Bicontinuous electrolytes via thermally initiated polymerization for

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Bicontinuous electrolytes via thermally initiated polymerization for structural lithium ion batteries Lynn Maria Schneider, Niklas Ihrner, Dan Zenkert, and Mats Johansson ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00563 • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019

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ACS Applied Energy Materials

Bicontinuous electrolytes via thermally initiated polymerization for structural lithium ion batteries Lynn M. Schneider1, 2, Niklas Ihrner 2, Dan Zenkert 1, Mats Johansson* 1

2

Department of Aeronautical and Vehicle Engineering, KTH Royal Institute of Technology, SE-

100 44 Stockholm, Sweden 2

Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, SE-100 44

Stockholm, Sweden Keywords: thermal polymerization; lithium-ion conductivity; polymerization induced microphase separation; bicontinuous morphology; polymer electrolyte matrices ABSTRACT

Structural batteries (SB) are a growing research subject worldwide. The idea is to provide massless energy by using a multifunctional material i.e. structural battery. This technology can provide a new pathway in electrification and offer different design opportunities and significant weight savings in vehicle applications. The type of SB discussed here is a multifunctional material that can carry mechanical loads and simultaneously provides an energy storage function. It is a composite material that utilizes carbon fibers (CF) as electrodes and structural reinforcement which are embedded in a multifunctional polymer matrix (i.e. structural battery electrolyte). A feasible composite manufacturing method still needs to be developed to realize a full cell SB. UV

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initiated polymerization induced phase separation (PIPS) has previously been used to make bicontinuous structural battery electrolytes (SBE) with good ionic conductivity and mechanical performance. However, UV curing cannot be used for fabrication of a full cell SB since a full cell is made of multiple layers of non-transparent CFs. The present paper investigates thermally initiated PIPS to prepare a bicontinuous SBE and a SB half-cell. In addition, the effect of curing temperature was examined with respect to curing performance, morphology, ionic conductivity, mechanical and electrochemical performance. The study revealed that thermally initiated PIPS provides a robust and scalable process route to fabricate SBs. The results of this study are an important milestone in the development of SB technology as they allow for the SB fabrication for an actual application. However, other challenges still remain to be solved before this technology can be introduced into an application.

INTRODUCTION Multifunctional structures present an innovative concept to increase system efficiency as two or more functions are combined in one material (1,2). A SB is one example for a multifunctional lightweight material that enables energy storage and structural integrity simultaneously (3). A SB is a technological concept that resembles a standard fiber composite laminate. CF composites are lightweight materials with high structural performance. In a SB, this structural composite can simultaneously function a battery, creating a multifunctional material. In this technology, multifunctionality is introduced by using multifunctional battery constituents such as CFs embedded in an electrolyte/ matrix (i.e. structural battery electrolyte). CFs are used as high performance structural backbone and as electrodes. In a SB concept, fibers and separator are laid up in the sequence: negative electrodes/ separator/ positive electrodes. For the negative electrode,


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commercial CFs can be used since Li ions can be intercalated, similar to a state-of-art Li-ion battery. The capacity also depends on the type of CF used and for most intermediate modulus polyacrylonitrile based CFs, the capacity stabilizes around 200-250 mAh g-1 in commercial liquid electrolyte (4,5). The separator could be a thin glass fiber weave or veil. The positive electrode can be made of carbon fibers coated with an electrochemically active material, e.g. LiFePO4 (6). The fiber stack is embedded in a SBE, which is a key component to achieve multifunctionality since it has to transfer mechanical loads between the fibers and provide ionic conductivity. The SB technology is particularly interesting for applications in the vehicle sector as it allows for significant weight savings on a system level. Substantial weight savings could be realized by using SBs in loadbearing structures (7), e.g. in the body panel of a car, the interior of an airplane or for electric ferries. Weight reduction, e.g. in an airplane would improve fuel economy and thus reduce carbon dioxide emissions. For electric vehicles, SBs could increase driving range and also open up new design opportunities. The SB technology is still in an early development stage and before a full cell SB can be established into an application some challenges have to be solved (8). One of the major engineering challenges to realize a full cell SB is to develop a feasible composite manufacturing process. The manufacturing process should be robust, scalable, provide high system performance and it should be suitable for curing a multilayer structure (full cell SB). A feasible process allowing to fabricate a multiple layer stack of full cells while maintaining electrochemical and mechanical properties of the SB has not been developed yet. Liquid composite molding (LCM) is a well-established composite processing method for thermosets that is suitable for a large variety of geometrical shapes and sizes and requires low financial investment (9). Important aspects of LCM include the infusion and consolidation stages. Hence, the flow of the uncured resin must be controlled as well as the subsequent curing steps in order to manufacture a



