The Effect of Phosphoric Acid on the Preparation of High-Performance Li₃InCl₆ Solid-State Electrolytes by Water-Mediated Synthesis

Abstract

The halide solid-state electrolyte Li₃InCl₆ (LIC) is an excellent solid-state electrolyte for lithium-ion batteries. In this study, phosphoric acid (H₃PO₄) solution was added during the preparation of Li₃InCl₆ to obtain LIC samples with high ionic conductivity, and the effect of different concentrations of H₃PO₄ on increasing the ionic conductivity of LIC was investigated. The results showed that the LIC prepared in 1% H₃PO₄ had the least impurities and the highest ionic conductivity of 1.15 × 10⁻³ S/cm, and the all-solid-state LiCoO₂-LIC/LIC-1%H₃PO₄/Li₆PS₅Cl/In/Li batteries assembled with this sample could be stably cycled for 14 cycles at 30 °C, presenting a high initial charge capacity of 128 mAh/g at 0.05 C. The Li-In/LPSC/LIC-1% H₃PO₄/LPSC/Li-In symmetric cell can be stably cycled at current densities of 0.05 mA/cm², 0.1 mA/cm², and 0.3 mA/cm². Further studies showed that LIC samples prepared in H₃PO₄ contained small amounts of LiIn(P₂O₇). The structure of LiIn(P₂O₇) is a continuous skeleton containing a large number of vacancies for Li⁺ transport, thus improving the ionic conductivity of LIC.

1. Introduction

Due to the flammability of traditional liquid lithium-ion batteries, liquid leakage, and other safety issues, the use of solid-state electrolytes (SSEs) instead of liquid electrolytes and separators in the all-solid-state lithium-ion batteries (ASSLIBs) has become the most promising development in next-generation energy storage devices [1,2]. Solid-state electrolytes combine the lithium-ion transport capability of traditional liquid electrolytes with the separator function to isolate electrodes, effectively suppressing lithium dendrite penetration and short-circuit-induced fire hazards [3].

Solid-state electrolytes are categorized into organic polymer solid-state electrolytes and inorganic solid-state electrolytes. The latter further includes oxide solid-state electrolytes, sulfide solid-state electrolytes, and halide solid-state electrolytes [3]. Polymer solid-state electrolytes exhibit favorable mechanical processing properties, but their ionic conductivity is generally low (room-temperature ionic conductivity: 10⁻⁸–10⁻⁵ S/cm) [4]. Oxide solid-state electrolytes demonstrate excellent air stability and electrochemical stability; however, they suffer from poor mechanical processability and require high-temperature sintering above 1000 °C for preparation [5]. Sulfide solid-state electrolytes possess high ionic conductivity and relatively good mechanical processing performance, but they generate toxic H₂S gas when exposed to air [6]. Halide solid-state electrolytes, in contrast, not only achieve high ionic conductivity at room temperature but also avoid releasing toxic gases into the air, positioning them as promising competitors for next-generation solid-state electrolytes [7].

In 2019, Li et al. reported a halide solid-state electrolyte, Li₃InCl₆ (LIC), which exhibits an ionic conductivity of 1.49 × 10⁻³ S/cm and can be synthesized at scale in water, paving the way for the industrialization of halide solid-state electrolytes [8]. However, during the dehydration process in the water-mediated synthesis of Li₃InCl₆, the material is prone to oxidation, leading to a rapid decline in ionic conductivity [9]. Acidic environments can suppress hydrolysis and the oxidation of the sample [10], and mixed crystals of Li₃PO₄ and Li₄P₂O₇ have also been identified as promising solid-state electrolytes [11]. Macarie et al. have demonstrated that phosphonic groups exhibit favorable properties in water-mediated synthesis methods by effectively suppressing the formation of scale-forming salts [12] and investigated the synthesis methods of phosphate compounds [13]. Iliescu et al. demonstrates that phosphorus can enhance the ionic conductivity of polymer solid-state electrolytes [14]. Phosphoric acid was selected as an additive due to its ability to suppress hydrolysis via pH modulation and generation of phosphate derivatives (e.g., P₂O₇⁴⁻) that enhance ionic conductivity.

This study employs water-mediated synthesis to prepare LIC, during which phosphoric acid (H₃PO₄) solutions of varying concentrations are introduced to suppress hydrolysis and oxidation of the sample while simultaneously generating a small amount of LiIn(P₂O₇) to form a composite with LIC, thereby enhancing the ionic conductivity of the material. Furthermore, a full solid-state battery LiCoO₂-LIC/LIC-1%H₃PO₄/Li₆PS₅Cl/In/Li was assembled to evaluate its cycling performance.

