A High Performance All-solid-state Polymer Electrolyte with

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A High Performance All-solid-state Polymer Electrolyte with Controllable Conductivity Pathway Formed by Self-assembly of Reactive Discogen and Immobilized via a Facile Photopolymerization for Lithium-ion Battery Shi Wang, Xu Liu, Ailian Wang, Zhinan Wang, Jie Chen, Qinghui Zeng, Xiaorui Jiang, Heng-Hui Zhou, and Liaoyun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04672 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Applied Materials & Interfaces

A High Performance All-solid-state Polymer Electrolyte with Controllable Conductivity Pathway Formed by Self-assembly of Reactive Discogen and Immobilized via a Facile Photopolymerization for Lithium-ion Battery Shi Wang,a Zeng,a a

Xu Liu,a Ailian Wang,a

Xiaorui Jiang,c

Zhinan Wang,a

Henghui Zhou,*b

Jie Chen,a

Qinghui

Liaoyun Zhang*a

School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049,

China b

c

College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

Pulead Technology Industry Co., Ltd, Beijing 102200, China.

*Emails: [email protected]; [email protected]

1

ACS Paragon Plus Environment

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ABSTRACT: All-solid-state polymer electrolytes (SPEs) have aroused great interests as one of the most promising alternatives for liquid electrolyte in the next generation high-safety and flexible lithium-ion batteries. However, some disadvantages of SPEs such as inefficient ion transmission capacity and poor interface stability result in unsatisfactory cyclic performance of the assembled batteries. Especailly, the solid cell is hard to be run at room temperature. Herein, A novel and flexible discotic liquid crystal (DLC)-based cross-linked solid polymer electrolyte (DLCCSPE) with controlled ion-conducting channels is fabricated via a one-pot photopolymerization of oriented reactive discogen, poly(ethylene glycol) diacrylate (PEGDA) and lithium salt. The experimental results indicate that the macroscopic alignment of self-assembled columns in the DLCCSPEs are successfully obtained under annealing and effectively immobilized via the UV photopolymerization. Because of the existence of unique oriented structure in the electrolytes, the prepared DLCCSPE films exhibit higher ionic conductivities, better comprehensive electrochemical properties than that of the DLCCSPEs without controlled ion-conductive pathways. Especially, the assembled LiFePO4/Li cells with oriented electrolyte show initial discharge capacity of 164 mAh g-1 at 0.1 C, average specific discharge capacity of 143, 135 and 149 mAh g-1 at the C-rates of 0.5, 1 and 0.2 C, respectively. In addition, the solid cell also shows the first discharge capacity of 124 mAh g-1 (0.2 C) at room temperature. The outstanding cell performance of the oriented DLCCSPE should be originated from the macroscopically oriented and self-assembled DLC, which can form ion-conducting channels. Thus, combining the excellent performance of DLCCSPE and the simple one-pot fabricating process of the DLC-based all-solid-state electrolyte, it is believed that the discotic liquid crystal-based electrolyte can be one of the most promising electrolyte materials for the next-generation high-safety solid lithium-ion batteries. KEYWORDS: discotic liquid crystal, orientation, all-solid-state polymer electrolyte, crosslinking, lithium-ion batteries

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1. INTRODUCTION Li metal (anode material) shows a great potential in meeting the ever-growing demands for higher energy density batteries because it has a superior high theoretical capacity (3860 mAh g-1) and an unprecedented low redox potential of -3.04 V vs standard hydrogen electrode.1-2 However, when electrolyte is liquid, the Li-metal anode has long been thought “unsafe” because the formation of dendritic Li during electrodeposition can pierce separator, cause internal short circuit and safety hazards, which prevents the practical use of a Li metal anode in lithium-ion battery (LIB).3 To suppress Li dendrite growth, it is necessary to form a uniform solid electrolyte interphase (SEI) layer on Li metal.4 Up to now, it has been found that many methods have good advances in stabilizing the SEI layer, such as adding of functional solvents5 and electrolyte additives,6 modifying the surface of the lithium metal,7 preparing of novel separators8-9 and solid electrolytes.10-11 Recently, a gel polymer electrolyte (GPE) or solid polymer electrolyte (SPE) has presented effectively dendrite-suppressing ability.12-13 This is because both GPEs and SPEs exhibit stronger adhesion to Li surface and less reactive with Li metal (compared with liquid electrolytes). However, gel electrolyte systems usually show poor mechanical properties,14 leakage of the added massive combustible liquid electrolyte (inevitably sacrifices the safety of cells).15-16 Of particular interest, the advantages of high safety and flexibility, lightweight, good mechanical strength characteristic, the ability to replace toxic/flammable organic liquids and electrode separators make SPEs as one of the most attractive candidates for LIBs.17-19 Up to now, poly(ethylene oxide) (PEO) and its derivatives have been widely explored as the polymer electrolyte matrices for preparing SPEs due to their flexibility and their (the ether chains) strong interactions with lithium-ion.20 However, the semi-crystalline characteristic of linear PEO hampers the migration of lithium ions through the electrolytes.21-23 To suppress the 3



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crystallization of PEO for improving the ionic conductivity, various methods have been supplied, such as bending modification of PEO with inorganic substance,24

copolymerization

modification

of

PEO,25-26

crosslinking

modification of PEO.27-28 Although the crystallinity of PEO can be restrained and the electrochemical performances of PEO-based electrolytes can be improved, these methods are very limited. Obviously, more effective measures are needed to be taken to further perfect the comprehensive electrochemical performances of PEO-based polymer electrolytes. Recently, the construction of self-organized structures of liquid crystals (LCs) has been paid much more attention as conductors of electrons29 and ions30-31. These materials have potential as electrolytes in LIBs owing to the improvement of ionic conductivity by liquid crystalline molecular order.32-33 To design and develop these materials, it needs to control of intermolecular interactions and construction of ion-conducting channels. For example, nano-segregated polymeric electrolyte film based on phase-segregated layers of rigid aromatic (LC) cores and alternating mobile tetra(ethylene oxide)s with high

ionic

conductivity

was

designed

and

prepared

by

in-situ

photopolymerization.30 It was also demonstrated that liquid crystalline block copolymers combined with lithium salts can form ordered electrolyte membranes (induced by magnetic fields) and the ionic conductivity can be improved.34-35

