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Article  |  Open Access  |  10 Jan 2024

Binary anion and cation co-doping enhance sulfide solid electrolyte performance for all-solid-state lithium batteries

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Energy Mater 2024;4:400009.
10.20517/energymater.2023.78 |  © The Author(s) 2024.
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Abstract

Sulfide solid electrolytes are regarded as a pivotal component for all-solid-state lithium batteries (ASSLBs) due to their inherent advantages, such as high ionic conductivity and favorable mechanical properties. However, persistent challenges related to electrochemical stability and interfacial compatibility have remained significant hurdles in their practical application. To address these issues, we propose an anion-cation co-doping strategy for the optimization of Li7P3S11 (LPS) through chemical bonding and structural modifications. The co‐doping effects on the structural and electrochemical properties of SiO2-, GeO2-, and SnO2-doped sulfide electrolytes were systematically investigated. Cations are found to preferentially substitute the P5+ of the P2S74- unit within the LPS matrix, thereby expanding the Li+ diffusion pathways and introducing lithium defects to facilitate ion conduction. Concurrently, oxygen ions partially substitute sulfur ions, leading to improved electrochemical stability and enhanced interfacial performance of the sulfide electrolyte. The synergistic effects resulting from the incorporation of oxides yield several advantages, including superior ionic conductivity, enhanced interfacial stability, and effective suppression of lithium dendrite formation. Consequently, the application of oxide-doped sulfide solid electrolytes in ASSLBs yields promising electrochemical performances. The cells with doped-electrolytes exhibit higher initial coulombic efficiency, superior rate capability, and cycling stability when compared to the pristine LPS. Overall, this research highlights the potential of oxide-doped sulfide solid electrolytes in the development of advanced ASSLBs.

Keywords

All-solid-state batteries, sulfide solid electrolyte, binary co-doping, Li+ conductivity, electrochemical stability

INTRODUCTION

With the burgeoning expansion in electronic devices, electric vehicles, and energy storage solutions, the demands placed on lithium-ion batteries have grown significantly[1-5]. Traditional lithium-ion batteries utilizing liquid electrolytes face safety concerns, and their energy density is approaching a plateau. By transitioning to solid electrolytes, all-solid-state lithium batteries (ASSLBs) have emerged as a promising solution to address these challenges. It shows great potential for these batteries to offer increased safety, durability, higher energy density, and greater flexibility in battery packaging design[6-10].

Solid electrolytes, a critical component of ASSLBs, have garnered increasing attention. Among various solid electrolytes, sulfide solid electrolytes have gained prominence due to their exceptional characteristics, including high ionic conductivity (> 1 mS cm-1), favorable mechanical properties, and minimal grain boundary resistance. Nonetheless, solid-state batteries relying on sulfide electrolytes face significant hurdles, including poor air stability, chemical/electrochemical stability, and issues related to interfacial compatibility[11-15].

One of the most effective strategies for enhancing the stability of sulfide electrolytes is through element doping. Anion doping strategies have witnessed prominence, with partial substitution of sulfur by oxygen being the prevailing approach[16,17]. This strategy is widely employed because it confers notable benefits. Firstly, by incorporating oxygen atoms, the system forms non-bridging oxygen (O), which is more stable than bridging sulfur (S), thereby improving the overall stability of the electrolyte. Furthermore, the lattice mismatch between oxygen ions and oxide-based positive electrodes is minimal. Partially replacing sulfur with oxygen can effectively prevent oxygen from the positive electrode from infiltrating the sulfide electrolyte, thereby significantly inhibiting undesirable interface side reactions in sulfide-based ASSLBs. However, the incorporation of oxygen impedes the migration of Li+. Notably, as the level of oxygen doping increases, the conductivity of the electrolyte may experience a gradual decline. On the other hand, when considering cation doping, the selection of dopants aligns with the Hard and Soft Acid-Base Theory (HSAB). According to this theory, hard acids have a preference for reacting with hard bases, while soft acids tend to react more readily with soft bases. Combining a softer acid, such as Sn4+, Ge4+, As5+, and Sb5+, with the soft base sulfur (S) can significantly enhance the air stability of the sulfide electrolyte[18-20].

