Failure Mode Analysis of Li-Ion Batteries Using in-Situ Scanning Electron Microscopy

High cycle life is a key parameter for any lithium-ion battery technology to obtain a commercial success.  In this regard, multiple electrochemical (Impedance, ASI, floating current, etc.) and non-electrochemical techniques (XRD, FT-IR, SEM/TEM, etc.) are used and developed. The most complete techniques involve measurements while the battery is cycling (in-situ experiment).  Among them, in-situ SEM/TEM is able to focus on the same region/particle at all steps of the oxidation/reduction process.  Hydro-Quebec has developed a complete expertise for in-situ techniques, especially in situ SEM.

 Lithium Polymer Battery (LPB), a solid-state battery, is the preferred configuration for SEM in-situ experiment.  We present in Figure 1 micrographs taken during in-situ experiments of a Li/PEO-based solid polymer electrolyte (SPE)/Li_1.2V₃O₈ LPB showing the variation in thickness for the Li, SPE and cathode layers with the cell voltage. Only the Li thickness shows an important variation during cycling. The Li plating rate can also be measured. This experiment can clearly be used to better understand the Li plating mechanism. 

 In figure 2 we present micrographs taken during an in-situ SEM cycling of a Li/PEO-based SPE/SiO cell using backscattered electron signal.  We clearly see an important decrease in the beam intensity of the back-scattered electrons (BSE) as a function of the cell voltage as a result of the Si phase transformation upon cycling (upon Li ‘insertion’, the BSE intensity is lower and the particle becomes darker).  We also note that a region of the big particle remains white (i.e. the Li⁺ ions did not have sufficient time to ‘diffuse’ inside their core region).  This result explains the rather low capacity observed with such particles, and it gives evidence that the system is out of thermodynamic equilibrium.  This in-situ experiment also shows no cracking of the relatively big SiO particles since the voltage was kept higher than 0.1V (stop at the Li₂₂Si₇ phase instead of Li₂₂Si₅) when compared to previous results [1].  We will also present in-situ SEM of nano-Si particles that did not suffer any cracking, but agglomerate (electrochemical sintering), which results in a more rigid and fractured electrodes [2]

  This presentation will also use in-situ SEM to obtain a better understanding of the failure mechanism of new electrode materials, including Li-S.

 References:

[1]  http://batt.lbl.gov/blog/research-tasks/in-situ-sem-seeing-battery-cycling-in-action/?utm_source=rss&utm_medium=rss&utm_campaign=in-situ-sem-seeing-battery-cycling-in-action

[2] Hovington et al, (2014) ‘in situ Scanning electron microscope study and micriostructural evolution of nano silicon anode for high energy Li-ion batteries’, Journal of Power Sources 248, 457-464