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Advanced Fiber Materials

, Volume 1, Issue 1, pp 46–60 | Cite as

High-Performance 3-D Fiber Network Composite Electrolyte Enabled with Li-Ion Conducting Nanofibers and Amorphous PEO-Based Cross-Linked Polymer for Ambient All-Solid-State Lithium-Metal Batteries

  • Chaoyi Yan
  • Pei Zhu
  • Hao Jia
  • Jiadeng Zhu
  • R. Kalai Selvan
  • Ya Li
  • Xia Dong
  • Zhuang Du
  • Indunil Angunawela
  • Nianqiang Wu
  • Mahmut DiricanEmail author
  • Xiangwu ZhangEmail author
Research Article
  • 542 Downloads

Abstract

Solid electrolytes have gained attention recently for the development of next-generation Li-ion batteries since they can fundamentally improve the battery stability and safety. Among various types of solid electrolytes, composite solid electrolytes (CSEs) exhibit both high ionic conductivity and excellent interfacial contact with the electrodes. Incorporating active nanofibers into the polymer matrix demonstrates an effective method to fabricate CSEs. However, current CSEs based on traditional poly(ethylene oxide) (PEO) polymer suffer from the poor ionic conductivity of PEO and agglomeration effect of inorganic fillers at high concentrations, which limit further improvements in Li+ conductivity and electrochemical stability. Herein, we synthesize a novel PEO based cross-linked polymer (CLP) as the polymer matrix with naturally amorphous structure and high room-temperature ionic conductivity of 2.40 × 10−4 S cm−1. Li0.3La0.557TiO3 (LLTO) nanofibers are incorporated into the CLP matrix to form composite solid electrolytes, achieving enhanced ionic conductivity without showing filler agglomeration. The high content of Li-conductive nanofibers improves the mechanical strength, ensures the conductive network, and increases the total Li+ conductivity to 3.31 × 10−4 S cm−1. The all-solid-state Li|LiFePO4 batteries with LLTO nanofiber-incorporated CSEs are able to deliver attractive specific capacity of 147 mAh g−1 at room temperature, and no evident dendrite is found at the anode/electrolyte interface after 100 cycles.

Graphic Abstract

A highly ionic-conductive 3-D fiber network composite solid electrolyte is introduced based on Li-ion conducting nanofibers and amorphous poly(ethylene oxide) (PEO) cross-linked polymer. With the reinforcement of Li0.3La0.557TiO3 (LLTO) nanofibers, the continuous 3D conduction network formed within the polymer matrix greatly enhances the electrochemical and mechanical properties of resultant composite solid electrolytes. Consequently, the lithium dendrite is effectively controlled after long cycles, and the all-solid-state Li|LiFePO4 prototype cells demonstrate excellent cycling stability at room temperature.

Keywords

Cross-linked PEO polymer Li0.33La0.55TiO3 nanofibers Composite solid electrolyte All-solid-state batteries Ambient working temperature 

Introduction

Current rechargeable Li-ion batteries are essential constituents of portable electronics, grid storages, and electric vehicles. With rapidly growing demands in energy storage, batteries with higher energy and power densities beyond the state-of-the-art Li-ion batteries are urgently needed [1, 2]. Li metal is believed as a promising anode for high-energy and high-power batteries due to its highest theoretical capacity (3860 mAh g−1) and lowest electrochemical potential (− 3.04 V versus standard hydrogen electrode). Despite these attractive features, the use of Li metal anode in practical battery applications has been hindered by ever growing safety concerns, which are originated from high chemical reactivity of organic liquid electrolytes and irregular Li depositions (dendrites) during charge–discharge cycles. The Li dendrites can ultimately short-circuit the cells and cause explosion hazards. Solid-state electrolytes have been considered as a fundamental strategy to address aforementioned problems because they are chemically stable and mechanically robust to suppress Li dendrite growth and prevent safety hazards.

Poly(ethylene) oxide (PEO) is one of the most commonly used solid polymer electrolytes, but the low room-temperature conductivity of PEO restricts its use in practical batteries. In PEO based electrolytes, Li+ is coordinated with ether oxygen groups and transferred by the local relaxation of polymer chains in the amorphous region [3, 4, 5]. Generally, ions in solid polymers are believed to be conducted via the segmental motion of polymer chains in the amorphous region under the help of local free volume generated when the temperature is above Tg. Ions are transferred or hopped into adjacent coordinated sites by the polar groups in polymer chains [6, 7]. Therefore, the conduction mechanism in organic solid electrolytes can be considered as the combination of short-range ion hopping and long-range motion of polymer chains while the long-range segmental motion only occurs in the amorphous region. Accordingly, the high crystallinity of traditional PEO based polymer electrolytes leads to their low ionic conductivity. Cross-linking is one of the effective ways to suppress the crystallization of polymer chains, and many PEO-based cross-linked polymers have been investigated as solid polymer electrolytes including copolymers [8, 9, 10] and polyether networks [11, 12, 13]. However, the ionic conductivities of those network polymers remain low at room temperature (~ 1.0 × 10−5 S cm−1) [14].