Oxide doping constitutes a synergistic amalgamation of anion and cation doping methodologies, demonstrating the capability to concurrently enhance the conductivity and stability of sulfide electrolytes[21-25]. Liu et al. presented a pioneering endeavor by doping of Li3PS4 glass-ceramic electrolytes with ZnO, demonstrating the synthesis of a new Li3+3xP1-xZnxS4-xOx solid electrolyte[26]. This advancement featured the incorporation of Zn, replacing a fraction of P, while O replaced a segment of S within the Li3PS4 matrix. The resultant electrolyte showcased enhanced air stability alongside improved ionic conductivity. Notably, there was no obvious structure change in the Li3+3xP1-xZnxS4-xOx electrolyte after exposure to air. Ahmad et al. synthesized a Li6.988P2.994Nb0.2S10.934O0.6 glass-ceramic electrolyte with Nb and O co-doping[27]. Wherein oxygen partially supplanted the sulfur bridge bond within the P2S74- group, thereby engendering the formation of P2OS64- entities, which contributed significantly to augmenting the air stability of sulfide electrolyte. In addition, the increase in the crystallinity of the conductive phase coordinated a reduction in the activation energy associated with the migration of lithium ions, which culminated in an appreciable enhancement of ionic conductivity within the glass-ceramic electrolyte. Xu et al. introduced a congener substitution strategy to optimize the electrochemical performance of Li7P3S11 (LPS) via chemical bond and structural regulation[28]. The as-obtained Li7P2.9Sb0.1S10.75O0.25 solid electrolyte shows improved solid-solid interfacial compatibility compared to LPS. Such enhancements are conducive to augmenting the reversible capacity, rate capability, coulombic efficiency, and cycling stability of ASSLBs. The oxide doping strategy has emerged as an effective methodology for mitigating the air sensitivity and instability issues associated with sulfide electrolytes without compromising the critical ionic conductivity. A systematic investigation of the effects of doping on the ionic conductivity, electrochemical stability, and interfacial stability is essential to guide the selection of appropriate doping elements and concentrations for optimizing the performance of the solid electrolytes.

In this work, the concurrent anion and cation co-doping strategy is proposed for optimizing the structural and electrochemical properties of LPS-based solid electrolytes employing oxides as deponents [Figure 1A]. We systematically investigate the effects of SiO2, GeO2, and SnO2 doping on the structure, ionic conductivity, oxidative stability, and lithium plating/stripping behavior of Li7+xP3-xMxS11-2xO2x (M = Si, Ge, Sn) (LPS-xMO2) electrolytes. The electrochemical performance of the doped LPS-xMO2 electrolytes is evaluated in ASSLBs. It reveals that this co-doping strategy yields improved ionic conductivity and enhanced electrochemical stability. Specifically, SnO2-doped LPS-xSnO2 electrolytes exhibit higher ionic conductivity, while SiO2-doped LPS-xSiO2 electrolytes exhibit superior oxidative stability. Consequently, ASSLBs based on NCM811 cathodes and LPS-0.05SnO2 electrolytes demonstrate higher initial capacity, while those utilizing LPS-0.05SiO2 electrolytes exhibit superior initial coulombic efficiency, rate capability, and cycling stability.

Binary anion and cation co-doping enhance sulfide solid electrolyte performance for all-solid-state lithium batteries

Figure 1. (A) Schematic illustrating the preparation process of sulfide electrolyte. X-ray diffraction patterns of the doped sulfide electrolytes (B) LPS-xSiO2, (C) LPS-xGeO2, (D) LPS-xSnO2 (x = 0, 0.02, 0.05, 0.08, 0.1) and Raman spectra of the doped sulfide electrolytes (E) LPS-xSiO2, (F) LPS-xGeO2, (G) LPS-xSnO2 (x = 0, 0.02, 0.05, 0.08, 0.1).

EXPERIMENTAL

Preparation of solid-state electrolytes

Li7+xP3-xMxS11-2xO2x (M = Si, Ge, Sn, x = 0, 0.02, 0.05, 0.08, 0.1) were synthesized through the high-energy ball-milling method combing with heat treatment techniques. Li2S (99.9%, Alfa Aesar), P2S5 (99%, P ≥ 27%, Macklin), and SiO2, GeO2, SnO2 (99.995%, Aladdin) were used as raw materials, which were weighed based on the stoichiometric ratio in an Ar filled glove box. The weighed raw materials and the zirconia balls (10 mm in diameter) were subjected to a zirconia ceramic vial and ball-milled (PULVERISETTE 7, Fritsch, Germany) at 400 rpm for 10 h. The weight ratio of zirconia balls to materials was 20:1. The as-obtained precursors were sealed in a quartz tube and annealed at 210 °C for 3 h and 250 °C for 1 h with a heating rate of 1 °C min-1. All the above processes were performed under Ar atmosphere.

