inorganic nanocomposites with star-shaped structure for high performance lithium ion battery

Author’s Accepted Manuscript Solid polymer electrolyte membranes based on organic/inorganic nanocomposites with star-shaped structure for high performance lithium ion battery Jinfang Zhang, Cheng Ma, Jiatu Liu, Libao Chen, Anqiang Pan, Weifeng Wei www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(16)30107-7 http://dx.doi.org/10.1016/j.memsci.2016.02.049 MEMSCI14315

To appear in: Journal of Membrane Science Received date: 23 October 2015 Revised date: 13 February 2016 Accepted date: 21 February 2016 Cite this article as: Jinfang Zhang, Cheng Ma, Jiatu Liu, Libao Chen, Anqiang Pan and Weifeng Wei, Solid polymer electrolyte membranes based on organic/inorganic nanocomposites with star-shaped structure for high performance lithium ion battery, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2016.02.049 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Solid polymer electrolyte membranes based on organic/inorganic nanocomposites with star-shaped structure for high performance lithium ion battery

Jinfang Zhang,a Cheng Ma,a Jiatu Liu,a Libao Chen, a Anqiang Pan b and Weifeng Wei a

a

*

State Key Laboratory of Powder Metallurgy, Central South University, Changsha,

Hunan, 410083, P. R. China. b

School of Materials Science and Engineering, Central South University, Changsha,

Hunan, 410083, P. R. China.

* Corresponding author: Fax/Tel: +86 73188877876; E-mail: [email protected]

Abstract Solid polymer electrolyte membranes (SPEMs) with star-shaped structure, consisting of octavinyl octasilsesquioxane (OV-POSS) with nanoscale organic/inorganic hybrid structure and poly(ethylene glycol) methyl ether methacrylate (PEGMEM), have been prepared by one-step free radical polymerization. OV-POSS with eight functional corner groups was used to produce star-shaped nanocomposites and PEGMEM was used as the polymer matrix to dissolve lithium ion. For comparison, SPEMs based on the corresponding linear copolymers, containing POSS moieties and PEGMEM segments, were also prepared. The SPEM with star-shaped structure containing 5.1 1

mol% POSS exhibits the highest ionic conductivity of 1.13×10-4 S cm-1 and Li+ transference number of 0.35 at 25 oC, which are about two times of that of SPEM with linear structure containing the same POSS content. Moreover, the Li/LiFePO4 cell assembled with such star-shaped SPEM delivers the highest discharge capacity of 137.1 mAh g-1 under a current density of 0.5 C at 25 oC. At an evaluated temperature of 80 oC, this cell exhibits an initial discharge capacity of 163.8 mAh g-1 at 0.5 C, and even at high discharging C-rates of 5 and 10 C, the discharge capacity of 75.6 and 45.7 mAh g-1 can still be obtained, respectively. Keywords Lithium-ion batteries; Solid polymer electrolyte membrane; Poly (ethylene oxide); POSS; Star-shaped structure. Introduction Lithium ion batteries (LIBs) have attracted intensive attention in recent years due to the higher energy and power density and longer cycle life over conventional battery technologies [1, 2]. The electrolyte, as an important component of LIBs, plays a critical role in aspects of electrochemical performance and safety. However, traditional organic electrolytes for commercial LIBs could cause safety concerns due to the leakage and explosive nature of volatile organic electrolytes, especially at high temperature (>60 oC) [3]. Recently, solid polymer electrolytes (SPEs) are considered as the most promising candidates for high-temperature LIBs by virtue of enhanced safety and good thermal and dimensional stability [4]. Poly(ethylene oxide) (PEO) is a well-known polymer matrix for SPEs since the highly flexible EO segments in 2

amorphous phase could provide channels for lithium ion transport. However, the pristine PEO electrolyte could hardly be applied for practical LIBs, due to its low ionic conductivity of 10-7-10-6 S cm-1 that stems from the crystallization propensity at temperatures below 65 oC [5, 6]. Many efforts have been dedicated to suppress the crystallization of PEO via developing block/graft copolymer electrolytes based on PEO matrix [7-9]. Although the conductivity has been improved, most of these copolymers are usually in a waxy state and cannot be used for SPEs directly due to the poor mechanical strength [10]. Therefore, it is desirable for SPEs based on PEO matrix to improve ionic conductivity as well as dimensional stability [11]. Many polymer electrolytes with special structure have been synthesized to achieve this balance, such as cross-linking/interpenetrating-network/blends copolymer electrolytes [12-15] and inorganic nanocomposite electrolytes [16-18]. Polyhedral oligomeric silsesquioxane (POSS), containing an inner inorganic silicon/oxygen core (SiO1.5)n (n = 4, 6, 8, 10 and 12) and external organic groups at each corner, has attracted intensive attention as an effective nanofiller to enhance mechanical strength of polymer matrix owing to its well-defined nanoscale organic/inorganic hybrid structure [19, 20]. Lee’s group developed a variety of free-standing copolymers by incorporating or copolymerizing POSS monomer and PEG groups, while the glass temperatures (Tgs) of the PEG domains in these copolymers were found to be close to that of the homopolymer containing only PEG segments [21-24]. Despite the rigidity of the POSS group, it provides additional free volume for the motion of EO segments due to the steric effect of POSS groups, 3

