Improving the Li-Electrochemical Properties of Monodisperse Ni2P

Article pubs.acs.org/cm

Improving the Li-Electrochemical Properties of Monodisperse Ni2P Nanoparticles by Self-Generated Carbon Coating S. Carenco,†,§ C. Surcin,‡,§ M. Morcrette,‡,§ D. Larcher,‡,§ N. Mézailles,∥ C. Boissière,† and C. Sanchez†,* †

Laboratoire de Chimie de la Matière Condensée, Collège de France, University Pierre et Marie Curie (UPMC), CNRS UMR 7574, Place Berthelot, 75005 Paris, France ‡ Laboratoire de Réactivité et Chimie des Solides, Université de Picardie Jules Verne, CNRS UMR6007, 33 rue Saint Leu 80039 Amiens, France § ALISTORE−European Research Institute ∥ Laboratoire Hétéroéléments et Coordination, Ecole Polytechnique, CNRS UMR7653, Route de Saclay, 91128 Palaiseau, France S Supporting Information *

ABSTRACT: Carbon coating of electrode materials is nowadays a major tool to improve the electronic percolation of the electrode. In this study, a self-generated carbon coating is described as a new way to deposit a regular thin layer of carbon on the surface of nanoparticles. It relies on the soft decomposition of the nanoparticles surface native ligands, containing alkyl chains, under inert atmosphere at 400 °C, a route particularly suited for oxidation-sensitive nanoparticles. Using 25 nm monodispersed Ni2P nanoparticles as a model phase, we succeeded in forming nonsintered and nonoxidized carbon-coated nanoparticles. The carbon coating is then tuned in thickness by modifying the ligands set. Electrochemical properties of the resulting Ni2P/C nanoparticles vs Li are compared with those of bulk Ni2P. Both materials are shown to undergo a conversion reaction. The capacity of the bulk material collapses after a few cycles while Ni2P/C nanoparticles show much better retention. The self-generated carbon coating is thus found to promote Li uptake by providing a Li-permeable electron-conductive percolating network and by improving the mechanical integrity of the electrode. KEYWORDS: carbon coating, nickel phosphide nanoparticles, electrochemical properties, negative electrode, nanostructured electrode, ligand decomposition, self-generated carbon shell

1. INTRODUCTION The quest for high-capacity lithium batteries is rooted in the growing need for electronic portable devices and their potential use in electric vehicles. Improving the negative electrode materials is a major challenge in this field.1,2 Since the early 2000s, metal phosphides have raised interest in this purpose, because they can undergo either intercalation3 or conversion4 reactions, associated with high gravimetric capacities5 despite a high reaction voltage (ca 1 V vs Li+/Li0), relative to the Ligraphite system. So far, numerous phases of binary and ternary metal phosphides have been used to build negative electrodes,6 such as MnP4,3 CoP3,4 ZnP2,7 Zn3P2,8 InP, GaP,9 Cu3P,10 VP,11 VP2,12 VP4,13 FeP,14 FeP2, and FeP4.15 Meanwhile, nanoscaling of electrode materials has been attempted both for negative and positive electrodes, with sometimes impressive increase in performances. P. Bruce et al. and A. S. Aricò et al. have summarized the major features that have been encountered so far.16,17 In particular, nanoscaling of the electrode material is expected to enable reactions that would not occur in the bulk, increasing the kinetics of Li incorporation/removal because of shorter transport distance and higher contact area with the electrolyte. Additionally, it may provide a wider range of solid solution for the material investigated.18 © 2012 American Chemical Society

Yet, using nanoparticles as electrodes is not straightforward. Indeed, their cohesion has to be achieved from both mechanical and electronic point of view to ensure long-term cyclability and efficient electron wiring to the current collector (electronic percolation). This has been achieved in the past few years by coating the nanoparticles with a layer of carbon. Several methods were developed for the fabrication of well-percolated conductive carbon/active nanopowder composites or even graphene-wrapped nanoparticles. Most of them rely on the deposition or growth of nanoparticles on nanostructured carbon precursors, such as graphite layers,19 carbon nanotubes,20 graphite oxide,21 and mesoporous carbon.22 Fewer routes rely on the use of molecular sources of carbon, such as glucose,23 sucrose in supercritical CO2,24 pluronic copolymer (P123),25 Li2CO3,26 and polyacene.27 More importantly for our purpose, a large majority of reported carbon-coated and carbon-supported systems28 are positive and low-conductivity electrode materials (such as LiFePO4/C composites, as exemplified by the references chosen in the last sentence). These electrodes are generally less sensitive to oxidation or Received: October 23, 2011 Revised: January 25, 2012 Published: January 25, 2012 688

