Ionic Rectification across Ionic and Mixed Conductor Interfaces

Abstract

In electrochemical devices, it is important to control the ionic transport between the electrodes and solid electrolytes. However, it is difficult to tune the transport without applying an electric field. This paper presents a method to modulate the transport via tuning of the electrochemical potential difference by controlling the electronic states at the interfaces. We fabricated thin-film solid-state Li batteries using LiTi2O4 thin films as positive electrodes. The spontaneous Li-ion transport between the solid electrolyte and LiTi2O4 is controlled by tuning the electrochemical potential difference via use of an electrically conducting Nb-doped SrTiO3 substrate. This study establishes the foundation for rectifying the ionic transport via electronic energy band alignment.

Anionic Flow Valve Across Oxide Heterointerfaces by Remote Electron Doping

“…As an analogue of charged electron flows across chemically inert heterointerfaces, charged ions could be transferred across chemically reacting heterointerfaces made of ionic solids. − In particular, the chemical potential mismatch for oxygen (Δμ O ) across oxide heterointerfaces leads to oxygen ionic flow ( J O 2 − ) , which promotes the equilibrium of the system with the heterojunction, provided the kinetics of the charged oxygen ions ( k false[ D̃ normalO 2 − false] ) is sufficient to mitigate Δμ O at the interface ( J O 2 − = k false[ D̃ normalO 2 − false] × f false[ normalΔ μ normalO false] ) . − Due to the enhanced kinetics of the oxygen ions at the growth temperature of the oxide heterostructures, the redistribution of the charged oxygen ions has been frequently observed during the epitaxial growth of oxide heterostructures with dissimilar materials. ,, This redistribution of charged oxygen ions across oxide interfaces enables the control of defect-induced properties at the interfaces due to mobile ionic defects. ,,,− …”

“…In ionic crystals, avoiding long-range charge separation during ionic transport leads to ambipolar diffusion through the coupling of charged species; ,,,− hence, the flux of the diffusing ionic species has to be coupled with the electronic charge species (i.e., electrons or holes) to maintain electroneutrality. For example, in the case of oxides, the flux of the negatively charged oxygen ions should be matched by an equivalent flux of electronic charges. , The ambipolar diffusion of the oxygen ions ( D̃ O 2 − ) could be manipulated by electron supply through the Fermi level alignment in the internal oxide heterostructures. , Thus, this internal interaction between the ionic and electronic charges may offer a new route for the control of the massive ionic diffusion across solid–solid heterointerfaces.…”

“…From a thermodynamic viewpoint, the chemical potential mismatch of oxygen (Δμ O ) caused the spontaneous driving of the upward oxygen ionic flux ( J O 2 − ) during the growth of the TiO 2 capping layers at the TiO 2 /VO 2 heterointerfaces ( J O 2 − = k false[ D̃ normalO 2 − false] × f false[ normalΔ μ normalO false] ) . , Moreover, the diffusion of the oxygen ions ( D̃ O 2 − ) was kinetically facilitated along the growth direction (i.e., [001]), which has open channels for ionic diffusion in anisotropic rutile VO 2 and TiO 2 , ,, as long as a sufficient electron supply is allowed. However, the flux of the negatively charged oxygen ions would be limited by the supply of the electrons due to the ambipolar nature of ionic diffusion (i.e., influence of the electric charges on ionic diffusion). ,,,,, The amount of transferred electric charges controlled the ionic diffusion across the TiO 2 /VO 2 heterointerfaces.…”

“…could be manipulated by electron supply through the Fermi level alignment in the internal oxide heterostructures. 15,23 Thus, this internal interaction between the ionic and electronic charges may offer a new route for the control of the massive ionic diffusion across solid−solid heterointerfaces.…”

“…However, the flux of the negatively charged oxygen ions would be limited by the supply of the electrons due to the ambipolar nature of ionic diffusion (i.e., influence of the electric charges on ionic diffusion). 9,15,17,20,22,23 The amount of transferred electric charges controlled the ionic diffusion across the TiO 2 /VO 2 heterointerfaces.…”