Peer review of: Stable high-capacity and high-rate silicon-based lithium battery anodes upon two-dimensional cov (2020)


RR2
 Reviewers' comments:
Reviewer #1 (Remarks to the Author):
  The author gives a reasonable explanation for the effect of covalent bonding on the cycle performance
 and for the difference of its nature from previous works. That said, the authors still need to clarify
 some critical points and provide the following dataset, without which it is hard to say the work can
 give a reasonable impact to the silicon-based battery community as well as the corresponding
 industry. From an industry viewpoint, Si-C composite, as well as pre-lithiated SiOx-C composite, is
 already commercialized and battery manufacturers are day-by-day increasing the Si concentration:
 currently up to 5w% of 1700-2500mAh/g Si-C and/or prelithiated SiOx-C composite is already mixed
 in the commercial products, and the ratio is to be increased to 7-10w% in a few years. Thus, without
 specifically clarifying and shaping the point of the finding, the work cannot be distinguished from many
 of previously elaborated nano-engineered Si-C works which is never put on the market. In other
 words, by fully addressing the points below and if the consequently extrapolated numbers are
 meaningful compared to the state-of-the-art, I think the editor work may want to consider this work
 qualifies the publication in Nature Communications.
 1. Please provide a plot of “mAh/cc vs. volume expansion (VE) rate at fully litiated state” so that the
 2D structure in this work has intrinsically different nature of volume expansion from the prior-arts.
 As I mentioned earlier, mAh/cc and VE rate (at fully lithiated state) have some linear correlation. As
 far as I notice this trend is more universal regardless of Si/C composite structures (0D, 1D, 2D, and
 3D). So, the authors need to provide direct evidence that the 2D material here has intrinsically
 different natures of VE from the prior arts. For this, it is very important for the authors to set a
 reasonably higher areal-capacity loading level (typically >5mAh/cm2, which is the current Li-ion
 industry standard) in the system. Also, when the authors make such a plot, the authors need to clarify
 a specific capacity of the anodes (mAh/g) and corresponding volumetric density (g/cc), multiplying
 these leads to mAh/cc. Please provide at least three points for mAh/cc to extrapolate linear
 correlation.
 2. Please provide the volume expansion rate under the following definition
 (Intrinsic volume expansion rate) = 100*(A thickness of fully lithiated anode at 100cycle)/(A thickness
 of as-is electrode at 0 cycle)
 The way the authors define the volume expansion in the text Figure S12 does not deliver intrinsic
 insights into the volume expansion when the cell is assembled into a commercially compatible cell.
 The volume change needs to be referenced to the thickness when the electrode is fully lithiated at the
 target final cycle. Please fix the areal-capacity more than 5mAh/cm2 and the volumetric capacity more
 than 2000mAh/cc, and then please calculate the volume expansion rate exactly in the way that I
 propose. If the expansion is still less than 5-10% at fully lithiated state after 100cy compared to the
 anode thickness at 0 cycle, that is very impressive.
 3. The author should amend the way energy density (Wh/kg) is tabulated in Table S1.
Cathode areal capacity: >5mAh/cm2
 The battery industry never calculates the density just based on active materials. For example, the
 density in the state of the art LCO vs Gr+3w% SiOx commercial full cell is 260Wh/kg at 3500mAh cell
 level, which has a huge deviation from what the authors provide in the table. Providing such
 parameters has actually meaningless. So, please re-calculate the density based on the following
 parameters.
 
 Cathode specific capacity (NCA): 198mAh/g
 Cathode press density: 3.5g/cc
 Cathode active material ratio: 96w%
 Al thickness: 12um
 Separator thickness: 9um (50% porosity of PE, 2side 1.5um MgO+PvDV coating)
 Pouch areal density: 0.05g/cm2
 Anode specific capacity: 2000mAh/g
 Anode active material rate: 96w%
 Negative/positive electrode ratio: 1.05
 Anode density: define-by-yourself g/cc
 Cu thickness: 8um
 Electrolyte amount: 1.8cc/Ah
 Full cell voltage window: 3.0-4.3V (you can set 2.5-4.3V, but lowering the voltage can degrade Si-
 based anode more rapidly)
 Nominal discharge voltage to calculate Wh: 3.7-3.75V (the more Si the more voltage drop you expect
 in the full cell)
 Reviewer #3 (Remarks to the Author):
 The authors responded my comments well. Therefore, the paper is acceptable for publication.
 Congratulation.

REVIEWERS' COMMENTS:
Reviewer #4 (Remarks to the Author):
The authors addressed the reviewer's comments.