Fracture in lithium ion battery cathode materials is a serious degradation mechanism which can lead to capacity loss and lifetime reduction [1]–[3]. Currently new cathode materials are being developed which operate at higher potentials for applications such as battery electric vehicles. Fracture exposes fresh material to electrolyte, which at higher potentials can increase the effects of corrosion leading to active material loss [1]–[3]. Consequently, there is currently a drive to develop fracture resistant particle architectures for next generation cathodes.

Typically, microscopy methods are used to assess particle fracture resistance; where cracking is observed post-cycling using methods like XCT, SEM, or TEM [4], [5]. Imaging materials post-mortem to observe fracture can make de-coupling mechanical from chemical effects more challenging. Nanoindentation has been previously used measure Young’s modulus of cathode particles in electrodes, demonstrating that the Young’s modulus varies with de-lithiation [6]. However, fracture stress measurements are more challenging as cathode particles are embedded in electrodes when cycled.

Here we present an in situ SEM microindentation method to measure the fracture stress of individual cathode particles, both in a pristine and cycled state. A microindentation stage is mounted in an SEM, allowing compression of cathode particles in real time whilst concurrently imaging fracture mechanisms and recording the load-displacement curves. A method developed to extract cathode particles from electrode tapes enables measurement of the fracture stress of individual particles at different states of charge.

Performing in situ SEM microindentation has a number of advantages for cathode particle systems. Assessment of the particle state prior to indentation is possible, including particle size, identification of surface flaws, and observation of surface films such as residual binder. Dynamical imaging of fracture as it occurs also allows the fracture mechanism to be quantified, enabling correlations between uniaxial compression during SEM microindentation and fracture effects observed post-mortem after battery cycling to be made.

Individual Li(Ni_0.8Mn_0.1Co_0.1)O₂ (NMC811) ‘meatball’ structured secondary particles were mechanically tested by in situ SEM microindentation with a sharp diamond tip (Figure 1). In situ uniaxial compression testing of both pristine and cycled NMC811 particles demonstrated that mechanical deformation resulted in intergranular fracture along primary grain boundaries, a mechanism typically seen during cycling of NMC811 materials [2]-[3]. The NMC811 intergranular fracture stress is shown to decrease with increasing state of charge, correlating well with reported increases in NMC fracture with after cycling [2]-[3].

We anticipate that future applications of in situ SEM microindentation will help evaluate the fracture resistance of future cathode particle morphologies.

References

[1] W. H. Woodford, Y.-M. Chiang, and W. C. Carter, “‘Electrochemical Shock’ of Intercalation Electrodes: A Fracture Mechanics Analysis,” J. Electrochem. Soc., vol. 157, no. 10, p. A1052, 2010.

[2] H.-H. Ryu, K.-J. Park, C. S. Yoon, and Y.-K. Sun, “Capacity fading of ni-rich Li[Ni_xCo_yMn_1-x]O₂ (0.6 ≤ x ≤ 0.95) Cathodes for High-Energy-Density Lithium-Ion Batteries: Bulk or Surface Degradation?,” Chem. Mater., vol. 30, no. 3, pp. 1155–1163, 2018.

[3] S. Schweidler, L. de Biasi, G. Garcia, A. Mazilkin, P. Hartmann, T. Brezesinski, and J. Janek, “Investigation into Mechanical Degradation and Fatigue of High-Ni NCM Cathode Material: A Long-Term Cycling Study of Full Cells,” ACS Appl. Energy Mater., vol. 2, no. 10, pp. 7375–7384, 2019.

[4] T. M. M. Heenan, A. Wade, C. Tan, J. E. Parker, D. Matras, A. S. Leach, J. B. Robinson, A. Llewellyn, A. Dimitrijevic, R. Jervis, P. D. Quinn, D. J. L. Brett, P. R. Shearing, “Identifying the Origins of Microstructural Defects Such as Cracking within Ni‐Rich NMC811 Cathode Particles for Lithium‐Ion Batteries,” Adv. Energy Mater., vol. 10, no. 47, p. 2002655, 2020.

[5] C. Tan, A. S. Leach, T. M. M. Heenan, H. Parks, R. Jervis, J. Nelson Weker, D. J. L. Brett, P. R. Shearing, “Nanoscale state-of-charge heterogeneities within polycrystalline nickel-rich layered oxide cathode materials,” Cell Reports Phys. Sci., p. 100647, 2021.

[6] J. G. Swallow, W. H. Woodford, F. P. McGrogan, N. Ferralis, Y.-M. Chiang, and K. J. Van Vliet, “Effect of electrochemical charging on elastoplastic properties and fracture toughness of Li_XCoO₂,” J. Electrochem. Soc., vol. 161, no. 11, pp. f3084–f3090, 2014.