Star-shaped

diblock

liquid-crystalline

copolymer

and

hyperbranched star liquid crystal copolymer were synthesized and used as polymer electrolyte matrices, the corresponding electrolytes show improvement of ionic conductivities due to the formation of ion transport channels after annealing.36-37 In addition, a nanostructured, liquid-electrolyte-filled polymer was developed that has many of the best characters of LC-based electrolytes and gelled polymers.38 Yet, the ionic conductive performances of LC systems have been mainly studied using rod-like LC molecules. Up to now, the development of discotic liquid crystal (DLC)-based polymer electrolytes is very limited in spite of the remarkable orientation/self-assembly ability of DLC. 4


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Triphenylene-based DLC with C3 rotational symmetry (represents one of the numerous discogens) can form the self-assembled nanocolumns.39 Specifically, the self-assembled nanocolumns appear and increase due to the production of strong π-π interactions between aromatic cores upon reducing temperature to anisotropic phase, resulting in the super-structures with positional orders.40-42 The formation of self-assembled nanocolumns of discotic liquid crystal after annealing means the DLC can form columnar orientation. We consider that if functional flexible segments (such as poly(ethylene glycol) diacrylate segments) which can migrate charge carriers are introduced into the DLC by a facile photopolymerization technique, meanwhile, the highly ordered arrangement of discotic liquid crystal by π-π interactions can spontaneously form after annealing, it can be inferred that highly orderly conductive channels may be generated. Moreover, To effectively immobilize such oriented columns-based polymer, it is better to decorate the DLC by active group (such as vinyl, then the DLC monomer can be transformed to discotic vinyl monomer), because the cross-linked polymerization between functional PEGDA and functional DLC can make the ionic channels extremely stable. In this paper, we construct an ion-conductive cross-linked polymer electrolyte film that has ordered ion-conductive pathways. Through orientation control of self-assembled nanosegregated structures of discotic vinyl monomer and incorporation of flexible PEGDA, lithium bis(trifluoromethanesulfonimide) (LiTFSI) and photoinitiator, the ordered ion-conductive polymer electrolyte precursor is designed and prepared. At last, the ion conductive film that has ionic channels is fixed by photopolymerization, which can result in effective suppression of Li dendrite growth and excellent cycle performance and rate capacity for the assembled LiFePO4 (LFP)/Li all-solid-state polymer cells.

2. RESULTS AND DISCUSSION 2.1. Characterization of DLC-CH=CH2 and the DLC-based cross-linked solid electrolytes 5



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In this study, 2,3,6,7,10,11-hexamethoxytriphenylene (I) was firstly prepared via trimerization reaction of veratrole (Figure S1 shows the characterization).

Then

2,3,6,7,10,11-hexahydroxytriphenylene

1

HNMR

(II)

was

synthesized by demethylation of I using HBr and ACOH (See Figure S1 of the 1

HNMR

characterization).

2,3,6,7,10,11-hexakis(but-3-enyloxy)triphenylene

Next (III,

DLC-CH=CH2)

was

obtained via williamson etherization of (II) and 4-bromo-1-butene (Figure S1 and Figure S2 show the

1

HNMR and

13

CNMR, respectively). At last,

DLC-based solid electrolytes were fabricated by one-pot double bond radical photopolymerization of DLC-CH=CH2, LiTFSI and PEGDA ([EO]/[Li] = 20). To construct the ordered ion-conductive pathways, the mixtures are annealed before UV photopolymerization. The fabrication processes of DLC-CH=CH2 and the cross-linked electrolytes with controlled ion-conductive pathways are presented in Scheme 1a and b, respectively. Macroscopically, as presented in Scheme

1b

(right),

the

free-standing

cross-linked

electrolyte

film

DLCCSPE-1000-8-A (Eight types of DLC-based cross-linked solid polymer electrolyte (DLCCSPEs), abbreviated as DLCCSPE-x-y-A/UA, were prepared. The compositions characteristics are presented in Table S1. Here, x denotes the molecular weight (Mn) of PEGDA segments, y is the weight ratio of PEGDA and

DLC-CH=CH2

(mPEGDA/mDLC-CH=CH2 =

y),

and

A/UA

represents

annealed/unannealed sample) can be freely twisted and bended without cracking, suggesting that the prepared electrolyte film has good film-forming ability and can be applied as SPE in lithium batteries. In addition, DLCCSPE-1000-8-UA also exhibits the superior flexibility (Figure S3a and b). Nevertheless, PPEGDA-1000/LiTFSI shows brittleness and breaks after the same degree of curvature (Figure S3d). We used IR spectra to characterize the chemical structure change before and after photopolymerization of the series samples (Figure S4, see the discussion in the supporting information). It indicates that no detectable monomer is 6


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existed in the SPEs and the reaction of the double bond is sufficient after UV irradiation.

Scheme 1. (a) Synthesis route of DLC-CH=CH2. (b) One-pot fabrication procedure of DLC-based cross-linked solid polymer electrolyte with controlled ion-conductive pathways via photopolymerization under UV irradiation. The right image shows the free-standing cross-linked electrolyte film and the film can be freely bended.