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SB (i.e. multifunctional composite material) with desired performance. These aspects are both related to the properties and processing of the polymer matrix. The SBE is a crucial component in the SB since it needs to conduct Li-ions and transfer mechanical loads between the CFs. Previous work has investigated homogenous (i.e. uniform) electrolyte systems such as liquid electrolytes, gel polymer electrolytes (GPE) and solid polymer electrolytes (SPE). Liquid electrolytes are most commonly used due to their high ionic conductivity around 10-2 S cm-1 at ambient temperature (10). However, they do not provide any mechanical performance and also give rise to safety issues such as leakage, flammability and thermal instability. SPEs and GPEs are being researched as alternative electrolytes mainly to address the safety issues but also to offer multifunctionality (1,11). GPEs can achieve higher conductivities but lack mechanical performance (12,13) for utilization in a SB concept. SPEs show better mechanical performance and are safer to use but the ionic conductivity is lower than 10-4 S cm-1 (12,14,15) A common correlation for SPEs is that as the mechanical performance of the electrolyte increases, the ionic conductivity decreases (16). Advantageous alternatives for SBEs are heterogeneous electrolytes. Heterogenous electrolytes are systems that combine at least two different material phases to form at least two percolating structures on a sub-micron scale. A type of heterogeneous electrolyte is for example a liquid-solid system. The liquid phase provides adequate ionic conductivity while an amorphous polymer network provides mechanical integrity. These systems can be made using different routes involving, for example, vinyl ester or epoxy chemistries (chain- or step wise polymerization, respectively) combined with commercial liquid electrolytes or ionic liquids (17-20). Lodge and co-workers presented different combinations of systems containing ionic liquids providing high ionic conductivity and high mechanical performance simultaneously (21-24). However, the use of


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ionic liquids can be problematic for battery applications due to low Li-ion transfer numbers (25,26). Ihrner et al. (27) established a phase-separated methacrylate based SBE involving organic electrolyte for SBs. This system shows an elastic modulus around 360 MPa and an ionic conductivity around 2 x 10-4 S cm-1 at ambient temperature. The utilized monomers are close in resemblance to conventional vinyl ester composite matrices. Therefore, a direct comparison with respect to mechanical performance with conventional vinyl ester matrices is feasible. Heterogeneous electrolytes can be prepared using PIPS (i.e. reaction induced phase separation) based on an UV-initiated radical chain polymerization. Lodge and co-workers introduced PIPS as a concept to make heterogeneous electrolytes on step-wise polymerizing systems in combination with ionic liquids (21,22). PIPS is a one-pot reaction that is based on solubility differences between the polymer and the monomer used to build it. The phase separation is influenced by a number of factors including: relative solubility parameters between ingoing components, reaction mechanism (step- vs chain-wise polymerization), temperature, and kinetics. These parameters also change when changing the processing method e.g. from UV curing to thermal curing. The concept of PIPS has previously been investigated in other multifunctional material concepts such as extended wear contact lenses (28) and dental composite materials (29). Ihrner et al. (27) used an UV-initiated PIPS with a free radical chain-wise polymerization mechanism. A chain-wise polymerization differs fundamentally from the step-wise polymerizing system (30) used by Lodge and co-workers (21,22). Chain-wise polymerizing systems for example often produce a significant amount of reaction heat that will determine the thermal history of the system, which will affect the materials properties and the process design (31). Composites made of multiple layers of non-transparent CF (e.g. a full cell SB) cannot be manufactured using UV curing because the UV light will just penetrate the surface and thus curing of a full cell SB would



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be incomplete and inhomogeneous. Therefore, UV initiated PIPS cannot be used to fabricate a full cell SB. An alternative process route is to use thermal curing which is a well-known technique in the composite industry (32). Thermally initiated PIPS is likely to affect the final properties of the SBE system since reaction kinetics and thermal history of the sample during cure are inherently different compared to UV curing. In thermal curing, the curing temperature is increased by additional heat of the exothermic reaction which influences the thermal history of the sample. Glauser et al. (31) showed that thermo-mechanical properties of the systems are affected by the thermal history of the sample during cure. Thermal curing of thermosets also makes it more unlikely for vitrification to occur in an early stage of the reaction. Sample geometry, molds and packaging as well as other components in the system will also influence the curing behavior. By way of example, CFs can act as heat sinks that reduce the heat in the system caused by the exothermic reaction (31). A lower temperature in the specimen during cure can be beneficial to avoid thermal degradation of the material, particularly for thick specimens that do not allow for sufficient heat dissipation (33). The present work demonstrates a suitable process to fabricate multilayer stacks of full cell SBs using thermal curing. This process allows to use an ordinary composite manufacturing that is scalable and complies with limitations set by the battery constituents and to form a multifunctional CF electrode. The phase separation of the bicontinuous SBE and multifunctional performance was studied on pure SBE films and on a UD laminae negative electrode. The multifunctionality was evaluated with respect to morphology, curing kinetics, mechanical properties and electrochemical performance both as a free-standing film and on a composite level.

EXPERIMENTAL


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Materials Bisphenol A ethoxylate dimethacrylate (Mn: 540 g mol-1) was supplied by Sartomer Company, Europe. Dimethyl methylphosphonate (97%) (DMMP), ethylene carbonate (99% anhydrous) (EC), lithium trifluoromethanesulfonate (LiTFS) (96%) and 2,2′-Azobis(2-methylpropionitrile) (AIBN) were purchased from Sigma-Aldrich. CFs of the type T800S from Toray were provided as 10 mm spread tows from Oxeon AB. The linear weight of the fibers was 0.52 g m-1. Before use, DMMP, EC and LiTFS were heated to remove residual moisture. All other materials were used as received. Techniques and Procedures Structural Battery Electrolyte Sample Preparation SBE mixtures were prepared in a glovebox under argon atmosphere and dry conditions (

Bicontinuous electrolytes via thermally initiated polymerization for

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