2. Materials and Methods

2.1. Materials Synthesis

As depicted in Figure 1, precursor powders of LiCl (99%, Tianjin Xiensi, Tianjin, China) and InCl₃ (98%, Macklin, Shanghai, China) were precisely weighed at a 3:1 molar ratio and mechanically homogenized via agate mortar grinding. The resulting mixture was dissolved in phosphoric acid (H₃PO₄, Hushi, Shanghai, China, 85.0%) solutions with mass concentrations of 0%, 0.5%, 1%, and 2%, respectively. H₃PO₄ concentrations (0–2%) were chosen to systematically investigate the trade-off between crystalline water suppression and LiIn(P₂O₇) over formation. According to the water-mediated synthesis route for Li₃InCl₆ reported by Li et al. [8], the solutions were dried in a forced-air drying oven at 100 °C for 4 h to obtain hydrated precursors. To prevent oxidation, the hydrated precursors were ground into powder under an inert atmosphere in a glovebox and subsequently dried in a vacuum-drying oven at 250 °C for 4 h, yielding the final Li₃InCl₆ powders. Samples synthesized in pure water and H₃PO₄ solutions of varying concentrations were labeled as LIC-H₂O, LIC-0.5% H₃PO₄, LIC-1% H₃PO₄, and LIC-2% H₃PO₄.

2.2. Structural Characterization

X-ray diffraction analysis (Rigaku Ultima IV, 2θ range: 10–80°, scan rate: 10°/min) was employed to characterize the crystallographic phase composition of the specimens. The micro-morphology of the samples was characterized using a scanning electron microscope (SEM; Hitachi TM3030, Tokyo, Japan). The samples were mixed with KBr at a mass ratio of 1:100, and the infrared spectra of the samples were obtained using a Nicolet Is5 FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). Simultaneous TG-DSC measurements (Netzsch STA449F5, Selb, Germany) were conducted under dynamic heating (25–400 °C, 15 °C/min) in a nitrogen atmosphere (flow rate: 80 mL/min) to probe thermal stability. The TG temperature range was capped at 400 °C to prevent thermal decomposition of LiCl and InCl₃ [15].

2.3. Electrochemical Measurements

Electrochemical impedance spectroscopy (EIS) measurements were conducted on the synthesized LIC powder using a Metrohm Autolab PGSTAT204 workstation (Ota, Japan). Moreover, 0.15 g of LIC powder was placed between two stainless steel rods of 10 mm diameter in a PEEK (polyether ether ketone) mold (Figure 2), and a pressure of 300 MPa was applied to form a symmetric cell of the powder in the mold. EIS measurements were conducted under an argon atmosphere at ambient temperature (25 °C) across a frequency range of 0.01 Hz to 1 MHz to assess interfacial and bulk resistance. Subsequently, cyclic voltammetry (CV) was performed using a electrochemical workstation (Shanghai CHI760E, CH Instruments, Shanghai, China) on asymmetric cells with a configuration of Li-In/Li₆PS₅Cl/LIC/stainless steel, which were encapsulated in polyetheretherketone (PEEK) molds. The CV scans spanned a potential window of −1 to 3 V (vs. Li⁺/Li-In) at a controlled sweep rate of 0.025 V/s, enabling characterization of redox behavior and interfacial stability. The Blue Battery Test System (LAND-CT2001A, Wuhan Baxter Bao, Wuhan, China) was programmed to cycle cells between 1.9 V and 3.8 V (Li⁺/Li-In) with thermal regulation maintained at 30 ± 0.5 °C.

2.4. Assembly of ASSLIBs

The prepared LIC and lithium cobaltate powder (LiCoO₂, Canrd) were ground uniformly at the mass ratio of 3:7 to obtain the composite cathode powder. The all-solid-state batteries (ASSLIBs) consisted of composite cathode powder, LIC electrolyte powder, Li₆PS₅Cl (LPSC, Shenzhen Cogent, Shenzhen, China), In foil (99.999%, Φ = 10 mm, thickness = 0.1 mm) and Li foil (99.95%, Φ = 10 mm, thickness = 0.1 mm). Firstly, 0.15 g of LIC powder was weighed with an electronic balance and added to a PEEK mold, and the powder was pressed into discs in the mold with a pressure of 300 MPa. Then, LPSC powder and composite positive electrode powder were added at both ends of the discs, and 300 MPa pressure was again applied to the sample in the mold. Finally, indium (In) and lithium (Li) foils were positioned at the LPSC interface to construct the multilayer cell architecture (LiCoO₂-LIC/LIC/LPSC/In/Li).