Material characterizations

X-ray diffraction (XRD) was used to analyze the crystalline phase of the sulfide solid-state electrolytes (SSE) by using a Rigaku Ultima IV X-ray Diffractometer (Rigaku Corporation, Japan) equipped with Cu Ka radiation. Raman measurements were performed with a confocal Raman microscope (WITec Alpha, Ulm, Germany) with a 532 nm laser. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) measurements were conducted using a SEM (ZIESS SUPRA 55, Germany). X-ray photoelectron spectroscopy (XPS) analysis was performed on a K-Alpha X-ray photoelectron spectrometer system (Thermo Scientific).

Assembly of ASSLBs

Li7.05P2.95M0.05S10.9O0.1 (M = Si, Ge, Sn) and LPS powders were used as solid electrolytes; single crystal NCM811 was used as the cathode material, and the Li/In alloy was used as the anode. The composite cathode was prepared by mixing NCM811, acetylene black (AB), and sulfide solid electrolytes with a weight ratio of 70:1:29. For the assembly of the ASSLBs, first, 100 mg of pristine LPS electrolyte powders were introduced into a PEEK tube (internal diameter of 10 mm) and subjected to cold-pressing at a pressure of 360 MPa. Subsequently, 10.0 mg of composite cathode material was uniformly distributed atop the electrolyte and cold-pressed under 360 MPa for three minutes. Then, the Li/In alloy (Li/In = 3:7) was positioned on the other side of the electrolyte, applying a pressure of 100 MPa for three minutes. All the battery assembly steps were conducted within an argon-filled glove box.

Electrochemical measurements

The ionic conductivities of the sulfide solid electrolytes were measured by electrochemical impedance spectroscopy (EIS) using an Autolab PGSTA302 electrochemical workstation (Eco Chemie, Netherland) in a frequency range from 1 MHz to 0.1 Hz. Cyclic voltammetry (CV) was applied to evaluate the oxidative stability of the sulfide solid electrolytes, which was carried out with the sulfide solid-state batteries with composite material of LPS-xMO2 and AB (LPS-xMO2:AB = 6:4) as cathodes and lithium foil as anodes. The CV tests were carried out within the voltage range spanning from 2 to 4.5 V (vs. Li+/Li), employing a scan rate of 0.1 mV s-1. Li/LPS-xMO2/Li symmetric cells were used to assess the electrochemical stability of the LPS-xMO2 electrolytes against lithium metal anodes. The critical current density of the symmetric cells was assessed at 30 °C with the applied current density incrementally increasing in steps of 0.06 mA cm-2, and each Li plating/stripping step lasting 0.5 h. Galvanostatic charge-discharge tests were carried out using a battery test system (LAND CT2001A, Wuhan LAND Electronics. Ltd.) at room temperature within the voltage range of 2-3.7 V (vs. Li-In).

RESULTS AND DISCUSSION

The crystal structure and structural units of the electrolytes were characterized through XRD and Raman spectroscopy. The XRD results [Figure 1B-D] show that when the doping amount is ≤ 0.05 mol, the diffraction patterns of the doped sample remain in congruence with that of the pristine LPS electrolyte, thus preserving an intact body-centered cubic structure. However, for doping concentration exceeding 0.08 mol, unidentified new diffraction peaks are evident, signifying the limits of dopant concentration. In addition, the lattice constant variation of LPS with an increasing concentration of MO2 [Supplementary Table 1] and XRD patterns for reference materials (LPS, MO2) [Supplementary Figure 1] was analyzed. It shows that as the doping concentration of MO2 increases, the lattice parameters exhibit a gradual increase within the range of 0 ≤ x ≤ 0.08, indicative of the successful incorporation of MO2 into LPS. Raman spectra [Figure 1E-G] show peaks at approximately 407, 421 and 386 cm-1 across all spectra. The peaks centered at 407 and 421 cm-1 correspond to the symmetric stretching vibration of the P-S bonds in the P2S74- and PS43- units, respectively, indicating the formation of a highly conductive crystalline LPS phase. A minor peak at approximately 386 cm-1 corresponds to the low-conductivity phase of Li4P2S6[29-31]. Furthermore, it is noteworthy that the moderate co-doping of anion and cation atoms into LPS exerts a relatively modest impact on the fundamental framework of the LPS-xMO2 glass-ceramic electrolytes in comparison to the unaltered LPS electrolyte. Supplementary Figure 2 shows the morphology and element distribution of the typical LPS-0.05MO2 (M = Si, Ge, Sn) electrolytes characterized by SEM. A typical spherical powder morphology is discernible, with diameters of about 1-3 µm. Notably, Si, Ge, Sn, and O elements are evenly distributed in the electrolyte particles.