dx.doi.org/10.1021/cm203164a | Chem. Mater. 2012, 24, 688−697

Chemistry of Materials

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Figure 1. Electrochemical behavior of the electrodes. (a) Bulk Ni2P (15% C-SP) at C/20 rate (first 4 cycles on top and gravimetric capacity evolution over 50 cycles on the bottom). (b) Ni2P-amorph sample after calcination under N2 at ca 400 °C. Cycling rate: C/10.

cycles. Cubic NiP2 exhibited interesting performances by undergoing conversion to Li3P and Ni. However, monoclinic NiP2, obtained at high temperature (900 °C) from elemental Ni and P, was found to be the most interesting candidate so far.38 Indeed, it starts to react though an insertion process and switches to a conversion reaction.46,42,43 The latter charge/ discharge cycles work with a conversion reaction and are fairly reversible for 5 Li per Ni, but drops because of electrode fragmentation. Optimization of the performances was obtained by supporting the NiP2 monoclinic phase on a nickel foam, giving a very good reversible capacity of ca 900 mAh/g.38 Nanostructuration of the electrode into nanowires was also proposed, as a first step toward nanoscaling.44 Tatsumigo and co-workers proposed the use of Ni5P4 and NiP2 submicronic nanoparticle aggregates in all-solid-state batteries with (Li2S)80(P2S5)20 to enhance the performances of the electrodes.45 In contrast with this, NixPy phosphorus-poor phases (Ni/P ≥ 1) were only scarcely evaluated as negative electrodes. In the phases obtained by solid-state reaction at 900 °C, Ni3P, Ni2P, Ni12P5, Ni5P4, and NiP, only the last two phases were reported to react with lithium.46 Additionally, Ni3P thin films (400 nm) were electrodeposited and treated at 500 °C, giving agglomerates of nanosized particles (70 nm).47 The films underwent incomplete conversion reaction to Li3P and Ni, providing a capacity of 2 Li per Ni (ca 260 mAh/g) at the first discharge. However, only a limited capacity was recovered at the first charge (75 mAh/g). Interestingly, no plateau could be observed during the charge and the discharge. Altogether, metal-rich phosphides (Ni3P, Ni5P4, and NiP) exhibit a stronger metallic character and a higher proportion of Ni−Ni bonds vs Ni−P and P−P bonds than monoclinic NiP2, which makes the reaction potential lower in average, a feature of the negative electrode that is intrinsically interesting for

carbide formation than negative electrode materials, for which only a few systems have been reported (mainly Si/C,29,30 SnO2/C31,32 and Si/C33), often using glucose or sucrose as the main carbon source. Yet, there is still a need for new carboncoating methods that would be compatible with nanoparticles highly sensitive to oxidation and that would ensure a conformal coating, percolating through the electrode, without causing nanoparticles sintering. In the present work, we described a straightforward selfgenerated carbon coating protocol, using well-defined 25 nm Ni2P nanoparticles as a model system. It relied on a relatively soft thermal treatment (400 °C, 30 min) that decomposed the nanoparticles native ligands to form a thin carbon layer (ca 2−3 nm) without sintering of the nanoparticles. Electrochemical properties of the Ni2P/C composite negative electrode were evaluated and compared with those of bulk Ni2P and noncoated nanoparticles, this phase being reported here as electrochemically active vs Li for the first time. The transformations of the nanoparticles (size, shape, and crystal structure) were scrutinized by systematic post-mortem analyses, in order to propose a reaction pathway for Ni2P.

2. RESULTS AND DISCUSSION Only a few studies have been conducted using metal phosphide nanoparticles (MoP2,34 GaP,35 SnP0.9436) by J. Cho and coworkers, and have illustrated a range of behaviors (intercalation or alloying with Ga). However, the influences of particles nanometric size and surface state were not discussed at that time. On the other hand, several bulk nickel phosphide phases were reported as negative electrodes, such as Ni3P,37 NiP2,38−40 or NiP3.41 Among them, phosphorus-rich phases (Ni/P < 1), were the most studied: a large initial capacity of 1475 mAh/g was obtained for NiP3 but it dropped dramatically after a few 689



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Figure 2. Electrochemical behavior of the electrodes. (d) PITT cycling mode of the calcined Ni2P-amorph sample, containing carbon-coated Ni2P nanoparticles (samples 1 and 2 come from the same batch). (e) Ni2P-PBu3 after calcination.

