Development of safe, green and high performance ionic liquids-based batteries (ILLIBATT project)
Introduction
Lithium batteries (LBs) are nowadays one of the most popular energy storage devices. Depending on their cell chemistry LBs can show high energy, high power and high cycle life. For these characteristics, they dominate the consumer portable electronic and telecommunications market. In the last years, LBs have been indicated as the most promising option for the next generation of hybrid and electric vehicles (HV, EV) as well as an attracting candidate for the realization of high performance delocalized energy storage units. Clearly, the wide deployment of lithium batteries in these applications would have tremendous consequences on the battery-market and it would further strengthen the central role of these systems in the field of energy storage. For that, considerable efforts are now focused on the development and realization of lithium batteries able to fulfil the requirement necessary for these applications.
When the present lithium ion battery technology is considered, the safety and the temperature range of use represents the main drawbacks holding the introduction of these systems in the new applications. As a matter of fact, the commercial systems nowadays available use electrolytes commonly based on organic carbonates (e.g. ethylene carbonate, EC, diethyl carbonate, DEC, ethyl methyl carbonate, EMC) but, since these electrolytes are flammable and volatile, their use poses a serious safety risk and strongly reduces the battery operative temperature range [1], [2]. Moreover, liquid electrolytes are always a possible source of cell leakage. For that, alternative electrolytes with improved safety and able to work in a broader operative temperature range are today urgently needed. However, in order to be really effective, the introduction of such advanced electrolytes cannot lead to a reduction of the battery performance.
In the past, many R&D projects were devoted to the development of “solid” and “dry” polymeric electrolytes [3], [4], [5], [6]. Polymer electrolytes have low volatility and flammability and thus they may be considered as safer than liquid organic electrolytes. Unfortunately, polymeric electrolytes show lower (for many applications insufficient) ionic conductivities than liquid organic solvent-based electrolytes [7]. More recently, the use of ionic liquids (ILs) as electrolyte for lithium batteries have jumped into the centre of interest. The main advantages of ILs towards organic solvents are the non-flammability, the negligible vapor pressure, the high chemical and thermal stability and, in some cases, hydrophobicity [8], [9], [10], [11]. For these characteristics, ILs have attracted a large attention for use as “green” solvents and recently have been intensively investigated as electrolytes and/or electrolyte components for lithium batteries [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. The results of these studies indicated ILs as promising electrolytes. However, when the performance of IL-based systems is compared with that of systems containing conventional electrolytes, further improvement appears still necessary. With the aim to combine the favorable properties of polymer electrolyte and ionic liquids, also the realization of polymer electrolytes containing ionic liquids has been investigated in the past. The results of these studies showed that the addition of ionic liquids to conventional polymer electrolytes increase dramatically their ionic conductivity, without negative effects of their mechanically stability [22], [23], [24], [25], [26]. However, also in this case, further improvement in term of ionic conductivity appears still necessary.
Considering the results of these studies, the replacement of organic electrolytes with ionic liquids based electrolytes (both, liquid or solid) certainly represents an attracting and promising strategy to improve the safety of the lithium battery technology. Nonetheless, further research is necessary to achieve the performance required for the next generation of batteries when these electrolytes are used.
In this paper we report about some of the results obtained within the European project ILLIBATT (2007–2010, contract no. NMP3-CT-2006-033181), which was dedicated to the development of green, safe and high performance IL-based lithium batteries. Four universities, two research centres and two industries from 7 different European countries were involved in the ILLIBATT Consortium (for more details, see Table 1).
Considering the limitation of the state of the art reported above, the scientific objectives of ILLIBATT project were:
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Synthesis and characterization of solid polymeric electrolytes, either formed by an Ionic Liquid (or a mixture of Ionic Liquids) integrated in a polymer matrix or being a Polymeric Ionic Liquid having properties (ionic conductivity, (electro-)chemical, and thermal stabilities) better than the present polymer electrolytes especially at ambient and lower temperatures. This included the synthesis and processing of novel ILs, polymers, and their composites.
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Synthesis of nano-structured metallic electrode materials, which are able to reversibly store lithium, via electroplating.
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Thorough investigation of interfacial reactions of commercial and newly synthesized cathode and anode materials with ILs and IL-based solid electrolytes.
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Realization of battery concept cells of lithium-ion and lithium metal configurations and investigation of their electrochemical performance and safety.
