Residual alkali effect in LLZO-modified composite solid polymer electrolyte
0 Graphical Abstract
Abstract
Solid polymer electrolytes (SPEs) are regarded as ideal solid electrolytes for next-generation lithium-ion batteries due to their high safety, but their low ionic conductivity limits practical applications. The incorporation of garnet-type Lithium Lanthanum Zirconium Oxide (LLZO)-based electrolytes with high Li+ conductivity is an effective strategy to enhance the performance of polyethylene oxide (PEO)-based SPEs. In this study, highly Li+-conductive Ga-doped LLZO (Li6.4Ga0.2La3Zr2O12) was prepared by wet and dry methods, Ga-doped (Ga = 0.2) LLZO is referred to as LLZO for brevity throughout this manuscript, yielding samples denoted as wet-LLZO and dry-LLZO, with abundant and minimal surface residual alkali, respectively. These LLZO nanoparticles with different residual alkali were incorporated as fillers in composite solid polymer electrolytes (CSPEs). Studies demonstrate that the residual alkali, such as Li2CO3, on the LLZO surface effectively disrupts the ordered arrangement of PEO segments, significantly reducing their crystallinity and thus facilitating Li+ transport. Based on this, CSPE with 15 wt% wet-LLZO achieves a high ionic conductivity of 0.80 mS cm-1 and a lithium-ion transference number of 0.36 at 60 °C. The assembled Li/LiFePO4 all-solid-state battery maintains a discharge specific capacity of 142.5 mAh g-1 and a Coulombic efficiency exceeding 99% after 300 cycles at a 1.0 C rate. This work reveals the role of residual surface alkali in the LLZO composite solid electrolyte, providing novel insights for designing high-performance solid-state batteries through filler surface engineering.
Keywords
Download PDF
INTRODUCTION
Solid-state lithium batteries (SSLBs) have become a key focus in next-generation energy storage technology development due to their inherent safety and high energy density. The performance of their core component - solid-state electrolytes (SSEs) - directly determines the overall battery performance[1]. Among various solid-state electrolytes, polyethylene oxide (PEO)-based solid polymer electrolytes (SPEs) have been extensively studied for their excellent flexibility, good interfacial compatibility, and ease of processing[4]. However, the high crystallinity of PEO results in low room-temperature ionic conductivity (typically 10-6-10-5 S cm-1), which severely limits its practical applications at room temperature[8]. To overcome this bottleneck, introducing inorganic fillers into the PEO matrix - has been proven to be one of the effective strategies[11].
Among various inorganic fillers, garnet-type Lithium Lanthanum Zirconium Oxide (LLZO) has garnered significant attention due to its outstanding bulk lithium-ion conductivity and excellent electrochemical stability[16]. There is much literature on the addition of LLZO nanoparticles and nanowires to polymer electrolytes to improve electrolyte performance[22]. However, LLZO materials face a common practical challenge: their extreme sensitivity to moisture (H2O) and carbon dioxide (CO2) in the air, leading to frequent H+/Li+ exchange reactions on the surface, resulting in residual alkali layers such as LiOH and Li2CO3[25,26]. Traditional research has typically regarded residual surface alkali as insulating impurity phases, believing they increase interfacial impedance and hinder ion transport[27,28]. Consequently, extensive efforts have focused on removing residual alkali through post-processing techniques such as acid washing and heat treatment, aiming to achieve the filler’s intrinsic high conductivity[29,30].
In this work, we unveil the positive role of residual surface alkali in optimizing electrolyte interface transport. Through precise control of the synthesis pathway, we prepared two LLZO fillers with distinct surface chemical states. Scheme 1A demonstrates the dry-LLZO preparation using a solvent-free method (dry method)[31], where ball milling and calcination were conducted under fully sealed, air-free conditions, yielding a clean surface with minimal residual alkali. As shown in Scheme 1B, wet-LLZO was synthesized via solid-state synthesis and ground in isopropanol solvent and air (wet method)[32], resulting in a surface rich in residual Li2CO3 alkali layer. Both fillers were incorporated into the PEO/LiTFSI system at equal proportions (15 wt%) to prepare the corresponding composite electrolytes CSPE-15 wet-LLZO and CSPE-15 dry-LLZO, enabling systematic investigation of the differential effects of surface residual alkali on electrolyte structure and performance. Comprehensive characterization reveals that residual alkali, such as Li2CO3 on the wet-LLZO surface, disrupts the regularity of PEO chain segments, thereby substantially decreasing their crystallinity. Benefiting from this, CSPE-15 wet-LLZO achieves a high ionic conductivity of 0.80 mS cm-1 and a lithium-ion transference number of 0.36 at 60 °C. The assembled Li/LiFePO4 all-solid-state battery maintains a discharge specific capacity of 142.5 mAh g-1 and a coulombic efficiency exceeding 99% after 300 cycles at a 1.0 C rate. This study deepens the understanding of the surface characteristics of LLZO in SPEs and provides a novel, promising strategy for designing high-performance composite solid-state electrolytes.