Figure 3. Electrochemical behavior of the electrodes. (c) First cycles of the three as-made nanoparticles samples (cycling rate: C/10). (f) Calcined Ni2P-PBu3 nanoparticles; the carbon SP was added after the calcination. (g) Calcined Ni2P-PBu3 nanoparticles; the carbon SP was added before the calcination. (h) Calcined Ni2P-OA sample at a C/20 cycling rate.

the effect of nanoscaling and nanoparticle surface treatment on the electrochemical properties of metal phosphides. 2.1. Surface-Clean Bulk Ni2P Phase. Prior to the study of Ni2P nanoparticles, we investigated the electrochemical

energy storage. Because the Ni2P phase has been the most studied as the nanoscale and is now mature (highly reproducible syntheses, fine control of the nanoparticles size and surface),48,49 we choose it as a model material to investigate 690



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and the electrode capacity collapsed after a few cycles, the activity of the Ni2P phase vs lithium could be observed. 2.2. Electrochemical Properties of Nanoparticles Covered with a Ligand Shell. Advantageously, the comparison between bulk and nanoscaled Ni2P could be made in the best conditions because the same synthetic methodology, using P4 as a phosphorus source, can be applied to produce Ni2P nanoparticles. Monodisperse Ni2P nanoparticles (25 nm diameter) were indeed synthesized, as described in a previous work (Figure 5), by adding white phosphorus on nickel nanoparticles.53 Very briefly, monodisperse 25 nm nickel nanoparticles were reacted at 220 °C on 1 /8 equiv. P4. The reaction resulted in the quantitative insertion of phosphorus in the nanoparticles and the recrystallization of the Ni2P phase, each nanoparticle being a single crystal. Aggregation and extensive growth of the nanoparticles during the synthesis was prevented by the presence of tri-noctylphosphine ligands on their surface (Ni2P-TOP sample). For the purpose of comparison, an amorphous Ni2P sample was also prepared, by heating at 120 °C for 15 min instead of 220 °C for 2 h (Ni2P-amorph; see Figure 1 in the Supporting Information). Lastly, when TOP was not introduced, larger polycrystallized Ni2P nanoparticles (50−80 nm diameter) were obtained (Ni2P-OA sample; OA: oleylamine).53 All samples were tested after copious washing of the excess surface ligands. It must be noted that the closest layers of ligands, coordinated to the surface, resisted this washing, as described in a previous work.54 In the absence of an electronic conducting additive, no reaction was observed. As in the case of bulk Ni2P, the powders were thus mixed with 15% mass carbon SP by soft manual grinding in the glovebox. Upon cycling, all samples exhibited similar behavior: an irreversible reaction with modest amounts of Li, followed by a very poor reversible contribution, which came from the carbon SP part (Figure 3c). XRD analyses of the samples after cycling indicated no modification of the structure (whether crystalline or amorphous), which correlates with the fact that the nanoparticles did not react with Li+ in these conditions. In an attempt to understand this failure, TEM observations were performed (Figure 6). They showed the presence of large aggregates of nanoparticles that were not directly in contact with carbon SP. Thus, we reasoned that the conductive additive may not percolate properly in the electrode, leaving most of the nanoparticles without an electronic connection, even in the presence of carbon SP. Several procedures were attempted to promote a better physical mixing of the two powders: a short ball-milling (30 min), an impregnation of the nanoparticles with the carbon in cyclohexane under stirring, and the addition of larger amounts of carbon (20% mass). However, they were not successful, suggesting that a incomplete percolation was not the sole problem. We therefore reasoned that the layer of aliphatic ligands on the nanoparticles surface may be responsible for the lack of Li reaction. Indeed, the presence of long alkyl chains from TOP and/or OA may prevent the electrolyte from wetting the nanoparticles surface and the solvated Li+ ions from accessing the inorganic core of the nanoparticles. This ligand layer could not be washed more thoroughly, as mentioned before, because it was strongly coordinated to the surface. 2.3. Self-Generated Carbon Coating by Calcination. Because its complete removal is not reachable, we attempted to transform this organic coating without altering the Ni2P phase, the samples were treated by a short calcination under inert

