Printable, Non-aqueous Emulsion-templated Polymer Separators for Li Ion Batteries
Natasha Shirshova1, Stephen Johns1, Alexander Bismarck2, Jérémie Salomon3, Hélène
Rouault3, Joachim H. G. Steinke1
1.
Department of Chemistry, Imperial College London, London SW7 2AZ, UK. e-mail:
j.steinke@imperial.ac.uk
2. Department of Chemical Engineering and Chemical Technology, Imperial College London, London
SW7 2AZ, UK.
3. Commissariat à l'énergie atomique (CEA) Grenoble, DRT/DTEN/SCSE/LSEM, 17 rue des Martyrs, F38054 Grenoble, France.
Introduction
Lithium ion batteries are one of the key device components for portable energy storage.
Mayor interests in lithium ion batteries are concerned with “greener” transport options such
as hybrid cars and are ubiquitous in portable electronic devices and equipment. In
conjunction with solar cells energy generation and storage can be performed locally leading
to new ways of delivering energy for applications requiring mobility and power.
With the intense interest in energy storage, research into lithium ion batteries addresses
essentially all aspects of performance improvements: (i) New materials for higher charge
densities which are at the same time environmentally more benign, (ii) electrode materials
with better cyclability, (iii) greener electrolytes such as those based on ionic liquids and (iv)
separators with higher ion flux for faster charging and discharging kinetics. Overarching as a
theme to all those activities is the production of Li ion batteries is the need for low energy
processing routes combined with simpler production techniques and more environmentally
benign deposition routes. In this contribution we will present our efforts in integrating more
desirable processes for Li ion battery production with the development of new materials,
which can be processed more easily, are environmentally friendly, but do not sacrifice
performance.
Results and Discussion
The focus of our research is to develop a separator membrane filled with environmentally low
impact electrolytes such as ionic liquids, which can be printed directly and be used in a rollto-roll process, leading to significant environmental and cost advantages in the production of
thin film batteries. To achieve high energy and power densities, the separator has to be thin
and highly porous, to be able to absorb the electrolyte, while still remaining mechanically
strong1. Separators can be divided into three groups: macroporous polymers, non-woven
fabric mats and inorganic composite membranes. Typically, the manufacturing of separators
is a multistep process. Our approach to a printable separator is based on the formation of a
highly porous polymer network which can be generated in a single step. As pore- and
network forming strategy we have chosen emulsion templating to produce highly porous
polymer matrices.2,3
Non aqueous high internal phase emulsions were prepared (Figure 1) by addition of the
dispersed phase (consisting of an ionic liquid doped with Li salt) into the external phase (a
combination of thermal or UV radical polymerisation initiators, monomer, crosslinker and a
surfactant).
PolyHIPE +
electrolyte
Electrodes
Dispersed phase
(org solvents, IL)
Monomer +
Xlinker +
surfactant
+
printing
UV, T (oC)
Figure 1. Schematic representation of in situ synthesis of a porous separator membrane prefilled with liquid
electrolyte for printable thin-film Li ion batteries.
The emulsion was then cast as film and polymerised using thermal or photochemical radical
initiation. By varying the monomer to crosslinker ratio, the surfactant concentration and the
external phase volume, we were able to control morphology, ionic conductivity and
mechanical properties of the resulting separator materials.
Stable HIPE conditions were identified and both thermal and photo-polymerisation led to
highly porous thin films pre-filled with polymer electrolyte (an example of the pore
morphology of such a polyHIPE is shown in Figure 2). Without the need of filling the
separator membrane in a post-polymerisation step with electrolyte, we obtained
conductivities ranging between 1-10 mS/cm. Details on the properties of the synthesised
HIPEs and polyHIPEs as well as the outcome of printing trial of these separator/electrolyte
inks as part of a thin film battery production route will be presented at the meeting.
Figure 2. SEM image of a polyHIPE morphology representative for emulsion template Li ion separator
membranes with ionic liquid-based electrolyte as dispersed phase.
Conclusions
In conclusion, appropriate conditions and monomer/crosslinker pairs have been identified,
which allowed us to produce a (highly porous) polyHIPE thin films. These thin films do not
requiring post-filling with electrolyte as is common practice, but are already prefilled with
ionic liquid electrolyte. Our approach will simplify the way in which thin film lithium ion
batteries can be produced while we are were able to achieve conductivities very similar to
that of pure liquid electrolyte. We believe this demonstrates an important advance in the
ability to realise more environmentally friendly and more cost effective thin film batteries via
multi-layer printing as the emulsion-templated separator material is suitable as an ink for high
throughput roll-to-roll production processes.
Acknowledgements
This work was funded as part of the GREENBAT project under the seventh framework
programme (FP 7) of the European Commission (FP7-ICT-2007-2).
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