Origin of the Voltage Hysteresis in the CoP Conversion Material for

Article pubs.acs.org/JPCC

Origin of the Voltage Hysteresis in the CoP Conversion Material for Li-Ion Batteries R. Khatib,† A.-L. Dalverny,† M. Saubanère,† M. Gaberscek,‡ and M.-L. Doublet*,† †

Institut Charles Gerhardt, CNRS − Université Montpellier 2, Place Eugène Bataillon, 34 095 Montpellier, France National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia

‡

S Supporting Information *

ABSTRACT: The electrochemical activity of the CoP conversion electrode was investigated through the combination of computational and experimental techniques. The carbon-free CoP electrode shows better performances than the carbon-coated electrode, in sharp contrast with the beneficial role of carbon coating reported in many insertion materials. A two-step insertion/conversion process associated with the exchange of 3Li is predicted for this system from the T = 0 K phase stability diagram performed on bulk structures within the DFT framework. The voltage hystereses measured for these two processes through a seven-day relaxation procedure (GITT) are 1 order of magnitude higher for the exp conversion process (ΔVexp conv = 0.44 V) than for the insertion process (ΔVins = 0.04 V). The various elementary reactions susceptible to occur at the surface of the electrode were investigated by means of surface DFT calculations. This mechanistic study shows that the insertion mechanism is not significantly affected by the electrode nanosizing (ΔVth ins = 0.04 V), while the conversion reaction does. Asymmetric responses are expected upon charge and discharge for this system, due to the growth of different interfaces. This induces different electrochemical equilibriums and then different voltages in charge and discharge. The hysteresis voltage computed for the conversion of LiCoP into Li3P + Co0 is again in very good agreement with experiments (ΔVth conv = 0.41 V). Such results are very encouraging and open new routes to the rationalization of the microscopic mechanisms acting as limiting reactions in electrode materials for Li-ion batteries.

■

INTRODUCTION The principle of Li-ion batteries relies on simultaneous Liinduced electrochemical reactions occurring at both electrodes to transform the chemical energy in electricity.1−4 The electrochemical reactions need to be fully reversible to allow recharging the device after a discharge. Today, the electrode materials used in commercialized accumulators are insertion materials capable of accommodating lithium in their hosts without breaking bonds.3 This structural integrity is essential for the thermodynamic reversibility of the reactions but also for the kinetics, since insertion reactions do not involve any other element than lithium in the mass transport. This typically leads to good battery performances, since the energy delivered in discharge is very close to the energy required to recharge the battery. However, the energy density of an insertion material which is proportional to the integrated product of the working voltage and the current generated by lithium exchangeis generally limited by the small number of lithium atoms that can be accommodated in the material host structure. Hence, no matter how successful we are in increasing the working voltage of a battery, the small capacity (number of exchanged lithium per formula unit) generally associated with insertion materials ineluctably limits their energy density. This is particularly true for high-potential cathodes for which lithium insertion involves the redox couple of strongly correlated transition metal (TM).5−7 To overcome this issue, conversion reactions © 2012 American Chemical Society

appeared in the early 2000s as an alternative to insertion reactions.8−11 A pure electrochemical conversion reaction is supposed to consist of a single step whereby the starting material, generally designated as MXy (M = transition metal; X = ligand of the p-main group) is decomposed into a composite made of M0 nanoparticles embedded into a yLinX matrix.9 The theoretical capacity which is directly linked to the oxidation state of the transition metal in the MXy starting material can then be formally as high as y*n (n being the formal charge of the ligand). In real systems, however, a certain amount of lithium can be dissolved into the starting electrode, and only after a critical concentration has been reached, the MXy decomposition will be triggered. This first discharge mechanism is typical for all conversion materials,9 and characterized by a large overpotential. This is required to transform the microsized starting electrode MXy into a nanosized discharged electrode yLinX + M0. After the first cycle, however, the MXy material remains in the nanosized form rather than returning to the microsized morphology. For this reason, the first lithiation (discharge) exhibits, almost without exception, a different voltage pattern than the next lithiations. In addition to that, several ternary phases of general formula LixMXy can be formed Received: October 19, 2012 Revised: November 30, 2012 Published: December 11, 2012 837

