Studying Li-Ion Solid-State Batteries with In Situ TEM

Researchers were forced to search for an alternative to the flammable organic liquid electrolytes in the Li-Ion Batteries (LIBs) after safety issues were raised.  This article reports the successful preparation of an all-solid-state LIB sample directly on the MEMS-based Nano-Chip. DENSsolutions biasing system was used to achieve atomic-scale HAADF- and ABF-STEM imaging of the cathode during in situ electrochemical delithiation of the working battery cell.

Purpose

  • To address the safety hazard of current LIB’s and improve the power density by switching to solid-state electrolytes
  • To understand the behavior of the LiCoO2 cathode during in situ high-voltage delithiation of all-solid-state-battery inside TEM

Challenges

  • Using FIB to prepare a solid-state battery cell onto a MEMS-based sample carrier
  • Avoiding any damage when final thinning directly on the MEMS-based sample carrier
  • Obtaining a working device with no electrical shortcuts
  • Obtaining high-quality lamella suitable for atomic resolution, which is thin but without any FIB artifacts
  • Biasing while resolving Li-ions and atomic resolution imaging stability

HAADF micrograph of the delithiated polycrystalline LiCoO2 cathode colored using the Geometric Phase Analysis (GPA) method, showing two grains orientations colored with green and red. The inset shows the SEM image of the FIB fabrication.

Figure 1. HAADF micrograph of the delithiated polycrystalline LiCoO2 cathode colored using the Geometric Phase Analysis (GPA) method, showing two grains orientations colored with green and red. The inset shows the SEM image of the FIB fabrication.

Results

1. Sample Preparation

Constructed on the MEMS Nano-Chip was an all-solid-state LIB with a gold anode, a LiCoO2 cathode, and Y and Ta-doped LLZO (Li6.75La2.84Y0.16Zr1.75Ta0.25 O12) as solid-state electrolyte (SSE). This was done using FIB milling. As shown schematically in Figure 2, step by step assembly was followed to position the battery cell directly on the Nano-Chip. Low energy ions (5 kV and 2 kV) were used to perform the final thinning, allowing reduced FIB-generated damage and achieving a thickness below 50 nm.

The accuracy of the measurement is not affected by any potential damage of the supporting membrane during final thinning onto the Nano-Chip.

The supporting information provides a detailed description of the sample preparation: http://pubs.acs.org/doi/suppl/10.1021/jacs.6b13344/suppl_file/ja 6b13344_si_001.pdf

Step by step schematics of the battery cell fabrication process in the FIB.

Figure 2. (a – d) Step by step schematics of the battery cell fabrication process in the FIB.

2. In Situ Biasing and STEM Imaging of LiCoO2 Cathode

As shown in Figure 3(b), an in situ charging cycle of the battery cell was performed in the TEM using the Lightning D6+ system and 2601B Source Measure Unit from Ketihley. It was observed for the first time that the pristine single-crystal transforms into a polycrystalline structure having grains of 5-15 nm in size when the voltage bias reaches 2.1 V. The polycrystalline structure has two crystal orientations and therefore two types of grain boundaries. These are coherent twin and antiphase boundaries as shown in Figure 3 (c & d).

Both light elements, lithium and oxygen, were identified in the structure before and after the delithiation through collected atomically resolved ABF images (Figure 3 (a, c & d)). Confirmation of the delithiation process was two-fold:

  1. a weakening of the lithium contrast and
  2. layer spacing expansion in the sample;

Furthermore, as seen in Figure 3(c), Co ion migration into lithium-ion layers was observed.

(a) ABF micrograph of the pristine LiCoO2 sample. (b) Measured voltage-current-time plot of the delithiation process with an inset showing corresponding schematic of the battery cell. (c & d) ABF micrographs of the delithiated cathode, showing coherent twin and antiphase boundaries, respectively.

Figure 3. (a) ABF micrograph of the pristine LiCoO2 sample. (b) Measured voltage-current-time plot of the delithiation process with an inset showing corresponding schematic of the battery cell. (c & d) ABF micrographs of the delithiated cathode, showing coherent twin and antiphase boundaries, respectively.

The occurrence of the polycrystallization phenomena can be explained through the collected atomic-scale in situ STEM results together with the theoretical calculations. The created nanosized domains are connected to the preferred form of the coherent domain boundaries with low lithium-ion diffusion interface energy-barrier under applied bias as well as with the physical contact between the electrode and the electrolyte.

3. Key Message

  • The all-solid-state battery operation is now better understood and can now be used for designing safer, more energy-efficient energy storage devices.
  • Exciting new avenues in Nanoscale research have been opened up through in situ biasing &/or heating stimuli, along with the capability of functional devices preparation directly on the MEMS-based Nano-Chip with no effect on the measurement accuracy.

This information has been sourced, reviewed and adapted from materials provided by DENSsolutions .

For more information on this source, please visit DENSsolutions.

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