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Deposition of metallic Li is a severe aging mechanism in Lithium-ion cells. This study evaluates the influence of the main operating parameters leading to deposition of Li: temperature, charging C-rate, and end-of-charge voltage. Therefore both, graphite anodes and NMC cathodes from commercial 16Ah pouch cells are reconstructed into 3-electrode full cells Due to their comparably high energy and power densities, Lithiumion batteries are currently used in state-of-the-art electric cars.1-3 In automotive applications, battery life-times of 10 years are expected for customer acceptance. However, it is known that life-time of Lithiumion batteries is limited by aging mechanisms.4-11 One of these aging mechanisms is deposition of metallic Li on anodes. 8,[12][13][14][15] Due to the high chemical reactivity of metallic Li, it readily reacts with electrolyte leading to capacity loss of the cell. 8,9,14,16 It is known that Li deposition mainly depends on (i) charging C-rate, 14,16,17 (ii) temperature, 8,9,12,14,[16][17][18] and (iii) end-of-charge voltage/state-of-charge.14,17 Several authors reported trends for variation of only one of these parameters respectively. E.g. low temperatures during charging are reported to lead to Li plating on graphite anodes. 8,9,14,16,19 However, there is a dearth of experimental results on the topic how specific combinations of operating conditions affect Li deposition. Since such experiments are highly interesting for extension of cycle life 14,20 and improvement of cell safety, 21 this is the topic of the present paper.The reason for deposition of Li on anodes are negative anode potentials E anode vs. (Li/Li + ). 8,12,14,20 Unfortunately, in commercial cells only the cell voltageand not the anode potential vs. (Li/Li + ) can be measured. The reason is that commercial cells do not contain a third reference electrode.Measurements of E anode in full cells would allow detection of Li deposition on anodes (condition: E anode vs. (Li/Li + ) < 0 V). Indeed, by introducing an additional reference electrode, such as metallic Li, the electrode potentials are accessible. 8,12,14,20,22,23 We note that such measurements are also not possible in a halfcell configuration with e.g. either only a graphite anode vs. a Li counter electrode or only a NMC cathode vs. a Li counter electrode, since the interaction between anode and cathode is not taken into account. Instead, 3-electrode full cells with the following electrodes are required to detect Li deposition correctly: (i) anode (e.g. graphite), (ii) cathode (e.g. NMC), and (iii) reference electrode (e.g. metallic Li).Furthermore, it is well-known that the reference electrode has to be positioned near the anode in order to minimize the Ohmic drop and to measure its potential accurately. 24 For example, in aqueous z E-mail: thomas.waldmann@zsw-bw.de systems, this is achieved by a Luggin-Haber capillary. 24 Simulations by Dees et al. also showed that the best position for a reference electrode in Lithium-ion cells is between anode and cathode. 25 The same...

Li plating is a severe and safety relevant aging mechanism which has to be avoided. A new 4-electrode operando pouch full cell, with two Li reference electrodes is developed. The voltage shifts between the two Li metal electrodes are correlating directly with changes in the Li+ activity a[Li+] in the electrolyte in front of the anode surface. We take advantage of the Li+ activity dependency of the Li+/Li redox potential as described by the Nernst equation to measure the Li+ activity in the electrolyte. On this basis, the mismatch between reaction and transport kinetics in anode and cathode is discussed. Finally, optimized fast-charging procedures based on strictly positive anode potentials are deduced from the measurements in cells with reference electrodes. These charging protocols allow reducing charging time while enhancing cycle life significantly. The second reference electrode is helpful to understand these measurements in terms of Li+ activity during charging. The applicability of such fast-charging protocols is demonstrated on the example of commercial 16 Ah pouch and lab cells.