Improving the Stability of Ball-Milled Lead Halide Perovskites via Ethanol/Water-Induced Phase Transition.

Recently, lead halide perovskite nanocrystals have been considered as potential light-emitting materials because of their narrow full width at half-maximum (FWHM) and high photoluminescence quantum yield (PLQY). In addition, they have various emission spectra because the bandgap can be easily tuned by changing the size of the nanocrystals and their chemical composition. However, these perovskite materials have poor long-term stability due to their sensitivity to moisture. Thus far, various approaches have been attempted to enhance the stability of the perovskite nanocrystals. However, the required level of stability in the mass production process of perovskite nanocrystals under ambient conditions has not been secured. In this work, we developed a facile two-step ball-milling and ethanol/water-induced phase transition method to synthesize stable CsPbBr3 perovskite materials. We obtained pure CsPbBr3 perovskite solutions with stability retention of 86% for 30 days under ambient conditions. Our materials show a high PLQY of 35% in solid films, and excellent thermal stability up to 80 °C. We believe that our new synthetic method could be applicable for the mass production of light-emitting perovskite materials.

Understanding the Surface Film Formation on Si Electrodes in Lithium Secondary Batteries with Atomic Force Microscopy.

The solid electrolyte interphase formation on the negative electrodes of lithium secondary batteries has been considered as one of the principal issues limiting the performance of batteries. Si is an attractive electrode material for improving energy density of lithium secondary batteries because of its high specific theoretical capacity (4200 mAh g-1). However, solid electrolyte interphase formation on Si-based electrodes have not been clearly understood in spite of its significance. Herein, the solid electrolyte interphase formation on Si electrodes in electrolyte solutions containing ethylene carbonate or propylene carbonate was investigated by using in-situ atomic force microscopy. Large and irreversible capacity fade in SiO electrodes was confirmed in both electrolyte solutions through cyclic voltammetry and charge/discharge testing. The in-situ atomic force microscopy results indicated that the decomposition reaction occurred in the ethylene carbonate-based electrolyte solution at a potential of ~0.68 V, while the lithium alloying reaction occurred below 0.25 V during the first reduction process. The decomposition reaction was more vigorous and occurred at a higher potential in the propylene carbonate-based electrolyte solution, resulting in the formation of a thick solid electrolyte interphase film. These results suggest that the solid electrolyte interphase formation on Si electrodes is strongly influenced by the composition of the electrolyte solution.

High Lithium Ion Transport Through rGO-Wrapped LiNi0.6Co0.2Mn0.2O2 Cathode Material for High-Rate Capable Lithium Ion Batteries.

In this work, we show an effective ultrasonication-assisted self-assembly method under surfactant solution for a high-rate capable rGO-wrapped LiNi0.6Co0.2Mn0.2O2 (Ni-rich cathode material) composite. Ultrasonication indicates the pulverization of the aggregated bulk material into primary nanoparticles, which is effectively beneficial for synthesizing a homogeneous wrapped composite with rGO. The cathode composite demonstrates a high initial capacity of 196.5 mAh/g and a stable capacity retention of 83% after 100 cycles at a current density of 20 mA/g. The high-rate capability shows 195 and 140 mAh/g at a current density of 50 and 500 mA/g, respectively. The high-rate capable performance is attributed to the rapid lithium ion diffusivity, which is confirmed by calculating the transformation kinetics of the lithium ion by galvanostatic intermittent titration technique (GITT) measurement. The lithium ion diffusion rate (D Li) of the rGO-wrapped LiNi0.6Co0.2Mn0.2O2 composite is ca. 20 times higher than that of lithium metal plating on anode during the charge procedure, and this is demonstrated by the high interconnection of LiNi0.6Co0.2Mn0.2O2 and conductive rGO sheets in the composite. The unique transformation kinetics of the cathode composite presented in this study is an unprecedented verification example of a high-rate capable Ni-rich cathode material wrapped by highly conductive rGO sheets.

Interfacial Reactions Between Graphite and Diethyl Carbonate Solutions Containing Different Concentrations of Li.

The electrochemical reactions occurring at graphite electrodes in lithium-ion batteries using diethyl carbonate (DEC) electrolytes containing different concentrations of LiClO4 (0.85 and 2.82 mol kg-1) were examined. The poor charge-discharge performance of the electrode in the 0.85 mol kg-1 LiClO4 in DEC was improved considerably when employing 2.82 mol kg-1 LiClO4 in DEC as the electrolyte. In situ electrochemical atomic force microscopy analysis revealed that an effective film had formed on the graphite surface after the first potential cycle when using the more concentrated solution. Charge-discharge analysis revealed that the nature of the film was significantly different from that formed in ethylene carbonate-based solutions.

Effects of Electrolyte Concentration on Surface Film Formation on Graphite in Ethylene Carbonate-Based Solutions.

