The authors present a timely study on some previously unreported ionic materials that exhibit plastic crystal behaviour (as organic ionic plastic crystals, OIPCs) within a certain temperature range, that would likely be of interest for possible application as solid-state electrolytes for e.g. rechargeable alkali-ion batteries. Following syntheses with comprehensive chemical analyses (NMR, MS, elemental analysis), they perform a detailed investigation on the thermal properties (TG, DSC) of these materials. The study then moves on to an investigation of the ionic conductivity (impedance method) and ion dynamics (wideline NMR).
It was an informative and generally enjoyable read, and while the concept is not particularly original, the reported materials are apparently new and have significance. Indeed, the topic is important and of recent interest.
The manuscript is in general very well written, and well structured. The title is clear, concise and suitable (but please see notes below regarding “borate”). The length of the manuscript is also reasonable and suitable. The figures and tables in the main text are in general clear, and all necessary for the understanding of the work.
The work gives the impression of being scientifically sound, and also lies very firmly within the scope of the journal, being an experimental study that develops new understanding, properties and syntheses of materials. This research article will have a modest impact on the field, and furthermore has the potential to have a substantial impact on the field, pending the outcome of further research on the application of the reported materials.
The abstract is suitable, and describes well the contents of the paper. The introduction describes the current understanding and knowledge gaps with respect to OIPC materials, and in particular highlights the important efforts to eliminate fluorine from such materials.
The “materials and methods” and “characterisation and analysis” sections are generally sound, although there are some omissions (see comments below). The results and discussion section is logically arranged, and for the most part thorough. The Conclusions section sums up the contents well, and provides some useful future advice/directions.
The references are in general necessary and for the most part up to date.
However, whilst the manuscript appears to be in general sound, and I would tend to give a recommendation that it be published, there are some important points/concerns that I feel need addressing. These are summarized below. I would be pleased to receive and consider the authors’ response to these points.
Page number refer to page numbers of the manuscript itself (i.e. title, abstract and start of introduction is page 1).

1. This article presents the synthesis and characterisation of two materials. Whilst these two ionic compounds have the same cation, the anions are quite different (both are boron-centred, but structurally very different). Would the authors be able to provide some description as to why these two materials in particular were chosen for this study?
   Throughout the manuscript, the reader is presented with a comparison of data for these materials. Moreover, comparison of the [P4444][B(bdo)4] with the previously reported [P4444][B(edo)4] is also made. Since the structure of [B(pyr)4]− is quite different, the direct comparison of P4444][B(bdo)4] with [P4444][B(pyr)4] is questionable.

2. There are some instances in the manuscript where observation of ionic conductivity is equated with confirmation of the presence of a plastic crystal phase. Namely, (i) in the abstract “which was confirmed by the observation of ionic conductivity in the solid state”, (ii) page 4, (right column, second paragraph) “the retention of ionic conductivity even in solid phases”, and (iii) page 4 (right column, last paragraph) “it was apparent that both salts were conductive in the solid state at temperatures below the transitions shown in the DSC analysis…high degree of disorder or defects below their respective endothermic transitions…both salts are indeed OIPCs with high levels of solid-state conductivity”. I think it’s just a matter of phrasing in these cases – whilst significant ionic conductivity may be indeed be indicative of a plastic crystal phase, more generally, an ionic solid-state material may exhibit ion conduction through various phenomena, and a polycrystalline sample of the true crystal phase of such a material may also exhibit ionic conductivity by various mechanisms. So the fact that an ionic material exhibits modest ionic conductivity in the solid phase is not a definitive indication of a plastic crystal phase.
   On that point, the suggestion that there may be another, lower temperature solid-solid transition not observed in the DSC is interesting (page 4, right column, last paragraph, “It is possible that a low temperature plastic->ordered crystal transition occurs below the lower limit of scanning on the DSC”). It is reasonable to then conclude that these are OIPCs over a broad range of temperatures (including “at, and below, ambient temperature”, same paragraph). It should be noted that if 25 °C is taken as being ambient temperature, then only one conductivity data point for each of the two materials is reported at or below ambient temperature. Nevertheless, it seems a valid suggestion, and an alternate/additional way of putting it would be that the absence of a large (entropy >> 20 J K−1 mol−1) solid-solid transition in the lower measured temperatures (below the II-I transition for the [P4444][B(bdo)4] complex, and below the III-II transition for [P4444][B(pyr)4]) can lead to the tentative conclusion that these materials are also plastic crystals at lower temperatures.