The optimal doping concentration was determined by measuring the ionic conductivity of LPS-xMO2(M = Si, Ge, Sn, x=0, 0.02, 0.05, 0.08, 0.1) electrolytes through EIS employing a symmetrical SS/SSE/SS cell configuration [Figure 2A-C]. The ionic conductivity of pristine LPS was determined to be 1.07 × 10-3 S cm-1[Supplementary Figure 3]. As the doping level increased, the ionic conductivities exhibited a gradual increment followed by a decline. Remarkably, LPS-0.05SiO2, LPS-0.08GeO2, and LPS-0.05SnO2 electrolytes reached a peak ionic conductivity of 1.32 × 10-3, 2.06 × 10-3, and 2.53 × 10-3 S cm-1, respectively [Supplementary Figure 4]. The LPS-0.05SnO2 electrolyte exhibits a 2.5 times improvement in ionic conductivity compared to pristine LPS. However, with further increasing doping levels, the EIS curves displayed a distinctive semicircle in the high-frequency region due to the presence of impurities in the electrolytes, consistent with XRD results [Supplementary Figure 5]. Moreover, the activation energies of the electrolytes were derived from Arrhenius plots based on the Arrhenius equation:

$$ \begin{equation} \begin{aligned} \sigma=\mathrm{A} \exp (-\mathrm{Ea} / \mathrm{KT}) \end{aligned} \end{equation} $$

where σ, T, k, and A represent the Li+ conductivity, the absolute temperature, the Boltzmann constant, and the pre-exponential factor, respectively. The Arrhenius plots of activation energy are presented in Figure 2D-F. Notably, at higher doping concentrations, the electrolytes exhibited elevated activation energies, indicative of more pronounced barriers to lithium-ion migration.

Binary anion and cation co-doping enhance sulfide solid electrolyte performance for all-solid-state lithium batteries

Figure 2. Nyquist plots for the solid electrolytes measured at room temperature. (A) LPS-xSiO2 (x = 0.02, 0.05, 0.08, 0.1). (B) LPS-xGeO2 (x = 0.02, 0.05, 0.08, 0.1). (C) LPS-xSnO2 (x = 0.02, 0.05, 0.08, 0.1). Corresponding Arrhenius plots of ionic conductivity of solid electrolytes. (D) LPS-xSiO2 (x = 0, 0.02, 0.05, 0.08, 0.1). (E) LPS-xGeO2 (x = 0, 0.02, 0.05, 0.08, 0.1). (F) LPS-xSnO2 (x = 0, 0.02, 0.05, 0.08, 0.1).

It was found that the ionic conductivity of glass-ceramic electrolytes is correlated to the proportion of the high conductive crystalline LPS phase characterized by the typical local structural units of P2S74- and PS43-[32]. When cations displace P5+ ions and bond with S2-, two possible scenarios emerge: the replacement of either P2S74- or PS43-. Importantly, the energy required for substituting P2S74- is lower than that for PS43-, thus favoring cation doping to favorably target the P2S74-[33,34]. Additionally, the ionic radii of Si4+, Ge4+, and Sn4+ are larger than that of P5+ ions, which means that cation-doping will broaden the lithium-ion conducting channels and generate lithium defects, thereby facilitating lithium-ion transport.

XPS was employed to assess the effects of doping on the content of P2S74- and PS43- structural units. Taking SnO2 doping as an example, Figure 3A shows that in bare LPS, the ratio of P2S74- (131.2 eV) to PS43-(132.1 eV) is approximately 1.93:1, closely resembling the theoretical ratio of 2:1[35,36]. With increasing doping levels, a portion of P2S74- is progressively replaced, resulting in diminishing ratios of P2S74- to PS43- of 1.87:1, 1.76:1, 1.4:1, and 1.23:1 for LPS-0.02SnO2, LPS-0.05SnO2, LPS-0.08SnO2, and LPS-0.1SnO2, respectively. Assuming that the LPS structure remains unaltered, the theoretical ratios of P2S74- and PS43- in LPS-0.08SnO2 and LPS-0.1SnO2 should be 1.84:1 and 1.8:1, respectively. However, the substantial decrease in the proportion of P2S74- groups in LPS doped with 0.08 mol and 0.1 mol SnO2 indicates that the optimal doping amount to maintain the highly conductive LPS phase structure is approximately 0.05 mol. Similarly, the peak intensity of P-S-P in the P2S74- unit [Figure 3B] decreases, while the Sn4+ peak in the Sn 3d spectra and O2- peak in the O 1s spectra [Figure 3C and D] increase significantly with increasing the doping contents, indicating an elevated oxide doping level in the electrolytes. Similar trends are also observed in the XPS spectra for samples doped with SiO2 and GeO2 [Supplementary Figures 6 and 7]. Moreover, the XPS peak positions of Si, Ge, and Sn in the doped electrolytes align with the peaks of Si-S, Ge-S, and Sn-S bonds of Li10SiP2S12, Li10GeP2S12, and Li10SnP2S12[37-39], further confirming the successful incorporation of SiO2, GeO2, and SnO2 into the LPS electrolyte.

