Hong Gan

Visualizing non-equilibrium lithiation of spinel oxide via in situ transmission electron microscopy

Spinel transition metal oxides are important electrode materials for lithium-ion batteries, whose lithiation undergoes a two-step reaction, whereby intercalation and conversion occur in a sequential manner. These two reactions are known to have distinct reaction dynamics, but it is unclear how their kinetics affects the overall electrochemical response. Here we explore the lithiation of nanosized magnetite by employing a strain-sensitive, bright-field scanning transmission electron microscopy approach. This method allows direct, real-time, high-resolution visualization of how lithiation proceeds along specific reaction pathways. We find that the initial intercalation process follows a two-phase reaction sequence, whereas further lithiation leads to the coexistence of three distinct phases within single nanoparticles, which has not been previously reported to the best of our knowledge. We use phase-field theory to model and describe these non-equilibrium reaction pathways, and to directly correlate the observed phase evolution with the batterys discharge performance.

A Study of Capacity Fade in Cylindrical and Prismatic Lithium-Ion Batteries

Identical electrodes wound in both a cylindrical and a prismatic configuration provided different capacity fade rates. At the completion of 300 cycles with a C/2 charge and 1C discharge, the prismatic cells faded 24% and the cylindrical cells faded 16%. In the cylindrical-type cell, the fade was primarily caused by a reduction in the capacity of the cathode. The higher fade rate in the prismatic cell was attributed to lower cell stack pressure which resulted in more anode swelling during cycling. The increased porosity of the anode in this cell reduced the integrity of the electrical conduction network causing many graphite particles to become isolated or less accessible.

Anode Passivation and Electrolyte Solvent Disproportionation: Mechanism of Ester Exchange Reaction in Lithium‐Ion Batteries

Carbonate-based electrolytes used in lithium-ion electrochemical cells were found to undergo ester exchange reactions, where the use of dimethyl carbonate and diethyl carbonate resulted in the in situ formation of ethyl methyl carbonate. The reaction was found to be reversible and occurs during the first charge cycle of the LiCoO 2 /petroleum coke lithium-ion system. Mechanistic studies were carried out and determined that the ester exchange reaction is reductively initiated at the carbon anode. A mechanism has been deduced, with an intermediate alkoxide species responsible for the ester exchange reaction. Electrode passivation was found to limit the extent of the reaction during subsequent cycles, with the choice of electrolyte solvent impacting the passivation of the electrode.

Lithium electrodes with and without CO2 treatment : electrochemical behavior and effect on high rate lithium battery performance

Abstract The ionic resistivity and integrity of a solid-electrolyte interphase (SEI) film on a lithium electrode surface was investigated. The performed lithium carbonate film on the surface of the lithium electrode was found to improve the electrode behavior by maintaining a low ionic resistance. In lithium/silver vanadium oxide batteries, voltage delay can be eliminated with the use of a lithium anode pretreated with CO 2 . An SEI consisting of lithium carbonate appears to be responsible. Unlike the surface film formed from lithium-electrolyte reactions, the lithium carbonate film is relatively strong and can withstand high current density pulses (∼ 20 mA/cm 2 ) without significant damage. An ion exchange mechanism involving the carbonate anion is proposed.

Kinetic Phase Evolution of Spinel Cobalt Oxide during Lithiation

Spinel cobalt oxide has been proposed to undergo a multiple-step reaction during the electrochemical lithiation process. Understanding the kinetics of the lithiation process in this compound is crucial to optimize its performance and cyclability. In this work, we have utilized a low-angle annular dark-field scanning transmission electron microscopy method to visualize the dynamic reaction process in real time and study the reaction kinetics at different rates. We show that the particles undergo a two-step reaction at the single-particle level, which includes an initial intercalation reaction followed by a conversion reaction. At low rates, the conversion reaction starts after the intercalation reaction has fully finished, consistent with the prediction of density functional theoretical calculations. At high rates, the intercalation reaction is overwhelmed by the subsequently nucleated conversion reaction, and the reaction speeds of both the intercalation and conversion reactions are increased. Phase-field simulations show the crucial role of surface diffusion rates of lithium ions in controlling this process. This work provides microscopic insights into the reaction dynamics in non-equilibrium conditions and highlights the effect of lithium diffusion rates on the overall reaction homogeneity as well as the performance.

Kinetically-Driven Phase Transformation during Lithiation in Copper Sulfide Nanoflakes

Two-dimensional (2D) transition metal chalcogenides have been widely studied and utilized as electrode materials for lithium ion batteries due to their unique layered structures to accommodate reversible lithium insertion. Real-time observation and mechanistic understanding of the phase transformations during lithiation of these materials are critically important for improving battery performance by controlling structures and reaction pathways. Here, we use in situ transmission electron microscopy methods to study the structural, morphological, and chemical evolutions in individual copper sulfide (CuS) nanoflakes during lithiation. We report a highly kinetically driven phase transformation in which lithium ions rapidly intercalate into the 2D van der Waals-stacked interlayers in the initial stage, and further lithiation induces the Cu extrusion via a displacement reaction mechanism that is different from the typical conversion reactions. Density functional theory calculations have confirmed both the thermodynamically favored and the kinetically driven reaction pathways. Our findings elucidate the reaction pathways of the Li/CuS system under nonequilibrium conditions and provide valuable insight into the atomistic lithiation mechanisms of transition metal sulfides in general.