Several approaches have been devised to improve the conductivity of solid polymer electrolytes. Adding plasticizers, such as ionic liquids [15, 16] or low-molecular-weight oligomers [17, 18, 19], were proven to increase the ionic conductivity by decreasing the glass transition temperature (Tg) of the polymers and thereby facilitating the segmental motion, but may come at the cost of increased reactivity. Another approach is to introduce inorganic fillers in the polymer matrix, which leads to the formation of composite solid electrolytes (CSE). Among various inorganic fillers, active Li-ion conductors such as Li0.3La0.557TiO3 (LLTO) [20], Li7La3Zr2O12 (LLZO) [21], and Li1.3Al0.3Ti1.7(PO4)3 [22] were proposed to have better performance than the inert fillers such as SiO2 [23], TiO2 [24], and Al2O3 [25]. Active Li-ion conductor incorporated CSEs show remarkably higher ionic conductivity than polymer electrolytes owing to the suppression of crystallinity of polymer matrix and benefiting from the fast ion transport pathways on the surface of these active nanofiller [21, 26, 27]. The Li+ conduction in CSEs is realized via the combination of the segmental motion of polymer chains and the activated hops along the interfaces of active fillers and polymer matrix [26, 28]. However, in many studies [21, 27, 29, 30], the ionic conductivity decreased after adding 10 wt% or more nanofillers because agglomerated filler particles enhance the local crystallinity of PEO. Compared to the commonly-used particulate nanofillers, one-dimensional (1D) active nanofibers [21, 26, 31] is preferable due to the long-range and continuous Li+ diffusion pathways created within the polymer matrix. Therefore, creating a continuous nanosized Li+ conductive network within a low crystalline or non-crystalline polymer matrix by using active nanofibers may eliminate the agglomeration effect and lead to a highly ionic-conductive CSE [32].

Herein, we synthesized a highly ionic-conductive cross-linked poly(ethylene oxide) (PEO) polymer. After plasticized with poly(ethylene glycol) (PEG), the solid polymer electrolyte shows a high ionic conductivity of 2.40 × 10−4 S cm−1 at room temperature. Based on the synthesized cross-linked polymer, a composite solid electrolyte was developed by incorporation of Li0.3La0.557TiO3 (LLTO) nanofibers within the polymer matrix. Since LLTO nanofibers provide a 3D network and continuous Li+ transfer channels [21] within the polymer matrix, the composite solid electrolyte shows high ionic conductivity, high lithium transference number, and excellent mechanical properties. Owing to these improvements, the all-solid-state Li-ion battery prototype based on this new composite solid electrolyte can work at room temperature and show remarkable cycle-ability.

Experimental Section

Fabrication of Cross-Linked Poly(Ethylene Oxide) Solid Polymer Electrolytes (CLPSPEs)

All chemicals were purchased from Sigma-Aldrich and used without further purification. Lithium perchlorate (LiClO4) was dried at 80 °C for 24 h before use to remove the moisture. For CLP synthesis, prepolymer solution was prepared by adding monomer (poly(ethylene glycol) methyl ether acrylate, PEGMEA Mn = 360), crosslinker (poly(ethylene glycol) dimethacrylate, PEGDMA Mn = 550), photoinitiators (hydroxycryclohexyl phenyl ketone, HCPK), and lithium salts (LiClO4), sequentially. The amount of crosslinker and initiator was controlled at 2 wt% and 0.1 wt% based on the total weight of monomers, and the amount of lithium salts was controlled at [EO]/[Li+] = 20. After stirring, the solution was sonicated for 10 min to eliminate the air bubbles. For the preparation of unplasticized polymer (CLP), the prepolymer solution was directly exposed to 312-nm UV light for 5 min at 3 mW cm−2 in the Argon-filled glove box. For plasticized polymers (CLP-Px), certain amount of poly(ethylene glycol) methyl ether (mPEG Mn = 250) was added in the prepolymer solution, and then the same synthesis procedure was followed. The amount of plasticizer was set as 10, 20, 30, and 40 wt% based on the weight of prepolymer solution and the as-prepared plasticized polymers were denoted as CLP-P1, CLP-P2, CLP-P3, and CLP-P4, respectively. During polymerization, A quartz plate weighing of 25 g was placed on the top to control the thickness of solid electrolyte membrane at 50–80 µm.

Fabrication of Li0.33La0.557TiO3 (LLTO) Nanofibers

LLTO nanofibers were fabricated by electrospinning. LLTO precursor solution was first prepared by dissolving stoichiometric amounts of 3.3 mmol lithium nitrate (LiNO3), 5.6 mmol lanthanum nitrate hexahydrate (La(NO3)3·6H2O), 10 mmol titanium butoxide (Ti (OC4H9)4) in 20 ml of dimethylformamide (DMF) with 15% acetic acid (volume ratio). After stirring for 30 min, 2 g polyvinylpyrrolidone (PVP, Mw = 1,300,000) was added. The solution was mechanically stirred for overnight and then sonicated for 10 min before use to eliminate the air bubbles. The electrospinning was run at 15 kV with a constant feeding rate of 0.75 ml h−1. As-spun fibers were later calcined at 800 °C for 2 h to obtain LLTO nanofibers.

