Interplay of Operational Parameters on Lithium Deposition in Lithium-Ion Cells: Systematic Measurements with Reconstructed 3-Electrode Pouch Full Cells

Abstract

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...

Low‐Temperature Charging and Aging Mechanisms of Si/C Composite Anodes in Li‐Ion Batteries: An Operando Neutron Scattering Study

“…Below 0 V versus Li/Li + , Li metal deposition on the anodes surface becomes thermodynamically possible . Li deposition is a severe side reaction, which is caused by overpotentials favored by charging at low temperatures, high current densities, and high lithiation states . Li metal deposition on anodes leads to fast aging by reaction with the electrolyte .…”

Electrochemical, Post-Mortem, and ARC Analysis of Li-Ion Cell Safety in Second-Life Applications

“…7,16,18,20,27 In this case the anode potential vs. Li/Li + is mostly positive. 20,22 In contrast, critical combinations of low temperatures, high charging C-rates, and high SOCs lead to negative anode potentials vs. Li/Li + and therefore to deposition of metallic Li on graphite anodes, often called Li plating. [20][21][22][23][24][28][29][30] Due to the high reactivity of Li metal and the possibility of forming mechanically decoupled Li, cycling under Li plating conditions leads to fast capacity fade.…”

“…20,22 In contrast, critical combinations of low temperatures, high charging C-rates, and high SOCs lead to negative anode potentials vs. Li/Li + and therefore to deposition of metallic Li on graphite anodes, often called Li plating. [20][21][22][23][24][28][29][30] Due to the high reactivity of Li metal and the possibility of forming mechanically decoupled Li, cycling under Li plating conditions leads to fast capacity fade.a Present address: BMW AG, D-80788 München, Germany. z E-mail: thomas.waldmann@zsw-bw.de However, there is still a dearth of knowledge on the effect of aging mechanisms on the safety properties of Li-ion cells.…”

Effects of Biphenyl Polymerization on Lithium Deposition in Commercial Graphite/NMC Lithium-Ion Pouch-Cells during Calendar Aging at High Temperature

“…A previous study with the same cell type showed that charging at C/25 does not lead to negative anode potentials vs. Li/Li + and therefore no Li deposition is expected. 18 Standard electrochemical measurements are performed in order to characterize cells before dismantling. Cells are discharged at a rate of C/10 until 2.7 V is reached.…”