P system as efficient anode for Li and Na- batteries

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The versatile Si/P system as efficient anode for Li and Na- batteries: understanding of an original electrochemical mechanism by a full XRD-NMR study. Gaël Coquil, Bernard Fraisse, Nicolas Dupre, and Laure Monconduit ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00567 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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ACS Applied Energy Materials

The Versatile Si/P System as efficient Anode for Li and Na- batteries: Understanding of An Original Electrochemical Mechanism by a Full XRD-NMR Study. Gaël Coquil †; Bernard Fraisse†; Nicolas Dupré₸, ‡,*; Laure Monconduit †,§, ‡,* † Institut Charles Gerhardt, Université de Montpellier, 34095 Montpellier, France. ₸ Institut des Matériaux Jean Rouxel (IMN), CNRS UMR 6502, Université de Nantes, 44322 Nantes Cedex 3, France § Réseau sur le Stockage Electrochimique de l’Energie (RS2E), CNRS FR3459, 33 Rue Saint Leu, 80039 Amiens, Cedex, France. ‡ ALISTORE European Research Institute, Université de Picardie Jules Verne, 80039 Amiens, France KEYWORDS: Li-ion Battery, Phosphides alloys, Negative electrodes, Operando x-ray diffraction, NMR study, Reaction mechanism

ABSTRACT: As illustrated by recent extensive crystallographic studies, the structural richness of the ternary Li/Si/P system offers a unique variety of physical properties. Taking advantage of 3D networks enabling fast ionic conduction properties, new phosphorus silicides based electrodes have recently emerged, exhibiting attractive performance in Li-ion as well as in Na-ion batteries. To date SiP2 reaction mechanism vs Li or Na has nevertheless not been elucidated. The combination of operando X-ray diffraction and ex situ 31P, 7Li, 23Na, 19F MAS NMR spectroscopy allows here probing both long range structural information and evolution of local environments. These complementary techniques lead to deeper understanding and depiction of the mechanism behind the lithiation/delithiation and sodiation/desodiation of SiP2. The versatility of connection of tetrahedral SiP4 anions, from three-dimensional network to isolated SiP4 units, stabilized by an increasing numbers of counterions as a function of the state of charge/discharge is the driving force of the new SiP2/Li and SiP2/Na batteries. This richness of SiP4 anions crystallographic network allows step wise phase transitions during lithiation/delithiation which limit the detrimental effects of volume expansion.

INTRODUCTION Li-ion batteries present an unrivalled combination of high energy and power density that pushes them as the technology of choice for powering portable electronics and electric vehicles (EVs). However, Li-ion batteries (LIBs) have not yet reached full maturity1,2 and the exploration of new electrode materials is required to push the boundaries of energy and power density, cycle life and cost. Moving beyond classical intercalation reactions (such as for graphite and LiCoO2), a variety of low cost compounds providing much higher specific energy such as alloy 3,4 or conversion materials5,6 have emerged in the last 15 years. Unfortunately, such materials suffer from high volume expansion/contraction occurring during the drastic phase transitions of lithiation/delithiation, causing particle fracturing, loss of electronic contact upon cycling and finally leading to

low cycle life. Even though electrode formulation (modification of the relative proportions of active material, conductive additive and polymeric binder in the electrode) is one of the proposed strategies to circumvent these issues,7 efforts have to be pursued to concomitantly reach high capacities and limit the effects of volume changes. The exploration of structure/property relationships in new materials should allow identifying new electrochemical reaction mechanisms vs. Li (or Na). In particular, the detailed inspection of the crystallographic and physicochemical properties of new materials is a necessary step for the identification of new electrodes and mechanisms for LIBs as well as for Na-ion batteries (NIBs).8 Silicon- and phosphorus-based electrodes have been extensively studied in lithium-ion batteries (LIBs) on the basis of their high theoretical capacities of 3600 mAh/g and 2500 mAh/g, respectively, and their mod-

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erate cost. Only phosphorus is electrochemically active versus sodium in Na-ion batteries (NIBs)9,10,11 and up to now no electrochemical activity could be measured for silicon vs Na.12, 13 However, maintaining cycling stability still remains a challenge for thick electrodes and full cells in both LIBs and NIBs. The limited cycling performance, usually observed for this class of material, comes from i) the large volume changes during lithiation/delithiation of phosphorous (320%) and silicon (280%) and sodiation/desodiation of phosphorous (500%) which result in the mechanical pulverization of the electrode in long cycling and ii) the low electronic conductivity of P (10-9 S.m-1) and Si (2,5.10-4 S.m-1).14 Recently in a pioneer study a satisfying specific capacity was however obtained with SiP2 with up to 1000 and 550 mAh/g sustained after 30 and 15 cycles for Li and Na batteries, respectively.15,16 Only a very recent work by Reinhold et al. proposed a mechanism for a SiP2 exhibiting a specific capacity rapidly fading upon cycling. 17 Based on XPS results, they proposed that the formation of a homogeneously covering layer of LiP is responsible for the rapidly decaying performance. On the other hand the understanding of the mechanism of the electrochemical process of SiP2 vs Na has never been investigated. For SiP2, a simple formation of Li3P and LixSi was ruled out, and the formation of Li/Si/P ternary phases, was suggested as formed during the discharge.15 Similar hypotheses were considered for the mechanism occuring in Na batteries.15 Nevertheless no proofs allowed confirming these hypotheses. For decades, the only known compound in the Li-Si-P system was Li5SiP3, which was described by R. Juza et al. in 1954.18 On the basis of powder X-ray diffraction data, this phase has been assigned to the antifluorite structure type (space group Fm-3m), where the P atoms build an anionic ccp lattice, and Li and Si fill the tetrahedral voids in a 5:1 mixed occupancy ratio. Recently a series of new ternary phosphidosilicates, namely LiSi2P3, Li2SiP2, Li5SiP3, Li3Si3P7 and Li8SiP4 were discovered.19,20 The crystallographic description realized on this series of phosphidosilicates demonstrated the versatility of these crystallographic structures based on tetrahedral SiP4 anions more or less connected and Li counterions. The lithium ions are located and mobile in the open spaces between the SiP4 tetrahedra or large supertetrahedral clusters. Moreover Li ions were demonstrated as having very fast motion in these networks. Starting from this new highlight on Li/Si/P phases the SiP2 electrochemical mechanism is here investigated through the operando XRD and ex situ NMR full study. The emerging knowledge about various existing lithium phosphidosilicates is crucial to