resulting in a high mobility of EO chains [22]. Therefore, it is possible that the POSS moieties in SPEs could enhance dimensional stability as well as ionic conductivity. Although there are many reports on the synthesis and properties of copolymers for SPEs containing POSS segments, most efforts were focused on their linear copolymers, such as the block copolymers and the random copolymers [25-27]. Recently, star-shaped copolymers containing POSS segments, with its unique multiple-chain-ended structure, have attracted considerable interest due to their larger segments mobility and more free space than the corresponding linear polymers [20, 22-24, 28-30]. However, previous studies on POSS-containing SPEs were limited to the POSS macromonomers with only a single functional group, and the SPEs based on POSS macromonomers with multifunctional corner groups were little studied. Besides, all of these star-shaped copolymers reported were synthesized by atom transfer radical polymerization (ATRP)/reversible addition-fragmentation chain transfer (RAFT) polymerization. Both of these two methods require a polyfunctional initiator specially prepared and certain amounts of transition-metal catalyst to produce star-shaped structure [22, 23]. However, the purification of the final products is a tricky matter since it is difficult to remove the residual of transition-metal catalysts completely. Also, the reaction process requires several steps and is rather complex [31]. Different from the reported star-shaped SPEs synthesized via complicated procedures using POSS macromonomers with only a single functional corner group, we developed a series of organic/inorganic hybrid star-shaped copolymers as matrices 4

for solid polymer electrolyte membranes (SPEMs) by one-step simple free radical polymerization using POSS macromonomer with multifunctional corner groups. Compared with the ATRP/RAFT polymerization, simple free radical polymerization includes an easily acquired initiator of 2,2-azobisisobutyronitrile (AIBN) but not the polyfunctional

macroinitiators

and

transition-metal

catalyst.

Octavinyl

octasilsesquioxane (OV-POSS) with eight functional corner groups was used to produce star-shaped nanocomposites while poly(ethylene glycol) methyl ether methacrylate (PEGMEM) was used as the polymer matrix to dissolve lithium ion. Linear copolymers with POSS moiety and PEGMEM segments were also synthesized for comparison. The effects of the polymer microstructure and the POSS content on the properties of SPEMs were systematically investigated, such as the thermal property, ionic conductivity, lithium ion transfer number, electrochemical stability and charge/discharge performance. The results show that star-shaped copolymers were favourable candidates for the polymer matrices of SPEMs owing to the presence of many star-shaped ends that facilitate lithium-ion conduction. 2. Experimental section 2.1. Materials. Poly(ethylene glycol) methyl ether methacrylate (PEGMEM, Mn = 936 g mol-1), octavinyl octasilsesquioxane (OV-POSS, Mn = 633 g mol-1) and lithium bis(trifluoromethane sulfonamide) (LiTFSI) were purchased from Sigma-Aldrich. Methacryl-cyclohexyl-POSS (MA-POSS) was purchased from Hybrid Plastics and stored in a desiccator. Tetrahydrofuran (THF), 2,2-azobisisobutyronitrile (AIBN), 5

ethyl acetate and petroleum ether, obtained from Sinopharm Chemical Reagent Co., Ltd., were used as received. 2.2. Synthesis of star-shaped PEGMEM-co-OV-POSS copolymer (SCP) We select the macromonomer of OV-POSS with eight functional corner groups for the synthesis of star-shaped copolymers. Star-shaped poly(ethylene glycol) methyl ether

methacrylate-co-octavinyl

octasilsesquioxane

(PEGMEM-co-OV-POSS)

copolymers were synthesized via free radical polymerization, as shown in Scheme 1a. The star-shaped copolymer is abbreviated to SCPX, where X corresponds to the mole percentage of POSS in resulting copolymer. SCP5.1, containing 5.1 mol% of POSS in resulting copolymer, was synthesized via free radical polymerization as follows. PEGMEM (2.5704 g, 2.70 mmol) and the initiator of AIBN (0.0135 g, 0.082 mmol) were dissolved in 10 mL of ethyl acetate and used as A solution. OV-POSS (0.1353 g, 0.214 mmol) was dissolved in 6 mL of THF and used as B solution. A and B solution were mixed in a 50 mL three-neck flask equipped with a condenser. This solution was then heated to 70 oC in an oil bath under constant stirring and N2 atmosphere for 10 h. The mixture after reaction was dissolved in THF and precipitated in petroleum ether for three times to remove the unreacted monomers. The as-precipitated copolymer was dried at 70 oC under vacuum condition for 24 h and a transparent solid product was obtained with 56.3% yield. The POSS content in as-precipitated copolymer was determined by 1H NMR was 5.1 mol%. Other SCPs with different POSS mole contents were also prepared using the same synthesis procedure as shown in Table 1. 2.3. Synthesis of linear PEGMEM-co-MA-POSS copolymer (LCP) 6

