Constructing phase boundary in AgNbO3 antiferroelectrics: pathway simultaneously achieving high energy density and efficiency

Dielectric capacitors with high energy storage density (Wrec) and efficiency (η) are in great demand for high/pulsed power electronic systems, but the state-of-the-art lead-free dielectric materials are facing the challenge of increasing one parameter at the cost of the other. Herein, we report that high Wrec of 6.3 J cm-3 with η of 90% can be simultaneously achieved by constructing a room temperature M2–M3 phase boundary in (1-x)AgNbO3-xAgTaO3 solid solution system. The designed material exhibits high energy storage stability over a wide temperature range of 20–150 °C and excellent cycling reliability up to 106 cycles. All these merits achieved in the studied solid solution are attributed to the unique relaxor antiferroelectric features relevant to the local structure heterogeneity and antiferroelectric ordering, being confirmed by scanning transmission electron microscopy and synchrotron X-ray diffraction. This work provides a good paradigm for developing new lead-free dielectrics for high-power energy storage applications.

1. P4/lines 92-93: "the M2-M3 phase boundary is shifted downward to room temperature"; P4/lines 104-105: "the M2-M3 phase boundary was shifted to room temperature": in these sentences, "M2-M3 phase boundary" is mixed up with "M2-M3 phase transition". While the M2-M3 phase boundary is around 0.55-0.6, the M2-M3 phase transition takes place at room temperature (RT); in other word, when the Ta content is around 0.55-0.6 in Ag(NbTa)O3, the M2-M3 phase transition temperature is RT (i.e., the M2 and M3 phases are coexisting at RT). 2. Why do the peaks around 575 cm-1 firstly shift to lower wavenumber and then go upward at around 45 mol%? The author attributed it to a possible M2-M3 phase transition that happened in the ANTx ceramics. The article would be more interesting if the detailed M2-M3 phase transition process was analyzed. As M2-M3 phase transition at RT (or M2 and M3 phases coexisting at RT) is the key for the enhanced relaxor antiferroelectric behavior of Ag(Nb0.45Ta0.55)O3, this is of most interest to see whether it is indeed the order-disorder or alternatively composition inhomogeneity responsible for the relaxor behavior. 3. As known, dielectric thin films and nanocomposites have been widely investigated in the field of energy storage recently. Can the authors comment on the advantages of ANTx solid solution for application in energy storage compared with the film and nanocomposite materials? 4. The two arrow indicators in Fig. 3b are probably mixed up.
Reviewer #4 (Remarks to the Author): Authors presented an excellent work entitled "Constructing phase boundary in AgNbO3 antiferroelectrics: pathway simultaneously achieving high energy density and efficiency" This work provides much awaited breakthrough in high energy density and high efficiency capacitor material for energy storage. This is specially important for futuristic miniaturized devices. The developed material clearly exhibit not only high temperature stability but also high cycle stability. Furthermore, this work provide fundamental understanding of the phenomenon, which could be used as guiding principle for developing such materials for various applications. This paper is suitable for nature communications.
Here is some suggestions for further improving manuscript: 1. It will be great if authors could generate a graph showing their results versus other reports on the same topic.
2. Please add some recent and more relevant references. For example: (a) Relaxor behavior and electrothermal properties of Sn-and Nb-modified (Ba,Ca)TiO3 Pb-free ferroelectric.
(b) A new method for achieving enhanced dielectric response over a wide temperature range Reviewer #1 (Remarks to the Author): N. Luo and co-authors present high performance lead-free dielectric materials based on ANO AFE-ATaO solid solution system for high energy storage capacitor application. The authors introduce new material design concept i.e., highly stabilized AFE with relaxor feature to achieve both high energy storage density and high efficiency simultaneously. The obtained data are remarkable among lead-free based dielectric materials, and the proposed material design strategy, analysis and interpretation of the obtained data, material characterizations (phase, microstructure, etc.) are sound and not misleading. Though there are some reports on the ANO based AFE materials with dopant, the work presented in this study has a unique concept of AFE-RFE phase coexistence for enhanced energy storage and efficiency. Therefore, I recommend the publication of this manuscript. However bellow concerns should be considered before publication.