Fabrication of LLTO Incorporated CLP Composite Solid Electrolytes (L-CLPCSEs)

L-CLPCSEs were fabricated by dispersing Li0.33La0.557TiO3 (LLTO) ceramic nanofibers in a prepolymer solution (PEGDMA, PGMEA, HPCK, and LiClO4). To prepare the composite solid electrolytes, CLP-P4 was selected as the polymer matrix since it gave the highest ionic conductivity among all CLPs. LLTO nanofibers (10, 20, and 30 wt% based on the monomer amount) were dispersed in the prepolymer solution. For samples with a high amount of LLTO nanofibers, the solution was allowed to stir for 24 h and then sonicated for 5 min to ensure uniform dispersion of the nanofibers. The crosslinking process was performed using the same procedure as the CLP synthesis, but the exposure time was extended to 15 min to react the monomers completely. The obtained L-CLPCSEs were denoted as CLP-P4-LLTO-1, CLP-P4-LLTO-2, and CLP-P4-LLTO-3, respectively.

Characterization Methods

Fourier transform-infrared spectroscopy (FT-IR, Thermo Scientic™ Nicolet™ iS™10) was used to study the UV curing conditions and LiClO4 dissociation of CLPSPEs and L-CLPCSEs. The IR spectra were collected under absorbance mode from 425 to 4000 cm−1 with 32 scans and resolution of 4 cm−1. 1H NMR spectroscopy analysis was conducted with Varian Inova 400 spectrometer, and the samples are dissolved in d6-DMSO solvent. Differential scanning calorimetry (DSC) was carried out at a heating/cooling rate of 2 °C min−1 using a TA Instrument Discovery Series. The second heat cycles were used to characterize the thermal properties of CLPSPEs. Thermo-gravimetric analysis (TGA) was conducted by Perkin Elmer Pyris 1 with a heating rate of 20 °C min−1 under air atmosphere to examine the thermal stability of CLPSPEs and L-CLPCSEs. X ray diffraction (XRD) was used to identify the crystal structures of synthesized LLTO nanofibers, CLPSPEs, and L-CLPCSEs by Rigaku D/Max 2400 (Japan) with Cu Ka radiation (λ = 1.5418 Å) in a 2-Theta angle range from 10° to 70°. The morphology of LLTO nanofibers, L-CLPCSEs, and Li foil were characterized by the field-emission scanning electron microscopy (FE-SEM, FEI Verios 460L, USA).

Electrochemical Performance Tests

Liner sweep voltammetry (LSV) was carried out to test the stability of CLPSPEs at a scan rate of 10 mV s−1. The total lithium-ion conductivity was characterized by electrochemical impedance spectroscopy (Garmy Reference 600 device) over a frequency range of 0.1 HZ to 1 MHZ. The solid electrolyte membranes were sandwiched between two stainless steel blocking electrodes. On the Nyquist plot, the intercepts of extended semicircles with real axis represent the bulk resistance of the solid electrolyte, and the ionic conductivity was calculated by the following equation:
$$\sigma = \frac{1}{R}\frac{t}{A}$$
(1)
where R is the bulk resistance, t the sample thickness, and A the sample area. Activation energy (Ea) was calculated by Arrhenius equation:
$$\sigma = A\exp \left( { - E_{a} /RT} \right)$$
(2)
with Arrhenius plot of ionic conductivities. Li transference numbers (tLi+) were determined by the chronoamperometry test on symmetric lithium cells with an applied DC voltage of 10 mV. EIS was also performed both before and after the polarization with the frequency ranging from 1 MHz to 1 Hz. The tLi+ value was calculated by Bruce’s equation:
$$t_{{Li^{ + } }} = \frac{{I_{ss} \left( {\Delta V - I_{0} R_{0} } \right)}}{{I_{0} \left( {\Delta V - I_{ss} R_{ss} } \right)}}$$
(3)
where \(\Delta V\) is the polarization voltage, \(I_{0}\) the initial current, \(\varvec{ }I_{ss}\) the steady state current, \(R_{0}\) the initial total resistance, and \(R_{ss}\) the steady state total resistance. Galvanostatic cycling of symmetric Li cells was conducted to evaluate the structural stability of solid electrolytes and mimic a charging and discharging operation in lithium metal batteries. All cells were cycled at current densities of 0.2 and 0.5 mA cm−2 for 30 min at room temperature.