We select the macromonomer of MA-POSS with only a single functional corner group for the synthesis of linear copolymers. Linear poly(ethylene glycol) methyl ether

methacrylate-co-methacrylisobutyl-POSS

(PEGMEM-co-MA-POSS)

copolymer is abbreviated to LCPX, where X corresponds to the mole percentage of POSS in resulting copolymer. LCPs were synthesized via free radical polymerization, as shown in Scheme 1b. LCP5.1, containing 5.1 mol% of POSS in resulting copolymer, was synthesized via free radical polymerization as follows. PEGMEM (2.5704 g, 2.70 mmol) and the initiator of AIBN (0.0140 g, 0.085 mmol) were dissolved in 10 mL of ethyl acetate and used as A solution. MA-POSS (0.2235 g, 0.236 mmol) was dissolved in 6 mL of THF and used as B solution. A and B solution were mixed in a 50 mL three-neck flask equipped with a condenser. This solution was then heated to 70 oC in an oil bath under constant stirring and N2 atmosphere for 10 h. The mixture after reaction was dissolved in THF and precipitated in petroleum ether for three times to remove the unreacted monomers. The as-precipitated copolymer was dried at 70 oC under vacuum condition for 24 h and a transparent solid product was obtained with 62.5% yield. The POSS content in as-precipitated copolymer was determined by 1H NMR was 5.1 mol%. Other LCPs with different POSS mole contents were also prepared using the same synthesis procedure as shown in Table 1.

7

Scheme 1 Synthesis of (a) star-shaped (SCPs) and (b) linear copolymers (LCPs) via simple free radical polymerization. 2.4. Preparation of SPEMs The copolymers and LiTFSI, with a desired [EO]/[Li] mole ratio, were dissolved in a THF solution and then casted onto a Teflon plate. Lithium salt concentrations are determined as the ratios of the number of lithium cations (Li+) to that of ethylene oxide (EO) repeating units in the copolymers ([EO]/[Li]). After dried in a vacuum oven at 80 oC for 24 h, the copolymer films could be peeled off from the Teflon plate and then stored in a glove box for measuring the ionic conductivity or other electrochemical testings. The thickness of the films measured using a micrometer was around 200 μm. 2.5. Characterization The structure of star-shaped and linear copolymers was characterized by 1H NMR 8

and FTIR spectroscopy. 1H NMR spectra were taken on an Avance III 400 MHz Digital NMR spectrometer at 25 ºC. Tetramethylsilane (TMS) was used as the internal standard and DCCl3 was the solvent. FTIR spectra were taken on a Nicolet 6700 spectrometer over the range of 4000-500 cm-1 at 25 ºC. Gel permeation chromatography (GPC) was taken on a Waters 515–2410 instrument by using tetrahydrofuran (THF) as elution solvent at 25 ºC. The molecular weight calibration was performed using polystyrene (PS) standards. Differential scanning calorimetry (DSC) was performed on a TA-SDTQ600 with a heating rate of 10 ºC min-1 from -100~90 ºC under N2 flow. Thermogravimetric analysis (TG) was carried out on a TA-SDTQ600 with a heating rate of 10 ºC min-1 from 30 to 600 ºC under N2 atmosphere. The microstructure was investigated using a Nova Nano SEM230 field emission scanning electron microscope (FE-SEM) and a field emission JEOL JEM-2100F transmission electron microscope (TEM). The ionic conductivity of star-shaped and linear SPEMs was analyzed by using a PARSTAT 4000 system (Ametek Advanced Measurement Technology INC) in the frequency range of 0.1–100 kHz with a small perturbation voltage of 10 mV from 25 to 80 ºC via a two-probe method. The samples were sandwiched between two stainless steel discs (d = 16 mm) to form a symmetric stainless steel/SPEM/stainless steel cell. The lithium-ion transference number (t+) was taken on a Li/SPEM/Li symmetric coin cell by using the dc-polarization method. The electrochemical stability of the solid electrolyte was tested on a PARSTAT 4000 system with a stainless steel/SPEM/Li coin cell from 2 to 8 V (V versus Li/Li+) at 25 and 80 ºC. To 9

investigate the electrochemical performance of star-shaped and linear SPEMs in LIBs, coin cells containing a lithium anode, a LiFePO4 cathode (the mixture of 80 wt% LiFePO4, 10 wt% carbon black and 10 wt% PVDF was coated on an aluminum foil) and SPEMs, were assembled in an Ar-filled glove box. The charge/discharge testing was performed in a battery test system (LANHE CT2001A) between 2.5 and 4.0 V at different temperatures and C-rates. Results and discussion 3.1 Structural characterization of SCPs and LCPs As shown in Fig. 1a, the absorption peak at 1720 cm-1 is assigned to the C=O bonds of PEGMEM, while the characteristic absorption at 1112 cm-1 comes from the C-O-C bonds of PEGMEM and the Si-O-Si bonds of OV-POSS. To determine the POSS content in resulting copolymers, PEGMEM/OV-POSS mixtures with different POSS contents were dissolved in THF for FTIR testing [32]. Using the strong absorption intensity of C=O bonds in the PEGMEM segments at 1720 cm-1 as an internal standard, we plotted the ratios of integral areas of absorption bands (A1112/A1720) against the POSS mole contents in the mixtures, and a calibration curve was obtained, as shown in Fig. 1b. The POSS mole percentage held a linear relationship with the ratio of A1112/A1720 and could be expressed as follows (Equation 1): POSS mol% = 1.21907(A1112/A1720) – 4.97218

(1)

By detecting A1112/A1720 values of resulting SCPs, POSS mole contents of resulting SCPs can be calculated by Equation 1 and the results were shown in Table 1. 10

