Dr. J. Yu, Dr. C. Wang, Dr. S. Li, Prof. J. Zhu, Prof. Z. LuNational Laboratory of Solid State MicrostructuresCollege of Engineering and Applied SciencesJiangsu Key Laboratory of Artificial Functional MaterialsNanjing UniversityNanjing 210093, P. R. ChinaE-mail: [email protected]; [email protected]. C. Wang, Prof. N. LiuSchool of Chemical and Biomolecular EngineeringGeorgia Institute of TechnologyAtlanta, GA 30332, USAE-mail: [email protected]. Z. LuResearch Center for Environmental Nanotechnology (ReCENT)Nanjing UniversityNanjing 210023, P. R. China
The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.201902729.
DOI: 10.1002/smll.201902729
solid-electrolyte interphase formation,[4,5] separators failure, and internal short cir-cuits.[6,7] The flammable liquid electrolytes further exacerbate the safety hazards and result in fires or even explosions.[8] Non-flammable solid-state electrolytes (SSEs) as a promising alternative to liquid elec-trolytes have been intensively studied over decades, due to their great operation safety, comparable high ionic conductivity, and large electrochemical window.[9–13] Moreover, SSEs as robust mechanical barriers can effectively suppress the den-drites growth and reduce side reactions enabling high-energy Li batteries.[12,13] Among the various types of SSEs, polymer solid electrolytes with excellent flexibility and processability have attracted much interests, while their poor ionic conduc-tivity and mechanical strength impede the further applications.[10–12] Composite polymer electrolytes (CPEs) composed of polymer electrolytes and inorganic fillers, which can provide significantly enhanced ionic conductivity and mechanical proper-ties, have gained increasing attention over
years.[14–19] Polyethylene oxide (PEO)-based composite electro-lyte is one of the most investigated CPEs since its birth.[16–19] Generally, inorganic fillers (active Li+ conductors or passive non-Li+ conductors) are introduced into the PEO-based polymer matrix to change its crystallization kinetics, creating local amor-phous regions at the filler/polymer interfaces for efficient Li+ conduction.[17–22] Meanwhile, the incorporation of ceramic fillers also boosts the electrochemical stability and mechanical properties.[18,19]
However, there are two major limitations of conventional PEO-based CPEs. First, the discontinuity of the inorganic filler is not ideal for Li+ transport across the membrane. As illustrated in Figure S1a (Supporting Information), the clas-sical CPE using low-aspect-ratio nanoparticle fillers, presents relatively low Li+ conductivity due to the short and isolated Li+ transport pathways.[20,21] When using higher-aspect-ratio nanorod or nanowire fillers (Figure S1b, Supporting Infor-mation), the Li+ paths are comparatively prolonged but still discontinuous.[14,23] A latest study shows that the capability to suppress Li dendrites highly relies on the mechanical strength of CPEs,[24] while the above two categories of CPEs demon-strate poor mechanical strength due to the absence of strong skeleton. Thus developing continuous networks reinforcement (Figure S1c, Supporting Information) is urgently needed to
Solid-state electrolytes have recently attracted significant attention toward safe and high-energy lithium chemistries. In particular, polyethylene oxide (PEO)-based composite polymer electrolytes (CPEs) have shown outstanding mechanical flexibility and manufacturing feasibility. However, their limited ionic conductivity, poor electrochemical stability, and insufficient mechanical strength are yet to be addressed. In this work, a novel CPE supported by Li+-containing SiO2 nanofibers is developed. The nanofibers are obtained via sol–gel electrospinning, during which lithium sulfate is in situ introduced into the nanofibers. The uniform doping of Li2SO4 in SiO2 nanofibers increases the Li+ conductivity of SiO2, generates mesopores on the surface of SiO2 nanofibers, and improves the wettability between SiO2 and PEO. As a result, the obtained SiO2/Li2SO4/PEO CPE yields high Li+ conductivity (1.3 × 10−4 S cm−1 at 60 °C, ≈4.9 times the Li2SO4-free CPE) and electrochemical stability. Furthermore, the all-solid-state LiFePO4-Li full cell demonstrates stable cycling with high capacities (over 80 mAh g−1, 50 cycles at C/2 at 60 °C). The Li+-containing mesoporous SiO2 nanofibers show great potential as the filler for CPEs. Similar methods can be used to incorporate Li salts into other filler materials for CPEs.
