Thin Film RuO₂ Lithiation: Fast Lithium‐Ion Diffusion along the Interface

Abstract

Although lithium‐ion batteries that run on the conversion reaction have high capacity, their cyclability remains problematic due to large volume changes and material pulverization. Dimensional confinement, such as 2D thin film or nanodots in a conductive matrix, is proposed as a way of improving the cyclic stability, but the lithiation mechanism of such dimensionally controlled materials remains largely unknown. Here, by in situ transmission electron microscopy, lithiation of thin RuO2 films with different thicknesses and directions of lithium‐ion diffusion are observed at atomic resolution to monitor the reactions. From the side‐wall diffusion in ≈4 nm RuO2 film, the ion‐diffusion and reaction are fast, called “interface‐dominant” mode. In contrast, in ≈12 nm film, the ion diffusion–reaction only occurs at the interface where there is a high density of defects due to misfits between the film and substrate, called the “interface‐to‐film” mode. Compared to the side‐wall diffusion, the reaction along the normal direction of the thin film are found to be sluggish (“layer‐to‐layer” mode). Once lithiation speed is higher, the volume expansion is larger and the intercalation stage becomes shorter. Such observation of preferential lithiation direction in 2D‐like RuO2 thin film provides useful insights to develop dimensionally confined electrodes for lithium‐ion batteries.

The Origin of Fast Lithium‐Ion Transport in the Inorganic Solid Electrolyte Interphase on Lithium Metal Anodes

“…To improve the carrier concentration and ionic conductivity of solid conductors, strategies such as heteroatom doping and interface design have been proposed. [13,29,42] The former is widely adopted in designing fast ionic conductors [29] while the latter is very promising in constructing composite solid electrolytes. [42] On the one hand, SEI is one kind of composite solid electrolyte and the interface design is supposed to enhance the ionic conductivity of SEI.…”

“…[13,29,42] The former is widely adopted in designing fast ionic conductors [29] while the latter is very promising in constructing composite solid electrolytes. [42] On the one hand, SEI is one kind of composite solid electrolyte and the interface design is supposed to enhance the ionic conductivity of SEI. On the other hand, inorganic SEI components have a very low bulk ionic conductivity and the interface is the only possible pathway for Li þ conduction.…”

Suppression of Oxygen Vacancies in Rutile Ruo2 via In Situ Exsolution for Enhanced Water Electrocatalysis

“…Rod‐shaped particles can be observed in the selected area as shown in Figure a and Figures S11–S14 in the Supporting Information. The SAED pattern for particle 1 in Figure 4a can be assigned to ($$0\bar{1}1$$), ($$\bar{1}10$$), and ($$\bar{1}01$$) planes of tetragonal RuO 2 (the Joint Committee on Powder Diffraction Standards (JCPDS) # 00‐040‐1290) (Figure 4b), [ 50 ] while the electron diffraction rings recorded from the adjacent particles can be assigned to cubic SrO phase (JCPDS # 00‐006‐0520) (Figure 4c). [ 51 ] TEM image on a selected rod clearly indicates the existence of a thin SrO layer on the {110} planes of RuO 2 (Figure 4d).…”

“…The high‐resolution transmission electron microscopy (HRTEM) analysis provides more information on the high structural stability of the RuO 2 phase. Figure 4h shows the HRTEM images of RuO 2 [ 50 ] on the bottom and the cubic SrO phase on the top‐right part of the picture. [ 51 ] Interestingly, we occasionally observed the in situ exsolution process during the electron beam illumination.…”

Fundamental understanding of electrochemical energy storage devices via in situ or operando characterization

“…As observed by Kim et al, for a 4.1 nm thick layer the conversion occurs uniformly leading to an expansion of 124.4% but with a thicker film of 12.4 nm Li ions intercalated preferentially at the RuO 2 /substrate interface. 231 The RuO 2 regions away from the interface were lithiated later due to a concentration gradient resulting in a final expansion of 99.2%. It is suggestive that the region where the strain is maximum is the point where lithiation will occur to release it.…”