Binary anion and cation co-doping enhance sulfide solid electrolyte performance for all-solid-state lithium batteries

Figure 3. XPS spectra of (A) P 2p, (B) S 2p, (C) Sn 3d and (D) O 1s for LPS, LPS-0.02SnO2, LPS-0.05SnO2, LPS-0.08SnO2 and LPS-0.1SnO2.

The oxidative decomposition of solid electrolytes has adverse effects on cathode impedance and overall cell performance. To probe the impact of oxide doping on the oxidative decomposition of LPS-based electrolytes, we employed a half-cell setup, with Li-In serving as the anode and a composite of carbon black (AB) (40 wt.%) mixed with LPS-xMO2 (60 wt.%) on the cathode, composite cathode structures greatly increase the contact area and accelerating the kinetic process of electrolyte decomposition, while close contact greatly reduces the interfacial charge transfer resistance and effectively suppresses polarization. Compared to semi-blocking electrodes, mixed electrodes more accurately reflect the conditions in all-solid-state batteries[40]. CV was conducted on the Li-In|LPS-xMO2|LPS-xMO2-AB cells within a potential range between 2.0 to 4.5 V (vs. Li/Li+) at a scan rate of 1 mVs-1 [Figure 4]. In the case of the pristine LPS electrolyte, the initial positive sweep from open circuit voltage (OCV) revealed a substantial oxidative peak between 3.0 and 4.5 V, with no discernible reductive peak during the subsequent negative sweep from 4.5 to 2.0 V. During the second cycle, the oxidative peak shifted to a higher voltage, and the intensity of the peak significantly diminished, indicating the passivation of the carbon/LPS-xMO2 electrolyte interphase after oxidation. All the doped LPS-xMO2 electrolytes exhibited a higher onset oxidative potential, coupled with a lower anodic peak current density during the first cycle compared to the LPS electrolyte. Furthermore, the onset oxidative potential exhibited an increasing trend with rising doping content, concomitant with a noticeable reduction in oxidative peak current density [Supplementary Figure 8]. Similar trends were also observed in the subsequent second cycle. These indicate that the incorporation of SiO2, GeO2, and SnO2 consistently enhanced the oxidative stability of the LPS electrolyte. Among these, LPS-SiO2 demonstrated the lowest oxidative peak current density due to the non-metallic properties of Si[32]. In order to analyze the impact of oxide doping on the interface, EIS and XPS were tested on the (LPS + AB/LPS/Li) cells at different voltages. From Supplementary Figures 9-12, it can be evident that upon charging to 4.5 V, the interfacial impedance experiences a significant increase. Subsequent discharge to 2.0 V results in a decrease in the interfacial impedance. Notably, compared to pristine LPS, the cells with doped electrolytes exhibit a substantial reduction in interfacial impedance. Moreover, XPS spectra reveal that electrolytes are oxidized at high voltage, forming oxidation products such as elemental sulfur and P2S5. In the doped electrolyte, sulfate ions are detected. During the subsequent discharge to 2.0 V, the oxidation products decrease. It is worth noting that a small amount of oxidation products is still detected in the XPS spectra of a pristine electrolyte after the charge/discharge process, indicating the irreversible oxidative decomposition of the electrolyte at high voltage. These findings align with the CV results, demonstrating that the incorporation of SiO2, GeO2, and SnO2 can effectively suppress the oxidative decomposition of the LPS-xMO2 electrolytes during cycling to high voltage. As illustrated in Supplementary Figure 13 and Supplementary Table 2, the electronic conductivity of the LPS-xMO2 electrolytes was assessed through direct current (DC) polarization, employing an ion-blocking cell configuration comprising two blocking electrodes at a bias voltage of 0.5 V. Both the pristine LPS and doped LPS-xMO2 electrolytes show a low electronic conductivities of about 1.1~1.2 × 10-9 S cm-1, which are far lower than the ionic conductivities. The similar values of the pristine and doped LPS-xMO2 electrolytes suggest that the electronic conductivity does not serve as the decisive factor influencing the electrochemical properties in the doped electrolytes[41-43].

