Calendar aging of commercial graphite/LiFePO4 cell – Predicting capacity fade under time dependent storage conditions

169

107

For reliable lifetime predictions of lithium-ion batteries, models for cell degradation are required. A comprehensive semi-empirical model based on a reduced set of internal cell parameters and physically justified degradation functions for the capacity loss is developed and presented for a commercial lithium iron phosphate/graphite cell. One calendar and several cycle aging effects are modeled separately. Emphasis is placed on the varying degradation at different temperatures. Degradation mechanisms for cycle aging at high and low temperatures as well as the increased cycling degradation at high state of charge are calculated separately. For parameterization, a lifetime test study is conducted including storage and cycle tests. Additionally, the model is validated through a dynamic current profile based on real-world application in a stationary energy storage system revealing the accuracy. Tests for validation are continued for up to 114 days after the longest parametrization tests. The model error for the cell capacity loss in the application-based tests is at the end of testing below 1% of the original cell capacity and the maximum relative model error is below 21%. Today, stationary energy storage systems utilizing lithium-ion batteries account for the majority of new storage capacity installed. 1In order to meet technical and economic requirements, the specified system lifetime has to be ensured.For reliable lifetime predictions, cell degradation models are necessary. Physicochemical models that include aging mechanisms are based on a detailed set of parameters which are often not readily available, computationally costly and require experimental parameterization of degradation rates.2-4 Instead, purely empirical models can be parameterized without knowledge of internal cell setup through extensive testing. Several purely empirical studies capture calendar aging 5,6 or cycle aging 7,8 without evaluating interdependencies. Through superposition, some empirical model approaches combine calendar and cycle aging 9-12 but tend to neglect the temperature dependence of the cycle aging mechanisms and are prone to extrapolation errors due to the utilized mathematical functions.Due to the limited knowledge about degradation mechanisms, empirically based models conventionally lump multiple degradation effects into single functions. This leads to the aforementioned prediction errors when deviating from the parameterization test conditions. E.g. for cycle aging, Waldmann et al. reported a transition of dominating aging mechanisms at 25• C. 13 The aging for temperatures above 25• C was attributed to the solid-electrolyte interface (SEI) growth and cathode degradation, while below 25• C the aging was attributed to lithium plating. In fact, for an improved understanding of cell internal degradation, model development should aim for a separation of the degradation mechanisms wherever possible. The respective mechanisms can then be modeled through functions that are suitable for the degradation driving factors.In this work, ...

Section: Calendar Agingmentioning

confidence: 99%

Comprehensive Modeling of Temperature-Dependent Degradation Mechanisms in Lithium Iron Phosphate Batteries

Schimpe

Kuepach

Naumann

et al. 2018

169

107

“…These results are consistent with previously reported results. [41][42][43][44] For high temperatures, the strong difference observed between SOC=90% and SOC=100% in calendar ageing of Graphite/NMC Li-ion cells has already been observed in the literature. The influence of the SOC on the ageing is not linear with the SOC.…”

mentioning

confidence: 56%

Effects of Biphenyl Polymerization on Lithium Deposition in Commercial Graphite/NMC Lithium-Ion Pouch-Cells during Calendar Aging at High Temperature

Matadi

Géniès

Delaille

et al. 2017

Self Cite

Metallic lithium deposition is a typical aging mechanism observed in lithium-ion cells at low temperature and/or at high charge rate. Lithium dendrite growth not only leads to strong capacity fading, it also causes safety concerns such as short-circuits in the cell. In applications such as electric vehicles, the use of lithium-ion batteries combines discharging, long rest time and charging phases. It is foremost a matter of lifetime and safety from the perspective of the consumer or the investor. This study presents the post-mortem analyses of commercial 16 Ah Graphite/NMC (Nickel Manganese Cobalt layered oxide) Li-ion pouch cells. The cells were degraded by calendar aging at high temperature with or without periodic capacity tests. Unexpected local depositions of metallic lithium were confirmed on graphite electrodes by Nuclear Magnetic Resonance (NMR). Biphenyl, a monomer additive present in the liquid electrolyte, generates gas during its polymerization reaction occurring at high temperature and at high state of charge. As a result, dry-out areas are present between the electrodes leading to high impedance regions and no charge transfer between the electrodes. It is at the border of these areas that lithium metal is deposited. Lithium-ion batteries have the particularity of addressing applications as diverse as mobile electronic devices, electric vehicles or stationary applications of renewable energy. The use of rechargeable electrochemical cells is first and foremost a question of autonomy, lifetime and safety.1-3 So it is necessary to further increase the current performance of lithium-ion batteries.Deposition of metallic lithium or "lithium plating" is one of the aging mechanisms that is limiting the lifetime of lithium-ion batteries. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] In the literature, it is reported that the mechanism of lithium deposition generally occurs during the charge at low temperatures and/or at high current-rates. The intercalation of lithium ions into the structure of the negative electrode is in competition with the deposition of metallic lithium on top of the Solid Electrolyte Interphase (SEI). The morphology of the negative electrode, the porosity, the conductivity of the electrolyte and the state of lithiation are parameters to be considered. 8 This aging mechanism depends on intrinsic criteria of the components of the cell (active material, electrodes and electrolyte composition [21][22][23][24][25][26][27][28][29][30] ) and extrinsic to the system (temperature and charging current).Contact between the deposited lithium and the electrolyte (above the surface of the SEI) leads to the oxidation of lithium to form species such as ROCO 2 Li, Li 2 CO 3 or LiF. 15 The deposited lithium becomes isolated and electrochemically inactive and cannot participate in the electrochemical process at the electrodes. The deposit can grow in height from graphite particles and form dendrites which can pass through the separator and cause short circuit. This phenomena in combinatio...

