19 research outputs found
Understanding capacity fade in silicon based electrodes for lithium-ion batteries using three electrode cells and upper cut-off voltage studies
Commercial Li-ion batteries are typically cycled between 3.0 and 4.2 V. These voltages limits are chosen based on the characteristics of the cathode (e.g. lithium cobalt oxide) and anode (e.g. graphite). When alternative anode/cathode chemistries are studied the same cut-off voltages are often, mistakenly, used. Silicon (Si) based anodes are widely studied as a high capacity alternative to graphite for Lithium-ion batteries. When silicon-based anodes are paired with high capacity cathodes (e.g. Lithium Nickel Cobalt Aluminium Oxide; NCA) the cell typically suffers from rapid capacity fade. The purpose of this communication is to understand how the choice of upper cut-off voltage affects cell performance in Si/ NCA cells. A careful study of three-electrode cell data will show that capacity fade in Si/NCA cells is due to an ever-evolving silicon voltage profile that pushes the upper voltage at the cathode to >4.4 V (vs. Li/Liþ). This behaviour initially improves cycle efficiency, due to liberation of new lithium, but ultimately reduces cycling efficiency, resulting in rapid capacity fade
Electrolyte Volume Effects on Electrochemical Performance and Solid Electrolyte Interphase in Si-Graphite/NMC Lithium-Ion Pouch Cells
Functioning Mechanism of AlF<sub>3</sub> Coating on the Li- and Mn-Rich Cathode Materials
We
report systematic studies of the microstructural changes of
uncoated and AlF<sub>3</sub>-coated Li-rich Mn-rich (LMR) cathode
materials (Li<sub>1.2</sub>Ni<sub>0.15</sub>Co<sub>0.10</sub>Mn<sub>0.55</sub>O<sub>2</sub>) before and after cycling using a combination
of aberration-corrected scanning/transmission electron microscopy
(S/TEM) and electron energy loss spectroscopy (EELS). TEM coupled
with EELS provides detailed information about the crystallographic
and electronic structure changes that occur after cycling, thus revealing
the fundamental improvement mechanism of surface coating. The results
demonstrate that the surface coating reduces oxidation of the electrolyte
at high voltage, suppressing the accumulation of a thick solid electrolyte
interface (SEI) layer on electrode particle surface. Surface coating
significantly enhances the stability of the surface structure and
protects the electrode from severe etching/corrosion by the acidic
species in the electrolyte, reducing the formation of etched surfaces
and corrosion pits. Moreover, surface coating alleviates the undesirable
voltage fade by mitigating layered to spinel-like phase transformation
in the bulk region of the material. These fundamental findings may
also be widely applied to explain the functioning mechanisms of other
surface coatings used in a broad range of electrode materials
Deciphering the oxygen absorption pre-edge: a caveat on its application for probing oxygen redox reactions in batteries
The pre-edges of oxygen-K X-ray absorption spectra have been ubiquitous in transition metal (TM) oxide studies in various fields, especially on the fervent topic of oxygen redox states in battery electrodes. However, critical debates remain on the use of the O-K pre-edge variations upon electrochemical cycling as evidences of oxygen redox reactions, which has been a popular practice in the battery field. This study presents an investigation of the O-K pre-edge of 55 oxides covering all 3d TMs with different elements, structures and electrochemical states through combined experimental and theoretical analyses. It is shown unambiguously that the O-K pre-edge variation in battery cathodes is dominated by changing TM-d states. Furthermore, the pre-edge enables a unique opportunity to project the lowest unoccupied TM-d states onto one common energy window, leading to a summary map of the relative energy positions of the low-lying TM states, with higher TM oxidation states at lower energies, corresponding to higher electrochemical potentials. The results naturally clarify some unusual redox reactions, such as Cr3+/6+. This work provides a critical clarification on O-K pre-edge interpretation and more importantly, a benchmark database of O-K pre-edge for characterizing redox reactions in batteries and other energy materials.</div
