Dissection and quantitative description of aging of lithium-ion batteries using non-destructive methods validated by post-mortem-analyses

• Sezierung und quantitative Beschreibung der Alterung von Li-Ionen-Batterien mittels zerstörungsfreier Methoden validiert durch Post-Mortem-Analysen

Lewerenz, Meinert; Sauer, Dirk Uwe (Thesis advisor); Eichel, Rüdiger-Albert (Thesis advisor)

Aachen : Institut für Stromrichtertechnik und Elektrische Antriebe (ISEA) (2018)
Book, Dissertation / PhD Thesis

In: Aachener Beiträge des ISEA 110
Page(s)/Article-Nr.: 1 Online-Ressource (iv, 139 Seiten) : Illustrationen, Diagramme

Dissertation, RWTH Aachen University, 2018

Abstract

In this thesis over 50 cylindrical LiFePO4|Graphite cells with a capacity of 8 Ah are analyzed, utilizing several non-destructive methods that are validated with post-mortem-analyses. The scope of the analyses is to dissect the degradation of capacity and performance into disjunct dominating aging effects. This includes different irreversible aging mechanisms like the formation of solid-electrolyte-interphase (SEI), particle cracking, lithium plating or transition metal deposition. Beside the irreversible capacity losses, superposed reversible effects can be observed, too. As reversible losses and gains, the flow of active lithium from and to the anode overhang has been identified. Another reversible effect could be found in the homogeneity of lithium distribution (HLD) that is measurable evaluating the peak height in differential voltage analysis (DVA). The HLD is a measure for the SOC spread within the cell during a charge or a discharge. A reduced or increased HLD leads to higher or lower amount of extractable lithium due to the limits of the cut-off voltages. Moreover, passivated lithium plating leads to an irreversible capacity fade. It is associated to the occurrence of a dense covering layer that impedes lithium-ions to pass. This leads to an additional increased loss of active lithium trapped within the deactivated active materials. This is eye-catching by a significantly increased slope of capacity fade and internal resistance. The covering layer evolution can be measured beside DVA, using capacity difference analysis (CDA) introduced during this work for the first time. It describes the lateral flow of lithium between passive and active electrodes and is helpful to detect massive plating. Another irreversible effect is the loss of anode active material (LAAM) for cells cycled from 0-100%.As a result most probably the loss takes place inhomogeneously over the cell and is pronounced where the counter pressure in the cell is low. Moderate LAAM leads to no direct additional loss of capacity. Only if the lost particles are charged, an influence on the extractable capacity is observable. This could be estimated from test results to 10% SOC or lower. The losses, according to the cell design, have not been presented before. Considering temperature, the major loss mechanism, besides well-known SEI formation, has been associated to Fe dissolved from the cathode and thereafter deposited on the anode. The deposition on top of the anode leads to an increased SEI formation and to a sealing of graphite pores. As discussed before, trapped lithium reduces additionally extractable capacity and lowers performance. Once the pores are sealed, the further aging, due to Fede position, reduces pace of degradation. The temperature threshold where Fe dissolution begins could be identified for this cell to 45-50 °C. The floating currents are a measure of irreversible loss of lithium in the SEI for calendaric aging, as could be shown in this thesis. The irreversible losses at moderate temperatures (<45 °C) can be fitted in a good approximation with a linear function. The slope of the function represents the aging and follows with respect to state-of-charge the half-cell potential of the cathode.

Institutions

• Chair of Electrochemical Energy Conversion and Storage Systems [618310]
• Institute of Power Electronics and Electrical Drives [614500]