1. Page 6, after line 136, Authors mentioned that AFE-FE phase transition is shifted to higher E with the addition of Ta and this P-E loop evolution is the reveal of the improved stability of AFE phase. In addition, the authors argued in the Fig. SI  The present manuscript reports that high energy storage density (6.3 J cm-3) and efficiency (90%)can be simultaneously achieved by constructing a room temperature M2-M3 phase boundary in the Ag(NbTa)O3 system. Moreover, the material was found to show high stability of energy storage density and efficiency over a wide temperature range and excellent cycling reliability. The excellent energy storage performance and stability are attributed to relaxor antiferroelectric features relevant to the local structure heterogeneity and antiferroelectric ordering, which are supported by scanning transmission electron microscopy and synchrotron x-ray diffraction. It is a solid work with systematic characterizations, measurements and reasonable discussions. However, I am not well convinced that the paper deserves publication in Nat. Communi. While the basic idea of using relaxor antiferroelectrics (highly stabilized antiferroelectricity with relaxor feature) to develop dielectric capacitors with high energy storage performance has already been demonstrated in several solid-solution systems, e.g., (Bi0.5Na0.5) TiO3-NaNbO3 (Adv. Funct. Mater. 2019, 29, 1903877), and (Na0.5Bi0.5)TiO3-x(Sr0.7Bi0.2)TiO3 (Adv. Mater. 2018, 30, 1802155), the superior energy storage performance of the Ag(NbTa)O3 system has also been explored and reported (e.g., Adv. Mater. 2017, 1701824). The key point/novelty of the present work could be determining of the room temperature M2-M3 phase boundary, i.e. Ag(Nb0.45Ta0.55)O3, but this info was already well The present work can be regarded as an extension of previous work (Adv. Mater. 2017, 1701824), and verifying that Ag(Nb0.45Ta0.55)O3 is in the M2-M3 phase boundary at room temperature, and possesses enhanced relaxor antiferroelectric behavior, and thus has further improved energy storage performance. In addition, I have the following points that need author's attention.
2. Why do the peaks around 575 cm-1 firstly shift to lower wavenumber and then go upward at around 45 mol%? The author attributed it to a possible M2-M3 phase transition that happened in the ANTx ceramics. The article would be more interesting if the detailed M2-M3 phase transition process was analyzed. As M2-M3 phase transition at RT (or M2 and M3 phases coexisting at RT) is the key for the enhanced relaxor antiferroelectric behavior of Ag(Nb0.45Ta0.55)O3, this is of most interest to see whether it is indeed the order-disorder or alternatively composition inhomogeneity responsible for the relaxor behavior. Authors presented an excellent work entitled "Constructing phase boundary in AgNbO3 antiferroelectrics: pathway simultaneously achieving high energy density and efficiency" This work provides much awaited breakthrough in high energy density and high efficiency capacitor material for energy storage. This is specially important for futuristic miniaturized devices. The developed material clearly exhibit not only high temperature stability but also high cycle stability. Furthermore, this work provide fundamental understanding of the phenomenon, which could be used as guiding principle for developing such materials for various applications. This paper is suitable for nature communications.
Here is some suggestions for further improving manuscript: 1. It will be great if authors could generate a graph showing their results versus other reports on the same topic.
2. Please add some recent and more relevant references. For example: (a) Relaxor behavior and electrothermal properties of Sn-and Nb-modified (Ba,Ca)TiO3 Pb-free ferroelectric.
(b) A new method for achieving enhanced dielectric response over a wide temperature range We thank the reviewers for their valuable reviews, insightful comments and constructive suggestions. We appreciate their positive comments, such as "obtained data are remarkable," "solid work with systematic characterizations, measurements and reasonable discussions," "specially important for futuristic miniaturized devices," and "breakthrough." We have revised the manuscript and Supplementary Information accordingly.
Below, we present point-by-point responses to the reviewers' comments. Our responses to the reviewers' comments are highlighted in blue. In the revised manuscript, we highlighted our modifications in red.

