Morphology-controlled synthesis of novel nanostructured Li₄P₂O₇ with enhanced Li-ion conductivity for all-solid-state battery applications

Abstract

Mechanical stiffness of oxide-type solid-electrolytes is a major drawback which has hindered their practical application in all-solid-state Li-ion batteries to date. Despite their enhanced structural and electrochemical stabilities, lack of deformability of fast-ion conducting oxides impedes the integration of these materials in bulk-type solid-state cells. Deformable solid-electrolytes such as sulfides, on the other hand, lack sufficient electrochemical stability in contact with conventional cathodes. This has recently triggered a search for new materials that combine high ion-conductivity, deformability and sufficient electrochemical stability. Here, we report the synthesis of a novel form of Li₄P₂O₇ that can be densified by cold-pressing and possesses an ion conductivity that is two orders of magnitude higher than conventional Li₄P₂O₇ phases. The material is synthesized by a combination of microwave synthesis and chemical lithiation and adopts a nanostructured morphology with a small amorphous component. The material is electrochemically stable at voltages >5 V vs. Li⁺/Li, which suggests safe use with high-voltage cathodes. The newly-synthesized material is therefore a bulk, deformable analogue of LiPON, with comparable ion conductivity and phase stability. This research highlights the potential of using novel low-temperature synthetic routes to control the morphology and enhance the electrochemical performance of conventional functional materials.

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Introduction

Sulfide-type solid electrolytes such as Li₇P₃S₁₁ and Li₁₀GeP₂S₁₂ are deformable superionic conductors^1,2 which can readily form dense fast-ion conducting electrolyte-layers by cold/warm pressing. These materials, however, have limited electrochemical stability when in contact with conventional cathodes, which is a major drawback that limits their application in bulk-type all-solid-state batteries. Oxide-type solid electrolytes, on the other hand, are more compatible with high-voltage cathodes due to their better electrochemical stability window,^3,4 but, unfortunately, lack deformability that allows the build-up of bulk-type solid-state cells. The search for new solid-electrolytes that possess good ion-conductivity, deformability, and an appropriately wide electrochemical stability window, is underway and has recently been extended to explore new classes of materials such as halide-type solid electrolytes.⁵ For oxide-based solid-electrolytes (e.g., garnets and LISICONs), sintering at elevated temperatures is crucial to deliver crystalline phases with sufficient ion-conductivity. This, however, leads to stiff, brittle materials which are difficult to integrate in bulk-type all-solid-state batteries. Recently, we have employed low-temperature synthetic techniques such as microwave synthesis to develop materials with controlled morphology and electrochemical properties.^6–8 Here, we describe our approach to achieve a deformable, nanostructured Li₄P₂O₇ phase with promising electrochemical properties.

Li₄P₂O₇ is conventionally prepared by solid-state reactions at 700–800 °C (ref. 9 and 10) or by hydrothermal methods (often as multiple-phase mixtures).^11,12 Li₄P₂O₇ prepared by solid-state reactions adopts a triclinic symmetry^9,10,13,14 and can be described in terms of P₂O₇ units, built up of two PO₄ tetrahedra sharing vertex, and LiO₄ units. LiO₄ tetrahedra share common edges or vertices, forming a continuous framework containing large voids that are typically available for Li-ion transport. The ionic conductivity of the triclinic Li₄P₂O₇ polymorph is reported only at elevated temperatures (e.g. 5.6 × 10⁻³ S cm⁻¹ at 450 °C (ref. 10)), suggesting minimal conductivity at ambient conditions. This triclinic polymorph transforms to a high-temperature monoclinic phase at ∼640 °C.^10,15 The formation of the high-temperature phase is accompanied by an increase in the conductivity by approximately a factor of three.¹⁰ It has been suggested that the structural modifications within the monoclinic symmetry lead to a step-wise increase in the rate-limiting apertures that permit passage of Li⁺ through the material.¹⁰ Other thermodynamic metastable structural modifications of Li₄P₂O₇ are also known in the literature, including a high-pressure monoclinic phase,¹¹ pseudo-monoclinic (triclinic) phases^12,16 and a trigonal phase.¹⁶ However, these phases are often obtained as multiphase mixtures and have not been isolated in bulk form. In this report, we have employed a combination of microwave synthesis and chemical lithiation to obtain a novel form of Li₄P₂O₇ at temperature as low as 400 °C. In addition to an enhanced electrochemical stability window, this phase can be densified by cold-pressing, possessing ion conductivity that is two orders of magnitude higher than that of conventional Li₄P₂O₇ phases.

