The high cost of solid-state electrolytes (SSEs) poses a significant barrier to the industrial adoption of all-solid-state lithium-ion batteries (ASSLBs). This cost is primarily driven by two factors: the use of expensive raw materials and energy-intensive synthesis processes [
1]. For instance, elements such as La in Li
7La
3Zr
2O
12 [
2–
6], Ge in Li
10GeP
2S
12 [
7–
9], and Sc/In Li
3ScCl
6/Li
3InCl
6 [
10–
12] are scarce and significantly increase material costs. Although earth-abundant elements like P, S, and Cl are used in sulfide SSEs (
e.g., Li
6PS
5Cl), the complex preparation process of Li
2S still elevates the overall cost [
13]. Similarly, oxide electrolytes with garnet and NASICON structures require sintering temperatures exceeding 1000 ℃, leading to high energy consumption [
4,
14–
17]. Halide SSEs, such as Li
2ZrCl
6, often involve inefficient synthesis methods like long time high-energy ball milling (500 rpm for 45 h) [
18–
23]. These challenges underscore the urgent need to develop low-cost SSEs composed of earth-abundant elements and scalable synthesis methods.
Aluminum (Al), the most abundant metallic element in the Earth's crust (Figs. S1a and b in Supporting information), offers a promising solution. Aluminum chloride (AlCl
3), a low-cost precursor with a melting point of 194 ℃, enables the synthesis of SSEs (
e.g., LiAlCl
4) at relatively low temperatures (< 200 ℃) [
24–
27]. However, the ionic conductivity of solid LiAlCl
4 remains low (10
-6 S/cm at 25 ℃), limiting its applicability in ASSBs [
24,
28].
The ionic conductivity of SSEs is closely tied to their structural properties. Short distances between adjacent charge carriers, particularly in face-sharing polyhedra, are critical for achieving rapid ion conduction. Notable examples include body-centered sulfides [
29–
33] and LaCl
3-based Li
+/Na
+ conductors [
34,
35]. In LiAlCl
4, however, all Li
+ ions occupy octahedral sites (4e, O site), which are either corner-sharing or edge-sharing, with a Li-Li distance exceeding 4.22 Å (
Fig. 1a). The migration of Li
+ ions between adjacent octahedral sites involves passing through one or two tetrahedral sites (T site), following an O-T-O or O-T-T-O pathway. The higher site energy of tetrahedral sites increases the activation energy for ion diffusion, resulting in low ionic conductivity (
Fig. 1b and Fig. S2 in Supporting information) [
25]. Previous studies have shown that structural distortion induced by high-energy ball milling can reduce the energy of Li
+ ions in tetrahedral sites, enabling partial relocation of Li
+ ions to these sites [
24]. This approach increases the ionic conductivity of LiAlCl
4 by approximately 20 times, reaching 2.1 × 10
−5 S/cm [
24]. Nevertheless, this value remains insufficient for practical applications.
Anion mixing has emerged as an effective strategy for tuning the site energy and distribution of Li
+ ions in SSEs. Since anions directly coordinate with Li
+ ions, they exert a strong influence on Li
+ site energy. Anion mixing (
e.g., Cl
−/O
2−, S
2−/Cl
−, and S
2−/O
2−) enables Li
+ ions to occupy a wider range of sites, shortening the migration distance between charge carriers and enhancing ionic conductivity [
18,
36–
45]. In this study, we employ Cl
−/O
2− mixing to significantly improve the ionic conductivity of Li
xAlCl
3-xO
x. This strategy increases the number of Li
+-occupied sites and reduces the distance between adjacent sites, leading to a lower ion migration activation energy (0.30 eV) and a remarkable enhancement in ionic conductivity (1.5 × 10
−3 S/cm).