Scheme 1. Schematic diagram of (A) dry-LLZO preparation via solvent-free method, (B) wet-LLZO preparation via solid-phase synthesis. LLZO: Li6.4Ga0.2La3Zr2O12, dry-LLZO: LLZO by a dry method; wet-LLZO: LLZO by a wet method; YSZ: yttria-stabilized zirconia
EXPERIMENTAL
Materials
PEO (Aladdin, MV = 600,000 g mol-1) and LiTFSI (Aladdin, 99%) were dried under vacuum at 65 °C and
Preparation of Ga-LLZO particles
In this work, the LLZO particles were added into the SPEs. LLZO particles with a Ga = 0.2 doping level are confirmed as having the highest Li+ conductivity, which is consistent with our previous work[33].
Ga-doped (Ga = 0.2) LLZO was prepared via solid-state synthesis, with a stoichiometric ratio of lithium hydroxide monohydrate to lithium oxide. The starting materials - LiOH·H2O (AR, 98%), La2O3 (99.99%, preheated at 900 °C for 12 h), ZrO2 (AR, 3N), and Ga2O3 (AR, 99.99%) (total
The powder was then subjected to wet ball milling at 120 rpm for 6 h. After drying, it was sieved through a 200-mesh sieve to obtain LLZO powder with particle sizes approaching the nanoscale. The powder was vacuum-dried at 65 °C for 3 h and stored in a glovebox for further use.
Dry-LLZO was prepared by a solvent-free method: 53.71 g of LiOH·H2O (AR, 98%),
Preparation of composite solid electrolytes
PEO and LiTFSI lithium salt were dissolved in anhydrous acetonitrile at a ratio of EO:Li+ = 18:1. Wet-LLZO or dry-LLZO fillers (15 wt%) were added to the mixture. The mixture was stirred at 60 °C for 12 h to form a homogeneous slurry, which was then cast into a film using solution casting. After initial drying at room temperature, the film was dried in a vacuum at 60 °C for 24 h, yielding composite electrolyte films with a thickness of approximately 80 μm (labeled as CSPE-15 wet-LLZO and CSPE-15 dry-LLZO).
Material characteristics
The LLZO fillers with varying surface residual alkali content were first prepared through solid-phase synthesis (wet method) and solvent-free synthesis (dry method). The wet synthesis method used isopropanol as the solvent, with the mixture calcined at 850 °C in an air environment for 6 h to form a rich LiOH/Li2CO3 residual alkali layer on the filler surface (denoted as wet-LLZO). The dry synthesis method involved solvent-free ball milling and calcination under strict sealed conditions to avoid air contact, yielding LLZO with minimal surface residual alkali (denoted as dry-LLZO). Subsequently, the composite electrolyte was prepared via solution casting: PEO and LiTFSI (EO:Li+ = 18:1) were dissolved in anhydrous acetonitrile, with 15 wt% wet-LLZO or dry-LLZO fillers added, respectively. The mixture was stirred at 60 °C for 12 h to form a homogeneous slurry, which was cast into a film and then vacuum-dried to obtain an electrolyte film with a thickness of approximately 80 μm (labeled as CSPE-15 wet-LLZO and CSPE-15 dry-LLZO, respectively). The physical properties were characterized by X-ray diffraction (XRD) to analyze the crystal structure of the filler and the crystallinity of PEO, Scanning Electron Microscopy - Energy Dispersive X-ray Spectrometry (SEM-EDS) to observe the morphology and dispersion of the filler, and X-ray photoelectron spectroscopy (XPS)/Fourier Transform Infrared Spectroscopy (FTIR)/Raman to characterize the surface chemistry (e.g., the lattice Li-O component of wet-LLZO exhibits a Li 1s binding energy of 54.2 eV, and the Li2CO3 peak position of C 1s is 289.85 eV). In Supplementary Figure 1, Brunauer-Emmett-Teller (BET) was used to measure the specific surface area (wet-LLZO is 10.14 m2 g-1, higher than dry-LLZO’s
Electrochemical testing
All coin cells were assembled in a glove box and stored in a constant temperature chamber awaiting testing. Electrochemical impedance spectroscopy (EIS) tests were performed using an Alternating Current (AC) impedance spectrometer (IVIUM nSTAT) with a frequency range of 0.1 HZ to 1 MHZ and a bias voltage of 10 mV. Coin cell cases of type CR2016 were used for assembly, and the electrolyte film (thickness ~80 µm) was sandwiched between two stainless steel plates (diameter = 16 mm) and tested at a temperature range of 30 °C to 80 °C. The AC impedance spectra (EIS) of the electrolyte films at different temperatures were obtained by this method to calculate the Li+ conductivity of the corresponding electrolytes [Supplementary Figure 2]. The calculation equation is as follows[38]:
where σ is the Li+ conductivity, L is the thickness of the electrolyte membrane, R is the intrinsic impedance of the electrolyte (intercept of the Nyquist plot on the x-axis), and S is the effective area of the electrolyte.
