Interactions Between Positive and Negative Electrodes in Li-Ion Cells Operated at High Temperature and High Voltage

Abstract

Introduction When NMC/graphite Li-ion cells are operated at high temperature and a cut-off potential higher than 4.2 V, generated gaseous products and oxidized species which could passivate the positive electrode and/or dissolve in the bulk electrolyte could reach the negative electrode and be reduced there.1,2 In order to study electrode/electrode interactions, pouch bags were used to separate the two electrodes, so that interactions between two electrodes would be impossible. Pouch bags containing only the charged negative or positive electrode can be used as a simple tool to obtain valuable information concerning the consequences of the absence of an interaction with the other charged electrode. Experimental Before electrolyte filling, dry (no electrolyte) NMC442/graphite (240 mAh) pouch cells were cut below the seal and vacuum dried at 80°C for 14 h. The pouch cells were filled with 0.9 g of 1M LiPF6 in EC:EMC (3:7 by weight) (control electrolyte) plus 2 % vinylene carbonate (VC), 2% prop-1-ene-1,3-sultone (PES) or 2% pyridine-boron trifluoride (PBF) in an argon-filled glove box and vacuum sealed in the same glove box. All the additives were added by weight percentage. After electrolyte filling, cells were placed in a temperature box at 40. ± 0.1°C and held at 1.5 V for 24 h. They were then charged to 3.8 V at C/20, and transferred to a glove box for degassing (cut open below the seal and re-sealed under vacuum). After degassing, they were charged to either 4.4 V, then discharged to 2.8 V and charged back to the same charge cutoff voltage until the holding period for the NMC442/graphite cells reached 30 hours. Four cells either at 4.2 or 4.4 V were moved to a 60°C temperature box for storage. The other cells were transferred to the glovebox and dissembled there. The delithiated NMC442 electrodes collected from the pouch cell were inserted into pouch bags with 0.3 g EMC. The addition of 0.3 g EMC to the pouch bags created a similar electrolyte environment as in the original pouch cells, because EC and LiPF6 were still left in the electrode and only about 0.3 g of EMC evaporated from the positive electrodes. Figure 1 shows photographs to illustrate the process used to make and test pouch cells and pouch bags. Results and discussion Figure 2 shows gas evolution for pouch bags containing lithiated graphite electrodes which were taken from pouch cells with control electrolyte, electrolyte with 2% PES, 2% VC and 2% PBF. No volume changes were detected for these pouch bags during approximately 500 h storage at 60°C. This suggests that parasitic reactions between the lithiated graphite and electrolyte do not contribute to gas generation at 60 °C. Figure 3 shows the volume change versus time for pouch cells with control electrolyte, or electrolyte with 2% PES, 2% VC or 2% PBF and pouch bags containing charged positive electrodes taken from brother pouch cells during a 500 h storage period at 60°C. Figure 3 shows that the volume of the pouch cells and pouch bags containing the charged positive electrodes increased with time. Figure 3 also shows that the pouch bags containing the delithiated NMC442 electrodes produced almost two times more gas than the corresponding pouch cells during the storage period. This suggests that some gaseous products generated at the positive electrode are consumed at the negative electrode in a full cell. Pouch bags with the delithiated NMC442 electrode taken from pouch cells at 4.4 V produced more gas than those taken from pouch cells at 4.2 V. This means, unsurprisingly, that delithiated NMC442 electrodes at a higher potential oxidize electrolyte at a higher rate. Pouch bags containing the washed charged NMC442 electrode (DMC washing) + EC/EMC produced gas at a higher rate than those containing a charged NMC442 electrode without washing. This suggests that the presence of LiPF6 can affect the parasitic reactions at the positive electrode that cause gaseous products. The above results suggest that oxidized species such as gaseous products generated at the positive electrode can be consumed at the negative electrode leading to a smaller volume expansion in pouch cells than pouch bags. The “clean-up” of other oxidized species by the lithiated graphite electrode resulting in a lower rate of impedance increase at the positive electrode will be discussed.