2.2. Thermal behaviours and liquid crystallinity Thermal stability of monomer (DLC-CH=CH2), PPEGDA-1000/LiTFSI and DLCCSPE-1000-8-A was evaluated by TGA analysis, as shown in Figure 1a. With the increase of temperature (from 50 to 600 oC at 20 oC min-1 under N2), the 5 wt % weight loss of DLC-CH=CH2, PPEGDA-1000/LiTFSI ([EO]/[Li] = 20) and DLCCSPE is detected at 367, 356 and 361 oC, respectively, suggesting that the monomer and the polymers have good thermal stability. In addition, the weight loss at ~50 oC for PPEGDA-1000/LiTFSI and DLCCSPE-1000-8-A, and the weight loss for PPEGDA-1000/LiTFSI at around ~180 oC are because of the evolution of absorbed water as LiTFSI is very easy to absorb water. 43-44

7



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Figure 1. (a) The TGA analysis of monomer, PPEGDA-1000/LiTFSI and DLCCSPE-1000-8-A. (b) DSC cooling and heating thermal diagrams of DLC-CH=CH2 at the scanning rate of 10 oC min-1. (c) POM image of DLC-CH=CH2 after annealing from 140 to 30 oC, the inset shows the POM image of PPEGDA-1000/LiTFSI. (d) and (e) are POM morphological observations of DLCCSPE-1000-8-UA

and

DLCCSPE-1000-8-A,

respectively.

(f)

XRD

patterns

of

DLCCSPE-1000-8-UA and DLCCSPE-1000-8-A.

The phase transition behaviours of DLC-CH=CH2 were investigated by DSC (Figure 1b). During the heating process, the sample shows a strong crystal melting point at 54 oC with the melt enthalpy (△H) of 37.9 J g-1. The appearance of minor endothermic peaks at 67 (0.71 J g-1) and 146 oC (8.7 J g-1) suggests the formation of hexagonal columnar LC and hexagonal columnar LC-isotropic phase transition, respectively.39, 45 The cooling process shows two 8


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exothermic peaks at 140 (8.7 J g-1) and 5 oC (38.4 J g-1). Compared with the heating process, the low temperature exothermic transition peak (cooling process) presents the supercooling effect, which often happens during a crystallization process. The formation of ordered phase usually can be directly monitored under polarizing optical microscopy (POM). The phase transformation behaviours of DLC-CH=CH2 are shown in Figure S5. At the original 30 oC, the typical mosaic texture is clearly presented. A dendritic structure is observed and it becomes more and more obvious with the decrease of temperature (cooling from 140 to 30 oC). Especially, the disk-shaped texture is seen after annealing DLC-CH=CH2 from 140 to 30 oC (Figure 1c), which is consistent with the DSC analysis (Figure 1b). It is also reported that the mosaic, dendritic and disk-shaped textures are the typical POM images for the columnar liquid crystal phases of discotic liquid crystal molecules.45-46 The self-assembled ability of DLC-CH=CH2 after annealing inspires us to construct ordered channels for more efficient transport of ions. Specifically, DLC is used for orientation while PPEGDA is applied as ions transmission part. The mixtures of DLC-CH=CH2, PEGDA, LiTFSI, photoinitiator and polymerization inhibitor are cooled from 140 to 30 oC. During this process, DLC-CH=CH2 is transformed from isotropic to the columnar LC phase while PEGDA insets into the interspace of the columnar LC. Subsequently, the ordered structure is immobilized by UV photopolymerization. Compared with the POM image of pure PPEGDA-1000/LiTFSI (inset of Figure 1c, the film shows smooth surface and no other feature in a wide temperature range) and DLCCSPE-1000-8-UA (Figure 1d), the annealed and then cross-linked film (DLCCSPE-1000-8-A) shows more ordered morphology and symmetrical distribution of discotic liquid crystal and PPEGDA (Figure 1e), indicating that the orientation of discotic liquid crystal has been fixed via the UV photopolymerization of functional PEGDA and functional columnar DLC. 9



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The structure-sensitive XRD experiments were conducted to further confirm the above conclusion. As shown in Figure 1f, no obvious diffraction peak is seen for the DLCCSPE-1000-8-UA (the XRD pattern of PPEGDA-1000/LiTFSI is also presented in Figure S6 and no distinct diffraction peak is observed). In contrast, for the electrolyte film of DLCCSPE-1000-8-A, two obvious diffraction peaks appear at 2θ = 5.08 (d-spacing = 1.74 nm) and 6.39o (d-spacing = 1.38 nm). The diffraction peaks likely originate from the organization

of

the

self-assembled

DLC-CH=CH2

columns,45

further

demonstrating the orientation of DLC has been achieved and fixed. 2.3. Electrochemical performances Ionic conductivity is an important parameter needs to be evaluated for the application of SPEs in energy storage devices.47 The typical impedance profiles for the PPEGDA-1000/LiTFSI and the DLCCSPEs at 30 oC are shown in Figure S7a and b, respectivley. According to the EISs and Equation (2), the ionic conductivities can be calculated. Figure 2 presents the temperature dependence (from 30 to 80 oC) of ionic conductivities for the DLCCSPEs and the PPEGDA-1000/LiTFSI. Notably, the plots of logσ versus 1000/T show a nonlinear relationship for the DLCCSPEs and the PPEGDA-1000/LiTFSI, which can be explained by Vogel-Tamman-Fulcher (VTF) empirical equation:

σ = A ⁄ exp

( )

(1)

Where A represents a pre-exponential factor related to ionic conductivity, while Ea is the activation energy for coupled ions and local segmental motion, To is a parameter correlated to the Tg and R is the ideal gas constant. Table S2 exhibits the related parameters A, Ea and To, which is obtained by a nonlinear least-squares fitting regression on the experimental data. After constructing of ionic channels and fixing by UV radiation, the ionic conductivities of DLCCSPE-1000-4-A, DLCCSPE-1000-6-A and DLCCSPE-1000-8-A are significantly improved (Figure 2a, b, and c) in the whole test temperature range 10


