Analysis of Transport Properties of Polycarbonate-Based Polymer Electrolytes Containing Lithium (perfluoroalkylsulfonyl)Imides for Possible Application in Lithium-Ion Batteries

The search for safe electrolytes for the use in lithium metal batteries and high energy density batteries has driven the interest in alternative polymer electrolytes with enhanced performance. The central interest focuses on transport properties as good ionic conductivities and on high electrochemical stability towards lithium metal anodes and cathodes for high voltage batteries. To gain insight in the Li-ion movement in polycarbonates, as dry or gel electrolyte, is necessary for understanding how to improve these systems and achieve better performances. In this context, we present a critical study on polycarbonates as alternative to polyethylene oxide (PEO) in salt-in-polymer electrolytes. [1]Polymer electrolytes based on PEO as well as some related with grafted PEO side chains along polymer backbones such as polyphosphazenes, polysiloxanes and others are known to reach ionic conductivities in the range of 10-3 - 10-4 S·cm-1 in a favourable temperature range and typical Li+ transference numbers around 0.2.[2,3] In contrast to that, polycarbonates were reported to exhibit transference numbers up to about 0.5 as well as higher ionic conductivities and electrochemical stability.[4]In this work, the commercially available polycarbonates polyethylene carbonate (PEC) and polypropylene carbonate (PPC) were prepared as flexible self-standing membranes with dissolved fluorinated lithium salts such as LiTFSI and others. With the use of propylene carbonate (PC) novel gel-polymer electrolytes have been fabricated. The ion conducting polymer membranes were examined regarding their electrochemical and transport characteristics with respect to possible battery applications. We studied ionic conductivity and Li-ion transference number as well as the coordination of lithium with different dissolved lithium salts on carefully purified polycarbonate samples. We obtained ionic conductivities in dry state around 10-6 S·cm-1 at room temperature and up to 10-3 in gelled systems. The novel gel systems based on PEC and PPC perform significantly better than the dry alternatives

New Insights into the Behavior of Polycarbonate-Based Lithium Salt Containing Electrolytes Regarding Thermal Long-Term and Electrochemical Stability

For lithium ion batteries solid-state electrolytes like polymers gained more and more attention in the last years, especially due to safety reasons. Polyethylene oxide (PEO) based systems are still state of the art. One of its major drawbacks is the highly crystalline structure, which inhibits fast migration of lithium ions and results in a low transference number and low conductivity [1]. Recently, polycarbonate based polymer electrolytes gained some interest [2,3]. Polycarbonates were assumed to show weaker ion-dipole interactions in contrast to PEO, given rise to higher transference numbers [2]. With suitable additives and high salt concentrations, higher ionic conductivities were reported at moderate temperatures (>60 °C) [3].Nevertheless, there is still a lack of knowledge concerning long-term performance of the polycarbonate-salt-mixtures as electrolytes in terms of thermal stability in the temperature range of solid-state batteries. Therefore, we prepared lithium salt containing polycarbonate samples using either solution casting with additional removal of the solvent or the hot press method. We examined the behavior of polycarbonate salt mixtures over a period of time at given temperatures. Investigation methods such as differential scanning calorimetry (DSC), nuclear magnetic resonance spectroscopy (NMR) and electrochemical impedance spectroscopy (EIS) were used. The results of the experiments indicate that e. g. the mixture LiTFSI in polyethylene carbonate (PEC) suffers from a salt catalyzed decomposition for longer times whereas polypropylene carbonate (PPC) is stable.Furthermore, candidates with good thermal long-term stability (e. g. PPC/LiTFSI) were analyzed concerning their ionic conductivity and their electrochemical stability window. If necessary, improvements were made to increase the electrochemical stability (e. g. down to lithium stripping / plating) or the conductivity by use of different additives, e.g. plasticizers and SEI additives

Polypropylene carbonate-based electrolytes as model for a different approach towards improved ion transport properties for novel electrolytes

Linear poly(alkylene carbonates) such as polyethylene carbonate (PEC) and polypropylene carbonate (PPC) have gained increasing interest due to their remarkable ion transport properties such as high Li+ transference numbers. The cause of these properties is not yet fully understood which makes it challenging to replicate them in other polymer electrolytes. Therefore, it is critical to understand the underlying mechanisms in polycarbonate electrolytes such as PPC. In this work we present insights from impedance spectroscopy, transference number measurements, PFG-NMR, IR and Raman spectroscopy as well as molecular dynamics simulations to address this issue. We find that in addition to plasticization, the lithium ion coordination by the carbonate groups of the polymer is weakened upon gelation, leading to a rapid exhange of the lithium ion solvation shell and consequently a strong increase of the conductivity. Moreover, we study the impact of the anions by employing different conducting salts. Interestingly, while the total conductivity decreases with increasing anion size, the reverse trend can be observed for the lithium ion transference numbers. Via our holistic approach, we demonstrate that this behavior can be attributed to differences in the collective ion dynamics