dx.doi.org/10.1021/jp310366a | J. Phys. Chem. C 2013, 117, 837−849

The Journal of Physical Chemistry C

Article

nitrogen. X-ray diffraction (XRD) patterns were recorded on a PANanalytical X'pert PRO X-ray diffractometer (Cu Kα 1.5406 Å). Scanning electron microscopy (SEM) imaging was performed on the as-obtained samples, using an FE-SEM SUPRA 35VP device. Electrochemical Measurements. Before being used as a cathode material, the as-obtained powder was grinded using an agate mortar or was ball-milled for 30 min at 300 tr/mn (Retsch). The assembly of CoP/Li° Swagelok-type cells was done in an argon filled glovebox. These cells contained a 1 M LiPF6/EC+DEC (1:1 in volume) electrolyte, a metallic lithium counter electrode, and a Wathman GF/A borosilicate glass fiber sheet as a separator. The electrode loading for electrochemical tests was about 2 mg/cm2. The CoP/Li0 electrodes were cycled using a Maccor’s series 4000 or a VMP3 (Biologic Co.) at room temperature over the range 0.02−2 V. The galvanostatic intermittent titration technique (GITT) was also carried out to quantify the voltage hysteresis associated with the conversion of CoP. A seven-day relaxation procedure was necessary to achieve the thermodynamical equilibrium of the electrode. Carbon black (Printer XE) and PVdF were added to the starting CoP material in a weighted 8:1:1 ratio in order to compare the electrochemical response of carbon-free and carbon-coated CoP electrodes. DFT Calculations. Calculations were performed using the Vienna ab initio simulation package23,24 within the generalized gradient approximation (GGA) of Perdew−Burke−Ernzerhof (PBE)25 for the exchange-correlation potential. Projector augmented wave (PAW)26 pseudopotentials were applied with an energy cutoff of 500 eV. The k-point meshes were generated with the Monkhorst−Pack scheme to regularly decompose the Brillouin zone. For bulk calculations, atoms and cell parameters were allowed to relax, while, for surface calculations, only the atomic positions were relaxed. The convergence criterion was set to 2 × 10−5 eV and 2 × 10−3 eV·Å−1 for the energy and atomic forces, respectively. Surfaces have been represented within the three-dimensional periodic boundary conditions by slabs of CoP, LixCoP, Co0, and Li3P separated by a vacuum layer. The layer thickness was optimized to avoid unphysical interactions between consecutive surfaces. Each slab consists of n formula units of the initial material (cleaved in one hkl plane) surrounded by two stoichiometric or two non-stoichiometric surfaces. The surface energies are obtained through eqs 1 and 2:

during charge/discharge. This is generally observed when the transition metal oxidation state in the starting MXy is high enough to allow some intermediate oxidation states to be reached in reduction, prior to the complete M0 reduction.9,11−13 Altogether, the mechanism of conversion systems can get relatively complex. Indeed, there are hardly examples in the literature, neither experimental nor theoretical, where the conversion mechanism would be fully rationalized. A common feature to all conversion systems is the relatively wide gap between the charge and discharge voltages. In all cases, this gap persists at several hundreds of mV even when the kinetics is slowed down to rates as low as C/100.14 This fact blurs the explanation of the actual mechanism because it sounds as if the gap is an inherent feature of the system, independent of the transport rate. In other words, this gap has a thermodynamic origin and is thus a voltage hysteresis, in opposition to a voltage polarization which has a kinetic origin. So far, various authors have recognized one rule as regards the extent of hysteresis: it depends on the nature of the M−X chemical bonds in the starting material (ionic vs covalent) which is directly linked to the material electronic behavior.9−12 For example, the insulating CoO material shows a much wider voltage hysteresis8 than the metallic FeP material,11 at a comparable rate. Our recent attempts to rationalize this phenomenon led us to propose a new methodology based on first-principles DFT calculations to account for interface electrochemistry in conversion materials.15,16 Applied to CoO, this methodology has allowed the origin of the voltage hysteresis to be identified as arising from an asymmetric response of the Co/CoO (metal/oxide) and Li2O/CoO (oxide/oxide) interfaces upon charge and discharge. In the FeP material, we showed from phase stability diagram DFT calculations that the conversion reaction is preceded by an insertion mechanism to form the LiFeP phase, thus leading to small and large voltage hysteresis for the insertion and conversion mechanisms, respectively.11 In the present work, we want to extend this methodology to the impact of surface reactivity on the electrochemical mechanism and voltage hysteresis of conversion materials. The study is performed on the CoP material whose electrochemical behavior has never been reported. CoP17 is iso-structural to FeP18 and presents covalent Co−P bonds in its structure, in contrast to the more ionic Co−O bonds occurring in CoO. It is a paramagnetic metal19,20 in contrast to the antiferromagnetic insulating behavior of CoO.21,22 This study is the first report on firstprinciples DFT investigations of bulk vs surface reactivity in a conversion material. It is presently combined with appropriate experimental observations in order to rationalize the electrochemical mechanism of the CoP electrode and quantify its voltage hysteresis between charge and discharge. In this context, the appearance of surface/interface effects is shown to be the crucial underlying phenomenon causing the hysteretic behavior.

