Scientists are expanding our fundamental understanding of how electrodes in alkali-ion batteries work by shedding light on mechanisms that govern reversible electrochemical processes, known as conversion reactions, which take place inside electrodes between alkali metals and transition metal oxides (TMOs). A research team, from the University of Texas at Austin and Argonne National Laboratory, has explored a new general synthetic strategy for two-dimensional (2D) TMO nanosheets, with a pierced (holey) structure that resembles a grid, using graphene oxide (GO) as a sacrificial-template. The results are published in a recent issue of Nature Communications.

Alkali metal electrodes with a 2D nanoarchitecture can boost a battery’s performance. TMOs are deemed desirable for the production of 2D nanosheets because of their distinct reaction mechanism, abundance of active sites, and short ion diffusion distance. Previous research revealed two main challenges in the production of 2D TMO nanostructures: First, the existing top-down exfoliation methods cannot be used, because TMOs are intrinsically nonlayered materials. Furthermore, when these nanomaterials are used in electrodes, they suffer from severe morphology changes and structural degradation.

Guihua Yu, a professor at the University of Texas at Austin, together with PhD students Lele Peng and Pan Xiong, conceived the idea for a synthetic route for “holey” oxide nanosheets to address these issues. Mixed TMOs (ZnMn₂O₄, ZnCo₂O₄, NiCo₂O₄, CoFe₂O₄) as well as simple TMOs (Fe₂O₃, Co₃ O₄, and Mn₂O₃) were chosen for evaluation. For the study of mechanical properties, morphology evolution, and oxidation state changes during electrochemical processes, they turned to Jun Lu from Argonne National Laboratory for characterization using in situ transmission electron microscopy (TEM), operando x-ray diffraction (XRD), and absorption spectroscopy (XAS), in order to understand the mechanism of the conversion reactions.

An elegant two-step synthesis was developed for the preparation of the 2D nanostructures. The researchers first prepared sheets of reduced graphene oxide (rGO), with TMO ion precursors uniformly anchored on them, by means of a solution-phase reaction: transition metal cations were mixed with a suspension of graphite oxide in ethylene glycol, stirred well, and collected by centrifugation. In the second step, post-calcination was used to remove the rGO templates: the samples were heated at relatively high temperature (i.e., 400°C for Zn metal oxides). Thus, 2D holey nanosheets composed of interconnected TMO nanocrystals were formed.  According to Yu, developing this process was a challenge because uniform deposition of the metal oxide precursors on the GO template for different materials is difficult.

The in situ TEM results showed that TMO 2D nanostructures inherit strong mechanical properties from graphene oxides. “These nanosheets are very robust during charging and discharging cycles, and this surprised us in a way,” Lu says. “These structures do not crack, do not fall off, they are free-standing structures with no binders and this knowledge will provide guidance to the future study of these nanomaterials,” he says. TEM also revealed tunable pore and particle sizes, which, together with their strong mechanical stability, enables greatly enhanced alkali-ion storage properties.  

Operando XRD and state-of-the-art XAS were used to look at the elements involved in the electrochemical processes during the charge/discharge cycles. The results showed that the TMO nanosheets have better electrochemical properties (high reversible capacity, excellent rate capability and cycling stability), for both lithium and sodium ion storage, due to increased surface areas and interfaces and easier interfacial transport and shortened diffusion paths.

Jang Wook Choi, a professor at the Korea Advanced Institute of Science and Technology who did not take part in the study, says, “Electrodes in typical rechargeable batteries contain spherical active particles that can store carrier ions. The isotropic morphologies of active particles usually make it difficult to analyze active materials during their battery operation. Yu’s team found that holey 2D morphologies are more suitable for robust and fast battery operation, by utilizing their superiority in maintaining the structures and facilitating carrier ion transport.”

Yu’s team is now focused on expanding the developed general method to the preparation of other materials systems based on phosphides, sulphides, and selenides. “We are also devoting our attention to the exploration of the electrocatalytic properties of the TMO nanosheets,” Yu says. According to the research team, the holey TMO nanosheets can be used as highly efficient catalysts to fabricate next-generation, high-performance rechargeable metal-air (Li-air and Zn-air) batteries and fuel cells.

Read the article in Nature Communications.