Effect of microcracking on ionic conductivity in LATP

Because of high thermal expansion anisotropy, lithium aluminum titanium phosphate (LATP) is prone to microcrack generation. While much attention has been given to the role of grain boundary phases on the ionic conductivity of LATP, the effect of microcracking is also expected to lower conductivity. While all LATP materials characterized had grains larger than the critical size for microcracking, fine-grained (1.7 ± 0.7 μm) Li1.3Al0.3Ti1.7(PO4)3 had twice the ionic conductivity of the same purity of coarse-grained (4.8 ± 1.9 μm) LATP at 323 K due to less extensive cracking at grain boundaries. While the increase in Young’s modulus (81–115 GPa) and biaxial strength (26 ± 2 MPa to 123 ± 17 MPa) is consistent with the reduction in grain size, the increase in conductivity is a strong driving force for avoiding large LATP grains. Lower purity degraded ionic conductivity for similar grain sizes, as expected. Single-edged precracked beam fracture toughness measurements showed that the fine-grained, high-purity LATP has a KIc value of 1.1 ± 0.3 MPa√m. Flexural strength measurements (147 ± 14 MPa) indicated that the critical flaw size was on the order of 30 μm, with processing flaws still limiting strength. It is expected that as grain size is further reduced, strength and ionic conductivity will continue to improve in tandem until microcracking is eliminated. ► Microcracking leads to high grain boundary impedance in LATP. ► Optimum total ionic conductivity of 0.7 mS cm−1 and activation energy of ∼0.35 eV. ► d.c. conductivity in aqueous LiOH measured as 1.6 mS cm−1 at 50 °C. ► Biaxial membrane strength measured as 123 MPa with 1.1 MPa√m fracture toughness.