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get a more precise insight into the lithiation process of SiP2 anode material. Then electrochemical properties of SiP2 are investigated in the present study and supported by operando X-ray diffraction and full 31P, 7Li, 19F NMR spectroscopy study, providing original insights on the lithiation and sodiation mechanisms of these new phosphidosilicates anodes. EXPERIMENTAL SECTION Synthesis and characterization. SiP2 were prepared by mechanosynthesis of Si powder and red phosphorous powder using high energy ball milling under Ar atmosphere as described elsewhere.13 After testing different grinding durations, the optimized active-milling time of 16 h was determined (Fig. S1) to obtain a pure SiP2 sample. References samples for the NMR study, Li3P, Li5SiP3, Na3P, NaP, Na5SiP3 were also prepared by mechanosynthesis and characterized by XRD. The single phase nature of SiP2 was confirmed by indexing the whole diffraction pattern in the Pa-3 space group with the cell parameter a of 5.707(1) Å (Panalitical Empyrean diffractometer equipped with Cu Kα radiation), similar to what is reported in literature ((ICSD 24333) a = 5.701(4) Å)21) (Fig 1-a). Scanning electron microscopy (Hitachi 4800) indicates clusters of 1 to 30 µm composed of submicronic particles. An EDX mapping allowed evaluating the Si/P ratio in sample to 1/2 and showing that Si and P are distributed evenly in the whole sample (Fig 1-b). Electrochemical Tests. A slurry with a 50 wt % concentration of SiP2 powder and 12 wt % carboxymethyl cellulose (CMC, Aldrich, MW = 250 000, DS = 0.7) as the binder and 38 wt % carbon black (CB) (SN2A, Y50A) as the conductive additive was prepared and spread on a copper foil. The electrode mass loading was about 1.82 mg of active material/cm2. The slurry was tape cast onto a 20 µm thick copper foil and dried for 24 h at room temperature. Pellets of the as-prepared films were cut and dried for 12 h at 80 °C under vacuum. The used electrolyte was a solution of 1 M LiPF6 dissolved in ethylene carbonate/propylene carbonate/dimethyl carbonate (EC/PC/DMC, 1:1:3, v/v/v) with 1% vinylene carbonate (VC) and 5% Fluoroethylene carbonate (FEC) for SiP2/Li and a solution of 1 M NaPF6 dissolved in diglyme for SiP2/Na. The separator was a glass microfiber filter (Whatman, GF/D). Coin cells


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were assembled in an argon-filled glovebox with a crimper machine, and electrochemical tests were performed on a MPG (Biologic) in galvanostatic mode at a current rate of 148 (mA)/g (one lithium insertion every 2 h, C/2). X-ray diffraction (XRD). The synthesized phases were analyzed by XRD using a Panalytical Empyrean diffractometer, Cu-Kα1/Kα2 radiation, theta–theta configuration, with a silicon sample holder covered with a Kapton foil for the airsensitive Li-P and Na-P powders. The measured 2θ range was 10º-60º, during 1 hour. In situ XRD : For the in situ experiments a self-supported electrode, with composition 60/38/2 wt % of active material/carbon black/PTFE, was used. The in situ XRD experiments were conducted using a cell with a beryllium window.22 The electrolyte and the cycling rates used were the same as for the electrochemical measurements. NMR 7 Li and 31P MAS NMR experiments were carried out at room temperature on a Bruker Avance-500 spectrometer (B0 = 11.75T, Larmor frequency ν0(7Li) = 194.45 MHz and ν0(31P) = 202.45 MHz). MAS spectra were obtained by using a Bruker MAS probe with a cylindrical 2.5 mm o.d. zirconia rotor. Spinning frequencies up to 25 and 30 kHz were utilized. 7Li and 31P NMR spectra were acquired by making use of a single π/2 pulse sequence of 1.4 µs and 4 µs, respectively. Recycle delays of 5s were determined to ensure quantitative measurements for both 7Li and 31P. The isotropic shifts, reported in parts per million, are relative to external 1M solutions of LiCl and H3PO4 in water set at 0 ppm. 23Na and 19F NMR spectra were acquired at room temperature on a Bruker Avance-500 spectrometer (B0 = 11.8T, Larmor frequency ν0(23Na) = 132.29 MHz and ν0(19F) = 470.0 MHz), using an echo sequence to discard the significant contribution from the probe signal, with a π/2 pulse of 1 µs and 3.6 µs, respectively. The isotropic shifts, reported in parts per million, are relative to external 1M solutions of NaCl and CFCl3 in water set at 0 ppm. NMR integrated intensities were determined by using spectral simulation (Dmfit software).23

Figure 1. a) XRD pattern with indexed peaks of SiP2 and crystallographic description. The red line corresponds to the experimental pattern, the black line to the calculated pattern, the green sticks to the theoretical SiP2 Bragg position and the blue line to the difference between experimental and calculated patterns, b) SEM picture of the ball milled SiP2 powder, and corresponding EDS mapping. RESULTS AND DISCUSSION SiP2 vs. Li Electrodes were prepared with an aqueous formulation described in the technical part. The galvanostatic curve of SiP2 vs. Li and the corresponding derivative curve are presented in Fig.2-a. In first discharge at C/2 (148 mA/g) rate more than 9 Li react with SiP2 which corresponds to 2730 mAh/g-SiP2 (or 1550 mAh/g when reported to the SiP2/C electrode). Only 7 Li (/SiP2) are deinserted in the following charge, amounting to a reversible high capacity of 2055 mA h/g-SiP2, a value far from the theoretical capacity (2900 mAh/g-SiP2), and an irreversibility around 25%. After 35 cycles a capacity of 750 mAh/g-SiP2 (see the Fig. S2 to follow the capacity of the composite SiP2/C) was sustained with a coulombic efficiency of 98.9% (Fig.2-b). The derivative curve, displayed three peaks during the first discharge at 0.52 V, 0.38 V and 0.05 V, and one main oxidation peaks at 0.75 V for the first



charge. During the second discharge only two peaks at 0.36 V and 0.05 V are maintained.

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(P63/mmc, ICSD 2688) was reported as function of the lithiation/delithiation (Fig. 4).

Figure 3: Operando XRD pattern for SiP2/Li recorded at a C/8 rate during the first discharge and charge.

Toward the end of the lithiation the cell volume increases to 125.5 Å3. Once the subsequent charge starts, a decreasing of the hexagonal cell is observed from 125.5 to 117.5 Å3, the latter corresponds well to the expected volume for Li3P.25 It is noteworthy that the hexagonal cell of Li3P was also given in the literature with a slightly higher volume, 120 Å3 (red star)26 which could be explained by a non-stoichiometry in Li or P. Figure 2. a) Galvanostatic curve of SiP2/Li at a C/2 rate and the derivative curve in inset, and b) corresponding capacity retention as a function of the number of cycles. Operando XRD analysis: To get deeper insight into the lithiation/delithiation mechanism of the SiP2 electrode, operando XRD was collected during the first electrochemical cycle (Fig. 3). During the insertion of the first 3 Li, the Bragg peaks characteristic of SiP2 (200), (201), (211) and (220) disappear and no new peak appears indicating the transformation of the pristine SiP2 into amorphous species. During the further 3 Li insertion, the XRD stays flat even though a hump seems to grow around 23°. During the last part of the discharge, 3 Li are inserted and well defined peaks grow at 23.1°, 23.6°, 26.4°, 41.8° and 42.6°. In this angular range fall the main Bragg peaks of Li3P (P63/mmc, ICSD 26880).24 However, simultaneously with their growth, these peaks shift to lower angles and move away from positions expected for Li3P. To better illustrate this phenomenon the volume cell (by considering a hexagonal cell isostructural of Li3P

