Peer review of: Low-temperature paddlewheel effect in glassy solid electrolytes (2020)

Editorial Note: This manuscript has been previously reviewed at another journal that is not operating a 
transparent peer review scheme. This document only contains reviewer comments and rebuttal letters 
for versions considered at Nature Communications. 

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

Coupling of the anion rotation with the lithium translation is examined in details from the MD 
simulations of sulfide glasses that serve as a prototype for the conductive solid state amorphous (and 
crystalline) electrolytes. This study is well-performed, careful and clearly articulated. I expect taht 
many researchers in the field will find it inspirational. While reviewer 3 points that oxygen and other 
anion deformations are often coupled with the cation motion in cathodes and solid electrolytes, 
making transitions softer and faster, I agree with the point made by authors that these local vibrations 
or small re-arrangements are quite different from the more pronounced Paddle-Wheel Mechanism 
discussed in this manuscript. I recommend this manuscript for publication in Nature Communications 
after authors address my specific comments below. 

Was protocol shown in Figure S1 applied to all “6 independent MD runs performed at 300 K”. How to 
authors know that systems were decorrelated and independent? 

What is the distance “a” in eq. 3? 

In Fig. S7 and Fig. 4, the initial and final positions of the Li+ cations are labeled but the initial and 
final positions of the sulfur of PS4 are not. Could author label S with 1,2,3,4 in Fig. S7 to make it 
possible to see how the PS4 anions rotated during this simulation from the initial to final positions. 
This should clearly show that a “new” S comes to the same position after helping a Li+ to move via 
the PS4 rotation. Such diagram will help to differentiate this work from the other work mentioned by 
the 3d reviewer (Chem. Mater. 2015, 27, 6016−6021) where oxygens deformed but returned to the 
same positions after assisting cation to overcome a barrier. Rotation of the PS4, OH or B12H12 anions 
is different assuming it is not a partial rotation with a return to the same position. Figure 4b indicates 
that it is a combination of rotations that “complete” and “return” to the same position and it varies 
with density according to Figure S19. 

Figure 4 shows that some anions have displacements of 2 A while the most active Li move 5 A (Figure 
3). Could authors add MSD for anions vs. different temperatures in SI to convenience readers that 
anions do not move much during MD simulations, especially, at high temperature. If anions move at 
high temperature, their diffusion coefficient vs. temperature should be included in Figure 6 as it could 
also be coupled to Li translation (in addition to anion rotation). 
Anion translation is also important because freezing anions (Figure S21) freezes both anions 
translation and rotation. The former is not mentioned. I think that this could be an important 
depending on the magnitude of the anion MSD at higher temperatures. 

Is there a correlation of the anion MSD with the anion rotations shown in Figure S19? If there is, it is 
important to quantify it. 

A cooperative motion of anions with lithium might also account to the lower experimental conductivity 
compared to the predicted value that is based only on the lithium diffusion without including anions. 
Again, if anions do not move at all, it is not an issue. 

There is a significant overlap with the analysis of paddlewheel reported for (Li/Na)B12H12 solid state 
conductor (power spectrum, rotational relaxation, etc) reported in Chem. Mater. 2017, 29, 
9142−9153. It could be helpful to specify in the discussion the main differences in the mechanism 
between sulfide glasses and this system could help readers contrast and better understand both works. 

Page 2:.“Unfortunately, exploiting these rotational degrees of freedom presents a challenge, as the 
paddlewheel mechanism is typically observed in high-temperature polymorphs having expanded 
volumes. 39–43,45–54,61 
Note that a paddlewheel effect was also observed for Li2+xOH(1-x)Cl solid state conductors for cubic 
phase near to room temperature but called “rotating door” mechanism, see Song Protons Enhance 
Conductivities in Lithium Halide Hydroxide/Lithium Oxyhalide Solid Electrolytes by Forming Rotating 
Hydroxy Groups. Adv. Energ. Mater. 2018, 8 (3), 1700971-8, followed by Howard Fundamental 
aspects of the structural and electrolyte properties of Li2OHCl from simulations and experiment. Phys. 
Rev. Mater. 2017, 1 (7), 075406. A clear correlation between the anion rotation and lithium diffusion 
(space and time) was also shown for these systems, albeit with the DFT-based MD simulations 
performed at higher temperature but experiments indicate that this mechanism persists down to room 
temperature as Arrhenius scaling of conductivity was observed but because some of the analysis of 
the coupling of anion rotation with Li+ motion is similar, I think that this work should be referenced. 

It would be helpful to give in Figure S3b the neutron-weighted partial contributions to G(r) to clearly 
show which PDFs contribution to each peak. Otherwise, it is not clear by a strong peak for Li-S PDF 
around 2.5 A yields a minimum in Figure S3. This will also clarify sensitivity of the neutron weighted 
G(r ) to different contributions. 

Oleg Borodin 



Reviewer #2 (Remarks to the Author): 

The authors presented comprehensive diffusion mechanisms studies in glass Li3PS4 via ab initio MD 
simulations. The authors clearly demonstrated that the previously-proposed “Paddlewheel” mechanism 
contributes to the Li-ion diffusion at temperatures as low as 300K in the glass phase but not in the 
crystalline phase. This is not a trivial task, and the authors also did a very carefully job in generating 
representative amorphous structures. They considered the “Paddlewheel” mechanism is the key to the 
fast ionic conductivity in glass Li3PS4. 

While I applause for the detailed simulation work on identifying the Li diffusion mechanism in the glass 
Li3PS4 phase. I would suggest the authors could comment on ionic conductivity, not just the 
diffusivity. 

When Li hops from one site to another site, these sites will have different energy levels in the glass 
structure, like those shown in PCCP 11, 3210, 2009. The authors fit one activation energy for 
diffusivity, in fact, the amorphous structure has a distribution of the activation energies, as discussed 
in PRL 109, 075901, 2012. The distribution of the activation energies in amorphous was also used to 
explain the non-Arrhenius conductivity in Li-P-S glass (Solid State Ionics, 176, 2349, 2005 ). However, 
these theories on diffusion in glass structures are not discussed in the current manuscript. To advance 
the field, I hope the authors could comment on the distribution of site energy, the percentage of low 
energy migration pathway, and the distribution of activation energies in amorphous.