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

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

The authors synthesized nano-engineered Si-C structure, interface of which is encapsulated by 
covalent bonding, resulting in great improvement of both capacity retention and Coulombic efficiency 
of Si-anode based full cells. Various examination viewpoints are considered and systematically 
organized. That said, there are few things that needs to be clarified prior to me commening on 
whether or not this work deserves the publicaiton in Nature communicaitons. Thsese include, 

1. The positive effect of covalent interface is varified comparing SF, SF@G, and SF@G-HF. That said, 
as far as I notice, there are a few works that fundamentally deliver the covalent concept in previous 
arts such as 

Hasan, et. al 
https://www.nature.com/articles/ncomms9597 
Osaka et. al 
https://pubs.rsc.org/en/content/articlelanding/2014/ta/c3ta13080k#!divAbstract 

These works show the covalent effect is particulaly exhibited only when the base Si structure is quite 
small feature size, such as <10nm, i.e. cell performance improvement by covalent encapsulation is 
not global but synergized when combined with ultra-thin Si. Also the work, 

Ogata et. al 
https://www.nature.com/articles/s41467-018-02824-w 

shows in-situ stabilization of reversibility, showing the composite turns into covalent encapsulated sub 
3nm Si system. The author needs to clarify the intrinsic novelty of the work compared to the previous 
covalent works. 

2. There are hundreds of nano-engineered Si-C composites published for the last 10 years. However, 
unfortunately, most of them neglect density (g/cc) and volumetric/gravimetric energy density (Wh/L 
or Wh/kg) after volumet expansion when combined with cathode. Some S-C composites only shows 
0.5-1.0 g/cc despite its high capacity such as 2000mAh/g, ending up with 1000-2000mAh/cc. Further, 
volume expansion needs to be considered. There is an approximately linear correlationship between 
mAh/cc and volume expansion rate, thus the mAh/cc needs to be devided by volume expansion rate, 
e.g. if 180%, 2000mAh/cc devided by 1.8 = 1100mAh/cc effectively. This effective number needs to 
be taken into account into calculation of Wh/L and Wh/kg. Further, with increase of Si % in the anode, 
discharge voltage drop gets significant due to nature of Cap-Voltage profile of Si. This can reduce 
energy density indeed. In the actual commercialization stage, people always realize that after volume 
expansion and voltage drop in use of higher concentration Si there is not so much energy density gain 
in view of whole device. So the author needs to clarify advantage of their work in this view point. 



Reviewer #2 (Remarks to the Author): 

The authors synthesized silicon-graphene hybrids in the skin-like form where Si and C are covalently 
bonded and used the hybrids as anode for lithium batteries. They found that the covalent binding 
makes electrode material superior compared to other Si-C hybrid forms. 

A central, outstanding issue for silicon anodes is its large volume change upon charge and discharge. 
The authors acknowledge that. In literature, as long as the silicon loading is sufficiently large to make 
the volumetric capacity sufficiently high, this mechanics issue arises. However, throughout the 
manuscript, the authors did not mention about this, despite that they acknowledge volume change is 
a serious issue in previous studies. It is important to report to the readers and the community how 
this issue is resolved (or preserved). If this issue remains, i.e., if volume change exists for the Si-C 
hybrid material, the authors need to address why the volume change does not (or does) influence the 
performance of the material. 

I would be supportive to this manuscript to be accepted by Nature Comm, provided that the above 
issue is carefully addressed. 



Reviewer #3 (Remarks to the Author): 

The paper demonstrates a nice energy density and electrochemical stability. However, to verify the 
concept of the atuhros, further investigations are needed. Otherwise, the results are one of good 
results in the field of silicon based anodes for LIBs. So far, the paper does not reach the level of 
Nature Commun.. 

In the concept of the authors, the graphene layer is fixed by silicon oxide and carbon bonding. 
However, when lithiated, SiOx is reacted into Li2O and LixSi. The authors need to verify how the 
silicon layer and the graphene layer are bonded after lithiation. 
Acutely, The data of SF@G is superior to the data of SF@G-HF. However, this is not direct evidence to 
verify the concept of the authors. 

The authors explained that the electrolyte penetrates through pinholes and/or defects of graphene 
without direct contact of Si with the electrolyte. If the ionic pathway is limited like this, the authors 
need to verify how such high current operation (20 A/g) was achieved. 

These are critical points. And small questions are followings. 

The authors had better to explain what b value is. 

The magnitude of Fig 5a-c is not enough to tell us the flower-like structure. 

The SEIs on SF@G and "SF@G-HF and SF" are different. I understand that the reactivity changes the 
compactness of the SEI. But I don't understand why the composition of SEI changes. 

The authors should indicate absolute value (intensity) in the elemental composition results, not only 
concentration. Because even if inorganic layers were same, thick organic layer changes the 
concentration.


 Reviewer #4 (Remarks to the Author):
 The authors report a two-dimensional covalent binding strategy to improve Li storage performance of
 SF@G by enhancing the interface stability and forming conductive framework in SF@G. And the Si-O-
 C is very important for the Si-C hybrid materials to establish a robust and efficient electrical contact.
 The SF@G exhibited much improved electrochemical performances compared to SF@G-HF and SF.
 Although the paper is well written, the idea to form the area-to-area efficient electrical contact
 between Si and C by Si-O-C is not novel and the work cannot be distinguished from many of
 previously elaborated nano-engineered Si-C works such as J. Mater. Chem. A, 2019, 7, 12763–12772.
 The authors demonstrate that SF@G, SF@G-HF and SF could form different SEI, which is very
 interesting. However, the SEI components and distributions in Figure 5h are only deduced from the
 XPS results, which are not enough. The authors should provide more evidences such as cryo-EM to
 support the assumption. The current version is not acceptable for publication in Nature
 Communications. Some detailed comments as following.
 (1)The SF@G displays a very small volume change of 5.8~6.6%(Figure S12), which is very important
 for the practical applications of Si anodes, the authors should provide more discussions and
 explanations
 (2)What is the thickness of Si nanoplates? Since the authors insist that graphene nanosheets
 deposited on SF are typically 2~3 layers, however, the C content in SF@G is about 12 % as estimated
 by thermogravimetric analysis (TGA).
 (3)In Figure 4a, the SF@G and SF@G-HF have capacities about 2200 mAh g-1, however, in the long-
 term cycle in Figure 3c, their capacities are below 2000 mAh g-1, why?
 (4)Line 129-130, SF@G also offers a dramatically improved cycling stability at a high rate 130 of 2 A
 g-1 over 500 cycles compared to SF@G (here, the SF@G should be changed as SF@G-HF) and SF,
 (5)If the author investigated the effect of morphology and structure of graphene coating on the
 electrochemical properties of SF@G? the Raman results reveal the the ratio of D band to G band
 (ID/IG) to be around 1.5, if this is the optimized structure of graphene coating?
 (6)The SF@G exhibits larger b values in Figure 4g, however, the SF@G-HF and SF have the similiar b
 values. The authors should provide more explanations since the SF@G-HF and SF@G have the similar
 morphology and structure except Si-O-C bonding.
 (7)What is the tap density of SF@G? In Table S1, the electrode press density of SF@G is only 0.94 g
 cm-3.
 (8)The um should be corrected as μm in main text and supporting informations