Lithium-ion batteries have occupied a dominant position in secondary batteries for a long time. However, the traditional liquid lithium-ion batteries are restricted because of its safety problem and limited energy density. The use of solid electrolytes (SEs) will not have the risks of flammability and explosion. Meanwhile, the theoretical energy density of batteries is also improved by replacing graphite anode with lithium anode. Therefore, all-solid-state lithium metal batteries (ASSLMBs) are regarded as one of the ideal next-generation energy storage equipment [
1].
SEs are key component of ASSLMBs. Argyrodite electrolyte, as a typical sulfide solid electrolyte (SSE), has received widespread attention owing to its high ionic conductivity, fine mechanical property and stable electrochemical window. However, its poor air stability and interface compatibility should not be underestimated. SSEs are extremely easy to react with H
2O in air to form toxic gas H
2S. Besides, on cathode side, it undergoes side reactions with lithium transition metal oxides (
e.g., LiCoO
2 and LiMn
2O
4). On anode side, it also undergoes side reactions with lithium to generate by-products (
e.g., lithium sulfide and lithium phosphide) and cause the growth of lithium dendrites, leading to internal short circuits in battery. Therefore, it is important to develop an SSE with high ionic conductivity, good air stability and excellent interface compatibility [
2].
Element doping of SSEs is the simplest and most efficient strategy to solve above problems [
3]. So far, many researchers have doped SSEs with nitrides and halides. The N
3–, X
– doping [
4] may promote the formation of crystal defects and regulate the space charge layer, accelerating Li
+ transport. However, N
3– itself is easily hydrolyzed, and X
– (halide ion) cannot improve the air stability of SSEs. Therefore, to achieve high ionic conductivity and good chemical stability (including air stability and interface stability), better dopants should be explored.
Gallium oxide (Ga
2O
3), as an excellent semiconductor material, has broad applications in electronic devices, chip manufacturing and energy storage due to its conductivity and luminescence properties [
5]. Firstly, Ga
2O
3 enables suppressed electron transport due to its a wide bandgap of approximately 4.5–5.0 eV, expected to provide a good ion transport pathway in SEs. In addition, since Ga
2O
3 has good thermal and chemical stability, it will not undergo side reactions during material preparation and battery cycling, which is expected to improve the overall stability of the material and battery. At present, relevant studies have shown that because of the similarity in chemical properties between O and S, the O
2– easily replaces the sites of S
2– [
6]. Meanwhile, the moisture resistance of O
2– is better than that of S
2–, after the introduction of O
2–, the air stability of SSEs can be improved. As a positive valence ion, both Ga
3+ and P
5+ have the property of soft acid. Ga
3+ has a larger ionic radius than P
5+, and the charge of Ga
3+ is smaller than that of P
5+. According to soft hard acid-base theory, Ga
3+ is a softer acid compared to P
5+, while S
2– is a softer base. If Ga
3+ replaces P
5+, the binding strength of Ga–S is theoretically greater than that of P–S bond, which enhances the air stability after doping. At the same time, the introduction of Ga
3+ with larger radius will cause lattice expansion, promoting Li
+ transport [
7].
Herein, we prepared Li
6+2xP
1–xGa
xS
5–1.5xO
1.5xCl (for the convenience, the sample with
x = 0 is abbreviated as LPSC, and the samples with other doping contents are abbreviated as LPSC-
xGa
2O
3) electrolytes by ball milling and solid-state sintering methods. To characterize the surface morphologies and components of the as-prepared SSEs, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) analysis were carried out. The SEM images in
Fig. 1a and Fig. S1 (Supporting information) demonstrate that the Ga
2O
3-doped LPSC has a uniform particle size (average diameter is ~6.5 µm) without obvious agglomerations. In contrast, LPSC displays larger particle size (average diameter is ~20 µm) and obvious agglomeration phenomenon (Fig. S1), which may cause a bad interfacial contact between SSEs and electrodes during battery assembly. Furthermore, EDS mappings and line scans of the doped-LPSC show that, except for P, S, Cl elements from LPSC, O and Ga elements are also detected (Fig. S2 and Table S1 in Supporting information), which proves the successful introduction of Ga and O into LPSC.
To further investigate the microscopic doping mechanism, X-ray diffraction (XRD), Raman, and X-ray photoelectron spectroscopy (XPS) measurements were conducted.