Experimental section

Synthesis

LiPO₃ was initially synthesized by a microwave approach.⁶ Phosphorus pentasulfide (P₂S₅) and lithium tert-butoxide were used as the starting materials. Stoichiometric amounts of the materials were dissolved in tetraglyme at ∼130 °C till a clear solution was obtained. 5 ml of this solution (∼1 mM) was then heated in a sealed microwave tube for 20 min at 200 °C using a CEM Discover SP microwave synthesizer (2.45 GHz). The resulting product was washed thoroughly with THF (3 × 3 ml) and finally dried in air at 150 °C for 2 h. For chemical lithiation, LiPO₃ was mixed with n-butyllithium (2.5 M solution in hexane; Sigma Aldrich) in 1 : 2 molar ratio, and stirred overnight under an argon atmosphere. The resulting product was washed thoroughly with hexane (3 × 3 ml) and finally dried in air at 100 °C for 2 h, followed by calcination in air at 400 °C for 4 h.

Characterization

X-ray diffraction (XRD) studies were performed using a PANalytical X'Pert PRO diffractometer in reflection mode using Cu-Kα radiation. In situ X-ray diffraction studies were performed using an Anton Paar HTK 1200N heating stage where the samples were heated in the temperature range 25–800 °C under an argon atmosphere. Prior to each XRD measurement, the samples were equilibrated for 1 h at constant temperature followed by 1 h scan at each temperature. A heating rate of 5 °C min⁻¹ was employed, and samples were allowed to cool down naturally. Rietveld analyses of diffraction data (12 h scans at room temperature) were performed using the GSAS and EXPGUI programmes, employing a pseudo-Voigt profile function. ICP-MS analysis was performed using an Agilent 7700 ICP-MS instrument. SEM studies were performed using a Zeiss Sigma field emission scanning electron microscope. TEM images were obtained using an FEI Tecnai T20 machine operating at 200 kV. AC impedance and DC polarization measurements were conducted in Swagelok cells assembled under argon atmosphere, and the data were recorded using a Biologic VMP-300 potentiostat. AC impedance data were recorded in the frequency range of 1 MHz to 1 Hz (an electrical perturbation of 50 mV) using gold or lithium electrodes. Gold electrodes were gas-phase deposited on the circular sides of the pellets by thermal evaporation. Prior to each impedance measurement, the samples were equilibrated for 1 h at constant temperature. DC polarization measurements (gold electrodes) were performed by applying voltages in the range of 0.5–1.7 V (8 hours each) and recording the corresponding DC current. To prepare lithium symmetric cells, pellets were sandwiched between two Li foils, and assembled within a Swagelok cell in an Ar-filled glovebox. The surfaces of the used lithium foils were scratched using a stainless-steel blade to obtain clean surfaces before use. The constant current galvanostatic cycling of the symmetric cell was performed using a BioLogic VMP-300 potentiostat by applying a constant current for 2 h, followed by a 10 min OCV before switching to the opposing current direction. For reactivity tests using asymmetric solid-state cells, Li₄P₂O₇-C and Li₃PS₄-C composites were initially prepared by mixing either Li₄P₂O₇ or Li₃PS₄ with carbon black (10 wt%; KETJENBACK EC-600JD, AkzoNobel) using a pestle and mortar. Three-layer pellets of In/Li₃PS₄/Li₄P₂O₇-C and In/Li₃PS₄/Li₃PS₄-C were prepared by pressing In foil (Sigma Aldrich), 60 mg of Li₃PS₄, and 3 mg Li₄P₂O₇-C (or Li₃PS₄-C) at 2 tons for 30 s. Indium foil was thinned to ∼0.3 mm before use. Thin Cu-foil and Al-foil discs were used to support and collect the current from the anode and the cathode, respectively. Cyclic voltammetry (CV) tests on the assembled cells were performed in Swagelok cells using a BioLogic VMP-300 potentiostat at a scan rate of 0.1 mV s⁻¹ in the voltage range 0–5 V vs. In/InLi. For reactivity tests using liquid-electrolyte-based cells, electrodes were prepared by mixing Li₄P₂O₇ (or ZrO₂) with carbon black and PTFE in the ratio 70 : 20 : 10 wt% using a pestle and mortar, followed by pressing the mixture to form thin pellets. Charge/discharge tests were performed in Swagelok cell using 1 M LiPF₆ in EC/DMC (1 : 1) as electrolyte and lithium foil as counter and reference electrodes. Cells containing 3 mg of Li₄P₂O₇(ZrO₂)/C/PTFE composites were cycled in the voltage range 1.0–4.5 V at 10 mA g⁻¹ current density.