Fig. 1 (a) FTIR spectra of the macromonomers of PEGMEM and OV-POSS and the copolymer of SCP5.1; (b) calibration curve for determining the POSS contents in the as-synthesized SCPs. The structure of star-shaped (SCP5.1) and linear (LCP5.1) copolymers was characterized by 1H NMR analysis, as shown in Fig. 2. The resonance peaks of a at 1.0-1.25 ppm and b at 2.0-2.5 ppm could be assigned to the proton of CH2 and CH3 from the PEGMEM backbone, respectively. The resonance signals of c at 3.29–4.37 ppm in Fig. 2a and 3.44-4.37 ppm in Fig. 2b are ascribed to the protons of CH2–CH2–O units from PEGMEM segments [24]. The resonance peaks of d at 3.21 ppm in Fig. 2a and 3.40 ppm in Fig. 2b are assigned to the proton of terminal CH3 of PEGMEM units. The protons from isobutyl groups of MA-POSS could be observed at 1.95 ppm (g) and 0.98 ppm (h), and the peaks around 0.6 ppm (e and f) are attributed to the protons of CH2 from MA-POSS [21]. The multiple peaks observed at 5.88-6.24 ppm (i and k) are the unreacted vinyl protons of OV-POSS [32] and C=C bond of unreacted vinyls in SCP5.1 also can be observed in FTIR spectra of Fig. 1a. It's worth noting that eight vinyls of OV-POSS do not fully participate in the reaction.

11

Fig. 2 1H NMR spectrum of (a) SCP5.1 and (b) LCP5.1. For SCPs, the resonance peak b at 2.5 ppm is only assigned to the CH3 protons of PEGMEM while the protons of unreacted vinyl groups at 5.88-6.24 ppm originate from the OV-POSS units. Therefore, the number of unreacted vinyl groups (x) of POSS segments was estimated on the basis of the 1H NMR analysis and the POSS contents of resulting SCPs were obtained from calibration curve (Fig. 1b) using the following Equation: POSS mol%= (A5.88-6.24/3x)/[ (A5.88-6.24/3x)+(A2.5/3)]

(2)

In Equation 2, A5.88-6.24 and A2.5 represent the integrated areas of peaks at 5.88-6.24 and 2.5 ppm, respectively. On the basis of Equation 2 and the detected peak areas, the x values are calculated and listed in Table 1. The x values are in the range of 1.9-3.9 and thus averagely 4-6 vinyl groups from OV-POSS participated in the copolymerization process, demonstrating that the resulting SCPs are not linear 12

copolymers. In addition, it is noted that the as-prepared SCPs could be easily dissolved in most of the common solvents, such as THF, dimethyl formamide (DMF), CHCl3 and so on, indicating that the resulting SCPs are the star-shaped structure rather than the network structure since the copolymers with the network structure are barely dissolvable in any solvents. The POSS mole contents of resulting LCPs can be calculated based on the 1H NMR spectrum [24], as shown in the following Equation: POSS mol% = (Ae+f /16)/[(Ad / 3) + (Ae+f /16)]

(3)

In Equation 3, Ae+f represents the integrated areas of the peaks e and f at 0.6 ppm corresponding to the protons from CH2 of MA-POSS moieties while Ad represents the integrated area of the peak d at 3.40 ppm corresponding to the protons from CH2–CH2–O units of PEGMEM segments, respectively. GPC traces of the SCPs and LCPs are shown in Fig. S1 and the GPC results are listed in Table 1. The weight-average molecular weight (Mw) of the SCPs and LCPs is in the range of 13.0 ~ 15.8 ×103 g mol-1 and the polydispersity (PDI) is ranging from 1.21 to 1.45. Table 1 Synthesis results of SCPs and LCPs with different POSS contents. POSS(wt%) Samples

Feed

POSS(mol%) Feed

Resulting

Yied

Mwc

polymer

(wt%)

3 -1 (×10 g mol )

PDIc

xd

P(PEGMEM)

0

0

0

78.9

13.6

1.33

/

SCP3.2

2.5

3.7

3.2a

59.5

13.0

1.31

1.9

13

SCP5.1

5

7.3

5.1a

56.3

14.5

1.30

3.9

LCP5.1

8.0

7.9

5.1b

62.5

15.8

1.21

/

SCP7.6

10

14.3

7.6a

54.7

13.5

1.45

3.9

SCP11.2

12.5

17.6

11.2a

51.6

13.8

1.40

3.7

LCP11.2

18.2

18.0

11.2b

56.3

13.8

1.33

/

a

Obtained based on Equation 1.

b

Obtained based on Equation 3.

c

Determined by GPC results using the PS standard curve.

d

x is the number of unreacted vinyl groups of POSS segments, obtained based on

Equation 2. 3.2 Thermal properties Fig. 3a shows the typical DSC curves of P(PEGMEM), SCPs with various POSS mole contents and LCP5.1 in the temperature range of -100 to 90 oC. Glass transition temperature (Tg) and melting temperature (Tm) of PEGMEM domains in SCPs and LCP5.1 are summarized in Table 2. Tm of PEGMEM domains in the homopolymer of P(PEGMEM) is 27.56 oC, indicating that a semi-crystalline state presents in the P(PEGMEM) homopolymer below the melting temperature. However, the melting peaks of PEGMEM domains in SCPs and LCP5.1 were not observed since the bulky POSS groups could inhibit the crystallization of EO units from PEGMEM segments effectively [33]. Furthermore, as shown in Table 2, Tgs of PEGMEM domains in SCPs and LCP5.1 are generally lower than that in the homopolymer of P(PEGMEM). Although the POSS groups act as a barrier to restrict the movement of 14

the EO segments, they also provide additional free space for EO segments due to the steric effect of POSS groups, resulting in the decrease of Tgs in SCPs and LCP5.1. Meanwhile, the Tg of PEGMEM domains in SCP5.1 is slightly lower than that of LCP5.1 and it could be ascribed to larger free volume of star-shaped copolymers for EO segments than that of the corresponding linear copolymers with the same POSS content, as mentioned in other reports [34-36].