Figure 4. Volume of the hexagonal cell of the phase formed in the last part of the discharge and beginning of the subsequent charge, as a function of Li inserted in the SiP2 electrode. Yellow and red stars give the volume of the hexagonal cell of Li3P from the works of DiSalvo et Nazri et al respectively.24,25


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The larger hexagonal volume cell recorded at the end of discharge could be ascribed to an overlithiation as previously identified for other alloy type electrode materials such as Li15+x Si4.27,28,29 To get deeper insight in the mechanism of SiP2 vs Li, a ex situ 7Li and 31P MAS NMR study was performed. NMR study on SiP2/Li: Figure 5-a displays the 7Li MAS NMR spectra obtained during the first cycle. The spectrum corresponding to the end of the first discharge (red) shows clearly at least two distinct resonances at -1.5 ppm and 3 ppm, with a shoulder at 6 ppm, assigned to lithiated species in the SEI and species formed through the lithiation process of SiP2, respectively. 7Li signals from the species contained in the SEI and electrolyte overlap with signals corresponding to Li local environments stemming from the lithiation of SiP2. From our experience with low potential conversion materials,30 rinsing samples stopped at low potential prior to ex situ analysis proved problematic due to an exacerbated instability of samples. Providing that 7Li MAS NMR permits enough resolution to discriminate SEI from bulk species, samples were not rinsed here. The slightly negative shift of the signal attributed to the SEI species suggests a strong content of fluorinated species such as LiF, LixPFy, LixPOyFz and LiPF6 of the SEI. The signal at 3 ppm is quite broad and can possibly envelop several Li local environments.

Figure 5. a) 7Li MAS NMR spectra obtained at middischarge (green), end of discharge (red), at mid-charge (orange) and end of charge (blue), b) 19F MAS NMR spectra obtained at the end of discharge (violet) and end of the subsequent charge (orange) of SiP2/Li.

The 19F MAS NMR experiments (Fig. 5-b) confirm the presence of PF6- groups, at -72 ppm, coming most probably from non-degraded electrolyte trapped at the surface of the active material. A very weak signal at 204 ppm indicates also the presence of LiF. The signal at 3 ppm (7Li MAS NMR) is also visible in the spectra corresponding to a SiP2 electrode stopped at middischarge and mid-charge, as a shoulder of the SEI signal and illustrates the progressive reaction of Li with SiP2. This shoulder is nevertheless less marked for the electrode stopped at mid-charge, indicating a lower content of lithium in the electrode. The 7Li MAS NMR spectrum acquired after one full cycle at the end of charge displays only the resonance at -1.5 ppm previously assigned to SEI species.

a)



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completely rule the presence of various Li rich LixSi alloys from the discharged SiP2 samples out.

Figure 6. a) 31P MAS NMR spectra at different steps of the first cycle, b) 31P MAS NMR spectra of pristine SiP2 (green) and SiP2 electrode at end of charge (blue). No clear evidence of a signal at higher chemical shift can be seen, indicating that most of the irreversible capacity observed on the galvanostatic curves (Fig. 2a) corresponds to the formation of the electrode/electrolyte interphase. It suggests in particular that few or no lithium ions are trapped in the bulk of the active material during the oxidation process. The very sharp additional resonance at -1.6 ppm may correspond to some liquid electrolyte trapped in the porosity of the solid electrolyte interphase (SEI). The signal observed at 6 ppm as a shoulder of the main resonance in the spectrum at the end of discharge is tentatively assigned to the presence of lithium-silicon alloys. Considering Key et al. work,27 Li-poor LixSi alloys appear at 18-20 ppm while Li-rich alloys, in particular Li15Si4 appears at 6 ppm. The presence of Li-poor LixSi alloys at the end of the discharge of SiP2 can be eliminated but a contribution of Li15Si4 to the 7 Li NMR signal obtained at the end of discharge remains possible. This result appears in contradiction with the observation of Reinhold et al. who detected a sharp 7Li NMR resonance at 16 ppm, assigned to various Li poor LixSi such as Li12Si7, Li7Si3 or Li13Si4.17 This is suggesting here the formation of different species along the discharge process and possibly a different lithiation mechanism. For the present system, if we consider a SiP2 unit reacting, we should expect an equivalent of 6 Li reacting with phosphorus, to form Li3P and 3.75 Li reacting with silicon. This would yield a Li15Si4 7Li NMR contribution corresponding to approx. 40% of the 7Li NMR integrated intensity assigned to species formed during the discharge process. Taking into account a resonance at 6 ppm in the deconvolution of the 7Li MAS NMR spectrum at the end of discharge, a possible contribution of only 5-6% is found. It is therefore not possible to

Figure 6-a displays 31P MAS NMR spectra for SiP2 electrodes stopped at mid-discharge, end of charge for the first reduction as well as mid-charge and end of charge for the subsequent oxidation. All the spectra show an intense and sharp resonance at -146 ppm, assigned to PF6- groups. A sharp and well defined resonance is visible at 6 ppm at all stages of the electrochemical cycle except for the electrode stopped at the end of charge where the resonance appears at a slightly lower shift (1 ppm). The pristine SiP2 powder spectrum (Fig. 6-b) displays, in addition to the main broad and intense signal centered at approx. -100 ppm, assigned to bulk SiP2, a smaller sharp resonance in the same chemical shift range although slightly shifted toward negative chemical shifts at approx. -10 ppm and a smaller broad resonance at 170 ppm. The 31P MAS NMR spectrum of pristine SiP2 electrode (i.e. after electrode processing) displays also additional resonances at approx. -2 ppm (Fig. S3) and 170 ppm. In the 20 to -20 ppm chemical shift range, species containing phosphate groups or organic phosphates (such as HPO(OR)2) can be typically found. The broad secondary signal at 170 ppm has been found to increase when the SiP2 sample is oxidized in ambient atmosphere, evolving eventually in to a set of sharper resonances at 145 ppm and 80 ppm (Fig. S3). Again, these signals are minor contributions to the complete spectrum and may be attributed to the very reactive surface of SiP2 particle, probably oxidized by a contact with ambient atmosphere. Upon the electrochemical reaction of the electrode with lithium, the sharp signal initially at -2 ppm shifts towards a position at 6 ppm, suggesting that this particular surface phosphorus environment is also reacting with lithium. This chemical shift is comparable to that of Li3PO4,31 which is known to form from the reaction of traces of H3PO4 or surface phosphates at the surface of phosphorus based negative electrode upon electrolyte exposure and cycling.32 This shift is already detected during the early stages of discharge, in agreement with a surface reaction that would occur before the bulk reaction. At the end of the subsequent charge, the resonance moves back towards lower shifts and the 31P MAS NMR spectrum observed at the end of charge displays a resonance with a chemical shift intermediate between that of the electrode at the end of discharge and that of pristine SiP2 electrode (i.e. after electrode pro-