Assembled with two lithium metal cells (diameter = 16 mm) as non-blocking electrodes using CR2032-type coin cell cases. The Li+ mobility of the electrolyte membrane was measured by the chrono-current method (potential applied to the cell was 10 mV) and AC impedance spectroscopy, calculated as follows[21]:
where t+ is the Li+ transference number, I0 and Iss represent the initial and steady-state currents, respectively, ∆V denotes the external polarization voltage (10 mV), and R0 and Rss represent the electrolyte/Li interface resistance in the initial and steady states.
The electrochemical stability window was evaluated using linear sweep voltammetry (LSV) at a scan rate of 1 mV s-1, within a voltage range of 0-6 V, and at a test temperature of 60 °C. Critical current density (CCD) tests were conducted using Li/electrolyte/Li batteries, with current incrementally increased from
The full-cell performance evaluation utilized CR2032 battery casings to assemble Li/electrolyte/LiFePO4 batteries. The cathode active material loading was maintained at 1.5-4.0 mg cm-2, with a voltage window set between 2.5 and 4.0 V. Using the Neware battery testing system, constant-current charge-discharge tests were conducted at rate ranges of 0.2-3.0 C (1 C = 170 mA g-1), with test temperatures including 30 and 60 °C. Cycle performance testing was performed at 0.2 C rate for 100 cycles, evaluating capacity retention and Coulombic efficiency. All battery assemblies were fabricated in an argon glove box (H2O/O2 < 0.01 ppm) and pre-treated in a 60 °C vacuum oven for 12 h prior to testing to ensure interface stability. The experimental chemicals and reagents, as well as the experimental instruments and equipment used, are listed in Supplementary Tables 1 and 2, respectively.
RESULTS AND DISCUSSION
Supplementary Figure 3 illustrates the preparation process of the PEO-based composite electrolytes containing LLZO fillers. The digital photographs in Supplementary Figure 4 show the 18 mm-diameter SPE-blank, CSPE-15 dry-LLZO, and CSPE-15 wet-LLZO electrolyte membranes, confirming that flexible and self-standing films were successfully obtained. The morphologies of dry-LLZO and wet-LLZO were examined by SEM, as shown in Figure 1A and 1B. The particle-size distribution histograms inserted in these two figures were obtained from SEM image analysis. The average particle size of dry-LLZO is 0.908 ±
Figure 1. (A) Surface SEM image of LLZO prepared by the solvent-free method, (B) Surface SEM image of LLZO prepared by the solid-state synthesis method, and the insets in Figure 1A and B show the corresponding particle-size distribution histograms obtained from SEM image analysis. Cross-sectional SEM images of (C) SPE-blank, (D) CSPE-15 dry-LLZO, and (E) CSPE-15 wet-LLZO. SEM: Scanning electron microscopy; LLZO: Li6.4Ga0.2La3Zr2O12.
The XRD patterns of dry-LLZO and wet-LLZO particles are shown in Figure 2A. Both wet- and dry-LLZO particles exhibited cubic phases after calcination at 850 °C for 6 h. Notably, the dry-LLZO displayed a broad full width at half maximum (FWHM) in its XRD pattern, accompanied by weak La2O3 impurity peaks (PDF#05-0602). This phenomenon is attributed to the high hardness of La2O3, which led to incomplete pulverization and uneven mixing during the dry mixing process. However, trace amounts of La2O3 are not expected to significantly affect the final sintered powder[39]. The FTIR spectrum of wet-LLZO [Figure 2B] exhibits distinct characteristic peaks assigned to LiOH·H2O and Li2CO3 (labeled i-viii). The peaks corresponding to LiOH·H2O are identified as follows: the O-H stretching vibration at 3,566 cm-1 (i), the H-O-H bending vibrations at 1,579 cm-1 (ii) and 999 cm-1 (iii), and the O-H-related vibrational modes at 678 cm-1 and 632 cm-1 (iv and v). Meanwhile, the characteristic peaks of Li2CO3 are observed at 1,504 cm-1 and
Figure 2. (A) XRD patterns of wet-LLZO and dry-LLZO, (B and C) FTIR and Raman spectra of wet-LLZO, dry-LLZO, LiOH, and Li2CO3, (D-F) high-resolution spectra of Li 1s, C 1s, and O 1s for dry-LLZO and wet-LLZO. XRD: X-ray diffraction; dry-LLZO: LLZO by a dry method; wet-LLZO: LLZO by a wet method; FTIR: fourier transform infrared spectroscopy; LLZO: Li6.4Ga0.2La3Zr2O12.
