Ionic Conductivity of Lithium Phosphides

Round 1

Reviewer 1 Report

A theoretical work on lithium - phosphorous system is presented in this paper. The calculations involve structure stability assessment and lithium mobility study. I did not find any issues for corrections, so in my opinion the manuscript can be published in the present form.

Author Response

We thank referee for this positive report and high appreciation of our work

Reviewer 2 Report

The manuscript entitled “Ion Conductivity in Lithium Phosphides” submitted by Ilya V. Chepkasov and co-workers for consideration for publication in the MDPI journal Crystals presents a study of the ionic conductivity in lithium phosphides by using the combination of first-principles computations with machine learning interatomic potentials. Ionic conductivity of stable (Li3P, LiP, Li3P7, Li3P11, LiP7) and metastable (Li4P3, Li5P4, LiP5) compounds was studied as a function of temperature. This study may provide insights for developing Li-P materials in lithium-ion battery applications. The subject of the presented manuscript is really interesting. The manuscript is well written. It would be better to transfer data from SI to the manuscript. I can recommend the publication of the manuscript in the MDPI journal Crystals after minor revision and taking into consideration the development of the manuscript with an answer to the following questions:

1.               What does it add to the subject area compared with other published material?

2.               The references should be cited appropriately.

 

3.               Data from SI should be transferred to the manuscript.

Author Response

Dear Reviewer, thank you for your comments, here are our answers to the questions.

1. Phosphorous materials are considered in the literature as promising materials for lithium ion batteries anodes. Li-P compounds are promising materials for solid electrolyte applications. It is vital to know ionic conductivity of such compounds, however we did not find any information corresponding either experimental or theoretical ionic conductivity for Li-P compounds other than Li3P in other published materials.
2. We checked all the citing literature and made small changes, i.e. the sentence on line 21 no longer refers to publications 1, 6.
3. We transferred data from SI to the manuscript.

Reviewer 3 Report

In this work the authors studied Li⁺ ion conductivity in Li-P based compounds by means of DFT calculations. Overall, this is an interesting work which deserved to be accepted only after a minor revision:

1.      Figure 2: it is not clear which are the values calculated by the authors and which are taken from literature.

2.      Figure 2: is it possible that in this temperature range some of the investigated materials face a phase transition, which could explain the non-linearity of some curves?

3.      Is it possible to calculate the electronic conductivity of these materials? As explained by the authors in the introduction, this is an important parameter in the evaluation of the best electrolyte material.

4.      It is not clear why the authors discuss also about the application of these materials as anodes.

Author Response

Dear Reviewer, thank you for your comments, here are our answers to the questions.

 

1. Clarifications added to figure 2. Only ionic conductivity of Li₃P was taken from the paper [82]. All other ionic conductivities values were calculated in our work.
2. No phase transitions were occurred during simulations in the temperature range 200 K - 500 K. Non-linearity of some curves might be explained by statistical errors of the simulations and stochastic behavior of Li-jumps during diffusion. Statistical errors might be overcome by increased time and supercell size of the simulations, but requires much more computing resources.
3. Accurate electronic conductivity calculations are challenging task that was not within the scope of current work. However, there is evidence from the literature, indicating low electronic conductivity of Li-P compounds.

In example there are band gaps for Li-P compounds

https://materialsproject.org/materials/mp-736?formula=Li3P

https://materialsproject.org/materials/mp-9588?formula=LiP

https://materialsproject.org/materials/mp-28336?formula=Li3P7

https://materialsproject.org/materials/mp-27687?formula=LiP7

And electronic conductivity of black phosphorous is ≈10⁻¹² S m⁻¹ [42]. Low electronic conductivity makes such compounds suitable for solid electrolyte applications.

4. Phosphorous materials can be considered not only as solid electrolytes, but also as anodes. We provided some examples from the literature for lithium and sodium ion batteries [37-45]. Low electronic conductivity is one of issues, but it might be overcome modifying by various conductive agents, i.e. carbonaceous materials [42]. If phosphorus is used as an anode, then in the charge-discharge process it can transform into Li-P compounds, discussed in the paper. Ionic conductivity in this case is also essential. That is why we discuss also about the application of these materials as anodes.