3. Results and Discussion

Figure 3 presents the SEM images of dried LIC samples prepared with varying H₃PO₄ concentrations. As observed, LIC synthesized in pure water exhibits bulk-like stacking due to the difficulty in removing crystalline water during drying. The LIC-H₂O sample exhibits bulk-like aggregates with irregular shapes, ranging in size from 10 to 50 μm. When 0.5% H₃PO₄ is introduced, the samples form porous bulk structures, which facilitate crystalline water elimination. The porous structure in LIC-0.5% H₃PO₄ contains interconnected voids with diameters of 2–10 μm. With increasing H₃PO₄ concentration, the microstructure progressively transitions to a flake-like morphology. At 1% H₃PO₄, the samples partially display flake-like structures alongside residual porous bulk regions. Finally, at 2% H₃PO₄, the microstructure becomes entirely flake-shaped, indicating complete morphological transformation. The microstructure transitions to flake-like particles with lateral dimensions of 5–20 μm and thicknesses of 0.5–2 μm.

Figure 4 displays the XRD patterns of LIC powders synthesized with varying H₃PO₄ concentrations. During testing, the powders were sealed onto glass slides using Kapton polyimide tape to prevent air exposure. As shown, the XRD patterns of all four samples align well with most characteristic peaks of the LIC reference PDF card (ICSD No. 04-009-9027). The LIC-H₂O sample (prepared in pure water) exhibits weaker peak intensities compared to others, suggesting lower crystallinity or reduced LIC content. Upon H₃PO₄ addition, the XRD patterns reveal coexisting characteristic peaks of LIC and LiIn(P₂O₇), confirming the formation of a LIC/LiIn(P₂O₇) composite. Notably, the intensity of LiIn(P₂O₇) peaks increases progressively with higher H₃PO₄ concentrations, indicating enhanced formation of this phase. This is attributed to the thermal decomposition of PO₄³⁻ (from H₃PO₄) into P₂O₇⁴⁻ at 200 °C, which reacts with In³⁺ and Li⁺ in the sample to yield LiIn(P₂O₇). The P₂O₇⁴⁻ anion, owing to its favorable structural properties, has been reported to enhance ionic conductivity when composited with solid-state electrolytes [11].

The electrochemical impedance spectroscopy (EIS) of the samples, measured under 300 MPa pressure in an Ar atmosphere, is shown in Figure 5a. The inset in the upper-left corner depicts the equivalent circuit based on a simple R(RQ)Q model. By fitting the equivalent circuit and measuring the diameter and thickness of the pressed LIC electrolyte pellets, the ionic conductivity (σ) of the powders was calculated using the following formula:

σ = d/(S × R)

(1)

where σ is the ionic conductivity, d is the pellet thickness, R is the measured AC impedance value, and S is the pellet area. The calculated ionic conductivities are summarized in

Table 1. Ionic conductivity of LIC prepared in different concentrations of H₃PO_4.

Sample	Conductivity σ (×10−3 S/cm)
LIC-H2O	0.38
LIC-0.5% H3PO4	0.65
LIC-1% H3PO4	1.15
LIC-2% H3PO4	0.53

.

As shown in Table 1, the ionic conductivity follows the order of LIC-1% H₃PO₄ > LIC-0.5% H₃PO₄ > LIC-2% H₃PO₄ > LIC-H₂O. The LIC-H₂O sample demonstrates the lowest ionic transport efficiency among the tested materials, with a recorded conductivity of 0.38 × 10⁻³ S/cm. Upon H₃PO₄ addition, the ionic conductivity significantly increases, reaching a maximum value of 1.15 × 10⁻³ S/cm for the LIC-1% H₃PO₄ sample. The non-linear relationship between H₃PO₄ concentration and ionic conductivity (LIC-2% H₃PO₄ < LIC-1% H₃PO₄) is tied to excessive LiIn(P₂O₇) formation, which disrupts the LIC matrix. The ionic conductivity of the sample decreases when the H₃PO₄ concentration exceeds 2%, as the inherently low conductivity of LiIn(P₂O₇) (~10⁻⁵ S/cm) cannot compensate for the loss of the highly conductive LIC matrix. Thus, LiIn(P₂O₇) functions as a secondary phase rather than a primary conductor, a conclusion consistent with prior studies [11].