3. Nomenclature
   In this manuscript, there are a few instances of “orthoborate” (or variations): on page 2 (left column, first paragraph and fourth paragraph). Whilst “orthoborate” has been used to describe these types of boron-oxygen bond containing anions in the electrolyte field since at least Angell’s work in 2000 (Xu and Angell, Electrochem. Solid-State Lett, 2000, 3, 366 https://doi.org/10.1149/1.1391150), I’m not sure this is the right name – an orthoborate is perhaps a derivative of boric acid, BO3, i.e. B bonded to three O, not four.
   In this manuscript one of the prepared anions is “tetrakis(1-pyrazolyl)borate”. Whilst this anion has been described with the same name in published literature (e.g. Roy et al., Inorganica Chimica Acta, 1988, 150, 1, 47-49. https://doi.org/10.1016/S0020-1693(00)87623-9) , some chemical suppliers also use this same name to describe an anion with the same pyrazole-derived groups attached not directly to boron, but with an oxygen between each B and N (see for example “Tetrakis(1-pyrazolyl)borate, potassium salt”, CAS 14782-58-2). It may be worthwhile re-considering the name, or at least inserting a cautionary note to avoid confusion.
   More broadly, “borate” may (for some) imply the presence of B-O bonds. Thus, the use of the term “borate” in e.g. the title of this manuscript may cause some confusion, as one of the anions is not a borate in that sense.
   Note that it is not my intention to get into a debate over nomenclature. In any case, it’s probably fair to say that the nomenclature for boron-containing anions in the published literature is far from systematic. If, having considered the points in this comment (comment 3), the authors feel that as-is (i.e. no change) is their preferred response, that would also be fine.

4. [B(pyr)4]− has applications as a chelate for transition metal ions (see: Matsui et al., Analytical Sciences, 1991, 7, 7–10. https://doi.org/10.2116/analsci.7.Supple_7). Would this perhaps be a limitation regarding its potential application for a Li ion battery electrolyte e.g. a Mn, Ni, Co or Fe containing cathode? (as a solid-state electrolyte, but also as potentially as an IL electrolyte, as suggested in the last line of the Conclusion)

5. Introduction
   Page 1, right column, second paragraph: “Incorporation of boron…advantageous Solid Electrolyte Interphase chemistry" – I think the capitalisation is unnecessary here. Could the authors elaborate on what they mean by “advantageous SEI chemistry”, and is it due to the presence of boron, or e.g. B-O containing species specifically?

6. Materials and Methods section:
   First paragraph: Please include purities for all chemicals. Also, the following were apparently used, but not mentioned here: toluene, chloroform, acetonitrile, acetone, CDCl3, d6-DMSO, hexagonal boron nitride powder and NH4H2PO4. Please include the sources and purities of these.
   In the sub-title “Sodium bis(butane-1,4-dioxy)borate, (Na[B(C4H8O2)2]).”, the “C4H8O2” should probably be "bdo" for consistency.
   In the syntheses, 1:1 molar mixing was used for the ion exchange processes. An alternative would be to use a slight molar excess of one reagent e.g. [P4444]Cl and then washing away the excess (often performed by using a solvent in which the substrates are soluble, but the product insoluble). Whilst not particularly satisfying from the point of view of sustainable practises, this method may be effective to ensure complete reaction. Do the authors have any comment about their choice of method? It seems to be very dependent on the careful 1:1 stoichiometric mixing, which is in turn dependent on the substrate purities. Do the authors consider the residual ~200 ppm of chloride ion to be a reasonable level considering the possible electrochemical application of these materials?
   The NMR data should be reported in a format consistent with the journal requirements. (see: https://www.rsc.org/journals-books-databases/journal-authors-reviewers/prepare-your-article/experimental-data/) Namely, the solvent and reference should be written next to each isotope e.g. “δH(100 MHz; CDCl3; Me4Si) 2.3 (3 H, s, Me)…” Also regarding the journal requirements, are the reported MS data accurate to “within 5 ppm (EI and CI) or 10 ppm (FAB or LSIMS)”?

7. Characterisation and Analysis section:
   Did the authors measure the water content for these materials? It may have some influence on the DSC and ionic conductivity, and indeed diffusion, and thus measuring the water content is crucial.
   Please provide instrumental/experimental details for the mass spectrometer and elemental analysis instrument.
   Impedance: is 0.01V the peak, peak-to-peak or r.m.s. amplitude? Was temperature hysteresis investigated for the IC measurement? Equation 2 is not an equation. Should be perhaps "σ= ..."? The use of the “touchdown” data, without fitting the impedance spectra to a suitable equivalent circuit is a very approximate method. I would suggest that the authors amend “The conductivity of the solid electrolytes was determined by relating the touch-down (TD) point…” by changing “determined” to “estimated”. Would it be possible for the authors to provide the Nyquist plots in the supporting information?