Binary anion and cation co-doping enhance sulfide solid electrolyte performance for all-solid-state lithium batteries

Figure 4. CV curves of the Li/SSEs/SSEs + AB cells within the voltage range from 2 to 4.5 V (vs. Li/Li+), (A) the first and second cycles curves of LPS, and the second cycle curves of (B) LPS-xSiO2 (x = 0.02, 0.05, 0.08, 0.1), (C) LPS-xGeO2 (x = 0.02, 0.05, 0.08, 0.1), (D) LPS-xSnO2 (x = 0.02, 0.05, 0.08, 0.1)

Considering both ionic conductivity and electrochemical stability, LPS-0.05MO2 electrolytes were selected for further electrochemical characterizations. To assess the dendrite-suppressing capability of LPS and LPS-xMO2 electrolytes, symmetric Li/LPS-xMO2/Li cells were constructed to determine the critical current density[44]. As depicted in Figure 5, the critical current density of the pristine LPS electrolyte is 0.64 mA cm-2 at room temperature. In contrast, the doped LPS-xMO2 electrolytes exhibit higher critical current densities, with values of 1.14 mA cm-2 for LPS-0.05SiO2, 1.02 mA cm-2 for LPS-0.05GeO2, and 0.78 mA cm-2 for LPS-0.05SnO2, respectively. Notably, LPS-0.05SiO2 displays the highest critical current density, while LPS-0.05SnO2 exhibits a lower polarization voltage. The impact of doping on the stripping/platting behaviors of lithium metal was further assessed using symmetric Li/SSE/Li cells. Supplementary Figure 14 illustrates the time-dependent galvanostatic voltage profiles of the Li/SSE/Li cells operating at 0.1 mA cm-2. The Li/LPS/Li cell exhibited the highest overpotential of ~0.03 V. After 40 h of cycling, the voltage curve exhibited a sudden drop, which was attributed to a short circuit induced by the formation of lithium dendrites within the LPS electrolyte. In contrast, the stripping/plating behavior of the symmetric cell utilizing doped LPS-xMO2 electrolytes remained stable for 200 h cycling and displayed lower overpotential. This enhanced performance can be attributed to reduced interfacial resistance and improved interfacial stability, thus suppressing lithium dendrite formation. Importantly, among the three doped samples, the SSE doped with SiO2 exhibited the most robust cycling stability, whereas the SSE doped with SnO2 displayed lower polarization voltage.

Binary anion and cation co-doping enhance sulfide solid electrolyte performance for all-solid-state lithium batteries

Figure 5. Lithium plating/stripping voltage profiles of (A) Li/LPS/Li, (B) Li/LPS-0.05SiO2/Li, (C) Li/LPS-0.05GeO2/Li, and (D) Li/LPS-0.05SnO2/Li cells at different current densities.

To investigate the interfacial reactions between the electrolyte and lithium metal, XPS was employed for characterization[45]. The sample doped with SnO2 is used as an example; analysis of XPS spectra [Figure 6] reveals insightful information. In the O 1s spectrum of pristine LPS-0.05SnO2, two distinct peaks are discerned, attributed to P-O-P and Li-O-P interactions. Upon cycling, the O spectrum collected at the Li/LPS-0.05SnO2 interface additionally shows a Li2O peak at 526.6 eV. The presence of Li2O in the interphase would modulate interfacial lithium deposition behaviors and maintain the stability of the SSE/Li interphase, thus inhibiting lithium dendrite formation within the SSE[46,47]. The initial S spectrum of LPS-0.05SnO2 is ascribed to non-bridging sulfur Li-S-P and bridging sulfur P-S-P. However, after cycling, the S spectrum at the LPS-0.05SnO2/Li interface exhibits an additional peak at 161.0 eV, attributed to Li2S. Similarly, the P spectrum [Figure 6C] after cycling displays a Li3P peak (129.3 eV), which was absent in the original sample. As for the pristine LPS electrolyte, the XPS spectra of S 2p and P 2p show similar variations to that of LPS-0.05SnO2. The initial S spectrum is ascribed to non-bridging sulfur Li-S-P and bridging sulfur P-S-P. An additional peak at 161.0 eV, attributed to Li2S, is observed after cycling [Supplementary Figure 15A]. A Li3P peak (129.3 eV) absent in the original sample can be observed in the P spectrum [Supplementary Figure 15B] after cycling. In pristine LPS-0.05SnO2 electrolyte [Figure 6D], Sn is in the +4 valence state, with binding energies of the Sn 3d5/2 peak at 485.4 eV. After cycling, the collected Sn 3d spectrum at the LPS-0.05SnO2/Li interphase demonstrates an elemental Sn peak at binding energies of 483.7 eV, signifying the reduction of +4 valence Sn at the interphase to elemental Sn by lithium metal[48]. This reduction process results in the formation of a lithium alloy phase at the interphase. A similar phenomenon is observed in the XPS data of solid electrolytes doped with SiO2 and GeO2[Supplementary Figure 16]. The formation of Li2O, Li2S, and Li3P species facilitates the passivation of the Li/electrolyte interphase, which effectively inhibits the continuous reductive degradation of the solid electrolyte.

