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While nowadays lithium ion batteries are used in a wide range of consumer electronic devices and full electric cars, they still suffer from a limited energy density, required for larger driving ranges or long-lasting mobile phone batteries. One way to improve the energy density is to increase the electrode loading, i.e., the thickness of anode and cathode coating layers. By increasing the layer thickness, the ion transport ways are prolonged and rate limitations can occur if the drawn or applied current is too large, depending on the morphology of the coating and the ion transport properties in the liquid electrolyte (i.e., conductivity, diffusion coefficient, transference number, and thermodynamic factor).. Insufficient ion transport rates can lead to the buildup of concentration gradients within the liquid electrolyte phase of the cell and thus may cause large overpotentials, which in turn can limit the charge that can be extracted/inserted before the cut-off potentials are reached.1 In the case of the charging of graphite anodes, the buildup of concentration gradients across the electrode during lithium intercalation leads to an unequal distribution of overpotentials, which ultimately can result in lithium plating a high C-rates, particularly for thick electrodes and/or at low temperatures. The occurrence of lithium plating will decrease the coulombic efficiency and thus describes an important aging mechanism for long term cycling life. Our work focuses on the understanding of ionic transport properties of commonly used lithium ion battery electrolytes2 as well as on the quantification of geometric parameters3 of porous coatings. We will analyze the influence of the effective ionic transport on the rate performance of the cell as well as the onset of lithium plating in the anode depending on the cycling C-Rate. Figure 1 exemplarily shows the anode potential (vs. Li/Li+) of a graphite electrode in a simulated NMC/graphite full cell using the transport parameters determined for an electrolyte of LiPF6 in EC:EMC (3:7, w:w) at two different temperatures at the end of a 1C charge (cell potential of 4.3 V). It can be seen, that the critical point for lithium plating is reached at different fractions of the cell’s state of charge (SOC), which is a result of the different ionic transport properties at -10°C and 50°C. Our work will help to better understand the charging rate limitations of graphite anodes as a function of operating conditions, electrolyte transport properties, and electrode morphology. This can be used to improve electrode design and to optimize charging schemes, with the aim to prevent lithium plating and to decrease cell aging. Figure 1: Simulated graphite half-cell potentials at end of 1C charge of a 3 mAh/cm² NMC/graphite full cell (at 4.3 V) using temperature dependent transport parameters to illustrate the impact of ionic transport properties on the cell performance. References [1] K. G. Gallagher, S. E. Trask, C. Bauer, T. Woehrle, S. F. Lux, M. Tschech, P. Lamp, B. J. Polzin, S. Ha, B. Long, Q. Wu, W. Lu, D. W. Dees, and A. N. Jansen, J. Electrochem. Soc., 163, 138–149 (2016). [2] A. Ehrl, J. Landesfeind, W. A. Wall, and H. A. Gasteiger, J. Electrochem. Soc., 164, A826–A836 (2017). [3] J. Landesfeind, J. Hattendorff, A. Ehrl, W. A. Wall, and H. A. Gasteiger, J. Electrochem. Soc., 163, A1373–A1387 (2016). Acknowledgements We gratefully acknowledge the funding by the Bavarian Ministry of Economic Affairs and Media, Energy, and Technology for its financial support under the auspices of the EEBatt project. The authors thank rhd instruments GmbH & Co. KG for providing the measurement cell for determination of ionic conductivities. Figure 1