Composite Solid Electrolyte
1. Introduction
Conventional organic liquid electrolytes cooperating with highly reactive Li metal anodes, usually cause severe safety issues owing to the infinite dendritic Li growth,[1–3] unstable
improve both the Li+ conductivity and mechanical strength of CPEs for advanced Li batteries. The second limitation of con-ventional CPE is that typical inorganic fillers (e.g., TiO2, Al2O3, and SiO2) do not contain Li+, so their intrinsic ionic conduc-tivity is poor.[19–21] The major function of those passive fillers are merely prevent the crystallization of PEO. Their Li+-con-taining derivatives such as lithium silicates afford positive atti-tudes but are often costly and complicated to fabricate.[25,26] A well-designed and affordable network structure with improved intrinsic ionic conduction is therefore required.
In this work, we report a low-cost Li+-containing mesoporous silica nanofibers reinforced composite polymer electrolyte through in situ lithium salt introducing, and dem-onstrate its applications in lithium storage. Passive SiO2 as a low-cost filler is widely used in solid-state Li chem-istry.[19,27–29] A hierarchical SiO2 nanofibers network struc-ture with uniform Li+ doping and numerous mesopores has been successfully developed via Li2SO4-modified electro-spinning and calcination processes. Li2SO4 salt is selected to modify the SiO2 nanofibers, which yields multiple advan-tages: 1) the doped Li2SO4 as active Li+ conductor contrib-utes to intrinsic Li+ conductivity enhancement;[30–32] 2) the doping of Li salt creates mesopores in the nanofibers during the fabrication,[33,34] which significantly boost the polymer wettability promoting the absorption of anions;[35–37] 3) the water-soluble Li2SO4 additive is compatible during the fabrication procedures;[30,38] 4) moreover, Li2SO4 is stable within the typical electrochemical window;[31,39] 5) last but not least, low-cost lithium salt introducing combining with elec-trospinning technique offers new views for viable CPEs.[18] As shown in Figure 1b, after incorporating PEO polymer matrix, the interconnected nanofiber networks within the CPE provide fast and continuous Li+ pathways; and meanwhile, the robust
3D network functions as strong skeleton thereby supporting and strengthening the whole CPE. Considering the multiple advan-tages, the resultant SiO2/Li2SO4/PEO CPE is able to achieve greatly improved ionic conductivity and mechanical properties, and yields high performance in electrochemical cells.
2. Results and Discussion
As depicted in Figure 1a, a Li2SO4-modified electrospinning approach is employed to fabricate mesoporous SiO2/Li2SO4 nanofibers reinforcement.[40] Typically, a silica gel was well-prepared by the hydrolysis and polycondensation of precursors (see the Supporting Information), and then a moderate amount of poly(vinyl alcohol) (PVA) aqueous solution was added to enhance the electrospinnability. Various amounts of Li2SO4 salt were successively introduced to the electrospinning solution to further regulate the Li+ conduction behavior. The introducing of Li2SO4 greatly changes the fundamental physical and chemical properties of precursor solution,[41] which would result in poor spinnability (Figure S2, Supporting Information). A proper elec-trospinning parameter range with a maximum Li2SO4 doping amount of 0.15 g in 13 mL of aqueous solution (≈1.9 wt% in the calcined fibers, determined by inductively coupled plasma mass spectrometry) was successfully developed. The Li2SO4-added viscous solution was electrospun into flexible membrane (Figure S3, Supporting Information), followed by calcina-tion in air yielding the freestanding nanofibers reinforcement (Figure S4, Supporting Information).[33] Figure 2 presents the morphology variation of Li2SO4-doped SiO2 nanofibers exam-ined by scanning electron microscopy (SEM). The as-spun PVA-supported nanofibers show smooth surfaces and long-fiber morphology (Figure 2a) with an average diameter of ≈120 nm
Small 2019, 15, 1902729
Figure 1. Conceptual design and fabrication of SiO2/Li2SO4/PEO CPEs. a) Schematics of realization procedures of SiO2/Li2SO4/PEO CPEs. b) Sche-matics of Fast Li+ conduction enabled by the Li2SO4-derived SiO2 fibers CPEs.
(Figure 2b). After calcination, PVA was removed and Li2SO4-doped SiO2 nanofibers were obtained. As shown in Figure 2d, the calcined nanofibers present more rough surface with shrunken diameters (Figure 2e). Besides, the as-prepared SiO2 membrane shows no obvious change in thickness by the calci-nation processing (Figure 2c,f), suggesting the high mechanical stability. Notably, the Li+-containing SiO2 nanofibers are inter-connected with each other after calcination, forming a cross-linked continuous 3D network for fast Li+ conduction.