Binary anion and cation co-doping enhance sulfide solid electrolyte performance for all-solid-state lithium batteries

Figure 6. XPS spectra of (A) O 1s, (B) S 2p, (C) P 2p, and (D) Sn 3d for LPS-0.05SnO2 sulfide solid electrolytes before and after five cycles.

To further assess the electrochemical performance of the LPS-xMO2 electrolyte, ASSLBs were constructed utilizing NCM811 as the cathode material, LPS-xMO2 as solid electrolytes, and Li-In as the anode material, respectively. Figure 7A shows the initial charge-discharge voltage profiles of these cells employing various electrolytes. Compared to the bare LPS, the cells with doped LPS-xMO2 electrolytes exhibit higher initial coulombic efficiency and improved discharge capacity. This can be attributed to the improved ionic conductivity and enhanced anodic oxidative stability of the doped LPS-xMO2 electrolytes. Notably, among the doped LPS-xMO2 electrolytes, the cells with LPS-0.05SiO2 electrolytes show the highest initial coulombic efficiency, whereas the cell with LPS-0.05SnO2 electrolytes demonstrates the highest initial discharge capacity. In addition, Figure 7B illustrates the rate capability of the cells at varying current rates. All the cells employing doped LPS-xMO2 electrolytes exhibit improved rate capability compared to the cells with the pristine LPS electrolyte. Although the cells with LPS-0.05SnO2 electrolyte delivers the highest discharge capacity at 0.05 C, the cell with LPS-0.05SiO2 electrolytes shows the best rate capability with a high reversible capacity of 75 mAh/g at 3 C. Furthermore, as depicted in Figure 7C, the cells employing doped LPS-xMO2 electrolytes also exhibit enhanced cycling stability compared to the cells with the pristine LPS electrolyte. Following the formation cycles at 0.05C, the bare LPS exhibits an initial discharge capacity of 154.9 mAh·g-1 with a capacity retention of 74.63% after 200 cycles at 0.1 C. In comparison, LPS-0.05SiO2 showcases an initial discharge capacity of 160.5 mAh·g-1 with a capacity retention of 86%, LPS-0.05GeO2 exhibits an initial discharge capacity of 174.6 mAh·g-1 with a capacity retention of 81.8%, and LPS-0.05SnO2 displays an initial discharge capacity of 184.9 mAh·g-1 with a capacity retention of 81.1%. These results indicate that while the cell with LPS-0.05SnO2 electrolyte demonstrates higher initial capacity, LPS-0.05SiO2 electrolytes offer superior initial coulombic efficiency, rate capability, and cycling stability. This observation aligns with the previously conducted assessments of ionic conductivity and oxidative stability, wherein the LPS-0.05SiO2 electrolyte demonstrated the highest oxidative stability, while the LPS-0.05SnO2 electrolyte exhibited the highest ionic conductivity.

Binary anion and cation co-doping enhance sulfide solid electrolyte performance for all-solid-state lithium batteries

Figure 7. Electrochemical performance of the NCM811/SSEs/Li-In ASSLBs, (A) charge-discharge voltage profiles of the first cycle, (B) rate performance at 0.05C, 0.1C, 0.3C, 0.5C, 1C, and 3C, (C) cycling stability, and Coulombic efficiency.

