Structural regulation of halide superionic conductors for all-solid-state lithium batteries

Metal halide solid-state electrolytes have gained widespread attention due to their high ionic conductivities, wide electrochemical stability windows, and good compatibility with oxide cathode materials. The exploration of highly ionic conductive halide electrolytes is actively ongoing. Thus, understanding the relationship between composition and crystal structure can be a critical guide for designing better halide electrolytes, which still remains obscure for reliable prediction. Here we show that the cationic polarization factor, which describes the geometric and ionic conditions, is effective in predicting the stacking structure of halide electrolytes formation. By supplementing this principle with rational design and preparation of more than 10 lithium halide electrolytes with high conductivity over 10−3 S cm−1 at 25 °C, we establish that there should be a variety of promising halide electrolytes that have yet to be discovered and developed. This methodology may enable the systematic screening of various potential halide electrolytes and demonstrate an approach to the design of halide electrolytes with superionic conductivity beyond the structure and stability predictions.


REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): In this manuscript, the authors propose a polarization factor which can be utilized for the design of halide solid electrolyte Li3MCl6.While this work is intriguing, it does not meet the high-quality standards expected by Nature Communications.Before publishing, it is essential to tackle the following issues: 1) The phase transition of Li3MCl6 (among CCP-M, hcp-O, and hcp-T) and the high ionic conductivity of the monoclinic phase have been widely reported in the literature.[Science Advances, 2022, 8(36), eadc9516; ACS Energy Letters, 2022, 7 (5), 1776-1805.]The authors should clearly provide the novelty of their work.
2) The authors demonstrated a general design rule for Li3MCl6, but they only presented a limited number of new Li3MCl6.The reviewer concerns regarding the applicability of the structural design rule for Li3MCl6, suggesting that it may be limited to a specific region and may not offer new hypothetical Li3MCl6 structures.Please provide various new hypothetical Li3MCl6 structures that can be synthesized by other researchers.
3) The structural design rule was modified for Zr4+, but the reviewer concerns regarding the effectiveness of the design rule for M4+ 4) This study includes a limited range of Li3MCl6, with the elements primarily restricted to the costly lanthanides.Furthermore, the comprehensive examination of the effect of Br has not been conducted.
Reviewer #2 (Remarks to the Author): The present work deals with the systematic investigation of novel compositions of halide-based Liion conductors.Using a cationic polarization factor, the authors reveal compositions with increased ionic conductivity.The work presents a novel approach for the design and improvement of halide ion conductors and can be accepted for publication after minor revision.A point-by-point list of remarks is given in the following.
-At the beginning of page 2, there is an error in the citing literature.[...]different Li+ migration properties.Please correct this.
-In Figure 1d the crystallographic axis should be included to get a better understanding of the structural changes -As the cationic polarization factor is relatively new in the field of solid electrolytes, can the authors extend the explanation of how it was calculated (see page 4) -What does "weighted ionic potential of lithium ions" mean?Please explain this in more detail.
-Please add a paragraph into the conclusion section discussing the possibility to transfer such concept of the cationic polarization factor to other classes of ion conductors.
Reviewer #3 (Remarks to the Author): Halides are currently a hot topic for SEs.In this manuscript, the authors try to give and prove the structural regulation and evolution of halide SEs by using the cationic polarization factor, etc.They provide a predicted model of Li-M-X phase diagram, which is highly related to the Li-ion transport capability.This work is important for the halide design and may be a good reference for other systems.It is also interesting that they provide over 10 Li-halide electrolytes possessing high conductivities over 1 mS/cm.I think the prediction is reasonable.Therefore, I recommend to publish the manuscript in Nature Communication.

