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Direct tracking of solid polymer electrolyte (SPE) decomposition in comparison to a liquid analogue was accomplished by monitoring the evolution of volatile species using online electrochemical mass spectrometry (OEMS). Reduction of a poly(trimethylene carbonate)-based SPE was dominated by CO₂ formation. Detection of CO₂ and an absence of CO confirms a preferred reduction degradation pathway involving C−O bond cleavage at the carbonyl carbon, in correlation with earlier suggestions. In contrast, the alkyl carbonate-based liquid electrolyte exhibited extensive ethylene formation. Trace quantities of H₂ evolution ascribed to water impurities were also observed in both systems. During oxidation, the SPE and liquid electrolyte exhibited CO₂, CO and SO₂ evolution synonymous with electrolyte solvent and salt degradation, albeit at different potentials. Overall, gas evolution rates and redox currents were lower in the SPE system. OEMS revealed significant gas formation independent of current response, as such highlighting the limitations of the voltammetry technique commonly used today to assess electrochemical stability.

3D-microbatteries (3D-MBs) impose new demands for theselection, fabrication and compatibility of the different battery components, notleast the electrolytes. Herein, solid polymer electrolytes (SPEs) based on poly(trimethylene carbonate) (PTMC) have been implemented in 3D-MB systems. 3D electrodes of two different architectures, LiFePO₄-coated carbon foams and Cu₂O-coated Cu nanopillars, have been coated with SPEs and used in Li-cells. Functionalized PTMC with hydroxyl end groups was found to enable uniform and well-covering coatings on LiFePO₄-coated carbon foams, although the cell cycling performance was limited by the large SPE resistance. By employing a SPE prepared from a copolymer of TMC and caprolactone (CL), with higher ionic conductivity, Li-cells composed of Cu₂O-coated Cu nanopillars were constructed and tested both at room temperature and 60 °C. The footprint areal capacity of the cells was ca. 0.02 mAh cm^-2 for an area gain factor (AF) of 2.5, and 0.2 mAh cm^-2 for a relatively dense nanopillar-array (AF=25) at a current density of 0.008 mA cm^-2at ambient temperature (22±1 °C). These results provide new routes towards the realization of all-solid-state 3D-MBs.

Among the alternative host materials for solid polymer electrolytes (SPEs), polycarbonates have recently shown promising functionality in all-solid-state lithium batteries from ambient to elevated temperatures. While the computational and experimental investigations of ion conduction in conventional polyethers have been extensive, the ion transport in polycarbonates has been much less studied. The present work investigates the ionic transport behavior in SPEs based on poly(trimethylene carbonate) (PTMC) and its co-polymer with epsilon-caprolactone (CL) via both experimental and computational approaches. FTIR spectra indicated a preferential local coordination between Li+ and ester carbonyl oxygen atoms in the P(TMC20CL80) co-polymer SPE. Diffusion NMR revealed that the co-polymer SPE also displays higher ion mobilities than PTMC. For both systems, locally oriented polymer domains, a few hundred nanometers in size and with limited connections between them, were inferred from the NMR spin relaxation and diffusion data. Potentiostatic polarization experiments revealed notably higher cationic transference numbers in the polycarbonate based SPEs as compared to conventional polyether based SPEs. In addition, MD simulations provided atomic-scale insight into the structure-dynamics properties, including confirmation of a preferential Li+-carbonyl oxygen atom coordination, with a preference in coordination to the ester based monomers. A coupling of the Li-ion dynamics to the polymer chain dynamics was indicated by both simulations and experiments.

Pressing demands for high power and high energy densities in novel electrical energy storage units have caused reconsiderations regarding both the choice of battery chemistry and design. Practical concerns originating in the conventional use of flammable liquid electrolytes have renewed the interests of using solvent-free polymer electrolytes (SPEs) as solid ionic conductors for safer batteries.

In this thesis work, SPEs developed from two polymer host structures, polyethers and polycarbonates, have been investigated for all-solid-state Li- and Li-ion battery applications. In the first part, functional polyether-based polymer electrolytes, such as poly(propylene glycol) triamine based oligomer and poly(propylene oxide)-based acrylates, were investigated for 3D-microbattery applications. The amine end-groups were favorable for forming conformal electrolyte coatings onto 3D electrodes via self-assembly. In-situ polymerization methods such as UV-initiated and electro-initiated polymerization techniques also showed potential to deposit uniform and conformal polymer coatings with thicknesses down to nano-dimensions.