σ (J/m 2) =

μ(MX[MX]n MX − (n + 2)μ(MX) 2A

(1)

σ (J/m 2) =

μ(X[MX]n X) − nμ(MX) − 2μ(X) 2A

(2)

where A, μMX, and μX represent the surface area (at both layer sides) and the chemical potentials of the MX bulk and the X vacancy, respectively. This representation is the most numerically efficient way to simulate the reactivity of “nanoparticles”, i.e., systems whose reactivity is dominated by surface effects. Depending on the particle size, the electrochemical properties of the system vary as the surface/bulk ratio. For spherical particles of radius r, the fraction of atoms lying at the surface (ns) and in the bulk (nb) varies in 4πr2 and (4/3)πr3, respectively. In first approximation, the electrochemical voltage of a reaction involving nanoparticles of radius r is then a mixed voltage which depends on the particle size, i.e., on the surface vs bulk ratio ns/nb, through the following equation:

1. METHODS Synthesis and Characterization. The synthesis of the orthorhombic CoP was carried out using a high-temperature procedure as previously reported.11 Stoichiometric amounts of cobalt (Co Aldrich, 1). Attempts to identify this extra peak are still unsuccessful. The electrochemical measurements were carried out with and without additional carbon in the starting electrode. As shown in Figure 2, the electrochemical responses of the carbon-coated electrode CoP(C) (red curve) and the carbon-free electrode CoP (black curve) show significant differences. In agreement with the irreversible activity of the carbon black vs lithium, the first discharge capacity is higher for the CoP(C) electrode (3 Li) than for the CoP electrode (2.5 Li). Interestingly, the capacity fading is higher for the CoP(C) electrode than for the CoP electrode, showing that carbon coating does not benefit the cycling performance of this material. This could suggest a better interface migration upon charge/discharge when CoP is not coated by carbon. Different mechanisms also occur for the two electrodes: after a first cycle of discharge/charge, the CoP(C)

Figure 2. Galvanostatic measurements carried out on CoP/Li half-cells for the carbon-coated CoP(C) electrode (black curve) and carbon-free CoP electrode (red curve) at a C/20 rate (1 Li in 20 h) in the potential range [2.0, 0.0] V.

electrode undergoes two distinct processes, while the carbonfree CoP electrode exhibits one more process, clearly visible in the first part of the discharge. Given the better performance and the apparent more complex mechanism taking place in the carbon-free electrodes, solely these electrodes were investigated in further studies. As already mentioned in the Introduction section, the first discharge of a conversion material is far different from the following ones.9 In the present case, it shows one process occurring at a voltage close to 0 V, associated with the exchange of 2.5 Li and with a reversible capacity of ∼2 Li. This process, hereafter denoted D3, corresponds to a large plateau typical for a direct, although incomplete, conversion reaction. In further discharges, two additional processes are observed, hereafter denoted D1 and D2. They occur at ∼0.80 and ∼0.60 V and correspond to a global exchange of lithium around twice smaller than for the conversion D3 process. As shown in the −dx/dV derivatives of Figure 3, the occurrence and the shape of D1 and D2 clearly depend on the scan rate and on the cycle number. In the first two discharges, D1 is observed only at very 839



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Figure 3. dx/dV derivatives at cyles 1, 2, 7, and 10 for the carbon-free CoP electrode as a function of the C/n scan rate (1 Li in n hours). The different processes observed are denoted D1, D2, D3 and C1, C2, respectively, for the discharge and charge, along with their associated “average” voltages.