The calcined nanofibers were studied by the transmission electron microscopy (TEM) more carefully. As shown in Figure 3b, the Li2SO4-doped SiO2 fibers demonstrate a high-porosity structure with numerous mesopores, and some of them exhibit hollow structures (Figure 3c). While the bare SiO2 fibers without Li2SO4 doping present solid structure with smooth surface as reported (Figure 3d,e).[40,41] The porous nature of Li2SO4-doped SiO2 fibers was further confirmed by N2 adsorption–desorption analysis. The SiO2/Li2SO4 nanofibers present a large Brunauer–Emmett–Teller (BET) surface area of 160 m2 g−1 (Figure 3f), and a narrow pore size distribution of ≈40 nm is observed (Figure 3g). These mesopores might be pro-duced by the typical phase separation processing,[42] the intro-ducing of Li2SO4 into the electrospinning solution, significantly changes its fundamental properties via possible interactions, thereby disturbing the migration of PVA molecule toward fibers surface and resulting in localized uneven distribution of PVA in as-spun fibers as illustrated in Figure 3a.[33,34] The surface structures and morphologies of the substrate highly determine the liquid wettability.[35,37] In this regard, the polymer electrolyte wettability of mesoporous SiO2/Li2SO4 nanofibers was investi-gated by contact angle measurements (schematically illustrated in Figure 3h). As depicted in Figure 3i, the Li2SO4-doped SiO2
membrane demonstrates a much smaller contact angle of 11° against the PEO-lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte solution than that of bare SiO2 membrane (18°). The smaller contact angle indicates that the Li2SO4-derived mesoporous SiO2 fibers can be better wet by polymer electrolyte. The improved wettability facilitates the infiltration of PEO electrolytes into the porous networks and makes more effective contract with fibers,[43] promoting the Lewis acid-based interaction for anions adsorption, which dramatically boosts the Li+ conductivity.[17,19]
To further understand the Li2SO4 role, in-depth investiga-tions were employed on the Li2SO4-doped SiO2 nanofibers. X-ray diffraction results confirm the Li2SO4 species in the obtained membrane (Figure S5, Supporting Information). Energy-dispersive spectroscopy elemental mapping analysis was performed to study the spatial distribution of Li2SO4 in the fibers. As shown in Figure 4b, the uniform distribution of Si and S elements suggests the homogeneous doping of Li2SO4 in the mesoporous SiO2 fibers, and the weaker S signal indi-cates that the Li2SO4 doping is trace. Fourier transform infrared spectroscopy (FTIR, Figure 4c) was conducted to investigate the surface chemistry of the SiO2/Li2SO4 fibers. A clear peak at ≈643 cm−1 corresponding to the antisymmetric bending of SO4 group is found in the modified SiO2 sample (blue line),[44,45] indicating the presence of Li2SO4. And the shifted broad peak at ≈1104 cm−1 of the modified SiO2 fibers suggests new com-plex formation between SiO2 and Li2SO4 during the fabrica-tion.[44] X-ray photoelectron spectroscopy (XPS) analysis was carried out to verify the new chemical composition (Figure 4d,e, and Figure S6, Supporting Information). The high-resolution spectra of Li 1s and Si 2p demonstrate the formation of lithium silicates by the splitting of binding energy, consistent with the
Small 2019, 15, 1902729
Figure 2. Morphology of the Li2SO4-modified electrospun SiO2 nanofibers reinforcement. a) SEM image of the as-spun nanofiber network. b) Diameter distribution of the as-spun nanofibers. c) Cross-section SEM image of the as-spun membrane. Inset showing the network structure. d) SEM image of the calcined SiO2/Li2SO4 nanofiber network. e) Diameter distribution of the SiO2/Li2SO4 nanofibers. f) Cross-section SEM image of the SiO2/Li2SO4 membrane. Inset demonstrating the interconnected network structure.
FTIR results. In brief, the Li+-containing SiO2 fibers are homo-geneous bicontinuous-phase composites with strong interac-tions between the two species as depicted in Figure 4a, which is reasonable to yield fast Li+ conduction.
The Li+-containing SiO2 fibers afford multiple merits for Li+ conducting: uniformly doped Li2SO4 forming strong bonding with SiO2 enhances the intrinsic Li+ conductivity; and induces numerous mesopores in fibers which greatly improve the polymer wettability. SiO2/Li2SO4 nanofibers reinforced CPEs were produced by PEO/LiTFSI electrolyte infusion as shown in Figure 5a inset. Thermogravimetric analysis (TGA) meas-urements were carried out to study the thermal stability of the CPEs, bare PEO polymer electrolyte membrane was prepared as control. As shown in Figure 5a, polymers are unstable and
begin to decompose under continuous heating,[18] obvious mass loss is observed when temperature increases over 200 °C, and a drastic weight decline is shown at ≈400 °C indicating the full decomposition of PEO. Both the two SiO2 nanofibers reinforced CPEs (Li+-containing or free) are thermally stable to ≈200 °C, but the Li2SO4-doped CPE performs better with a higher ini-tial decomposition temperature. Meanwhile, the TGA profiles present a lower polymer content in the Li2SO4-doped SiO2 CPE compared to the Li2SO4-free one, which could be assigned to enhanced electrolyte wettability of mesoporous nanofibers, in which less polymer can effectively wet the reinforcement.