Figs. 1b and c show the XRD patterns of LPSC-
xGa
2O
3 (
x = 0, 0.05, 0.1, 0.15, 0.2). From the figures, six diffraction peaks at 25.5°, 30°, 31.5°, 45°, 48° and 52.5° are observed, which corresponds to (220), (311), (222), (422), (511) and (440) crystal plane of LPSC, respectively. It implies that the obtained SEs have great crystal structure, as confirmed by the clear P−S bonds of the
tetrahedron (420 and 570 cm
–1) in Raman spectra of LPSC-
xGa
2O
3 (
x = 0, 0.05, 0.1, 0.15, 0.2) in
Fig. 1d. Notably, as the doping content of Ga
2O
3 increasing to
x = 0.1, both XRD and Raman peaks of the SSEs gradually shift, indicating lattice distortion of the unit cell, which could be caused by the substitution of the smaller ionic radius of P
5+ (R (P
5+): 17 pm) by the larger ionic radius of Ga
3+ (R (Ga
3+): 47 pm), as confirmed by the increased unit cell parameters of Ga
2O
3-doped LPSC in Rietveld refinement (Fig. S3, Tables S2 and S3 in Supporting information). Moreover, a weak Raman peak due to Ga–S bond at 200–250 cm
‒1 more directly verifies that Ga
3+ successfully replaces P
5+ in
tetrahedron, forming
tetrahedra. Yet, as increasing the doped Ga
2O
3 to
x = 0.2, there is no conspicuous peak shift in XRD and Raman pattens, while the peak intensities decrease seriously, which can be attributed to more defects introduced into the original crystal structures of the SSEs when Ga
2O
3 is excessive [
8]. Furthermore, from the XPS analysis of LPSC and the Ga
2O
3-doped LPSC (
Figs. 1e and f), besides the P−S bonds of the
tetrahedron, a new P–O bond at 135.6 eV is probed in P 2p XPS spectrum of LPSC-0.05Ga
2O
3, implying O
2– of Ga
2O
3 replaces S
2– in LPSC successfully. All above results demonstrate that Ga
3+ partially replaces P
5+ and O
2– partially replaces S
2–, thus constructing a novel LPSC-0.05Ga
2O
3. Its crystal structure is shown in
Fig. 1g.
Ionic conductivity (
σ), as an essential index of SSEs, directly affects the performance of ASSLMBs. To ascertain the ionic conductivity of the prepared samples, electrochemical impedance spectroscopy (EIS) tests were conducted. The typical Nyquist plots for LPSC-
xGa
2O
3 (
x = 0, 0.05, 0.1, 0.15, 0.2) and the corresponding ionic conductivities are shown in Fig. S4 (Supporting information) and
Fig. 2a. It is found that LPSC-0.05Ga
2O
3 exhibits the highest ionic conductivity to be 1.81 mS/cm at room temperature, which is ≈1.4 times that of the original LPSC (1.33 mS/cm). The ionic conductivity improvement of the electrolyte can be attributed to the unit cell expansion caused by the doping of Ga
3+, which broadens the migration channel of Li
+. However, the ionic conductivity of the electrolyte reveals a decreasing trend when Ga
2O
3 doping content continues increasing (
x > 0.05). It is possibly due to the high electronegativity of O compared to S, which has a strong binding force with Li
+.
Compared to ionic conductivity, the activation energy (
Ea) of electrolytes can essentially reflect the Li
+ transmission kinetics properties of materials [
9]. Therefore, we further tested the impedance of LPSC-
xGa
2O
3 (
x = 0, 0.05, 0.1, 0.15, 0.2) at temperatures of 25, 35, 45, 55, and 65 ℃, respectively. The impedance is converted into ionic conductivity, and then the
Ea is calculated from the slope of the curve between ln
σ and 1000/
T (
Fig. 2b). As shown in
Fig. 2c, the
Ea of LPSC-
xGa
2O
3 (
x = 0, 0.05, 0.1, 0.15, 0.2) exhibits opposite change trend to the ionic conductivity. Among them, the LPSC-0.05Ga
2O
3 reveals the smallest
Ea, only 0.18 eV, indicating that the energy barrier overcome by Li
+ migration through LPSC-0.05Ga
2O
3 is the lowest [
10]. Besides, electronic conductivity (
σe), as the activity descriptor of electron migration, represents generally opposite trend with the ionic conductivity of LPSC-
xGa
2O
3 (
x = 0, 0.05, 0.1, 0.15, 0.2) (
Fig. 2d).