slow C/20 scan rates. This suggests the formation of an intermediate “LixCoP” electrode through a kinetically limited electrochemical reaction. In the further discharges, i.e., when the active material is nanosized, D1 progressively increases to the detriment of D2, while D3 remains almost constant. In charge, mainly two processes can be distinguished, hereafter denoted C1 and C2. While C2 occurs at around 0.8 V irrespective of the cycle number, C1 shifts from 1.05 to 1.17 V from cycle 1 to 10 (see Figure 3) and becomes more asymmetric. Whether or not C1 corresponds to the same reaction all along the cycle number is unclear at this stage. It should be noted here that we did not consider the electrolyte reduction as a possible contribution to the incremental peaks of the −dx/dV derivatives. In Sn-based electrodes, the EC reduction was recently shown to occur at 1.5 V.27 While this potential is luckily much higher than those presently observed for processes D1 and D2, the electrolyte reduction in the CoP electrodes cannot be fully discarded at present and other electrolytes should probably be tested to address this question. In situ XRD and magnetic measurements were investigated at different states of charge/discharge to characterize the electrode material. As shown in Figure 4, the in situ XRD patterns obtained for the half-discharged, fully discharged, half-charged, and fully recharged electrodes are unfortunately not very instructive. The diffraction peaks of the CoP starting electrode show a lower intensity in the fully discharged electrode. They are nevertheless still present in the diffraction patterns at the end of discharge which is indicative of a partial decomposition of the starting electrode. In contrast to the derivative curve of

Figure 4. In situ, but not in operando, X-ray powder diffraction patterns collected during charge/discharge in a homemade cell on a Simens D5000 diffractometer in reflection (Bragg−Brentano) mode using Cu Kα1 radiation with an Autolab cycling/data recording system. Data were collected in the range 20−45° in steps of 0.04°. The crosses correspond to the diffraction peaks of the Be window, while the circles, triangles, and squares correspond to the diffraction peaks of CoP, Li3P, and Co0.

Figure 3, no extra peaks corresponding to the growing of Co0 and Li3P are detected in the XRD patterns after the first discharge. In regard to the Co0 particle size which is expected to be very small (a few nanometers) in all conversion reactions,9 this result is not surprising. It is however more awkward for the Li3P which is generally detected upon discharge in other transition metal phosphides such as FeP,11 NiP2,13 and Cu3P.28 840



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During the first charge (half or fully charged curves), the CoP peak intensity slightly increases and no extra peaks are detected. This suggests that the electrode transformation associated with process C1 does not correspond to the formation of a wellcrystallized phase. After a full charge, no significant changes are observed in the diffraction peak, suggesting that the remaining CoP particles occurring in the electrode after the first discharge are nonreactive in further cycles. This is likely related to the electrode loading required for in situ XRD measurements which is 1 order of magnitude higher (∼20 mg/cm2) than the one used for electrochemical measurements (∼2 mg/cm2). In order to get more information from in situ XRD, we tried in operando techniques (see Supporting Information SI-1). The electrode loading is equivalent to the one used for in situ (but not in operando) XRD, but carbon black and PvDF were added to the active material to increase the electrical contacts between particles. The results here show the complete disappearance of the starting CoP material during the first discharge, but still no extra peak could be detected to characterize the expected Li3P matrix. Up to now, our attempts to observe the fully discharged Li3P/Co0 electrode were all unsuccessful. Preliminary magnetic measurements,29 however, reveal the existence of Co0 nanoparticles in the fully discharged electrode, showing that the conversion reaction does take place in the second part of the discharge (see Supporting Information SI-2a, SI-2b). These experimental results therefore lead to the assumption that the conversion of CoP should occur through a multistep insertion/ conversion mechanism, as for the previously reported FeP electrode.11 x Li

the thermodynamical equilibrium for this electrode. The voltage hysteresis measured for the second cycle discharge is found to be around 0.08 V for the global insertion process and 0.45 V for the conversion process. This result clearly supports our assumption that CoP reacts with Li through a two-step insertion/conversion mechanism. Nevertheless, our inability to characterize the intermediate and discharged electrodes led us to supplement our experimental analysis with a computational study. First-principles DFT calculations were then performed to elucidate both the bulk and surface reactivity of CoP vs Li and to rationalize the insertion/conversion mechanism of this electrode. Bulk Reactivity. The T = 0 K phase stability diagram of the “LixCoP” ternary system was computed using the double reference method, as previously reported for the isostructural FeP material.11 This method consists of computing the formation enthalpy of various hypothetical “LixCoP” phases, H(“LixCoP”), with respect to the formation enthalpy of the reference conversion electrode, H(ref), for x varying from 0 (starting material) to 3 (converted Li3P + Co0 electrode): ⎛ x⎞ ΔH(x) = H(“LixCoP”) − ⎜1 − ⎟H(CoP) ⎝ 3⎠ x − {H(Li3P) + H(Co0)} 3 = H(“LixCoP”) − H(ref)