CONCLUSIONS

In summary, an anion and cation co-doping strategy is proposed to functionally enhance both the ionic conductivity and electrochemical stability of the LPS-based sulfide electrolyte. A series of SiO2-, GeO2-, and SnO2-doped Li7+xP3-xMxS11-2xO2x (M = Si, Ge, Sn, x = 0, 0.02, 0.05, 0.08, 0.1) sulfide solid electrolytes are prepared via high-energy ball milling and sintering. The effects of doping on the structural and electrochemical properties were systematically screened. XPS characterization indicates that the cations demonstrate a preference for substituting the P5+ of the P2S74- unit within the LPS matrix, which expands the Li+ transport channels and generates lithium defects to facilitate ion conduction. This significantly enhances the ionic conductivity of the doped LPS-xMO2 electrolytes. The LPS-0.05SnO2 electrolyte exhibits a high ionic conductivity of 2.53 × 10-3 S cm-1, which is 2.5 times higher than the pristine LPS electrolyte. Further improving the doping levels hinders lithium-ion migration, resulting in elevated activation energy and decreased ionic conductivity. The oxide doping also significantly improves the oxidative stability of the LPS-xMO2 electrolytes, which is crucial for maintaining stable electrode/electrolyte interphases. While the LPS-0.05SnO2 electrolyte delivers higher ionic conductivity, the LPS-0.05SiO2 electrolyte demonstrates superior oxidative stability and higher critical current density. The NCM811-based ASSLBs utilizing the doped LPS-0.05MO2 electrolytes exhibit higher coulombic efficiency, superior discharge capacity, rate capability, and cycling stability compared to the pristine LPS electrolyte. The cell utilizing the LPS-0.05SnO2 electrolyte exhibits a higher initial capacity, whereas the LPS-0.05SiO2 electrolyte demonstrates superior initial coulombic efficiency, rate capability, and cycling stability. This is consistent with the characterization results of ionic conductivity and electrochemical stability, where the LPS-0.05SiO2 electrolyte shows higher oxidative stability and the LPS-0.05SnO2 electrolyte exhibits superior ionic conductivity. These findings shed light on doping strategies for optimizing solid electrolyte materials toward the development of high-performance ASSLBs.

DECLARATIONS

Authors’ contributions

Methodology, formal analysis, and writing of the manuscript: Li C, Lv Z,

Data analysis and technical support: Wu Y, Peng J

Data acquisition: Liu J, Zheng X

Analysis and interpretation of the results: Wu Y, Tang W

Supervision, writing - review and editing: Gong Z, Yang Y

All authors discussed the results and commented on the manuscript.

Availability of data and materials

The data supporting our findings can be found in the Supplementary Material.

Financial support and sponsorship

This work was supported by the National Natural Science Foundation of China (grants no. 22279108, 22261160570, 21935009, and 22021001) and the National Key R&D Program of China (grant no. 2021YFB2401800).

Conflicts of interest

All authors declared that there are no conflicts of interest.

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Supplementary Materials

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Cite This Article

OAE Style

Li C, Wu Y, Lv Z, Peng J, Liu J, Zheng X, Wu Y, Tang W, Gong Z, Yang Y. Binary anion and cation co-doping enhance sulfide solid electrolyte performance for all-solid-state lithium batteries. Energy Mater 2024;4:400009. http://dx.doi.org/10.20517/energymater.2023.78

AMA Style

Li C, Wu Y, Lv Z, Peng J, Liu J, Zheng X, Wu Y, Tang W, Gong Z, Yang Y. Binary anion and cation co-doping enhance sulfide solid electrolyte performance for all-solid-state lithium batteries. Energy Materials. 2024; 4(1): 400009. http://dx.doi.org/10.20517/energymater.2023.78

Chicago/Turabian Style

Li, Cheng, Yuqi Wu, Zhongwei Lv, Jinxue Peng, Jun Liu, Xuefan Zheng, Yongmin Wu, Weiping Tang, Zhengliang Gong, Yong Yang. 2024. "Binary anion and cation co-doping enhance sulfide solid electrolyte performance for all-solid-state lithium batteries" Energy Materials. 4, no.1: 400009. http://dx.doi.org/10.20517/energymater.2023.78

ACS Style

Li, C.; Wu Y.; Lv Z.; Peng J.; Liu J.; Zheng X.; Wu Y.; Tang W.; Gong Z.; Yang Y. Binary anion and cation co-doping enhance sulfide solid electrolyte performance for all-solid-state lithium batteries. Energy Mater. 2024, 4, 400009. http://dx.doi.org/10.20517/energymater.2023.78

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This article belongs to the Special Issue Interface Engineering in (Photo)electrochemical Systems
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