Improvement of lithium-ion battery performance at low temperature by

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Improvement of lithium-ion battery performance at low temperature by adopting ionic liquid-decorated PMMA nanoparticles as electrolyte component Yang Li, Ka-Wai Wong, Qianqian Dou, Wei Zhang, and Ka Ming Ng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00355 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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ACS Applied Energy Materials

Improvement

of

Lithium-Ion

Battery

Performance at Low Temperature by Adopting Ionic Liquid-Decorated PMMA Nanoparticles as Electrolyte Component Yang Li,† Ka Wai Wong,† Qianqian Dou,† Wei Zhang† and Ka Ming Ng＊†

†

Department of Chemical and Biological Engineering, The Hong Kong University of

Science and Technology, Clear Water Bay, Hong Kong 999077, China

*Corresponding Author: E-mail: [email protected]; Fax: +852-23580054; Tel: +852-23587238

ACS Paragon Plus Environment

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ABSTRACT A novel electrolyte system blending ionic liquid (IL)-decorated PMMA nanoparticles with 1 M LiTFSI dissolved in a mixture of propylene carbonate (PC) and methyl acetate (MA) is reported. With the addition of PMMA-IL-TFSI, this electrolyte exhibits an ionic conductivity of 9.15×10–4 S cm–1 even at –40 oC. The improved ionic conductivity at low temperature is attributed to the liquid component in the electrolyte and the unique grafting structure of IL groups on PMMA nanoparticles. Furthermore, it is proved that the presence of PMMA-IL-TFSI can improve the reversible capacity and rate capability of Li4Ti5O12 (LTO)/Li half cells at low temperature. In addition, the morphology change and the electrochemical impedance spectroscopy (EIS) results indicate that the enhancement in battery performances is mainly attributed to the increase of ion conduction via the formation of a stable and effective SEI film on the electrode. These attributes enable the developed PMMA-ILTFSI-based electrolyte to be a potential ingredient in high-performance lithium-ion batteries at low temperature.

KEYWORDS Ionic liquid, PMMA nanoparticle, electrolyte, low temperature, lithium-ion battery


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INTRODUCTION Rechargeable lithium-ion batteries (LIBs) as power sources have been intensively pursued for portable electronic equipment, electric transportation and grid-energy storage systems. The development of LIBs with high energy density, prolonged cycle life and satisfying environment constraints is an essential challenge for nextgeneration energy storage technology.1–5 Typically, a conventional electrolyte system used in LIBs is based on lithium salt dissolved in carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylene carbonate (EC), propylene carbonate (PC) and ethylmethyl carbonate (EMC), which works reasonably well under ambient conditions.6 However, at low temperature, their poor performance limits the applications of LIBs in extreme conditions, such as certain space and defense space systems,7 as well as large-scale transportation applications, including plug-in hybrid electric vehicles (PHEVs), electric vehicles (EVs) and hybrid electric vehicles (HEVs).8 There is often a significant loss in the power and energy capability when LIBs are operated below 0 oC at which the electrolyte solution becomes partially frozen.9,10 To improve the performance at low temperature, extensive research has been focused on increasing the ionic conductivity by developing multicomponent electrolytes such as binary, ternary or quaternary electrolyte systems composing of alkyl carbonate esters and alkyl carbonates, such as EC-DMC-EMC,11



EC-DMC-DEC12 and EC-DEC-DMC-EMC.13 Despite the higher ionic conductivity, the major problem with such electrolytes in LIBs is that the corresponding SEI films formed are usually unstable. This separate issue also leads to poor battery performance at low temperature. The use of additives in a liquid electrolyte is considered to be an economic and effective way to enhance the performance of LIBs, by performing various functions,14 such as film-forming, protection of electrodes,15,16 stabilizing and increasing cycle life17,18 and flame-retardation19,20 during repeated charge and discharge processes. Ionic liquids (ILs) have been extensively used as a new solvent or additive21-23 of electrolyte for LIBs because of their attractive properties, including high thermal and chemical stability, negligible vapor pressure, reasonably high conductivity and non-flammability.24 It has been pointed out that the wide working temperature range of ILs, from –81 oC to 280 oC,25 is advantageous for improving the performance of LIBs at low temperature. Among various ILs, imidazolium-based bis(trifluoromethylsulfonyl)imide (TFSI) ILs display good compatibility with cathodic and anodic materials, and improve the electrodeposited lithium (Li) morphology due to their pronounced electrochemical performance and high stability.26–28 In addition, theoretical studies indicate that if a small amount of anions in an electrolyte can be immobilized, the concentration gradient causing


