Consumption of Intentionally Added Gas in Lithium-Ion Cells

Abstract

Introduction Lithium-ion cells produce a considerable amount of gas during their first cycle, as electrolyte solvents react with the surface of the charging electrodes to form passivating films. In cells without electrolyte additives, this gas is largely a mixture of CO2 and C2H4. [1] If lithium-ion cells are packaged in flexible bags (commonly termed “pouch” cells), these gases must be removed by the manufacturer in a degassing step, to prevent deformation of the cell and ensure stack pressue on the electrodes. If the degassing step is omitted, as in the case of cells packaged in metal cans, some amount of these gases are consumed by the cell over time. The reaction that consumes gas, and the effects of gas consumption on cell performance are not fully understood. It has been suggested that consumption of CO2 gas in lithium-ion cells causes reversible self-discharge through a “chemical dialogue” between negative and positive electrodes: CO2 is reduced at the lithiated negative electrode to form lithium oxalate, which diffuses to the positive electrode and is oxidized to reform CO2. [2 ] To reveal the presence of a chemical dialogue, and determine the fate CO2 and C2H4 inside cells, the reaction of these gases with individual charged electrodes will be discussed. Experimental Machine-made lithium-ion pouch cells, containing Li[Ni0.4Mn0.4Co0.2]O2 (NMC442) positive electrodes and graphite negative electrodes, were obtained sealed, without electrolyte from LiFun Technologies (Zuzhou City, China). The cells were filled with a slight excess of electrolyte, composed of 1 M LiPF6, dissolved in a 3:7 wt blend of ethylene carbonate and ethyl methyl carbonate. The cells were charged to 4.5 V, at a rate of C/20 at 40°C. Several of of the cells were transferred to an Ar-filled glove box, where the charged electrodes were removed from the cell and sealed individually in aluminized polymer bags, made from the same material as the casing of the parent pouch cell. Each of these bags was equipped with a rubber septum, through which gases were injected. After injection, the bags were vacuum sealed below the septum and the septum was cut off. The inflated bags were stored at 40°C, in a similar manner to charged full cells which were not disassembled, and which were left at open-circuit voltage. The changes in volume of the charged cells and the inflated bags were measured at regular intervals, using Archimedes’ principle. Results and Discussion Figure 1 shows the voltage and volume change of a cell during its first charge cycle (formation cycle), followed by 100 hours of open-circuit voltage storage at 40°C, during which time over 0.5 mL of gas was consumed. Figure 2 shows the volume change versus time for pouch bags inflated with CO2, containing lithiated graphite negative electrodes or charged NMC442 positive electrodes, stored at 40°C. The volume of the bags containing lithiated graphite negative electrodes decreased steadily with time, indicating that CO2 was consumed by the lithiated graphite negative electrode. The volume of the bags containing charged NMC442 positive electrodes remained constant after equilibration at 40°C, indicating that CO2 was not consumed by the positive electrode. The amount of gas consumed by the full cell over 100 hours, shown by Figure 1, is approximately equal to the amount of CO2consumed by the lithated graphite electrode, shown by Figure 2. The consumption of gas in lithium-ion cells and the effect of gas consumption on the surface chemistry of electrodes will be discussed further.