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(compared with the corresponding unannealed samples). Moreover, the ionic conductivities of DLCCSPE-1000-8-A are much higher than that of DLCCSPE-1000-4-A, DLCCSPE-1000-6-A at a certain temperature (from 30 to 80 oC). For example, the ionic conductivity of DLCCSPE1000-8-A is 5.48×10-5

S

cm-1

at

30

o

C,

while

DLCCSPE-1000-6-A

and

DLCCSPE-1000-4-A have ionic conductivities of 3.77×10-5 and 4.53×10-5 S cm-1 at 30 oC, respectively. However, the pure PEGDA-1000/LiTFSI only shows the ionic conductivity of 7.93×10-6 S cm-1 at 30 oC, suggesting that addition of DLC-CH=CH2 and formation of ion-conductive channels are crucial for the improvement of ionic conductivity (we will further explain the reason in the later part). Generally，it is clearly seen that the annealed DLC-based electrolytes have higher ionic conductivities than that of the corresponding unannealed samples and the pure PEGDA-based electrolyte. The construction of ordered ion-conductive pathways of the DLC-based cross-linked electrolytes probably is the main reason for the improvement of ionic conductivities (as discussed in part 2.2). Furthermore, DLCCSPE-1000-8-A shows higher ionic conductivity than that of DLCCSPE-1000-y-UA (y = 4 and 6, respectively), which is likely because the samples with different contents of DLC have different levels of orderliness. SEM was further used to confirm the above speculates. The SEM characterization of DLCCSPE-1000-4-UA (Figure 3a) presents chaotic and irregular surface morphology. As predicted, DLCCSPE-1000-4-A shows more ordered morphology (Figure 3b). The SEM images of PPEGDA-1000/LiTFSI, DLCCSPE-1000-6-UA/A and DLCCSPE-1000-8-UA/A are also shown in Figure S8, Figure 3c, d and Figure 3e, f, respectively. The surface of PPEGDA/LiTFSI

is

very

smooth

while

the

SEM

images

of

DLCCSPE-1000-y-A (y = 6, 8) have more ordered morphology than that of DLCCSPE-1000-y-UA (y = 6, 8). Specifically, flower-like structure (inset of Figure 3f) is constructed for DSCCSPE-1000-8-A because of the self-assembly of discotic liquid crystal. So it can further verify that the formation of 11



ion-conductive

pathways

improve

the

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ionic

conductivities

of

DLCCSPE-1000-y-A (y = 4, 6 and 8, respectively).

Figure 2. (a) and (b) are the ionic conductivities of the DLCCSPE-1000-4-A/UA and DLCCSPE-1000-6-A/UA in the temperature range from 30 to 80 oC, respectively. (c) Ionic conductivities of the DLCCSPE-1000-8-A/UA in the temperature range from 30 to 80 oC, which

compares

with

PPEGDA-1000/LiTFSI.

(d)

Ionic

conductivities

of

DLCCSPE-1000/600/200-8-A in the temperature range from 30 to 80 oC, which also compares with PPEGDA-1000/LiTFSI.

Moreover,

we

want

to

know

why

the

ionic

conductivities

of

DLCCSPE-1000-y-A (y = 4, 6 or 8) are generally improved with decreasing the contents of DLC-CH=CH2. On the one hand, it is widely known that the segmental mobility becomes more violent with the increase of ethylene oxide segments contents within limits, then the ionic conductivity is improved 10, 48 In addition, the answer can also be found by comparing the SEM images of Figure 3b, d and f. With the decrease of the contents of discotic liquid crystal, the wrinkle obviously increases in the unit area, which is probably beneficial to the conduction of ions (the increase of wrinkle probably increases the 12


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ion-conducting channels). Thus, the ionic conductivity generally decreases with the increase of DLC-CH=CH2 contents. It is also necessary to compare the ionic conductivities of DLCCSPEs which contains PEGDA with different molecular weight. Figure 2d shows the ionic conductivities of DLCCSPE-x-8-A (x = 1000, 600, 200 g mol-1, respectively). The result indicates that the ionic conductivities decrease with the decrease of molecular weight of PEGDA (PPEGDA-1000/LiTFSI even shows slightly higher ionic conductivity than that of DLCCSPE-200-8-A, because too short PPEGDA segments is not beneficial to the transport of ions). SEM images in Figure S9 shows that the annealed samples have more ordered structures than that of the unannealed samples. Moreover, with the increase of the molecular weight of PEGDA, the wrinkle obviously increases in the unit area, which also likely increases the ion-conducting channels. So it can be conclude that the higher molecular weight of PEGDA is beneficial to construct ionic channels within limits, which causes the improvement of ionic conductivity. And more specifically, the enhance of ionic conductivity can be indirectly judged by a lower Tg due to the more relaxing polymer chains.49 Figure S10 shows the DSC curves of the PPEGDA-1000/LiTFSI and the DLCCSPEs, the results of DSC tests are consistent with the ionic conductivity analysis (the discussion are presented in the supporting information). In a word, the conductivity property of DLCCSPEs depends on a synergistic effect between weight ratio of discotic liquid crystal to PEGDA, chain length of PEGDA and whether ionic channels are constructed. DLC-based SPE with ordered structures after annealing (further immobilized by photopolymerization) likely has anisotropic ionic transport behaviour. We choose DLCCSPE-1000-8-A to further study the anisotropic ionic transport behaviours due to its optimal property as discussed above. We measured the ionic conductivities of perpendicular direction (σ⊥，relative to the orientation direction, see Figure S7c). 30-33 The ionic conductivities of the parallel direction 13



(σ//) are much higher than than of the σ⊥ at a given temperature (from 30 to 80 o