Results and discussion

Microwave-synthesized LiPO₃ was used as a precursor to synthesize Li₄P₂O₇ through chemical lithiation followed by calcination in air at 400 °C. The material resulted from this process is referred to as Li₄P₂O₇-400. Parent LiPO₃ was fully characterized in our previous report.⁶Fig. 1 shows how the morphology of as-synthesized LiPO₃ is changed by chemical lithiation and the calcination processes to form Li₄P₂O₇-400. The porosity is largely retained throughout the stages of the formation of Li₄P₂O₇-400, while a significant decrease in the angularity of the particles is observed in the final product. This suggests that the heating step facilitates mass transport that permits formation of particles of similar size but with smoother, less anisotropic shapes than the parent LiPO₃. Sintering at temperatures higher than 400 °C was avoided to maintain the nanostructured morphology (and deformability) of the material. Li₄P₂O₇-400 showed a large specific surface area of 31.1 m² g⁻¹, as determined from nitrogen adsorption using the Brunauer–Emmett–Teller (BET) method. Elemental analysis by ICP-MS revealed a Li : P molar ratio of 2.06(1) : 1, indicating the conversion of LiPO₃ to Li₄P₂O₇.

Fig. 1 Morphology evolution of LiPO₃ upon chemical lithiation and subsequent calcination. (a) As-synthesized LiPO₃; (b) the material after chemical lithiation; (c) the material after lithiation and calcination at 400 °C.

XRD data collected from Li₄P₂O₇-400 are shown in Fig. 2. Data collected from the as-made material show significant broadened peaks at similar d-spacings to those anticipated for the low-temperature triclinic form of Li₄P₂O₇ (Fig. 2b). In order to evaluate the structure of Li₄P₂O₇-400, additional diffraction patterns were collected in situ as this material was heated to 800 °C and subsequently cooled to room temperature (Fig. 2a). These data show a largely invariant diffraction pattern as the sample is heated from room temperature to 500 °C, but, between 500 and 600 °C the Bragg peaks sharpen and this permits resolution of multiple peaks that were extensively overlapped in the as-made material. Heating further to 700 °C shows a simplification of the pattern with fewer peaks being observed. Heating to 800 °C causes further sharpening of the peaks and enables comparison of the structure with the monoclinic phase previously observed for Li₄P₂O₇ above ca. 640 °C.¹⁰ On cooling this sample from 800 °C to room temperature, a diffraction pattern characteristic of conventional low-temperature Li₄P₂O₇ phase, with no traces of impurities, is obtained (see Fig. S1† for XRD refinement). A second cycle of heating/cooling (Fig. S2†) shows the anticipated conversion of the low-temperature Li₄P₂O₇ phase to the high-temperature modification,^10,15 with no appearance of the Li₄P₂O₇-400 phase. The appearance of the established triclinic to monoclinic phase transition in subsequent heating cycles indicates that Li₄P₂O₇-400 is a form of Li₄P₂O₇ that likely lacks full crystallinity. The X-ray diffraction data collected from Li₄P₂O₇-400 were readily indexed in a monoclinic crystal system, with unit cell parameters related to the high-temperature phase (with an appropriate consideration of the effects of lattice expansion due to temperature elevation). Rietveld structural refinement against the diffraction data were performed employing the structure model of the high-temperature phase,¹⁰ resulting in refined unit cell parameters of a = 8.6181(6), b = 5.1863(2), c = 13.1373(1) Å, and β = 103.624(5)°. The refined XRD data are depicted in Fig. 4, and the refinement parameters are summarized in Table S1.† The fact that the material displays considerable peak broadening leading to a larger degree of peak overlap, together with the limited accuracy of X-rays to locate light elements such as lithium and oxygen, however, limits the precision of the crystal structure analysis when compared to the refinement against the high temperature dataset shown in Fig. S1.†

Fig. 2 (a) In situ XRD analysis of Li₄P₂O₇-400 through heating from room temperature to 800 °C, then cooling back to room temperature. (b) A highlight of the XRD patterns of Li₄P₂O₇-400, high-temperature (HT) Li₄P₂O₇ phase and the conventional low-temperature (LT) Li₄P₂O₇ phase.