To further elucidate differences between LLZO particles synthesized by wet and dry methods, XPS was performed on both wet-LLZO and dry-LLZO samples. In the Li 1s spectrum [Figure 2D], Li2CO3 exhibits characteristic Li-O peaks at 54.83 eV (dry-LLZO) and 55.18 eV (wet-LLZO), while the peaks centered near 54.2 eV correspond to the Li-O bond within the LLZO lattice. Comparison of the two samples reveals a significantly stronger Li2CO3 peak for wet-LLZO relative to dry-LLZO. Analysis of the C 1s spectrum [Figure 2E] shows a peak at 289.85 eV, which, according to literature assignments, is associated with the C-O bond in Li2CO3[41,42]. This carbonate-related peak is markedly more intense in wet-LLZO than in dry-LLZO, consistent with the observations in the Li 1s spectrum. In the O 1s spectrum [Figure 2F], peaks centered near 531.0 eV are attributed to Li2CO3 and LiOH species, while the peak at approximately 529 eV arises from lattice oxygen within LLZO. Wet-LLZO exhibits a significantly higher peak intensity ratio of residual alkali (Li2CO3/LiOH) to lattice oxygen than dry-LLZO, indicating more abundant surface alkali residues in the wet-synthesized material[43]. Taken together, these XPS results demonstrate that wet- and dry-processed LLZO powders possess distinctly different surface alkali characteristics: wet-LLZO exhibits substantial residual Li2CO3 and LiOH, whereas dry-LLZO shows minimal surface alkali.
XRD was employed to evaluate the crystallinity of CSPE-15 wet-LLZO, CSPE-15 dry-LLZO, and SPE-blank [Figure 3A]. The SPE-blank sample displayed well-defined PEO crystalline peaks at the 2θ positions of 19° and 23°, corresponding to the (120) and (112) planes, respectively, as indexed in the PEO reference pattern (PDF card 45-0109). Incorporation of either wet-LLZO or dry-LLZO particles decreased the intensities of these PEO crystalline peaks, reflecting reduced polymer crystallinity. Notably, CSPE-15 wet-LLZO exhibited a more pronounced attenuation of PEO diffraction intensity compared to CSPE-15 dry-LLZO. DSC measurements [Figure 3B] further corroborated these findings. The glass transition temperature (Tg) of CSPE-15 wet-LLZO was -44.51 °C, lower than that of CSPE-15 dry-LLZO (-42.86 °C) and substantially lower than that of SPE-blank (-41.96 °C). This progressive decrease in Tg indicates enhanced disruption of PEO chain packing and reduced crystallinity upon LLZO incorporation, with wet-LLZO exerting a stronger effect than dry-LLZO. Collectively, XRD and DSC analyses consistently demonstrate that wet-LLZO incorporation is more effective than dry-LLZO in suppressing PEO crystallinity, which may be attributed to differences in particle-polymer interfacial interactions and dispersion behavior.
Figure 3. (A) XRD patterns of CSPE-15 wet-LLZO, CSPE-15 dry-LLZO, and SPE-blank, (B) DSC curves of CSPE-15 wet-LLZO, CSPE-15 dry-LLZO, and SPE-blank, (C) Lithium-ion conductivity curves at 30-80 °C; (D) LSV curves at 60 °C for SPE-blank, CSPE-15 dry-LLZO, and CSPE-15 wet-LLZO, (E-G) Chronoamperometry (CA) curves and EIS spectra before and after polarization at 60 °C and a potential of
The thermal stability of SPE-blank, CSPE-15 wet-LLZO, and CSPE-15 dry-LLZO was evaluated and analyzed using TGA curves [Supplementary Figure 5]. Before 200 °C, all three samples exhibited approximately 2% to 3% weight loss due to water vapor evaporation. Between 240 and 700 °C, significant weight loss was observed, caused by the decomposition of PEO and LiTFSI. Further TGA analysis using Derivative Thermogravimetry (DTG) revealed that the thermal decomposition temperature of the SPE-blank electrolyte was 402 °C. After adding the fillers, the decomposition temperatures of CSPE-15 dry-LLZO and CSPE-15 wet-LLZO increased to 417 and 414 °C, respectively. Therefore, the addition of wet-LLZO and dry-LLZO fillers can enhance the thermal stability of CSPEs.
To investigate the electrochemical effects of residual alkali on the surface of LLZO on composite electrolytes, fundamental electrochemical performance tests were conducted on SPE-blank, CSPE-15 dry-LLZO, and CSPE-15 wet-LLZO. The Li+ conductivity curves with temperature variation are shown in Figure 3C. At
The electrochemical stability of the PEO-based solid electrolytes was evaluated using LSV, as shown in Figure 3D. CSPE-15 wet-LLZO maintained a stable voltage plateau up to approximately 5.04 V, indicating superior oxidative stability. In contrast, CSPE-15 dry-LLZO and SPE-blank reached instability at lower potentials of about 4.87 and 4.31 V, respectively, highlighting their limited voltage tolerance. Figure 3E-G present the Li+ transference number (tLi+) for the three samples: CSPE-15 wet-LLZO exhibited a tLi+ of 0.36, significantly higher than that of CSPE-15 dry-LLZO (0.28) and SPE-blank (0.20). This trend suggests that the incorporation of wet-LLZO not only enhances ionic conductivity but also improves the selective transport of Li+ ions, which is critical for the performance of solid-state battery systems.