response of surface-clean bulk Ni2P, as a benchmark compound to be compared with carbon-coated nanoparticles. Considering the reaction modes of the other nickel phosphides, the following conversion reaction was expected for Ni2P: Ni2P + 3Li → 2Ni + Li3P (1) Surface-clean bulk Ni2P was prepared, as described in previous works, by the reaction of white phosphorus (P4), a reactive source of P atoms, on M(0) species.49,50 Very briefly, the reaction of Ni(COD)2 (COD: 1,5-cyclooctadiene) and 1 /8P4 produced an amorphous Ni2P powder, then all the COD was evacuated under vacuum. The remaining solid, which contains no more organic components, was annealed under nitrogen to induce the crystallization of Ni2P. Large micronic aggregates were obtained, though the crystallites size was only ca 20 nm.51 When introduced as such in a two-electrode Swagelok-type cell with a counter electrode of lithium, the Ni2P powder presented no electrochemical reaction in a potential window of 0 to 2 V vs Li. This was not surprising as Ni2P is not an electron conductor. The fine powder was therefore mixed with 15% mass carbon SP (a conductive additive) and cycled in the 0−2 V range. At the first discharge, a capacity of ca 500 mAh/g was reached (Figure 1a). The corresponding x value in LixNi2P was close to 3, as expected for the conversion reaction proposed in eq 1, although part of this lithium consumption is to be expected from reaction with the electrolyte. However, only 50% of the initial capacity was restored at the first charge. This lack of reversibility carried on over the next cycles, resulting in a dramatic drop of capacity. After 20 cycles, the remaining capacity corresponded to the contribution of carbon SP (30 mAh/g). Moreover, there was no well-defined plateau over the first cycles. This might suggest a solid solution behavior. Alternatively, this could be a consequence of the covalence of the Ni−P bond: the potential of reaction was reported to continuously decrease with the phase composition (x value in LixNi2P) because of the strong electronic exchanges between its components.5 XRD analysis after the last charge step showed a destruction of the initial Ni2P structure (Figure 4). The powder was now

Figure 4. XRD analysis of bulk Ni2P before and after cycling (sample taken out of the battery at 2 V).

majorly an amorphous Ni-based structure, as attested by the broad signal from 40 to 50°. It showed one relatively sharp peak at 45° that is likely due to the formation of poorly crystallized Ni−P mixed phases,52 while the broad peak centered at 47° likely corresponded to poorly crystallized Ni−P compounds. Altogether, this experiment showed that, in our conditions, the Ni2P phase was prone to react with lithium in a significant manner. Even though the reversibility of the reaction was poor 691



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Figure 5. TEM observation of the three Ni2P nanoparticles samples.

Interestingly, the appearance of a nice, relatively regular, coating of 2−3 nm thickness was evidenced by TEM observations (Figure 8). It appeared to be quite porous with

Figure 8. TEM observations of the calcined Ni2P-amorph sample before cycling. Figure 6. TEM observation of the Ni2P-TOP electrode after cycling. Aggregates of Ni2P nanoparticles that do not incorporate carbon SP (such as in the lower-right corner) are numerous. Regions where nanoparticles are in contact with the carbon SP, as illustrated on the center of the figure, are a minority.

pore size in the subnanometer range. This self-generated shell was relatively robust under electron beam, as expected from a graphitic-like carbon species. Some small domains of layered structures could be found (spacing of ca 0.4−0.5 nm). This shell is composed of carbon from the ligands decomposition (with some possible doping by nitrogen or phosphorus from the ligands). The formation of active graphitic-like species upon calcination is likely catalyzed by the Ni2P nanoparticles, as suggested by a report that mentions the catalytic growth of carbon nanotubes on NiP amorphous nanoparticles.55 To get information on the nature of this thin carbon coating, a surface-dedicated technique was required. XPS analysis was thus performed on the Ni2P/C nanoparticles (Figure 9). Surface composition confirmed the presence of graphite-like carbon (B.E. of 284.6 eV for C1s).56 Only a small proportion of the carbon (ca 23%) is in an oxidized state in carbonyl-like moieties. However, no carbide-like species was detected (Ni3C would have appeared at 283.90 eV), confirming that the Ni2P phase was not affected by the thermal treatment. Moreover, a relatively high proportion of phosphorus was observed on the surface of the sample (19% atom). It likely comes from the decomposition of the phosphine ligands. Interestingly, it is found both as phosphorus oxides (134.0 and 134.7 eV) and as reduced species (P elemental-like type at 130.4 eV and Ni2P phosphorus at 129.5 eV). Since the carbon-coating step was conducted in the absence of oxygen, phosphorus oxides were formed either before it, by oxidation of the surface phosphine that acted as a scavenger toward oxygen during the washing of the nanoparticles,54 or by secondary oxidation of P in the graphitic network upon air-exposure of the sample. Lastly, the Ni2P spectrum showed the presence of the expected Ni2P as the major component (48%). Altogether, XPS confirmed the