Following a comprehensive, manifold and multidisciplinary material approach (see Fig. 1), the ILLIBATT partners worked on four key objectives: (1) development of a green and safe solid-state electrolyte chemistry based on ionic liquids and unique ionic liquid-based composites with high performance; (2) use of novel nano-structured high capacity anodes, prepared with the help of novel ionic liquids; (3) investigation of the peculiar electrolyte properties and the specific interactions of these electrolytes with advanced commercial and self-prepared electrode (anode and cathode) materials with the goal to understand and improve the electrode and electrolyte properties and thus their interactions; and (4) construction of rechargeable lithium cells with optimized electrode and electrolyte components.
The final goal of ILLIBATT was to realize rechargeable lithium batteries with high performance and safety. Specifically, the targets of the project were:
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optimize positive electrodes with specific capacities of at least 150 mAh g−1 of electrode active materials and 80 mAh g−1 of electrode composite layers;
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develop all-solid-state 1 Ah concept cell batteries operating at room temperature with specific energy up to 200 W h kg−1 with respect to the overall weight of the concept cell;
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obtain a high coulombic efficiency in average higher than 99% during cycling at 20 °C;
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obtain a cycle life of up to 500 cycles with 20% maximum loss of capacity, cycling (at 20 °C) between 100% and 0% SOC;
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evaluate the integration of the developed batteries in renewable energy sources, especially their use in combination with photovoltaic cells (PVs).
Several materials have been developed and tested during the ILLIBATT project. However, this paper focus mainly on the components materials (electrolytes and electrodes) used for the realization of the ILLIBATT prototypes. The performance of the ILLIBATT prototypes and the safety test carried on the developed batteries are also illustrated.
Section snippets
Synthesis of electrolytes
N-Butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) and N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR14FSI) room temperature ionic liquids were synthesized and dried as reported in Ref. [21]. All the compositions of the electrolytes containing PYR14TFSI and PYR14FSI are given as molar concentrations.
The polymer electrolyte based on cross linked poly(ethyleneoxide) and ionic liquid (cl-PEO–LiTFSI–PYR14TFSI) was synthesized as reported in Refs. [27], [28].
Electrolytes
Table 2 compares the physical and electrochemical properties of three different types of IL-based electrolytes:
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Electrolyte (0.9PYR14TFSI – 0.1LiFSI)
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Polymeric electrolyte cl-PEO–PYR14TFSI–LiTFSI
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Polymeric ionic liquids PIL–PYR14TFSI–LiTFSI
As indicated in the table, all electrolytes displayed very promising performance in terms of low vapor pressure and high thermal stability (from 150 °C of (0.9PYR14TFSI–0.1LiFSI) to more than 300 °C for PIL–PYR14TFSI–LiTFSI). Furthermore, their overall
Conclusions
The results obtained within the European project ILLIBATT indicated that several cell chemistries can be used for the realization of safe and high performance IL-based batteries.
Using cl-PEO–PYR14TFSI–LiTFSI and PIL–PYR14TFSI–LiTFSI electrolytes in combination with LFP-based cathodes it is possible to realize high performance lithium-metal polymer batteries. Such batteries display high capacity and a remarkable cycling stability. For example, a prototype of 0.7 Ah containing cl-PEO–PYR14
Acknowledgments
The authors wish to thank the financial support of the European Commission within the FP6 STREP Projects ILLIBATT (Contract No. NMP3-CT-2006-033181).
References (50)
- et al.
Polymer
(1973) - et al.
Solid State Ionics
(1997) - et al.
Solid State Ionics
(1983) - et al.
Electrochim. Acta
(2008) - et al.
Electrochim. Acta
(2004) - et al.
Electrochem. Commun.
(2008) - et al.
J. Power Sources
(2008) - et al.
J. Power Sources
(2006) - et al.
J. Power Sources
(2006) - et al.
J. Power Sources
(2007)
J. Power Sources
Electrochem. Commun.
Electrochim. Acta
J. Power Sources
Eur. Polym. J.
J. Power Sources
J. Power Sources
J. Power Sources
J. Power Sources
J. Power Sources
J. Power Sources
Ultramicroscopy
Electrochim. Acta
Electrochim. Acta
J. Power Sources
Cited by (0)
- 1
Current address: Varta Micro Innovation GmbH, Stremayrgasse 9, A-8010 Graz, Austria.
- 2
Current address: IMDEA Energy, Universidad Rey Juan Carlos, C/Tulipán, s/n – 28933 Móstoles-Madrid, Spain.
- 3
Current address: Universidad del País Vasco/Euskal Herriko Unvertsitatea (UPV-EHU) Joxe Mari Korta Center, Avda. Tolosa, 72, 20018 Donostia-San Sebastian, Spain.