C, Figure S7d), suggesting that the ions can move faster in the well-oriented

structures of DLCCSPE-1000-8-A. The interfacial compatibility of DLCCSPE-1000-8-UA/A with electrode was investigated by testing the electrochemical impedance spectroscopy (EIS) of Li/SPEs/Li symmetrical cells after different storage times (Figure 4a and b, respectively). At the real axis, the semicircle of the ESI diagram in the medium frequency range signifies the interfacial resistance (Ri) between the solid polymer electrolyte and Li electrodes.50 It can be seen that both of the Ri have an obvious decrease after 3 days aging (from 33.5 kΩ at initial state to 14.5 kΩ (DLCCSPE-1000-8-UA) and from 7.2 kΩ at initial state to 2.6 kΩ (DLCCSPE-1000-8-A) at the third day, respectively), and then almost reach constant values (~7.5 and ~1.6 kΩ, respectively) with aging times. The increase of Ri can be suppressed when a stable resistive layer is formed on the surface of Li electrodes.51 Thus, the above result confirms that the DLCCSPE shows a satisfactory stability with storage times and good compatibility toward Li electrode. Furthermore, it can be noticed that the annealed SPE shows lower interface resistance than that of the unannealed SPE, which may result of better cycling performance for the solid polymer cells. Electrochemical stability is another necessary property for the application of SPEs in LIBs. Figure 4c and d present the electrochemical windows of DLCCSPE-1000-8-UA and DLCCSPE-1000-8-A, respectively. Both of the values are much higher than 4 V, which is suitable for the application of the electrolytes. Interestingly, the electrochemical window of DLCCSPE-1000-8-A (4.5 V) is much higher than that of DLCCSPE-1000-8-UA (4.2 V). This is probably because the annealing process can improve the regularity of the segments and make the chains more tightly packed.

14


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Figure 3. (a) and (b) are the SEM images of the solid electrolyte film of DLCCSPE-1000-4-UA and DLCCSPE-1000-4-A, respectively. (c) and (d) shows the SEM morphologies characterization of DLCCSPE-1000-6-UA and DLCCSPE-1000-6-A, respectively. (e) and (f) are the SEM images of DLCCSPE-1000-8-UA and DLCCSPE-1000-8-A, respectively.

15



Figure 4. (a) and (b) are the impedance profiles for the Li/DLCCSPE-1000-8-UA/Li and Li/DLCCSPE-1000-8-A/Li cells at 30 oC, respectively. (c) and (d) show the linear sweep voltammograms of DLCCSPE-1000-8-UA and DLCCSPE-1000-8-A at a scan rate of 1 mV s-1 at 30 oC, respectively. (e) and (f) present chronoamperometry of Li/DLCCSPE-1000-8-UA/Li and Li/DLCCSPE-1000-8-A/Li cells at a potential step of 10 mV (60 oC), respectively. The insets show the AC impedance spectra before and after polarization at 60 oC.

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Figure 5. Combustion testing of (a) a commercial separator which is dipped in 1 M LiPF6-EC/DMC liquid electrolyte for ~3 seconds and (b) DLCCSPE-1000-8-A. Comparison photographs of the thermal shrinkage of a commercial separator and the cross-linked DLCCSPE-1000-8-A membrane (c) before and (d) after exposure to 150 °C for 0.5 h. (e) Galvanostatic cycles for Li/DLCCSPE-1000-8-UA/Li and Li/DLCCSPE-1000-8-A/Li symmetrical cells. (f) and (g) are the SEM images of lithium anode polarized in DLCCSPE-1000-8-UA and DLCCSPE-1000-8-A after 1184 h cycling, respectively. (h) Proposed lithium dendrite growth in (h1 ) DLCCSPE-1000-8-UA and (h2) DLCCSPE-1000-8-A. It shows that the lithium dendrite can be effectively suppressed in DLCCSPE-1000-8-A.

Lithium ion transference number (tLi+) is also a crucial parameter for evaluating DLC-based electrolytes. The tLi+ of DLCCSPE-1000-8-UA obtained using a Li/ DLCCSPE-1000-8-UA /Li symmetrical cell is displayed in Figure 4e. It is seen that DLCCSPE-1000-8-UA presents a low tLi+ of 0.16. Interestingly, DLCCSPE-1000-8-A exhibits a higher tLi+ of 0.22 (Figure 4f, the corresponding values are tabulated in Table S3). The higher tLi+ as well as the much higher room temperature ionic conductivity of DLCCSPE-1000-8-A

17



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indicates that the formation of ionic channels is beneficial to improve the transmission efficiency of ions. Compared with traditional carbonate liquid electrolyte, higher safety is one of the most important advantages of SPEs. As shown in Figure 5a, the liquid electrolyte is highly combustible while the DLC-based SPE is hard to be burned (Figure 5b). In addition, DLC-based SPE shows excellent dimensional stability (Figure 5d, a negligible change in its dimensions is observed) after thermal treatment at 150 oC for 30 min. However, the commercial separator suffers a drastic shrinkage when it is treated at the same temperature for 0.5 h (Figure 5d, which is compared with Figure 5c). Thus, the above results indicate that DLC-based SPE is beneficial to enhancing the safety performance of LIBs. Furthermore, we carried out the Li stripping and plating experiments in the symmetric Li/DLCCSPE-1000-8-UA/Li and Li/DLCCSPE-1000-8-A/Li coin cells to assess the stability of the electrolytes to Li metal (Figure 5e). Figure 5e shows

the

voltage

response

of

DLCCSPE-1000-8-UA

and

DLCCSPE-1000-8-A at 60 oC with a current density of 0.01 mA cm-2. Both of the symmetric cells present slight voltage decrease during the initial ~20 h. However, Li/DLCCSPE-1000-8-UA/Li coin cell has a higher and less steady-state overpotential for over 1184 h than that of the coin cell of Li/DLCCSPE-1000-8-A/Li, revealing the more stable lithium stripping and plating cycling with the supressed lithium dendrite growth of the oriented polymer electrolyte (DLCCSPE-1000-8-A). Here, SEM was further used to observe the morphologies of the corresponding lithium tablets after cycling. As can be clearly seen in Figure 5f, a rougher surface of lithium anode is observed after

