Electrochemomechanical fatigue and fracture in electrode and electrolyte materials for Li-Ion batteries

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged student-submitted from PDF version of thesis.Includes bibliographical references (pages 179-199).In Li-ion batteries (LIBs), electrochemically driven dimensional changes in the electrodes lead to mechanical stress buildup during operation. Electrochemomechanical fatigue refers to both mechanical degradation (fracture) and the associated chemical degradation that is exacerbated by fracture as a result of this stress, accumulated during repeated electrochemical cycling. Such fracture can have serious consequences for the performance of LIBs over time in terms of capacity loss, growth of electrochemical impedance, and in all-solid-state batteries (ASSBs) even failure via short-circuiting. To better understand and predict mechanisms for electrochemically-induced fracture, we measured elastic, plastic, and fracture properties of electrode and solid electrolyte materials, focusing especially on sulfide electrolytes for ASSBs. We found that these electrolytes are extremely brittle and therefore vulnerable to fracture-assisted internal electrical shorting, an issue that currently limits commercialization of ASSBs. We built on these results with finite element modeling of electrolyte fracture in ASSBs, thus finding a strong dependence of fracture conditions on both electrolyte fracture toughness and plastic behavior of lithium metal. Using these results, we constructed electrochemomechanical failure maps to establish how microstructure, processing, and mechanical properties influence electrolyte fracture. We also studied how electrochemically induced fracture in turn affects battery performance, particularly for electrode materials. We implemented controlled fracture events in Li[subscript X]Mn₂O₄ cells employing liquid electrolytes and lithium anodes, and used acoustic emissions monitoring to confirm the timing of the fractures. We then used electrochemical impedance spectroscopy based on a distribution of relaxation times analysis method to isolate the fracture-based mechanisms leading to impedance growth, thereby observing sudden increases in electronic contact resistance concurrent with crack formation within the active particles. We also observed an increased rate of capacity fade following each fracture event, consistent with increased exposure of electrode surfaces to liquid electrolyte that promotes active material dissolution. Thus, within this thesis, we address complementary aspects of electrochemomechanical fatigue: how electrochemical changes promote fracture in electrodes and solid electrolytes, and how this fracture in turn affects electrochemical performance of LIB devices.by Frank Patrick McGrogan IV.Ph. D

Effect of transition metal substitution on elastoplastic properties of LiMn[subscript 2]O[subscript 4] spinel

LiMn[subscript 2]O[subscript 4] (LMO) derivatives partially substituted with transition metals (e.g., Ni) have received attention for their higher energy density achieved at higher charge voltage than pure LMO, and may be attractive cathode candidates for emerging all solid state batteries. Accurate mechanical properties of these high voltage spinels are required for prediction of electrode and electrolyte fracture that may compromise battery lifetime and performance. Here, we quantified the Young’s elastic modulus E and hardness H for LMO, LiMn[subscript 1.5]Ni[subscript 0.5]O[subscript 4] (LMNO), and LiMn[subscript 1.5]Ni[subscript 0.42]Fe[subscript 0.08]O[subscript 4] (LMNFO) spinel microparticles via instrumented grid nanoindentation. Elastic modulus E and hardness H increased by more than 40% (up to 145 and 11 GPa, respectively) as a result of Ni or Ni/Fe substitution; such substitution also reduces the lattice parameter and increases the oxidization state of Mn. These results demonstrate how changes in transition metal occupancy can significantly affect the mechanical properties of LMO spinel, and provide critical parameters for designing against fracture in all solid state batteries.United States. Department of Energy. Office of Basic Energy Sciences (Award DE-SC0002633

Electrochemomechanical Fatigue: Decoupling Mechanisms of Fracture-Induced Performance Degradation in Li X Mn 2 O 4

Decades of Li-ion battery (LIB) research have identified mechanical and chemical culprits that limit operational lifetime of LIB electrodes. For example, severe capacity fade of unmodified LiXMn2O4 electrodes has been linked historically with Mn dissolution and, more recently, fracture of the electrochemically active particles. Mitigation approaches targeting both effects have prolonged cycle and calendar life, but the fundamental mechanistic sequences linking fracture to capacity fade in LiXMn2O4 and many other cathode materials remain ambiguous. Here, we investigate specifically the temporal correlations of fracture, capacity fade, and impedance growth to gain understanding of the interplay between these phenomena and the time scales over which they occur. By conducting controlled excursions into the cubic-tetragonal phase transformation regime of LiXMn2O4, we find that fracture contributes to impedance growth and capacity fade by two distinct mechanisms occurring over different time scales: (1) poorly conducting crack surfaces immediately hinder electronic conduction through the bulk of the electrode, and (2) capacity fades at a faster rate over multiple cycles, due plausibly to dissolution reactions occurring at newly formed electrode-electrolyte interfaces. The deconvolution of these effects in a well-studied cathode material such as LiXMn2O4 facilitates understanding of the complex relationship between mechanics and electrochemistry in LIB electrodes. ©2018 The Author(s). Published by ECS.US DOE Office of Basic Energy Science for the Chemomechanics of Far-From-Equilibrium Interfaces (COFFEI) small group (award no. DE-SC0002633)RSEC Program of the NSF (award no. DMR-1419807

Compliant Yet Brittle Mechanical Behavior of Li₂S–P₂S₅ Lithium-Ion-Conducting Solid Electrolyte

Young's modulus, hardness, and fracture toughness are measured by instrumented nanoindentation for an amorphous Li₂S–P₂S₅ Li-ion solid electrolyte. Although low elastic modulus suggests accommodation of significant chemomechanical strain, low fracture toughness can facilitate brittle crack formation in such materials.United States. Department of Energy. Office of Basic Energy Sciences (Grant DE-SC0002633

Effect of Electrochemical Charging on Elastoplastic Properties and Fracture Toughness of Li[subscript X]CoO[subscript 2]

Mechanical degradation of lithium-ion battery (LIB) electrodes has been correlated with capacity fade and impedance growth over repeated charging and discharging. Knowledge of how the mechanical properties of materials used in LIBs are affected by electrochemical lithiation and delithiation could provide insight into design choices that mitigate mechanical damage and extend device lifetime. Here, we measured Young's modulus E, hardness H, and fracture toughness K[subscript Ic] via instrumented nanoindentation of the prototypical intercalation cathode, Li[subscript X]CoO[subscript 2], after varying durations of electrochemical charging. After a single charge cycle, E and H decreased by up to 60%, while K[subscript Ic] decreased by up to 70%. Microstructural characterization using optical microscopy, Raman spectroscopy, X-ray diffraction, and further nanoindentation showed that this degradation in K[subscript Ic] was attributable to Li depletion at the material surface and was also correlated with extensive microfracture at grain boundaries. These results indicate that K[subscript Ic] reduction and irreversible microstructural damage occur during the first cycle of lithium deintercalation from polycrystalline aggregates of Li[subscript X]CoO[subscript 2], potentially facilitating further crack growth over repeated cycling. Such marked reduction in K[subscript Ic] over a single charge cycle also yields important implications for the design of electrochemical shock-resistant cathode materials.United States. Dept. of Energy. Office of Basic Energy Sciences. Division of Materials Sciences and Engineering (Award DE-SC0002633)United States. Dept. of Energy. Office of Science Graduate Fellowship (Contract DE-AC05-06OR23100)Massachusetts Institute of Technology (Salapatas Fellowship