atmosphere. Amorphous nanoparticles (sample Ni2P-amorph) were heated as a powder in a Schlenk tube for 30 min at 400 °C, under N2. The powder stayed black under this treatment, which was consistent with the remaining of nonoxidized species. Complete characterization of the sample was then performed. After calcination, the XRD diagram indicated the formation of the Ni2P phase, as expected at such temperature (Figure 7, bottom).53 At this point, it must be noted that a

Figure 7. XRD diagram of calcined Ni2P-amorph sample, before and after cycling.

similar calcination process applied to crystallized Ni2P nanoparticles (Ni2P-TOP) lead to similar powders and electrochemical properties than exposed below. 692



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Figure 9. XPS analyses of Ni2P/C nanoparticles (area values are given in percent). Complementary spectra can be found in the Supporting Information.

Figure 10. TEM observation of the electrode after cycling (left) and typical autocorrelation analysis. (right) of the gray crystallized matrix in which the Ni nanoparticles are entrapped.

(Figure 1b) were very similar those observed with bulk Ni2P (Figure 1a). Finally, post-mortem analysis unraveled the transformation of the 25 nm Ni2P nanoparticles upon Li intake. Indeed, TEM observations after cycling revealed the presence of small contrasted nanoparticles in an amorphous beam-sensitive matrix (Figure 10 and Supporting Information Figure 2) that prevented the use of selected area electron diffraction (SAED). Their dark appearance indicated that they contained nickel atoms. Because a potential of 0 V was reached (Figure 1b), the conversion also yielded fully reduced nickel nanoparticles at the end of the discharge. Additionally, the increasing and broadening of the peak at 47.4° observed on the XRD patterns after cycling (Figure 7, top spectrum), which indicated the formation of back-formed NixPy nanoparticles, which may also be observed here. Additionally, the carbon shell was still observed by TEM and a gray matrix, surrounding the nanoparticles, showed the presence of crystallized domains sensitive to beam-exposure (Figure 10). They were thus identified using an autocorrelation treatment of the region of interest in a low-magnification picture (Gatan software). Analyses of several regions showed lattice distances of 0.47 and 0.38 nm (among others), suggesting the presence of the LiPO3 (d = 0.472 nm, major diffraction peak) and Li3PO4 (d = 0.380 nm, major diffraction peak) phases. These phases appeared from LiP and Li3P phases oxidation upon air exposure of the sample, during their transfer from the glovebox to the TEM sample-holder.

formation of graphitic-like carbon that did contain significant amounts of phosphorus. The Ni2P/C nanoparticles were gently crushed by manual milling and introduced in an electrochemical cell, without any carbon SP. The electrochemical behavior was profoundly affected by this carbon shell, as can be seen in Figure 1b. The self-generated carbon revealed to be permeable to Li+ ions and ensured good electron conductivity to the current collector, even in the absence of additional carbon SP. Thus, the calcined nanoparticles clearly reacted with Li, in contrast with noncalcined ones. Altogether, the capacity seemed to stabilize at ca 75 mAh/g, a capacity solely due to the carbon-coated Ni2P nanoparticles since no conductive additive (carbon SP) had been added to these samples. It could not be excluded at this point that this newly formed carbon shell would be partially responsible for the Li uptake of the composite electrode. To evaluate the carbon coating contribution on the total capacity of the electrode, 25 nm nickel(0) nanoparticles exhibiting the same ligand set were calcined and cycled at a C/20 rate to produce carbon-coated Ni nanoparticles. In these nanoparticles, Ni2P was not present, which made the carbon the only potentially active compounds. A capacity of 20 mAh/g was obtained at the first discharge. Then, the capacity immediately dropped to 2 mAh/g at the first charge. Thus, the contribution of the coating on the overall capacity was confirmed to be negligible in front of the contribution of the Ni2P phase. Accordingly, the potentials observed on charge and discharge for the calcined Ni2P sample 693



