Geometric Characteristics of Three-Phase Porous Microstructures for All Solid-State Lithium Ion Batteries

Fast lithium ion conducting inorganic solid materials are considered superior candidates for lithium ion batteries (LIBs) to overcome flammable characteristics, leakage, and tight electrochemical window issues of organic liquid electrolytes. Besides high ion conductivities of the inorganic materials, their wide electrochemical window and good chemical stability with electrode materials are great advantages as an electrolyte [1]. However, there are remained challenges to utilize the inorganic solid-state electrolytes for LIBs because of a low specific capacity, rate capability, and cycle life. Our previous study [2] showed a large capacity decay after the first charge and poor cycle life of an all solid-state LIB (Li_1+xMn₂O₂ + Li_1.3Ti_1.7Al_0.3(PO₄)₃ | Li_1.3Ti_1.7Al_0.3(PO₄)₃  | Li_1-xMn₂O₂ + Li_1.3Ti_1.7Al_0.3(PO₄)₃). We concluded that the large capacity decay was caused by dismantled ion and electron pathway due to the structural change after the first charge. The electrochemical impedance spectroscopy results showed that the internal resistance of the solid-state cell has reduced by applying higher pressing pressure. Therefore, it is necessary to investigate the realistic microstructure to understand the capacity decay and poor cycle life of the all solid-state LIB.

By using synchrotron nano-computed tomography (nano-CT) at Argonne National Lab, we implemented quantitative study of the microstructures of all solid-state LIBs. The three-phase electrode was fabricated from a 47:47:6 (wt%) mixture of Li(Ni_1/3 Mn_1/3 Co_1/3)O₂ as active material (NMC), Li_1.3Ti_1.7Al_0.3(PO₄)₃as Li-ion conductor (LTAP), and Super P carbon as electron conductor. The electrodes were packed under two different pressures (700 psi and 1300 psi) to control the volume fractions of void phase. To obtain the morphology data of the electrodes, TXM measurements were carried out adjusting energy levels of a synchrotron x-ray source. The x-ray generated images of solid-state electrodes with a 60 nm of spatial resolution at 8 keV are shown in Fig. 1. The gray-scaled intensity values of the images indicate the x-ray absorption rate of the materials. From the Fig. 1 (a) and (b), NMC particles, LTAP particles, and pores can be distinguished due to their different intensity values. Based on the reconstructed microstructure, the geometric characteristics will be presented to understand the lithium ion and electron pathways in all solid-state LIBs.

Acknowledgments: This work was supported by US National Science Foundation under Grant No. 1335850.

References

1. P. Knauth, Solid State Ionics, 180, 911 (2009).

2. N. M. Asl, J. Keith, C. Lim, L. Zhu, and Y. Kim, Electrochimica Acta, 79, 8 (2012).

Figure 1. Nano-CT images of solid-state electrode (NMC+LTAP) by full-field TXM measurements at an 8 keV of x-ray under packing pressures (a) 700 psi and (b) 1300 psi