TEM analysis of Li₄P₂O₇-400 indicates a good crystallinity of the material with a small contribution from an amorphous phase. Fig. 3 shows TEM images collected from selected parts of the sample. It is worth mentioning here that the samples were highly sensitive to the beam which prevented higher-magnification imaging. The sample contained rod-shaped particles of ∼200–400 nm length and smaller interconnected particles with undefined shape. The rod-shaped particles had sharp edges with no visible amorphous coating. The particles overlap showing a striped appearance in some parts which may be Moire fringes. Diffraction patterns collected from different areas of the sample showed irregular spot patterns indicative of single crystal ordering (Fig. 3c). The irregular appearance is due to imaging multiple particles together. In addition to the spot patterns, there is a weak diffuse ring pattern, which differs greatly from the diffraction pattern of the support material, suggesting a degree of amorphousness. A small contribution from an amorphous phase is attributed to the low synthesis temperature, and helps to preserve the deformability of the material as discussed below.

Fig. 3 TEM images (a and b) and SAED pattern (c) collected from a Li₄P₂O₇-400. Diffuse rings, which indicate a small degree of amorphousness, are arrowed.

Fig. 4 Observed (+), calculated (red) and difference (black) patterns of the Retiveld analysis of the XRD data collected from Li₄P₂O₇-400 at room temperature.

The structural characterisation, hence, shows that Li₄P₂O₇-400 represents a new modification of monoclinic Li₄P₂O₇ in a nanostructured morphology with a small contribution of an amorphous phase. This novel morphology has, interestingly, allowed the densification of the material by cold-pressing. Pellets of Li₄P₂O₇-400 cold-pressed at 250 MPa achieved a density that is approximately 84% of the theoretical density. These cold-pressed pellets were used to study ion and electron conductivities of the material using impedance spectroscopy and DC polarization measurements. For comparison, a sample of Li₄P₂O₇ heated at 800 °C was examined by impedance spectroscopy to offer conductivity data for the conventional low-temperature Li₄P₂O₇ phase (Li₄P₂O₇-800). Li₄P₂O₇-800 was an extremely poor conductor at room temperature, showing an undetectably small current in the impedance measurements. Raising the temperature to 80 °C, allowed an estimation of the conductivity as 2.7 × 10⁻⁸ S cm⁻¹. A cold-pressed sample of Li₄P₂O₇-400, on the other hand, showed a conductivity of 1.2 × 10⁻⁶ S cm⁻¹ at the same temperature. The two orders of magnitude increase in conductivity is evident in the comparison of the impedance plots of Li₄P₂O₇-400 and conventional Li₄P₂O₇ (Li₄P₂O₇-800) shown in Fig. 5. The impedance of Li₄P₂O₇-400 is composed of a single semicircle and a low-frequency linear increase in impedance. The latter observation typically arises from blocking of Li⁺ conduction at the gold electrodes and suggests that the observed transport effect is mainly ionic and corresponds to Li-ion mobility. The data were successfully fitted using a conventional equivalent circuit that employs a constant phase element (Q) in parallel to a resistance element (R) to represent the semicircle ([RQ]), and a constant phase element to represent the low frequency spike ([Q]). The capacitance (C) calculated from the fit parameter Q of the high-frequency semicircle is ∼8 × 10⁻¹¹ F. This value accounts well for a combination of bulk and grain-boundary resistances.^17,18 Hence, similar to deformable sulfide-type solid-electrolytes, Li₄P₂O₇-400 impedance showed a single semicircle representing bulk and grain-boundary resistances as an indication of a good sinterability achieved via cold pressing. The activation energy (E_a) for the total conductivity of the material in the temperature range 40–120 °C is obtained from the Arrhenius plot of σ = σ₀ exp(−E_a/kT), where σ₀ is the pre-exponential factor, k is the Boltzmann constant, T is the absolute temperature. The Arrhenius plot is shown in Fig. 5c, revealing an activation of 0.45(1) eV. In order to investigate the electronic contribution to the measured conductivity of Li₄P₂O₇-400, DC polarization tests were employed to estimate the electronic conductivity of the material by applying voltages in the range 0.7–1.5 V (Fig. 5d). These tests revealed an electronic conductivity of 1.8(3) × 10⁻⁹ S cm⁻¹ at 80 °C, which clearly suggests that the observed conductivity of Li₄P₂O₇-400 (1.2 × 10⁻⁶ S cm⁻¹ at 80 °C) is almost entirely ionic.

