The development of lithium-ion battery materials has a key challenge due to the formation of lithium dendrites on the surface of the electrode after repeated charge/discharge cycles. The formation of the solid electrolyte interphase (SEI) layer is fundamental to this process.
The SEI layer is an intermediate solid-liquid layer that forms at the electrode surface due to the interaction between lithium and the electrolyte. An unstable SEI layer can lead to crack formation, increasing the electrodes’ surface roughness and resulting in the formation of lithium dendrites.
As dendrites form over repeated cycles, the so-called “dead” lithium does not take part in ion transport, reducing battery capacity and simultaneously increasing the potential for the formation of a short circuit between the cathode and the anode.
With the Poseidon system, correlative electrochemical results such as cyclic voltammogram (CV) curves can be simultaneously acquired. CV curves enable the quantification of electrochemical cell capacity and process reversibility, which are critical parameters affecting the battery performance of many consumer devices.
The Poseidon electrochemistry liquid cell system enables the study of dynamic nanoscale processes in the transmission electron microscope (TEM) and simultaneously collects correlative electrochemical data. In order to better understand the basic mechanisms and develop new materials, researchers can probe complex processes, such as corrosion, fuel cells, and lithium-ion batteries at the nanoscale.
The Poseidon system features a dedicated in situ microfluidic TEM holder, syringe pump, and potentiostat. Samples are contained in a liquid environment between a large and a small microchip (known as E-chips), both of which have an electron beam transparent, amorphous 50 nm membrane of silicon nitride (SiN).
A three electrode circuit is used to simultaneously obtain images and electrochemical data in the TEM. Featuring a working, reference and counter electrode, the three electrode circuit is incorporated into an E-chip device loaded in the tip of the TEM holder (Figure 1).
When inverted, the large Poseidon system E-chip rests against the small E-chip (shown positioned in the tip of the Poseidon system TEM holder) and the integrated electrodes of the large E-chip make contact with the electrode pad provided in the holder to form a closed circuit.
The E-chip surface has a 500-nm insulating layer that prevents the electrolyte from interacting with the electrodes outside the viewing region, which not only enhances electrical signal but also provides a flow channel for the electrolyte.
Figure 1. Posiedon E-chips
At the Joint Center for Energy Storage Research at Pacific Northwest National Lab in Richland, WA, L. Mehdi and N. Browning used the Protochips Poseidon system to view the charge/discharge process of lithium-ion batteries in solution using scanning TEM (STEM).
With the Protochips Poseidon system, the researchers were able to obtain TEM images of hydrated samples, which allowed them to image and measure the growth and evolution of the SEI layer and also the resulting formation of lithium dendrites on the anode surface during successive charge/discharge cycles.
The battery cell configuration used in the study was as follows: The reference, counter, and working electrodes were platinum, patterned linearly on the surface of the Poseidon system E-chip, with the working electrode (anode) patterned onto the 50 nm SiN.
An insulating layer on the E-chip surface confined the electrolyte to the electrodes’ localized regions and provided a 500 nm spacer to allow the electrolyte to flow through the cell.
On the corresponding E-chip (Part # EPB-55BF), an additional 150 nm spacer was used for a 650 nm thick chamber liquid at the edge of the liquid cell. Lithium was introduced into the system through the electrolyte, 1.0 M lithium hexafluorophosphate in propylene carbonate (LiPF6/PC), at a flow rate of 3 µL/minute using a syringe pump.
Using a glove box, the entire system was assembled under argon because the electrolyte is sensitive to water and air. A Gamry Reference 600 potentiostat was used to supply current to the system.
Images were recorded using an FEI Cs corrected Titan operated in scanning TEM (STEM) mode at 300 kV. In order to prevent the breakdown of the electrolyte, bubble formation and beam induced precipitation, the electron dose was maintained below ≤0.3 electrons/Å2/s.