Comments:
1. Why do you use Shannon crystal radii rather than the Pauling radius of ions, as we know Pauling radius is also widely used?What are the differences for them and the scope of their respective application?I suggest to give more detailed information in the manuscript.
2. The structure model is based on the factors, e.g., ion polarization.I wonder how much influence the electronic structure of different elements has on this model?3. The phase structures of hcp-T, hcp-O, and ccp-M show different ion conduction rates.Why and what are the rules about them?Does it mean that the symmetry of the halide structure and/or the arrangement of the anion framework have a significant influence on ion transport?4. The oxychloride-based and UCl3-type SEs e.g., Li-La-Ta-Cl have been reported recently, how about to use this phase diagram model in these novel electrolytes?Have you considered to use the model in other systems say, sulfide SEs? 5.The references for the definition of ionic potential should be cited.Regarding the formulaΦLi = nLi × ϕLi, ϕLi is not defined.
to provide a principle for the rational design of halide SSEs and the determination of the phase transition of halide SSEs when regulating the composition, which further induces changed Li +conducting behavior.We highlight the novelties and importance of this work in the discussion below.
The development of halide SSEs has been very limited compared to other types of welldeveloped solid-state electrolytes (SSEs) such as oxides, polymers, borohydrides, and sulfides.
Until now, there are many successful examples of halide SSEs that can achieve room-temperature (RT) ionic conductivity over 10 -3 S cm -1 , i.e., Li3YBr6, Li3InCl6, and zirconium substituted Li3−xM1−xZrxCl6.How the composition determines the structural chemistry is crucial for the ionic conductivity of halide SSEs but very challenging to predict.Therefore, it is essential and urgent to make it possible to predict the stacking structures and conductive properties of halide SSEs by capturing the key interactions and other factors.
For the concern of the related previous reports, firstly, these two works are review papers and only limited sections that discuss the structure and conduction behavior of halide SSEs.Secondly, these two works only roughly mentioned the different cations/anions radii and the ratio of ionic radii of cation M and anion X (rM/X) effects on the final halide SSEs.Thirdly, the radii effect only accounts for the difference in cation/anion radii, which makes it impossible to predict the structure of halides SSEs with the same metal cations and anions (for example, hcp-T typed Li3HoCl6 and hcp-O typed Li2.727Ho1.091Cl6,hcp-T typed Li3ErCl6 and hcp-O typed Li2.727Er1.091Cl6),let alone give further guidance of structure regulation.
In our work, we introduce the "cationic polarization factor" which provides an estimate of the phase transition among the different structures of halide SSEs, which further induces different Liconducting behavior.By supplementing this principle with rational design and preparation of more than 10 lithium halide electrolytes with high conductivity over 10 -3 S cm -1 , we establish that there should be a variety of promising halide-based SSEs that have yet to be discovered and developed.
The key novelty and advancements of our study are that, for the first time, we proposed a cationic polarization factor, which captures the key role of cation/anion interactions on the final stacking structures that can chart the landscape of the existing and future lithium halide SSEs.The cationic polarization factor enables the classification of experimentally observed and constitutes the prediction of those still unexplored lithium halide SSEs.Guided by the cationic polarization factor, more than 10 lithium halide SSEs with RT ionic conductivities over 10 -3 S cm -1 have been identified and synthesized.The cationic polarization factor further allows the determination of the phase transition of halide SSEs when regulating the composition, which further induces changed Li + -conducting behavior.
(2) The authors demonstrated a general design rule for Li3MCl6, but they only presented a limited number of new Li3MCl6.The reviewer concerns regarding the applicability of the structural design rule for Li3MCl6, suggesting that it may be limited to a specific region and may not offer new hypothetical Li3MCl6 structures.Please provide various new hypothetical Li3MCl6 structures that can be synthesized by other researchers.
Response: Many thanks for the suggestions.As shown in Figure R1, we have proposed four strategies and related examples that follow the structural design rule for Li3MCl6, including cation regulation, anion regulation, cation concentration regulation, and combined regulation.Based on the proposed routes, we have synthesized more than twenty Li3MCl6 halides that follow the structural design rule (Table R1).Thus, we synthesized at least 27 halide SSEs, including 23 Li3MX6 compounds and 4 typical Li3-xM1-x 3+ Mx 4+ X6 compounds.All the synthesized materials are based on the proposed design rule for anion-sublattice-based halide SSEs.It is believed that the proposed design principles can be broadly applied to the synthesis of a wide variety of halide SSEs with high ionic conductivities.
We have further revised in the manuscript and supporting information and highlighted.
(3) The structural design rule was modified for Zr 4+ , but the reviewer concerns regarding the effectiveness of the design rule for M 4+ .
Response: Many thanks for the suggestions.
The structural design rule in this work mainly focuses on LiaMXb halides that developed from ccp-type LiX.The structure of LiX can be seen as the distribution of small Li + ions into the X - anion sublattice.Different types of M 3+ cations are further added to the system to form LiaMXb halides.The highly symmetrical ccp structure can be maintained if there's a minor difference in the cation radius (such as Li3InCl6, and Li3ScCl6 with the highest symmetry).Whereas Li3YbCl6 and Li3LuCl6 with moderate differences lead to decreased symmetry to hcp-O structure (moderate symmetry) and other Li3MCl6 halides (M = Y, Tb-Tm) with the largest difference result in a further decrease of symmetry to hcp-T structure.
Thus, for the Li3-xM1-x 3+ Mx 4+ X6 halides with a small cation radius difference (Table R3), their   As presented in Table R3, for these Li3-xM1-x 3+ Mx 4+ X6 halides with a large cation radius difference, a coefficient was introduced for the calculation of the cationic polarization factor τ for in our work.Examples include Li3-xYb1-xZrxCl6   We further added these tables and figures in the revised manuscript and supporting information and highlighted.
(4) This study includes a limited range of Li3MCl6, with the elements primarily restricted to the costly lanthanides.Furthermore, the comprehensive examination of the effect of Br has not been conducted.For Al, the formation of LiaAlXb follows the basic rules of ion stacking and can be divided simply by the cation/anion radius ratio as we presented in Figure 1e and Table R5.The Al 3+ in Li3AlF6 is six-coordinated while four-coordinated in LiAlCl4 and LiAlBr4.

