Multifunctional Composite Binder for Thick High-Voltage Cathodes in Lithium-Ion Batteries.

High-voltage LiNi0.5Mn1.5O4 (LNMO) spinel offers high specific energy and good rate capability with relatively low raw-material cost due to cobalt-free and manganese-rich chemical compositions. Also, increasing mass loading (mg/cm2) by thickening cathodes has been one of the focused areas to greatly improve the energy density of lithium-ion batteries (LIBs) at the cell level. The LNMO cathode made with a polyvinylidene fluoride (PVdF) binder, however, suffers from an oxidative decomposition of liquid electrolytes and cathode delamination from a current collector. This problem is exacerbated with an increase in thickness. In this study, we developed a lithium polyacrylate (LiPAA)-sodium alginate (Na-Alg) composite binder series that offer positive multifunctions such as enhancing cathode adhesion and cohesion, improving cycle life, creating an effective passivating layer at the cathode-electrolyte interface (CEI), and lowering cell impedance. Comprehensive design of systematic experiments revealed a close chemo-mechano-electrochemical relationship in the thick high-voltage cathodes. Among the various binder compositions, the LiPAA (30 wt %)-Na-Alg (70 wt %) binder offered a strong adhesion property and positive multifunctions at the CEI layer, which consequently stabilized the solid-electrolyte interfacial (SEI) layer on the graphite anode and improved LIB performances. This novel composite binder will be applicable to various types of thick cathodes in future studies.

Ambient Pressure X-ray Photoelectron Spectroscopy Investigation of Thermally Stable Halide Perovskite Solar Cells via Post-Treatment.

Long-term thermal stability is one limiting factor that impedes the commercialization of the perovskite solar cell. Inspired by our prior results from machine learning, we discover that coating a thin layer of 4,4'-dibromotriphenylamine (DBTPA) on top of a CH3NH3PbI3 layer can improve the stability of resultant solar cells. The passivated devices kept 96% of the original power conversion efficiency for 1000 h at 85 °C in a N2 atmosphere without encapsulation. Near-ambient pressure X-ray photoelectron spectroscopy (XPS) was employed to investigate the evolution of the composition and evaluate thermal and moisture stability by in situ studies. A comparison between pristine MAPbI3 films and DBTPA-treated films shows that the DBTPA treatment suppresses the escape of iodide and methylamine up to 150 °C under 5 mbar humidity. Furthermore, we have used attenuated total reflection Fourier transform infrared and XPS to probe the interactions between DBTPA and MAPbI3 surfaces. The results prove that DBTPA coordinates with the perovskite by Lewis acid-base and cation-π interaction. Compared with the 19.9% efficiency of the pristine sample, the champion efficiency of the passivated sample reaches 20.6%. Our results reveal DBTPA as a new post-treating molecule that leads not only to the improvement of the photovoltaic efficiency but also thermal and moisture stability.