The critical current density (CCD) is a key parameter for assessing a polymer electrolyte’s ability to suppress lithium dendrite penetration under high current conditions. Symmetric Li/SPEs/Li cells were assembled to determine CCD, with results illustrated in Figure 4A-C. The current density was increased stepwise at a rate of 0.05 mA cm-2 per hour (0.5 h stripping followed by 0.5 h plating), and the corresponding cell voltage was monitored continuously. Lithium symmetric cells incorporating CSPE-15 dry-LLZO and SPE-blank exhibited pronounced polarization at current densities of 0.4 mA cm-2 and 0.1 mA cm-2, respectively, indicative of internal short circuits that precluded sustained cycling. In contrast, the cell employing CSPE-15 wet-LLZO maintained stable operation until reaching a short-circuit event at 0.65 mA cm-2. This demonstrates that CSPE-15 wet-LLZO possesses markedly superior lithium dendrite suppression capability, enabling stable lithium stripping/plating at higher current densities compared with both dry-LLZO and blank polymer electrolyte systems.
Figure 4. (A-C) CCD curves of Li/SPE-blank/Li, Li/CSPE-15 dry-LLZO/Li, and Li/CSPE-15 wet-LLZO/Li batteries at 60 °C, (D and E) EIS spectra and voltage-time curves at 0.1 mA cm-2 before long cycling for the same batteries; and (insert) magnified view of local voltage distribution. CCD: Critical current density; EIS: electrochemical impedance spectroscopy; SPE: solid polymer electrolyte; LLZO: Li6.4Ga0.2La3Zr2O12.
EIS of the three lithium symmetric cells are shown in Figure 4D. The semicircular feature in the high- to mid-frequency range corresponds to the charge transfer resistance (Rct)[44]. Among the samples, the Li/CSPE-15 wet-LLZO/Li cell exhibited the lowest Rct value, indicating that the presence of wet-LLZO effectively buffers interfacial potential fluctuations and thereby promotes interfacial stability during cycling. The long-term galvanostatic cycling performance of the lithium symmetric cells was evaluated at 60 °C under a current density of 0.1 mA cm-2 with a fixed capacity cutoff of 0.1 mAh cm-2, as shown in Figure 4E. The Li/CSPE-15 wet-LLZO/Li cell demonstrated exceptional electrochemical durability, sustaining stable cycling for over 1,000 h with a steady polarization voltage stabilized at approximately 30 mV. In contrast, the Li/SPE-blank/Li and Li/CSPE-15 dry-LLZO/Li cells maintained relatively high initial polarization voltages and experienced a rapid decline in polarization voltage to 0 V after 248 and 381 h, respectively. This abrupt voltage drop is attributed to irreversible degradation of the electrolyte-electrode interfacial layer induced by repeated lithium deposition and stripping. Consequently, these results underscore that the CSPE-15 wet-LLZO composite effectively preserves the structural and electrochemical integrity of the electrolyte/lithium metal interface. This interfacial robustness is pivotal to the superior long-term stability observed in the wet-LLZO-based system.
The EIS measurement of Li|LiFePO4 full cells with different SPEs was tested at 60 °C [Figure 5A]. The obtained EIS spectra consist of ohmic impedance (first semicircle), charge transfer impedance (middle semicircle), and diffusion resistance (linear portion)[45]. Compared to the SPE-blank control, the full cell incorporating CSPE-15 wet-LLZO exhibited a significant reduction in ohmic impedance from 21.4 Ω to 18.2 Ω and charge transfer impedance from 101.0 Ω to 28.5 Ω. These findings indicate that the addition of wet-LLZO effectively facilitates Li+ migration and lowers the overall impedance within the SPEs. Figure 5B presents the rate performance of Li/SPEs/LiFePO4 full cells. At current rates of 0.2, 0.5, 1.0, and 3.0 C, the Li/CSPE-15 wet-LLZO/LiFePO4 cell delivered discharge-specific capacities of 167.7, 166.8, 159.9, and
Figure 5. (A and B) EIS spectra before cycling and rate performance of Li/SPE-blank/LiFePO4, Li/CSPE-15 dry-LLZO/LiFePO4, and Li/CSPE-15 wet-LLZO/LiFePO4 batteries, (C and D) Long-cycle performance and charge-discharge curves of Li/SPEs/LiFePO4 batteries at 0.2 C at 60 °C, with the 50th cycle included, (E and F) Long-term cycling performance of Li/SPEs/LiFePO4 batteries at 30 °C and 0.2 C, and the charge-discharge curves of the 50th cycle. EIS: Electrochemical impedance spectroscopy; SPE: solid polymer electrolytes; LLZO: Li6.4Ga0.2La3Zr2O12.