Fig. 3b compares the typical TG curves of P(PEGMEM), SCPs with different POSS contents and LCP5.1 in the temperature range of 30 to 600 oC. The degradation temperatures of 5% weight loss (Td,5%) and char yields at 600 oC are summarized in Table 2. The P(PEGMEM) copolymer has a Td,5% at 273 ºC and has almost no residue (0.7%) when the temperature reaches 600 ºC. However, for SCPs with different POSS contents, the Td,5% and char yield increase with increasing POSS contents, suggesting that the addition of POSS could enhance the thermal stability of the copolymers [32]. For example, SCP3.2 has Td,5% at 324 ºC and a char yield of 3.11% at 600 ºC, while SCP11.2 has Td,5% at 369 ºC and a char yield of 10.31% at 600 ºC. Furthermore, Td,5% for SCP5.1 at 357 ºC is higher than 339 ºC for LCP5.1, demonstrating that polymer matrix with star-shaped structure possesses enhanced thermal stability compared with its linear counterpart.

15

Fig. 3 (a) DSC heating traces and (b) TG thermograms of the homopolymer of P(PEGMEM), SCPs with various POSS contents and LCP5.1. Table 2 Thermal properties of the P(PEGMEM) homopolymer, SCPs with various POSS contents and LCP5.1. Tga

Tma

Td,5%b

Char yieldc

(ºC)

(ºC)

(ºC)

(%)

P(PEGMEM)

-52.01

27.56

273

0.79

SCP3.2

-53.19

/

324

3.11

SCP5.1

-54.72

/

357

4.84

SCP7.6

-53.56

/

361

7.16

SCP11.2

-53.15

/

369

10.31

LCP5.1

-53.17

/

339

1.95

Samples

a

Tg and Tm of PEGMEM domains in SCPs and LCPs.

b

The decomposition temperature (Td,5%) is determined as 5 wt% loss.

c

The char yield at 600 ºC.

3.3 Morphology of SPEMs Transmission electron microscopy (TEM) was employed to characterize the dispersity of the POSS units in SCPs and LCPs, as shown in Fig. 4a-4c. Some 16

irregular dark domains could be clearly observed in SCP5.1 (Fig. 4a) and LCP5.1 (Fig. 4c) without additional staining processes since the silicon in POSS has high electron density than the PEGMEM units and the dark region is corresponding to the POSS-rich domain [21, 24, 37]. Fig 4b shows the corresponding Energy Dispersive Spectroscopy (EDS) compositional mapping of Si in SCP5.1, indicating that the POSS units uniformly dispersed in SCPs. SPEMs containing the SCPs/LCPs and LiTFSI with a desired [EO]/[Li] mole ratio, were prepared by the solution casting method and used for electrochemical tests. As shown in Fig. 4d, a transparent and free-standing film was prepared from SCP5.1 and LiTFSI with [EO]/[Li] of 1/15. Fig. S2 shows the SEM results of SCP5.1 solid electrolyte membrane, and the element of C mainly from PEGMEM segments, F derived from LiTFSI and Si derived from OV-POSS evenly dispersed in SPEM, indicating that the SPEM was prepared successfully. The as-prepared SPEMs with various POSS contents (SCP3.2, SCP5.1, SCP7.6 and SCP 11.2) and LCP5.1 exhibit smooth surface and the thicknesses of the membranes are around 200 μm (Fig. S3).

17

Fig. 4 (a) TEM micrograph of SCP5.1; (b) TEM-EDS of Si; (c) TEM micrograph of LCP5.1 and (d) a photo of SCP5.1 membrane with [EO]/[Li] of 15/1. 3.4 Ionic Conductivity Fig. 5a shows the ionic conductivities of SCP5.1, SCP11.2 and LCP5.1 with [EO]/[Li] mole ratios of 25/1, 20/1, 15/1, 10/1 and 8/1 at 25 ºC. For all the samples, the ionic conductivity first increases with increasing LiTFSI content, up to a maximum corresponding to a [EO]/[Li] mole ratio of 15/1, and then decreases with a further increase of LiTFSI content. The ionic conductivity of SPEMs is affected by both the number of charge carriers and the mobility of the EO chains [38, 39]. When the LiTFSI content is low, the ionic conductivity depends mostly on the number of 18

charge carriers, so the ionic conductivity increases with increasing LiTFSI content since the addition of LiTFSI produces more charge carriers. However, when the LiTFSI content exceeds a certain threshold, the number of effective charge carriers decreases as ion pairs or ion aggregates increase with increasing LiTFSI content. Besides, the mobility of EO chains gradually decreases since the increasing of intermolecular interactions between the EO chains and the lithium cations[40]. Any further increase in the amount of LiTFSI will therefore reduce the ionic conductivity, since the number of effective charge carriers and the mobility of polymer segments decreases. The optimum salt concentration corresponding to the highest conductivities for SCP5.1, SCP11.2 and LCP5.1 was observed at [EO]/[Li] of 15/1.