8. NMR Spectroscopy experimental details (and liquid-state spectra):
   In the Materials and Methods section, can the authors include tentative assignments for the peaks in the 13C spectra in the data? For the [P4444][B(bdo)4] compound, there are 7 peaks listed, but apparently only 6 non-equivalent carbon environments. Perhaps one of the sets of two closely spaced peaks (either 22.41 and 22.21, or 17.39 and 16.92 ppm) are in fact a doublet due to 13C-31P coupling? For the [P4444][B(pyr)4] material, there is an unlisted peak at approximately 133 ppm (Fig.S13) This is presumably one of the heteroaromatic ring carbons. Then, there is one too many non-equivalent carbon environments, but again there is perhaps 13C-31P coupling leading to a doublet in the lower ppm range?
   In the Characterisation and Analysis section, how where the 1H and 13C NMR spectra referenced? It is a little unclear whether the section of text starting from “All experiments were run…” and ending in “…0.9 ppm).” applies to both solution-state and solid-state NMR, or just solid-state NMR. Are the referencing compounds reported for 11B (BN powder) and 31P (NH4H2PO4) used both for solution-state NMR and solid-state NMR? If not, please state the references used for solution state NMR. Does the statement “without any 1H decoupling” apply to all reported spectra (liquid-state too)? For example, the 13C spectra in the supporting information (and the data reported in the synthetic section) have the appearance of 1H decoupled 13C spectra. If so, please be sure to amend the relevant part of the figure captions for Figures S2, S5, S8, S13 i.e. “13C{1H} NMR spectrum” rather than “13C NMR spectrum”. The same goes for liquid-state 31P spectra. Please also amend in the list of peaks in the methods section (13C{1H} not 13C. Appears to be already correct for 31P).
   In the proton NMR spectrum for Na[B(pyr)4] (Fig S4), it appears that the protons adjacent to the N of the pyrazoyl ligand are equivalent - does it imply there is rapid exchange between the 2 nitrogens? If so, does this contribute somehow to the conductivity/disorder?
   In Fig S6, S9, S14: can the authors comment on the possible origin for the peak at around −2 ppm?
   In Fig S10, S15: can the authors comment on the possible origin for the peak at around 37 ppm?

9. Page 2, right column, first paragraph:
   There is a suggestion that [B(bdo)4]− has only been reported in the patent literature. I would suggest the authors undertake a more rigorous literature search. See for example: Gainsford and Kemmitt, Acta Cryst. 2005, C61, m136-m138 https://doi.org/10.1107/S0108270105002520.

10. TG data: The anion for [P4444][B(pyr)4] contains a lot of nitrogen. The presented TG data is of course measured in an inert atmosphere, but do the authors have any comment on the possible thermal stability in air (i.e. in the presence of oxygen)?
    In Fig 2, data up to 600 °C are shown, however according to the experimental description data were measured up to 1000 °C. Can the authors please include a brief justification for the omission of the data between 600–1000 °C?

11. DSC data:
    Page 3, right column, paragraph 4: "repeatable, reversible phase transitions in two consecutive cycles as shown in". The figure (Fig 3) apparently only shows one cycle for each material, not the consecutive cycles. Perhaps the authors can slightly amend the wording here?
    In Fig 3, the data particularly for [P4444][B(bdo)2] is rather noisy. I am not suggesting that the data be re-measured, but if the authors might have some comment (e.g. instrumental issue, or something specific to the sample) regarding that it may be helpful for the readers.
    In Fig 3, it would be helpful to have arrows showing the direction of heating/cooling, if possible.
    There appears to be four peaks, not three for the cooling curve of [P4444][B(pyr)4] (a small bump at around 48 °C). Do the authors have some comment on this?
    Sorry if I missed it, but was some explanation provided for the different measurement ranges for these two materials?
    Table 1: how was the value of onset temperature determined?

12. Conductivity data
    The Table 2 caption mentions Activation energy and R2, but these data do not appear in the table (may be an unintended omission, or perhaps somehow lost in the submission and pdf conversion?)
    Fig 4: If possible it would be help to have the temperature in °C also displayed on the graph using e.g. a second y-axis scale at the top, or labels. Whilst the data reported in Table 2 and in this plot are necessarily in K, the discussion in the text (and data in Table 1) are in °C, so it would make it easier for the reader if the °C temperatures were there too.
    The change in slope of the fitted Arrhenius plot for [P4444][B(pyr)4] is mentioned in the text (Page 5, left column, third paragraph), but could the authors provide some possible rationale?

13. Wideline NMR data
    A limitation in the work is that the wideline NMR data have been recorded down to 243 K (−30 °C), but the conductivity data only to a minimum of 293 K (20 °C). Nevertheless, the DSC data has been reported to lower than 243 K, so even without the conductivity data, it is sufficient to draw some conclusions.

14. Whilst the manuscript is in general well written, there are numerous typographical (and similar) errors.
    Below are some of those that I noticed. I’d like to ask the authors to thoroughly go through the manuscript.

Page 1, right column, second paragraph
“Most high conductivity materials OIPCs contain”. I think “materials” is redundant here?

Page 1, right column, second paragraph
“Small ion, or proton,.” perhaps “including” rather than “or” would be better here?

Page 1, right column, second paragraph,
“fluid ILs” seems somewhat odd, since the “L” of “IL” is liquid i.e. a fluid

Page 2, right column
In the NMR peak assignments (1H NMR) for [P4444][B(bdo)2] and [P4444][B(pyr)4] there are some inconsistencies in the way to write the bonds (including several instances of a superscript) e.g. “O-CH2”, “P−CH–”

Page 3, right column, last paragraph:
“Figs. 3” should be “Fig. 3”

Page 5, left column, last paragraph:
“deconvolution” should probably be “deconvoluted”

Thank you.