The physicochemical properties of a surface film generated on a graphite surface were greatly dependent on the concentration of the electrolyte solution. Cyclic voltammetry results obtained using four ethylene carbonate-based solutions of different electrolyte (i.e., LiN(SO2C2F5)2) concentrations showed that the amount of irreversible current used to generate the surface film between 0.8 and 0.1 V decreased upon increasing the electrolyte concentration. Transmission electron microscopy results revealed that the thickness and morphology of the surface film were greatly affected by the concentration of the electrolyte solution. In addition, electrochemical impedance spectroscopy revealed that the resistance of the surface film was also affected by the electrolyte concentration. These results therefore indicated that both the chemical and physical properties of the surface film were affected by the concentration of the electrolyte solution.

Electrochemical Solvent Cointercalation into Graphite in Propylene Carbonate-Based Electrolytes: A Chronopotentiometric Characterization.

Interfacial reactions strongly influence the performance of lithium-ion batteries, with the main interfacial reaction between graphite and propylene carbonate- (PC-) based electrolytes corresponding to solvent cointercalation. Herein, the redox reactions of solvated lithium ions occurring at the graphite interface in 1 M·LiClO4/PC were probed by chronopotentiometry, in situ atomic force microscopy (AFM), and in situ Raman spectroscopy. The obtained results revealed that high coulombic efficiency (97.5%) can be achieved at high current density, additionally showing the strong influence of charge capacity on the above redox reactions. Moreover, AFM imaging indicated the occurrence of solvent cointercalation during the first reduction, as reflected by the presence of hills and blisters on the basal plane of highly oriented pyrolytic graphite subjected to the above process.

Correlations Between Electrolyte Concentration and Solid Electrolyte Interphase Composition in Electrodeposited Lithium.

This study examined the electrochemical deposition and dissolution of lithium on nickel electrodes in propylene carbonate (PC) electrolytes containing different concentrations of lithium salts, including LiN(SO2C2F5)2 or LiPF6. The electrode reactions were significantly affected by the electrolyte concentration. The cyclability of the electrodes was considerably improved by increasing the electrolyte concentration. X-ray photoelectron spectroscopy (XPS) showed that the composition of the solid electrolyte interphase (SEI) was also affected by the electrolyte concentration. The SEI formed in the 1st cycle consisted mainly of LiF in 1 and 2.15 M LiN(SO2C2F5)2/PC solutions. After the 30th cycle in the former solution, there was a large decrease in the amount of LiF and a large increase in the amount of LiOH. On the other hand, in the latter solution there was a smaller decrease and a smaller increase in the amount of LiF and LiOH, respectively, as compared to the former solution after the 30th cycle.

Structural Properties of Solid Electrolyte Interphase on Lithium Metal.

To gain an understanding of the structural properties of the solid electrolyte interphase (SEI) created on the surface of lithium during an electrochemical deposition/dissolution reaction in propylene carbonate (PC) containing different concentrations of LiClO4 (1.09, 3.27 mol kg(-1)), the surface of lithium was examined through transmission electron microscopy (TEM) after 30 cycles. The poor cyclability of the electrodes in the 1.09 mol kg(-1) solution was improved considerably when employing 3.27 mol kg(-1) solution. TEM showed that the latter solution produced a thinner SEI with smaller resistance on the electrode posited lithium than the former solution, e.g., -120 nm in 1.28 mol kg(-1) versus -34 nm in 3.27 mol kg(-1) solutions. Results of chemical treatment using chloroform to dissolve part of the components constituting the SEI suggested that the principal component of SEI in direct contact with the lithium was an inorganic material, while the component of SEI directly contacting the electrolyte was an organic material. That is, the SEI consisted of 2 layers, which were almost of equal thickness. These results partly supported the compact-stratified layer (CSL) model previously presented. In this study, a modified CSL model of the SEI structure was presented based on the results obtained through TEM analyses.

Atomic force microscopy study on the stability of a surface film formed on a graphite negative electrode at elevated temperatures.

The stability at elevated temperatures of a solid electrolyte interphase (SEI) formed on a graphite negative electrode in lithium ion batteries was investigated by storage tests and in situ atomic force microscopy (AFM) observation. When the fully discharged graphite electrode was stored at elevated temperatures, the irreversible capacity in the following cycle increased remarkably. On the other hand, when the electrode was stored at the fully charged state at elevated temperatures, it was severely self-discharged during storage. AFM observation of the SEI layer formed on a model electrode of highly oriented pyrolytic graphite revealed two important facts on the stability of the SEI at elevated temperatures: (i) dissolution and agglomeration of the SEI layer at the discharged state and (ii) serious SEI growth at the charged state. These phenomena well explain the results of the charge and discharge tests. It was also shown that the addition of vinylene carbonate greatly improves the stability of the SEI at elevated temperatures, and gives good charge and discharge performance after storage.