3 research outputs found
Controlling the Intercalation Chemistry to Design High-Performance Dual-Salt Hybrid Rechargeable Batteries
We
have conducted extensive theoretical and experimental investigations
to unravel the origin of the electrochemical properties of hybrid
Mg<sup>2+</sup>/Li<sup>+</sup> rechargeable batteries at the atomistic
and macroscopic levels. By revealing the thermodynamics of Mg<sup>2+</sup> and Li<sup>+</sup> co-insertion into the Mo<sub>6</sub>S<sub>8</sub> cathode host using density functional theory calculations,
we show that there is a threshold Li<sup>+</sup> activity for the
pristine Mo<sub>6</sub>S<sub>8</sub> cathode to prefer lithiation
instead of magnesiation. By precisely controlling the insertion chemistry
using a dual-salt electrolyte, we have enabled ultrafast discharge
of our battery by achieving 93.6% capacity retention at 20 C and 87.5%
at 30 C, respectively, at room temperature
Investigating Local Degradation and Thermal Stability of Charged Nickel-Based Cathode Materials through Real-Time Electron Microscopy
In this work, we take advantage of
in situ transmission electron microscopy (TEM) to investigate thermally
induced decomposition of the surface of Li<sub><i>x</i></sub>Ni<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> (NCA)
cathode materials that have been subjected to different states of
charge (SOC). While uncharged NCA is stable up to 400 °C, significant
changes occur in charged NCA with increasing temperature. These include
the development of surface porosity and changes in the oxygen K-edge
electron energy loss spectra, with pre-edge peaks shifting to higher
energy losses. These changes are closely related to O<sub>2</sub> gas
released from the structure, as well as to phase changes of NCA from
the layered structure to the disordered spinel structure, and finally
to the rock-salt structure. Although the temperatures where these
changes initiate depend strongly on the state of charge, there also
exist significant variations among particles with the same state of
charge. Notably, when NCA is charged to <i>x</i> = 0.33
(the charge state that is the practical upper limit voltage in most
applications), the surfaces of some particles undergo morphological
and oxygen K-edge changes even at temperatures below 100 °C,
a temperature that electronic devices containing lithium ion batteries
(LIB) can possibly see during normal operation. Those particles that
experience these changes are likely to be extremely unstable and may
trigger thermal runaway at much lower temperatures than would be usually
expected. These results demonstrate that in situ heating experiments
are a unique tool not only to study the general thermal behavior of
cathode materials but also to explore particle-to-particle variations,
which are sometimes of critical importance in understanding the performance
of the overall system
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
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 compositionsî—¸this 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