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

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

Evaluation of cyclic aging tests of prismatic automotive LiNiMnCoO2-Graphite cells considering influence of homogeneity and anode overhang

Cyclic aging tests of 20 compressed prismatic automotive Li(NiMnCo)O2|Graphite cells are evaluated. The shallow cyclic aging tests are conducted around five average SOCs with respect to the anode. The cells are cycled at two DODs and two C-rates. The irreversible capacity loss is evaluated by the slope of the near-linear part at the end of aging test. The homogeneity of lithium distribution (HLD) is associated with peak height of differential voltage analysis (DVA) and to capacity difference analysis (CDA). The evaluations of DVA, CDA and capacity fade curve are depending mainly on the average SOC and hardly on DOD or C-rate. The trends correlate with the volume expansion originated from the graphite. The highest HLD and the lowest capacity fade are reached around 50% SOC where hardly any additional volume expansion occurs. In the SOC regions with high volume expansion of the graphite the HLD reduces dramatically and the capacity fade rises towards 0% and 100%, respectively. Due to smeared characteristics in DVA, capacity loss cannot be directly separated into shares related to anode overhang, HLD, loss of active material and residual irreversible losses. The combination of cell compression and high gradients of volume expansion during shallow cycling is found to be the root cause for the flattening of DVA curves

Float Current Analysis for Fast Calendar Aging Assessment of 18650 Li(NiCoAl)O2/Graphite Cells

Float currents are steady-state self-discharge currents after a transient phase—caused by anode overhang, polarization, etc.—is accomplished. The float current is measured in this study with a standard test bench for five 18650 cells (Samsung 25R) at potentiostatic conditions while the temperature is changed in 5 K steps from 5 °C to 60 °C. The entire test is performed in about 100 days resulting in 12 measurement points per cell potential for an Arrhenius representation. The float current follows the Arrhenius law with an activation energy of about 60 kJ/mol. The capacity loss measured at reference condition shows a high correlation to the results of float currents analysis. In contrast to classical calendar aging tests, the performed float current analysis enables determining the aging rate with high precision down to at least 10 °C. Returning from higher temperatures to 30 °C reference temperature shows reducing float currents at 30 °C for increasing temperature steps that may originate from an hysteresis effect that has to be investigated in future publications

Investigation of capacity recovery during rest period at different states-of-charge after cycle life test for prismatic Li(Ni1/3Mn1/3Co1/3)O2-graphite cells

In this publication two strategies are introduced to assess irreversible capacity loss during shallow cycling at different average SOCs. Due to superposed reversible capacity effects, a simple evaluation of capacity trend is not sufficient. Those reversible effects are related to contributions of the anode overhang (geometrical oversized anode) and to the homogeneity of lithium distribution (HLD). For both strategies the cycling test is additionally followed by a calendaric aging test to recover capacity. While the contribution of HLD can be assessed by storing the cells at the same average SOC as during cycling, the contribution of the anode overhang is evaluated for a defined low SOC. During the storage phase in all cases the extractable capacity rises supporting the reversible capacity theory. Moreover, the HLD, measured with differential capacity analysis and capacity difference analysis, rises as well; this is the case for all test conditions exhibiting the reversible nature of HLD and its influence on extractable capacity. The irreversible capacity losses are compared to an alternative method, called ‘slope method’, assuming that the aging is nearly linear and that the linear part at the end of test is mainly attributed to irreversible aging. While the results of both methods are in the same order of magnitude, the relaxation method can be applied, not only to static, but also to any dynamic aging profiles

Post-mortem analysis on LiFePO 4 |Graphite cells describing the evolution & composition of covering layer on anode and their impact on cell performance

During cyclic aging of lithium-ion batteries the formation of a μm-thick covering layer on top of the anode facing the separator is found on top of the anode. In this work several post-mortem analyses of cyclic aged cylindrical LFP|Graphite cells are evaluated to give a detailed characterization of the covering layer and to find possible causes for the evolution of such a layer. The analyses of the layer with different methods return that it consists to high percentage of plated active lithium, deposited Fe and products of a solid electrolyte interphase (SEI). The deposition is located mainly in the center of the cell symmetrical to the coating direction. The origin of these depositions is assumed in locally overcharged particles, Fe deposition or inhomogeneous distribution of capacity density. As a secondary effect the deposition on one side increases the thickness locally; thereafter a pressure-induced overcharging due to charge agglomeration of the back side of the anode occurs. Finally a compact and dense covering layer in a late state of aging leads to deactivation of the covered parts of the anode and cathode due to suppressed lithium-ion conductivity. This leads to increasing slope of capacity fade and increase of internal resistance