Thermal Stability and Degradation of NCA in Solid-polymer Batteries

Festkörperbatterien mit Polymerelektrolyt in Kombination mit Ni-reichen Kathodenmaterialien, wie zum Beispiel LiNi$_{0.8}$Co$_{0.15}$Al$_{0.05}$O$_{2}$ (NCA), sind vielversprechende Kandidaten für die nächste Generation von Energiespeichern. Angesichts der Vorteile hinsichtlich Sicherheit und der potentiellen Ermöglichung von hohen Energiedichten, haben sich zahlreiche (Start-up-) Unternehmen mit der Kommerzialisierung dieser Technologie auseinandergesetzt. Dennoch verbleiben einige technologische Herausforderungen, die auf die eingeschränkte Batterieleistung aufgrund des (thermisch) anfälligen Kathodenmaterials, die engen elektrochemischen Stabilitätsfenster nutzbarer Polymerelektrolyte, und andere, noch aufzuklärende, Degradationsprozesse zurückzuführen sind. Diese Arbeit ist daher auf die Analyse möglicher Degradationsphenomäne fokussiert, die auf unterschiedlichen Ebenen der Polymer-Festkörperbatterien in Kombination mit NCA vorkommen. Durch die Nutzung moderner Synchrotrontechniken, wird hierbei ein besonderer Schwerpunkt auf das chemische, thermische und mechanische Zusammenspiel der hierarchisch strukturierten Polymer-Festkörperbatterie gelegt. Die systematische Herangehensweise bestrebt hierbei die komplexe thermo-mechanische, chemo-thermische, und chemo-mechanische Stabilität von NCA im delithiierten Zustand, auf der Ebene eines einzelnen Partikels, aufzuklären. Dabei wird dargestellt, wie sich die durch thermische Belastungen ausgelöste Zersetzung in Form von freigesetztem Sauerstoff, Phasenumwandlung und gleichzeitiger Reduktion von Ni sowie durch die Entstehung von Mesoporen zeigt. Zudem wird die hohe Bedeutung des chemo-thermischen Zusammenspiels des Aktivmaterials, Polymerelektrolyten und Lithiumsalzes in der Kathode demonstriert. Letztendlich wird illustriert, dass die Entstehung von intergranularen Frakturen die Deaktivierung von Subpartikel-Domänen verursacht und folglich einen Verlust der Ionen- und Elektronenleitfähigkeit innerhalb von Partikeln, die wiederum eine lokale Impedanzerhöhung so wie Änderung der Transportwege von Ladungsträgern mit sich bringt

Effect of Liquid Electrolyte Soaking on the Interfacial Resistance of Li7La3Zr2O12 for All-Solid-State Lithium Batteries.

The impact of liquid electrolyte soaking on the interfacial resistance between the garnet-structured Li7La3Zr2O12 (LLZO) solid electrolyte and metallic lithium has been studied. Lithium carbonate (Li2CO3) formed by inadvertent exposure of LLZO to ambient conditions is generally known to increase interfacial impedance and decrease lithium wettability. Soaking LLZO powders and pellets in the electrolyte containing lithium tetrafluoroborate (LiBF4) shows a significantly reduced interfacial resistance and improved contact between lithium and LLZO. Raman spectroscopy, X-ray diffraction, and soft X-ray absorption spectroscopy reveal how Li2CO3 is continuously removed with increasing soaking time. On-line mass spectrometry and free energy calculations show how LiBF4 reacts with surface carbonate to form carbon dioxide. Using a very simple and scalable process that does not involve heat-treatment and expensive coating techniques, we show that the Li-LLZO interfacial resistance can be reduced by an order of magnitude

A Modified Electrochemical Model to Account for Mechanical Effects Due to Lithium Intercalation and External Pressure