Effects of Propylene Carbonate Content in CsPF<sub>6</sub>‑Containing Electrolytes on the Enhanced Performances of Graphite Electrode for Lithium-Ion Batteries
The effects of propylene carbonate
(PC) content in CsPF<sub>6</sub>-containing electrolytes on the performances
of graphite electrode in lithium half cells and in graphite∥LiNi<sub>0.80</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> (NCA) full
cells are investigated. It is found that the performance of graphite
electrode is significantly affected by PC content in the CsPF<sub>6</sub>-containing electrolytes. An optimal PC content of 20% by
weight in the solvent mixtures is identified. The enhanced electrochemical
performance of graphite electrode can be attributed to the synergistic
effects of the PC solvent and the Cs<sup>+</sup> additive. The synergistic
effects of Cs<sup>+</sup> additive and appropriate amount of PC enable
the formation of a robust, ultrathin, and compact solid electrolyte
interphase (SEI) layer on the surface of graphite electrode, which
is only permeable for desolvated Li<sup>+</sup> ions and allows fast
Li<sup>+</sup> ion transport through it. Therefore, this SEI layer
effectively suppresses the PC cointercalation and largely alleviates
the Li dendrite formation on graphite electrode during lithiation
even at relatively high current densities. The presence of low-melting-point
PC solvent improves the sustainable operation of graphite∥NCA
full cells under a wide temperature range. The fundamental findings
also shed light on the importance of manipulating/maintaining the
electrode/electrolyte interphasial stability in various energy-storage
devices
Simultaneous neutron and X-ray tomography for ex-situ 3D visualization of graphite anode degradation in extremely fast-charged lithium-ion batteries.
Extreme fast charging (XFC) of commercial lithium-ion batteries (LIBs) in ≤10-15 minutes will significantly advance the deployment of electric vehicles globally. However, XFC leads to considerable capacity fade, mainly due to graphite anode degradation. Non-destructive three-dimensional (3D) investigation of XFC-cycled anodes is crucial to connect degradation with capacity loss. Here, we demonstrate the viability of simultaneous neutron and X-ray tomography (NeXT) for ex-situ 3D visualization of graphite anode degradation. NeXT is advantageous because of the sensitivity of neutrons to Li and H and X-rays to Cu. We combine the neutron and X-ray tomography with micron resolution for material identification and segmentation on one pristine and one XFC-cycled graphite anode, thereby underscoring the benefits of the simultaneous nature of NeXT. Our ex-situ results pave the way for the design of NeXT-friendly LIB geometries that will allow operando and/or in-situ 3D visualization of graphite anode degradation during XFC
The Role of Cesium Cation in Controlling Interphasial Chemistry on Graphite Anode in Propylene Carbonate-Rich Electrolytes
The Role of Cesium Cation in Controlling Interphasial Chemistry on Graphite Anode in Propylene Carbonate-Rich Electrolytes
Despite the potential advantages
it brings, such as wider liquid
range and lower cost, propylene carbonate (PC) is seldom used in lithium-ion
batteries because of its sustained cointercalation into the graphene
structure and the eventual graphite exfoliation. Here, we report that
cesium cation (Cs<sup>+</sup>) directs the formation of solid electrolyte
interphase on graphite anode in PC-rich electrolytes through its preferential
solvation by ethylene carbonate (EC) and the subsequent higher reduction
potential of the complex cation. Effective suppression of PC-decomposition
and graphite-exfoliation is achieved by adjusting the EC/PC ratio
in electrolytes to allow a reductive decomposition of Cs<sup>+</sup>-(EC)<sub><i>m</i></sub> (1 ≤ <i>m</i> ≤ 2) complex preceding that of Li<sup>+</sup>-(PC)<sub><i>n</i></sub> (3 ≤ <i>n</i> ≤ 5). Such
Cs<sup>+</sup>-directed interphase is stable, ultrathin, and compact,
leading to significant improvement in battery performances. In a broader
context, the accurate tailoring of interphasial chemistry by introducing
a new solvation center represents a fundamental breakthrough in manipulating
interfacial reactions that once were elusive to control