12 800 sodium‐containing compounds was obtained from the Materials Project database and systematically refined using the following selection criteria: (i) compounds containing heavy elements (atomic number > 86), alkali metals in the +1 oxidation state (Li, K, Rb, Cs), or band gaps <0.5 eV were excluded; (ii) thermodynamically stable phases on the convex hull were retained; (iii) materials with reduction potentials >2.5 V or oxidation potentials lower than 3.5 V were discarded; and (iv) compounds exhibiting low reaction energies with both HSEs and high‐voltage cathodes were identified as promising coating candidates.

Thermodynamic stability was evaluated based on each compound's convex‐hull energy (E _hull), retaining only phases located exactly on the convex hull (E _hull = 0). Compounds with E _hull > 0, indicating a tendency to decompose into competing phases, were excluded from the dataset. The E _hull = 0 criterion was adopted as a conservative definition of thermodynamic stability to restrict the chemical space and enable meaningful high‐throughput screening. Although allowing a finite metastability window could include additional kinetically accessible phases, such an expansion would substantially increase the candidate pool and complicate systematic analysis. Because the subsequent K‐means clustering groups materials based on similarities in interfacial reaction‐energy profiles, rather than on individual compound identities, the extraction of general interfacial design principles remains robust even with a strict stability cutoff. Accordingly, a conservative E _hull criterion was employed to facilitate trend analysis rather than exhaustive materials enumeration.

Electrochemical stability thresholds of >2.5 V (reduction) and <3.5 V (oxidation) were applied as screening criteria for candidate coating materials. While sodium‐ion batteries can operate over a broad voltage range (∼2.0–4.0 V vs Na/Na⁺), enforcing this full window in thermodynamic screening would eliminate most candidates. In practice, interfacial overpotentials and kinetic effects shift effective operating voltages, making these limits reasonable for retaining viable coatings while excluding materials susceptible to rapid redox‐driven degradation.

Representative HSEs with high ionic conductivities (∼1 mS cm⁻¹) were investigated, including seven compounds: three Zr‐based chlorides (Na₂ZrCl₆, Na_0.7La_0.7Zr_0.3Cl₄, and Na_0.625Y_0.25Zr_0.75Cl_4.375) [17, 18, 19] one Ta‐based chloride (NaTaCl₆) [20], two oxychlorides (NaAlCl_2.5O_0.75 and NaTaOCl₄) [21, 22], and a fluorinated Zr‐based chloride (Na_0.5ZrCl₄F_0.5) [23], as summarized in Table 1. High‐voltage cathode candidates included polyanionic compounds and P2‐ and O3‐type layered oxides [36, 37, 38]. The set comprised two polyanionic compounds (Na₃V₂(PO₄)₃ and NaFePO₄), six P2‐type layered oxides (Na_0.6Mn_0.8Li_0.2O₂, Na_0.67Fe_0.5Mn_0.5O₂, Na_0.67Ni_0.33Mn_0.33Ti_0.33O₂, Na_0.67Co_0.5Mn_0.5O₂, Na_0.67Mn_0.65Co_0.2Ni_0.15O₂, and Na_0.67Mn_0.7Ni_0.2Mg_0.1O₂) as well as six O3‐type layered oxides (NaFe_0.5Co_0.5O₂, NaNi_0.5Mn_0.5O₂, NaNi_0.5Fe_0.5O₂, NaNi_0.33Fe_0.33Mn_0.33O₂, NaNi_0.33Mn_0.33Co_0.33O₂, and NaNi_0.61Co_0.12Mn_0.27O₂). as listed Table S1.

TABLE 1.

Sodium HSEs with high ionic conductivities and their reported values.

Type	HSEs	Ionic conductivity (mS/cm)	Reference
Zr‐based chlorides	Na2ZrCl6	0.018	[17]
	Na0.7La0.7Zr0.3Cl4	0.29	[18]
	Na0.625Y0.25Zr0.75Cl4.375	0.4	[19]
Ta‐based chloride	NaTaCl6	3.3	[20]
Oxychlorides	NaAlCl2.5O0.75	1.33	[21]
	NaTaOCl4	1.1	[22]
Fluorinated Zr‐based chloride	Na0.5ZrCl4F0.5	0.11	[23]

Funding

This work was supported by the institutional program of the Korea Institute of Science and Technology (No. 2E33941 and 2E33943); by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (RS‐2024‐00404414 and RS‐2024‐00427700); and by the National Supercomputing Center with supercomputing resources including technical support (KSC‐2025‐CRE‐0062).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting file: smtd70462‐sup‐0001‐SuppMat.pdf.

Jang M., Kwon E., Jeon C., Kim S., and Yu S., “Interfacial Stability and Design Strategies for Halide Solid Electrolytes in High‐Voltage All‐Solid‐State Sodium‐Ion Batteries.” Small Methods 10, no. 3 (2026): e02179. 10.1002/smtd.202502179

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.