Synthesis of LiPON Solid Electrolyte Films by Thermal Evaporation of Lithium Orthophosphate

Round 1

Reviewer 1 Report

The authors set up an optimized thermal evaporation equipment with a special designed discharge plasma generator. And the LiPON films’ quality were improved. The optimized growth condition and mechanism was discussed in detail. However, there are several suggestions the authors should consider to revise before the manuscript is accepted.

1.  The distribution of the globules on the surface of LiPON film diverges from normal Gaussian distribution in Fig. 3. The authors can consider the other distribution, such as Wald distribution, Rayleigh distribution, or Log-Normal Distribution, etc.

2. “2” of ~47 μF/cm2 in the last second line in the Conclusions should be superscript.

Author Response

Dear Reviewer,

We are very grateful to you for the careful reading of our manuscript and helpful comments that improved presentation and readability of our work. Please find below line-by-line answers to the comments. All changes made in our manuscript are shown in yellow. We hope that revised version of manuscript now is suitable for acceptance in Coatings.

Sincerely yours,

Dr. Kamenetskikh on behalf of authors

Reviewer

The authors set up an optimized thermal evaporation equipment with a special designed discharge plasma generator. And the LiPON films’ quality were improved. The optimized growth condition and mechanism was discussed in detail. However, there are several suggestions the authors should consider to revise before the manuscript is accepted.

1. The distribution of the globules on the surface of LiPON film diverges from normal Gaussian distribution in Fig. 3. The authors can consider the other distribution, such as Wald distribution, Rayleigh distribution, or Log-Normal Distribution, etc.

Author’s answer:

We decided to exclude Figure 3 from the article due to its insignificant information

2. “2” of ~47 μF/cm2 in the last second line in the Conclusions should be superscript.

Author’s answer:

The index position has been corrected.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors have proposed a method for LiPON films. I regret to mention that I cannot support this work in its current form. Here are my comments for the authors.

-        The authors have recently published a very similar work with a similar topic [10]. The results in the current manuscript are more like a few results left from that work. Authors need to emphasize on the importance of the current work and its advantages over their recently published work.

-        The authors mention “The imperfection of the film structure is one of the reasons limiting the achievement of high values of ionic conductivity of films”. So, their goal is to achieve a film with a homogenous surface. However, none of the images shown in Figure 2 present any homogeneity. This bad film quality is also confirmed in Figures 3 and 4. So it seems what is claimed is not really achieved in this work.

-        The term “thin film” typically refers to the films with nanometer thicknesses (i.e., less than ~100 nm). However, the films in this work are thicker than 0.5 microns and cannot be considered thin films.

-        How many samples are tested to show sample to sample variations?

-        The Abstract and Conclusions are too short. The deposition rates mentioned in these two are different. These two important sections need to be written more carefully.

 

Author Response

Dear Reviewer,

We are very grateful to you for the careful reading of our manuscript and helpful comments that improved presentation and readability of our work. Please find below line-by-line answers to the comments. All changes made in our manuscript are shown in yellow. We hope that revised version of manuscript now is suitable for acceptance in Coatings.

Sincerely yours,

Dr. Kamenetskikh on behalf of authors

Reviewer

The authors have proposed a method for LiPON films. I regret to mention that I cannot support this work in its current form. Here are my comments for the authors.

-        The authors have recently published a very similar work with a similar topic [10]. The results in the current manuscript are more like a few results left from that work. Authors need to emphasize on the importance of the current work and its advantages over their recently published work.

Author’s answer:

The material presented in this work is the result of a separate comprehensive study aimed both at creating a more advanced device for deposition of solid electrolyte films, and at providing conditions for controlled changes in parameters critical for achieving high ionic conductivity and a uniform microstructure of films. We have tried to note this in the revised version of the article (see Page 2):

The aim of this work was to determine the conditions for the synthesis of LiPON films with a homogeneous microstructure and high ionic conductivity in a low-pressure arc. To achieve this goal, the electrode system of the film deposition device used in previous experiments [10] was significantly modified. This made it possible to stabilize the evaporation process and ensure the uniformity of the plasma in the film deposition zone, as well as the possibility of independently adjusting the frequency of interaction of electrons with vapor by changing the vapor pressure or discharge current. The paper presents the results of studies of the influence of the degree of decomposition of Li₃PO₄ vapor on the ionic conductivity of films, cyclic tests of films and measurements of specific electrical capacitance.