Li

stripping/plating

experiment

in

the

symmetric

Li/DLCCSPE-1000-8-UA/Li (compared with the morphology of lithium tablet after stripping/plating experiment in the symmetric Li/DLCCSPE-1000-8-A/Li, Figure 5g), which is consistent with the results of the corresponding Li stripping/plating experiments (Figure 5e). In this case, it is due to that in the DLCCSPE-1000-8-UA membrane with unordered DLC, the growth of lithium 18


Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dendrite is not easy to be hindered ((Figure 5h1)); while in the DLCCSPE-1000-8-A membrane with ordered packing of DLC, the rigid benzene acted as a physical barrier that can restrict the free growth of lithium dendrite (Figure 5h2, more tightly packed PPEGDA segments may also help to inhibit dendritic growth). In addition, the much lower overpotential for the DLCCSPE-1000-8-A

(only

0.07

V,

while

overpotential

for

the

DLCCSPE-1000-8-UA is over 0.18 V, the values are obtained from Figure 5e) is mainly because of the higher ionic conductivity and the superior interfacial contact between DLCCSPE-1000-8-A and the Li electrode.52 2.4. Cell performance To demonstrate the usefulness of the DLCCSPEs, DLCCSPE-1000-8-A, which has the best comprehensive electrochemical properties among all samples, was used as the all-solid-state polymer electrolyte to assemble the LFP/Li cells (DLCCSPE-1000-8-UA was also employed as the solid electrolyte as a contrast). Figure 6a presents the charge/discharge property of Li/ DLCCSPE-1000-8-UA/LFP cell at 0.1 C. The initial discharge capacity is 136 mAh g-1, and the discharge capacity increases to 148 mAh g-1 after cycling for 5 cycles, then the cell retains a capacity of over 140 mAh g-1, resulting in coulombic efficiency of ~93%. Compared with Li/DLCCSPE-1000-8-UA/LFP cell, Li/DLCCSPE-1000-8-A/LFP cell exhibits more excellent cycling performance at the same condition. As can be directly observed in Figure 6b, the initial discharge capacity is 164 mAh g-1, which is much higher than that of the Li/DLCCSPE-1000-8-UA/LFP cell. Although the discharge capacity based on Li/DLCCSPE-1000-8-A/LFP coin cell slightly decreased with further increases the number of cycles, it is still much higher than the comparison sample. This result is also clearly illustrated in Figure 6c, the discharge capacity and coulombic efficiency based on Li/DLCCSPE-1000-8-A/LFP cell are higher than that of the cell based on Li/DLCCSPE-1000-8-UA/LFP in the whole cycling range. Moreover, the LFP/Li cell using DLCCSPE-1000-8-A is well 19



Page 20 of 34

run ~50 cycles (an average specific discharge capacity of ~140 mA h g-1 with coulombic efficiency close to 100 %, see Figure S11) after the original high C-rates cycling (the cell using DLCCSPE-1000-8-UA shows fast capacity attenuation at the same condition), indicating the cell has good cycling stability.

Figure 6. (a) Charge-discharge curves of Li/DLCCSPE-1000-8-UA/LFP at 0.1 C. (b) Charge-discharge

profiles of Li/DLCCSPE-1000-8-A/LFP with a certain current density as 0.1 C. (c) Cycle performance of Li/DLCCSPE-1000-8-UA/LFP and Li/DLCCSPE-1000-8-A/LFP cells during galvanostatic cycling at 0.1 C.

(d)

and

(e)

are

the

charge-discharge

curves

of

Li/DLCCSPE-1000-8-UA/LFP

and

Li/DLCCSPE-1000-8-A/LFP cells at different rates (0.5, 1 C and 0.2 C), respectively. (f) Rate capacities of the LFP/Li cells at varied current density. All the experiments were conducted at 60 oC.

The rate charge and discharge capacities of the two cells are investigated at the C-rates from 0.2 to 1.0 C (Figure 6f). For the cell using DLCCSPE-1000-8-A, average specific discharge capacities of 143, 135 and 149 mAh g-1 are obtained 20


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at the C-rates of 0.5, 1 and 0.2 C, respectively. The excellent discharge capacity of Li/DLCCSPE-1000-8-A/LFP cell indicates the good cycling stability and reversibility of the cell. However, much lower capacities are obtained from the cell that using DLCCSPE-1000-8-UA at the same C-rates (136, 95 and 144 mAh g-1, respectively). Figure 6d, e further show the charge and discharge voltage profiles of the two cells at different C-rates. It can be calculated that the cell based on DLCCSPE-1000-8-A presents lower overpotentials (0.19, 0.30 and 0.12 V at 0.5, 1 and 0.2 C, respectively) than that of the LFP/Li cell using DLCCSPE-1000-8-UA as electrolyte (0.23, 0.35 and 0.17 V at 0.5, 1 and 0.2 C, respectively). For all-solid-state polymer electrolytes, one of the most crucial problems need to be solved is their room temperature cycle performance. However, inefficient ion transmission capacity, poor interface stability (and so on) of SPEs make the cells hard to be run at room temperature. Here, The LIB based on DLCCSPE-1000-8-A/UA was performed at RT (28 oC) (Figure 7a). For the cell using DLCCSPE-1000-8-A, the initial discharge capacity is 124 mA h g-1 at 0.2 C and average specific discharge capacity reaches 110 mA h g-1 with coulombic efficiency of ~99%. Nevertheless, much lower discharge capacity and coulombic efficiency are obtained for the cell using DLCCSPE-1000-8-UA (average discharge capacity of ~ 20 mA h g-1 and coulombic efficiency of ~97%, respectively). The corresponding charge/discharge curves of the two cells at the 2nd, 10th and 35th cycles are shown in Figure 7b. It can also be seen that the LFP/Li cell using DLCCSPE-1000-8-A has lower overpotential than that of the DLCCSPE-1000-8-UA-based cell. To confirm the applicability of the SPE, the coin cell based on DLCCSPE-1000-8-A was further used to power a LED. As shown in the inset of Figure 7b, the cell can power a LED at RT. The outstanding cell performance delivered by DLCCSPE-1000-8-A can be attributed to the better comprehensive electrochemical performances (such as higher ionic conductivity, wider electrochemical window, higher t+, lower over 21



