Cross-Linked Polymer Electrolytes for Li-Based Batteries: From Solid

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Cross-linked polymer electrolytes for Libased batteries: From solid to gel electrolytes Victor Chaudoy, Fouad Ghamouss, Erwann Luais, and François Tran-Van Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02287 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on August 31, 2016

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Industrial & Engineering Chemistry Research

Cross-linked polymer electrolytes for Li-based batteries: From solid to gel electrolytes Victor Chaudoy1,2, Fouad Ghamouss1,*, Erwann Luais1, François Tran-Van1 1

Laboratoire de Physico-Chimie des Matériaux et des Electrolyte pour l’Energie (EA 6299),

UniversitéFrançois Rabelais de Tours, Faculté des Sciences et Techniques, parc de Grandmont, 37200 Tours, France. 2

STMicroelectronics, BP.7155, 37071 Tours Cedex 2, France

ACS Paragon Plus Environment

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KEYWORDS: Gel polymer electrolyte; cross-linked polymer electrolyte; room temperature ionic liquid; P14FSI; lithium battery

ABSTRACT

Herein, we report the preparation of solvent free polymer electrolytes via a free radical copolymerization of methacrylate-based oligomers in the presence of LiTFSI. Properties of the electrolytes were then studied as a function of their compositions. Furthermore, the incorporation of room temperature ionic liquid (RTIL) into the co-polymer electrolyte to form a gel polymer electrolyte (GPE) is also reported. The high miscibility of the oligomers in the RTIL enables the preparation of the GPEs by a one-step method using the in situ free radical co-polymerization. The GPEs have a dry aspect, and are free-standing, they also exhibit an ionic conductivity close to 4 x 10-4 S cm-1 at 25 °C and to 1.45 x 10-3 S cm-1 at 65 °C. Furthermore, the GPEs have been used as electrolytes in Li/Electrolyte/LiNi1/3Mn1/3Co1/3O2 battery. Specific capacities of 79 mAh g-1 and 118 mAh g-1 were reached at C/5 and C/10 rate respectively.

Introduction Polymer based electrolytes are today widely studied and reported for many electrochemical applications

1–4

. Polymer based electrolytes have been investigated as electrolyte for Li-ion and

more generally Li-based batteries, since the earlier works of Armand et al.5,6. Later, Gel Polymer Electrolytes (GPEs) that are consisting of a ternary mixture of polymer, salt, and high permittivity solvent have been reported

7,8

. GPEs can be dimensionally stable and also exhibit a


2

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relatively high conductivity (up to 10-3 mS cm-1 @ room temperature, RT). Moreover, GPEs show an extended operational temperature window by comparison to conventional all solid-state electrolytes

9,10

. Therefore, GPEs seem to be more appropriate to practical exploitations

1,11

.

Nowadays, the general trend in the context of GPEs consists in replacing the molecular solvent in the gel polymer electrolyte by room temperature ionic liquids (RTILs)

12

. Indeed, RTILs are

very promising liquid because of their intrinsic ionic conductivity, their negligible vapor pressure and their thermal stability 13. Moreover, with the use of an appropriate combination of cations and anions, RTILs can achieve an impressive electrochemical stability

14

. Today, a large

selection of RTILs is commercially available, and some of them have been already reported for Li-ion and Li-based batteries

14

. RTILs also meet the requirements of plasticizing solvents and

can be therefore blended with polymers to prepare gel electrolytes. In addition, if the ionic liquid and the polymer allow a compatible mixture without phase separation, the obtained polymer gel electrolytes should be distinguished from conventional polymer gels in terms of non-volatility and high thermal stability. During the last decade, several studies dealing with the preparation, the characterization, and the optimization of Polymer/RTIL/Li-salt ternary mixtures for Li-based batteries have been reported 10,12,15,16. GPEs can be divided into two classes of materials: i) GPEs made from a blend of liquid phase and linear polymers (also co-polymers) such as polyethylene oxide, polyvinylidene fluoride, polyacrylonitrile, etc.

4,12,17

, and ii) GPEs using cross-linked polymers or co-polymers . The

Cross-linked polymers and co-polymers are usually prepared via an in situ reaction of monomers or oligomers in the liquid phase leading to the formation of the GPEs. Using this method, the blend is suspected to be more efficient due to chains entanglement

10,15,16

and stronger van der

Waals forces between the liquid phase and polymer moieties. Among reported polymerization


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reactions, the free radical polymerization of compatible monomers in RTILs leads to a transparent, dimensionally stable and highly conductive GPEs 10. Liao et al. reported obtained a GPE via an in situ free radical co-polymerization of polyethylene glycol dimethacrylate and polyethylene glycol

methyl ether methacrylate in a (1-methyl-1-propylpyrrolidinium

bis(trifluoromethylsulfonyl)imide (P13TFSI) + LiTFSI) mixture

10

. Using this method, the

obtained GPE exhibits good ions transport and electrochemical properties. However, the gel electrolyte reported in that study was not self-supporting and was therefore transferred onto a cotton mesh before it use as a separator in lithium battery. By using this gel, the authors reported a discharge capacity of 130 mAh g-1 at 50 °C using a Li/lithium Iron Phosphate battery. Gerbaldi et al. also reported a GPE obtained via a free radical co-polymerization of bisphenol A ethoxylate dimethacrylate and polyethylene glycol methyl ether methacrylate in a (Nmethoxyethyl-N-methylpyrrolidinium TFSI + LiTFSI) mixture

16

. The GPE was transparent,

self-supporting and fair electrochemical performances using a Li/lithium Iron Phosphate battery at 25 °C was also claimed by these authors. In this study, we report the preparation and the characterization of Gel Polymer Electrolytes (GPEs) for Li-based batteries. The electrolytes were prepared via an in situ free radical copolymerization of poly(ethylene glycol)dimethacrylate (DMA550) and poly(ethylene glycol) methyl ether methacryate (MA475) in the presence of the 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide, P14FSI, in the LiTFSI-based mixture. Ionic conductivity and glass transition temperature (Tg) of the electrolytes are reported and discussed as function of salt concentration and co-polymer composition. The cyclability of selected GPEs on a Li/NMC-type battery has been also determined and their performances presented and discussed as function of gels composition.


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Experimental Materials Poly(ethylene glycol) methyl ether methacrylate (MA475, Mn: 475) and Poly(ethyleneglycol) dimethacrylate (DMA550, Mn: 550) were purchased from Aldrich. Radical initiator 2,2’azobis(2-methylpropionitrile) (AIBN) was purchased from Aldrich and was purified in methanol before use. MA475, DMA550 and AIBN were dried at 40 °C under vacuum for 5 days and stored in a glove box (mBRAUN, H2O

Cross-Linked Polymer Electrolytes for Li-Based Batteries: From Solid

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