Battery Performance Evaluation

LiFePO4/CLP-P4-LLTO-3/Li coin cells were assembled in an argon-filled glove box. To prepare the cathode, a slurry of LiFePO4, prepolymer solution (PEGMEA, PEGDMA, LLTO, LiClO4, HPCK), and carbon black (C65, TIMCAL Graphite & Carbon Ltd.) was mixed at a weight ratio of 65:25:10. The mixture slurry was coated on Al foil by using proper content of water as the solvent with a controlled thickness of 35–45 µm. The coated cathode was directly exposed to 312-nm UV light for 5 min to cross-link the polymer and then dried at 80 °C under vacuum for 48 h. The loading of active material in composite cathode was controlled at 1.5 mg cm−2. Lithium foil was then stacked on the composite solid electrolyte, and the coin cells were assembled in Argon-filled glovebox. The cycling performance of all-solid-state LiFePO4 cells was tested by Arbin battery tester in a potential range of 2.5–4.2 V.

Results and Discussion

Figure 1 illustrates the synthesis procedure of cross-linked poly(ethylene oxide) solid polymer electrolyte (CLPSPE) from poly(ethylene oxide) methyl ether acrylate (PEGMEA) monomers and poly(ethylene oxide) dimethacrylate (PEGDMA) cross-linker using photo-initiated polymerization. Cross-linking is able to inhibit the crystallinity of PEO based polymers because the reduced long-range movement of polymer chains restricts the formation of polymer crystallites. Acrylate-based PEO monomer was selected in this work because the atactic methacrylate backbones suppress the tendency of PEO to crystallize due to the formation of interconnected “cages” that spatially besiege the PEO molecules [33, 34, 35]. In this three-dimensional (3D) polymer framework, branched PEO side chains in PEGMEA absorb Li+ and provide the Li-conductive pathways. The oligoethylene oxide pendants provided by PEGDMA are attached to PEGMEA backbones and are connected to the network. Note that, the pendants can swing freely between the main chains like springs, which facilitates the Li+ transfer. To enhance the ion mobility, a certain content of PEG plasticizer was added. These small molecular weight PEG molecules cross-linked with the matrix by hydrogen bonding and spaced the long chains apart, which may increase the free volume and thus lower the glass transition temperature (Tg) [36]. In this work, lithium salt concentration was controlled at a molar ratio of [EO]/[Li+] = 20. The unplasticized cross-linked PEO polymer is denoted as CLP, and plasticized cross-linked PEO polymer is denoted as CLP-Px (“x” represents the plasticizer content). Figure S1 compares the typical Fourier-transform infrared (FTIR) spectra of the synthesized CLPs, monomers, and cross-linker. It is seen that the plasticized and unplasticized samples are very similar in FTIR since PEG is chemically the same as PEGMEA and PEGDMA, while all of the characteristic peaks for vinyl groups (986 and 812 cm−1) [37, 38] and acrylate groups (1190 and 1410 cm−1) [37, 38] disappear in the FTIR spectra of the synthesized polymers. Raman spectra of CLP, CLP-P4, and CLP-P4-LLTO-3 are shown in Fig. S1b. Compared to CLP and CLP-P4, CLP-P4-LLTO-3 exhibits additional peaks at Raman shift of 140, 238, 315, 452, 526, and 597 cm−1, which are in agreement with the six typical bands for the tetragonal LLTO structure [39, 40, 41]. Moreover, except from the intense and sharp peak at 931 cm−1, corresponding to LiClO4 salt, all other characteristic peaks are broad and less intense for all three synthesized solid electrolytes, which indicates the amorphous structure of cross-linked polymers [42, 43]. Furthermore, the structures of monomers and synthesized polymers are confirmed by 1H NMR spectroscopy, and all chemical shift peaks are illustrated with the corresponding proton resonance (Fig. S2). Similarly with the FTIR results, the multiplet signals at 5.91 and 6.33 ppm (alkene protons in PEGMEA) and triplet signals at 5.66 and 6.00 ppm (alkene protons in PEGDMA) disappear in 1H NMR spectra of synthesized polymer, revealing the complete conversion of monomers to a cross-linked network structure. Note that, the main signal of methylene protons (CH2–CH2–O) shifts to 3.31 ppm after polymerization due to the shielding effect of forming cross-linked macromolecules. Additionally, the PEG ratios in synthesized CLPs can be estimated by comparing the integration area of methylene protons with the integration area of methyl protons (CH3) since the huge difference of methyl protons concentration in PEG and monomers (PEGMEA and PEGDMA) (Fig. S3). The detailed calculation is stated in supporting information, and the obtained PEG ratio is listed in Table 1.
Fig. 1

Synthesis procedure of cross-linked poly(ethylene oxide) solid polymer electrolyte (CLPSPE)

Table 1

Compositions, thermal and electrochemical properties of CLPSPEs

 

PEG content (wt%)

Tg (°C)

Ionic conductivity 25 °C (S cm−1)

Activation energy (Ea) (eV)