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of the electrode (Figure 3h). An initial capacity of 200 mAh/g was obtained at the first discharge and a reversible capacity of ca 50 mAh/g was observed. There was no striking size-effect on the properties in this case, though the x value after the first discharge was slightly smaller here. This was likely due to diffusion limitation in larger nanoparticles, an effect that would be compensated by a slower cycling regime. The carbon coating that appeared on the Ni2P-TOP nanoparticles was quite thick, meaning longer diffusion pathways for Li and lesser accessibility of the nanoparticles. We reasoned that using lighter phosphine ligands, with shorter alkyl tails, it may be possible to enhance the performances of the electrode. New Ni2P nanoparticles were thus prepared using PBu3 instead of TOP (sample Ni2P-PBu3). The nanoparticles exhibited the same diameter range as before (ca 25 nm). After calcination, the carbon coating was found to be slightly thinner (ca 1.5 nm; see Supporting Information Figure 5). In terms of electrochemical properties, the initial capacity of 450 mAh/g (Figure 2e) was found to be higher than before and near those of surface-clean bulk Ni2P (ca 500 mAh/g), indicating a better accessibility of the nanoparticles to the Li+ ions than for the calcined Ni2P-amorph electrode. The cycling rate was found to influence mainly the first 10 cycles. Upon further cycling, the same slow decrease in capacity was observed. This correlates well with a conversion mechanism: the first discharge is critical because the kinetic and thermodynamic barriers for the formation of metal nanoparticles must be overcome. However, a slower cycling rate also promotes secondary reactions of the electrode with the electrolyte by increasing the reaction duration. Afterward, the materials are trapped within a matrix containing LixP phases and products of decomposition from the electrolyte. The overall properties were found to be interesting, but the capacity could not be stabilized. The self-generated carbon coating was indeed very efficient to improve interparticular electric contact, but it seemed that the contact with the current collector was not good enough yet. To improve it, carbon SP was reintroduced in the samples, either before, or after the ligand transformation by calcination; the order was found to have no consequence on the electrochemical properties. Figure 3f and Figure 3g show the electrochemical properties of calcined Ni2P-PBu3 nanoparticles with 15% carbon SP. With a slow cycling rate of C/20, better Li uptake were observed. XRD after cycling at C/10 (right) was performed (the cycling was stopped at 0 V). It showed the same feature as that noticed before (Supporting Information Figure 6): a striking consumption of the Ni2P phase and the appearance of a peak near 47°. TEM observations showed, as before, the formation of Ni nanoparticles (Supporting Information Figure 6). Altogether, the addition of carbon SP was slightly beneficial to the overall capacity of the electrode by improving long-range electron transfer, while the self-generated carbon coating provided local connectivity. 2.7. Mechanistic Considerations. Altogether, interesting electrodes could be obtained from carbon-coated Ni2P nanoparticles. Interestingly, two recent studies also mentioned it as an active phase,57,58 highlighting the growing interest in carbon-coated nanophosphides. A soft calcination step was found to be the key by (i) modifying the surface properties of the nanoparticles, (ii) making them accessible to Li+, and (iii) ensuring a good “electronic wiring”, even in the absence of carbon SP. In the presence of this last additive, a practically