Moreover, poly(trimethylene carbonate) (PTMC), an alternative to the commonly investigated polyether host materials, was synthesized for SPE applications and showed promising functionality as battery electrolyte. High-molecular-weight PTMC was first applied in LiFePO₄-based batteries. By incorporating an oligomeric PTMC as an interfacial mediator, enhanced surface contacts at the electrode/SPE interfaces and obvious improvements in initial capacities were realized. In addition, room-temperature functionality of PTMC-based SPEs was explored through copolymerization of ε-caprolactone (CL) with TMC. Stable cycling performance at ambient temperatures was confirmed in P(TMC/CL)-based LiFePO₄ half cells (e.g., around 80 and 150 mAh g^-1 at 22 °C and 40 °C under C/20 rate, respectively). Through functionalization, hydroxyl-capped PTMC demonstrated good surface adhesion to metal oxides and was applied on non-planar electrodes. Ionic transport behavior in polycarbonate-SPEs was examined by both experimental and computational approaches. A coupling of Li ion transport with the polymer chain motions was demonstrated.

The final part of this work has been focused on exploring the key characteristics of the electrode/SPE interfacial chemistry using PEO and PTMC host materials, respectively. X-ray photoelectron spectroscopy (XPS) was used to get insights on the compositions of the interphase layers in both graphite and LiFePO₄ half cells.  

Polymer electrolytes were prepared from an aliphatic polycarbonate with 10 mol% of repeating units having a hydroxyl-functional side group, with the addition of LiTFSI salt. The hydrogen bond-interacting side groups were found to be beneficial for improving adhesion to 2D planar electrode material surfaces. These favorable surface properties proved to be valid also for 3D-structured systems since thin, conformal coatings could be cast on 3D-microstructured electrodes. In addition, the electrolytes were found to have reasonable ionic conductivity (up to 2.7 x 10(-8) S cm(-1) at 25 degrees C and 2.3 x 10(-6) S cm(-1) at 60 degrees C) that was almost independent of salt concentration. This demonstrates how a hydroxyl-functional polymer approach is suitable for the creation of 3D-structured electrode-electrolyte assemblies for microbattery applications.

This paper presents atomistic molecular dynamics simulation studies of lithium bis(trifluoromethane) sulfonylimide (LiTFSI) in a blend of 1-ethyl-3-methylimidazolium (EMIm)-TFSI and poly(ethylene oxide) (PEO), which is a promising electrolyte material for Li- and Li-ion batteries. Simulations of 100 ns were performed for temperatures between 303 K and 423 K, for a Li:ether oxygen ratio of 1:16, and for PEO chains with 26 EO repeating units. Li+ coordination and transportation were studied in the ternary electrolyte system, i.e., PEO16LiTFSI center dot 1.0 EMImTFSI, by applying three different force field models and are here compared to relevant simulation and experimental data. The force fields generated significantly different results, where a scaled charge model displayed the most reasonable comparisons with previous work and overall consistency. It is generally seen that the Li cations are primarily coordinated to polymer chains and less coupled to TFSI anion. The addition of EMImTFSI in the electrolyte system enhances Li diffusion, associated to the enhanced TFSI dynamics observed when increasing the overall TFSI anion concentration in the polymer matrix. (C) 2015 AIP Publishing LLC.

The first characterization studies of the interface layer formed between a Li-ion battery electrode and a solid polymer electrolyte (SPE) are presented here. SPEs are well known for their electrochemical stability and excellent safety, and thus considered good alternatives to conventional liquid/gel electrolytes in high-energy density battery devices. This work comprises studies of solid electrolyte interphase (SEI) formation in SPE-based graphite|Li cells using X-ray photoelectron spectroscopy (XPS). SPEs based on high molecular weight poly(ethylene oxide) (PEO) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt are studied. Large amounts of LiOH are observed, and the XPS results indicate a correlation with moisture contamination in the SPEs. The water contents are quantitatively determined to be in the range of hundreds of ppm in the pure PEO as well as in the polymer electrolytes, which are prepared by a conventional SPE preparation method using different batches of PEO and at different drying temperatures. Moreover, severe salt degradation is observed at the graphite-SPE interface after the 1st discharge, while the salt is found to be more stable at the Li-SPE interface or when using LiTFSI-based liquid electrolyte equivalents.

This paper reports the synthesis and application of high-molecular-weight poly(trimethylene carbonate) (PTMC) as a new host material for solid polymer electrolyte-based Li-ion batteries. PTMC was synthesized through bulk ring-opening polymerization of the cyclic monomer to yield a high-molecular-weight polymer to serve as a base material for the electrolytes. The thermal properties and ionic conductivity of polymer electrolytes with different salt ratios were measured by TGA/DSC and electrochemical impedance spectroscopy, respectively. The most conductive systems were found at [Li⁺]:[carbonate] ratios of 1:13 and 1:8, which showed electrochemical stability up to 5.0 V vs. Li/Li⁺ and an ionic conductivity on the order of 10^− 7 Scm(-1) at 60 °C. LiFePO₄ half-cells using the electrolytes demonstrated a plateau in the specific discharge capacity around 153 mAhg(-1) after long-term cycling. The functionality of the electrolytes for three-dimensional microbatteries was also confirmed.