Using Real-Time Electron Microscopy To Explore the
Effects of Transition-Metal Composition on the Local Thermal Stability
in Charged Li<sub><i>x</i></sub>Ni<sub><i>y</i></sub>Mn<sub><i>z</i></sub>Co<sub>1–<i>y</i>–<i>z</i></sub>O<sub>2</sub> Cathode Materials
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Abstract
In this work, we use <i>in situ</i> transmission electron
microscopy (TEM) to investigate the thermal decomposition that occurs
at the surface of charged Li<sub><i>x</i></sub>Ni<sub><i>y</i></sub>Mn<sub><i>z</i></sub>Co<sub>1–<i>y</i>–<i>z</i></sub>O<sub>2</sub> (NMC) cathode
materials of different composition (with <i>y</i>, <i>z</i> = 0.8, 0.1, and 0.6, 0.2, and 0.4,and 0.3), after they
have been charged to their practical upper limit voltage (4.3 V).
By heating these materials inside the TEM, we are able to directly
characterize near surface changes in both their electronic structure
(using electron energy loss spectroscopy) and crystal structure and
morphology (using electron diffraction and bright-field imaging).
The most Ni-rich material (<i>y</i>, <i>z</i> =
0.8, 0.1) is found to be thermally unstable at significantly lower
temperatures than the other compositionsthis is manifested
by changes in both the electronic structure and the onset of phase
transitions at temperatures as low as 100 °C. Electron energy
loss spectroscopy indicates that (i) the thermally induced reduction
of Ni ions drives these changes, and (ii) this is exacerbated by the
presence of an additional redox reaction that occurs at 4.2 V in the <i>y</i>, <i>z</i> = 0.8, 0.1 material. Exploration of
individual particles shows that there are substantial variations in
the onset temperatures and overall extent of these changes. Of the
compositions studied, the composition of <i>y</i>, <i>z</i> = 0.6, 0.2 has the optimal combination of high energy
density and reasonable thermal stability. The observations herein
demonstrate that real-time electron microscopy provide direct insight
into the changes that occur in cathode materials with temperature,
allowing optimization of different alloy concentrations to maximize
overall performance