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...

No abstract

International audienc

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...

No abstract

This paper has the aim to demonstrate that Acoustic Emission (AE) is an appropriate technique to become a diagnostic tool for state of charge, state of health and state of security of lithium-ion cells. These issues are actually key parameters for both performance and durability improvement. The presented results deal with the monitoring of the SEI formation and the first lithium ion intercalation on the surface and inside the graphite electrode. AE events coming from different sources have thus been identified. A large number of acoustic emission events occur during the film formation corresponding to gas evolution and cracking processes happening at the electrode surface (lithium intercalation). The acoustic emission signals were analyzed and correlated with electrochemical phenomena taking place during the cell operation.

No abstract

Blended negative electrodes with a mix of graphite and silicon are attractive solutions to extend the energy density of lithium-ion batteries. Given the huge volume expansion of silicon, the relative lithiation/delithiation of both active materials upon cycling is crucial to determine the mechanical behavior of the electrode. It is complicated however by the potential hysteresis displayed by silicon. We focus on a blended anode with 16 wt% of silicon carbon composite (SiC-C) at a gravimetric capacity of 1500 mAh.g −1 mixed with 84 wt% graphite. A multi-scale porous electrode model with three different hierarchical units has been developed to account for the morphology of the SiC-C composed of nanoflakes of silicon embedded in a carbon matrix. The developed physicsbased model is parametrized with appropriate experiments and validated for two electrodes loadings and various operating conditions. The validated model allows to probe the lithiation competition between the silicon and the carbon within the composite and the graphite. It is able to capture the streaking features revealed by synchrotron experiments. In particular, we show that at higher C-rates, the lithiation of the silicon is delayed and reduced compared to the graphite one. Quantifying this relative competition is valuable as aging effects are significantly accelerated by the expansion of the silicon.