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cessing) showing the (at least) partial reversibility of the reaction of the oxidized surface of SiP2. The secondary broad signal at 170 ppm disappears completely during the electrochemical cycling of SiP2 and supports an irreversible reaction with the electrolyte during cycling. Reinhold et al. also detected a sharper resonance in the same chemical shift range disappearing irreversibly upon cycling and possibly playing a role in the irreversibility of the whole system. These results are in agreement with an oxidized SiP2 surface undergoing an irreversible reaction upon electrochemical lithiation. In the following section, we now focus on the evolution of the broad signal centered at -100 ppm and assigned to bulk SiP2. 31P MAS NMR allows nevertheless following the redox reaction of the SiP2 bulk by looking at this extremely broad signal, exhibiting a progressive shift during the first cycle. The sharp signal observed for the PF6- contribution from the electrolyte trapped in the SEI allows, by contrast, a clear discrimination between electrolyte/SEI contribution and phosphorous environments forming upon lithiation of SiP2. In the case of the electrode stopped at mid-discharge, the broad resonance centered at 170 ppm and spanning from -50 to -300 ppm appears as the main component of the 31P spectrum and is therefore assigned to lithiated environment in the SiP2 matrix. The extreme broadness of the signal suggests strongly a distribution of chemical shifts most probably due to a distribution of Li and P local environments implying a very disordered structure or a significant range of Li local compositions in the electrode. In addition, the signal appears to be nonsymmetric with still significant intensity in the -50 to -100 ppm range and showing up as a tail to the main signal. It suggests that part of the pristine material has not reacted with lithium. At the end of the discharge, although some intensity can still be detected in the 150 / -200 ppm range, the main signal rises at -270 ppm. The 31P MAS NMR spectrum of reference Li3P prepared by mechanosynthesis exhibits a single resonance at -270 ppm (Fig. S4). This signal (Fig. 6-a) is thus assigned to Li3P,33, 34 indicating clearly that it is the main final product of the redox process of Li with SiP2. Furthermore, the 7Li signal at 3 ppm (Fig. 6-a) is in agreement with the 7Li resonance of Li3P prepared by ball milling (Fig. S4). It is noteworthy that the signal is again very broad suggesting a possible distribution of Li and P local environments, and a nonstoichiometry of Li3P. In particular, the presence of LiP (typically rising at -190 ppm) is possible. LiP has been clearly identified in a previous study.17 The broad resonance observed here suggested nevertheless

more diversified local environments. The 7Li chemical shift range of diamagnetic species is quite narrow and the 7Li MAS NMR signal detected at the end of discharge could also contain minor contributions of LiP as well as lithiated phosphidosilicates such as Li3Si3P7 ( δ = 3.0 ppm), Li10Si2P6 (δ = −2.1 ppm), Li2SiP2 (δ = 2.1 ppm), Li5SiP3 (δ = 1.4 ppm) or Li8SiP4 ( δ = 3.3 ppm). The presence of these phases is discussed in the paragraph below in the light of 31P NMR. Even though they exhibit non negligible intensity that could be assigned to pristine SiP2, spectra of SiP2 electrodes stopped at mid-discharge and at the end of discharge are broad and span over wide and overlapping frequency ranges. The progressive shift observed upon reduction indicates nevertheless characteristic structural changes in the phosphorus local environments. It is interesting to compare the shift of the samples collected during the discharge/charge with those reported for Li/Si/P phases gathered in the Table1. The maximum of intensity at approx. -100 ppm is comparable with chemical shifts of -77 ppm that can be detected in Li3Si3P7 31P MAS NMR spectrum and assigned to phosphorus atoms interconnected forming a zig-zag chain. Such environments also exist in SiP2 structure. In addition, corresponding P atoms in Li3Si3P7 are in a Li-poor environment since only one Li can be found in their vicinity. 31P resonances at -121 ppm (Li3Si3P7), -124 ppm (Li10Si2P6) and -129 ppm (Li2SiP2) seem to be characteristics of connected SiP4 tetrahedra, and correspond to phosphorus atoms bridging two edge-sharing SiP4 tetrahedra, phosphorus atoms bridging three cornersharing SiP4 tetrahedra and phosphorus connecting different adamantane cages (composed of SiP4 tetrahedra) respectively. The corresponding phosphorus environments are also still quite poor in lithium, with 1 to 5 lithium in the vicinity. 31P NMR chemical shifts in the corresponding range seem then to be linked with the existence of connections between SiP4 tetrahedra and increasing lithium content. Phosphorus atoms bridging only two corner-sharing SiP4 tetrahedra, in Li3Si3P7 and Li2SiP2 structures, give rise to NMR resonances between -168 and -178 ppm and 240 ppm, respectively. The shift towards more negative ppm values is here indicative of a lower extent of the SiP4 tetrahedra connection and an increase in lithium content. Li10Si2P6 also contain terminal phosphorus atoms that are connected only to one silicon atom can be surrounded by up to 8 lithium and yield 31P NMR resonance between -188 and -200 ppm. Such chemical shifts could be indicative of environments both richer in lithium and characteristic of isolated SiP4 tetrahedra. The Li5SiP3 phase obtained by Juza et al.11 crystallizes with an antifluorite structure containing quasi-isolated SiP4 tetrahedra. The 31P MAS NMR



measurements that we performed on this material prepared by ball milling lead to a broad resonance with a maximum at -230 ppm (Fig. S4). Li8SiP4, containing only isolated SiP4 tetrahedra well separated by lithium (P-P distances between phosphorus atoms belonging to different SiP4 tetrahedra are over 4.18 Å), displays 31P resonances at -225 and -251 ppm. The evolution of the NMR shift is also here in accordance with the trend observed in 31P MAS NMR from the pristine SiP2 to partially lithiated material and finally to the resonance attributed to Li3P at the end of reduction. The observed evolution is therefore in agreement with an increase in lithium content but also with a progressive breaking of the SiP4 inter-connections, leading most probably to isolated SiP4 tetrahedra flooded by surrounding Li+, and finally to the breaking of the Si-P bonds to form Li3P (or close structure). Considering the multiplicity of environments leading to broad and overlapping resonances in 31P NMR, isolated SiP4 could not be discriminated using 31P MAS NMR. An unambiguous detection might be achieved using 29Si MAS NMR but this would require 29 Si enriched samples, which was not feasible in the scope of this work. The additional broad signal visible as a tail between -150/-200 ppm at the end of discharge is therefore assigned to region of the electrode with a lower composition in lithium and incomplete breaking of the Si-P bonds in SiP4 tetrahedra and probably presence of LiP. Finally, the absence of cross-peaks between the 6 and the -270 ppm resonances in the additional 31P 2D EXSY NMR spectrum (Fig. S5) confirms that the corresponding phosphorus environments do not belong to the same phase. Contrary to the various examples of Li/Si/P phases encountered in previous crystallographic studies and displaying relatively sharp 31P NMR resonances, the Li5SiP3 phase that we prepared by mechanosynthesis, as a model compound (Fig.S6) exhibits a much broader NMR signal and therefore might bear more structural resemblance to the disordered matrix involved in the electrochemical cycling of SiP2. Thus, it is also interesting to compare the 31P MAS NMR spectrum obtained at the end of discharge with that of Li5SiP3 obtained by mechanosynthesis. The 7Li MAS NMR spectrum of Li5SiP3 exhibit a single sharp resonance at 1.4 ppm while the corresponding 31P MAS NMR spectrum displays a broad signal spanning from -100 to -350 ppm with a maximum of intensity at -230 ppm along with broad shoulders at -170 and -300 ppm, suggesting a distribution of phosphorus local environments. Attempts of fitting the 31P MAS NMR spectrum of the electrode at the end of discharge using a combination of the spec-

