RESUMO
Lithium batteries with solid polymer electrolytes (SPEs) and mobile ions are prone to mass transport limitations, that is, concentration polarization, creating a concentration gradient with Li+-ion (and counter-anion) depletion toward the respective electrode, as can be electrochemically observed in, for example, symmetric Li||Li cells and confirmed by Sand and diffusion equations. The effect of immobile anions is systematically investigated in this work. Therefore, network-based SPEs are synthesized with either mobile (dual-ion conduction) or immobile anions (single-ion conduction) and proved via solvation tests and nuclear magnetic resonance spectroscopy. It is shown that the SPE with immobile anions does not suffer from concentration polarization, thus disagreeing with Sand and diffusion assumptions, consequently suggesting single-ion (Li+) transport via migration instead. Nevertheless, the practical relevance of single-ion conduction can be debated. Under practical conditions, that is, below the limiting current, the concentration polarization is generally not pronounced with DIC-based electrolytes, rendering the beneficial effect of SIC redundant and DIC a better choice due to better kinetical aspects under these conditions. Also, the observed dendritic Li in both electrolytes questions a relevant impact of mass transport on its formation, at least in SPEs.
RESUMO
Thanks to its high operating voltage, the LiNi0.5Mn1.5O4 (LNMO) spinel represents a promising next-generation cathode material candidate for Lithium ion batteries. However, LNMO-based full-cells with organic carbonate solvent electrolytes suffer from severe capacity fading issues, associated with electrolyte decomposition and concurrent degradative reactions at the electrode/electrolyte interface, especially at elevated temperatures. As promising alternatives, two selected LiTFSI/pyrrolidinium bis(trifluoromethane-sulfonyl)imide room temperature ionic liquid (RTIL) based electrolytes with inherent thermal stability were investigated in this work. Linear sweep voltammetry (LSV) profiles of the investigated LiTFSI/RTIL electrolytes display much higher oxidative stability compared to the state-of-the-art LiPF6/organic carbonate based electrolyte at elevated temperatures. Cycling performance of the LNMO/Li4Ti5O12 (LTO) full-cells with LiTFSI/RTIL electrolytes reveals remarkable improvements with respect to capacity retention and Coulombic efficiency. Scanning electron microscopy (SEM) images and X-ray diffraction (XRD) patterns indicate maintained pristine morphology and structure of LNMO particles after 50 cycles at 0.5C. The investigated LiTFSI/RTIL based electrolytes outperform the LiPF6/organic carbonate-based electrolyte in terms of cycling performance in LNMO/LTO full-cells at elevated temperatures.
RESUMO
To identify alternative single-solvent-based electrolytes for application in lithium-ion batteries (LIBs), adequate computational methods were applied to screen specified physicochemical and electrochemical properties of new cyanoester-based compounds. Out of 2747 possible target compounds, two promising candidates and two structurally equivalent components were chosen. A constructive selection process including evaluation of basic physicochemical properties as well assessing the compatibility towards graphitic anodes was initiated to identify the most promising candidates. With addition of a film-forming additive in a low concentration, the most promising candidate showed an adequate long-term cycling stability with LiNi1/3 Mn1/3 Co1/3 O2 [NMC(111)] in a full-cell setup using graphite as anode material. The main advantages of the new electrolyte formulation are related to its good thermal behavior, especially with regard to safety in combination with satisfying electrochemical performance.