Response
Table R5.The summary of the cation, halogen anion crystal radius, and their ratio of the typical Li-Al-X halides.For the effect of Br, we have presented that the structure transition from hcp-T to ccp-M can be realized by decreasing the ionic potential of X -with the substitution of a larger anion.Two representatives are hcp-T typed Li3YCl6 and ccp-M typed Li3YBr6.When Li + and Y 3+ cations stack into the anion framework, the larger size of the Br -anion will dilute the electronic cloud extension effect of cations, thus ensuring the high symmetry of the Br -anion framework in the ccp arrangement.As displayed in the manuscript, the target sample of Li3YCl5.6Br0.4still remains the hcp-T structure, while further Br-substitution induces structural transitions to the hcp-O and finally the ccp-M phases at compositions of Li3YCl4.8Br1.2 and Li3YCl4.5Br1.5, respectively.The reported Li3YCl3Br3 also possesses ccp-M structure 10, 11 , which consistent well with our results.

Halides
The similar structure and ionic conductivity evolution can be further confirmed in the Li3ErCl6-   We further added these tables and figures in the revised manuscript and supporting information and highlighted.

Referee: #2
Comments to the Author The present work deals with the systematic investigation of novel compositions of halide-based Li-ion conductors.Using a cationic polarization factor, the authors reveal compositions with increased ionic conductivity.The work presents a novel approach for the design and improvement of halide ion conductors and can be accepted for publication after minor revision.A point-by-point list of remarks is given in the following.
Response: Many thanks for your strong recommendation for publishing! 1.At the beginning of page 2, there is an error in the citing literature.[...]different Li+ migration properties.Please correct this.
Response: Many thanks for the suggestions.We have corrected this error in the revised manuscript and marked it yellow.
2. In Figure 1d the crystallographic axis should be included to get a better understanding of the structural changes.
Response: Many thanks for the suggestions.We have added the crystallographic axis in Figure 1d in the revised manuscript and marked it yellow.

Figure R1 .
Figure R1.Four strategies for the regulation of structures and conductivities of Li3MX6 halides.

:
Many thanks for the suggestions.The price and corresponding abundance in Earth's crust of different candidate elements are shown in Figure R6.It's apparent that the cheapest candidates include Al, La, Ce, and Zr.

Figure R6 .
Figure R6.The abundance in Earth's crust and the corresponding price of different candidate elements.
xBrx, Li3YCl3Br3 21 , and Li3YbCl6-xBrx halides as presented in FigureR7.All these samples consistent with the structural design rules and the structure transition from hcp-T (or hcp-O) to ccp-M can be realized by decreasing the ionic potential of X -with the substitution of a larger anion Br -instead of Cl -in the Li3MCl6-xBrx systems.

Figure R8 .
Figure R8.Cationic polarization factor of the Br-contained halides.

Table R1 .
Calculated molar content proportionally ionic potential of different ions and corresponding cationic polarization factors for LiaMXb halides.The reported conductivities of LiaMXb halides are also presented.