The long-term cycling stability of solid-state lithium metal batteries was systematically investigated at 60 °C and 30 °C. The Li|LiFePO4 battery using CSPE-15 wet-LLZO maintained a specific capacity of 156.8 mAh g-1 and a capacity retention rate of 99.7% after 100 cycles at 0.2 C and 60 °C [Figure 5C]. By contrast, under identical test conditions, the discharge specific capacities of the cells based on CSPE-15 dry-LLZO and SPE-blank decreased to 128.3 mAh g-1 and 116.8 mAh g-1, respectively, after 100 cycles. Furthermore, the Li/CSPE-15 wet-LLZO/LiFePO4 cell exhibited a more stable voltage plateau and lower polarization [Figure 5D]. Supplementary Figure 6 further compares the cycling performance of the three solid-state Li|LiFePO4 cells at 0.5 C and 1.0 C under 60 °C with a LiFePO4 loading of 1.5 mg cm-2. The long-cycle results over 100 cycles at 0.5 C [Supplementary Figure 6A] demonstrate that the CSPE-15 wet-LLZO battery delivers the highest discharge-specific capacity among all samples within these 100 cycles. Its charge-discharge profiles recorded up to 150 cycles [Supplementary Figure 6B] further reveal a stable voltage plateau from the 10th to 150th cycle. At 1.0 C, the long-cycle performance after 300 cycles [Supplementary Figure 6C] indicates that CSPE-15 wet-LLZO maintains a discharge specific capacity of 142.5 mAh g-1, with its coulombic efficiency exceeding 99%. In contrast, CSPE-15 dry-LLZO shows significant capacity degradation after only 200 cycles, while SPE-blank exhibits substantial capacity loss after merely 133 cycles, as evidenced by its charge-discharge curves [Supplementary Figure 6D], which confirm minimal polarization deterioration over 50-200 cycles. Collectively, the results from these subfigures demonstrate that the CSPE-15 wet-LLZO composite electrolyte achieves both high reversible capacity and excellent long-cycle stability. The long-term cycling stability of Li/SPEs/LiFePO4 full cells was also evaluated at 30 °C via galvanostatic charge-discharge at 0.2 C. As shown in Figure 5E,
CONCLUSION
This study investigates the positive effect of residual alkali on the surface of LLZO on CSPEs. Combined with characterization results from DSC and XRD, it was found that residual alkali on the LLZO surface can reduce the crystallinity of PEO, thereby enhancing the lithium-ion conductivity of CSPEs. Consequently, CSPE with 15 wt%wet-LLZO exhibits favorable Li+ conductivity (0.80 mS cm-1) and Li+ transference number (0.36) at
DECLARATIONS
Authors’ contributions
Study design, experimental validation, data curation and analysis, figure preparation: He, Y.
Experimental design, experimental validation, data analysis, writing of the original draft: Liu, G.
Research supervision, resource coordination, project administration, manuscript revision: Ao, X.
Research supervision, resource support, project management, manuscript review and revision: Tian, B.
All authors discussed the results and commented on the manuscript, and all authors have read and approved the final submitted version.
Availability of data and materials
The data supporting our findings can be found in the Supplementary Materials.
AI-assisted tools statement
During the preparation of this manuscript, the AI tool ChatGPT (version 1.0, released 2026-07-22) was used solely to generate part of the particle elements in the Graphical Abstract. The tool did not influence the study design, data collection, analysis, interpretation, or the scientific content of the work. All authors take full responsibility for the accuracy, integrity, and final content of the manuscript.
Financial support and sponsorship
The authors sincerely thank the National Natural Science Foundation of China (52262036), Jiangxi Science and Technology Major Program (20252ABG010005), and the Key Projects of Guangxi Natural Science Foundation (2024AB02044).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2026.
Supplementary Materials
REFERENCES
1. Joshi, A.; Mishra, D. K.; Singh, R.; Zhang, J.; Ding, Y. A comprehensive review of solid-state batteries. Appl. Energy. 2025, 386, 125546.
2. Liu, J.; Chen, Y.; Zhou, Z.; et al. Multifunctional polar polymer boosting PEO electrolytes toward high room temperature ionic conductivity, high-voltage stability, and excellent elongation. ACS. Appl. Mater. Interfaces. 2025, 17, 7821-9.
3. Du, A.; Lu, H.; Liu, S.; et al. Breaking the trade-off between ionic conductivity and mechanical strength in solid polymer electrolytes for high-performance solid lithium batteries. Adv. Energy. Mater. 2024, 14, 2400808.
4. Qiao, Q.; Li, Y.; Song, C.; et al. Recent advances and remaining challenges of solid-state electrolytes for lithium batteries. Prog. Mater. Sci. 2026, 156, 101559.
5. He, H.; Deng, N.; Wang, X.; et al. Design strategies, characterization mechanisms, and applications of MOFs in polymer composite electrolytes for solid-state lithium metal batteries. Adv. Funct. Mater. 2025, 35, 2421670.
6. Jiang, Y.; Yan, X.; Ma, Z.; et al. Development of the PEO based solid polymer electrolytes for all-solid state lithium ion batteries. Polymers 2018, 10, 1237.
7. Tan, S.; Zeng, X.; Ma, Q.; Wu, X.; Guo, Y. Recent advancements in polymer-based composite electrolytes for rechargeable lithium batteries. Electrochem. Energ. Rev. 2018, 1, 113-38.