CLP

0

− 39.2

3.38 × 10−5

0.53

CLP-P1

10

− 42.1

9.36 × 10−5

0.43

CLP-P2

20

− 45.3

1.28 × 10−4

0.43

CLP-P3

30

− 50.3

1.75 × 10−4

0.42

CLP-P4

40

− 56.5

2.40 × 10−4

0.40

The thermal and electrochemical properties of all synthesized CLPSPEs were investigated by differential scanning calorimetry (DSC) and electrochemical impedance spectroscopy (EIS), respectively (Fig. 2). DSC traces of the second heating cycle are demonstrated in Fig. 2a, and Table 1 also summarizes the Tg values of CLPs. Note that, all samples show no melting transition (Tm) of PEO segments up to 90 °C, indicating that the synthesized polymers are naturally amorphous. X ray diffraction (XRD) analysis further confirms the amorphous structure of the synthesized CLPs (Fig. S4). The XRD patterns show a broad carbon peak range near 2θ = 20°, and no characteristic peaks can be found corresponding to crystalline PEO structure (2θ = 19° and 23°). More importantly, addition of plasticizer leads to significant decrease in Tg from − 39.2 °C (0 wt% PEG) to − 56.5 °C (40 wt% PEG), which is benefited from incorporation of small PEG oligomers in the cross-linked polymer matrix. Polymers with lower Tg is believed to better facilitate Li+ transport due to the enhancement of segmental motion of polymer chains. Therefore, our plasticized CLP demonstrates to be a suitable base material for solid electrolytes with sufficient chain mobility and low activation energy (Ea). Moreover, the naturally amorphous structure of polymer matrix may prevent the formation of local crystalline phase when incorporating with a higher content of inorganic fibers.
Fig. 2

a DSC traces of second heat cycle, b EIS profiles, c Arrhenius plots, and d linear sweep voltammetry of CLPSPEs with different weight percentages of plasticizer

The EIS profiles and ionic conductivities of CLPSPEs are demonstrated in Fig. 2b and c, respectively. Additionally, the activation energy calculated according to Arrhenius equation and the room-temperature ionic conductivities of each solid polymer electrolyte are listed in Table 1. At room temperature, CLP-P4 gives the lowest impedance of 108 Ω and its corresponding ionic conductivity is 2.40 × 10−4 S cm−1. CLP-P3, CLP-P2, and CLP-P1 provide higher impedances of 133, 193, and 265 Ω, respectively, while the unplasticized CLP gives the highest impedance of 800 Ω at room temperature. In addition, the Ea value decreases from 0.53 eV (CLP) to 0.43 eV (CLP-P1) by adding plasticizer, indicating a faster Li+ migration in polymer due to the addition of small plasticizer molecules [44]. Increasing the plasticizer content further decreases Ea value to 0.40 eV (CLP-P4) because of the reduced Tg of the polymer matrix. Notably, adding plasticizer in the cross-linked polymer dramatically improves the ionic conductivity due to the addition of freely-moving small molecules and the increase of free volume. Among all samples, CLP-P4 shows the highest ionic conductivity of 2.40 × 10−4 S cm−1 at all temperature ranges, which is almost one order of magnitude higher than that of unplasticized CLP (3.38 × 10−5 S cm−1) at 25 °C. The electrochemical stability of solid polymer electrolytes was measured by liner sweep voltammetry (Fig. 2d). Both CLP and CLP-P4 exhibit stable voltage windows between 1 and 5 V vs. Li/Li+, indicating that this novel Li+ conducting polymer is suitable for most high-voltage lithium batteries and the PEG plasticizer does not influence the electrochemical stability.

Once the polymer matrix with the highest ionic conductivity was identified, composite solid electrolytes were fabricated by introducing electrospun LLTO nanofibers that were obtained according to the modified method as ref. [27]. Figure 3 illustrates the schematic of the resultant LLTO nanofiber-incorporated cross-linked poly(ethylene oxide) composite solid electrolyte (L-CLPCSE), in which the 3-D fiber network formed by rigid inorganic LLTO nanofibers is effective in suppressing lithium dendrites mechanically. Morphologies and crystal structure of LLTO nanofibers calcined at 800 °C are shown in Fig. 3b and c, respectively. The main peaks in diffraction pattern could be clearly indexed with the LLTO structure, and the calcined nanofibers maintained a well-defined 3D fiber network. Figure S5 shows the digital image of the free-standing L-CLPCSE with excellent flexibility and ductility. Although the ionic conductivity of CLPSPE has been improved significantly by adding plasticizers, the plasticized CLPSPE has reduced mechanical strength. Figure S6 compares the strain–stress curves of unplasticized CLP, CLP-P4 (i.e., CLPSPE), and CLP-P4-LLTO-3 (i.e., L-CLPCSE). With the introduction of plasticizer, the Young’s modulus of the resultant CLP-P4 decreases by nearly 90%, compared to that (12.8 MPa) of unplasticized CLP. Along with adding LLTO nanofibers, Young’s modulus and elongation improve dramatically to 8.5 MPa and 60%, respectively, which are considered as favorable mechanical properties for solid electrolyte membranes [45, 46]. Clearly, the nanofibers provide a mechanically robust framework within the polymer matrix and support the electrolyte membranes. Thermogravimetric analysis (TGA) was used to characterize the thermal stability of L-CLPCSE, which is demonstrated in Fig. S7. For all three samples, the thermal degradation temperature is about 400 °C, indicating that the addition of plasticizer and nanofibers has no obvious negative influence on the thermal stability.
Fig. 3

a schematic drawing of CLP-P4-LLTO composite solid electrolyte: cross-linked PEO polymer matrix is reinforced by LLTO nanofibers to effectively control the lithium dendrite, b XRD pattern and c SEM image of the LLTO nanofiber calcination at 800 °C for 2 h