Page 22 of 34

potentials). More importantly, the construction and immobilization of controllable conductivity pathways through the orientation of reactive discogen (resulting in more effective transportation of ions) is the main reason for the superior cell performance.

Figure 7. (a) Cycle performance of the Li/DLCCSPE-1000-8-UA/LFP and Li/DLCCSPE-1000-8-A/LFP

cells during galvanostatic cycling at 0.2 C (the tests were conducted at 28 oC). (b) Charge and discharge curves of Li/DLCCSPE-1000-8-UA/LFP and Li/DLCCSPE-1000-8-A/LFP cells at 0.2 C, the image in the inset shows the LIB based on DLCCSPE-1000-8-A powering a LED at 28 oC.

3. CONCLUSIONS In this work, we developed a discotic liquid crystal (DLC)-based cross-linked solid polymer electrolytes (DLCCSPEs) with controlled ion-conducting channels and reinforced safety for use in LIBs. Ionic channels are constructed through orientation of discotic liquid crystal and incorporation of flexible PEGDA, and then crosslinked by DLC and PEGDA via photopolymerization. It is demonstrated that macroscopic orientation of the nanostructures of the DLC in

the

polymeric

films

improves

the

comprehensive

electrochemical

performances (such as enhances the ionic conductivity and electrochemical stability).

Thus, the

resulted all-solid-state

polymer

electrolytes with

nanochannels can transport ions more effectively than that of the electrolyte films without ordered structures. Based on the special design of the DLC-based electrolytes, the assembled LiFePO4/Li cells with oriented electrolyte show excellent cycling performance (initial discharge capacity of 164 mAh g-1 at 0.1 22


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C, average specific discharge capacities of 143, 135 and 149 mAh g-1 at the C-rates of 0.5 C, 1C and 0.2 C, respectively).

4. EXPERIMENTAL SECTION 4.1. Materials Veratrole (99 %, Macklin), anhydrous ferric chloride (98%, Aladdin), sulfuric acid (98-95 %, Beijing Chemical Works), trichloromethane (≥ 99%, Beijing Chemical Works), 4-methoxyphenol (Admas, 99%), glacial acetic acid (≥ 95%, Beijing Chemical Works), potassium carbonate (99%, Aladdin), anhydrous magnesium sulfate (99%, Aladdin), 4-Bromo-1-Butene (98%, Adamas), hydrogen bromide (48%, Aladdin), poly(ethylene glycol) diacrylate (PEGDA, Mn = 200, 600, or 1000 g mol-1, Aladdin), p-methoxyphenol

(adamas) and

1-Hydroxycyclohexyl phenyl ketone (98%, Macklin) were used without further purification. Dichloromethane (≥ 99.5 %), tetrahydrofuran (THF) and N,N-Dimethylformamide (DMF) were purchased from Beijing Chemical Works and dried with CaH2, then, distilled before use. LiTFSI (99 %, Aladdin) was dried under a vacuum at 80 °C for 24 h before use. 4.2. Synthesis of 2,3,6,7,10,11-Hexakis(but-3-enyloxy)triphenylene (DLC-CH=CH2) The triphenylene central core was prepared according to literature.53 Firstly, 2,3,6,7,10,11-hexamethoxytriphenylene (I) was synthesized in the presence of CH3COOH (ACOH) and FeCl3 via trimerization reaction. Then (I) was completely demethylated to 2,3,6,7,10,11-hexahydroxytriphenylene (II) using ACOH and HBr at reflux. DLC-CH=CH2 (III) was further synthesized by williamson etherization of (II) and 4-bromo-1-butene in the presence of DMF and K2CO3 (see the detailed synthesis of the monomer in the supporting information). 4.3. Preparation of DLC-based cross-linked polymer electrolytes

23



The

hybrid

all-solid-state

photopolymerization

of

electrolytes

PEGDA

and

were

Page 24 of 34

fabricated

DLC-CH=CH2

while

through using

1-hydroxycyclohexyl phenyl ketone as the photoinitiator and p-methoxyphenol as inhibitor (100 to 200 ppm)54 and the [EO]/[Li] was kept at 20. The composition of the electrolytes was changed by varying the weight ratio of DLC-CH=CH2 and cross-linker (PEGDA) (see the detailed weight ratio in part 2.1 and Table S1). As an example, the particular preparation procedure of DLCCSPE-8-1000-A can be found in the supporting information. 4.4. Characterization 1