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tra of Li3P and Li5SiP3 were not satisfying due to the fact that the resonance assigned to the Li3P formed electrochemically is much broader compared to its asprepared parent. Nevertheless, when using a broader Li3P resonance, an acceptable fit can be obtained, including a possible contribution of Li5SiP3 up to 35 %. It appears thus clearly that even though Li5SiP3 is not the main product formed at the end of the lithiation, its presence or the presence of a disordered Li/Si/P environments cannot be ruled out. This result supports the existence of local lithium environments similar or close to those found in the various Li/Si/P phases mentioned earlier in this work at the end of the incomplete lithiation process. The 31P MAS NMR spectrum obtained at mid-charge is very similar to that of the middischarged electrode with a broad signal centered at 145 ppm, suggesting that the delithiation process seems to go through the formation of the same intermediate phosphorus local environments and that both SiP4 tetrahedra units and SiP4 tetrahedra interconnections could be reformed. Spectra of samples stopped at mid-discharge and mid-charge display visible differences in their respective maximum of intensity (from -170 to -145 ppm) and frequency span. The composition in lithium measured on the electrochemical curves is nevertheless the same with x=4.5 in the nominal composition LixSiP2. Although this discrepancy may be explained by irreversible parasitic reactions occurring at low potential upon reduction it could also be explained, in the light of the above comparison with phosphorus local environments in existing structures, by a hysteresis in the formation/breaking of Si-P bonds and SiP4 tetrahedra connections. The more negative shift and slightly narrower signal for the resonance at mid-discharge suggests that the overall lithiation is more homogenous and contains less connected SiP4 tetrahedra compared to the sample stopped at mid-charge. On the other side, the broader signal for the latter suggests that the extraction of lithium is significantly more difficult: the detection of intensity in the 30 to 100 ppm range indicates that delithiated or lithiumpoor local environments with P-P bonds may have already reformed while the significant intensity remaining in the -200 ppm regions indicates that lithium-rich environments with isolated SiP4 tetrahedra remain. The more heterogeneous character of the sample stopped at mi-charge could be then a consequence of an increased difficulty in the reformation of SiP4 connection and/or P-P bonds. At the end of charge, the maximum of intensity of the broad signal has shifted to -100 ppm as the lithium is extracted from the active material. The resonance detected at


Page 9 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the end of charge exhibits both similar chemical shift and spanning range with respect to the pristine SiP2 (Fig.6-c) indicating that no significant amount of lithium seems to be trapped in the active material and that the different P-P bonds and connections between adjacent SiP4 tetrahedra must have been reformed. This result is in agreement with the corresponding 7Li MAS NMR spectrum (Fig. 5-a) in which only the SEI signal can be detected. Moreover, contrary to the observations by Reinhold et al.17 no LiP or Li3P could be detected in the 31P spectrum at the end of charge, indicating clearly a good reversibility of the conversion/de-conversion reaction, in agreement with the better capacity retention we observed in the present study (75% of the first discharge capacity is retained for the second cycle vs 65% in ref. 17 and the capacity for the 10th cycle is 1250 mAh/gSiP2 vs 370 mAh/gSiP2 in ref.17). These results tend to indicate a more reversible lithiation mechanism in the case of the quite disordered SiP2 studied in the present work. The three extremely broad resonances observed at mid-discharge, mid-charge and end of charge are strongly overlapping (Fig.6-a). Each of them can be considered as the sum of several phosphorus local environments due to both structural disorder and lithium composition: spectra of the pristine SiP2 and electrode at the end of charge show indeed that non lithiated environments can be found in a wide range of chemical shift, ranging from approx. 20 to -160 ppm. It is most probable that some of these environments contribute to the intensity of the broad resonances observed at mid-charge and mid-discharge. In the case of the electrodes stopped at mid-charge and middischarge, the width of the apparent resonances is then due to both structural disorder and lithium variation, lithium-rich environments and increasingly isolated silicon-phosphorus units both appearing at more negative shifts. At the end of the discharge, no signal could be detected in the 20 to -160 ppm range, suggesting that all the pristine material reacted and is, although inhomogenously, lithiated.

To summarize and despite the very broad NMR spectra measured from the cycled samples, a reversible mechanism based on the formation of ternary more and more lithiated Li/M/P phases can be suggested upon the discharge/charge. The broadness of the spectra is in good agreement with the disorder in the amorphous phases formed from the lithiation of SiP2 which originated a flat XRD upon discharge (Fig. 3). Li3P already identified by XRD at the end of discharge is confirmed by 31P and 7Li NMR. The possible overlithiation of Li3P proposed from XRD analyses is consistent with the very broad 31P spectrum associated to Li3P. The operando XRD analysis during the discharge and charge shows the appearance/disappearance of i) amorphous phases, ii) Li3P and iii) an overlithiated Li3+xP phase, that attests to a reversible electrochemical mechanism. The NMR analysis also confirms the reversibility of the mechanism with a reversible shift of the 7Li and 31P spectra along the discharge and charge.

Figure 7. Outline of the SiP2 reaction with Li



Page 10 of 35

Table 1: Cristallographic data, 7Li and 31P NMR shift (in red: measured in the present study) of all Li/Si/P phases. Phases Li/Si/P

structure

SiP2 ICSD-24333

P a -3 (205) - cubic a=5.7050 Å V=185.68 Å3 Z=4

LiSi2P3 ICSD-431584

I 41/a (88) a = 18.4656 Å c = 35.0924 Å V= 11966 Å3 Z=100

Li3Si3P7

P21/m (11) monoclinic a = 6.3356(4) Å b = 7.2198(4) Å c = 10.6176(6) Å β = 102.941(6)° V= 473.33(5) Å3 Z=2 I41/acd (142) Tetragonal a=12.11(0) c= 18.63(0) V=2732.61(7) Å3 Z=32

Li2SiP2 ICSD-431573

Li10Si2P6

P21/n (14) monoclinic a= 7.2051(4) b=6.5808(4) c=11.64 β =90.580(4)° 05(7) V=551.91(6) Å3 Z=2

