There is a strong need in developing stretchable batteries that can accommodate stretchable or irregularly shaped applications including medical implants, wearable devices and stretchable electronics. Stretchable solid polymer electrolytes are ideal candidates for creating fully stretchable lithium ion batteries mainly due to their mechanical and electrochemical stability, thin-film manufacturability and enhanced safety. However, the characteristics of ion conductivity of polymer electrolytes during tensile deformation are not well understood. Here, we investigate the effects of tensile strain on the ion conductivity of thin-film polyethylene oxide (PEO) through an in situ study. The results of this investigation demonstrate that both in-plane and through-plane ion conductivities of PEO undergo steady and linear growths with respect to the tensile strain. The coefficients of strain-dependent ion conductivity enhancement (CSDICE) for in-plane and through-plane conduction were found to be 28.5 and 27.2, respectively. Tensile stress-strain curves and polarization light microscopy (PLM) of the polymer electrolyte film reveal critical insights on the microstructural transformation of stretched PEO and the potential consequences on ionic conductivity.

Flexible and stretchable energy storage devices, including batteries, supercapacitors, and ionic piezoelectrics, have garnered substantial research interest in recent years to address a wide range of applications such as smart textiles and medical implants. These devices are intended to undergo mechanical deformation, and the impact of deformation on electrochemical performance is not well understood. One important area of focus is studying how mechanical deformation influences ion conduction in polymer electrolytes. In this work, a dual theoretical and experimental approach is taken to further evaluate this phenomenon. A stretchable solid polymer electrolyte film subjected to tensile deformation (approximately 48% strain), through which ion diffusion occurs, is analyzed using a continuum approach treating ion diffusion and mechanical deformation as coupled. Thermodynamic laws are applied to obtain governing multiphysics equations accounting for large deformation mechanics and material nonlinearity. The theoretical solution obtained demonstrates how through-plane ionic conductivity changes when the polymer is subjected to stretching. Evolutionary materials deformation of the polymer electrolyte is considered to elucidate the underlying driving physical mechanisms of ion conduction. An experiment was also conducted to measure change in through-plane ionic conductivity with applied uniaxial strain in a sample of polyethylene oxide (PEO), a material commonly used as the electrolyte in solid polymer electrolyte lithium ion batteries. The experimental results show a greater than 1600% ionic conductivity enhancement for approximately 48% strain. The theoretical and experimental results are in good agreement and show that ion conduction is enhanced with increasing strain following an exponential function for a PEO electrolyte.

Stretchable batteries are needed to accommodate deformable geometries in tantalizing applications such as smart textiles, biomedical implants, and stretchable electronics. An increasing number of studies have focused on flexible and bendable batteries, but very few have investigated a stretchable lithium ion battery in which some or all components, including the electrodes, electrolyte, and encapsulation may be stretched. Here, we report the design, fabrication and characterization of a stretchable-sliding battery where the electrodes can slide, and the solid polymer electrolyte is stretched. The battery consists of a single solid polymer electrolyte film sandwiched between two sliding layered electrodes on each side. The two cathode layers are based on LiFePO4 active material, and the two anode layers are graphite based. The stretchable polymer electrolyte is composed of a specific blend of polyethylene oxide (PEO) of 100k and 600k molecular weights to enhance both the ionic conductivity and mechanical properties. Results show that the capacity of the stretchable-sliding battery increases at small tensile strains, but can degrade at larger strains. Tensile stress-strain curves of the stretchable battery and its components until failure are also presented. In situ strain-dependent electrochemical measurements provide critical insights on the stretching and sliding mechanisms in the battery. This study further validates the dual-functionality of the PEO solid electrolyte as both a stretchable film and a lithium ion conductor in a charged/discharged battery. This stretchable-sliding battery configuration can offer an experimental platform for in situ characterizations of solid polymer electrolyte films subjected to stretching inside an active electrochemical cell.

There is a strong need in developing stretchable batteries that can accommodate stretchable or irregularly shaped applications including medical implants, wearable devices and stretchable electronics. There has been a fair amount of work exploring the development and performance of stretchable electrodes, but very little for stretchable electrolytes.  Solid polymer electrolytes are ideal candidates for creating fully stretchable lithium ion batteries because of their mechanical and electrochemical stability, thin-film manufacturability and enhanced safety. However, the characteristics of ion conductivity of polymer electrolytes during tensile deformation are not well understood. We have investigated the effects of tensile strain on the ion conductivity of thin-film polyethylene oxide (PEO) through an in situ study.  The results of this investigation demonstrate that both in-plane and through-plane ion conductivities of PEO undergo steady and linear growth with respect to the tensile strain.  This increasing trend in conductivity infers that the structural changes induced in the polymer electrolyte results in altered and improved ion conduction.  The coefficients of strain-dependent ion conductivity enhancement (CSDICE) for in-plane and through-plane conduction were found to be 28.5 and 27.2, respectively. We hypothesize that the stretching and aligning of the amorphous polymer chains decreases the degree of tortuosity in the polymer, allowing for faster, and less obstructed ion transport. Semi-crystalline PEO consists of a crystalline phase and an amorphous phase.  The amorphous phase is generally present along the edges of the crystallites where the polymer chains are disordered, twisted and entangled and tie one crystallite to another.  The semi-crystalline conformation of PEO can be seen using polarization light microscopy, which confirms the growth and extension of the amorphous regions as the chains stretch and disentangle due to tensile strain. In conclusion, the present work confirms the feasibility of using solid polymer PEO as a stretchable electrolyte for next generation stretchable batteries.