When spatially resolved models are applied to describe the behavior of lithium-ion pouch cells, the surface temperature of a cell as well as its terminal voltage are usually considered for validation purposes. Examining a cell’s behavior whilst being placed within a climate chamber, the influential heat transfer from the cell to its surroundings can only be estimated due to undirected, turbulent air flow and unknown heat flux along the attached cables. In the work presented here, a laboratory test setup for infrared thermography measurements on lithium-ion cells is introduced which allows for well-defined thermal test conditions and a controlled, directed air flow of 1 m/s to 3 m/s, whereby an overall temperature measurement accuracy of ±0.1 K is achieved. By monitoring the temperature gradient along additional copper bars mounted on the cell’s terminals, the conductive heat flux between the cell and the attached cables can be calculated. The convective heat transfer from the cell’s surface to the surrounding air can be estimated by calculating characteristic dimensionless groups (i.e. Reynolds, Prandtl and Nusselt numbers) from empirical correlations resembling the studied geometry and flow condition. By combining the two approaches, the cell’s energy balance can be calculated for purposes of both cell characterization and model validation. It turns out that the temperature distribution within the cell can be shifted dramatically depending on the electrical and thermal contact conditions prevailing at the cell’s terminals (see Fig. 1, left). We used additional layers of nickel plated steel pads for provoking a resistance increase between the test bench terminals (copper bars) and the cell terminals (tabs). Further, the positive tab’s response towards the artificial resistance increase differs substantially from the negative one (see Fig. 1, right). In literature, a characteristic higher positive tab temperature is often explained as being dependent on the difference in electrical resistance of the positive and negative current collector and electrode layers based on their electrical conductivity and thickness.1 However, the difference in resistance of the positive and negative electrodes seems to be superimposed by an additional resistance arising from a varying goodness of electrical contact between the cables and the positive and negative tabs.2,3 In this work, we extend this approach and investigate four different mechanisms which we assume to be dominating the observed variation in temperature distribution: the contact resistance between the cables and the cell’s tabs, the tab’s electrical bulk resistance, the contact resistance between the tabs and current collectors, and the electrical bulk resistance of the electrodes. To prove our presumptions, a spatially resolved model is prepared for determining the main influencing factors of the studied characteristic inhomogeneous temperature distribution within lithium-ion pouch cells. With the aid of experimental studies carried out by using the presented test setup, the impact of the four mechanisms on the spatial temperature distribution within lithium-ion pouch cells is discussed. Figure 1. Infrared thermographic measurement data (left) and cut lines along the positive and negative terminal (right) of a 40 Ah pouch cell during a 4C discharge rate at an ambient temperature of 25 °C and an air flow speed of 1 m/s flowing parallel to the cell’s planar surface. Varying the electrical contact resistance by including two (2H) and four nickel-plated steel strips (4H) shows to have a dominant influence on the location of the maximum surface temperature being shifted clearly towards the positive cell terminal, however, not on the absolute value of the measured temperature maximum. 1. U. S. Kim, C. B. Shin, and C. S. Kim, J Power Sources, 180, 909 (2008). 2. J. Yi, U. S. Kim, C. B. Shin, T. Han, and S. Park, J Electrochem Soc, 160, A437 (2013). 3. B. Wu, Z. Li, and J. B. Zhang, J Electrochem Soc, 162, A181 (2015). Figure 1

Thermally and electrochemically driven imbalances might reduce the overall net energy within large format lithium-ion batteries. In this study, we designed a proprietary multi-tab pouch cell for investigation of these imbalances depending on cell design and temperature. The cell consists of a single-layered NMC/graphite electrode pair with a length of 50 cm and a width of 10 cm. 22 equidistantly distributed tabs are connected to each electrode giving insight to local potentials within the current collector domain (Fig. 1). Temperature variation is negligible due to the comparably small heat generation of the cell and the global influence of temperature can be analyzed. A high precision measurement setup is prepared for a μV resolution between tab pairs. By that, the internal current density distribution can be assessed. The