Oligomers/Anions type Connected SiP6 octahedra

7

Li NMR shift X

31

P NMR shift (ppm) -100

Connected SiP4 tetrahedra (3D network) by P atoms

2.7

X

double layers of SiP4 tetrahedra (2D)

3.0

-179 -169 -121 -77

supertetrahedral clusters

2.1

-129.1 -241.5

quasi isolated deformed SiP4 tetrahedra

-2.1

-199 -188 -124


Page 11 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Li8SiP4 ICSD 431370

Pa3¯ (205) cubic a=11.678(1) V=1592.7 Å3 Z=8 (ordered distribution of Li and Si atoms)

Li5SiP3 ICSD 44821

F m -3 m (225) – cubic Antifluorite (statistical distribution of the Li and Si atoms)

Li3P (Li6P2) ICSD 240861

P63/mmc (194)

isolated SiP4 tetrahedra

3.3

-225.3 -251.3

quasi isolated SiP4 tetrahedra

1.4

-230

4

-278

This original mechanism, schematized in Fig. 7, is

based on the high versatility of the LixSiP4 structural units that resemble the SiO4 building blocks of the well-known oxosilicates and in which the tetrahedra may interconnect via vertex- or edge-sharing to form dimers or one-, two- and three-dimensional anionic frameworks, with the alkali cations located in layers or channels.35 In addition, when compared to the mechanism described in an earlier work17 and involving a well defined SiP2 covered with a thick layer of oxidation products, the lithiation mechanism proposed here involves a bulk SiP2 exhibiting quite disordered P local environments. The two mechanisms seem closely related with the final formation of Li3P and LixSi alloys. Nevertheless, the disordered SiP2 seems to yield a more reversible reaction, involving intermediate LixSiyP type environments instead of only LiP. The initial disorder leading to a higher multiplicity of local environments might help the reversibility of the lithiation process, in particular hindering the formation of a significant amount of insulating LiP. SiP2/Na

Figure 8. a) Galvanostatic curve of SiP2/Na at a C/10 rate, the derivative curve in inset, b) capacity as function of the number of cycles and in blue the coulombic efficiency. Although to date the number of ternary phases in the Na/Si/P system is rather limited, Na5SiP3 being the only listed member,36 it is interesting to study the electrochemical reaction of SiP2 versus Na. It is specifically interesting to explore this system because to date Si has shown to be unreactive versus Na. The same electrodes were cycled prepared with an aqueous formulation described in the technical part. The galvanostatic curve of SiP2 vs. Na and the derivative curve are presented in Fig. 8. During the first discharge at C/10 rate more than 4.6 Na react with SiP2 which corresponds to 1370 mAh/g-SiP2 (or 780 mAh/g when considering the SiP2/C electrode), when only 3.7 Na are deinserted in the following charge, which corresponds to a reversible capacity of 1150 mA h/g-SiP2 and an irreversibility around 16%. Differently to SiP2/Li, the capacity fades slower, and is still



at 750 mAh/g after 35 cycles, with a coulombic efficiency close to 100% (Fig. 8-b, S7). The derivative curve is almost featureless and displays two main broad reduction peaks during the first cycle at 0.43 V and 0.05 V, three main oxidation peaks at 0.1 V, 0.25 V and 0.7 V for the first charge. During the second discharge three peaks at 0.01 V, 0.35 V and 0.61 V appear. Operando XRD was collected during the first electrochemical cycle (Fig. S8). During the insertion of the first 3 Na, the Bragg peaks characteristic of SiP2 disappear without any new peak appearing concomitantly, indicating the transformation of the pristine SiP2 into amorphous species. At the end of the discharge, differently to the LIB, no phase crystallized. Figure 9 displays the 23Na MAS NMR spectra obtained during the first cycle. For all spectra, only one set of strongly overlapping resonances is observed. The spectrum corresponding to the end of the first discharge (red) shows a maximum of intensity between 0 and -4 ppm, assigned to local Na environments in the species formed upon reduction of SiP2. The spectrum corresponding to the end of charge displays also a broad resonance which seems to be slightly shifted towards more negative chemical shifts and centered at -8.5 ppm. The ratio of the NMR integrated intensities for the electrodes stopped at the end of discharge and at the end of charge is in agreement with the ratio of specific capacity at the end of discharge and irreversible capacity measured at the end of charge. The set of broad resonances centered at 8.5 ppm is then assigned to the Na ions trapped irreversibly inside or at the surface of the electrode. Additional minor contribution can be seen on several 23Na MAS NMR spectra, in particular as a shoulder at 8 ppm and/or sharper resonances at -9.5 or -16 ppm.

Page 12 of 35

Figure 9. 23Na MAS NMR spectra obtained at middischarge (green), end of discharge (red), at midcharge (orange) and end of charge (blue). The 8 ppm resonance is close to that of NaF, a possible decomposition product of the NaPF6 electrolyte while the sharp resonance appearing between -9.5 and -16, depending on the sample is tentatively assigned to non degraded NaPF6 electrolyte salt. Delville et al. 37 studied the variation of the 23Na NMR chemical shift for solutions with different concentrations of NaPF6 in organic solvents and found that it varied between -9 and 15 ppm depending on the degree of complexation. In the case of non-complexation, a -15 ppm resonance was observed, shifting then towards lesser negative chemical shift to stabilize around -9 ppm for a complete complexation. It is then reasonable to assign the -16 ppm signal to non-rinsed NaPF6 salt at the surface of the electrode and the -9.5 signal to non-degraded NaPF6 salt trapped in the porosity of the SEI or with strong interactions with the SEI or the active material. In order to confirm the presence of the above fluorinated products, 19F NMR has been performed on the electrode stopped at the end of discharge, for which a significant amount of NaF could be expected considering the low potential and for which a signal at 8 ppm appeared clearly in the corresponding 23Na NMR spectrum. It can be seen in the inset of Fig. S9 that no NaF could be detected at its typical chemical shift of 225 ppm. On the contrary, an intense signal at -72 ppm indicates unambiguously the presence of PF6groups and therefore the presence of non-degraded electrolyte salt in the studied samples. Such signal was observed for all the samples, whatever their state