Point-by-point responses to the reviewers' comments
Reviewer #1 (Remarks to the Author): N. Luo and co-authors present high performance lead-free dielectric materials based on ANO-ATaO solid solution system for high energy storage capacitor application.
The authors introduce new material design concept i.e., highly stabilized AFE with relaxor feature to achieve both high energy storage density and high efficiency simultaneously. The obtained data are remarkable among lead-free based dielectric materials, and the proposed material design strategy, analysis and interpretation of the obtained data, material characterizations (phase, microstructure, etc.) are sound and not misleading. Though there are some reports on the ANO based AFE materials with dopant, the work presented in this study has a unique concept of AFE-RFE phase coexistence for enhanced energy storage and efficiency. Therefore, I recommend the publication of this manuscript. However bellow concerns should be considered before publication. AFE is enhanced with increase of Ta content. But it seems like AFE is not stabilized, but more smeared as RFE characteristic. It should be more clearly addressed in the revised manuscript. I am not sure whether we can claim this as 'highly stabilized'.

Response:
We really appreciate the reviewer's positive comment. In the manuscript, we used the phrase "improved stability of AFE phase" based on the reasons listed below.
Firstly, the M1-M2 phase transition temperature shifts to below room temperature, while the M2-M3 phase transition temperature also shifts downward to room temperature with increase of Ta content, which indicates that the AFE (M2 phase) region is expanded to room temperature. Secondly, the AFE-FE phase transition electric field in the P-E loops is increased obviously with increase of Ta content, which is generally observed in an AFE material with increased stability of AFE phase (need higher energy transform AFE to FE phase). Thirdly, electric field dependence of dielectric permittivity variation becomes more flattened with the increase of Ta concentration (see Supplementary Fig. 1), which has been demonstrated in other AFE materials. From the microstructure perspective, the decreased ionic displacements with the increase of Ta content, obtained from the structure refinement, also give the evidence of enhanced stability of AFE phase. All the phenomena above reveals the increased stability of AFE phase by increasing Ta content. It should be admitted that the relaxor characteristic cause the slanted and slim P-E loops, which also leads to the enhanced AFE-FE phase transition electric field in a certain degree, due to the disruption of the long-range ordered AFE domains and the weak interatomic interactions between the AFE nanodomain clusters. However, in our case, obvious improvement in AFE-FE phase transition electric field also occurs for a low Ta content, where no relaxor characteristic is observed. So, we prefer not ascribing the enhancement of AFE-FE phase transition electric field to the relaxor characteristic, notwithstanding relaxor characteristic also play a role in the enhanced AFE stability.
Based on the above consideration and knowledge, we used the phrase "highly stabilized". To make it more clearly to the readers, the reasons are also addressed in the revised manuscript. This superstructure features a tilting of [NbO 6 ] octahedra described in Glazer's notation as a − b − c − /a − b − c + . A sequence of two in-phase and two antiphase octahedral rotating around the c-axis produces 4a c periodicity, in which the two Ag sites (Ag1 in 4d site, Ag2 in 4c site) are different in crystal structure, as list in Table 1. Furthermore, the Ag1 and Ag2 ions shift along different directions, with Ag1 and Ag2 shifting along the b axis and a axis, respectively (Fig. 1).  Reviewer #2 (Remarks to the Author):

Comment: Authors have thoroughly reported high energy storage and efficiency in
AgNbO 3 -AgTaO 3 solid solution. I could find results interesting. However, I feel Nature Communication can not be appropriate platform for this manuscript as neither this topic/material is novel nor approach is unique as can be seen from reference list where AgNbO 3 -AgTaO 3 is well explored.

Response:
We thank the reviewer for considering our finding interesting. However, we respectfully disagree with the referee's statement that "neither this topic/material is novel nor approach is unique as can be seen from reference list where On the other hand, the concept of relaxor antiferroelectric has been demonstrated in some lead free systems, however either of them was achieved by shifting the antiferroelectric-paraelectric phase transition temperature (T m ) to room temperature, or forming less-stabilized antiferroelectricity, which are still unable to simultaneously achieve both high energy density and efficiency.

M2-M3 phase boundary in AgNbO 3 -AgTaO 3 solid solution to room temperature, by taking the advantages of highly stabilized antiferroelectric feature and relaxor behavior. As a result, ultrahigh energy storage density and efficiency are achieved in this work. The significance of this work not only lies in the high energy storage density and efficiency, but also unfolding the relaxor characteristic of M2-M3 phase boundary on atomic scale in AgNbO 3 based solution, opening a new direction
tuning the composition for specific applications. The present work can be regarded as an extension of previous work (Adv. Mater. 2017, 1701824), and verifying that Ag(Nb0.45Ta0.55)O3 is in the M2-M3 phase boundary at room temperature, and possesses enhanced relaxor antiferroelectric behavior, and thus has further improved energy storage performance. In addition, I have the following points that need author's attention.

Response:
We thank the reviewer for the positive comment of "It is a solid work with systematic characterizations, measurements and reasonable discussions", also appreciate for providing many valuable references.
Here we would like to clarify the significance of our research. We simultaneously achieved high energy storage density (6.3 J cm -3 ) and efficiency (90%) by constructing a room temperature M2-M3 phase boundary in the Ag(NbTa)O 3 system.
The M2-M3 phase boundary exhibits highly stable antiferroelectricity associated with good relaxor behavior which have never been studied. In addition to the good properties achieved, we provide a good paradigm for designing high-performance material on atomic scale to tailor the properties for specific applications.
We admitted that the concept of using relaxor antiferroelctric material to improve

Response:
Thanks for the constructive suggestion. We are sorry about the mistake and confusion. We actually mixed up "phase boundary" with "phase transition" in the previous manuscript. The sentence "the M2-M3 phase boundary was shifted to room temperature" is changed to "the M2-M3 phase transition temperature was shifted to RT" in the revised manuscript. interest to see whether it is indeed the order-disorder or alternatively composition inhomogeneity responsible for the relaxor behavior.