Figure 5b displays the cyclic voltammetry (CV) profiles acquired at 0.025 V/s over a potential window of −1.0–3.0 V (vs. Li⁺/Li-In). The LIC-1% H₃PO₄ composite demonstrates enhanced peak symmetry relative to counterparts, signifying superior electrochemical reversibility. During anodic polarization, a distinct oxidation peak is observed at 1.0 V, followed by a reduction peak at 2.2 V under cathodic conditions, collectively spanning a broad electrochemical stability window. These results confirm that the electrolyte synthesized with 1% H₃PO₄ delivers the best overall performance.

In summary, the LIC-1% H₃PO₄ electrolyte demonstrates superior electrochemical performance. To evaluate its practical applicability, the all-solid-state lithium-ion batteries (ASSLIBs) LiCoO₂-LIC/LIC-1%H₃PO₄/LPSC/In/Li were assembled as illustrated in Figure 6c. The cathode comprised a composite of LIC-1% H₃PO₄ and LiCoO₂ blended at a 3:7 weight ratio, while the anode consisted of stacked In and Li foils. LIC-1% H₃PO₄ served as the solid electrolyte, with a Li₆PS₅Cl interfacial layer inserted between the electrolyte and In foil to mitigate parasitic reactions at the halide–metal interface. The cell fabrication protocol adhered to the methodology outlined in the experimental section. The ASSLIBs underwent three initial activation cycles at 0.05 C within a voltage range of 1.9–3.8 V, followed by charge/discharge cycling at 0.1 C under the same voltage window.

Figure 6a displays the charge–discharge profiles of the ASSLBs at 30 °C under a 0.05 C rate. The first-cycle charge capacity reaches 128 mAh/g with an initial Coulombic efficiency of 72.95%. As shown in Figure 6b, the Coulombic efficiency stabilizes at 98.8% after 14 cycles, after which battery failure occurs due to electrolyte deliquescence in ambient air. Figure 6d presents the cycling performance of a Li-In/LPSC/LIC-1% H₃PO₄/LPSC/Li-In symmetric cell under varying current densities. The LPSC interlayers in the symmetric cell configuration are essential for isolating the halide electrolyte from Li metal, ensuring long-term cycling stability [16]. The symmetric cell demonstrates stable cycling over 1000 cycles at 0.05, 0.1, and 0.3 mA/cm², confirming that LIC-1% H₃PO₄ maintains stable contact with Li-In metal without detectable side reactions. The LIC-H₂O sample was excluded from symmetric cell testing due to its inherent instability and low ionic conductivity, which preclude meaningful cycling performance. The superior ionic conductivity of LIC-1% H₃PO₄ (1.15 × 10⁻³ S/cm, Table 1) and stable cycling of ASSLIBs (Figure 6a,b) are direct outcomes of phosphorus-enabled composite formation.

Figure 7a shows the Fourier transform infrared (FTIR) spectra of LIC powders synthesized with varying H₃PO₄ concentrations. The peak at 3500 cm⁻¹ corresponds to hydroxyl group vibrations. The LIC-H₂O sample exhibits the highest peak intensity at 3500 cm⁻¹, indicating the highest crystalline water content in the pure water-synthesized sample. Upon H₃PO₄ addition, the hydroxyl peak intensity gradually decreases, reaching its minimum at 1% H₃PO₄, which corresponds to the lowest crystalline water content. The progressive reduction of hydroxyl peaks (3500 cm⁻¹) with increasing H₃PO₄ concentration directly correlates with phosphorus-mediated suppression of crystalline water. However, further increasing the H₃PO₄ concentration to 2% results in no significant reduction in hydroxyl peak intensity, suggesting that H₃PO₄ concentrations above 1% have limited efficacy in reducing crystalline water. According to Sacci et al. [9], crystalline water in the sample undergoes hydrolysis during thermal dehydration, generating oxide impurities such as InOCl and In₂O₃ via the following reactions:

In³⁺ + H₂O + 3Cl⁻ → InOCl + 2HCl

(2)

2In³⁺ + 3H₂O + 6Cl⁻ → In₂O₃ + 6HCl_(g)

(3)

Therefore, the reduction of the water of crystallization content in the precursor is conducive to obtaining purer LIC samples.