-        The authors mention “The imperfection of the film structure is one of the reasons limiting the achievement of high values of ionic conductivity of films”. So, their goal is to achieve a film with a homogenous surface. However, none of the images shown in Figure 2 present any homogeneity. This bad film quality is also confirmed in Figures 3 and 4. So it seems what is claimed is not really achieved in this work.

Author’s answer:

On the Figure 4 we show an image of the surface of a homogeneous film. Such a film was obtained by optimizing the discharge current and the degree of decomposition of Li₃PO₄ vapor, and the stated goal was achieved.

-        The term “thin film” typically refers to the films with nanometer thicknesses (i.e., less than ~100 nm). However, the films in this work are thicker than 0.5 microns and cannot be considered thin films.

Author’s answer:

We excluded the term “thin film” from the article and limited ourselves to only mentioning “film”.

The upper limit of applicability of the term «thin film» is various for different authors, for example: «As an arbitrary delineation, the term “thin film” will generally be used for deposits less than about 0.5 microns (5,000 Ångstroms or 500 nanometers) in thickness». [Mattox, D. M. The Foundations of Vacuum Coating Technology, January 2003, Introduction, page 1]. Using “thin films”, we adhered to the conventional thickness criterion used in the well-known literature source [Handbook of Thin Film Technology, edited by L. Maissel and R. Glang. McGraw-Hill, 1970]: “Some of methods summarized in Table 1 are capable of producing both thin (<10 000 Å) and thick (>10 000 Å) films” (page 5-2).

-        How many samples are tested to show sample to sample variation?

Author’s answer:

Four samples were obtained simultaneously in each experiment. For each dependency point shown in Figure 6, the experiment was performed twice.

-        The Abstract and Conclusions are too short. The deposition rates mentioned in these two are different. These two important sections need to be written more carefully.

Author’s answer:

We have improved Abstract and Conclusion

Abstract

Lithium phosphorus-oxynitride (LiPON) films were deposited by the method of anodic evaporation of Li₃PO₄ in the nitrogen plasma of a low-pressure arc. A method for adjusting the degree of decomposition of vapors is proposed, based on a change in the frequency of interaction of electrons with vapors at a constant heating power of the anode-crucible. The conditions ensuring the formation of films with a homogeneous microstructure and ionic conductivity (1-2)*10^-6 S/cm at a deposition rate of 8 nm/min have been determined. It is shown that the degree of vapor dissociation critically affects the morphology of the films and the magnitude of their ionic conductivity. The results of cyclic tests of LiPON films deposited by anodic evaporation in a low-pressure arc are presented.

Conclusion

            In an electrode system with a self-heating hollow cathode, a crucible anode with autonomous heating in a discharge with a direct-heat thermocathode, an additional sectioned anode by thermal evaporation of Li₃PO₄ in Ar-N₂, lithium phosphorus oxynitride (LiPON) films are deposited in plasma at a rate of 8 nm/min. The electrode system pro-vides stable evaporation of Li₃PO₄, plasma uniformity in the deposition area of coatings and regulation of the degree of vapor decomposition.

A significant effect of the discharge current on the microstructure of films has been established, which consists in an increase in the degree of agglomeration of spherical inclusions on the surface of films with an increase in current due to Li segregation. By optimizing the magnitude of the discharge current and the degree of dissociation of Li₃PO₄ vapors in the discharge plasma, films with a homogeneous microstructure and ionic conductivity of 2*10^-6 S/cm were obtained.

The specific capacitance of the "steel/LiPON/steel" structure was estimated by cyclic voltammetry, which amounted to ~47 µF/cm² at a potential scan rate of 0.1 V/s for 0.5 mm thick LiPON film with an ionic conductivity of 1·10^-6 S/cm.

Author Response File: Author Response.docx

Reviewer 3 Report

In this manuscript, authors synthesized thin films of LiPON solid electrolyte using the thermal evaporation technique and achieved homogeneous microstructure and ionic conductivity between 1-2 * 10^-6 S/cm. 

