Diameter Change of Silicon Composite 18650 Lithium-Ion Batteries During Cycling

摘要

Silicon anodes are known for their high specific capacity and have the potential to increase the capacity of current lithium-ion technology significantly. However, the use of silicon in lithium-ion batteries is limited mainly due to its rapid mechanical degradation within the electrode caused by the volume change of the material during the intercalation process. To prevent mechanical aging in lithium-ion batteries, it is crucial to understand the volume change during cycling. To get a better understanding of the volume change and its influence on capacity loss in silicon composite electrodes, this work investigates commercial 18650 cells. The constructed test matrix consists of 32 cells with a nominal capacity of 3.4 Ah. The cathode consists of nickel and cobalt and the anode of silicon and graphite. Strain gauges were attached to all cells to measure the diameter change of the cells in operando over aging. The aging tests include 7 different SOCs and various ranges of DOD. Check-ups were conducted every 50th equivalent full cycle, including capacity determination, pulse resistance, and quasi-OCV characteristic. The experiments showed a diameter expansion of around 9 ?m for a fresh cell during a full discharge/charge cycle. The highest reversible diameter change we could observe during a full discharge/charge cycle was around 20 ?m. Two different expansion mechanism can be observed, a reversible caused by lithiation and delithiation of the electrodes and an irreversible volume expansion caused by the non-uniform formation of a covering layer or lithium plating. Furthermore, two effects can be observed over aging using strain gauges: 1. an increase of irreversible volume expansion and 2. an increase of reversible volume expansion. Overall, a rapid decline in capacity can be observed for most cells, which can partly be attributed to lithium plating. Our results reveal the two fundamental expansion mechanism in silicon-based composite electrodes and identify their origin. This is essential to understand the mechanical stress during cycling.