Fig. 5 (a) Impedance collected from low-temperature Li₄P₂O₇ phase (Li₄P₂O₇-800) at 80 °C. (b) Impedance collected from Li₄P₂O₇-400 at 80 °C. (c) The Arrhenius plot of the conductivity data of Li₄P₂O₇-400 collected in the temperature range 40–120 °C. (d) DC polarization tests and corresponding I/V curve (inset) for Li₄P₂O₇-400 at 80 °C.

The newly-synthesized Li₄P₂O₇, hence, possesses an enhanced ion conductivity compared with conventional Li₄P₂O₇ and γ-Li₃PO₄ (<10⁻¹² S cm⁻¹ at 100 °C (ref. 19)) phases. The improved conductivity can be attributed to the modified structure and morphology of the material. The modified morphology, including the nanostructured nature and a small contribution from an amorphous material, allowed good contact between the grains by simple cold-pressing which reduces of the grain-boundary resistance. The modified crystal structure also seems to favour an improved ion transport in the material. A monoclinic modification of the material is reported to trigger an enhanced Li-ion transport in the material through migration channels with extended cross-sections.^10,12 The conductivity of the new Li₄P₂O₇ form (1.2 × 10⁻⁶ S cm⁻¹ at 80 °C) remains too low to run a full solid-state cell at room temperature. However, considering the enhanced electrochemical stability of the material (see below), the new material clearly would have remarkable properties compared with other materials which fall into the same category. The conductivity of the new Li₄P₂O₇ at 80 °C is comparable to that of LiPON films at room temperature (∼10⁻⁶ S cm⁻¹ (ref. 20 and 21)), and that of recently-reported bulk Li_3.6PO_3.4N_0.6 (5.0 × 10⁻⁶ S cm⁻¹ at 70 °C (ref. 22)) and well above that of other bulk phosphorus oxynitrides such as Li_0.88PO_3.73N_0.14 (1.4 × 10⁻¹³ S cm⁻¹ at RT²³) and Li₂PO₂N (8.8 × 10⁻⁷ S cm⁻¹ at 80 °C (ref. 24)). None of the above materials, nevertheless, is densified by cold-pressing, which makes the new Li₄P₂O₇ phase more suitable for application in bulk-type all-solid-sate cells that operate at intermediate temperatures.

To study the possible use of Li₄P₂O₇-400 in contact with high-voltage electrodes, the electronic stability window of the material was checked by cyclic voltammetry (CV) in the voltage range 0–5 V vs. In/InLi. An asymmetric In/Li₃PS₄/Li₄P₂O₇-C cell was used to collect the CV data. In this cell, Li₃PS₄ is used as the solid-electrolyte because it has an enhanced ion conductivity compared with Li₄P₂O₇, which provides a reliable performance of the cell. Li₄P₂O₇ intimately mixed with carbon black (10 wt%) was used as the working electrode. The cyclic voltammetry of this cell is compared with the performance of an identical cell containing Li₃PS₄/C as the active material (In/Li₃PS₄/Li₃PS₄-C) in Fig. 6a. Unlike Li₃PS₄, Li₄P₂O₇ possess remarkable stability, showing no evidence of electrochemical reactivity up to 5 V vs. In/InLi (∼5.6 V vs. Li⁺/Li). The absence of any significant phase decomposition in this voltage range, compared with standard Li₃PS₄, suggests that the material is suitable for high-voltage applications. In order to further confirm the electrochemical stability of Li₄P₂O₇-400 at high voltages, a cell employing a liquid electrolyte and Li₄P₂O₇/C as an active material (Li/liquid-electrolyte/Li₄P₂O₇-C), was cycled at a low current density (10 mA g⁻¹) in the voltage range 1–4.5 V (vs. Li⁺/Li). Higher voltages were avoided to avoid a series decomposition of the liquid electrolyte. The cell was compared with an identical standard cell containing inert ZrO₂/C as the active material. The two cells showed identical charge/discharge curves under same experimental conditions, confirming the stability of Li₄P₂O₇ (Fig. 6b).