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of charge was. It seems thus that, contrary to the case of the cycling of SiP2 vs Li leading to the formation of LiF in the SEI, no NaF appears when cycled vs Na, at least during the first cycle. The additional contribution observed on the 23Na MAS spectra at 8 ppm is then assigned to another Na environment in a sodiated species formed upon the reduction of SiP2. The 31P MAS NMR spectra acquired during the cycling of SiP2 vs Na are overall very similar to their lithium counterpart (Fig. 10-a, -b). All the spectra show an intense and sharp resonance at -146 ppm, assigned to PF6- groups. Two sharp and intense resonances can be seen at 6 and 13 ppm for the electrodes stopped at mid-charge, mid-discharge and end of discharge. The latter is consistent with the 31P chemical shift of Na3PO4,38 in agreement with the formation of its lithium equivalent when SiP2 is cycled vs Li. The former matches well the chemical shift of Na2HPO4 39 suggesting a partial reaction of surface species such as H3PO4 30 with sodium ions in the electrolyte. Like in the case of their lithiated counterparts, the resonance observed at the end of charge is broader and rises with a slightly lower chemical shift at 4.5 ppm, supporting the partial reversibility of the reaction involving surface environments. Contrary to the lithium case, these signals are not minor contributions to the overall spectrum anymore, suggesting that the surface reaction is more pronounced compared to the bulk reaction, in the case of the cycling vs Na. For all stages of the electrochemical cycling, the extremely broad resonance at -100 ppm, assigned to bulk SiP2 is visible (Fig. 10) and is consistent with the much lower amount of sodium ions reacting per formula unit of SiP2 as measured by coulometry (2.5 Na vs 8 Li). It shows clearly that a significant part of the pristine SiP2 does not react upon the electrochemical process. As a matter of fact, the non-reacted SiP2 is still visible at the end of discharge (Fig. 10c). The figure S3 displays spectra corresponding to SiP2 sample oxidized in ambient atmosphere. The reaction with the ambient atmosphere alters drastically the 31P MAS NMR spectrum of bulk SiP2. The main and broad resonance at -100 ppm decreases and mostly disappears to the benefit of a set of sharp resonances between -20 and 7 ppm (typical 31P NMR chemical shift range for phosphates) and two broader resonances at 80 and 145 ppm are also present. The set of sharp resonances is already present in the spectrum of the pristine SiP2, although with much lower intensity, showing the very high reactivity of the surface of the SiP2 particles. The broad resonances at 80 and 140 ppm were already discussed in the SiP2/Li section and attributed to a consequence of the prolonged storage

of SiP2 in atmosphere. A clear link can be made between the set of sharp resonances in the -23 to 7 ppm range appearing along the oxidation of the pristine SiP2 and the sharp resonances observed in the 6 to 13 ppm range for the cycled samples, supporting the attribution of the latter to surface environments. It seems that some of the environments created at the surface upon reaction with ambient atmosphere can undertake partial reaction with sodium. As a matter of fact the corresponding signal detected at the end of charge is intermediate between that at the end of discharge and that of the “oxidized” surface of SiP2.

Figure 10. a) 31P MAS NMR spectra obtained at middischarge (green), end of discharge (red), end of discharge at 30kHz (violet), at mid-charge (orange) and end of charge (blue) of SiP2/Na. Sidebands are marked by an asterisk, b) magnification of the -220-300 ppm range and c) 31P MAS NMR spectra of pristine SiP2 (green) and SiP2 electrode at end of charge (red).



No shift of the broad -100 ppm resonance could be observed for the samples stopped at mid-discharge and mid-charge. Considering the analogy with the lithium system, it seems that the reaction of SiP2 with sodium ions is somehow more limited to the surface. In particular, the absence of significant intensity at more negative chemical shift suggests that P-P bonds and SiP4 tetrahedra connections are not broken upon discharge and that the structure of the pristine SiP2 keeps its integrity throughout the electrochemical cycling. Several attempts at discharging SiP2 vs Na were performed and analyzed using 31P MAS NMR, leading to slightly different spectra. These results emphasize the inhomogeneity and the difficulty of the reaction when involving sodium, in contrast with the lithium system. As illustrated with the two examples of electrodes stopped at the end of discharge (Fig. 10), the intensities of the broad SiP2 resonance and that of the two sharper resonances at 6 and 13 ppm clearly vary between the two samples indicating that the reaction with sodium occurred to a different extend. Present on the two spectra, appears nevertheless a signal at -235 ppm. This signal is not visible on spectra corresponding to mid-discharge, mid-charge and end of charge. Its presence only for low potential samples and a comparison with the behavior observed in the case of the cycling vs lithium lead to its attribution to a sodium rich phase. It is interesting to compare the spectra collected at the end of discharge with those measured from Na3P, NaP and Na5SiP3 reference samples (Fig. S10) prepared by ball milling (see experimental part). There is no good agreement neither for the 31P nor 23Na MAS NMR spectra. Differently to the lithium Li/Si/P system, the Na/Si/P phases has been less studied, likely due to lower stability of these ternary phases and only Na5SiP3 have been reported. A deeper understanding of the reaction mechanism would thus require syntheses and extensive characterizations of Na/Si/P containing various types of SiP4 unit connections. CONCLUSION Ternary Li/Si/P structures consist of tetrahedral SiP4 anions, which form versatile networks from isolated SiP4 units surrounded by Li counterions to threedimensional network based on corner- or edge sharing SiP4 tetrahedra, with the Li ions located in cavities and channels (See Fig. 7). While ternary silicon phosphides have attracted particular attention owing to their non-linear optical, thermoelectrical, the present study demonstrates that their structural variety allow

Page 14 of 35

very interesting electrochemical properties especially in LIBs. With various Li8SiP4, Li5SiP3, Li3Si3P7, and Li2SiP2 stoichiometries, the tetravalent Si atom is covalently bonded to four P atoms to form SiP4 units: these tetrahedral structural units resemble the SiO4 building blocks of the well-known oxosilicates and the tetrahedra may interconnect via vertex- or edgesharing to form dimers or one-, two- and threedimensional anionic frameworks, with the alkali cations located in layers or channels. On the other hand the substitution of O by P atoms leads to an increase of the anionic charge of the anionic entities in the phosphides. The specific structural feature of the system Li/Si/P is at the heart of the electrochemical mechanism of SiP2/Li leading to the good performance measured for SiP2 used as anode material. Needless to say that further work is needed to optimize this performance, especially by working on the electrode and electrolyte formulations. Furthermore, both lithium mobility and ion conductivity in these structures appear as an asset for their use in Li batteries. Although one phase is isostructural in the Na/Si/P system (Na5SiP3), we have not been able to identify the same electrochemical mechanism in SiP2/Na than for SiP2/Li. Further work is needed to clarify the electrochemical processes of SiP2 in NIB. While silicon, one of the most attractive negative electrode for LIBs, was not found to be electrochemically active in NIBs, its combination with phosphorus in the rich chemistry provided by phosphidosilicates phases could help in activating its electrochemical activity versus sodium. Finally, this study illustrates the relation linking knowledge of the crystal structure of intermediate Li(Na)/Si/P phases and the stability of connections between SiP4 units / intrinsic stability of SiP4 units towards electrochemical reaction vs Li (Na). This approach is crucial for the comprehension of the simultaneous electrochemical oxydo-reduction of Si and P in phosphides vs Li and its limitations vs Na. This new mechanism investigated here is also less detrimental than the classical alloying/dealloying or conversion reaction which lead to extreme volume expansion with dramatic consequences on the cycling performance. The study of possibilities offered by this new electrode family is in its infancy and an in-depth understanding of this original electrochemical mechanism will support the development and improvement of performance of these electrodes in the near future.