Fig. 5 (a) Ionic conductivities of SCP5.1, SCP11.2 and LCP5.1 with various LiTFSI concentrations at 25 ºC; (b) ionic conductivities of P(PEGMEM) homopolymer, SCPs 19

with various POSS contents and LCP5.1 with the same lithium salt concentration ([EO]/[Li] = 15/1) ranging from 25 to 80 ºC; (c) VTF fitting results of SCP5.1 and LCP5.1 with [EO]/[Li]of 15/1; (d) schematic of lithium ion conduction mechanism designed for the SPEMs based on SCP matrix. Fig. 5b shows the ionic conductivities of P(PEGMEM) homopolymer, SCPs with various POSS contents (SCP3.2, SCP5.1, SCP7.6, SCP11.2) and LCP5.1 with the same lithium salt concentration ([EO]/[Li] = 15/1) ranging from 25 to 80 oC. For a series of SCPs, it was revealed that the ionic conductivity increases with the increase of the POSS content, up to a maximum over the entire temperature ranges when the POSS content is 5.1 mol% (SCP5.1), and then decreases with the further increase of POSS content. This suggests that the introduction of POSS macromonomer plays an important role in improving the conductivity by increasing free volume for the mobility of EO chains, as estimated by the decrease of Tg of PEGMEM domains in SCPs. However, with the further increase of the POSS content in SCPs, the ionic conductivity decreases since the bulky POSS groups cannot provide channels for lithium ion transport and act as a barrier to restrain the mobility of EO chains. Meanwhile, the SCP5.1 solid electrolyte exhibits higher ionic conductivity over the entire temperature ranges than that of the LCP5.1 solid electrolyte. The ionic conductivity of SCP5.1 (1.13×10-4 S cm-1) is about two times of that of LCP5.1 (5.63×10-5 S cm-1) at 25 oC. This result could be ascribed to the larger free volume of star-shaped copolymers for the mobility of EO segments providing lithium ion transport than that of the corresponding linear copolymers when they have the same 20

POSS content, as estimated by the smaller Tg value of PEGMEM domains in SCP5.1. It is reasonable to conclude that additional free volume among POSS groups offers the necessary space for the motion of EO chains and the star-shaped arms of EO segments on the surface of the POSS particles provide continuous express pathways for the ion transport, resulting in higher ionic conductivity, as schematically illustrated in Fig. 5d. The thermal dependence of conductivity, measured at different temperatures, obeys the Vogel–Tamman–Fulcher (VTF) equation (Equation 4), which was used to analyze the conductive behavior of amorphous PEO based electrolyte: (4) where

is the pre-exponential factor; T is the operating temperature;

the activation energy for ion transport; K-1) and

stands for

is Boltzmann constant (8.314 10-3 kJ mol-1

is the equilibrium glass-transition temperature of the copolymer, which is

about 50 K below the real

[41]. These fitting parameters for the P(PEGMEM)

homopolymer, SCPs with various POSS contents and LCP5.1 ([EO]/[Li] = 15/1), derived from VTF equation are listed in Table 3. A reflects the number of charge carriers and the highest value of A (2.87 S cm-1 K-1/2) and the highest conductivity are observed for SCP5.1 solid electrolyte, indicating that the value of A is proportional to the ion conductivity. In addition, the lowest value of Ea for SCP5.1 solid electrolyte was also observed. A comparison between the experimental datas and the VTF fitting results for SCP5.1 and LCP5.1 solid electrolyte at different temperatures is showed in Fig. 5c. It was found that the experimental result of ionic conductive behavior is 21

definitely consistent with the fitting equation, indicating that the diffusion of lithium ions is primarily driven by the motion of segments in amorphous region rather than the crystalline region. Table 3 Ionic conductivity (σ) at 25 and 80 ºC and parameters for the P(PEGMEM), SCPs with various POSS contents and LCP5.1 ([EO]/[Li] = 15/1) derived from the VTF equation. Conductivity (S cm-1)

Sample

A

Ea

T0

25 oC

80 oC

(S cm-1 K1/2)

(KJ mol-1)

(K)

P(PEGMEM)

4.95×10-5

4.44×10-4

0.306 K-1/2)

9.69

231

SCP3.2 MMEM)

8.49×10-5

9.09×10-4

1.89

5.43

208

SCP5.1

1.13×10-4

1.05×10-3

2.87

2.58

198

SCP7.6

6.34×10-5

9.66×10-4

1.63

6.22

210

SCP11.2

5.18×10-5

4.40×10-4

1.06

7.35

213

LCP5.1

5.63×10-5

5.26×10-4

1.43

6.82

219

3.5 Li+ Transference Number Li+ transference number (t+) reflects the migration efficiency of lithium ion in the electrolyte and a high and unity t+ is desirable. The test of t+ was carried out by the methods

of

A.C.

impedance

and

chronoamperometry.

Fig.

6

shows

chronoamperometry of the symmetric Li/Li cell with SPEMs based on SCP5.1 and LCP5.1 matrix with [EO]/[Li] = 15/1 at a potential step of 10 mV at 25 oC. The inset is the AC impedance spectra before and after polarization and the equivalent circuit used to fit R0 and Rs. Then, the t+ was calculated using Equation 5 and the values are tabulated in Table 4.