Fig. 6 (a) CV data collected from the asymmetric (solid-state) cells In/Li₃PS₄/Li₄P₂O₇-C and In/Li₃PS₄/Li₃PS₄-C at room temperature with a scan rate of 0.1 mV s⁻¹. (b) Discharge–charge curves of two liquid-electrolyte-based half cells employing Li₄P₂O₇/C or ZrO₂/C as the working electrode and Li metal as counter electrode (current density = 10 mA g⁻¹).

In order to examine the compatibility of Li₄P₂O₇-400 with Li metal anodes, a symmetric Li/Li₄P₂O₇/Li cell was studied. A notable increase of the impedance of the cell, compared with Au/Li₄P₂O₇/Au cells, was observed which suggests a resistive interface between Li metal and Li₄P₂O₇. The area specific resistance (ASR) for the Li/Li₄P₂O₇ interface is estimated to be greater than the resistance of the pellet by a factor of three (Fig. S3†). Similar behaviour was observed for related phosphate-based solid-electrolytes such as LiZr₂(PO₄)₃.²⁵ The observed high interfacial resistance may be partially related to an insufficient contact between Li metal and the pellet, but also clearly indicates instability of Li₄P₂O₇ in contact with Li. This could also be visually observed as a darkening of the pellet's surface after contact with Li. This is consistent with the electrochemical stability data of related phosphates, including LiZr₂(PO₄)₃ ²⁵ and LiPON,²⁶ which revealed the formation of a passivation layer at the surface of the solid-electrolyte in contact with Li metal. The stability of the Li/Li₄P₂O₇ interface upon lithium dissolution/deposition was tested by cycling a symmetric Li/Li₄P₂O₇/Li at current density of 2 μA cm⁻² at 80 °C (Fig. 7). A low current density was employed to compensate for the high resistance of the material (see ref. 22). A very stable cycle performance, with insignificant increase of the cell voltage upon cycling for 100 h, is observed which indicates a good stability of the interface. This result is consistent with the behaviour of related phosphate-based solid-electrolytes such as LiZr₂(PO₄)₃,²⁶ LiPON²⁶ and bulk Li_3.6PO_3.4N_0.6,²² which are believed to form stable interfaces with Li metal.

Fig. 7 Galvanostatic cycling data of a Li/Li₄P₂O₇/Li symmetric cell at 80 °C and 2 μA cm⁻² current density.

Conclusions

A nanostructured form of Li₄P₂O₇ has been successfully synthesized by chemical lithiation of microwave-synthesized LiPO₃ followed by calcination at 400 °C. The new Li₄P₂O₇ form preserves the nanostructured morphology of parent LiPO₃, and exhibits a monoclinic structure which is related to the high-temperature Li₄P₂O₇ phase. The novel morphology of the material, including a nanostructured nature and a small contribution from an amorphous phase, allowed the densification of the material by cold-pressing, exhibiting an ion conductivity that is approximately two orders of magnitude higher than that of Li₄P₂O₇ formed by conventional syntheses. This improved conductivity is attributed to both the modified morphology and the modified crystal structure of the material. The material also showed no evidence of electrochemical decomposition at voltages up to 5.6 V vs. Li⁺/Li, which suggests possible use in contact with high-voltage cathodes. In contact with Li metal, Li₄P₂O₇ forms a resistive interface, which was found to be very stable upon Li⁺ dissolution/deposition. The newly-synthesized Li₄P₂O₇, hence, possesses ion conductivity and phase stability comparable to LiPON phases, but eventually can be prepared in a bulk, deformable form.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the ICSF Faraday Challenge project SOLBAT, UK [grant FIRG007], and the Academy of Scientific Research and Technology, Egypt [grant RESPECT-10025].

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3dt04377k

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