Response:
Thanks for the valuable and constructive suggestion. The structure evolution of M2-M3 phase transition is actually very important in this work, as it is closely related to the antiferroelctricity and relaxor characteristic that contribute to the achieved promising properties. To further understand this part, we added more careful and detailed analysis on the SXRD and Raman spectra. octahedral tilting angles θ and Φ (the θ and Φ are tilting angles along b and c axes, respectively) also decrease with increase of Ta content (Fig. 3d) with wavenumber at 500-650 cm -1 were fitted by using Gaussian function. Of particular interest is that the three fitted peaks in AN are merged into two peaks with Ta content over 40 mol% (Fig. 3f), revealing a possible M2-M3 phase transition above this composition, in agreement with the dielectric and SXRD analysis regardless of the small deviation in composition.
The discussions can also be found in the revised manuscript.

Response:
Thanks for the valuable suggestion. As demonstrated in this work, the ANTx solid solution exhibits excellent energy storage performance, which make it promising for energy storage application. Generally, bulk ceramics show higher total energy compared with film and nanocomposite, due to the large thickness. Moreover, the preparing process for bulk ceramics is generally simpler without special equipment.
The total energy can be further improved if multilayer ceramics are fabricated. It should be admitted that film and nanocomposite generally show higher energy density than that of bulk ceramics due to the ultrahigh breakdown strength resulting from the significantly decreased sample thickness and the addition of polymer. The good flexibility is another advantage of nanocomposite, which make it promising in flexible electronic device. Recently, film prepared on flexible matrix (mica, for example) was also developed, which opens another way for flexible electronic device application. If ANTx film can be prepared, very good energy density and efficiency can be expected due to its relaxor antiferroelectric feature. Furthermore, the ANTx-PVDF "0-3" nanocomposite may be another promising flexible material system for achieving ultrahigh energy density and efficiency. In this materials system, how to activate the antiferroelectric double-like P-E loops is a challenge and should be addressed.
Based on this suggestion, we added the following outlook paragraph discussing the potentials in the revised paper, also cited recent research on thin film and nanocomposite dielectrics for energy storage and other possible applications.
Outlook: We proposed a strategy of constructing M2-M3 phase boundary with RAFE feature in ANTx solid solution, to simultaneously achieve high energy storage density and efficiency. The energy storage density would be further improved in ANTx multilayer ceramics and film capacitors, due to the significantly increased breakdown strength. In addition, the unfolding of RAFE characteristic of M2-M3 phase boundary on atomic scale in AN-based solid solution gives a solid proof to the long-term confusion on the broad dielectric anomaly over M2-M3 phase transition temperature, which opens a wide range of applications where relaxor feature is desired, such as electrocaloric solid-state cooling devices and hysteresis-free actuators.

Comment 5:
The two arrow indicators in Fig. 3b are probably mixed up.

Response:
Thanks for the careful reading and pointing out this mistake. Fig. 3b is updated in the revised manuscript.

Reviewer #4 (Remarks to the Author):
Authors presented an excellent work entitled "Constructing phase boundary in AgNbO3 antiferroelectrics: pathway simultaneously achieving high energy density and efficiency" This work provides much awaited breakthrough in high energy density and high efficiency capacitor material for energy storage. This is especially important for futuristic miniaturized devices. The developed material clearly exhibit not only high temperature stability but also high cycle stability. Furthermore, this work provide fundamental understanding of the phenomenon, which could be used as guiding principle for developing such materials for various applications. This paper is suitable for nature communications.
We really appreciate the positive comments and acknowledge the breakthroughs we made in our research.
Here is some suggestions for further improving manuscript: Comment 1: It will be great if authors could generate a graph showing their results versus other reports on the same topic.

Response:
Thanks for the valuable suggestion. A graph comparing ANTx with other reported dielectrics in the field of dielectric energy storage was added in the revised paper, as shown in Fig. 4d in the revised manuscript. The results reported in this work show great advantage in energy density and efficiency.

Comment 2:
Please add some recent and more relevant references. For example: (a) Relaxor behavior and electrothermal properties of Sn-and Nb-modified (Ba,Ca)TiO3 Pb-free ferroelectric.
(b) A new method for achieving enhanced dielectric response over a wide temperature range.

Response:
Thanks for the good suggestion. Some references related to relaxor behavior and dielectric properties were added in the revised manuscript, to make the research background and discussion of this work more solid and sound.
The cited references are listed as follows.