Previously, the same group of authors had fabricated LiPON using Anodic Evaporation in N2 Plasma of a low-pressure arc discharge and achieved ionic conductivity 1 *10^-6 S/cm (doi.org/10.3390/membranes12010040) with a deposition rate of 5 nm/min. In this manuscript, the authors modified the electrode system and achieved a slightly higher deposition rate of ~8 nm/min with similar ionic conductivity.

I have a few comments which need to be addressed before publication.

1. Are the as-deposited films amorphous or crystalline? Any XRD?

2. If possible, could you please add a schematic diagram comparing the original and modified electrode systems in Figure 1?

3. Can you provide more information about the secondary compounds formed ( possible compound types) by the presence of lower P and N atomic percentages in the globules mentioned on page 4, line 166? How do these compounds affect the ionic conductivity of the film, and do you have any references on this topic?

 

Author Response

Dear Reviewer,

We are very grateful to you for the careful reading of our manuscript and helpful comments that improved presentation and readability of our work. Please find below line-by-line answers to the comments. All changes made in our manuscript are shown in yellow. We hope that revised version of manuscript now is suitable for acceptance in Coatings.

Sincerely yours,

Dr. Kamenetskikh on behalf of authors

Reviewer

The authors have In this manuscript, authors synthesized thin films of LiPON solid electrolyte using the thermal evaporation technique and achieved homogeneous microstructure and ionic conductivity between 1-2 * 10^-6 S/cm. 

Previously, the same group of authors had fabricated LiPON using Anodic Evaporation in N2 Plasma of a low-pressure arc discharge and achieved ionic conductivity 1 *10^-6 S/cm (doi.org/10.3390/membranes12010040) with a deposition rate of 5 nm/min. In this manuscript, the authors modified the electrode system and achieved a slightly higher deposition rate of ~8 nm/min with similar ionic conductivity.

I have a few comments which need to be addressed before publication.

1. Are the as-deposited films amorphous or crystalline? Any XRD?

Author’s answer:

The as-deposited films had an amorphous structure. Crystallization of LiPON took place after annealing at a temperature of ~300 ^oC. We noted in the revised version of the article that the films deposited under the experimental conditions were amorphous and added a link to a previous work [Gavrilov, N.; Kamenetskikh, A.; Tretnikov, P.; Nikonov, A.; Sinelnikov, L.; Butakov, D.; Nikolkin, V.; Chukin, A. Synthesis of Lithium Phosphorus Oxynitride (LiPON) Thin Films by Li3PO4 Anodic Evaporation in Nitrogen Plasma of a Low-Pressure Arc Discharge. Membranes, 2022, 12(1), 40. DOI: 10.3390/membranes12010040.] in which XRD results were previously presented (see Page 4):

The as-deposited films had an amorphous structure, however, the XRD results presented in the previous work [10] indicate the presence of traces of the LiOH crystalline phase, the appearance of which is due to the formation of globules.

2. If possible, could you please add a schematic diagram comparing the original and modified electrode systems in Figure 1?

Author’s answer:

We have added a schematic diagram of the original electrode system (see Figure 1b).

3. Can you provide more information about the secondary compounds formed ( possible compound types) by the presence of lower P and N atomic percentages in the globules mentioned on page 4, line 166? How do these compounds affect the ionic conductivity of the film, and do you have any references on this topic?

Author’s answer:

Similar secondary inclusions on the surface of films were observed in the works [Nimisha, C.S.; Rao, G. M.; Munichandraiah, N.; Natarajan, G.; Cameron, D.C. Chemical and microstructural modifications in LiPON thin films exposed to atmospheric humidity. Solid State Ion. 2011, 185, 47–51. DOI: 10.1016/j.ssi.2011.01.001; H. Xia, H. L. Wang, W. Xia et al Thin film Li electrolytes for all-solid-state micro-batteries. Int. J. Surf. Sci. Eng. 2009, 3, 23–43. DOI: 10.1504/IJSURFSE.2009.024360], their formation was associated with lithium segregation in films. Rapid diffusion of Li atoms to the surface and subsequent reactions with water vapor and CO₂ lead to the formation of compounds LiOH, Li₂O, Li₂CO₃, etc. [H. Xia, H.L. Wang, W. Xia et al Thin film Li electrolytes for all-solid-state micro-batteries. Int. J. Surf. Sci. Eng. 2009, 3, 23–43. DOI: 10.1504/IJSURFSE.2009.024360]. The presence of traces of LiOH is evidenced by the results of film analysis presented earlier in [Gavrilov, N.; Kamenetskikh, A.; Tretnikov, P.; Nikonov, A.; Sinelnikov, L.; Butakov, D.; Nikolkin, V.; Chukin, A. Synthesis of Lithium Phosphorus Oxynitride (LiPON) Thin Films by Li3PO4 Anodic Evaporation in Nitrogen Plasma of a Low-Pressure Arc Discharge. Membranes, 2022, 12(1), 40. DOI: 10.3390/membranes12010040].

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have answered some of my comments. However, the main problem with this work is not still answered.

-        It is clearer now that the authors have not achieved the goal they claimed. The authors mention “The aim of this work was to determine the conditions for the synthesis of LiPON films with a homogeneous microstructure and high ionic conductivity in a low-pressure arc.” In the answer letter authors mention that the achieved goal is shown in Figure 4. However, they have mentioned in the text that it presents low ionic conductivity. So, the goal is not really achieved. Additionally, this figure lacks a scale bar to show the morphology.

I regret to mention that I cannot support this work. But I will be happy to read the revised manuscript, if the authors revise the manuscript and show they really achieve their above-mentioned goal.

 

 

Author Response

Dear Reviewer,

Please find below line-by-line answers to the comments. All changes made in our manuscript are shown in yellow. We hope that revised version of manuscript now is suitable for acceptance in Coatings.

Reviewer 

The authors have answered some of my comments. However, the main problem with this work is not still answered.

-        It is clearer now that the authors have not achieved the goal they claimed. The authors mention “The aim of this work was to determine the conditions for the synthesis of LiPON films with a homogeneous microstructure and high ionic conductivity in a low-pressure arc.” In the answer letter authors mention that the achieved goal is shown in Figure 4. However, they have mentioned in the text that it presents low ionic conductivity. So, the goal is not really achieved. Additionally, this figure lacks a scale bar to show the morphology.

I regret to mention that I cannot support this work. But I will be happy to read the revised manuscript, if the authors revise the manuscript and show they really achieve their above-mentioned goal.

Author`s answer

This result is achieved by optimizing the conditions, namely, the current of the sectioned anode, which regulates the plasma concentration in the film deposition region, and the ratio of discharge currents with a self-heating hollow cathode and a thermionic cathode, which determines the degree of decomposition of Li₃PO₄ vapors.

Heating of the crucible in a discharge with a self-heating hollow cathode and heating using a discharge with a thermionic cathode correspond to modes with a maximum and minimum degree of vapor dissociation. In the first mode, as a result of segregation of free Li, films with a coarse-grained structure are synthesized, in the second mode homogeneous films with low ionic conductivity are formed. Combined heating of the crucible with an adjustable ratio of discharge currents with a self-heating hollow cathode and with a wire cathode made it possible to implement intermediate modes with varying degrees of vapor dissociation, the absolute values of which depend on the discharge current with a self-heating cathode. Since the concentration of free lithium atoms in the discharge plasma varies proportionally to the vapor pressure and discharge current, and the degree of dissociation is inversely proportional to the vapor pressure, an increase in vapor pressure with increasing discharge current with a hollow cathode does not affect the degree of vapor dissociation, which is determined only by the current of the discharge with a self-heating cathode

The dependence of the ionic conductivity of films on the ratio of discharge currents with a hollow (IHC) and direct heated cathode in the crucible circuit (ITC) at a constant heating power of the crucible (360 W) and the hollow cathode current of 15 A is shown in Figure 5. The ionic conductivity of synthesized LiPON films increases monotonically over the entire range of changes in the current ratio. The films deposited under these conditions are homogeneous and have high ionic conductivity. An increase in the hollow cathode current up to 40 A led to formation of granular films with low ionic conductivity.

Author Response File: Author Response.docx