3, The University of Tokyo, Kashiwa, , Japan
2, National Institute of Advanced Industrial Science and Technology, Tsukuba, , Japan
Li3PO4 based materials (e.g., Li3PO4-xNx) are used for solid electrolytes in thin film Li ion batteries. Recently, its application as non-volatile memory devices is also being explored: Au/Li3PO4/Li as well as Ni/Li3PO4/Li stacked structures were found to exhibit two different voltage states, viz., high and low voltage states, which can be controlled by applied voltages [1, 2]. To develop novel memory devices using this phenomenon, the understanding on Li ion distribution near the metal/Li3PO4 interfaces and atomic structures of the two voltage states are crucially important.
In this study, first we investigated the Li ion distributions in the γ-Li3PO4 connected with the metal electrodes, using the combination of the defect formation energy calculations from first principles and the continuum-model calculations : A one-dimensional continuum-model was adopted, and the charge density of Li defects (vacancy and interstitial) and the electrostatic potential in the solid electrolyte were determined self-consistently. We found that Li vacancies are hardly formed near the Au(111)/γ-Li3PO4 and Ni(111)/γ-Li3PO4 interfaces. On the other hand, the accumulation of formation of Li+ interstitials near the interfaces is seen, and its depth reaches ca. 10 Å from the interface. When the possibility of Au-Li alloy formation in the electrode is considered, the amount of Li+ interstitials decreases as compared to the pure Au case. This suggests the possibility of Au-Li alloy formation using the accumulated Li+ interstitials at the interface .
Next, we investigated the relation between the atomic structure of the interface and high- and low- voltage states of the system. The voltages were estimated using the method widely used for studies in Li ion batteries . For simplicity, Li adsorption on the metal electrode was considered without taking account of the Li3PO4 electrolyte. We found that the amount of Li atoms staying at the surface determine the two voltage states. For the Ni case, higher and lower Li coverages are likely to correspond to the low and high voltage states, respectively . On the other hand, for the Au case, a higher Li ratio in the Au electrode indicates the low voltage state and a partially alloyed Au-Li surface exhibits the high voltage state. Although preliminary calculations taking account of the Li3PO4 electrolyte suggest that the details of atomic arrangements of the two states may change by considering the electrolyte, we conclude that the switching mechanism between the two voltage states can be attributed to the formation of Li+ interstitials at the interface and the variation of the Li densities at the metal electrode.
This work was supported by CREST, JST.
 I. Sugiyama et al., APL Mater. 5, 046105 (2017).
 W. Liu et al., in preparation.
 S. Kasamatsu et al., Solid State Ionics 183, 20 (2011).
 K. Shimizu et al., in preparation.
 M.K. Aydinol et al., Phys. Rev. B 56, 1354 (1997).