Aging Mechanisms of Electrodes in LiFePO4/Graphite Batteries
Translated title
Aldringsmekanismer af Elektroder i LiFePO4/Grafit Batterier
Authors
Sørensen, Jonas Ilum ; Søgaard, Alex Juul
Term
4. term
Education
Publication year
2021
Submitted on
2021-06-03
Pages
183
Abstract
This thesis investigates aging mechanisms in commercial cylindrical lithium-ion batteries with lithium iron phosphate (LiFePO4) cathodes and graphite anodes, focusing on how cycle depth and average state of charge (SOC) at constant temperature influence electrode degradation. A post-mortem methodology was established for safe cell disassembly and electrode preparation. The electrodes were examined using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy (EIS) to assess morphology, elemental composition, structure, intercalation reactions, capacity, and resistance contributions. Findings indicate that deeper cycling promotes crack formation in graphite anodes, while higher average SOC accelerates lithium-ion loss, including via growth of the solid electrolyte interface (SEI), contributing to capacity fade. For the LiFePO4 cathode, results suggest a link between changes in elemental composition and structural modifications and show increased resistance of the carbon coating in cycle-aged cells. Overall, the majority of capacity loss is attributed to anode aging. This work provides a basis for further studies on the impact of cycle depth, cathode structural changes, and the composition of the SEI layer.
Dette speciale undersøger aldringsmekanismer i kommercielle cylindriske lithium-ion akkumulatorer med lithium-jern-fosfat (LiFePO4) katode og grafit anode ved at analysere, hvordan op-/afladningsdybde og gennemsnitlig ladningstilstand (SOC) ved konstant temperatur påvirker elektrodernes degradering. En post-mortem metode blev etableret for sikker adskillelse af cellerne og forberedelse af elektroder til videre analyse. Elektroderne blev karakteriseret med skannende elektronmikroskopi (SEM), energidispersiv røntgenspektroskopi (EDS), Raman spektroskopi, cyklisk voltammetri, kronopotentiometri og elektrokemisk impedansspektroskopi (EIS) for at belyse morfologi, elementar sammensætning, struktur, interkalationsreaktioner, kapacitet og modstandsbidrag. Resultaterne viser, at større op-/afladningsdybde fremmer revnedannelse i grafitanoden, mens høj gennemsnitlig SOC accelererer tabet af lithiumioner, bl.a. via vækst af et faststof-elektrolyt kontaktfladelag (SEI), hvilket bidrager til kapacitetstab. For LiFePO4-katoden antyder data en sammenhæng mellem ændringer i elementar sammensætning og strukturelle ændringer samt en øget elektrisk modstand i karbonbelægningen i cyklisk aldrende celler. Overordnet peger resultaterne på, at hovedparten af kapacitetstabet stammer fra anoden. Arbejdet kan danne grundlag for videre undersøgelser af effekten af cykeldybde, katodens strukturelle ændringer og SEI-lagets sammensætning.
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