(1)

Provided that the configuration space of the hypothetical phases is properly sampled, this method has already proven to be powerful for identifying novel intermediate composition phases in insertion30 and conversion reactions.11,13 To ensure a representative sampling of the potential energy surface (PES), hypothetical intermediate structures, “LixCoP”, were investigated following both local and global optimization procedures. The local scheme is the most common method used in solid state science. It starts from a given crystal structure and composition and searches the closest minimum of the PES in following the steepest downhill around the starting point. Such procedures do not, however, guarantee reaching the global minimum of the PES (i.e., the thermodynamically most stable phase/polymorph for the composition of interest), meaning that their success relies on the “quality” of the starting structures. When the structure of the PES global minimum is not known a priori, global minimization procedures can be used as a complementary approach to finely explore the PES.31,32 Starting from an arbitrary structure, these methods resort to statistical mechanics to randomly generate new structures (e.g., using, for instance, the basin hopping scheme33). The asobtained structures may now be far different from the initial one and are selected/discarded using an energy criterion (e.g., based, for instance, on the Metropolis algorithm34). All selected structures are then used as input structures in a local optimization procedure using accurate DFT calculations. Three main groups of starting electrodes were investigated: (i) single-phase LixCoP electrodes extracted from the basin hopping random procedure33 with the CoP crystal structure as the guess structure, (ii) single-phase LixCoP electrodes built on P-based fcc networks filled with Li and Co in the tetrahedral and/or octahedral sites,9 and (iii) two-phase mixed electrodes made of metal-rich Co2P and partially lithiated LiyP phases. The two former ones check the ability of the system to partially reduce the Co3+ ions of the starting CoP material during the discharge, while the latter one checks the ability of the system

(3 − x)Li

CoP... ⎯→ ⎯ ...LixCoP ⎯⎯⎯⎯⎯⎯⎯→ Li3P + Co0

Insertion mechanisms involving small bond reorganizations are generally associated with small voltage hysteresis.14 In contrast, conversion reactions involve large structural reorganizations including bond breaking and bond formation, and are associated with larger voltage hystereses.9 This is confirmed by the GITT measurements given in Figure 5 for the CoP/Li halfcell. A seven-day relaxation procedure was required to achieve

Figure 5. Galvanostatic intermittent titration technique (GITT) obtained after a seven-day relaxation procedure during the insertion and conversion processes in discharge and charge. The OCV steps were done at five different points: (i) after 20 h of discharge at a voltage close to 0 V, (ii) at 0.80 and 1.0 V during the first charge, (iii) at 0.60 and 0.02 V during the second discharge. The hysteresis measured for the insertion and conversion processes (ΔVexp ins and ΔVexp conv) is highlighted in red dotted lines and compared to the computed average (bulk) voltages in blue lines and hysteresis (ΔVth ins and ΔVth conv). 841



Article 0.5Li

to reconvert a thermodynamically stable Co2P phase rather than the initial CoP during the charge. Both metal-rich Co2P phases, the one crystallizing in the orthorhombic and the other having the hexagonal symmetry,17,35 have been considered in our calculations. The reference energy of the phase stability diagram of Figure 6 corresponds to the enthalpy variation associated with the