The electrolyte developed in this study uses AlCl
3 and LiOH as raw materials and can be prepared with heating at 200 ℃ for only 0.5 h (Fig. S1c in Supporting information). In contrast to crystalline LiAlCl
4, the synthesized Li
xAlCl
3-xO
x exhibits a pure amorphous structure (Fig. S3 in Supporting information). The incorporation of O
2− into Li
xAlCl
3-xO
x likely facilitates the formation of highly stable Al−O−Al fragments. During cooling from the molten state, the presence of these Al−O−Al fragments restricts atomic rearrangement (crystallization), resulting in the retention of an amorphous structure at room temperature [
43]. However, when x exceeds 0.8, a LiCl phase emerges in the product, and its content increases with higher O
2− concentrations (Fig. S3). This is because the enthalpy (
H) and entropy (
S) of Li
xAlCl
3-xO
x increases upon the incorporation of O
2− (Li
+) and formation of defects (O
2− substituted Cl
−, interstitial Li
+). When ∆
G = ∆
H -
T∆
S < 0, the structure of Li
xAlCl
3-xO
x would be thermal-dynamically stable (
x ≤ 0.8). Otherwise, the structure would be unstable and ultimately decompose to impurities such as LiCl (
x > 0.8). In the main glass phase, more anion vacancies and fewer Li
+ interstitial defects would be formed (Li
x-yAlCl
3-x-yO
x). This indicates that the high energy of interstitial Li
+ ions is the main defect leading to instability of Li
xAlCl
3-xO
x.
The Cl−/O2− mixing in the amorphous phase is further confirmed by X-ray photoelectron spectroscopy (XPS) (Fig. S4 in Supporting information). The Li 1s and Al 2p spectral peaks for Li0.8AlCl2.2O0.8 are situated between those of LiAlCl4 and LiAlO2, providing clear evidence of Cl−/O2− anion mixing in Li0.8AlCl2.2O0.8.
Solid-state nuclear magnetic resonance (ssNMR) spectroscopy is a powerful tool for probing the local structure and distribution of Li
+ ions [
46–
48], providing critical insights into ion conduction mechanisms in LiAlCl
4 and Li
xAlCl
3-xO
x (
Figs. 1c-f). In LiAlCl
4, a single
7Li resonance signal is observed, confirming that Li
+ ions exclusively occupy octahedral sites (4e sites) coordinated by six Cl
− ions (
Fig. 1a and Fig. S5 in Supporting information) [
43]. The introduction of O
2− significantly alters the local Li
+ environment in the halide SSE. The
6Li NMR spectra of Li
xAlCl
3-xO
x reveal three distinct resonance signals: one corresponding to the LiCl impurity (~−1.2 ppm) and two others (~−0.88 and −0.65 ppm) arising from different Li
+ sites within the Li
xAlCl
3-xO
x structure (
Fig. 1c). The presence of cross-peaks in the 2D exchange spectroscopy (EXSY) NMR spectra further confirms the rapid exchange of Li
+ ions between these two distinct sites (
Figs. 1d-f). In the 1D spectra, all signals representing different Li
+ environments are clearly resolved. At a short mixing time (0.05 ms), the exchange of Li
+ ions between the two sites is negligible (
Fig. 1d). However, as the mixing time is extended to 1 ms and 100 ms, distinct cross-peaks emerge between the two Li
+ chemical environments (−0.88 and −0.65 ppm), as highlighted in the dotted boxes in
Figs. 1e and f. This observation demonstrates that Li
+ ions dynamically migrate between neighboring sites, creating efficient conduction pathways.
The incorporation of O
2− into Li
xAlCl
3-xO
x significantly enhances its ionic conductivity, reaching values on the order of 10
−3 S/cm at 25 ℃. The composition is optimal at
x = 0.8, when it is phase-pure amorphous without significant impurities (based on XRD), and the ionic conductivity reaches an optimum of 1.5 × 10
−3 S/cm (
Figs. 2a and c). For
x < 0.8, the improvement in ionic conduction with increasing O
2− content is attributed to the rising defect density resulting from O
2−/Cl
− mixing. Conversely, for
x > 0.8, the decline in ionic conductivity is due to the formation of impurities (Fig. S3). Additionally, the amorphous structure induced by O
2- doping should also facilitates the distribution of Li
+ ions across multiple sites, further contributing to the enhanced ionic conductivity [
24].
