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...

Durability and performance of Li-ion cells are impaired by undesirable side reactions, observed as capacity decreases and resistance increases during their usage. This degradation is caused by aging mechanisms on the material level including surface film formation, especially in the case of graphite-based anodes. The present study evaluates the applicability of glow discharge optical emission spectroscopy (GD-OES) as a powerful tool to study aging-induced film formation on graphite anodes of Li-ion cells, including deposition of metallic Li. The technique provides depthresolved information on the elemental distribution in the samples from the anode surface to the current collector (through-plane resolution). Additionally, conducting GD-OES depth profiling at different positions of an aged graphite anode reveals differences in surface film growth across the anode plane (in-plane resolution). After verification of the GD-OES method by well-established analytical techniques, aged anodes from commercial state-of-the-art Li-ion cells are analyzed. The results show through-plane and in-plane inhomogeneity in surface film growth: local island-like Li deposition is revealed for 16Ah pouch cells cycled at 45 °C and high charging current density while a more homogeneous Li plating gradient is found for cycling 26650-type cells at −20 °C.

Material degradation is an issue limiting the life-time of Lithium-ion batteries. This study conducts quantitative observations of performance and material degradation in a commercial high-power Lithium-ion battery as a function of aging time and ambient temperature. Batteries are cycled until different states-of-health (SOHs) in the range of 100% to 78% are obtained before being disassembled. Inductively coupled plasma (ICP-OES) analysis of the electrodes from cells in different SOHs reveals a linear increase in Li, P, and Mn on anodes with aging time and thus allows conclusions on the kinetics of the aging reactions. The vertical distribution of these decomposition products in the anode is investigated by glow discharge optical emission spectroscopy (GD-OES) depth profiling. Following this, the chemical data from Post-Mortem analysis are correlated to the electrochemical performance of the cells. Combining chemical data sets from aged cells in different SOHs with data from cells aged at different ambient temperature reveals an Arrhenius-like behavior of chemical changes on the anodes.

Glow discharge optical emission spectroscopy (GD-OES) is employed to detect and quantify Li deposition as a function of depth on graphite electrodes. Commercial cells with graphite anodes were subject to Li plating by being cycled at 5 Li-ion batteries suffer from aging reactions which limit their lifetime in applications.1,2 It is known that chemical reactions such as SEI growth on anodes and dissolution of cathode active material are accelerated with increasing temperature, following an Arrhenius-like behavior. [3][4][5] In contrast, Li deposition and subsequent reaction with electrolyte is usually observed after charging at low temperatures and/or high rates. 4,[6][7][8][9][10][11] Li deposition causes rapid capacity decay 4,11 and can significantly reduce cell safety. 12The reason for lithium deposition is anode polarization, which drops below 0 V vs. Li/Li + in the respective cases. 4,6 However, measuring the anode potential in full cells requires a reference electrode that is not present in commercial cells. Therefore, non-destructive methods for detection of Li deposition and quantification are under development by several groups. 7-10In order to validate non-destructive detection methods, one has to be able to detect Li chemically after cell opening. Petzl et al. recently used the reactivity of Li with water as a sign of metallic Li on graphite anodes. 7 In post-mortem analysis, the high reactivity of Li metal makes it challenging to open the cell in charged state, especially in the presence of deposited Li. Analysis of graphite anodes subject to Li deposition can be done using methods such as inductively coupled plasma spectroscopy (ICP) and atomic absorption spectroscopy (AAS), however, distinguishing between metallic Li and intercalated Li is not possible. Therefore, a quantification of Li deposition in postmortem analysis is not available in literature at the moment.Glow Discharge Optical Emission Spectroscopy (GD-OES) was introduced by Grimm in 1968 13 and later used as a standard method for characterization of non-porous metallic materials, e.g. in steel industry. Similar to X-ray photoelectron spectroscopy (XPS) depth profiling, GD-OES employs an inert gas (usually Ar) as discharge source to perform a depth-resolved elemental analysis sensitive to light elements like Li. XPS is, however, a surface-sensitive method while GD-OES can be applied through the whole electrode coating.Recently, GD-OES has been also applied to porous electrodes from Li-ion batteries by Saito et al.14 The technique was further developed by Takahara et al., focusing on reactive sputtering, Li quantification in the electrodes and analysis of surface deposition on graphite anode (SEI). 15-18The present study introduces a novel method to prove Li deposition on graphite anodes of Li-ion batteries by means of GD-OES depth profiling. For comparison, anodes from cells where Li plating is expected (cycling at low temperatures) are compared to anodes without Li plating but initial SEI formation (anodes from fresh cell). ExperimentalG...

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