0.5Li

2Li

CoP ⎯⎯⎯⎯→ Li 0.5CoP ⎯⎯⎯⎯→ LiCoP ⎯→ ⎯ Li3P + Co0

The crystal structures of the newly identified phases (Li0.5CoP and LiCoP) are given in Figure 7. They both describe cobalt ions in a tetrahedral P coordination, as do most of the LixMPy (y = 1−4) phases so far reported in the literature for the M(3d) elements.36−39 Both phases can be described in tetragonal cells and consist of edge-shared CoP4 tetrahedra surrounded by lithium ions along the c-axis. The main difference between Li0.5CoP and LiCoP lies in the shift of every other CoP plane, leading to a unit cell twice larger for Li0.5CoP compared to LiCoP. Note that several polymorphs of Li0.5CoP and LiCoP were obtained with slightly different arrangements of the edge-shared CoP4 units in the unit cells but similar energies. Among these polymorphs, those with the most symmetric unit cells were considered as the referent structures for Li0.5CoP and LiCoP. Three consecutive processes are then predicted by our calculations to fully convert the CoP electrode into the Li3P + Co0 composite. A two-phase process should take place to transform the CoP starting material (based on CoP6 octahedra) into the Li0.5CoP ternary phase (based on CoP4 tetrahedra) with the breaking of two Co−P bonds. This is in good agreement with the D1 peak observed in the −dx/dV derivatives of Figure 3 and showing a kinetically limited reaction. Given the very close crystal structures of Li0.5CoP and LiCoP, a solid solution should then occur in this composition range. Again, this is in good agreement with the D2 peak observed in the −dx/dV derivatives of Figure 3. Then, the conversion of LiCoP into Li3P + Co0 should end the discharge with the breaking of the remaining Co−P bonds and the complete reduction of cobalt in Co0. The bulk voltages computed for the three processes are V1 = 0.98 V for the insertion of 0.5 Li in CoP, V2 = 0.70 V for the insertion of 0.5 more Li in Li0.5CoP, and V3 = Vconv = 0.41 V for the full conversion of LiCoP into Li3P + Co0. These values are reported on the GITT plot of Figure 5 (blue lines), along with the intermediate voltage computed for the direct insertion from CoP to LiCoP, Vins = 0.84 V (dotted blue line). At this stage, it is difficult to state whether the insertion process occurs in one or two steps. Obviously, this should

Figure 6. Phase stability diagram computed at T = 0 K from DFT calculations using the double reference method reported for the FeP electrode (ref 11). The horizontal line refers to the reference energy of the conversion reaction. All calculations were performed within the DFT formalism using the GGA-PBE functional for the exchange and correlation potential.

direct conversion of CoP into Li3P and Co0. It was accurately determined from the formation energies of CoP, Li3P, Co0, and Li0 (see Supporting Information SI-3) prior to being set as the zero energy in the phase stability diagram of Figure 6. Among the various intermediate “LixCoP” phases considered in the calculations, six are found to be thermodynamically more stable than the direct conversion of CoP into Li3P + Co0. Assuming a thermodynamical control of the reactions and following the convex hull of the phase stability diagram, the redox mechanism predicted by our calculations for the CoP starting electrode is then

Figure 7. (a) Crystal structure of the Li0.5CoP and LiCoP phases as obtained from the local and global optimization procedures. Various polymorphs were obtained, for instance, for LiCoP which differ by the way the edge-shared CoP4 tetrahedra are packed in the structure. An example is given in part b. 842



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Table 1. Surface Energies (in J/m2) for Three Different hkl Planes for the CoP, LixCoP (x = 0.5, 1), Li3P, and Co0 Phasesa

