The Role of Structural Features and System Relaxations on the Electrical Response of La1/2+1/2xLi1/2-1/2xTi1-XAlxO3 Perovskites

Abstract

The all-solid-state batteries (ASSBs) using solid electrolytes are excellent candidates for the next generation of commercial devices for energy storage. Among these, the perovskite-type family of Li-ion conducting oxides Li3xLa2/3−xTiO3 (LLTO) is one of the most studied [1] and it is shown as the most promising option for solid electrolytes because of its high bulk conductivity (10−3 S·cm−1) [2], negligible electronic conductivity [3] high stability, wide electrochemical window (larger than 4 V) [4] and easy preparation. [5] However, there are still many challenges to be solved: (i) the high impedance associated with grain boundaries which reduce the overall lithium ionic conductivity below 10-5 S/cm at 298 K; (ii) and the instability of LLTO in direct contact with metal Li, [6],[7] or graphite electrodes.[8], [9],[10] In this contribution, we have focused on the substitution of B-site of LLTO perovskite, La1/2+1/2xLi1/2-1/2xTi1-xAlxO3 (0 ≤ x ≤ 1) studying the influence of vacancy distribution on percolative phenomena. Along this series, (1/2Li+ + Ti4+) cations were substituted by (1/2La3+ +Al3+), preserving both the charge balance and the nominal A-site vacancies (nA = 0). Accordingly, the main goal of this study is to confirm the influence of effective vacancies and their distribution, instead of nominal vacancies, on the Li conduction mechanism. The presence of order/segregation must influence the conductivity of these materials and by performing Broad band Electric Spectroscopy (BES), which is a powerful technique for unravelling the complexities of a polycrystalline ceramic, the analysis of the different contributions to the electric response of this complex materials can be achieved[11], [12].Indeed, here the aim is to study the correlation between structural features, determined by HRTEM/STEM, and relaxation phenomena characterizing the electric response of these perovskites by means of the BES. To better elucidate the interplay between structural features, conductivity and relaxation phenomena from the electric response of these materials, Density Functional Theory (DFT) and dynamic molecular modelling simulations studies are carried out.Taking all together, it is shown that the charge migration processes occurring along the different conductivity pathways for Li+ long range migration phenomena are very effective when in these perovskites: a) a sufficient density of charge carriers is present; and b) in the Ti4+ based BO6 backbone octahedra, Al3+ ions acts as a defect, thus enhancing the dynamics characterizing their relaxation modes. This latter effect is obviously dependent on the existence of an efficient coupling between the host medium relaxations of the inorganic network and the relaxation events characterizing the long range migration processes of lithium conductivity pathways.In conclusion, this study permits to shed light precisely into the conduction mechanism in Li3xLa2/3−xTiO3 –type perovskites. Acknowledgements This work has been supported by the European Union’s Horizon 2020 research and innovation program under grant agreement No 829145 (FETOPEN-VIDICAT) and by the Agencia Española de Investigación /Fondo Europeo de Desarrollo Regional (FEDER/UE) for funding the projects PID2019-106662RBC43 and C44. V. Di Noto thanks the University Carlos III of Madrid for the "Cátedras de Excelencia UC3M-Santander" (Chair of Excellence UC3M-Santander).