4-Electrode Full Cells for Operando Li⁺ Activity Measurements and Prevention of Li Deposition in Li-Ion Cells

Abstract

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.

Fast Charging of Lithium‐Ion Batteries: A Review of Materials Aspects

“…[ 98 ] The increase of the diffusion barrier of Li + in graphite with SOC is unfavorable for the fast‐charging capability since it leads to a higher probability of Li metal deposition for high SOCs. [ 102–105 ]…”

“…[ 170 ] Consistent with this, post‐mortem analysis after cycling showed that lithium metal is mostly deposited on the anode surface. [ 173–175 ] The anode potential and therefore the tendency of lithium metal deposition is affected by electrolyte additives, [ 176 ] anode active materials, [ 177 ] the anode coating thickness, [ 38 ] operating parameters such as temperature, C‐rate, and SOC [ 38,103–105,177 ] as well as the ratio of the areal capacities between anode and cathode (N/P ratio). [ 19,178–181 ]…”

“…In literature, the mechanism of lithium metal deposition is mostly investigated for graphite anodes, [ 102,105,172–175,177,184,185 ] although it is also studied in Si/C composite anodes. [ 186 ] The phenomenon of lithium metal deposition on anodes is highly critical since it can reduce safety by dendrite growth [ 187 ] and exothermic reactions.…”

“…Unfavorable deposition of lithium metal on the anode becomes thermodynamically possible if the anode potential becomes lower than 0 V versus Li + /Li, [ 103,169–171 ] corresponding to an overpotential of (0.065–0.2) V in the case of graphite. [ 171 ] This condition is more likely to be fulfilled in front of the anode surface (near the separator) due to higher Li + activity [ 105 ] and less likely near the current collector due to depletion of Li + ions. [ 153 ] For pouch cells, Li metal deposition on the anode surface leads to measurable thickness changes.…”

“…[ 175,188 ] On graphite anodes, lithium metal deposition is favored during charging at low temperatures, [ 38,102,103,185 ] high charging C‐rates, [ 103,185 ] and high SOCs, [ 103 ] as well as their combination. [ 104,185 ] These effects can be counteracted by avoiding negative anode potentials, for example, by optimized charging protocols [ 103–105 ] or by internal heating of cells during charging. [ 189 ]…”