Low and high magnification cryogenic TEM images of electrochemically deposited Li metal along different directions. SEM and cryogenic STEM images of electrochemically deposited Li metal on a Cu current collector using conventional carbonate electrolytes (under 2.0 mA cm-2 for 10 s). SEM and cryogenic STEM images of electrochemically deposited Li metal on Cu current collector with highly concentrated LiFSI-DME electrolyte (below 2.0 mA cm-2 for 10 s).
High-magnification cryogenic TEM image and its fast Fourier transform (FFT) image of electrochemically deposited Li metal on a copper-based TEM band current collector using highly concentrated LiFSI-DME electrolyte (below 2.0 mA cm-2 for 10 s). SEM image and magnified areas of electrochemically deposited Li metal on Cu current collector with highly concentrated LiFSI-DME electrolyte (below 2.0 mA cm-2 for 10 s). SEM and cryogenic STEM images of electrochemically deposited Li metal on Cu current collector and Cu TEM grid using highly concentrated LiFSI-DME electrolyte (below 2.0 mA cm-2 for 30 min, 1.0 mAh cm-2).
High magnification cryogenic TEM image and its FFT image of electrochemically deposited Li metal on Cu TEM grid using highly concentrated LiFSI-DME electrolyte (under 2.0 mA cm- . 2 for 30 min, 1.0 mAh cm-2) . Low and high magnification cryogenic STEM/TEM images and their FFT images of electrochemically deposited Li metal (1.0 M LiFSI-DME electrolyte).
Research background
Chemistry of Lithium-ion secondary batteries
Significant improvements in increasing the energy density of battery systems have been made in the last 20 years. The first commercialized battery system consisting of lithium cobalt oxide cathode material (LiCoO2, LCO) and graphite anode materials showed energy densities of about 150 Wh/kg. The energy density of the battery system can be determined by the electrode materials that make up the electrochemical cell (Figure 5).
The gravimetric energy density (Wh/kg) is calculated by the total electrode capacity (Ah) x operating voltage (V) / unit weight (kg). The total capacity of the electrode, one of the important things for increasing energy density, can be calculated by the specific capacity of materials (Ah/g) x weight of active materials (g). These types of issues arising from the Li dendrites are directly related to the safety, cycling, and energy density issues of the battery system.
As discussed above, a heterogeneous interfacial state at the electrode can hinder the full utilization of active materials, leading to a loss of energy density. It is necessary to improve the homogeneity of the electrode to increase the use of active materials that can directly improve the overall energy density of the battery system.
Principle and examples of in-situ TEM analysis of diverse materials
Moreover, the results of cyclic voltammetry (CV) suggested that coating layers did not show any electrochemical activity to increase the specific capacitance in the same voltage range as the galvanostatic cycle (Figure 35b). a) The first and tenth charge/discharge profiles of the (black) bare SnO2, (blue) SnO2@MnO2 and (red) SnO2@ppy electrode (b) Cyclic voltammetry results of each electrode. At the first (0.1 C), the delithiation capacity was 780 mAh g-1 and decreased slightly as the C rate increased. The electron-received salt or solvent can be decomposed during the continuous charging process and form the SEI layer on the Li metal anode.
A schematic of the formation and role of a lithiophilic SEI layer that can reduce the surface free energy and induce uniform Li metal deposition. Image of cryogenic TEM holder to investigate meta-stable materials such as Li-metal anode and SEI layer (Double Tilt LN2 Atmos Defend Holder, Mel-Build). Applying the cryogenic TEM analysis to Li metal anode and its SEI layer can reveal the exact role of the SEI layer on the electrochemical performance improvements.
Heterogeneous Li flux and deposition can also occur with the SEI layer in the current collector or Li metal anode. The indistinct SEI layer on the deposited Li metal as shown in Figure 69 confirmed that hypothesis. The formed SEI layer composed of various inorganic crystalline grains was confirmed by cryogenic TEM analysis as shown in Figure 71.
This composition of the unique inorganic-rich bilayer SEI layer was quite different from the SEI layer from carbonate electrolytes as shown in Figure 79. TEM-EDS results of electrochemically deposited Li metal and the SEI layer formed from conventional carbonate electrolyte (under 2.0 mA cm-2 for 30 min, 1.0 mAh cm-2). TEM-EDS results of the electrochemically deposited Li metal and the SEI layer formed by the highly concentrated LiFSI-DME electrolyte (below 2.0 mA cm-2 for 30 min, 1.0 mAh cm-2).
XPS depth profile results of SEI layer formed by the highly concentrated LiFSI-DME electrolyte (0, 1, and 7 minutes of etching from the bottom). XPS depth profile analysis of the SEI layer formed by the conventional carbonate electrolyte was also performed as shown in Figure 87.
Density functional theory (DFT) studies have been conducted to reveal the origin of the inorganic-rich bilayer SEI layer from the highly concentrated LiFSI-DME electrolyte system. The prediction from the DFT calculation showed consistency with the nanostructure of the formed SEI layer, as shown in Figure 93. An additional DFT study was conducted to reveal the superior electrochemical performance of the Li metal anode with the highly concentrated LiFSI-DME electrolyte targeting the unique nanostructure. of the inorganic rich bilayer SEI layer (Figure 96).
The electrochemical behavior of alkaline and alkaline earth metals in non-aqueous battery systems: the interphase model for solid electrolytes.