depend on the reaction kinetics vs thermodynamics which are closely related to the electrode texture, and therefore to the cycle number. As deduced from the phase stability diagram of Figure 6, very close thermodynamical forces (slopes) are obtained to transform CoP in the partially lithiated Li0.5CoP electrode (two-step insertion) or directly in the lithiated LiCoP (one-step insertion). Kinetics should then govern the number of steps for the insertion mechanism. This result actually matches the experimental evidence outlined for D1 and D2, showing that D1 is more kinetically limited than D2. During the first cycles, the texture of the CoP electrode does not allow the two-step insertion mechanism to take place (or only at very slow scan regimes less than C/20). Then, both D1 and D2 are observed in further discharges, i.e., when the CoP electrode exhibits a more homogeneous nanosized texture. Interestingly, the average potentials computed for the direct insertion Vins = 0.84 V and for the conversion Vconv = 0.41 V are in very good agreement with the GITT measurements carried out on cycle 2, i.e., when the electrode is not perfectly homogeneous and D1 is kinetically hindered (at least partially). Indeed, assuming an equivalent hysteresis in charge and discharge, the experimental equilibrium voltages for the insertion and conversion processes exp correspond to Vexp ins = 0.81 V and Vconv = 0.44 V, respectively (see Figure 5). Despite this good agreement, bulk calculations are inappropriate to access voltage hysteresis. From the fundamental viewpoint, a voltage hysteresis arises from a partial irreversibility of the electrochemical reactions taking place upon charge/discharge. Conversion reactions requiring the creation of surfaces to increase the reaction extent, surface/interface effects are likely to be responsible for this irreversibility. In a recent work, some of us have investigated the interface electrochemistry in the CoO conversion electrode and suggested that the partial irreversibility could arise from the formation of a metastable electrode as a mandatory step to initiate the reaction.15,16 More precisely, defective CoO1−ε or Co1−εO electrodes were proposed to be formed during the electrochemical process, as a direct consequence of a kinetically hindered migration of the CoO/Co0 and CoO/Li2O interfaces, simultaneously. To identify the possible metastable electrodes for the CoP electrode and hopefully rationalize the microscopic mechanisms responsible for the voltage hysteresis of the CoP/ Li half-cell, the surface reactivity of the starting, intermediate, and ending electrodes was also investigated through firstprinciples DFT calculations. Surface Reactivity. All phases identified in our bulk phase stability diagram were considered in this study, i.e., CoP, LixCoP (x = 0.5 and 1), Co0, and Li3P. For the different LixCoP polymorphs, only the most symmetric phases were investigated to avoid too large unit cells. For each surface, three different hkl-planes were considered, i.e., (100), (010), and (001). As shown in Table 1, the hkl-planes leading to the lowest surface energies are the (001) planes and correspond to stoichiometric surfaces. These surfaces were then primarily considered to investigate the elementary processes susceptible to occur at the surface of the electrode during charge and discharge. In principle, the number of elementary processes one may envision to study the surface reactivity of a multiphased conversion electrode is very large and increases with the surface area. However, combining our bulk results with chemical intuition, this number can be easily reduced to a few events. These processes correspond to partial insertion/conversion reactions associated with the stabilization of nonstoichiometric

hkl σ (J/m ) 2

CoP

(100)

Li0.5CoP

1.93 2.27 1.45

P Co mixed

LiCoP

1.19

mixed

Li3P

0.52 0.89 2.30

Li mixed Co

Co0

(010) 1.47

mixed

(001) 1.43 2.24 0.68 2.80 0.43 2.92 0.49

P Co Li Co mixed Co Li

2.15

Co

a

The surface termination is given on the right side of the surface energy. The thickness of the vacuum layer at both sides of each surface was set from 9.7 to 10.7 Å depending on the surface considered.

interfaces due to the creation of surface vacancies or defects. They are schematically illustrated in Figure 8 and will be denoted Ri (i = 1−6) for the reduction processes (discharge) and Oj (j = 1−5) for the oxidation processes (charge). In the first part of discharge, the reaction of the CoP surface with lithium can lead to either an adsorption of lithium to form an amorphous ternary interface with various lithium compositions (R1), the creation of P vacancies at the CoP surface to form Li3P particles (R2), or the Co for Li substitution at the CoP surface to extrude Co0 particles (R3). The formation of Co0 by simply creating Co vacancies at the surface of CoP does not imply lithium exchange (voltage not defined) and was thus disregarded. The Co/Li substitution (R3) here corresponds to a displacement mechanism (i.e., partial conversion mechanism) which has already been reported for transition metal phosphides such as Cu3P, for instance.40 In the second part of the discharge, further lithiation at the surface of LiCoP should lead to the formation of a Li-rich Lix+ξCoP interface (R4) or a partial conversion mechanism associated with the formation of Li3P (R5) or the formation of Co0 through Co/Li substitution (R6). In charge, starting from the Li3P + Co0 electrode, one may consider the following reactions: the creation of Li vacancies in Li3P to form defective Li3−ξP interfaces (O1), the Li/Co substitution on the Li3P surface to form the LiξCoP (ξ = 0.5, 1) interfaces (O2), the adsorption of P or Li/P on Co0 to form CoP or LiξCoP interfaces (O3 and O4, respectively), and the creation of Li vacancies at the surface of LiξCoP (ξ = 0.5, 1) to form a Li-poor interfaces (O5). The surface voltages associated with these elementary reactions were computed for various surface areas and compositions. They are reported in Table 2 along with their respective electrochemical reactions. For each process, we used the notation (X)−Y where (X) and Y stand for the surface composition and for the bulk cleaved to create the initial stoichiometric surface, respectively. As an example, (LiξCoP)− CoP and (Li1−ξCoP)−LiCoP correspond to CoP−(001) and LiCoP−(001) surfaces which were partially reduced or oxidized through Li adsorption and Li desorption, respectively. These two surfaces may have equivalent compositions but not necessarily equivalent structures, since they arise from different bulk structures. It is worth noting here that all surface reactions are associated with states of charge/discharge which depend on the particle size, i.e., on the ns/nb ratio (see eq 3). In other words, the surface/interface compositions presently considered 843