For a battery cell, both the porosity of the electrodes/separator and the transport distance of charged species can evolve due to mechanical deformation arising from either lithium intercalation-induced swelling and contraction of the active particles or externally applied mechanical loading. To describe accurately the coupling between mechanical deformation and the cell\u27s electrochemical response, we extend Newman\u27s DualFoil model to allow variable, non-uniform porosities in both electrodes and the separator, which are dynamically updated based on the electrochemical and mechanical states of the battery cell. In addition, the finite deformation theory from continuum mechanics is used to modify the electrochemical transport equations to account for the change of the charged species transport distance. The proposed coupled electrochemomechanical model is tested with a parameterized commercial cell. Our simulation results confirm that mass conservation is satisfied with the new formulation. We further show that mechanical effects have a significant impact on the cell\u27s electrochemical response at high charge/discharge rates

Effect of Liquid Electrolyte Soaking on the Interfacial Resistance of Li7La3Zr2O12 for All-Solid-State Lithium Batteries.

The impact of liquid electrolyte soaking on the interfacial resistance between the garnet-structured Li7La3Zr2O12 (LLZO) solid electrolyte and metallic lithium has been studied. Lithium carbonate (Li2CO3) formed by inadvertent exposure of LLZO to ambient conditions is generally known to increase interfacial impedance and decrease lithium wettability. Soaking LLZO powders and pellets in the electrolyte containing lithium tetrafluoroborate (LiBF4) shows a significantly reduced interfacial resistance and improved contact between lithium and LLZO. Raman spectroscopy, X-ray diffraction, and soft X-ray absorption spectroscopy reveal how Li2CO3 is continuously removed with increasing soaking time. On-line mass spectrometry and free energy calculations show how LiBF4 reacts with surface carbonate to form carbon dioxide. Using a very simple and scalable process that does not involve heat-treatment and expensive coating techniques, we show that the Li-LLZO interfacial resistance can be reduced by an order of magnitude

Mesoscale Chemomechanical Interplay of the LiNi 0.8 Co 0.15 Al 0.05 O 2 Cathode in Solid-State Polymer Batteries

Complex chemomechanical interplay exists over a wide range of length scales within the hierarchically structured lithium-ion battery. At the mesoscale, the interdependent structural complexity and chemical heterogeneity collectively govern the local chemistry and, as a result, critically influence the cell level performance. Here we investigate the morphology and state of charge (SOC) inhomogeneity within secondary NCA particles that were cycled in solid polymer batteries. We observe substantial inhomogeneity in the nickel oxidation state (a proxy for SOC) and loss of structural integrity within secondary particles after only 20 cycles due to significant intergranular cracking. The formation of mesoscale cracks causes loss of ionic and electrical contact within cathode particles, triggering increases in local impedance and rearrangement of transport pathways for charge carriers. This can eventually lead to deactivation of subparticle level domains in solid-state lithium-ion batteries. Our findings highlight the importance of proper mesoscale strain and defect management in polymer lithium-ion batteries

A Study of Model‐Based Protective Fast‐Charging and Associated Degradation in Commercial Smartphone Cells: Insights on Cathode Degradation as a Result of Lithium Depositions on the Anode

The ever expanding mobile consumer electronic market has accelerated the need for safe and efficient fast-charging approaches that improve the overall speed of battery charging without hastened deterioration of the battery performance. Herein, the impact of a resource inexpensive, physics-based, electrochemically optimized fast-charging algorithm (charging time < 2 h) for mobile devices is investigated. A critical difference in the amount and morphology of lithium deposits on the anode for cells fast-charged without an optimized algorithm is observed and found to be the main cause of capacity decay. An in-depth study of the LiCoO2 cathode regions opposite to pronounced lithium deposits on the anode reveals a “mirroring” phenomenon, i.e., a frozen monoclinic phase, and inactivity to relithiation. In operando hard X-ray absorption spectroscopy reveals that degraded spots on harvested cathodes seem to be activated again and participate in the intercalation process when lithiated at low rates from lithium foil counter electrodes. On the other hand, tests at higher C-rates, closer to the actual fast-charging rate, reveal only negligible oxidation state changes and therefore poor performance