To sum up, the carbon-coated Ni2P nanoparticles underwent a conversion reaction in the electrochemical cell. This reaction yielded Ni nanoparticles and LixP phases. The electrode had enough mechanical coherence to undergo several cycles without collapsing. The good electrochemical behavior achieved here was a direct consequence of the calcination under N2 of the monodispersed nanoparticles: (i) During the thermal treatment, there was no direct sintering of the particles, which would have led to polydispersity; instead, a percolating carbon shell, from ligand decomposition, wrapped the nanoparticles. It not only provided an intimate contact between the carbon and the active phase but also between nanoparticles. (ii) This carbon coating ensured Li+ diffusion, electronic conduction, and a certain degree of mechanical robustness upon cycling, while the initial alkylphosphine ligands were not permeable to Li+ and/or prevented the electron conduction even in the presence of carbon SP. As a result, the properties of the nanoparticles-based electrode were much better than the properties of the reference Ni2P-bulk compound, in terms of reversibility. However, less lithium could be accommodated in the electrode, which will be discussed below. To gain understanding on this last point, another electrode was prepared and cycled using the potentiostatic intermittent titration technique (PITT). In this technique, the potential is imposed but it describes very small steps (10 mV); a new potential is applied when the current reaches a very small value, fixed a priori. Thus, the electron injection in the electrode is much softer, and the system tends to its equilibrium at each step. In these very slow conditions, a reversible capacity of 150 mAh/g was reached (Figure 2d). XRD analyses on the electrodes stopped at 0 or 2 V were performed (Supporting Information Figure 3). For both samples, a broad signal (40−50°) was present and testified of the formation of additional species (small Ni nanoparticles or other amorphous phases). For sample 1 (only one cycle), only the Ni2P phase was observed. For sample 2 (10 cycles), the Ni2P phase practically disappeared, which was compatible with its quantitative consumption in the conversion reaction. A new peak appeared at 47.4°, the same position than noticed before. The theoretical Li uptake (x = 3) was not reached, which suggested that, in the absence of carbon SP, some portions of the electrode were not surrounded by a good electron conductor. Additionally, the charge−discharge plots did not show any potential plateau, indicating that the reaction could work through a solid-solution mechanism. However, this could also be due to the highly covalent character of the Ni2P phase, compared with the NiP2 ones, which may favor a direct implication of the P centers in the redox reaction.5 2.6. Flexibility and Tailoring of the Self-Generated Carbon Coating. To better understand the formation of the carbon coating, the Ni2P-OA sample (50−80 nm nanoparticles with oleylamine ligands and without phosphine) was also calcined under nitrogen and compared with the native sample. A relatively regular carbon coating was observed on the polydispersed nanoparticles (Supporting Information Figure 4). This indicated that amine ligands, as well as phosphine ligands, were well suited for self-generated carbon coating. Here also, this coating was associated with drastically enhanced properties 694



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though the gravimetric capacities are still limited, (iv) the percolated carbon coating was found not only to restore but also to improve the electrochemical properties of the Ni2P as the nanoscale.

complete consumption of the Ni2P was obtained at the first discharge, producing Ni(0) nanoparticles and Li3P. At this point, it may be argued that the nickel nanoparticles might only act as catalysts for the further transformation of Li3P to LiP upon cycling (eq 2). Li3P → LiP + 2″P″

4. EXPERIMENTAL SECTION

(2)

Nanoparticles Synthesis. All reactions were carried out under nitrogen atmosphere using standard air-free techniques.60 First, nickel nanoparticles were synthesized according to a literature procedure.54 Briefly, Ni(acac)2 (2.00 g, 7.80 mmol) was added to 78.0 mmol of oleylamine (20.8 g, 10 equivalents) and 6.24 mmol of TOP (2.30 g, 0.8 equivalents). The mixture was degassed at 100 °C, and heated at 220 °C for 2 h under inert atmosphere, giving quickly a black solution. After 2 h, the heating was stopped and the solution left to cool down to room temperature. The Ni(0) nanoparticles solution was used as such for the second step. Second, Ni2P nanoparticles were obtained according to a literature procedure.49,53 Very briefly, a P4 in solution in toluene (13.0 mL, concentration in P: 0.3 mol/L, 0.5 equivalent in P) was added to the as-synthesized solution of Ni nanoparticles.61 The toluene was evaporated at 60 °C under vacuum and the solution was heated under nitrogen at 220 °C for 2 h for the preparation of crystallized Ni2P nanoparticles. To obtain Ni2P amorphous nanoparticles, the solution was heated at 120 °C for 15 min. The mixture was cooled to room temperature and centrifuged after the addition of 40 mL of acetone to give a black product. The nanoparticles could be easily redispersed in hexanes to prepare the TEM grids (deposition of one drop of the colloidal solution on a copper grid). The carbon coating was obtained by heating of the nanoparticles black powder in a Schlenk tube under N2, at 400 °C for 30 min. After cooling down, the resulting black powder aggregates were gently crushed manually for 10 s and stored under inert atmosphere. Characterization. Powder XRD measurements were performed with a Bruker D8 X-ray diffractometer operating in the reflection mode at Cu Kα radiation with 40 kV beam voltage and 40 mA beam current. Transmission electron microscopy (TEM): Samples were prepared by evaporating a drop of hexanes diluted suspension of the nanoparticles on a carbon-coated copper grid. The nanoparticles were studied using a TECNAI 120 (120 kV) apparatus with a GATAN camera and the Digital Micrograph software. XPS was performed with a SPECS GmbH PHOIBOS 100-5 MCD hemispherical analyzer and a monochromatic Al Kα X-ray source (energy 1486.6 eV). High resolution spectra were recorded at a transient energy of 10 eV under a pressure of 3.10−10 Torr in the chamber. Electrochemical Measurements. Electrochemical discharge and charge runs were carried out at 20 °C in Swagelok-type cells connected to a VMP automatic cycling and data recording system. The cells were assembled in a glovebox under argon. They contained a lithium foil (Aldrich) as the negative electrode, a Whatman GF/D borosilicate glass fiber sheet saturated with a LP30 electrolyte (solution of 1 M LiPF6 in ethylene carbonate/dimethyl carbonate (1/1 by weight)). The positive electrode was composed of the powder of metal phosphide nanoparticles and, when indicated, 15 wt % carbon black (SP). Typically, 10 mg of mixed metal phosphide and carbon black (85/15 wt %) were used, and the tests were carried out at C/10 scan rates (1 Li in 10 h per formula unit) in a potential window between 2.0 and 0.0 V versus Li+/Li.

