Bicontinuous Electrolytes via Thermally Initiated Polymerization for Structural Lithium Ion Batteries

Abstract

Structural batteries (SBs) are a growing research subject worldwide. The idea is to provide massless energy by using a multifunctional material. 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 provide an energy storage function. It is a composite material that utilizes carbon fibers (CFs) 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 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 nontransparent CFs. The present paper investigates thermally initiated PIPS to prepare a bicontinuous SBE and an SB half-cell. In addition, the effect of curing temperature was examined with respect to curing performance, morphology, ionic conductivity, and 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.

A Structural Battery and its Multifunctional Performance

“…The battery cell stack was then impregnated with an SBE mixture using a pipet before the pouch bag was vacuum heat sealed. The SBE mixture was prepared in accordance with the procedure described in the study by Schneider et al, [ 23 ] with the only differences being that DMMP in the electrolyte was replaced with PC, the mixing ratio of monomer versus liquid electrolyte was taken, and that a LiBoB salt was added to prevent unwanted side reactions induced by the exclusive use of LiTf, as discussed in Supporting information. The SBE solution was made by mixing 50:50 wt% of 1) a liquid electrolyte solution made from the mixture of LiBoB and LiTf at concentrations of 0.4 and 0.6 M, respectively, in EC:PC 1:1 w/w (50:50 wt%) and 2) monomer bisphenol A ethoxylate dimethacrylate and the thermal initiator AIBN (1 wt% of the monomer weight).…”

“…Recently, a bicontinuous polymer electrolyte system, referred to as a structural battery electrolyte (SBE), was developed for structural battery composites by Ihrner et al [ 22 ] and later optimized for manufacturing by Schneider et al [ 23 ] The SBE consists of a porous methacrylate polymer (for mechanical load transfer) impregnated with a liquid electrolyte mixture containing Li salt for ionic conductivity. The SBE has a Young's modulus around 0.5 GPa and an ionic conductivity of 2 × 10 −4 S cm −1 .…”

Structural Batteries: A Review

“…Notwithstanding the notable results obtained with the UV-initiated polymerization-induced phase separation technique, this approach has a limitation: it cannot be used for the production of non-transparent and thick SBEs. Starting from this consideration, Schneider et al [ 19 ] demonstrated the possibility of manufacturing SBEs and unidirectional laminae electrodes via the thermally initiated polymerization-induced phase separation (PIPS) process. In this framework, the liquid electrolyte was mixed with bisphenol A dimethacrylate in a ≈40/60 wt% ratio; the compound was then placed in a mold and transferred in a preheated oven to undergo a curing cycle.…”

“…The first one focuses on adding functionalities to structures by embedding off-the-shelf thin batteries into composite laminates or sandwich panels [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 ]. The other one aims to realize multifunctional composite materials where the reinforcement elements act as the electrodes while the polymeric matrix works as the electrolyte and as a structural binder for the fibers [ 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. An alternative path could come from the use of all-solid-state electrolytes.…”

Shape-morphing carbon fiber composite using electrochemical actuation

“…However, these are structurally low performing and cannot be used to transfer mechanical load. An SBE has been developed recently for structural battery applications that adheres to carbon fibers, transfers mechanical load, conducts Li ions, and allows Li insertion into carbon fibers (25)(26)(27). The SBE consists of two phases: a structural polymer backbone for mechanical load transfer and a liquid for ion conductivity, forming an interpenetrating and percolating network on a nanoscale (Fig.…”