Conductivity measurements also reveal a significant reduction in activation energy to 0.30 eV for Li
0.8AlCl
2.2O
0.8 (
Figs. 2b and c, Fig. S6 in Supporting information). Furthermore, the electronic conductivity of Li
0.8AlCl
2.2O
0.8, determined from DC polarization curves (
Fig. 2d and Fig. S7 in Supporting information), is calculated to be 6.98 × 10
−10 S/cm. This value is substantially lower than that of many widely studied solid-state electrolytes, highlighting the material's potential for minimizing electronic leakage in solid-state battery applications [
49–
51].
To elucidate the local structure of amorphous Li
xAlCl
3-xO
x and gain deeper insights into Li
+ ion conduction pathways, density functional theory (DFT) calculations were performed (
Figs. 3a-d). The initial structure of Li
0.75AlCl
2.25O
0.75 was constructed by replacing 0.75 Cl
− with O
2− in AlCl
3 and adding 0.75 Li
+ ions to maintain charge neutrality. The hypothesized structure was heated to 800 K for 10 ps with a time step of 1 fs to obtain a fully melted configuration. Subsequently, the amorphous structure was annealed at lower temperatures by adjusting the model size to achieve the appropriate pressure. The final structure of Li
0.75AlCl
2.25O
0.75 was energy-minimized (
Fig. 3a and Table S1 in Supporting information). The simulated structure reveals that Al
3+ ions remain in tetrahedral sites, with partial replacement of Cl
− by O
2− leading to the formation of Al-O-Al fragments (
Fig. 3a and Fig. S8 in Supporting information). The framework of Li
0.75AlCl
2.25O
0.75 consists of AlCl
nO
4-n units, while Li
+ ions occupy various interstitial sites, coordinated by different combinations of Cl
− and O
2− ions (
e.g., 3Cl, 1O2Cl, 4Cl, 2O3Cl). Compared to LiAlCl
4, the coordination environments of Li
+ ions in Li
0.75AlCl
2.25O
0.75 are more complex, resulting in two broad peaks in the NMR spectra (
Fig. 2b). The substitution of Cl
− with the more electronegative O
2− reduces the electron cloud density around the Li
+ nucleus, leading to a larger chemical shift (toward the left) [
36,
52]. Consequently, the left-shifted
6Li peak (−0.65 ppm) likely corresponds to Li
+ ions coordinated with a higher number of O
2− anions.
Compared to LiAlCl
4, where Li
+ ions predominantly occupy octahedral sites, the increased distribution of Li
+ sites in Li
xAlCl
3-xO
x suggests a reduction in the distance between adjacent Li
+ sites. A segment of the simulated Li
0.75AlCl
2.25O
0.75 structure (
Fig. 3b) confirms that the Li–Li distance (3.03–3.80 Å) is significantly shorter than that in LiAlCl
4 (4.22–4.88 Å). This observation is further supported by the radial distribution function (RDF) of Li–Li (
Fig. 3c), which reveals shorter Li-Li distances in Li
0.75AlCl
2.25O
0.75. Additionally, the peaks corresponding to short Li-Li distances gradually diminish with increasing temperature, indicating structural collapse at elevated temperatures. At 600 K, the loss of short-range order confirms that Li
0.75AlCl
2.25O
0.75 transitions to a molten state.
In solid-state electrolytes (SSEs), the distribution of Li
+ ions is governed by their site energy at different interstitial positions, which is primarily determined by the bond energy between Li
+ ions and surrounding anions. This bond energy can be described by the equation
E(r) =
, where
ε0 is the dielectric constant,
B and
n are constant,
Zi and
Zj are ion charge number,
r is the distance between Li
+ ions and surrounding anions [
44]. When the anion type changes, the site energy and Li
+ distribution in SSEs undergo significant modifications. In chloride-based SSEs, Li
+ ions predominantly occupy octahedral interstices, where the distance between adjacent sites is relatively large. However, upon the introduction of O
2− ions, the site energy of Li
+ in tetrahedral and interstitial sites decreases, increasing the number of favorable distribution sites. Consequently, the distance between adjacent Li
+ sites is substantially reduced.
