In operando measurement of surface strain of 18650 Li-ion cells during cycling

The strain and temperature responses of commercial high capacity (3.4 Ah) 18650 lithium-ion cells were investigated under normal operation and overcharge conditions. Strain measurements were carried out on the casing of the 18650 cells using surface-mounted strain gauges while cycling the cells in a temperature-controlled environment. During a normal C/10 charge/discharge cycle, the hoop strain at the midsection of the cylindrical cell reached a maximum value at 100 % state-of-charge (4.2 V). Strain returned to a baseline value upon discharge. Differential capacity (dQ/dV) data showed several characteristic peaks corresponding to a full cell with a Ni-rich layered oxide cathode and graphite-Si anode. A differential strain analysis (dε/dV) showed peaks in strain response that coincided with the dQ/dV peaks. In all the cycling scenarios, the peaks associated with the electrochemical reactions in the cell coincided with change in strain response. Repeated measurements across multiple cycles and multiple cylindrical cells have produced similar strain behavior. Finally, an overcharge test was conducted at a C/5 rate to measure strain up to CID activation. During the overcharge, strain increases significantly, i.e., more than 2.5 times the peak strain of a normal C/5 charge process. The results from this study indicate that strain measurements on a cell's surface are directly correlated to the reversible electrochemical behavior of a cell during normal cycling, and therefore can be used as a diagnostics tool for battery operation to detect irreversible phenomenon.