SEM images were taken to study the morphology and struc-ture of the derived CPEs. As shown in Figure 5b, the SiO2-reinforced CPE demonstrates a smooth surface compared to
Small 2019, 15, 1902729
Figure 3. Characteristics of mesoporous SiO2/Li2SO4 nanofibers. a) Schematic of proposed mechanism of mesopores formation by the doping of Li2SO4. b,c) TEM images of Li2SO4-doped SiO2 nanofibers showing mesoporous structure. Inset is the as-prepared electrospinning solution with Li2SO4 additive. d,e) TEM images of Li2SO4-free SiO2 nanofibers showing solid structure. Inset is the Li2SO4-free electrospinning solution. f) BET surface area and g) pore size distribution of the SiO2/Li2SO4 membrane, showing large surface area with numerous mesopores. h) Schematic of contact angle showing the wettability of reinforcements against the PEO-based polymer electrolyte. i) Contact angle between the PEO-based polymer electrolyte and Li2SO4-free membrane (left) or Li2SO4-doped membrane (right), respectively.
Figure 4. Verifying of the Li2SO4 species within the SiO2 nanofibers. a) Schematic of the structure and composition of mesoporous SiO2 fiber. b) TEM elemental mapping analysis of Li2SO4-doped SiO2 nanofibers. c) FTIR spectra of SiO2 membranes. The peaks at ≈1095, ≈960, and ≈803 cm−1 are assigned to the antisymmetric stretching of Si-O-Si, in-plane stretching of Si-OH, and symmetric stretching of Si-O, respectively. d,e) High-resolution XPS spectra of Li 1s and Si 2p of SiO2 membranes, respectively.
Figure 5. Characteristics of SiO2/Li2SO4/PEO CPEs. a) TGA curves of as-prepared CPEs. Inset showing the photograph of SiO2/Li2SO4/PEO CPE disk. b) SEM image of the surface morphology of the SiO2/Li2SO4/PEO CPE membrane. c–e) Cross-section morphology and the corresponding enlarged images of the SiO2/Li2SO4/PEO CPE. f) Stress–strain curves of the SiO2/Li2SO4/PEO CPE and PEO/LiTFSI electrolyte. g) EIS plots of the SiO2/Li2SO4/PEO CPE with the critical doping amount of Li2SO4 at different temperatures from RT to 80 °C. Inset showing the equivalent circuit. h) Arrhenius plots of the SiO2/Li2SO4/PEO CPEs with different doping amounts of Li2SO4. i) LSV profile of SiO2/Li2SO4/PEO CPE sandwiched between a stainless steel electrode and a piece of Li metal.
the pristine reinforcement (Figure 2d), which is ascribed to the fluidity of PEO polymer. Cross-section SEM morphology (Figure 5c) shows that the derived CPE is able to maintain the 3D network structure. PEO/LiTFSI electrolyte was fully infused into the porous silica networks with enhanced wetta-bility, as shown in Figure 5d,e, the nanofibers are completely encapsulated by the PEO matrix, enabling fast Li+ conduction. As aforementioned, high mechanical strength of the SSEs is vital for dendrites suppression in Li metal batteries. Hence, the mechanical properties of the CPEs were examined and summa-rized in Figure 5f. After embedded in robust SiO2/Li2SO4 rein-forcement, the resultant SiO2/Li2SO4/PEO CPE delivers much higher tensile strength and Young's modulus with decreased elongation, compared to the bare PEO polymer membrane. The enhance mechanical strength could be ascribed to the adhe-sion effect between the PEO-based matrix and SiO2 fibers rein-forcement,[46,47] which contributes a lot to the safe operation of working cells.