Such transformation, with no well-defined plateau of potential, was described for a black P/carbon SP electrode, and no crystallized intermediate was observed.59 Yet, the charge (ca 1.3 V) and discharge (ca 0.7 V) potentials were much higher in this study than the one observed here (ca 1.0 V and ca 0.5 V). Moreover, XRD analyses showed the appearance of a peak at ca 47°, attributed to the formation of intermediate Ni− P phases.52 Therefore, it is likely that the Ni element was involved in the phase transformation upon cycling and not just as a catalyst. Accordingly, the broadness of the Ni2P peaks in Figure 6 of Supporting Information pleaded for the presence of newly formed smaller Ni2P nanoparticles with smaller crystallite sizes.

3. CONCLUSION We here reported that bulk Ni2P can undergo a partially reversible reaction with Li. This material was thus chosen as a model to assess the influence of carbon coating on nanoparticles. Indeed, as-made monodisperse nickel phosphide nanoparticles were found to be inactive, in contrast with bulk Ni2P, until a suitable method of “self-generated carbon coating” was developed: a soft calcination step was found to provide carbon-coated active Ni2P nanoparticles. Not only did it avoid any growth of the nanoparticles by sintering but also it provided a very good conductive coating, ensuring electron wiring. Further optimizations lead to interesting properties: Ni2P nanoparticles with a reversible capacity of ca 200 mAh/g, and insertion of ca 3 Li per lattice unit could be obtained in the best cases. The conversion mechanism could be confirmed by post-mortem analyses of the electrodes, even though it could still be competitive with a solid-solution insertion mechanism, as suggested by the absence of charge/discharge potential plateau. The self-generated carbon coating method is original is the sense that it uses the ligands that come with the nanoparticles as a carbon source. Several other methods were developed in previous or simultaneous works, as mentioned in the Introduction, for the fabrication of well-percolated conductive carbon/active nanopowder composites or even graphenewrapped nanoparticles. Most of them deal with cathode materials (such as LiFePO4/C composites), which are less sensitive to oxidation and carbide formation than metal-like structures. In opposition with this, the protocol developed here relied on a relatively soft thermal treatment and did not entail a deep preliminary ligand removal that may be associated with oxidation or aggregation of the nanoparticles. It may then be easily extended to other nanoscaled negative electrodes that involve nanoparticles synthesized from solution routes. In this domain, ligands variety offers an attractive tool for further optimization of the carbon coating. Altogether, our study unraveled several relevant features of the nanoscale for lithium batteries: (i) a self-generated carbon coating method that avoids nanoparticles sintering was proposed, based on the thermal decomposition of coordinated ligands in inert atmosphere, (ii) the Ni2P phase was chosen as a model to probe the effect of this coating, (iii) it has proven to be electrochemically active vs lithium as a bulk phase, even

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ASSOCIATED CONTENT

* Supporting Information S

Complementary XRD, TEM, and XPS data. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 695



Article

Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Christophe Méthivier (LRS, UPMC−CNRS) is gratefully acknowledged for the XPS measurements. Marie-Liesse Doublet (Institut Charles Gehrardt) is gratefully acknowledged for fruitful discussions. The CNRS, the UPMC, the College de France, and the Ecole Polytechnique are acknowledged for financial support. The ALISTORE network is also acknowledged for financial support. S.C. thanks the DGA for financial support.

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Improving the Li-Electrochemical Properties of Monodisperse Ni2P

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