Advanced Polymer Electrolyte with Enhanced Electrochemical

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Advanced Polymer Electrolyte with Enhanced Electrochemical Performance for Lithium-Ion Batteries: The effect of Nitrile-Functionalized Ionic Liquid Xiaoli Zhan, Jiawen Zhang, Mingzhu Liu, Jianguo Lu, Qinghua Zhang, and Fengqiu Chen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01733 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019

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ACS Applied Energy Materials

Advanced Polymer Electrolyte with Enhanced Electrochemical Performance for Lithium-Ion Batteries: The effect of NitrileFunctionalized Ionic Liquid Xiaoli Zhan†, Jiawen Zhang†, Mingzhu Liu†, Jianguo Lu‡, Qinghua Zhang*,†, and Fengqiu Chen† †Zhejiang

Provincial Key Laboratory of Advanced Chemical Engineering Manufacture

Technology. College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, P. R. China ‡State

Key Laboratory of Silicon Materials, School of Materials Science and

Engineering, Zhejiang University, Hangzhou 310027, PR China

Corresponding Author *E-mail: [email protected]. Tel: +86-571-8795-3382. Fax: +86-571-8795-1227. Abstract: High-voltage electrode materials are beneficial for developing high power density lithium-ion batteries. The anodic voltage stability of electrolytes is a major challenge for high-voltage lithium batteries. Herein, a polysiloxane-based solid polymer electrolyte is fabricated by grafting ionic liquid and poly(ethylene oxide) chains onto flexible polysiloxane backbone. Ionic liquid with strong electronwithdrawing nitrile group and positively charged imidazole possesses good resistance against anodic oxidation, which endows the solid polymer electrolyte with a wide electrochemical window. The electrolyte integrates advantages of ionic liquid and poly(ethylene oxide) exhibiting a high ionic conductivity (3.56×10-4 S cm-1 at room temperature) and excellent resistance to dendrite growth. In addition, both Li/LiFePO4 and high-voltage Li/LiNi0.5Mn1.5O4 cells with the as-prepared solid polymer electrolyte show excellent cycling performance and superior rate capability at 30 °C. Thus the 1

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modified polysiloxane-based electrolyte is a promising electrolyte candidate for next generation high-energy-density batteries. Keywords: High-voltage, Lithium-ion batteries, Solid polymer electrolyte, DendriteFree, Ionic liquid.

1. Introduction Lithium-ion batteries (LIBs) are promising candidates as power source for electronic devices and emerging smart grids owing to their features of high specific energy and light weight.1-6 In order to obtain higher energy density of LIBs, spinel LiNi0.5Mn1.5O4 (LNMO) is considered as an ideal cathode material for high-energydensity LIBs due to its high theoretic capacity and working voltage plateau (~4.7 V, vs. Li+/Li).7-9 However, it is difficult for LNMO battery with liquid electrolyte-soaked PP separator to sustain high specific capacity at high voltage.10,11 Therefore, developing high-voltage tolerant electrolytes is significant for the development of high-energydensity batteries. Lithium metal batteries with lithium metal anode have been intensively pursued due to their high theoretical specific capacity. Nevertheless, the use of lithium metal electrode and liquid electrolyte often leads to the formation of lithium dendrites during repeated charge-discharge cycles, causing short circuiting and serious safety problem.12,13 Compared with the liquid-soaked separators, solid polymer electrolytes (SPEs) with sufficient mechanical properties and good electrochemical compatibility could effectively solve the above-mentioned problem.14-17

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Poly(ethylene oxide) (PEO) with lithium salt is the most widely studied SPE in the past few years.18-21 PEO can act as either solvating agents for lithium salts or structural materials for SPE, thus ensuring Li-ion transport and the chemical stability of SPE .22,23 Unfortunately, most of the PEO-based solid electrolytes are not suitable for practical applications because of their relatively low ionic conductivity and poor mechanical stability.24,25 However, the ionic conductivity could be significantly improved by suppressing the crystallinity and increasing the segmental mobility of PEO through adding plasticizer, modifying the PEO chains or cross-linking PEO oligomer.26-30 Meanwhile, the mechanical strength of SPEs could be enhanced by introducing crosslinked structures.31-32 Therefore, ion-conducting polymer materials with cross-linked structures and short PEO side chains, which possess high ionic conductivity and sufficient mechanical strength, have been widely studied as high-performance polymer electrolyte.33-36 In solid polymer electrolytes, ionic conductivities are governed by both the segmental motion of the polymer chains and the number of dissociated carrier ions and their mobility.37 To enhance ionic conductivity, the ionic liquids (ILs) have been used to incorporate in SPEs.38 ILs could act as plasticizers and substitute the conventional plasticizing agent. ILs exhibit several advantages including non-flammability, negligible vapor pressure, high ionic conductivity, wide oxidation stability window and high thermal stability.39-41 ILs composed of large organic cations and organic/inorganic anions are molten salts at room temperature. ILs as plasticizers can increase the