22

(5) Where V is the potential applied to the cell, 10 mV; I0 and Is are the initial and steady-state currents; R0 and Rs are the initial and steady-state interfacial resistances between the electrolyte and the lithium electrode before and after chronoamperometry, respectively. From the insets of Fig. 6a and 6b, interception at high frequency represents the bulk resistance of the solid electrolyte (Rb) and the semicircle at medium frequency is related to the interfacial resistances, which is composed of the charge transfer resistance (Rct) and the resistance of the passivating film resistance (Rpf) [42]. As shown in Table 4, it can be found that the value of t+ for SPEM based on SCP5.1 matrix is 0.35, which is about two times of that for LCP5.1 at 25 oC. This suggests that the larger free volume of star-shaped copolymers for the mobility of EO segments is beneficial to facilitate the lithium ion transport.

Fig. 6 Chronoamperometry of the symmetric Li/Li cells with SPEMs based on (a) SCP5.1 and (b) LCP5.1 matrix with [EO]/[Li] = 15/1 at a potential step of 10 mV at 25 oC. The inset: the AC impedance spectra before and after polarization and the equivalent circuit used to fit R0 and Rs. Table 4 Li+ transfer number (

) of the SPEMs based on SCP5.1 and LCP5.1 matrix 23

with [EO]/[Li] = 15/1 at 25 oC. Sample (Ω)

(Ω)

(mA)

(mA)

SCP5.1

128

161

0.059

0.036

0.35

LCP5.1

198

239

0.026

0.0081

0.19

3.6 Electrochemical stability The electrochemical stability of the SPEMs is considered to be a crucial factor related to the safety issues of LIBs. The electrochemical stability windows for SPEMs based on SCP5.1 and LCP5.1 matrix with [EO]/[Li] = 15/1 were evaluated by using linear sweep voltammetry (LSV) at 25 and 80 oC with a scan rate of 0.5 mV s-1 in the potential range of 2 to 8 V (vs. Li/Li+), as shown in Fig. 7. The oxidative decomposition potentials of SPEM based on SCP5.1 matrix are 5.81 and 5.35 V vs. Li/Li+ at 25 and 80 oC respectively, suggesting that SCP5.1 SPEM is less stable at high temperatures [10]. For comparison, the oxidative decomposition potentials of LCP5.1 SPEM were also measured and the lower oxidative decomposition potentials of 5.31 and 5.04 V vs. Li/Li+ at 25 and 80 oC were estimated, respectively. Besides, the reversible lithium plating/stripping peaks for SPEMs based on SCP5.1 and LCP5.1 matrix with [EO]/[Li] = 15/1 at 25 and 80 oC are observed between -0.5 and 0.2 V (Fig. S4), which are well consistent with previous reports on other PEO-based SPEs [43]. In addition, wide reversible peaks at 0.5/1.3V are visible for two samples at 25 and 80 oC, which are probably associated with the side reaction of the TFSIanions and impurities [10, 44]. 24

Fig. 7 Linear sweep voltammograms with a scan rate of 0.5 mV s-1 for SPEMs based on SCP5.1 and LCP5.1 matrix with [EO]/[Li] = 15/1 at (a) 25 and (b) 80 oC. 3.7 Performance of the Li/LiFePO4 cells The Li/LiFePO4 cells incorporating SPEMs based on SCP5.1/LCP5.1 matrix ([EO]/[Li] = 15/1) were assembled to further investigate the electrochemical performance of these solid electrolyte membranes. Fig. 8a and b are the cycling performances of Li/LiFePO4 cells with SCP5.1 and LCP5.1 SPEM at 25 oC under a current density of 0.5 C. The cell based on SCP5.1 SPEM (Fig. 8a) demonstrates an initial discharge capacity of 116.2 mAh g-1 and the highest discharge capacity of 137.1 mAh g-1 with a coulombic efficiency of ~99%. However, for the cell based on LCP5.1 SPEM (Fig. 8b), about 72.6 mAh g-1 was delivered at the first cycle. The discharge capacity begun to level off at ~100 mAh g-1 after 30 cycles and then droped to 91.1 mAh g-1 at 100th cycle. Besides, severe fluctuation of discharge capacity and coulombic efficience is observed during the whole process. Compared the corresponding charge/discharge curve of the cell based on SCP5.1 SPEM (inset of Fig. 8a) with that of LCP5.1 SPEM (inset of Fig. 8b) at different cycle numbers, it is apparent that serious polarization was observed in the cell with LCP5.1 SPEM. The 25

results suggest that the cell based on SCP5.1 solid electrolyte possesses improved cycling performance at 25

o

C, which could be attributed to its higher ionic

conductivity and Li+ transference number.