Figure 7b presents the thermogravimetric (TG) curve of the LIC precursor powder synthesized in 1% H₃PO₄. The dehydration process concludes at 250 °C, justifying the selection of this temperature for precursor dehydration. The TG curve exhibits three endothermic peaks across the temperature ranges of 100–150 °C, 150–200 °C, and 200–250 °C. The weight loss in the interval of 100–150 °C mainly originates from the free water and H₃PO₄ solution not removed from the sample surface, while 150–200 °C is the process of dehydration of the water of crystallization in the precursor samples, in which the precursor Li₃InCl₆·nH₂O removes the water of crystallization to form Li₃InCl₆. Since the sample is susceptible to oxidation by the water of crystallization to form In₂O₃ impurities during the dehydration process, resulting in a drastic decrease in the ionic conductivity of the sample, a high degree of vacuum must be ensured during the dehydration process. The heat absorption peaks in the range of 200–250 °C correspond to the reaction of PO₄³⁻ in the sample with the In³⁺ and Li⁺ in the sample to form LiIn(P₂O₇). The lack of a 200–250 °C DSC peak in LIC-H₂O (Figure 7c) confirms that the observed thermal event in H₃PO₄-containing samples arises from phosphate-specific reactivity. The dehydration mechanism involving PO₄³⁻ → P₂O₇⁴⁻ transformation is attributed to H₃PO₄-derived phosphorus species [17,18]. This process generates LiIn(P₂O₇), a key contributor to ionic conductivity enhancement [19].

Kartini et al. demonstrated that compositing P₂O₇⁴⁻ with solid-state electrolytes can significantly enhance ionic conductivity due to the exceptional crystal structure of P₂O₇⁴⁻ [11]. LiIn(P₂O₇) crystallizes in the triclinic system with the space group P-1, and its crystal structure is illustrated in Figure 8a [19]. In contrast, Li₃InCl₆ adopts a monoclinic structure with the space group C2/m, as shown in Figure 8b [8]. The composite of LiIn(P₂O₇) and LIC introduces a mixed-anion effect, where In³⁺ ions coordinate with P₂O₇⁴⁻ groups to form a continuous framework rich in vacancies. This framework facilitates efficient Li⁺ ion transport, thereby improving the overall ionic conductivity of the electrolyte. While PO₄³⁻ reacts with a fraction of In³⁺/Li⁺ to form LiIn(P₂O₇), the composite’s synergistic Li⁺ transport mechanisms ensure net conductivity enhancement, as evidenced by the 1.15 × 10⁻³ S/cm value for LIC-1% H₃PO₄ (Table 1). In summary, H₃PO₄ addition achieves dual objectives; on the one hand, H₃PO₄ can minimize hydrolysis-derived impurities (InOCl/In₂O₃), and on the other hand, addition of H₃PO₄ generates LiIn(P₂O₇) with P₂O₇⁴⁻ enabled Li⁺ pathways. The optimal 1% H₃PO₄ concentration balances these effects, yielding the highest conductivity.

4. Conclusions

This study demonstrates that the addition of H₃PO₄ during the water-mediated synthesis of LIC significantly enhances its ionic conductivity, with a systematic investigation into the relationship between H₃PO₄ concentration and LIC performance. Experimental results reveal that the LIC-1% H₃PO₄ sample, synthesized with 1% H₃PO₄, exhibits the lowest crystalline water content and the highest ionic conductivity of 1.15 × 10⁻³ S/cm. When assembled into ASSLIBs with the LiCoO₂-LIC/LIC-1%H₃PO₄/LPSC/In/Li configuration, the battery achieves stable cycling for 14 cycles at 30 °C, delivering a first-cycle charge capacity of 128 mAh/g. Additionally, Li-In/LPSC/LIC-1% H₃PO₄/LPSC/Li-In symmetric cells demonstrate stable cycling over 1000 cycles under current densities of 0.05, 0.1, and 0.3 mA/cm². Mechanistic studies indicate that PO₄³⁻ in the solution thermally decomposes into P₂O₇⁴⁻ at elevated temperatures, ultimately forming LiIn(P₂O₇) with a vacancy-rich framework that facilitates Li⁺ transport, thereby enhancing the ionic conductivity of LIC. This work highlights that LIC synthesized with 1% H₃PO₄ achieves optimal performance and elaborates on how the P₂O₇⁴⁻ anion (derived from H₃PO₄) creates a vacancy-rich framework in LiIn(P₂O₇) (Figure 8a), facilitating Li⁺ transport and suppressing interfacial side reactions (e.g., In₂O₃ formation), offering a low-cost and scalable strategy for producing high-conductivity LIC electrolytes.