Peer review of: Titanium-based potassium-ion battery positive electrode with extraordinarily high redox potentia (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

It is interesting to solve the problems in front of the development of potassium ion batteries (PIBs), 
especially to develop high-performance cathode materials to boost the fast migration of potassium ion 
batteries and to understanding the mechanisms behind. 
In this work, a novel polyanion cathode materials KTiPO4F is developed for PIBs with a relatively 
higher capacity. it is also interesting to see the introduction of Ti3+/Ti4+ redox pair for energy storage. 
The crystal structure and electrochemical performances were investigated. This work can be a good 
reference for other researchers who are working on cathode materials for potassium ion batteries. 
However, there are still some important issues should be clarified before accepted for publication with 
a major revision. 
1. The valance evolution during the extraction/insertion of potassium ion within the crystal structure of 
KTiPO4F is recommended to be investigated by in situ or ex-situ measurement for further 
understanding the energy storage mechanism. 
2.it can be seen from the CV and charge/discharge profile that there are three main oxidation process 
and caused two electrochemcial energy storage mechanism, namely, two-phase mechanism (>3.5 V) 
and solid solution mechanism (2~3.5 V). However, this observation is contradict with the claim given 
in Figure 2E. The operando XRD of KTiPO4F in PIBs present in Figure 3a is not clear enough to see the 
new phase growth during the two phase evolution during the charge/discharge process above 3.5 
V.Please clarify the concerns properly. 
3. Please check the whole manuscript to avoid any similar mistakes 
found from line 335 in Page 15. The claimed applied titanium sources is titanyl phosphate, but the 
provided chemical formula was TiOSO4. 



Reviewer #2 (Remarks to the Author): 

High Ti4+/3+ redox potential realized in phosphate fluoride system is very astonishing to see its 
operation at over 2.5 V. The authors also know how to produce trivalent Ti3+ that is formed in a 
strong acidic medium. 

The material was well characterized by many method and the authors have confirmed insertion of the 
large K+ into the host structure. 

Can the authors provide moisture-sentivity of the KTiPO4F? 

What is the used carbon sources? 

Line 399. N should be italic. 



Reviewer #3 (Remarks to the Author): 

The work by Stanislav S. Fedotov et al. reported the synthesis and characterization of KTiPO4F, which 
showed the possibility of delivering a high voltage around 3.6 V (vs. K+/K) by utilizing the Ti3+ Ti4+ / 
redox. A cumulative inductive effect from the phosphate and fluoride anions and the K/vacancy 
ordering were proposed for the high voltage character. It’s encouraging, although not fully out of 
expectation, to observe that the key electrochemical parameter of the charge/discharge plateau could 
be efficiently modulated through materials design to ensure a favorable battery performance, which is 
helpful to envision new protocols for materials innovation for KIBs. Despite of the large amount of 
characterizations described in the manuscript, the manuscript shows shortcomings in provide 
convincing evidence to support the key conclusions claimed by the authors. Therefore I’m not able to 
suggest an acceptance of this work for its publication in Nat Commun. in consideration of the high 
standard enforced by the journal. Below are some detailed comments I have during reading the 
manuscript. 

1. Electrochemical characterization is needed to confirm the activity of Ti3+/Ti4+ redox. For example, 
EELS and EPR data at different charge states, particularly charged to 4.2 V or discharged to 2 V, 
should be provided to validate the electrochemical transition of Ti3+/ Ti4+ redox. The Ti contribution 
to the high voltage character seems to be the key argument of this work. Unfortunately, no data 
related to the charge compensation mechanism is provided, which makes the claim highly suspicious. 
2. In fluorophosphates, fluorine: oxygen ratio seems to play a critical role in determining the 
electrochemical properties. However, the authors failed to provide experimental evidence and 
necessary discussion about the key factor. Hence, more systematic research may be required for this 
work, including the influence of fluorine on the operating voltage, galvanostatic curves and reversible 
capacity. 
3. The author considered the charge/discharge during 2.0-3.4V as solid-solution-like process, despite 
of clearly two oxidation peaks at 2.81 and 3.21 V for charge and two reduction peaks at 2.58 and 2.99 
V for discharge, which demonstrate multiple phase transitions during the reaction. A close look in the 
operando XRD data did show the emergence of tiny peaks during the disappearance of (212) peak of 
the pristine sample. For example, some tiny ones at 29.7° can be observed when charged to 3.0V. A 
detailed discussion is needed to facilitate the understanding of structural evolution during the 
electrochemical reaction. 
4. For the rate capability of the tested sample as shown in Figure 4c, the authors used different 
voltage windows, is there a special reason for such an operation? How about the Coulombic efficiency 
of the sample? The authors should compare at least the first 2 charge-discharge curves at 0.1 C of the 
samples, which are critical to understand the structural stability and transition of the electrode 
materials during cycling.