Electron energy loss spectroscopy (EELS) is one of most suitable methods to detect lithium ion itself because Li-K edge peak due to the existence of lithium ions can be detected. Furthermore, recent quantitative analysis of EELS can visualize the lithium concentration change simply [5]. In this study, we performed operando EELS mapping for understanding the lithium transport by using our developed nano battery, which consists of LiMn2O4 nanowire cathode, ionic liquid electrolyte and Li4Ti5O12 anode.
The nano battery was charged or discharged in TEM during operando EELS observation as loaded in our homemade in-situ TEM sample holder. Charge-discharge was performed by cyclic voltammetry of which scan speed was 0.55mV/s. EELS spectrum was recorded by Gatan Tridiem ER spectrometer (Gatan, Inc.) in aberration corrected TEM, R005 [6] which is equipped with cold field emission gun. The acceleration voltage was 200kV and the convergent semi-angle of electron probe was 23 mrad. The entrance aperture size was 60 mrad and the energy dispersion of 0.2eV/ch was used. The full width half maximum of zero loss peak was 1 eV. The pixel size of EELS images was around 9 x 9 nm2. The electron probe current was less than 5 pA in order to suppress electron beam damage. The dwell time per pixel was 0.2 s and the probe size was adjusted to pixel size.
During fast battery cycles, we found that lithium concentration changed with time delay with the cell current change in both of charge and discharge. Furthermore, the lithium concentration was imaged to change from the area far from the cathode/electrolyte interface. The observed lithium concentration change cannot be explained by conventional diffusion model, but the lithium drift model that lithium moves due to electric potential gradient inside cathode caused by high charge rate. In addition, after the first battery cycle lithium concentration changed only slightly around the range of x~1 in LixMn2O4 near the interface. It is attributed that solid electrolyte interface formation at high voltage after charge makes the chemical potential changed locally as an interface effect. We believe that these findings open the new way to understand the phase transformation under non-equilibrium state such fast charge as well as the new strategy to design fast chargeable batteries.
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