H NMR spectra were recorded on a JNM-ECZ400S (JEOL, 400 MHz)

spectrometer at 25 oC with TMS as the inner reference. We used a Thermo Nicolet AVATAR 360 infrared instrument (using the attenuated total reflectance (ATR) technique) from 4000 to 500 cm-1 to obtain the Fourier transform infrared (FTIR) spectra of samples. Thermal gravimetric analyser (Perkin–Elmer TGA 1 series instrument) was used to test the thermal stability of samples from 50 to 600 oC. The tests were performed under a N2 gas flow rate of 50 ml min-1 while the heating rate was 20 oC min-1. To determine phase-transition temperature of samples, we conducted the differential scanning calorimetry (DSC, Q2000 calorimeter manufactured by TA Instruments) studies of the samples (~5 mg, sealed at aluminium pans) in a N2 atmosphere (50 ml min-1). The heating/cooling rate was kept at 10 oC min-1. To eliminate thermal history, the date from DSC tests was acquired from the second heating curve (unless otherwise stated). The morphology and microstructure of the polymer electrolyte films and Li metal were probed using Hitachi SU8010 Field Emission Scanning Electron Microscope (FE-SEM). The electrolyte films were sputtered with Au for 30 s. To observe texture structures of samples at different temperature, we observed the texture changes using a Leica DM2700P Polarizing optical microscopy (POM) with an LNP95 cooling and heating stage. The detailed information about the POM characterization can be found in our 24


Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


recent report.55 We performed the X-ray diffraction (XRD) measurements by using an Automated Multipurpose X-ray Diffractometer Smartlab 9kW (using a copper target with λ of 1.54 Å). The scanning rate was 2o min-1 from 2 to 15o while the operated voltage and current were 45 kV and 200 mA, respectively. The electrolyte, initiator, and polymerization inhibitor were dissolved in THF and dropped on the silicon wafer and UV-irradiation for 5 min for XRD test (the unannealed film was dried at 50 oC in a vacuum while the annealing temperature for the annealed film was 140 oC, the thickness of the films is about 200 µm). 4.5. Electrochemical measurements A Li/DLC-based electrolyte/stainless steel coin cell was assembled and used to obtain the electrochemical stability window of the as-prepared DLC-based electrolyte

film.

Here,

linear

sweep

voltammetry

(ZahnerEnnium

Electrochemical workstation was used) was employed to measure the electrochemical stability window at 30 oC. The voltage range was 0 to 6 V (vs. Li+/Li) and the scan rate was 1 mV s-1. Electrochemical impedance spectroscopy (EIS) of the DLC-based SPE film was determined by ZahnerEnnium Electrochemical workstation via the two-probe method (frequency range between 1 Hz and 1 MHz and AC amplitude of 5 mV). The SPE was sandwiched between two stainless steel sheets and sealed in a stainless steel mold for the test. Then, the ionic conductivity was calculated according to the following equation (2):

=

(2)

Where L is the thickness of the electrolyte films, R is the bulk resistance of SPEs, and S is the contact area of electrolytes and electrode, respectively. In addition, the EIS of the sample at different temperature (30 to 80 oC) was obtained after the sample was kept at each temperature over 30 min.

25



To obtain the Li+ transference number (t+) of the electrolyte, EIS test was carried out at 60 oC before and after potentiostatic DC polarization with a symmetric cell of Li/ electrolyte/Li. The oscillation voltage is 5 mV and the frequency is in the range of 10 mHz to 1 MHz). t+ can be calculated using Equation (3): =

(∆ ) (∆ )

(3)

Where the initial (Io) and steady (Is) current flowing through the cell were measured by DC polarization with a voltage (∆V = 10 mV). Rlo and Rls are the initial and steady resistances (Ω) of the passivating layers of electrolyte and Li metal electrode, respectively. Furthermore, interfacial compatibility was recorded by monitoring the interfacial resistance of the Li symmetric cell (Li/electrolyte/Li) at 30 oC at various times. The tests were conducted on Zennium Electrochemical workstation in the frequency range from 100 mHz to 1 MHz with an oscillation voltage of 5 mV. Lithium plating/striping experiments for cross-linked membranes in symmetric lithium coin cells were cycled on the LANHE CT2001A battery testing system at a current density of 10 µA cm-2 (60 oC). Both the charge and discharge time are 8 h. To evaluate the electrochemical performance of the SPE, LFP/Li coin cells were assembled using the DLC-based cross-linked polymer electrolyte. Specifically, the process was conducted in an Ar-filled glovebox (MB-Labstar 1200/780, both O2 and H2O contents were lower than 0.5 ppm. The LFP-based cathode consisted of LFP (70 wt%), super P (20 wt%) and PVDF (10 wt%). In addition, the LFP content of the cathode was 1-2 mg cm-2. We used the LANHE CT2001A battery testing system to test the performance of the all-solid-state cells at 60 oC or room temperature (28 oC). The charge-discharge cycles of the cells were carried out between 2.6 to 3.8 V at different C-rates.

ASSOCIATED COTENT 26


Page 26 of 34

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Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org Synthesis of DLC-CH=CH2, preparation of DLCCSPE, characterization of materials by 1H NMR (Figure S1),

13

C NMR (Figure S2), FT-IR (Figure

S4), POM (Figure S5), XRD (Figure S6), EISs and anisotropic ionic conductivity (Figure S7), SEM (Figure S8-S9), DSC (Figure S10), long cycle performance and charge/discharge curve (Figure S11). Pictures of electrolyte films (Figure S2), the composition, glass transition temperature and ionic conductivities of DLCCSPEs (Table S1), VTF fitting results (Table S2) and related parameters of lithium ion transference numbers (Table S3)

AUTHOR INFORMATION Corresponding Auther *E-mail: [email protected] *E-mail: [email protected];

ORCID Liaoyun Zhang: 0000-0002-3682-0516

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors express thanks for the support of the National Natural Science Foundation of China (No.51073170) and Innovation Program of CAS Combination of Molecular Science and Education.

REFERENCES

27



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Hybrid

Solid

Electrolytes

Formed

via

Thiol-acrylate

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Discotic liquid crystal-based all-solid-state polymer electrolyte with ordered ion-conducting channels is designed and synthesized for LIBs, which shows excellent performances. 35x21mm (600 x 600 DPI)


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A High Performance All-solid-state Polymer Electrolyte with

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