The synthesized L-CLPCSEs were then investigated with series of electrochemical evaluation, which are shown in Fig. 4. Figure 4a demonstrates the Arrhenius plots of L-CLPCSEs with various contents of LLTO nanofibers. The room-temperature ionic conductivities of the composite solid electrolytes are summarized in Table 2. Among all samples, CLP-P4-LLTO-3 shows the highest ionic conductivity of 3.13 × 10−4 S cm−1 at room temperature. Note that, the improvement in ionic conductivity after adding LLTO nanofibers is not as significant as pure PEO systems reported elsewhere [21, 27, 32] because the primary influence of inorganic nanofillers in PEO polymer is the suppression of crystallinity. In this work, the cross-linked PEO polymers are intrinsically amorphous and have been plasticized with PEG, the introduction of inorganic nanofibers cannot further influence the crystallinity of polymer matrix. Herein, the ionic conductivity improvement achieved for CLP-P4 based composite electrolytes with the addition of LLTO nanofibers may be caused by the enhancement of Lewis base-acid interactions with the negatively charged Li+ vacancies along the surface of nanofibers acting as a strong Lewis base center. Moreover, the interconnected conductive network provides more Li+ hoping channels with lower migration energy [32, 47]. Notably, agglomeration effect was not found when increasing the LLTO nanofibers content to 30 wt%. The ionic conductivities increase accordingly with the filler content from CLP-P4-LLTO-1 to CLP-P4-LLTO-3, which results from the well-distribution of nanofibers (Fig. S8) and the naturally amorphous polymer structure of CLP-P4. XRD (Fig. S9) analysis confirms the amorphous phase of LLTO incorporated composite electrolytes. The intensity of characteristic peaks indexed with LLTO structure increases with the increase of the nanofibers content, but no characteristic peaks appear for the crystalline PEO phase in all three samples. These results provide an important idea in designing composite electrolytes and eliminating the negative influence when high content of fillers is incorporated, which summarizes as: (1) retaining the interconnected conductive pathway within the composite electrolytes provided by the 3-D fiber network, and (2) use of naturally amorphous polymer matrix to prevent the local crystallinity even with the existence of agglomerated particles. L-CLPCSEs with higher nanofiber concentrations were not studied because it was almost impossible to disperse 40 wt% LLTO in the prepolymer solution for the cross-linking reaction due to the high solution viscosity.
Fig. 4

a Arrhenius plots of CLP-P4 and CLP-P4-LLTO composite solid electrolytes, b lithium plating/striping cycles of symmetric Li|CLP|Li and Li|CLP-P4-LLTO-3|Li cells, FTIR spectra at 610–640 cm−1 and corresponding Gaussian–Lorentzian fitting of ClO4 absorbance for c CLP, d CLP-P4, and e CLP-P4-LLTO-3, f DC polarization curve of symmetric Li|CLP-P4-LLTO-3|Li cell, and g schematic showing the Li+ environment in polymer electrolyte and composite electrolyte

Table 2

Compositions and electrochemical properties of L-CLPCSEs

 

LLTO amount (wt %)

Ionic conductivity at 25 °C (S cm−1)

t Li +

LI+ dissociation (%)