cell is operated at 5, 25 and 40 °C and 0.1, 0.5, 1 and 2C current rate, whereby three different tab patterns are applied: Standard pouch cell, counter-tab and center-tab design. It turns out that higher potential drops along the cell as well as larger potential variation occur at elevated temperature. Further, the differentiation of potential difference between the tabs indicates a larger state of charge inhomogeneity at higher temperature, even at low current rates. It might be concluded that the interaction of an increased current collector resistance and an increased electrochemical performance in between the collector foils provokes a more distinct inhomogeneity in current density at higher temperature. Corresponding to our previous work [1,2], we apply a multi-dimensional modeling approach to the presented multi-tab cell to describe the temperature dependent current spread for purpose of optimized cell design. Based on these simulations, a maximum tab-to-tab distance depending on temperature can be estimated under the objective of an acceptable potential drop along the current collectors. References: [1] P.J. Osswald, S.V. Erhard, J. Wilhelm, H.E. Hoster and A. Jossen: Simulation and Measurement of Local Potentials of Modified Commercial Cylindrical Cells - Part I: Cell Preparation and Measurements, Journal of The Electrochemical Society 162 (10), A2099, 2015 [2] S.V. Erhard, P.J. Osswald, J. Wilhelm, A. Rheinfeld, S. Kosch and A. Jossen: Simulation and Measurement of Local Potentials of Modified Commercial Cylindrical Cells - Part II: Multi-Dimensional Modeling and Validation, Journal of The Electrochemical Society 162 (14), A2707, 2015 Figure 1

Non-destructive studies to investigate Li-ion batteries in situ/operando are a challenge although they show much more details on the processes during charging/discharging and aging. Especially, the high sensitivity of neutrons to light elements as Li and easier distinction of neighbor elements in comparison to X-rays lead to a powerful tool in battery research. With neutron diffraction, changes in a commercial 18650-type NMC (LiNi1/3Mn1/3Co1/3O2)/graphite cylindrical cell can be followed nicely at the graphite anode during intercalation/de-intercalation in the charging/discharging process by phase detection starting from pure graphite via LiCx phases [1,2]. In addition, the influence of temperature, C-rate or any relaxation if holding at a certain state of charge can be described in detail [2]. For direct visualization of electrodes, neutron radiography or neutron tomography was applied to observe directly spatial inhomogeneities down to 1/3Mn1/3Co1/3O2 (NMC) lithium pouch cells using the PGAA (prompte gamma activation analysis) technique [5]. This method exploits the irradiation of sample material with neutrons and the subsequent detection of prompt gamma rays emitted during de-excitation of the compound nuclei. The method determines elemental composition and concentration of sample materials down to the ppm range. Thus PGAA can detect even trace amounts of elements on electrodes. If compositional changes during charging/discharging occur in thin pouch cells, the transmission method of small-angle neutron scattering can be applied to describe the gradual particle lithiation with the measured total integrated intensity in graphite/LiNi1/3Mn1/3Co1/3O2cells [6]. In addition if present objects as particles on the nanoscale level (1-300 nm) can be characterized. 1.) M.A. Rodriguez, M.H. Van Benthem, D. Ingersoll, Powder Diffraction, 2010, 25, 2,143. 2.) V. Zinth, C. von Lüders, M. Hofmann, J. Hattendorf, I. Buchberger, S. Erhard, J. Rebelo-Kornmeier, A. Jossen, R. Gilles, Journal of Power Sources, 2014, 271, 152. 3.) M. Hofmann, R. Gilles, Y. Gao, J.T. Rijssenbeek and M.J. Mühlbauer, Journal of the Electrochemical Society, 2012, 159 (11), A1827. 4.) V. Zinth, S. Seidlmayer, N. Zanon, G. Crugnola, M. Schulz, R. Gilles, M Hofmann, Journal of The Electrochemical Society, 2015, 162(3), A384. 5.) I. Buchberger, S. Seidlmayer, A. Pokharel, M. Piana, J. Hattendorff, P. Kudejova, R. Gilles, H.A. Gasteiger, Journal of the Electrochemical Society, 2015, 162(14), A2737. 6.) S. Seidlmayer, J. Hattendorff, I. Buchberger, L. Karge, H. A. Gasteiger, R. Gilles, Journal of The Electrochemical Society, 2015, 162(2), A3116.