Performance Buster: Can Two-Dimensional Materials Truly Speed Up Ion Diffusion?

The speed of ion diffusion from electrolytes into electrodes largely dictates the charge-storage performance of rechargeable batteries, e.g., Li-ion batteries. Typically, the capacity of a battery decreases under rapid charging or discharging, since electrodes cannot hold as many ions as in slow-operation scenarios due to the limited ion diffusion rate inside electrodes.

The boom of two-dimensional (2D) materials as electrodes in rechargeable batteries started about a decade ago, marked by the growing interest of an iconic 2D material, graphene. With their ultrathin thicknesses and large surface areas, 2D materials exhibit large capacity as electrodes in Li-ion and Na-ion batteries. They have also been claimed to allow for ultrafast ion diffusion, which sustains their high capacity at elevated charging and discharge rates, a property crucial for fast-charging applications.

Do these claims stand true? Jonathan N. Coleman and coworkers at Trinity College Dublin, Ireland, said no. They recently challenged the promise of 2D materials for fast-charging batteries. By analyzing the reported data from 59 papers published in the past decade, the authors reached a conclusion perhaps unexpected for most materials researchers: The ion diffusion rate in 2D electrodes is at least one order of magnitude slower than that in non-2D-materials. Their results have been published in ACS Nano (DOI: 10.1021/acsnano.9b08304).

Specifically, the authors extracted capacity values at different discharging rates from previously published papers. They then fit these data using their reported equations and derived a figure-of-merit (FoM, in s/m²) for each material:

where τ is a time constant associated with the rate at which capacity starts to fade (in s), and L_E stands for electrode thickness (in m). FoM is a metric quantifying the performance of batteries at ultrafast charging and discharging rates. Comparing the FoMs of 2D materials with those of non-2D materials, the authors discovered that the fast-charging performance of most 2D materials was appreciably inferior to non-2D materials. On average, the FoMs of 2D materials were ~40 times lower than those of other materials analyzed (Fig. A).

Figure. (A) Comparison of the figure-of-merits of 2D materials (red bars) and a set of electrodes including mostly non-2D materials (grey bars). (B and C) Ion-diffusion pathways within (B) in-plane oriented and (C) multi-layered, vertically aligned 2D materials. Red and blue dashed arrows represent sluggish and rapid diffusion directions, respectively. Credit: ACS Nano.

*I believe the unit of the horizontal axis is mistyped. It should be s/m².

What factors should account for the discrepancy in fast-charging battery performance? The authors believed that the typical in-plane-aligned 2D nanosheets were the culprit slowing ion diffusion. In these structures, ions must meander through the tortious inter-sheet slits to interact with the entire electrode (Fig. B). This ion-percolation process is time-consuming and difficult. Since the unfavorable in-plane alignment results from vacuum filtration used to assemble 2D materials into electrodes, it is imperative to revolutionize synthesis techniques to convert 2D materials into game-changers. The authors suggest that the morphologies of "multiple layers of vertically aligned, small nanosheets" are competent to facilitate ion diffusion in 2D materials (Fig. C).

Now the blueprint is there. The next step is to figure out ways to achieve it.

Tianyu Liu acknowledges Dr. N. Balu of Bangalore, India and Chiung-Wei huang at the University of North Carolina – Chapel Hill for their valuable revision suggestions.

Comments

[Mark]

How about the reliability of their fitted equations? Does it really make sense to get conclusions by summarizing the published data? How about the reliability of the published data they used? Do you really believe the published batteries data?
I found another review which also published on Acs Nano (ACS Nano 2020, 14, 3, 2628-2658 https://pubs.acs.org/doi/10.1021/acsnano.9b08396) holds a different opinions that 2D materials could improve the rate performance of electrodes for batteries.

[Tianyu Liu]

Thanks Mark for your insightful comment. I remain neutral to the scientific findings of any papers I write about, but believe that your questions are valid. It is not surprising to me that there are papers holding contradictory views of a topic. I believe that it is beneficial to propel the development of science.
I greatly appreciate that you have brought up another informative paper to our readers.