As is known, the inherent humidity sensitivity of SSEs will deteriorate electrolyte structure and performance and release H
2S, where the P‒S bond in SSEs will break and form P‒O bonds when SSEs encounter water molecules. After doping with Ga
2O
3, the S
2– is replaced by O
2–, and the binding force between Ga
3+ and S
2– is stronger than that between P
5+ and S
2– [
11], which can alleviate the generation of H
2S to a certain extent [
12]. As a result, the volume of H
2S released per unit mass of LPSC-
xGa
2O
3 (
x = 0.05, 0.1, 0.15, 0.2) within 1 h at 25 ℃ and 70% humidity is less than what LPSC released (
Fig. 2e).
Fig. 2f and Fig. S5 (Supporting information) show the ionic conductivity retention that LPSC-0.05Ga
2O
3 is as high as 39.2%, which is > 3 times of LPSC (11.9%), indicating the air stability of the electrolyte has indeed been improved after doping Ga
2O
3.
To verify the interface stability between SSEs and electrodes, CV curves of SSEs@C|SSEs|Li cells were measured within 1–4.5 V at 0.1 mV/s. Here, carbon (C) was introduced into the electrolyte with low electronic conductivity to enhance the redox reaction kinetics. Obviously, from
Figs. 3a and b, both LPSC and LPSC-0.05Ga
2O
3 have similar redox peaks. During the process of the cell sweeping from open circuit voltage (OCV) to 1 V, the first peak appearing near 2 V can be considered as the process of SSE decomposition. In addition, the oxidation of the both electrolytes start at ~2.5 V. The results indicate that the "true" electrochemical stability window of both SSEs is 2–2.5 V [
13]. Meanwhile, the CV curves can further reflect the decomposition of SSEs at electrode interface. It can be observed that the degree of LPSC redox decomposition is more severe than that of LPSC-0.05Ga
2O
3. Besides, the cell assembled with LPSC-0.05Ga
2O
3 has a better overlap degree in the subsequent cycling than the cell with LPSC, which proves that LPSC-0.05Ga
2O
3 inhabits the decomposition of SSEs and has better interface stability [
14].
To evaluate the effect of Ga
2O
3 on the interface compatibility of Li anode, the CV curves of Li|SSE|SS (stainless steel) half-cells were tested. As shown in
Fig. 3c, the electrolytes exhibit a pair of redox peaks near 0 V, corresponding to the insertion of Li
+ (Li
+ + e
− → Li) and the extraction of Li
+ (Li ‒ e
− → Li
+). Furthermore, by observing the local magnified at 2.5–5 V (
Fig. 3d), LPSC shows a more active current response, while LPSC-0.05Ga
2O
3 remains relatively stable, indicating that LPSC may have a side reaction with Li, which can be alleviated by Ga
2O
3 doping.
The compatibility of Li metal interface is not only reflected by the side reactions at the electrolyte/Li interface, but also by the growth of Li dendrites. As is well known, electrolytes with high electron conductivity are more likely to induce Li dendrites growth. Therefore, direct-current polarization tests were conducted on LPSC-
xGa
2O
3 (
x = 0, 0.05, 0.1, 0.15, 0.2), and their electron conductivities were calculated based on it. As shown in
Fig. 2d and Fig. S6 (Supporting information), the electronic conductivities of the LPSC after doping Ga
2O
3 decrease significantly, which can imply that the doped-LPSC have capability to restrain the formation of Li dendrites more efficiently. Furthermore, since critical current density (CCD) can reflect the maximum current density that an electrolyte can withstand without soft short circuits caused by Li dendrites, we assembled Li|SSEs|Li symmetric cells and measured their CCD values to quantify the Li dendrite inhibition capability by Ga
2O
3-doping.
Figs. 3e and f show the polarization voltage curves of Li|SSEs|Li cells cycled at different current densities. The current density corresponding to the voltage drop is considered as the CCD for the formation of Li dendrites in the SSEs. The CCD of the symmetrical cell assembled using LPSC-0.05Ga
2O
3 is 1.2 mA/cm
2, much higher than 0.5 mA/cm
2 of LPSC, disclosing Ga
2O
3-doping can enhance the ability of SSEs against Li dendrite growth under high current [
15].
