Inhomogeneous temperature distribution in a large-format lithium-ion cell or between cells in a module/pack may cause a non-uniform current distribution, causing a local difference in aging, and potentially faster global aging (capacity fade and impedance rise) of the module. To study this effect, LiNi1/3Mn1/3Co1/3O2/graphite lithium-ion pouch cells were cycled at 32, 36, and 40 degrees C as single cells and in parallel connection, representing uniform and non-uniform temperature distributions. The results show that the current distribution becomes less uniform after cycling at a higher rate and in a narrower state-of-charge range. Cycling with non-uniform temperature at 3C rate results in aging similar to that at the maximum uniform temperature, while at 1C rate the non-uniform aging follows the trend at the average temperature. The performance decay of the cells cycled at 3C is mainly driven by the cell at 40 degrees C which shows 30 % more capacity loss than the corresponding cell cycled singularly. This leads to additional considerations when designing for cycle life and reliability in fast charging applications and high-power applications such as in electric vehicles or frequency regulation in stationary storage.

The reactions in energy-optimized 25 Ah prismatic NMC/graphite lithium-ion cell, as a function of fast charging (1C4C), are more complex than earlier described. There are no clear charging rate dependent trends but rather different mechanisms dominating at the different charging rates. Ageing processes are faster at 3 and 4C charging. Cycling with 3C-charging results in accelerated lithium plating but the 4C-charging results in extensive gas evolution that contribute significantly to the large cell impedance rise. Graphite exfoliation and accelerated lithium inventory loss point to the graphite electrode as the source of the gas evolution. The results are based on careful post-mortem analyses of electrodes using: scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical impedance spectroscopy (EIS). SEM results show particle cracking independent of the charging rate used for the cycling. XPS and EIS generally indicate thicker surface film and larger impedance, respectively, towards the edge of the jellyrolls. For the intended application of a battery electric inner-city bus using this type of cell, charging rates of 3C and above are not feasible, considering battery lifetime. However, charging rates of 2C and below are too slow from the point of view of practical charging time.

Internal resistance is a key parameter that affects the power, energy, efficiency, lifetime, and safety of a lithium-ion battery. It grows due to chemical and mechanical battery wear during ageing. In this work, the effect of the jelly-roll winding curvature on impedance rise is investigated. NMC electrode samples, harvested from the curved as well as the flat regions of the jelly-roll from cycle-aged and calendar-aged prismatic cells (25 Ah, hard casing) are investigated by electrochemical impedance spectroscopy. After cycling, larger impedance rise is observed at the outer radius (concave) of the curved region compared to the inner radius (convex) or the flat region of the jelly-roll, and the difference increases with a decrease in the jelly-roll radius of curvature, from the cell skin towards the core. To identify the causes behind the observed difference in the impedance rise, investigations at different external compression (0 and 2.5 MPa) and temperature (5 and 25 degrees C) are performed. The results show that contact loss between the current collector and the active layer is the main source of the difference in impedance rise. Mechanical mechanisms that may cause the contact loss are discussed and design recommendations to mitigate the rise in impedance are given.

The effects of external compression on the performance and ageing of NMC(1/3)/Graphite single-layer Li-ion pouch cells are investigated using a spring-loaded fixture. The influence of pressure (0.66, 0.99, 1.32, and 1.98 MPa) on impedance is characterized in fresh cells that are subsequently cycled at the given pressure levels. The aged cells are analyzed for capacity fade and impedance rise at the cell and electrode level. The effect of pressure distribution that may occur in large-format cells or in a battery pack is simulated using parallel connected cells. The results show that the kinetic and mass transport resistance increases with pressure in a fresh cell. An optimum pressure around 1.3 MPa is shown to be beneficial to reduce cyclable-lithium loss during cycling. The minor active mass losses observed in the electrodes are independent of the ageing pressure, whereas ageing pressure affects the charge transfer resistance of both NMC and graphite electrodes and the ohmic resistance of the cell. Pressure distribution induces current distribution but the enhanced current throughput at lower pressures cell does not accelerate its ageing. Conclusions from this work can explain some of the discrepancies in non-uniform ageing reported in the literature and indicate coupling between electrochemistry and mechanics.

