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Tracing Interfacial Reactivity of Lithium Transition Metal Oxides Through Outgassing

  • Author(s): Renfrew, Sara
  • Advisor(s): McCloskey, Bryan D
  • et al.
Abstract

Lithium transition metal oxides are Li host structures used as cathodes for Li-ion batteries. Li is removed (deintercalated) from the oxide during charge and is inserted (intercalated) on discharge. The reversibility of the process is enabled by the transition metal redox and relative stability of the oxide. The achievable reversible capacities of typical Li-ion cathodes are only approximately half of the total possible capacity, as calculated by the total Li content. Attempts to increase the capacity, i.e., intercalate and deintercalate more Li from the oxide host, lead to poor cycling stability and limited working lifetime of the battery due to inherent instabilities at the cathode/electrolyte interface at high potentials and/or high extents of delithiation (charge). These instabilities include loss of active material due to decomposition, transition metal dissolution, particle cracking and isolation, irreversible surface reconstruction, as well as resistive surface film formation.

Many of these interfacial instabilities evolve signature gases. This dissertation aims to trace these fundamental instabilities by monitoring outgassing and quantifying surface changes during electrochemical modification. The main technique used in this work is differential electrochemical mass spectrometry (DEMS), which allows detection and quantification of gases evolved in-situ during electrochemical measurements. Additionally, the DEMS is modified to also quantify gases evolved from ex-situ acid titrations, which allow identification and quantification of surface changes of cathode materials after electrochemical modification.

One difficulty in understanding the fundamental reactivity of the cathode/electrolyte interface is the uncertainty in surface structure of lithium transition metal oxides. Due to incomplete sold-state reaction in synthesis or even inevitable reaction with atmospheric H2O and CO2, the surface of lithium transition metal oxides are contaminated with metal hydroxides and carbonates.

A major result of this dissertation is that rigorous quantification of surface contaminants is needed to understand outgassing mechanisms of the cathode/electrolyte interface. For example, in the first charge-discharge (deintercalation-intercalation) cycle of lithium transition metal oxides, the surface contaminant Li2CO3 decomposes above 3.8 V vs. Li/Li+ to CO2 and, depending on the identity of the electrolyte, can directly account for 15-100 % of the total evolved CO2 in the first cycle.

To that end, isotopic labeling of the cathode with 18O is used throughout this work to distinguish the cathode from the electrolyte and the outgassing exhibited by electrolyte degradation, cathode surface degradation, and any mixed cathode/electrolyte reactivity. This dissertation introduces titration protocols that allow accurate determination of the 18O-enrichment of surface carbonates on lithium transition metal oxides.

In addition, also presented is a new titration that quantifies surface peroxo-like character of lithium transition metal oxides that arises due to irreversible surface reconstruction, which has been traditionally measured by electron microscopy techniques. Use of this new peroxide titration shows that in the first cycle the irreversible transformation on the surface of lithium transition metal oxides is due to the extent of delithiation (charge capacity) and is a material property of the prepared oxide that largely does not depend on the identity of the electrolyte. It also shows that some extent of the peroxo-like character developed on charge is reversible, allowing a new quantification of near-surface oxygen redox.

Lastly, this dissertation explores the effects of surface modifications of lithium transition metal oxides on the cathode/electrolyte reactivity. Surface contaminants and local defects fundamentally alter electrolyte decomposition and the irreversible surface reconstruction of lithium transition metal oxides. The main conclusions are that increased contaminants and increase lithium defects on the surface of lithium transition metal oxides increase the electrolyte decomposition.

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