In addition, constant current charge and discharge tests were also conducted on Li|SSEs|Li symmetric cells to further investigate the compatibility of the anode interface with LPSC and LPSC-0.05Ga
2O
3 [
16], setting the current densities at 0.2, 0.5 and 1 mA/cm
2, respectively (
Fig. 3g and Fig. S7 in Supporting information). Taking 0.2 mA/cm
2 as example, the overpotential of Li|LPSC|Li continuously increased within 134 h of cycling, and the voltage dropped sharply in the subsequent cycles, demonstrating short circuit of the cell occurs stemming from Li dendrite growth. In stark contrast, the Li|LPSC-0.05Ga
2O
3|Li symmetric cell reveals a low overpotential and no sudden voltage drop caused by Li dendrite growth after 350 h of stable cycle, which further confirms that the growth of Li dendrites is better inhibited by LPSC-0.05Ga
2O
3, and the interface of Li anode is optimized [
15].
To illustrate the practicality and functionality of LPSC-0.05Ga
2O
3, the ASSLMBs were assembled by mold cold pressing method, where NCM was used as the cathode and Li was used as the anode.
Figs. 4a and b show the Nyquist plots of the batteries before and after cycling [
17]. After analyzing by the equivalent circuits (Fig. S8 in Supporting information), it is found that the
Rgb and
Rint of the NCM|LPSC|Li battery increases significantly after cycling (Table S4 in Supporting information). For comparison, the impedances of NCM|LPSC-0.05Ga
2O
3|Li do not increase significantly after cycles, suggesting the solid-solid interfaces maintain stability and compatibility during cycling. Such a good interface stability can also be confirmed by the CV results. In
Figs. 4c and d, the NCM|LPSC-0.05Ga
2O
3|Li exhibits three oxidation peaks located at 3.82, 4.05, and 4.22 V, respectively, corresponding to the transition of the cathode material from H1 phase to M phase, M phase to H2 phase, and H2 phase to H3 phase [
18], implying the reaction of the cathode active material is more completed in NCM|LPSC-0.05Ga
2O
3|Li. Moreover, its CV profiles have high coincidence in the first 5 cycles, reflecting the excellent electrochemical reversibility of the battery, which also reflects a good cathode interface compatibility of NCM|LPSC-0.05Ga
2O
3|Li. In comparison, the coincidence of the NCM|LPSC|Li CV curves are not so good, where the oxidation peak corresponding to the third phase transition cannot be clearly observed. Such phenomenon may be caused by the side reactions at the SSEs/cathode interface.
Fig. 4e shows the rate performance of ASSLMBs [
19]. The NCM|LPSC-0.05Ga
2O
3|Li battery has discharge specific capacity of 138, 122, 110, 95, and 70 mAh/g at 0.1, 0.2, 0.3, 0.5, and 1 C, respectively. When it recovers to 0.1 C, the specific capacity can be restored to 123 mAh/g. By contrast, the NCM|LPSC|Li battery shows relatively low capacity and reversibility. The above results indicate that the ASSLMBs based on LPSC-0.05Ga
2O
3 has good rate reversibility. With the aim of studying the cycling stability of ASSLMBs, long cycling tests were conducted at 0.5 C on two batteries. As shown in
Fig. 4f, after gradient activation from 0.1 C to 0.5 C, the initial specific capacity of NCM|LPSC-0.05Ga
2O
3|Li battery is 135.68 mAh/g. After cycling for 200 cycles, the specific capacity still remains at 123.85 mAh/g, and the capacity retention rate reaches 91.2%. On the contrary, the initial specific capacity of NCM|LPSC|Li battery is only 89.06 mAh/g, and the capacity retention rate after 200 cycles is only 67.3%. The above results indicate that Ga
2O
3 doped SSEs can significantly improve the performance of ASSLMBs [
20]. Benchmarking against other argyrodite systems reveals that our battery not only matches but often surpasses the electrochemical performance of many reported counterparts (Table S5 in Supporting information).
On the whole, LPSC-xGa2O3 SSEs were synthesized successfully by high-energy ball milling combined with solid-state sintering. Through physical characterizations, it is found that O2– replaces S2– and Ga3+ replaces P5+. Further researches reveal that LPSC-0.05Ga2O3 has higher ionic conductivity (1.81 mS/cm) and fine air stability. More encouragingly, LPSC-0.05Ga2O3 shows the inhibition capability against Li dendrites growth (CCD = 1.2 mA/cm2) and suppressed side reactions in both anode and cathode interfaces. Therefore, the ASSLMBs based on the LPSC-0.05Ga2O3 exhibits high specific capacity with 135.68 mAh/g at 0.5 C and good cycle stability with a 91.2% retention. This work introduces a novel sulfide-based electrolyte materials, showing emerging prospect for future applications in advanced all-solid-state energy storage devices.