22 research outputs found
REAL-TIME INVESTIGATION OF INDIVIDUAL SILICON NANOSTRUCTURED ELECTRODES FOR LITHIUM-ION BATTERIES
Silicon-based anode materials are an attractive candidate to replace today's widely-utilized graphitic electrodes for lithium-ion batteries because of their high gravimetric energy density (3572 mAh/g vs. 372 mAh/g for carbon) and relatively low working potential (~ 0.5V vs. Li/Li+). However, their commercial realization is still far away because of the structural instabilities associated with huge volume changes of ~300% during charge-discharge cycles. Recently, it has been proposed that silicon nanowires and other related one-dimensional nanostructures could be used as lithium storage materials with greatly enhanced storage capacities over that for graphite in the next generation of lithium-ion batteries. However, the studies to date have shown that the nanomaterials, while better, are still not good enough to withstand a large number of lithiation cycles, and moreover, there is little fundamental insight into the science of the improvements or the steps remaining before widespread adoption. This dissertation seeks to understand the basic structural properties and reaction kinetics of one dimensional silicon nanomaterials, including Si-C heterostructures during electrochemical lithiation/delithiation using in-situ transmission electron microscopy (TEM).
I present my work in three parts. In part I, I lay out the importance of lithium-ion batteries and silicon-based anodes, followed by experimental techniques using in-situ TEM. In part II, I present results studied on three different nanostructures: Si nanowires (SiNWs), Si-C heterostructures and Si nanotubes (SiNTs). In SiNWs, we report an unexpected two-phase transformation and anisotropic volume expansion during lithiation. We also report an electrochemically-induced weld of ~200 MPa at the Si-Si interface. Next, studies on CNT@α-Si heterostructures with uniform and beaded-string structures with chemically tailored carbon-silicon interfaces are presented. In-situ TEM studies reveal that beaded-string CNT@ α-Si structures can accommodate massive volume changes during lithiation and delithiation without appreciable mechanical failure. Finally, results on lithiation-induced volume clamping effect of SiNTs with and without functional Ni coatings are discussed. In Part III, a conclusion and a brief outlook of the future work are outlined. The findings presented in this dissertation can thus provide important new insights in the design of high performance Si electrodes, laying a foundation for next-generation lithium ion batteries
The Anode Challenge for Lithium-Ion Batteries: A Mechanochemically Synthesized Sn-Fe-C Composite Anode Surpasses Graphitic Carbon
Carbon-based anodes are the key limiting factor in increasing the volumetric capacity of lithium-ion batteries. Tin-based composites are one alternative approach. Nanosized Sn-Fe-C anode materials are mechanochemically synthesized by reducing SnO with Ti in the presence of carbon. The optimum synthesis conditions are found to be 1:0.25:10 for initial ratio of SnO, Ti, and graphite with a total grinding time of 8 h. This optimized composite shows excellent extended cycling at the C/10 rate, delivering a first charge capacity as high as 740 mAh g(-1) and 60% of which still remained after 170 cycles. The calculated volumetric capacity significantly exceeds that of carbon. It also exhibits excellent rate capability, delivering volumetric capacity higher than 1.6 Ah cc(-1) over 140 cycles at the 1 C rate
Revisiting conversion reaction mechanisms in lithium batteries: lithiation-driven topotactic transformation in FeF2
Intercalation-type electrodes have now been commonly employed in todayâ s batteries due to their capability of storing and releasing lithium reversibly via topotactic transformation, conducive to small structural change, but they have limited interstitial sites to hold Li. In contrast, conversion electrodes feature high Li-storage capacity, but often undergo large structural change during (de)lithiation, resulting in cycling instability. One exception is iron fluoride (FeF2), a conversion-type cathode that exhibits both high capacity and high cycling stability. Herein, we report a lithiation-driven topotactic transformation in a single crystal of FeF2, unveiled by in situ visualization of the spatial and crystallographic correlation between the parent and converted phas-es. Specifically, conversion in FeF2 resembles the intercalation process but involves transport of both Li+ and Fe2+ ions within the F-anion array, leading to formation of Fe preferentially along specific crystallographic ori-entations of FeF2. Throughout the process, the F-anion framework is retained, creating a checkerboard-like structure, within which the volume change is largely compensated, thereby enabling the high cyclability in FeF2. Findings from this study, with unique insights into conversion reaction mechanisms, may help to pave the way for designing conversion-type electrodes for the next-generation lithium batteries
Atomic Insight into the Layered/Spinel Phase Transformation in Charged LiNi<sub>0.80</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> Cathode Particles
Layered
LiNi<sub>0.80</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> (NCA) holds great promise as a potential cathode material
for high energy density lithium ion batteries. However, its high capacity
is heavily dependent on the stability of its layered structure, which
suffers from a severe structure degradation resulting from a not fully
understood layered → spinel phase transformation. Using high-resolution
transmission electron microscopy and electron diffraction, we probe
the atomic structure evolution induced by the layered → spinel
phase transformation in the NCA cathode. We show that the phase transformation
results in the development of a particle structure with the formation
of complete spinel, spinel domains, and intermediate spinel from the
surface to the subsurface region. The lattice planes of the complete
and intermediate spinel phases are highly interwoven in the subsurface
region. The layered → spinel transformation occurs via the
migration of transition metal (TM) atoms from the TM layer into the
lithium layer. Incomplete migration leads to the formation of the
intermediate spinel phase, which is featured by tetrahedral occupancy
of TM cations in the lithium layer. The crystallographic structure
of the intermediate spinel is discussed and verified by the simulation
of electron diffraction patterns
Using In-Situ Methods to Characterize Phase Changes in Charged Lithium Nickel Cobalt Aluminum Oxide Cathode Materials
Tuning the Activity of Oxygen in LiNi<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> Battery Electrodes
Layered transition metal oxides such
as LiNi<sub>0.8</sub>Co <sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> (NCA) are highly desirable battery electrodes. However, these materials
suffer from thermal runaway caused by deleterious oxygen loss and
surface phase transitions when in highly overcharged and overheated
conditions, prompting serious safety concerns. Using in situ environmental
transmission electron microscopy techniques, we demonstrate that surface
oxygen loss and structural changes in the highly overcharged NCA particles
are suppressed by exposing them to an oxygen-rich environment. The
onset temperature for the loss of oxygen from the electrode particle
is delayed to 350 °C at oxygen gas overpressure of 400 mTorr.
Similar heating of the particles in a reducing hydrogen gas demonstrated
a quick onset of oxygen loss at 150 °C and rapid surface degradation
of the particles. The results reported here illustrate the fundamental
mechanism governing the failure processes of electrode particles and
highlight possible strategies to circumvent such issues