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■ AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions All authors contributed equally to this work. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This research was performed in the framework of ALISTORE-ERI, “Réseau sur le Stockage Electrochimique de l’Energie” (RS2E) and the ANR program no. ANR-10-LABX-76-01. ■ ASSOCIATED CONTENT Supporting Information Available: Details on the synthesis method (ball milling), the performance in batteries, NMR measurement, XRD characterisation.



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Density Distribution. Zeitschrift für Krist. 1984, 167 (1–2), 1–12. Leriche, J. B.; Hamelet, S.; Shu, J.; Morcrette, M.; Masquelier, C.; Ouvrard, G.; Zerrouki, M.; Soudan, P.; Belin, S.; Elkaïm, E.; Baudelet, F. An Electrochemical Cell for Operando Study of Lithium Batteries Using Synchrotron Radiation. J. Electrochem. Soc. 2010, 157 (5), A606. Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calvé, S.; Alonso, B.; Durand, J. O.; Bujoli, B.; Gan, Z.; Hoatson, G. Modelling One- and Two-Dimensional Solid-State NMR Spectra. Magn. Reson. Chem. 2002, 40 (1), 70–76. Brauer, G.; Zintl, E. Konstitution von Phosphiden, Arseniden, Antimoniden Und Wismutiden Des Lithiums, Natriums Und Kaliums. Zeitschrift für Phys. Chemie 1937, 37B (1). Dong, Y.; Disalvo, F. J. Reinvestigation of Trilithium Phosphide, Li3P. Acta Crystallogr. Sect. E Struct. Reports Online 2007, 63 (4), i97–i98. Nazri, G. Preparation, Structure and Ionic Conductivity of Lithium Phosphide. Solid State Ionics 1989, 34 (1–2), 97–102. Ogata, K.; Salager, E.; Kerr, C. J.; Fraser, A. E.; Ducati, C.; Morris, A. J.; Hofmann, S.; Grey, C. P. Revealing Lithium–silicide Phase Transformations in Nano-Structured Silicon-Based Lithium Ion Batteries via in Situ NMR Spectroscopy. Nat. Commun. 2014, 5, 1–11. Key, B.; Bhattacharyya, R.; Morcrette, M.; Seznec, V.; Tarascon, J. M.; Grey, C. P. RealTime NMR Investigations of Structural Changes in Silicon Electrodes for Lithium-Ion Batteries. J. Am. Chem. Soc. 2009, 131 (26), 9239–9249. Loaiza, L. C.; Salager, E.; Louvain, N.; Boulaoued, A.; Iadecola, A.; Johansson, P.; Stievano, L.; Seznec, V.; Monconduit, L. Understanding the Lithiation/delithiation Mechanism of Si 1−x Ge X Alloys. J. Mater. Chem. A 2017, 5 (24), 12462–12473. Johnston, K. E.; Sougrati, M. T.; Stievano, L.; Darwiche, A.; Dupré, N.; Grey, C. P.; Monconduit, L. Effects of Relaxation on Conversion Negative Electrode Materials for Li-Ion Batteries: A Study of TiSnSb Using 119 Sn Mössbauer and 7Li MAS NMR

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19


Figure 1. a) XRD pattern with indexed peaks of SiP2 and crystallographic description. The red line corresponds to the experimental pattern, the black line to the calculated pattern, the green sticks to the theoretical SiP2 Bragg position and the blue line to the difference between experimental and calculat-ed patterns, b) SEM picture of the ball milled SiP2 powder, and corresponding EDS mapping. 338x190mm (96 x 96 DPI)


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Figure 1. a) XRD pattern with indexed peaks of SiP2 and crystallographic description. The red line corresponds to the experimental pattern, the black line to the calculated pattern, the green sticks to the theoretical SiP2 Bragg position and the blue line to the difference between experimental and calculat-ed patterns, b) SEM picture of the ball milled SiP2 powder, and corresponding EDS mapping. 338x190mm (96 x 96 DPI)



Figure 2. a) Galvanostatic curve of SiP2/Li at a C/4 rate and the derivative curve in inset, and b) corresponding capacity retention as function of number of cycles. 338x190mm (96 x 96 DPI)


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Figure 2. a) Galvanostatic curve of SiP2/Li at a C/4 rate and the derivative curve in inset, and b) corresponding capacity retention as function of number of cycles. 338x190mm (96 x 96 DPI)



Figure 3: Operando XRD pattern for SiP2/Li recorded at a C/8 rate during the first discharge and charge. 297x209mm (75 x 75 DPI)


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Figure 4. Volume of the hexagonal cell of the phase formed in the last part of the discharge and beginning of the subsequent charge, as function of Li inserted in the SiP2 electrode. Yellow and red stars give the volume of the hexagonal cell of Li3P from the works of DiSalvo et Nazri et al respectively.24,25 338x190mm (96 x 96 DPI)



Figure 5. a) 7Li MAS NMR spectra obtained at mid-discharge (green), end of discharge (red), at mid-charge (orange) and end of charge (blue), b) 19F MAS NMR spec-tra obtained at the end of discharge (violet) and end of the subsequent charge (orange) of SiP2/Li. 338x190mm (96 x 96 DPI)


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Figure 5. a) 7Li MAS NMR spectra obtained at mid-discharge (green), end of discharge (red), at mid-charge (orange) and end of charge (blue), b) 19F MAS NMR spec-tra obtained at the end of discharge (violet) and end of the subsequent charge (orange) of SiP2/Li. 338x190mm (96 x 96 DPI)



Figure 6. a) 31P MAS NMR spectra at different steps of the first cycle. 338x190mm (96 x 96 DPI)


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Figure 6. a) 31P MAS NMR spectra at different steps of the first cycle, b) 31P MAS NMR spectra of pristine SiP2 (green) and SiP2 electrode at end of charge (blue). 338x190mm (96 x 96 DPI)



Figure 7. Outline of the SiP2 reaction with Li 297x209mm (75 x 75 DPI)


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Figure 8. a) Galvanostatic curve of SiP2/Na at a C/10 rate, the derivative curve in inset, b) capacity as function of the number of cycles and in blue the coulombic efficiency. 338x190mm (96 x 96 DPI)



Figure 8. a) Galvanostatic curve of SiP2/Na at a C/10 rate, the derivative curve in inset, b) capacity as function of the number of cycles and in blue the coulombic efficiency. 338x190mm (96 x 96 DPI)


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Figure 9. 23Na MAS NMR spectra obtained at mid-discharge (green), end of discharge (red), at mid-charge (orange) and end of charge (blue). 338x190mm (96 x 96 DPI)



Figure 10.a)

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P MAS NMR spectra obtained at mid-discharge (green), end of discharge (red), at mid-charge (orange) and end of charge (blue). Sidebands are marked by an asterisk. 338x190mm (96 x 96 DPI)


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Figure 10. b) magnification of the -220-300 ppm range and c) 31P MAS NMR spectra of pristine SiP2 (green) and SiP2 electrode at end of charge (red). 338x190mm (96 x 96 DPI)


P system as efficient anode for Li and Na- batteries

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