CLP

0

3.38 × 10−5

0.15

91.0

CLP-P4

0

2.40 × 10−4

0.15

91.3

CLP-P4-LLTO-1

10

2.48 × 10−4

0.26

CLP-P4-LLTO-2

20

2.82 × 10−4

0.40

CLP-P4-LLTO-3

30

3.31 × 10−4

0.51

99.1

Symmetric Li cells were assembled to mimic the charging and discharging operation in lithium metal batteries, which is also an indicator to evaluate the mechanical strength of solid electrolytes. Figures 4b and S10a show the time-dependent voltage profiles of the cells with CLP, CLP-P4, and CLP-P4-LLTO-3 solid electrolytes cycled for 15 min per cycle with constant current densities of 0.2 and 0.5 mA cm−2 successively at room temperature. At the current density of 0.2 mA cm−2, all cells exhibit stable cycling for 20 days. Note that, the overpotential is much lower for CLP-P4 and CLP-P4-LLTO-3 cells, which is around 44 mV and 42 mV, respectively. For CLP symmetric cell, the overpotential is around 148 mV. The lower and smooth cycling voltage indicates a higher ionic conductivity and stable interfacial resistance between Li and solid electrolyte membranes. To further demonstrate the reinforcement of LLTO nanofibers in the polymer matrix, the cells were cycled at an increased current density of 0.5 mA cm−2 for another 20 days. As seen in Fig. S10b, CLP-P4 cell get short-circuited after 712 h. The sudden voltage drop indicates the penetration of Li due to the mechanical failure of this solid electrolyte. After disassembling the symmetric Li cell, we observed that CLP-P4 solid electrolyte membrane was severely deformed and contained clear cracks without sufficient mechanical rigidity (Fig. S10c). On the other hand, CLP and CLP-P4-LLTO-3 cells show stable plating and stripping process for additional 20 days at the high current density of 0.5 mA cm−2 without short-circuiting, which is attributed to the high modulus of these two electrolytes. Similarly, the overpotential of CLP cell is extremely large, indicating a limited ionic conductivity in battery application. Overall, CLP-P4-LLTO-3 cell exhibits relatively low overpotential of 124 mV and excellent stability, which demonstrates the positive influence of LLTO nanofibers on both ionic conductivity and mechanical property. Figure S11 shows SEM images of Li metal electrodes cycled in symmetric Li cells with CLP, CLP-P4, CLP-P4-LLTO-3 solid electrolytes, respectively. It is clear that the surface of the Li metal after cycling with CLP-P4 solid electrolyte is rougher than those of Li metal foils cycled with CLP and CLP-P4-LLTO-3 solid electrolytes. Therefore, it could be concluded that the relatively higher mechanical strength of CLP-P4-LLTO-3 solid electrolyte helped suppress the Li dendrite growth in long-term cycling.

Based on the Lewis acid–base theory, active Li-ion nanofibers might act as anionic receptors that restrict the delocalized anions and improve free Li+ mobility. The highly-conductive surface of LLTO nanofibers has a stronger affinity with ClO4 that could help disassociate Li+-ClO4 ion pairs and increase the concentration of free Li+ [48], which could be directly revealed by FTIR spectra with the wavenumber in range of 610–650 cm−1. According to the previous studies [49, 50, 51], the peak at ~ 622 cm−1 is assigned to the free ClO 4 - anions and the peak at ~ 633 cm−1 corresponds to the bonded Li+–ClO4 ion pairs. Figure 4c–e show the FTIR spectra and the corresponding Gaussian–Lorentzian fitting results of CLP, CLP-P4, and CLP-P4-LLTO-3 solid electrolytes. The peak area ratio of free anion versus bonded ion pair is 99.1% for the CLP-P4-LLTO-3 composite solid electrolyte, which is significantly higher than those of CLP-P4 (91.3%) and CLP (91.0%). The whole peak shifts to 622 cm−1 for CLP-P4-LLTO-3, but the shoulder remains at 633 cm−1 for CLP and CLP-P4 (Fig. S11). These results clearly demonstrate that LLTO nanofibers help release more free Li+ and improve the active surface area for Lewis acid–base interaction, which ensures great contribution on the Li+ conductivity. Note that, the degree of ion dissociation of CLP is also higher than that (85%) of pristine PEO solid electrolytes reported previously [51] due to the naturally amorphous polymer matrix. Additionally, the degree of free Li+ within the solid electrolyte directly influences the lithium transference number (tLi+). High tLi+ is favorable to facilitate the stabilization of solid electrolytes during cycling, which prevents the formation of the large electric field by immobilizing anions [52, 53]. The tLi+ values of solid electrolytes were measured and calculated by the Bruce’s equation. Figure 4f shows the typical DC polarization curves for Li|CLP-P4-LLTO-3|Li cell, and all related values used to calculate tLi+ in other Li|solid electrolyte|Li symmetric cells are summarized in Table S1. The calculated tLi+ values are summarized in Table 2. The tLi+ value of CLP-P4-LLTO-3 reaches 0.51. CLP-P4-LLTO-1 and CLP-P4-LLTO-2 also show higher tLi+ values (0.26 and 0.40, respectively) than CLP-P4 (0.15). It is clear that adding inorganic nanofibers dramatically increases the lithium transference number, which may be due to the surface interaction of LLTO nanofibers with the polymer matrix and the fully dissociation of Li salt.