8. Xu, Q.; Wang, Y.; Feng, X.; et al. Monoclinic Li2ZrO3 with cationic vacancy-based ion transport channels enhanced composite polymer electrolytes for high-rate solid-state lithium metal batteries. Nano. Energy. 2026, 147, 111571.
9. Polu, A. R.; Kim, K.; Kareem, A. A.; et al. Impact of tetracyanoethylene plasticizer on PEO based solid polymer electrolytes for improved ionic conductivity and solid-state lithium-ion battery performance. J. Power. Sources. 2025, 625, 235742.
10. Wang, Z.; Chen, J.; Fu, J.; Li, Z.; Guo, X. Polymer-based electrolytes for high-voltage solid-state lithium batteries. Energy. Mater. 2024, 4, 400050.
11. Fu, Y.; Gu, Z.; Gan, Q.; Mai, Y. A review on the ionic conductivity and mechanical properties of composite polymer electrolytes (CPEs) for lithium batteries: insights from the perspective of polymer/filler composites. Mater. Sci. Eng. R. Rep. 2024, 160, 100815.
12. Liu, S.; Liu, W.; Ba, D.; et al. Filler-integrated composite polymer electrolyte for solid-state lithium batteries. Adv. Mater. 2023, 35, e2110423.
13. Wen, W.; Zeng, Q.; Chen, P.; et al. Enhancing Li-ion conduction and mechanical properties via addition of fluorine-containing metal-organic frameworks in all-solid-state cross-linked hyperbranched polymer electrolytes. Nano. Res. 2022, 15, 8946-54.
14. Xiao, Z.; Long, T.; Song, L.; Zheng, Y.; Wang, C. Research progress of polymer-inorganic filler solid composite electrolyte for lithium-ion batteries. Ionics 2022, 28, 15-26.
15. Ji, K.; Moon, H.; Kim, J.; Park, J. Role of functional nano-sized inorganic fillers in poly(ethylene) oxide-based polymer electrolytes. J. Power. Sources. 2003, 117, 124-30.
16. Ayalew, K. H.; Palaniyandy, N.; Mathe, M. K.; Msomi, P. F. Garnet-type LLZO electrolytes for solid-state lithium batteries: Interfaces, conductivity, in-situ processing, and industrial prospects. Chem. Eng. J. 2025, 524, 168098.
17. Wang, L.; Wu, J.; Bao, C.; You, Z.; Lu, Y.; Wen, Z. Interfacial engineering for high-performance garnet-based solid-state lithium batteries. SusMat 2024, 4, 72-105.
18. Wang, Y.; Chen, Z.; Jiang, K.; Shen, Z.; Passerini, S.; Chen, M. Accelerating the development of LLZO in solid-state batteries toward commercialization: a comprehensive review. Small 2024, 20, e2402035.
19. Chu, J.; Li, Z.; Wang, J.; Huang, G.; Zhang, X. Optimization strategies for key interfaces of LLZO-based solid-state lithium metal batteries. Mater. Chem. Front. 2024, 8, 2109-34.
20. Samson, A. J.; Hofstetter, K.; Bag, S.; Thangadurai, V. A bird’s-eye view of Li-stuffed garnet-type Li7La3Zr2O12 ceramic electrolytes for advanced all-solid-state Li batteries. Energy. Environ. Sci. 2019, 12, 2957-75.
21. Löwe, R.; Hanemann, T.; Zinkevich, T.; Hofmann, A. Structure-property relationship of polymerized ionic liquids for solid-state electrolyte membranes. Polymers 2021, 13, 792.
22. Counihan, M. J.; Lee, J.; Mirmira, P.; et al. Improved interfacial li-ion transport in composite polymer electrolytes via surface modification of LLZO. Energy. Mater. 2025, 5, 500032.
23. Seo, J.; Nasir, M.; Park, H. J. Lithium-ion conduction pathways in LLZO-PEO composite solid electrolytes. ACS. Appl. Energy. Mater. 2025, 8, 1518-25.
24. Go, W.; Doeff, M. M.; Tucker, M. C. Investigation of MgO additives on microstructure and properties of thin LLZO electrolytes for all-solid-state batteries. J. Mater. Chem. A. 2025, 13, 8835-42.
25. Ye, R.; Ihrig, M.; Imanishi, N.; Finsterbusch, M.; Figgemeier, E. A review on Li+/H+ exchange in garnet solid electrolytes: from instability against humidity to sustainable processing in water. ChemSusChem 2021, 14, 4397-407.
26. Zhang, N.; Ren, G.; Li, L.; et al. Dynamical evolution of CO2 and H2O on garnet electrolyte elucidated by ambient pressure X-ray spectroscopies. Nat. Commun. 2024, 15, 2777.
27. Dubey, R.; Sastre, J.; Cancellieri, C.; et al. Building a better Li-garnet solid electrolyte/metallic li interface with antimony. Adv. Energy. Mater. 2021, 11, 2102086.
