Peer review of: Strain engineering of two-dimensional multilayered heterostructures for beyond-lithium-based re (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

In this manuscript, the authors present a promising strategy for achieving a negligible volume 
change for the cathode material of VOPO4. The authors also systematically investigate this 
material for energy storage, which shows good performance beyond lithium ion batteries. The 
zero-strain properties of this rationally designed 2D VOPO4 are further validated by DFT 
calculations and in-situ XRD. It would be a meaningful contribution in this field. However, some 
concerns listed below needs to be addressed before it can be considered for publishing on Nature 
Communications. 

1. Cyclic voltammetry (CV) data is very important to analyze the electrochemical process upon 
charge/discharge. The peaks in CV curves can reveal the reversibility of the redox couples involved 
in the (de)lithiation process. The authors did not provide any CV data in this manuscript to show 
the reversibility of VOPO4 and supporting information for each energy storage system. 
2. A superior rate performance of VOPO4-graphene in potassium ion batteries is highlighted in this 
manuscript. However, based on the details the authors presented (active materials loading: 1.5 
mg cm-2), the testing current reaches about 2.5 mA at 5C (assuming 16mm electrode in CR2032 
coin cell). It has been widely accepted that potassium metal is not a good quasi-reference 
electrode especially at high current density condition. Thus, to make the C rate and high current 
density cycling data more convincing, data obtained from a consistent and stable K-K symmetrical 
cell should provide (at least 5C/10C/20C K-K symmetrical cell data. 
3. As the superior rate performance of VOPO4-graphene has been reported by the authors, it 
would be better to clarify the capacity contribution mechanism. Considering the nanosheets 
structure of VOPO4-graphene and its superior rate performance, a pseudocapacitive capacity 
contribution is likely existed (iScience, 6, 212-221). 



Reviewer #2 (Remarks to the Author): 

In this manuscript, the authors synthesized the multilayered VOPO4-graphene composite materials 
for K-, Na-, Zn- and Al-ion batteries, and exhibited excellent electrochemical performance. And the 
authors used various characterization methods along with theoretical calculation. However, I still 
have some concerns as follows and thus suggest that the manuscript would be accepted after 
major revision. 
1. The as-synthesized VOPO4-graphene cathode exhibited impressive cycling performance, which 
is mainly attributed to the multilayered structure. The authors used Cross-section HRTEM image 
(Figure 2f) to show unique microstructure, which seems to be not convincing. Could the authors 
provide more solid experimental results? 
2. We estimated the theoretical capacity of VOPO4-graphene cathode for K-ion battery based on 
the V4+/V5+ valence change and the value is 162 mAh/g. In the work, the measured capacity is 
160 mAh/g, very close to the theoretical value. That means that in the full charged state the value 
of element V is +5, which is inconsistent with Figure 3g. in Figure 3g, there exists V4+ except for 
V5+. 
3. The as-synthesized VOPO4-graphene cathode contains no K ion, thus the cell is initially 
discharged (potassiated). Based on the Figure 2 a and d, the first discharge capacity is lower than 
the first charge (depotassiated) capacity. Could the authors explain for it? 
4. As shown in Figure 3, The 001 peak of as-synthesized 2D VOPO4-graphene and restacked 
VOPO4 nanoflakes shifted to lower 2θ values upon discharge (potassium intercalation) and change 
reversibly after charge, representing increased interlayer distances when K-ions inserted into the 
layers and decreased interlayer distances when K-ions extracted. However, for similar layer 
materials, the interlayer d-spacing would decrease upon discharge (ions intercalation into host 
materials), owing to an attractive force between the inserted metal ions and the lattice oxygen 
atoms of stacked layers. And when ions extracted from host materials, the interlayer d-spacing 
would change in reverse[1-5]. Could the author explain this change in crystal structure that is 
different from conventional understanding? 
[1] Hyoung J, Heo J W, Chae M S, et al. Electrochemical Exchange Reaction Mechanism and the 
Role of Additive Water to Stabilize the Structure of VOPO4 2 H2 O as a Cathode Material for 
Potassium-Ion Batteries. ChemSusChem, 2019, 12(5): 1069-1075. 
[2] Liao J, Hu Q, Che B, et al. Competing with other polyanionic cathode materials for potassium-
ion batteries via fine structure design: new layered KVOPO4 with a tailored particle morphology. 
Journal of Materials Chemistry A, 2019, 7(25): 15244-15251. 
[3] Tian B, Tang W, Su C, et al. Reticular V2O5·0.6H2O Xerogel as Cathode for Rechargeable 
Potassium Ion Batteries. ACS Applied Materials & Interfaces, 2017, 10(1): 642-650. 
[4] Wang P, Chen Z, Wang H, et al. A high-performance flexible aqueous Al ion rechargeable 
battery with long cycle life. Energy Storage Materials, 2020, 25: 426-435. 
[5] Wang J, Tan S, Xiong F, et al. VOPO4·2H2O as a new cathode material for rechargeable Ca-ion 
batteries. Chemical communications (Cambridge, England), 2020.