Identifying the Cause of Rupture of Li-Ion Batteries during Thermal Runaway

Abstract

As the energy density of lithium-ion cells and batteries increases, controlling the outcomes of thermal runaway becomes more challenging. If the high rate of gas generation during thermal runaway is not adequately vented, commercial cell designs can rupture and explode, presenting serious safety concerns. Here, ultra-high-speed synchrotron X-ray imaging is used at >20 000 frames per second to characterize the venting processes of six different 18650 cell designs undergoing thermal runaway. For the first time, the mechanisms that lead to the most catastrophic type of cell failure, rupture, and explosion are identified and elucidated in detail. The practical application of the technique is highlighted by evaluating a novel 18650 cell design with a second vent at the base, which is shown to avoid the critical stages that lead to rupture. The insights yielded in this study shed new light on battery failure and are expected to guide the development of safer commercial cell designs.

Keywords: Li‐ion batteries; X‐ray CT; high‐speed imaging; thermal runaway; venting.

Figure 1

Greyscale XZ orthoslices from X‐ray CT reconstructions of a) LG‐S3 and b) Panasonic cells. Exploded 3D reconstructions of c) LG‐S3 and d) Panasonic cells, showing the placement of the integrated safety devices. Greyscale XZ orthoslices from e) Sanyo and f) Samsung cells. Exploded 3D reconstructions of g) Sanyo and h) Samsung cells, showing the placement of the integrated safety devices.

Figure 2

Time‐stamped radiographs of a) LG‐S3, b) LG‐B4, and c) Sanyo cells undergoing thermal runaway. Radiography movies of (a), (b), and, (c) are provided as Movies S1, S3, and S5, respectively, in the Supporting Information. The red arrows highlight the radial direction of propagation of thermal runaway, and the yellow arrows highlight the direction of peeling and shift of the electrode assembly.

Figure 3

Postmortem 3D reconstructions and corresponding XZ and YZ orthoslices of a) Sanyo, b) Panasonic, and c) Samsung cells showing the damage to the top button after thermal runaway. The cylindrical mandrel in (a) and (b) is seen to protrude after puncturing the top button. The radiography movies showing the process of thermal runaway for the Sanyo, Panasonic, and Samsung cells are provided as Movies S5, S7, and S8 in the Supporting Information.

Figure 4

Time‐stamped radiographs taken at a) 2000 fps showing the propagation of thermal runaway within the Samsung cell and the top button melting, and b) taken at 20 272 fps showing the stages of the Samsung cell bursting. The thin yellow line on each image is a reference line at the shoulder of the spin groove. Radiography movies of (a) and (b) are provided as Movies S8 and S10, respectively, in the Supporting Information. The red arrows highlight the radial direction of propagation of thermal runaway, and the yellow arrows highlight the shift of the electrode assembly and extension of the spin groove.

Figure 5

Time‐stamped radiographs taken at a) 2000 fps showing the mandrel piercing the crimp‐components during thermal runaway where the yellow arrows highlight the direction of shift of the mandrel, and b) taken at 20 272 fps showing the stages of the 18650 Panasonic cell bursting. Radiography movies of (a) and (b) are provided as Movies S12 and S13, respectively, in the Supporting Information.

Figure 6

Postmortem photographs of the 18650 Samsung cells where a) the top button melted, and b) the cell burst, as well as the Panasonic cells that underwent c) piercing of the top button by the mandrel, and d) bursting.

Figure 7

a) Photograph showing the vent disk at the base of the 18650 prototype cell. b) Photograph showing the position of the 18650 cell with clearance at its base for the second vent. c) Postmortem photograph showing ruptured vent disk. d) Time‐stamped radiographs showing the activation of the secondary vent during thermal runaway. e) Time‐stamped radiographs showing the crimp‐region of the 18650 cell during thermal runaway. Radiography movies of (d) and (e) are provided as Movies S14 and S16, respectively, in the Supporting Information.

Figure 8

a) NiCr wire with a length corresponding to a resistance of ≈22 Ω wrapped around the base of an 18650 cell. The wire is separated from the cell casing by glass cloth tape. b) An additional layer of glass cloth tape was wrapped around the NiCr wire to prevent short circuiting. c) A failed 18650 cell secured in place by hydraulic clamps and insulation plates.