28. Liu, M.; Song, A.; Zhang, X.; et al. Interfacial lithium-ion transportation in solid-state batteries: challenges and prospects. Nano. Energy. 2025, 136, 110749.
29. Siniscalchi, M.; Gibson, J. S.; Tufnail, J.; et al. Removal and reoccurrence of LLZTO surface contaminants under glovebox conditions. ACS. Appl. Mater. Interfaces. 2024, 16, 27230-41.
30. Zhang, H.; Paggiaro, G.; Okur, F.; et al. On high-temperature thermal cleaning of Li7La3Zr2O12 solid-state electrolytes. ACS. Appl. Energy. Mater. 2023, 6, 6972-80.
31. Park, G.; Hwang, J.; Song, M.; Song, W.; Lee, K. J. Recent progress on solvent-free electrode fabrication for lithium-based batteries. Chem. Eng. J. 2025, 511, 161888.
32. Muhammad, T.; Zhao, D.; Guerreiro, A.; et al. Solid phase synthesis of molecularly imprinted polymers for analytical and life science applications. TrAC. Trends. Anal. Chem. 2025, 184, 118134.
33. Li, J.; Luo, H.; Liu, K.; et al. Excellent stability of Ga-doped garnet electrolyte against Li metal anode via eliminating LiGaO2 precipitates for advanced all-solid-state batteries. ACS. Appl. Mater. Interfaces. 2023, 15, 7165-74.
34. Su, J.; Huang, X.; Song, Z.; et al. Overcoming the abnormal grain growth in Ga-doped Li7La3Zr2O12 to enhance the electrochemical stability against Li metal. Ceram. Int. 2019, 45, 14991-6.
35. Aguesse, F.; Manalastas, W.; Buannic, L.; et al. Investigating the dendritic growth during full cell cycling of garnet electrolyte in direct contact with Li metal. ACS. Appl. Mater. Interfaces. 2017, 9, 3808-16.
36. Yi, E.; Wang, W.; Kieffer, J.; Laine, R. M. Key parameters governing the densification of cubic-Li7La3Zr2O12 Li+ conductors. J. Power. Sources. 2017, 352, 156-64.
37. Afyon, S.; Krumeich, F.; Rupp, J. L. M. A shortcut to garnet-type fast Li-ion conductors for all-solid state batteries. J. Mater. Chem. A. 2015, 3, 18636-48.
38. Yu, X.; Zhao, L.; Li, Y.; et al. Weakening ionic coordination for high ionic conductivity composite solid electrolytes. ACS. Energy. Lett. 2024, 9, 2109-15.
39. Huang, X.; Song, Z.; Xiu, T.; et al. Sintering, micro-structure and Li+ conductivity of Li7-xLa3Zr2-xNbxO12/MgO (x= 0.2-0.7) Li-garnet composite ceramics. Ceram. Int. 2019, 45, 56-63.
40. Huang, X.; Lu, Y.; Niu, Y.; et al. From protonation & Li-rich contamination to grain-boundary segregation: evaluations of solvent-free vs. wet routes on preparing Li7La3Zr2O12 solid electrolyte. J. Energy. Chem. 2022, 73, 223-39.
41. Yao, K. P. C.; Kwabi, D. G.; Quinlan, R. A.; et al. Thermal stability of Li2O2 and Li2O for Li-air batteries: in situ XRD and XPS studies. J. Electrochem. Soc. 2013, 160, A824-31.
42. Huo, H.; Chen, Y.; Zhao, N.; et al. In-situ formed Li2CO3-free garnet/Li interface by rapid acid treatment for dendrite-free solid-state batteries. Nano. Energy. 2019, 61, 119-25.
43. Zhang, H.; Okur, F.; Cancellieri, C.; et al. Bilayer dense-porous Li7La3Zr2O12 membranes for high-performance Li-garnet solid-state batteries. Adv. Sci. 2023, 10, e2205821.
44. Sun, C.; Wang, Z.; Yin, L.; et al. Fast lithium ion transport in solid polymer electrolytes from polysulfide-bridged copolymers. Nano. Energy. 2020, 75, 104976.
Cite This Article
Open AccessHow to Cite
Download Citation
If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click on download.
Export Citation File
Type of Import
Tips on Downloading Citation
Citation Manager File Format
Type of Import
Direct Import: When the Direct Import option is selected (the default state), a dialogue box will give you the option to Save or Open the downloaded citation data. Choosing Open will either launch your citation manager or give you a choice of applications with which to use the metadata. The Save option saves the file locally for later use.
Indirect Import: When the Indirect Import option is selected, the metadata is displayed and may be copied and pasted as needed.
About This Article
Special Topic
Copyright
Data & Comments
Data
0
Comments
Comments must be written in English. Spam, offensive content, impersonation, and private information will not be permitted. If any comment is reported and identified as inappropriate content by OAE staff, the comment will be removed without notice. If you have any queries or need any help, please contact us at [email protected].