“…In general, one has to differentiate between cycling 7,16,18,21,[23][24][25]32 and calendar aging. 7,19,[21][22][23][24]27 Since commercial Li-ion cells can be subject to calendar aging in the time between manufacturing and delivery, it is good practice to measure the discharge capacity at t = 0 for each cell that undergoes an aging test. Since the discharge capacity depends mainly on temperature, depth-of-discharge (DOD), and discharge current, the SOH is usually monitored by regular check-ups with defined parameter sets, 7,16,21,23,24 which can vary depending on the application.…”

Section: 5mentioning

confidence: 99%

“…• C, 16,22,24 DOD of 100%, 16,21 and discharge rates of 1C 7,16,21,22,24 or lower 23 are used in check-ups. The performance decrease on the cell level is mainly based on chemical degradation reactions on the material and on the electrode level (see Figure 9).…”

Section: 5mentioning

confidence: 99%

Review—Post-Mortem Analysis of Aged Lithium-Ion Batteries: Disassembly Methodology and Physico-Chemical Analysis Techniques

Waldmann

Iturrondobeitia

Kasper

et al. 2016

247

149

Improvement of life-time is an important issue in the development of Li-ion batteries. Aging mechanisms limiting the life-time can efficiently be characterized by physico-chemical analysis of aged cells with a variety of complementary methods. This study reviews the state-of-the-art literature on Post-Mortem analysis of Li-ion cells, including disassembly methodology as well as physicochemical characterization methods for battery materials. A detailed scheme for Post-Mortem analysis is deduced from literature, including pre-inspection, conditions and safe environment for disassembly of cells, as well as separation and post-processing of components. Special attention is paid to the characterization of aged materials including anodes, cathodes, separators, and electrolyte. More specifically, microscopy, chemical methods sensitive to electrode surfaces or to electrode bulk, and electrolyte analysis are reviewed in detail. The techniques are complemented by electrochemical measurements using reconstruction methods for electrodes built into half and full cells with reference electrode. The changes happening to the materials during aging as well as abilities of the reviewed analysis methods to observe them are critically discussed. Li-ion batteries are currently used in everyday objects such as smart-phones, power tools and tablet computers as well as in the growing fields of light electric vehicles (LEVs), unmanned aerial vehicles (UAVs), battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). [1][2][3][4] Furthermore, the rise of renewable energy sources like wind and solar power, which are only intermittently available, demands reliable and highly flexible stationary energy storage solutions, which provide high capacities and predictable life-times. 2,5Aging of Li-ion batteries is a general problem for manufacturers as they have to guarantee long-term reliability of their products. For state-of-the-art cells, degradation effects on the material level lead to capacity fade and resistance increase on the cell level. The aging state of a battery is often characterized by the state-of-health (SOH) in % according to 3,16,22,[29][30][31] SO H(t) = discharge capacity (t) discharge capacity (t = 0)[1]where t represents the aging time. In general, one has to differentiate between cycling 7,16,18,21,[23][24][25]32 and calendar aging. 7,19,[21][22][23][24]27 Since commercial Li-ion cells can be subject to calendar aging in the time between manufacturing and delivery, it is good practice to measure the discharge capacity at t = 0 for each cell that undergoes an aging test. Since the discharge capacity depends mainly on temperature, depth-of-discharge (DOD), and discharge current, the SOH is usually monitored by regular check-ups with defined parameter sets, 7,16,21,23,24 which can vary depending on the application. Typically, a temperature of 25• C, 16,22,24 DOD of 100%, 16,21 and discharge rates of 1C 7,16,21,22,24 or lower 23 are used in check-ups. The performance dec...