The Li
+ ion conduction in Li
0.75AlCl
2.25O
0.75 was also simulated (
Fig. 3d). The Li
+ diffusion coefficients range from 10
−6 cm
2/s to 10
−5 cm
2/s between 300 K and 800 K, demonstrating rapid ion migration across this temperature range [
38]. The calculated activation energy (0.16 eV) and ionic conductivity (26.6 mS/cm at 300 K) further confirm the presence of fast ion-diffusion channels and efficient ion conduction in Li
0.75AlCl
2.25O
0.75. Notably, the Li
+ ion probability densities reveal potential diffusion pathways (Fig. S9 in Supporting information). At 300 K, Li
+ ions exhibit partial long-range diffusion through low-energy pathways. As the temperature increases to 600 K and 800 K, the more uniform probability densities suggest a flatter energy landscape in Li
0.75AlCl
2.25O
0.75.
The simulated structure was further validated by neutron pair distribution function (nPDF) analysis. The PDF profile features a peak at 2.165 Å, corresponding to Al-Cl pairs, while strong peaks at 3.595 Å and 6.425 Å are attributed to Al-Al pairs. Below 6 Å, Li
0.8AlCl
2.2O
0.8 exhibits multiple strong peaks, indicative of its characteristic short-range order. Beyond 10 Å, the peak intensity diminishes, reflecting the material's long-range disorder and amorphous nature, consistent with XRD results. To improve refinement accuracy, the structural unit of Li
0.75AlCl
2.25O
0.75 was expanded to a 3 × 3 × 3 supercell. The refined nPDF spectrum yields an good fit (χ
2 = 4.8), validating the simulated 3 × 3 × 3 structure (
Fig. 3e) [
53]. The refined structure is shown in
Fig. 3f, which closely matches the initial simulation.
Like other chloride solid electrolytes, Li
0.8AlCl
2.2O
0.8 is susceptible to deliquescence in natural air [
54], but the electrolyte is well stabilized in dry air. After exposing Li
0.8AlCl
2.2O
0.8 to a dry room atmosphere (dew point: −40 ℃) for 24 h, the ionic conductivity decreased slightly from 1.5 mS/cm to 1.1 mS/cm, while the amorphous structure remained intact (Fig. S10 in Supporting information). A minor mass increase from 0.3346 g to 0.3449 g further confirms its stability in dry air, attributed to the strong Al-O and Li-O bonds. Linear sweep voltammetry (LSV) of Li
0.8AlCl
2.2O
0.8-Super P||Li
0.8AlCl
2.2O
0.8||Li reveals an electrochemical stability window ranging from 1.12 V to 3.96 V
vs. Li
+/Li (Fig. S11 in Supporting information), comparable to other halide solid-state electrolytes [
34,
55].
To assess the practical applicability of Li
0.8AlCl
2.2O
0.8 as a solid-state electrolyte (SSE), an all-solid-state battery (ASSB) was assembled using a commercial LiCoO
2 composite cathode and a homemade Li-Si anode. A transition layer of homemade Li
6PS
5Cl was incorporated to isolate the Li
0.8AlCl
2.2O
0.8 SSE from the Li-Si anode (
Fig. 4a). Given the electrochemical stability window of Li
0.8AlCl
2.2O
0.8, it was employed as a catholyte in the composite cathode. To enhance stability, Li-Si (with a higher potential than Li metal) was selected as the anode [
56], while Li
6PS
5Cl (Fig. S12 in Supporting information), known for its compatibility with anodes, was used as the anolyte to separate Li
0.8AlCl
2.2O
0.8 from the Li-Si anode. To minimize oxidation and decomposition of Li
0.8AlCl
2.2O
0.8 above its oxidation potential (3.96 V
vs. Li
+/Li), conductive carbon was excluded from the composite cathode, enabling LiCoO
2 to be charged to 4.2 V
vs. Li-Si [
57].
