Operando Energy Dispersive X-Ray Diffraction (EDXRD) of Bulk Cathode Material for Multivalent Secondary Batteries
Authors: Kim Matthew A , Jadhav Ankur , Hawkins Brendan E. , Okasinski John , Gallaway Joshua 
Published: February 2020

Lithium-ion batteries have been at the forefront of secondary battery technology for the last decade and a half, being the preferred means for powering almost all small electronics. The characteristics of lithium-ion batteries (high energy density, high power) are extremely well suited to these applications. However, for many emerging applications such as solar and wind power backup, other characteristics are desired, including high safety, low toxicity, and low cost. It is for these reasons that new alternatives to lithium-ion must be explored.
Energy dispersive X-ray diffraction (EDXRD) from a high energy source allows the evolution of materials to be tracked from deep within large specimens, due to (a) the use of highly penetrating X-rays and (b) the ability to define a well-controlled diffraction gauge volume in space.1,2 This non-destructive characterization tool lends itself to the study of batteries, whose electrodes are generally composites of compressed particles, with electrolyte filling the pore space, and a sealed containment surrounding the cell designed to be as compact as possible. Using EDXRD, is it possible to study the structure of the compressed particles that make up these electrodes. Normal angle dispersive X-ray diffraction (ADXRD) is not capable of probing past the sealed containers of batteries making it mandatory to take out the active material of the battery to be analyzed. This means it is impossible to view the materials in disequilibrium states and risks exposing the sample to atmospheric gas or other contaminants. In contrast, EDXRD uses X-rays of high enough energy to pervade into the bulk material during charge/discharge cycles making it possible to observe dynamic crystallographic changes of the active material.
EDXRD opens new doors into the ability to correlate voltage and current data with structure to better understand how a material behaves during chemical reactions. Specifically, it offers the opportunity to gain further insight into mechanism of intercalation of working cations into anionic frameworks such as the Mo6S8 chevrel phase. Uniquely, this phase has been shown to intercalate divalent cations such as Zn2+ and even trivalent ions such as Al3+. This makes it an important material of study. However, the discharge products of these batteries are exceedingly susceptible to oxidation, making ex situ XRD incredibly difficult. It is known that two second order phase transitions are expected during intercalation of a guest ion A into the host chevrel: Mo6S8 → AMo6S8 → A2Mo6S8.3 Figure 1 shows operando XRD data of Zn2+ inserting into the chevrel phase. The peaks initially around 2θ = 30.5°, 2θ = 34° and 2θ = 41.7° on the red curve (bottom curve) show two distinct shifts or steps. This supporting a two-step phase transition mechanism . Further experiments can shed new light onto how the effects of different working ion and doping of the chevrel phase wit selenium in place of sulfur can modify the charge/discharge characteristics of secondary batteries using the chevrel phase as a cathode material.

Figure 1: Operando XRD data of the chevrel phase during Zn2+ intercalation.

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Figure 1