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undesirable polarization effect can be much reduced during operation. Therefore, uneven Li electrodeposition can be alleviated and better battery performance can be achieved.29,30 As such, successful immobilization of IL anions within the electrolyte system is expected to significantly enhance the battery characteristics, and shed light on better performance even at low temperature.31–33 Inspired by these ideas, we have synthesized a novel electrolyte component, ILtethered PMMA (PMMA-IL-TFSI) nanoparticles, and developed an electrolyte system for LIBs at low temperature by blending these nanoparticles with 1 M LiTFSI dissolved in the mixed solvents of PC and methyl acetate (MA). PC was chosen as the basic solvent owing to its high dielectric index (64.92), low melting point (–49 oC) as well as good miscibility with ILs. Due to the low freezing point (–96 oC) of MA and the specific IL-grafted structure of PMMA-IL-TFSI nanoparticles, the developed electrolyte was found to exhibit an enhanced ionic conductivity of 9.15×10–4 S cm–1 even at –40 oC. Further, more conducting and stable SEI films were formed on the electrodes with this unique electrolyte system. As expected, the corresponding Li4Ti5O12 (LTO)/Li half cells showed improved reversible capacity and rate capability at low temperature with the presence of PMMA-IL-TFSI. These attributes demonstrate the promising potential of this developed electrolyte system for use in high-performance LIBs at low temperature.



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EXPERIMENTAL SECTION Preparation of PMMA nanoparticles. Soap-free emulsion polymerization was used to synthesize PMMA nanoparticles in a three-necked flask34. Typically, 20 mL methyl methacrylate was mixed with 200 mL DI water with continuous stirring and the mixture was heated to boiling. 0.25 g potassium persulfate dissolved in water was added and reacted for 1 h. Afterwards, 4 mL 2-hydroxyethyl methacrylate was mixed in and the solution was heated for another 2 hours. Subsequently, the products were filtered with DI water and finally dried at 80 oC for one day under vacuum. Synthesis of PMMA-IL-TFSI nanoparticles. Procedure used to graft IL groups to PMMA

nanoparticles

was

illustrated

in

Scheme

1.

Typically,

(3-

chloropropyl)trimethoxysilane and 1-methylimidazole were mixed and stirred in a three-necked flask with N2 atmosphere at 80 oC for 48 h to prepare IL-Cl precursor (1-methyl-3-trimethoxysilaneimidazolium chloride). The yellow solution was purified with ether and the solvent was finally removed by evaporation. IL-Cl grafted PMMA nanoparticles were prepared by firstly dispersing as-prepared PMMA nanoparticles in DI water and then adding IL-Cl precursor of 1.5 times weight excess to the suspension. The suspension remained at 80 oC overnight with magnetic stirring. Finally, the resultant IL-Cl grafted PMMA nanoparticles were washed and separated with DI water by centrifugation. The trace of water in the sample was finally removed


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by freeze-drying. Substitution of the chloride ion (Cl–) with bis(trifluoromethanesulfoneimide) (TFSI) anion was carried out by an ion exchange reaction. The dispersion of 4 g as-obtained PMMA-IL-Cl in DI water was prepared, followed by the dropwise addition of 8 g LiTFSI water solution to the suspension under mechanical stirring. Since TFSI anion is hydrophobic, the resultant PMMA-IL-TFSI nanoparticles immediately sedimented to the bottom of the beaker. Eventually, the product was repeatedly washed with DI water, collected by centrifugation and finally freeze-dried32. Scheme1. Synthetic route for the preparation of PMMA-IL-TFSI nanoparticles.

Preparation of electrolyte mixtures. 1 M LiTFSI solution was firstly prepared by dissolving the desired amount of LiTFSI in a mixture of PC and MA. 11 wt% PMMA-IL-TFSI nanoparticles was dispersed to the solution because the electrolyte containing such an amount of nanoparticles reached its maximum in ionic conductivity at low temperature (Figure S1 in the Supporting Information). Then the mixture was sonicated to form a homogeneous solution. All procedures were carried



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out in a glove box filled with argon (≤1 ppm H2O and O2). Characterization.