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amorphous phase of the SPEs and improve Li-ion transportation.42,43 For example, the polymer electrolyte containing 100wt% of PYR13TFSI shows a significantly improved conductivity of ~10-4 S/cm comparing with that of analogous polymer electrolyte without ionic liquid (2×10-6 S/cm).44,45 However, high amount of ionic liquids might jeopardize the mechanical properties of polymer electrolytes. Fortunately, this problem can be avoid by introducing chemical bonding between ionic liquids and polymer electrolyte substrate. The nitrile group (-C≡N) with lower LUMO and high electrochemical stability is a strong electron withdrawing group.46 Nitrile-based SPEs exhibit high anodic oxidization potential, high dielectric constant, and strong coordination ability.46-48 The interactions between Li-ions and -C≡N could effectively improve the ionic conductivity of electrolytes.49 For example, the succinonitrile/lithium perchlorate-based SPEs show a high ionic conductivities (>10-3 S/cm) at room temperature.50 PAN exhibits an electrochemical window over 4.5 V (vs. Li+/Li).51 The PVA-CN SPEs prepared by in situ synthesis method exhibit high lithium ion transference number, high ionic conductivity, enhanced mechanical strength and reinforced safety.46,52 It is also known that introduction of nitrile groups into an IL structure would lead to the formation of SEIs on electrodes.53 ILs and nitrile groups have positive effects on improving the electrochemical properties of polymer electrolytes. In order to take the synergistic advantages of nitrile groups and ionic liquids, we proposed a tunable bifunctional polysiloxane electrolyte for LIBs. The bifunctional polysiloxane contains

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poly(ethylene oxide) and 1-vinyl-3-cyanopropylimidazolium bis(trifluoromethyl sulfonyl)imide ionic liquid (IN-CN). After mixing with LiTFSI and cross-linking, the thermal stability and electrochemical properties are carefully studied. The strong electron-withdrawing nitrile group and positively charged imidazole in IN-CN could enhance the compatibility of electrolyte while enabling high voltage operation.

2. Experimental section 2.1 Materials Poly(ethylene glycol) allyl methyl ether (PEO, Mw=400 g mol-1) was obtained from Hangyuan Chemical Industry Co. Ltd. Polymethylhydrosiloxane (PMHS, Mn: 17003200) was purchased from Aladdin. Toluene (≥ 99.5%) and acetonitrile (CH3CN, ≥ 99.9%) were purchased from Sinopharm. The above reagents were dried in 4A molecular sieves before use. 1-vinyl-3-butylimidazolium bis(trifluoromethyl-sulfonyl) imide

ionic

liquid

(IL,

≥

99%)

and

1-vinyl-3-cyanopropylimidazolium

bis(trifluoromethylsulfonyl)imide ionic liquid (IL-CN, ≥ 99%) were received from Lanzhou Yulu Fine Chemical Co. Ltd.1-methyl-2-pyrrolidinone (NMP, 99.5%), Hexachloroplatinic acid/isopropanol solution (1g H2PtCl6·6H2O dissolved in 50 ml isopropanol) and tri(ethylene glycol) divinyl ether (98%) and

were purchased from

Sigma Aldrich. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, ≥ 99%) was purchased from Aladdin and dried at 120 °C in a vacuum oven for 24 h before use. The liquid electrolyte was 1.0 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate

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(DMC)/ethyl methyl carbonate (EMC) (1:1:1, v/v/v) (Shenzhen Kejing Co., Ltd). All reagents were used as received. Microporous polypropylene separators (PP, 26 μm, Celgard) were used for comparison.

2.2 Synthesis of bifunctional polysiloxane (PIN-PMHS)

The process of synthesis of the bifunctional polysiloxane (PIN-PMHS) are shown in Scheme 1. Firstly, a poly (ethylene glycol)allyl methyl ether (PEO) was grafted onto PMHS (P-PMHS) via a hydrosilylation reaction with the Speier's catalyst, following the method previously reported by our group.54 PMHS (2 g) was dissolved in anhydrous toluene (2 g). PEO (4 g) and Speiers's catalyst (150 μL) with anhydrous toluene (4 g) were mixed well and then added to the PMHS solution. The mixture was stirred in an argon atmosphere at 50 °C for 2 h. Then, keep stirring at 80 °C until the stretching vibration absorption of CH2=CH2 disappeared in situ FT-IR spectrum. Remove the solvent by rotary evaporator at 55 °C, and the obtained viscous polymer was denoted as P-PMHS. The above prepared P-PMHS and IL-CN were mixed with anhydrous acetonitrile (6 g). The amount of IL-CN was controlled to obtain different ratios of side chains. The reaction mixture was stirred at 80 °C for 24 h under reflux. After reaction, the solvent was removed through rotary evaporation at 50 °C and the products were dried under vacuum at room temperature for 24 h. Finally, the polysiloxane with different content of side chains (PIN-PMHS) were obtained.

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Table 1. Targeted compositions of synthesized samples. IL content

IL-CN content

EO/Li+ mole

(wt% PMHS)

(wt% PMHS)

ratio

SPE0

0

0

20

SPE1

10

0

20

SPE2

0

5

20

SPE3

0

10

20

SPE4

0

15

20

SPE5

0

20

20

SPE6

0

10

10

SPE7

0

10

15

SPE8

0

10

25

SPE9

0

10

30

Samples

2.3 Preparation of solid polymer electrolytes

Solid polymer electrolytes were prepared by the following process illustrated in Scheme 1, and the whole procedure was performed in a glove box filled with argon (H2O

Advanced Polymer Electrolyte with Enhanced Electrochemical

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