At electric vehicle fast-charging stations, it is generally recommended to avoid charging beyond ∼80% State-of-Charge (SOC) since topping-off to full capacity disproportionately increases the charging time. This necessitates studying its long-term impact compared to slower rate charging to full capacity typical of home or residential charging. Here we present the long-term ageing effects on commercial 18650 NMC-LMO/graphite cell cycled between 2.6–4.2V at three different charging protocols: 1.5 C-rate fast-partial charging (to 82.5% SOC), 0.5 C-rate slow standard charging without or with a constant-voltage step (to 93% or 100% SOC). Quantitative discharge-curve and postmortem analyses are used to evaluate ageing. The results show that ageing rate increases in the order: fast-partial charging < standard charging < standard charging with constant-voltage period, indicating that higher SOC-range near full capacity is more detrimental to battery life than fast-charging. The capacity fade is totally dominated by cyclable-lithium loss. The ∼8% NMC-LMO active material loss has negligible impact on the cell capacity fade due to the electrodes excess material in the fresh cell and its moderate loss rate with ageing compared to the cyclable-lithium. Similar ageing modes in terms of capacity fade and impedance rise are found irrespective of the charging protocol.

Lithium-ion batteries can age non-uniformly posing additional challenge in managing larger battery cells. For instance, a non-uniform distribution of solid electrolyte interphase (SEI) or plated lithium has been observed in cylindrical cells along the jelly roll length (1-2). The authors have suggested pressure distribution as a cause of this non-uniform ageing. This necessitates investigation of the effect of pressure on ageing. With different goals in mind, the effect of pressure on the rate of lithium-ion battery ageing has been studied previously (3-4).The work by Rubino et al.(3) indicates that the higher capacity fade in prismatic cells as compared to cylindrical cells of the same chemistry is due to the lower pressure in the former. However, the pressure was not a directly controlled parameter in this study. In contrast, the work by Arnold et al. (4) on commercial pouch cells shows that high stack pressure causes higher capacity fade, and that a small stack pressure is important to extend the life-time. However, the pouch cells were constrained along the thickness inducing non-uniform pressure on the different electrode layers and varying amplitudes of cyclic stack pressure during charge and discharge that is proportional to the external pressure. Hence, the above discrepancy of the effect of pressure calls for further investigation on ageing of model systems with single layer cells subjected to different levels of constant pressure. As an additional contribution, postmortem analysis is performed in order to understand how pressure level affects ageing mechanism. Here we present a study on commercial NMC positive and graphite negative electrodes in small laboratory pouch cells. The pouch cells are subjected to different stack pressure levels at controlled temperature using spring loaded stainless steel plates. Additionally, in order to monitor any differences in current distribution due to pressure and ageing, groups of cells are connected in parallel as the current distribution is measured using high precision shunt resistors and a Keithly differential multimeter. Capacity and impedance are measured periodically in order to track the performance changes with aging. A preliminary result indicates higher capacity fade in cells subjected to lower stack pressure. Results from EIS test indicate an increased ohmic resistance for high stack pressure cells, while low stack pressure cells show an increase in charge transfer resistance. Furthermore, different stack pressure on the cells of a parallel connected cell configuration results in current distribution with the cell subjected lower pressure taking slightly larger share of current. Details of the experimental techniques, and results from electrochemical and post mortem analysis will be presented and discussed during the meeting. Conclusions from this study will contribute to better understanding of the causes of non-uniform aging and suggestion for solutions to this problem through better cell design. References [1] M. Klett, R. Eriksson, J. Groot, P. Svens, K.S. Högstöm, R.W. Lindström, H. Berg, T. Gustafson, G. Lindbergh, K. Edström , J.Power Sources 257 (2014) 126-137. [2] M. Petzl, M. Kasper, M.A. Danzer, J.Power Sources 275 (2015) 799-807. [3] R.S. Rubino, H. Gan, E.E. Takeuchi, J. Electrochem. Soc. 148 (2001) A1029-A1033. [4] J. Cannarella, C.B. Arnold, J.Power Sources 245 (2014) 745-751.