Fourier

transform

infrared

(FT-IR)

spectrophotometer

(FTS6000, Bio-Rad, US) was applied to identify the functional groups on PMMA nanoparticles. The structure of PMMA-IL-TFSI nanoparticles was observed by an ISM-6700F scanning electron microscope (SEM) (JEOL Ltd., Japan). The same SEM was used to observe the surface morphology of Li anodes after several cycles at low temperature. Differential scanning calorimeter (DSC Q1000) was used for thermal analysis. Viscosity was measured by a Brookfield viscometer (RVDV–II+Pro). The ionic conductivities of prepared electrolytes were determined as follows, 𝜎=

𝑙 𝑅𝑎

(1)

where a (cm2) is the area and 𝑙 (cm) is the thickness of electrolyte. R (Ω) is bulk resistance of the cell which was determined by AC impedance method using a CHI 760E electrochemical workstation in a low-temperature test chamber within a temperature range between 20 oC to –40 oC with frequency from 100 kHz to 10 Hz. Electrochemical impedance spectroscopy (EIS) measurements was implemented on the same electrochemical workstation. The electrochemical performance of the electrolyte was tested with a LTO/Li half cell. LTO was used as an electrode because of its stable cycling performance and excellent rate capability.28,35 For the preparation of electrode, LTO nanopowder (D50: 0.5–1.5 μm, BTR New Energy Material Ltd.),


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PVDF (polyvinylidene difluoride) and Super P carbon black were mixed in Nmethylpyrrolidone (NMP) in 80:10:10 weight ratio to form homogeneous slurry that was coated on copper foil and remained at 80 oC for 12 h under vacuum prior to use. The loading of active material on the electrode is around 1.5 mg cm–2. Lithium foil with diameter of 16 mm and thickness of 0.6 mm (MTI Corporation) was used as counter electrode. Charge and discharge curves of the cells were determined over 1.0–3.0 V at various temperatures in a low-temperature chamber equipped with a Neware CT– 4008 battery tester after staying at low temperature for 8 h. The C-rate was determined according to the theoretical capacity of 175 mA h g–1 for LTO/Li cell. RESULTS AND DISCUSSION SEM image observed in Figure 1a shows uniform PMMA-IL-TFSI nanospheres with an average particle size of around 260 nm. Successful grafting of IL to PMMA nanoparticles was confirmed by FT-IR analysis (Figure 1b). Typically, strong peaks at 1157 and 1732 cm−1 showed in PMMA-IL-TFSI correspond to C–O–C and C=O stretching and deformation, respectively. The broad band corresponding to –OH vibration from surface modification at 3467 cm−1 is clearly identified. The peaks observed at 989 and 843 cm−1 are ascribed to C–H bending, while the band at 746 cm−1 is ascribed to the polymer chains vibration.36 Moreover, the absorption bands in IL and PMMA-IL-TFSI nanoparticles displayed at 1078 and 1167 cm−1 confirm the



presence of –O–Si–C bond and Si–R (R=alkyl) groups37 of IL. The band attributing to Si–CH2 stretching at 657 cm−1 and the peak assigned to C–H stretching at 2959 cm−1 can be observed in the spectra of both IL and PMMA-IL-TFSI nanoparticles. The anion exchange is indicated by the characteristic absorption peaks of TFSI anion located at 1340, 1200 and 1056 cm−1, which are also highly intensified on PMMA-ILTFSI.38 All these demonstrate the successful grafting of IL to PMMA nanoparticles.

Figure 1. (a) SEM image of PMMA-IL-TFSI nanoparticles. (b) FT-IR spectrum of PMMA-IL-TFSI, IL and LiTFSI. Figure 2a shows the temperature dependence of ionic conductivity of electrolyte solutions which is of the Vogel-Tammann-Fulcher (VFT) type in all cases with and without PMMA-IL-TFSI. It shows a decrease in ionic conductivity with temperature decreasing to low temperature region (

Improvement of lithium-ion battery performance at low temperature by

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