We emphasize that the in situ introducing of Li2SO4 into SiO2 nanofibers contributes a lot upon the Li+ transport, which is pro-posed to provide three pathways in the CPEs including contin-uous filler/polymer interfaces, active Li+-containing networks, and the pristine Li salt/PEO matrix, enabling high Li+ conduc-tivity.[18] Ionic conductivity was investigated by electrochemical impedance spectroscopy (EIS) measurements on stainless-steel blocking cells at various temperatures. CPEs with different doping amounts of Li2SO4 were studied to understand the role of Li2SO4 in Li+ conduction. Figure 5g shows the Nyquist plots of SiO2/Li2SO4/PEO CPE with the highest Li2SO4 doping as an example (others see Figure S7, Supporting Information). In the fitted spectra, the semicircle during the high- and middle-frequency region represents the parallel combination of bulk resistance and bulk capacitance, which are assigned to the migration of Li+ and the immobile polymer chains within the CPEs, respectively; while the straight tail is ascribed to the interfacial impedance from Li+ migration at the double-layer region between the electrode/electrolyte.[14,16–19] Li+ conductivity is calculated by the bulk resistance. Figure 5h shows the corre-sponding temperature-dependent Arrhenius plots of the SiO2/Li2SO4/PEO CPEs. Increasing Li+ conductivity is achieved with the increasing of Li2SO4 doping within the acceptable spinna-bility, which is in accordance with above conclusions. Notably, the CPE with the highest Li2SO4 doping demonstrates a high Li+ conductivity of 3.9 × 10−5 S cm−1 at room temperature (RT, 25 °C), which is ≈4.2 times higher than that of Li2SO4-free CPE and much higher than the reported bare PEO polymer electrolyte.[20] Moreover, the above SiO2/Li2SO4/PEO CPE yields a higher Li+ conductivity of 1.3 × 10−4 S cm−1 at 60 °C (≈4.9 times of the Li2SO4-free CPE). With the temperature increasing, there are more amorphous regions formation and more Li salt dissociation, resulting in significantly enhanced Li+ conductivity. Note that the high Li+ conductivity is achieved by using low-cost Li2SO4-doped SiO2 fillers, rather than the costly Li+ conductors such as garnet or NASICON ceramics.
The activation energy (Ea) was also calculated as shown in Figure S8 (Supporting Information), where smaller Ea suggests faster Li+ migration within the CPE.[46] The Li2SO4-doped SiO2 CPEs demonstrates lower Ea values than that of the Li2SO4-free one, indicating improved Li+ conduction behaviors by Li-salt
introducing. The Li+ transference number (tLi+) of the SiO2/Li2SO4/PEO CPEs was evaluated by the potentiostatic polari-zation (PP) method.[23,48] As shown in Figure S9 (Supporting Information), the Li2SO4-doped SiO2 CPEs yield higher tLi+ values (0.25, 0.32, 0.45, and 0.52 for 0, 0.05, 0.10, and 0.15 g Li2SO4, respectively) than the reported bare PEO/LiTFSI elec-trolyte (0.12).[49] The enhanced tLi+ could be assigned to two reasons. First, the incorporating of mesoporous SiO2/Li2SO4 nanofibers into the PEO/LiTFSI matrix, promotes the anions absorption and relaxation of polymer local chains for more free Li+;[23,50] second, in situ introduced Li salt within the nanofibers affords considerable intrinsic Li+ conduction. Besides, time-related ionic conduction was investigated to determine the com-posite electrolyte stability over duration. As shown in Figure S10 (Supporting Information), the SiO2/Li2SO4/PEO CPE main-tains a stable Li+ conduction at 30 °C over 15 d storage, suggesting the huge application foreground.
Linear sweep voltammetry (LSV) measurement was carried out to evaluate the electrochemical stable window of the Li2SO4-doped SiO2 CPE, where a piece of Li foil and a stainless steel electrode were used as the counter and the working electrodes, respectively. As shown in Figure 5i, there is no obvious oxida-tion occurring until 5.0 V versus Li/Li+, indicating that the SiO2/Li2SO4/PEO CPE can be employed in wide lithium storage sys-tems even high-voltage system. Besides, combustion tests were performed to study the flammability of SiO2-reinforeced CPE before working cells assembly. As shown in Figure S11a (Sup-porting Information), the SiO2/Li2SO4 fibers reinforced CPE shows low flammability and maintains its structure stability after flame attack. While the bare PEO membrane is ignited instantly when close to the fire and quickly burned to ashes (Figure S11b, Supporting Information). The high Li+ conduc-tivity, wide electrochemical window and low flammability make the CPE possible to support the safe high-energy Li batteries.