Cross-sectional SEM images of the ASSB (
Figs. 4b and c) reveal layer thicknesses of ~24 µm (LiCoO
2 composite cathode), 300 µm (Li
0.8AlCl
2.2O
0.8 SSE), 110 µm (Li
6PS
5Cl SSE), and 130 µm (Li-Si anode). High-magnification SEM images (Fig. S13 in Supporting information) confirm the dense morphology of the Li
0.8AlCl
2.2O
0.8 layer. Under a pressure of 150 MPa, Li
0.8AlCl
2.2O
0.8 appears compact and pinhole-free (Fig. S13b), while Li
6PS
5Cl exhibits a porous structure (Fig. S13c). The total resistance of the ASSB before cycling exceeds 400 Ω (Fig. S14 in Supporting information), primarily attributed to the composite cathode (Fig. S15 in Supporting information). This high resistance stems from the high content (75 wt%) of poorly conductive LiCoO
2 in the cathode layer.
The LiCoO
2||Li
0.8AlCl
2.2O
0.8||Li
6PS
5Cl||Li-Si ASSB was cycled at various current densities between 2.5 V and 4.2 V under an operating pressure of 120 MPa at 25 ℃ (
Figs. 4d-g). At 0.1 C (25 ℃), the battery delivers a discharge capacity of 150 mAh/g with a high initial Coulombic efficiency (CE) of 97%, indicating highly reversible reactions (
Fig. 4d). This performance underscores the effectiveness of the battery configuration and the excellent oxidation stability of Li
0.8AlCl
2.2O
0.8, which minimizes side reactions during cycling. As the current density increases, the discharge capacities decrease to 138 mAh/g (0.2 C), 128 mAh/g (0.3 C), 108 mAh/g (0.5 C), and 73 mAh/g (1 C). The rate capability is currently limited by the high cathode resistance, necessitating further optimization of the cathode composition [
67]. Remarkably, the ASSB retains 94.4% of its discharge capacity after 180 cycles at 0.3 C (
Fig. 4g). Even after 1000 cycles at 1 C, the capacity retention reaches 70.6% (Fig. S16 in Supporting information), demonstrating exceptional cycling stability. Post-cycling resistance measurements (Fig. S14) reveal a slight reduction in total resistance after 180 cycles at 0.3 C or 1000 cycles at 1 C, further confirming the outstanding oxidation stability of Li
0.8AlCl
2.2O
0.8 SSE.
In conclusion, we have developed a low-cost Li
xAlCl
3-xO
x solid-state electrolyte (SSE) through an innovative anion mixing strategy, which offers a compelling combination of low cost, high oxidation stability, excellent interfacial compatibility, and superior ionic conductivity (
Fig. 4h). The use of earth-abundant elements significantly reduces material costs, while the short sintering time (30 min) at a low temperature (200 ℃) further minimizes production expenses. The O
2−/Cl
− dual-anion design enables Li
+ ions to occupy multiple sites within the Li
xAlCl
3-xO
x lattice, reducing Li
+ migration distances and enhancing ionic conductivity. The optimized Li
xAlCl
3-xO
x SSE achieves a room-temperature ionic conductivity of 1.5 mS/cm, representing a three-order-of-magnitude improvement over LiAlCl
4. The incorporation of O
2− into the AlCl
3-based halide not only improves oxidation stability but also enhances air stability, owing to the stronger bonding between O
2− and Li
+/Al
3+. The dense morphology of Li
xAlCl
3-xO
x SSE ensures excellent interfacial contact with the cathode, further supporting its practical application. The exceptional oxidation stability and compactability of Li
xAlCl
3-xO
x make it a promising catholyte candidate for all-solid-state batteries (ASSBs). In a proof-of-concept demonstration, a LiCoO
2/Li-Si ASSB incorporating Li
0.8AlCl
2.2O
0.8 as the catholyte delivered outstanding electrochemical performance, achieving a high initial Coulombic efficiency of 97% and retaining 94.4% of its discharge capacity after 180 cycles at 0.3 C. These results highlight the significant potential of Li
xAlCl
3-xO
x SSEs for practical applications in next-generation solid-state batteries. However, the absence of conductive carbon and the high weight ratio of low conductive LiCoO
2 (75%) at the cathode make the ASSB have high resistance which affects the rate performance. Further research could be enhancement of oxidation stability of SSE, adding conductive carbon if stability allows, or surface modify the LiCoO
2 cathode to improve the battery performance.