Results and Discussion
Figure 2A-D. Lithium dendrite growth and dissolution over repeated battery cycling
As the electrochemical potential of the cell was simultaneously cycled from 0 to -4 volts, a series of real-time STEM images of the working electrode (WE) was obtained. Figure 2 shows high angle dark field (HAADF)-STEM images of the formation of lithium dendrites on the platinum WE surface over the course of three charge/discharge cycles.
For each cycle (Figure 2 A-C), lithium deposition (charging) is displayed in the first four frames and lithium dissolution (discharging) is shown in the final two frames.
At the beginning of the first cycle, the surface of the platinum WE is pristine and the entire lithium is dissolved in the electrolyte solution. As the battery is charged, lithium is deposited on the WE surface and surface roughening is seen as the lithium is not deposited in an even layer.
During the first cycle, discharging of the battery reduces lithium on the surface of the electrode, but lithium dendrites remain on the electrode surface and do not re-dissolve in the electrolyte. This process is continued in cycles two and three, and each subsequent cycle increases the size and number of the lithium dendrites on the surface of the electrode.
Regions of “dead” lithium are also present after the second and third cycles. These are deposits of lithium metal that is no longer in contact with the surface of the electrode, and as a result, do not take part in charge transport. Capacity fading occurs due to the formation of these regions of dead lithium in an electrochemical cell, leading to shorter battery lifetimes between charging cycles.
Real-time movies of the charge/discharge cycle are available online at: http://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5b00175.
Figure 2D shows the corresponding electrochemical data (CV curves). As the number of charge cycles increases, there is an increased irreversibility of the electrochemical cell, which corresponds to the formation of the so-called “dead” lithium regions and lithium dendrites observed in the STEM images.
Characteristic peaks at -2 and -2.5 volts are also visible in the CV curves, indicating alloying between the lithium in the electrolyte and the platinum electrode.
To accurately interpret the electrochemical results, it was important to understand the distribution of the electric field inside the liquid chamber of the Poseidon system. The researchers used the dimensions of the electrodes and liquid chamber to simulate the electrical field resulting from a constant galvanostatic discharge 0.1 mA/cm2 in a LiPF6/PC electrolyte by conducting an Ansoft Maxwell static 3D simulation.
Figure 3A shows the Maxwell simulation of the working and reference electrode configuration of the E-chip used in this study. In the original work, a region of hotspot, or high electric field gradient, is likely to occur at the right hand tip of the working electrode (anode) between the cathode and anode. In the experimental data, this region of increased electric field density is confirmed.
Shown in Figure 3 B-E, the HAADF-STEM images of the working electrode (area indicated by the blue line) following the fifth cycle (B), sixth (C), seventh (D) and the eighth cycle (E) demonstrate the localized formation of a lithium dendrite at the same location as the predicted hotspot position, which increases in size with each subsequent charge/discharge cycle.
Figure 3. Modeled Electrode Hotspot and Corresponding Experimental Data. A: Maxwell simulation reference and working electrode of the E-chip used in this study. Dark field STEM image of the working electrode after the fifth (B), sixth (C), seventh (D) and the eight cycle (E).
A region of bright contrast at the edge of the electrode (indicated by the red line) is also seen in the images. This is the SEI layer region, which with each subsequent cycle becomes more and more irregular, corresponding to the increased growth of the lithium dendrites on the surface of the electrode.
Using the Poseidon electrochemistry liquid cell system, researchers successfully imaged the formation of lithium dendrites during successive battery charge/discharge cycles and simultaneously obtained electrochemical data using STEM.
Computer simulations were used to further confirm the experimental results, and they helped detect regions on the working electrode in which dendrites may form. In situ S/TEM analysis of fundamental battery mechanisms together with quantitative electrochemical results, such as the study described here, represents a key advancement toward the development of next generation, high-performance battery materials.
This information has been sourced, reviewed and adapted from materials provided by Protochips.
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