Fig. 8 Cycling performance of the Li/LiFePO4 cell with (a) SCP5.1 and (b) LCP5.1 solid electrolyte at 25 oC with a current density of 0.5 C. The inset: the corresponding charge/discharge curves at different cycles. Compared with liquid organic electrolyte, solid polymer electrolyte could work at a high temperature without worrying about safety issues. The SPEM based on SCP5.1 presents a comparative tensile stress of ~4 Mpa and there exists nearly no thermal shrinkage for the SPEM after thermal treatment at 80 °C for 5 days (Fig. S5), implying that the SPEM could be used in high temperature LIBs without considering the danger of short circuit. In order to further confirm the battery performance of SCP5.1 solid electrolyte ([EO]/[Li] = 15/1) at 80 oC, cells were cycled at different C-rates. Fig. 9a shows the cycling performance of the Li/LiFePO4 cell based on SCP5.1 solid electrolyte at 80 oC with a current density of 0.5 C and the inset shows the corresponding charge/discharge curves at different cycle numbers. The cell exhibits an initial discharge capacity as high as 163.8 mAh g-1 and 147.8 mAh g-1 in 26

the 100th cycle with a retention ratio of 90.2%. The coulombic efficiency presents a constant value close to 100%. Compared to other LIBs with PEO-based solid electrolyte reported previously [9, 45-47], the cell based on SCP5.1 solid electrolyte shows higher initial discharge capacity at 80 oC under a current density of 0.5 C. In addition, Fig. 9b shows the AC impedance of the Li/LiFePO4 cell at different cycles during cycling process in order to evaluate the interfacial properties in the cell. The interception at high frequency represents the bulk resistance (Rbulk) of the cell and the semicircle at medium frequency is related to the interfacial resistances. It is apparent that the interfacial resistance after 100 cycles is about two times of that before cycle. The increase of the interfacial resistance in the cell during charge/discharge process is the main reason for capacity fading, which is closely related to the deteriorate interfaces between the solid electrolyte and solid electrodes [48, 49] and can also be observed in other solid LIB devices [9, 42]. In addition, the increase of the interfacial resistance between the electrolyte and lithium anode interface also could cause the capacity fading (Fig. S6). Fig. 9c is the discharge profiles obtained at different C-rates for the Li/LiFePO4 cell with SPEM based on SCP5.1 matrix at 80 oC with a charging rate of 0.1 C. It is clear that the solid-state cell shows a higher discharging capacity at 80 oC than that of the liquid cell at 25 oC (Fig. S7). The results indicate that the solid-state cells demonstrate excellent rate capability at high temperatures, whereas the liquid cells cycled at 80 oC exhibit no discharge capacity since the liquid electrolyte tends to be decomposed at temperatures above 60 oC. The discharge capacity of the solid-state 27

cell decreased with an increase of the discharging C-rates and the flat potential plateaus was observed up to 1 C rate in Fig. 9c. Even at high discharging C-rates of 5 and 10 C, the discharge capacity of 75.6 and 45.7 mAh g-1 can still be obtained, respectively. As shown in Fig. 9d, when the cell was operated under different circulatory rates, the specific discharge capacity decreased slightly with an increase of the current density. The cell delivered a specific discharge capacity of about 113 mAh g-1 at a high current density of 2 C, and recovered to its original value around 160 mAh g-1 as the current density was set back to 0.2 C. This implies that the cells based on such SPEM exhibit not only good cycle performance but also enhanced rate capability at high temperature.

Fig. 9 (a) Cycling performance of the Li/LiFePO4 cell with SPEM based on SCP5.1 matrix ([EO]/[Li] = 15/1) and the inset in (a) is the corresponding charge/discharge curves at different cycle numbers at 80 oC with a current density of 0.5 C; (b) 28

impedance profiles (100 kHz to 10 mHz) for the Li/LiFePO4 cell after different cycles at 80 oC with a current density of 1 C; (c) discharge profiles obtained at different discharging rates for the Li/LiFePO4 cell at 80 oC with a charging rate of 0.1 C; (d) discharge performance of Li/LiFePO4 at 80 oC under different circulatory rates and the inset in (d) is the corresponding charge/discharge curves at different cycle numbers.

4 Conclusions A series of organic/inorganic nanocomposites with star-shaped structure for the matrices of SPEMs, containing octavinyl octasilsesquioxane (OV-POSS) with nanoscale organic/inorganic hybrid structure and poly(ethylene glycol) methyl ether methacrylate (PEGMEM), have been prepared by simple free radical polymerization. When compared with the corresponding linear copolymers, star-shaped copolymers exhibit enhanced ionic conductivity and Li+ transference number since larger segments mobility and more free space could be achieved in the star-shaped polymer matrix. Moreover, the Li/LiFePO4 cells incorporating SPEMs based on SCP5.1 matrix exhibit not only good cycle performance but also enhanced rate capability at 80 oC. It is anticipated that such SPEMs with star-shaped structure are promising candidates for advanced electrolyte materials in high performance LIBs.

Acknowledgments This work was supported by the Recruitment Program of Global Youth Experts, the National Natural Science Foundation of China (51304248), the Program for New 29

Century Excellent Talents in University (NCET-11-0525), the Doctoral Fund of Ministry of Education of China (20130162110002), the Program for Shenghua Overseas Talents from Central South University, Grants from the Project of Innovation-driven Plan in Central South University and the State Key Laboratory of Powder Metallurgy at Central South University.

Supporting Information

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Highlights 

Organic/inorganic nanocomposites electrolyte membranes have been prepared.



The highest ionic conductivity of 1.13×10-4 S cm-1 at 25 oC was achieved.



The membrane exhibits the highest Li+ transference number of 0.35 at 25 oC.



At 25 oC, the highest discharge capacity of 137.1 mAh g-1 at 0.5 C was obtained.



At 80 oC, the cell exhibits an initial discharge capacity of 163.8 mAh g-1 at 0.5 C.

37