Electrochemical cells were fabricated to evaluate the elec-trochemical performance of SiO2/Li2SO4/PEO CPEs. Li sym-metric cells were assembled in the argon-filled glove box by sandwiching the SiO2-derived CPE between two Li foils as illus-trated in Figure 6a, followed by long-term Li plating/stripping tests to study the cyclability. Routine liquid electrolyte cooper-ating with PP separator (Celgard 2400) was chosen as control. As shown in Figure 6b, the time-dependent voltage profiles show that the cell using SiO2/Li2SO4/PEO CPE achieves a stable deep cycling with smaller polarization at a fixed current density of 0.2 mA cm−2 over 400 cycles at 30 °C, indicating stable electrode/electrolyte interface maintained. While the cell using liquid electrolyte underwent larger polarization and suffered from serious Li dendrites growth suggested by the terrible internal short circuits within 80 cycles. In addition, a thin layer of gold was sputtered on the surface of SiO2-derived composite solid electrolyte to further improve the electrode/electrolyte contact. The modified Li symmetric cell demon-strates a much more stable cycling at a high current density of 1.0 mA cm−2 after hundreds of cycles without obvious voltage variation (Figure 6c,d). The distinguished plating/stripping stability further demonstrates the high mechanical strength of SiO2/Li2SO4/PEO CPE against Li dendrites growth.
All-solid-state full cell was fabricated to verify the bat-tery performance using LiFePO4 (LFP)-Li system. LFP
electrodes composed of LFP, PEO, LiTFSI, and acetylene black (60:18:12:10 by weight) were prepared with a mass loading of 1.2–1.5 mg cm−2. The cells were cycled during 2.7–3.9 V at RT and 60 °C by a two-stage charge/discharge process. As shown in Figure 6e, the assembled solid cell exhibits stable full-cell cycling, over 80 mAh g−1 capacity with high Coulombic effi-ciency of 99.8% was maintained over tens of cycles at C/2 (1 C = 0.17 A g−1 LFP) at 60 °C. Long flat plateaus with small polarization of ≈50 mV are observed in the charge/discharge profiles at C/10, even cycled at high current rate of C/2, flat plateaus with acceptable polarization shift are still maintained (Figure 6f). Even cycled at RT, the assembled full cell still achieved considerable high capacities. After cycling, the cell was disassembled and washed to study the morphology of SiO2/Li2SO4/PEO CPE. As shown in Figure 6g, the SiO2-based
CPE maintains a smooth surface without obvious morphology failure after cycles. Moreover, the magnified cross-section SEM image shows the rough SiO2 networks within the CPE dem-onstrating the effective suppression of Li dendrites growth (Figure 6g inset). These indicate the great potential of SiO2/Li2SO4/PEO CPE in practical applications.
3. Conclusion
We have successfully developed a novel Li+-containing silica nanofibers reinforced CPE through Li2SO4-modified electro-spinning and simple wetting processes, and demonstrate its advantages in various electrochemical configurations. Li2SO4 salt is in situ introduced into the electrospun SiO2 nanofibers,
Small 2019, 15, 1902729
Figure 6. Electrochemical performance of the SiO2/Li2SO4/PEO CPEs in different cell configurations. a) Schematic of the symmetric cell for the lithium plating/stripping tests. b) Voltage profile of the lithium plating/striping cycling with a current density of 0.2 mA cm−2 at 30 °C. c) Voltage profile of the lithium plating/striping cycling using Au-coated CPE with a current density of 1.0 mA cm−2 at 30 °C. d) Voltage profiles of the 1st, 10th, 100th cycle of the Au-coated CPE. e) Cycling performance of all-solid-state LFP-Li cell at RT and 60 °C, capacities are based on LFP mass. f ) Voltage profiles of LFP-Li cell at different current rates at 60 °C. g) SEM image of SiO2/Li2SO4/PEO CPE after LFP-Li cycling. Inset showing the cross-section morphology.
resulting in numerous mesopores formation with uniform Li+ doping. The mesoporous SiO2/Li2SO4 fibers reinforced CPE is expected to yield fast Li+ conduction: uniformly doped active Li2SO4 boosts the intrinsic Li+ conductivity; the intercon-nected fibers network within the polymer matrix provide fast and continuous Li+ pathways; sufficient mesopores promote the polymer wettability leading to more anions adsorption. The resultant SiO2/Li2SO4/PEO CPE demonstrates high Li+ conduc-tivity (3.9 × 10−5 S cm−1 at 25 °C and 1.3 × 10−4 S cm−1 at 60 °C) and large electrochemical window (0–5 V versus Li/Li+) as well as high mechanical strength. With these merits, all-solid-state Li cells were constructed in both symmetric cell and full cell systems. Li symmetric cell with SiO2/Li2SO4/PEO CPE yields stable long-term cycling over deep stripping/plating, and pre-sents high mechanical properties to inhibit dendrites growth. LFP-Li full cell exhibits stabilized cycling with high capacities achieved (over 80 mAh g−1 capacity maintained after 50 cycles at C/2 at 60 °C). Consequently, we propose this novel low-cost SiO2/Li2SO4/PEO CPE as one promising solid electrolyte candi-date toward safe high-energy lithium batteries.
Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.
AcknowledgementsJ.Y. and C.W. contributed equally to this work. This work was jointly supported by National Key R&D Program of China (No. 2016YFA0201100), Thousand Talents Program for Young Researchers, National Natural Science Foundation of China (No. 21601083), the Fundamental Research Funds for the Central Universities, Jiangsu Innovative and Entrepreneurial Talent Award and Natural Science Foundation of Jiangsu Province (No. BK20160614). The authors acknowledge Technical Center of Nano Fabrication and Characterization, Nanjing University for the TEM characterization.
Conflict of InterestThe authors declare no conflict of interest.
[1] Y. Li, Y. Li, A. Pei, K. Yan, Y. Sun, C.-L. Wu, L.-M. Joubert, R. Chin, A. L. Koh, Y. Yu, J. Perrino, B. Butz, S. Chu, Y. Cui, Science 2017, 358, 506.
[2] K. J. Harry, D. T. Hallinan, D. Y. Parkinson, A. A. MacDowell, N. P. Balsara, Nat. Mater. 2014, 13, 69.
[3] D. Lu, Y. Shao, T. Lozano, W. D. Bennett, G. L. Graff, B. Polzin, J. Zhang, M. H. Engelhard, N. T. Saenz, W. A. Henderson, P. Bhattacharya, J. Liu, J. Xiao, Adv. Energy Mater. 2015, 5, 1400993.
[4] X.-B. Cheng, R. Zhang, C.-Z. Zhao, F. Wei, J.-G. Zhang, Q. Zhang, Adv. Sci. 2016, 3, 1500213.
[5] Y. Li, W. Huang, Y. Li, A. Pei, D. T. Boyle, Y. Cui, Joule 2018, 2, 2167.[6] I. C. Halalay, M. J. Lukitsch, M. P. Balogh, C. A. Wong, J. Power
Sources 2013, 238, 469.[7] M. D. Tikekar, S. Choudhury, Z. Tu, L. A. Archer, Nat. Energy 2016,
1, 16114.[8] X. Feng, M. Ouyang, X. Liu, L. Lu, Y. Xia, X. He, Energy Storage
Mater. 2018, 10, 246.[9] J. B. Goodenough, P. Singh, J. Electrochem. Soc. 2015, 162, A2387.
[10] L. Yue, J. Ma, J. Zhang, J. Zhao, S. Dong, Z. Liu, G. Cui, L. Chen, Energy Storage Mater. 2016, 5, 139.
[11] A. Manthiram, X. Yu, S. Wang, Nat. Rev. Mater. 2017, 2, 16103.[12] S. Xia, X. Wu, Z. Zhang, Y. Cui, W. Liu, Chem 2019, 5, 753.[13] V. Thangadurai, S. Narayanan, D. Pinzaru, Chem. Soc. Rev. 2014, 43,
4714.[14] W. Liu, S. W. Lee, D. Lin, F. Shi, S. Wang, A. D. Sendek, Y. Cui, Nat.
Energy 2017, 2, 17035.[15] W. Zhou, S. Wang, Y. Li, S. Xin, A. Manthiram, J. B. Goodenough,
J. Am. Chem. Soc. 2016, 138, 9385.[16] J. Bae, Y. Li, J. Zhang, X. Zhou, F. Zhao, Y. Shi, J. B. Goodenough,
G. Yu, Angew. Chem., Int. Ed. 2018, 57, 2096.[17] X. Zhang, J. Xie, F. Shi, D. Lin, Y. Liu, W. Liu, A. Pei, Y. Gong,
H. Wang, K. Liu, Y. Xiang, Y. Cui, Nano Lett. 2018, 18, 3829.[18] K. K. Fu, Y. Gong, J. Dai, A. Gong, X. Han, Y. Yao, C. Wang, Y. Wang,
Y. Chen, C. Yan, Y. Li, E. D. Wachsman, L. Hu, Proc. Natl. Acad. Sci. USA 2016, 113, 7094.
[19] D. Lin, P. Y. Yuen, Y. Liu, W. Liu, N. Liu, R. H. Dauskardt, Y. Cui, Adv. Mater. 2018, 30, 1802661.
[20] F. Croce, G. B. Appetecchi, L. Persi, B. Scrosati, Nature 1998, 394, 456.
[21] F. Croce, L. Persi, B. Scrosati, F. Serraino-Fiory, E. Plichta, M. A. Hendrickson, Electrochim. Acta 2001, 46, 2457.