To evaluate the electrochemical performance of L-CLPCSEs (Fig. 5), CR2032 coin type cells with LiFePO4 cathode, Li metal anode, and the optimal solid composite electrolyte membrane of CLP-P4-LLTO-3 were assembled in the argon-filled glovebox. For achieving a decreased interfacial resistance between the cathode and solid electrolyte membrane, the polymer precursor solution was cast and photopolymerized directly on the electrodes as illustrated in Fig. 5a. A quartz plate weighing of 25 g was placed on the top during the polymerization to control the thickness of solid electrolyte membrane at 50–80 µm. The assembled cells were galvanostatically charged and discharged at 25 °C between 2.5 and 4.2 V at a current density of 17 mA g−1, i.e., 0.1 C-rate. As shown in Fig. 5b, the all-solid-state LiFePO4|CLP-P4-LLTO-3|Li cell demonstrates high capacity retention of ~ 98% after 100 cycles, indicating that the CLP-P4-LLTO-3 composite solid electrolyte can sustain stable cycling in lithium-ion batteries at room temperature. The slight increase of Coulombic efficiency before 40 cycles might be attributed to the Li deposition and hence improved interfacial contact between solid electrolyte and electrodes. The Li deposition first helped improve the interfacial contact by filling the interfacial gaps, while continuous Li deposition led to diminished interfacial contact and resulted in decreased Coulombic efficiency in latter cycles. Similar trend was also observed in solid-state LiFePO4 batteries using PEO-based solid electrolytes [54, 55, 56, 57]. Overall, the Coulombic efficiency is 97.8% after 100 cycles. A detailed comparison to the aforementioned reported literatures is listed in Table S2 [54, 55, 56, 57, 58, 59, 60, 61]. It is remarkable that our solid-state Li-ion battery delivers high capacity at room temperature, and more importantly, the cell shows superior cycling stability after 100 cycles. This excellent cycling performance is contributed by the perfect harmony of conductive nanofibers and the in situ plasticized cross-linked polymer matrix, tuning the solid electrolytes with high Li+ conductivity and moderate mechanical properties. The incorporation of LLTO nanofibers effectively reduces or eliminates the ClO4 transfer between the electrode interface, which improves the poor cycle-stability encountered in most solid polymer electrolytes [45, 58, 62]. Moreover, the all-solid-state cell shows excellent rate capability. Discharge capacities of 154, 147, 138, 115, and 90 mAh g−1 can be obtained at varied current densities of 0.05, 0.1, 0.2, 0.5, and 1 C, respectively. After applying cycles at higher current densities, the discharge capacity increases back to as high as 153 mAh g−1 when the current density returns back to 0.05 C (Fig. 5c). Figure 5d shows typical charge–discharge curves of all-solid-state cells at different C rates. Stable charge and discharge plateaus of all-solid-state cells are consistent with those of conventional liquid cells, indicating no mechanical failure and excellent stability of the composite solid electrolytes at high C rates. Additionally, Li dendrite formation was evaluated after 100 cycles. The Li foil shows small cracks during the plating/stripping process (Fig. S12a), but the majority of Li electrode surface is flat without obvious dendrite formation (Fig. S12b). Therefore, the introduced all-solid-state L-CLPCSE shows great suppression of lithium dendrites after long cycles due to the presence of 3-D fiber network.
Fig. 5

a Schematic illustration of the preparation of all-solid-state LiFePO4|CLP-P4-LLTO-3|Li cells. b Cycling performance (at 0.1 C), c rate capability (0.05–1 C), and d charge–discharge profiles (at different C rates) of all-solid-state LiFePO4|CLP-P4-LLTO-3|Li cells operated at 25 °C

Conclusions

In conclusion, we have developed a novel Li conductive PEO cross-linked polymer matrix with low glass transition temperature (− 56.5 °C) and high ionic conductivity (2.40 × 10−4 S cm−1) at room temperature. The amorphous structure ensures the further incorporation with LLTO nanofibers without filler agglomeration effect. The addition of high content of LLTO nanofibers provides robust mechanical support to polymer matrix and modifies the local structural environment within the polymer, resulting in a higher lithium transference number and better electrochemical stability. All-solid-state Li|LiFePO4 cells using this composite solid electrolyte provides remarkable specific capacity and stable cycle performance at room temperature. We believe that this design provides a general strategy in dealing with multiple safety challenges of Li-ion batteries. We anticipate that our composite electrolytes will be extended to other Li battery systems and flexible storage devices.

Notes

Acknowledgements

This work was supported by the Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE), under Award Number DE-EE0007806.

Compliance with Ethical Standards

Conflict of interest

The authors declare no conflict of interest.

Supplementary material

42765_2019_6_MOESM1_ESM.docx (2.7 mb)
Supplementary material 1 (DOCX 2762 kb)
42765_2019_6_MOESM2_ESM.rar (2.9 mb)
Supplementary material 2 (RAR 3012 kb)

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Copyright information

© Donghua University, Shanghai, China 2019

Authors and Affiliations

  • Chaoyi Yan
    • 1
  • Pei Zhu
    • 1
  • Hao Jia
    • 1
  • Jiadeng Zhu
    • 1
  • R. Kalai Selvan
    • 1
  • Ya Li
    • 1
  • Xia Dong
    • 1
  • Zhuang Du
    • 1
  • Indunil Angunawela
    • 2
  • Nianqiang Wu
    • 3
  • Mahmut Dirican
    • 1
    Email author
  • Xiangwu Zhang
    • 1
    Email author
  1. 1.Fiber and Polymer Science Program, Department of Textile Engineering, Chemistry and Science, Wilson College of TextilesNorth Carolina State UniversityRaleighUSA
  2. 2.Department of Physics and Organic and Carbon Electronics LabNorth Carolina State UniversityRaleighUSA
  3. 3.Department of Mechanical and Aerospace EngineeringWest Virginia UniversityMorgantownUSA

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