Dynamics in blended electrode materials for Li-ion batteries: coupling electrochemistry and synchrotron based operando techniques

Abstract

Blending different active materials in the same electrode is a strategy used in commercial Li-ion batteries for electric vehicles, the aim being achieve better performance than what can be attained with a single component thanks to the so called “synergistic effects”. Yet, fundamental understanding of these synergistic effects has progressed at a slower pace. The main aim of this thesis has been to get further understanding of interaction between components and specific contributions to the performance of blended electrodes by combining advanced electrochemical methods (“decoupled blend setup” specifically designed which involves the use of three electrode cell, with two short-circuited working electrodes each containing one of the blend component) to operando (mostly synchrotron X-ray diffraction, XRD and absorption, XAS) characterization. The focus has been placed on both the development of methodologies and experimental protocols and the study of a range of materials already present in commercial batteries, mostly at the positive electrode. Electrodes comprising equivalent amounts of lithium-ion battery active materials, namely LiNi0.5Mn0.3Co0.2O2 (NMC), LiMn2O4 (LMO), LiFe0.35Mn0.65PO4 (LFMP) and LiFePO4 (LFP)) has been studied. The distribution of current between blend components was followed during continuous and pulsed charge and discharge processes. Pulsed decoupled electrochemical testing reveals the exchange of charge between blend components during relaxation, which has also been captured through time-resolved operando XRD. The directionality and magnitude of the charge transfer were found to depend on the nature of the components and the cell SoC, being also influenced by temperature. These findings can be rationalized considering both thermodynamics (voltage profile) and reaction kinetics of the blend constituents and contribute to advancing the understanding of internal dynamics in blended electrodes. Mixtures of LMO and NMC in different amounts have been also studied in more detail, with the composition with 25% LMO exhibiting the best electrochemical performance. The effective current load on each blend component can be significantly different from the nominal rate and also varies as function of SoC. Operando studies enabled to monitor the evolution of oxidation state and changes in the crystal structure, which are in agreement with the expected behaviour of the individual components considering the material specific electrochemical current loads. Blends containing lithium rich manganese rich layered oxides (LRO), which exhibits a significant irreversible capacity upon the first cycle, have been also studied. Mixing with delithiated LFP enables to mitigate this aspect while at the same time improving thermal stability. Finally, the methodology has been also extended to silicon/graphite blends, which are starting to be implemented at the negative electrode in commercial Li-ion cells, and the relative contribution of each component as a function of SoC has been followed at different rates and temperatures (0C to 45C). Since the blend components have different potential vs. capacity profiles, direct reaction between them to reduce/oxidize to achieve equilibrium is possible. Differences in reaction kinetics can lead to complex situations in which both compounds contribute to the overall capacity at a given potential, especially at high rates, and internal lithium redistribution between components takes place during relaxation periods. The findings reported in this thesis should contribute to achieve a better understanding of lithium dynamics in blended electrodes and help in its rational design and achieve optimal performance to match application requirements.

Date of Award	15 Jan 2025
Original language	English
Supervisor	Montserrat Casas Cabanas (Director) & Rosa Palacin Peiro (Director)