[22] M. Krutyeva, A. Wischnewski, M. Monkenbusch, L. Willner, J. Maiz, C. Mijangos, A. Arbe, J. Colmenero, A. Radulescu, O. Holderer, M. Ohl, D. Richter, Phys. Rev. Lett. 2013, 110, 108303.
[23] P. Yao, B. Zhu, H. Zhai, X. Liao, Y. Zhu, W. Xu, Q. Cheng, C. Jayyosi, Z. Li, J. Zhu, K. M. Myers, X. Chen, Y. Yang, Nano Lett. 2018, 18, 6113.
[24] H. Huo, Y. Chen, J. Luo, X. Yang, X. Guo, X. Sun, Adv. Energy Mater. 2019, 9, 1804004.
[25] D. Cruz, S. Bulbulian, E. Lima, H. Pfeiffer, J. Solid State Chem. 2006, 179, 909.
[26] L. Zhou, Y. Wu, J. Huang, X. Fang, T. Wang, W. Liu, Y. Wang, Y. Jin, X. Tang, J. Alloys Compd. 2017, 724, 991.
[27] G. Jiang, S. Maeda, H. Yang, Y. Saito, S. Tanase, T. Sakai, J. Power Sources 2005, 141, 143.
[28] S. Delacroix, F. Sauvage, M. Reynaud, M. Deschamps, S. Bruyère, M. Becuwe, D. Postel, J.-M. Tarascon, A. N. V. Nhien, Chem. Mater. 2015, 27, 7926.
[29] D. Zhou, R. Liu, Y.-B. He, F. Li, M. Liu, B. Li, Q.-H. Yang, Q. Cai, F. Kang, Adv. Energy Mater. 2016, 6, 1502214.
[30] E. A. Secco, J. Solid State Chem. 1992, 96, 366.[31] S. Kondo, K. Takada, Y. Yamamura, Solid State Ionics 1992, 53–56,
1183.[32] D. R. MacFarlane, J. Huang, M. Forsyth, Nature 1999, 402, 792.[33] H. He, J. Wang, X. Li, X. Zhang, W. Weng, G. Han, Mater. Lett. 2013,
94, 100.[34] J. Saha, G. De, Chem. Commun. 2013, 49, 6322.[35] A. B. D. Cassie, S. Baxter, Trans. Faraday Soc. 1944, 40, 546.[36] M. T. Khorasani, H. Mirzadeh, Z. Kermani, Appl. Surf. Sci. 2005,
[37] B. Xin, J. Hao, Chem. Soc. Rev. 2010, 39, 769.[38] T. Uma, T. Mahalingam, U. Stimming, Mater. Chem. Phys. 2004, 85,
131.[39] K. E. Johnson, H. A. Laitinen, J. Electrochem. Soc. 1963, 110, 314.[40] Y.-E. Miao, R. Wang, D. Chen, Z. Liu, T. Liu, ACS Appl. Mater.
Interfaces 2012, 4, 5353.[41] S. W. Lee, Y. U. Kim, S. S. Choi, T. Y. Park, Y. L. Joo, S. G. Lee, Mater.
Lett. 2007, 61, 889.[42] M. Bognitzki, W. Czado, T. Frese, A. Schaper, M. Hellwig,
M. Steinhart, A. Greiner, J. H. Wendorff, Adv. Mater. 2001, 13, 70.[43] Q. Ni, Y. Bai, Y. Li, L. Ling, L. Li, G. Chen, Z. Wang, H. Ren, F. Wu,
C. Wu, Small 2018, 14, 1702864.
[44] T. Uma, T. Mahalingam, U. Stimming, J. Mater. Sci. 2004, 39, 2901.[45] S. Ramesh, T. F. Yuen, C. J. Shen, Spectrochimica Acta, Part A 2008,
69, 670.[46] X. Zhang, T. Liu, S. Zhang, X. Huang, B. Xu, Y. Lin, B. Xu, L. Li,
C.-W. Nan, Y. Shen, J. Am. Chem. Soc. 2017, 139, 13779.[47] D. Li, L. Chen, T. Wang, L.-Z. Fan, ACS Appl. Mater. Interfaces 2018,
10, 7069.[48] P. G. Bruce, C. A. Vincent, J. Electroanal. Chem. Interfacial Electro-
chem. 1987, 225, 1.[49] L. Chen, Y. Liu, L.-Z. Fan, J. Electrochem. Soc. 2017, 164, A1834.[50] W. Zhang, J. Nie, F. Li, Z. L. Wang, C. Sun, Nano Energy 2018, 45,