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Figure 8. Elementary surface reactions considered upon discharge (reduction) and charge (oxidation) starting from the fully charged CoP electrode and from the fully discharged Li3P + Co0 electrode. The voltages associated with each of these reactions are computed following the electrochemical reaction given in Table 2. The Co, P, and Li atoms are illustrated with blue, red, and gray balls.

respectively. Interestingly, the voltages associated with the nucleation of LixCoP nanoparticles (x = 0.5 or 1) from the lithiation of CoP nanoparticles (R1′) are found very similar to the R1 values, at equivalent surface compositions. They were computed from the total energies of the stoichiometric CoP001 and LixCoP-001 surfaces (x = 0.5, 1) and correspond to VR1′ = 1.09 and 0.86 V, respectively, for x = 0.5 and 1. The

(ξ) are expected to get closer and closer to the true electrode composition (x) as far as the particles size decreases. In reduction (discharge), the most probable elementary process, i.e., the process associated with the highest voltage, is R1. It corresponds to the adsorption of Li on the CoP-001 surface with associated voltages VR1 = 1.12, 1.09, 0.87, and 0.86 V for lithium coverage 1/4 Li, 1/2 Li, 3/4 Li, and 1 Li per CoP, 844



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Table 2. Reduction (Ri) and Oxidation (Oi) Elementary Processes Occurring at the Surface of the Electrode upon Discharge and Charge and Their Associated Voltages in Voltsa ξ coverage

V (V)

1

/4 Li per CoP

1.12

1

1.09 0.87 0.86

discharge: elementary reactions Li Adsorption on the CoP Surface to Form a LixCoP Interface R1

(CoP)−CoP + ξ Li → (LiξCoP)−CoP

/2 Li per CoP /4 Li per CoP 1 Li per CoP 3

Nucleation of LixCoP “Nanoparticles” from CoP “Nanoparticles” R1′

(CoP)−CoP + 0.5Li → (Li 0.5CoP)−Li 0.5CoP

1.09

(CoP)−CoP + 1Li → (LiCoP)−LiCoP

0.86

Creation of P Vacancies on the Surface of CoP to Form Li3P R2

(CoP)−CoP + 3ξ Li → (CoP1 − ξ)−CoP + ξ Li3P

1

0.60

1

/2 P per Co 3 /4 P per Co 1 P per Co

0.48 0.44 0.33

1

0.37

/4 P per Co

Li Substitution for Co at the Surface of CoP to Form Co0 R3

(CoP)−CoP + 3ξ Li → (Li3ξCo1 − ξ P)−CoP + ξCo0

/4 Co per CoP

Li Adsorption on the LixCoP Surface (x = 0.5 or 1) to Form Lix+ξ Interfaces R4

(LixCoP)−LixCoP + ξ Li → (Lix + ξCoP)−LixCoP

1

/2 Li (x = 1/2)

1.05

1 Li (x = 1/2) 1 /2 Li (x = 1)

0.34 1/2

0.04 1). This R4′ process is kinetically more efficient than R1′ and is expected to occur around VR4′ = 0.73 V, i.e., very close to the bulk transformation from Li0.5CoP to LiCoP (V2 = 0.70 V). In the second part of the discharge, assuming that the starting electrode now consists of LiCoP particles, the creation of P vacancies at the surface of these particles appears to be the only achievable process. This process is expected to occur around VR5 = 0.14 V and is associated with the formation of a P-defective (LiCoP1−ξ)−Li3P interface in which Li3P nanoparticles nucleate. As the number of P vacancies increases in the interface, the surface of LiCoP is progressively reduced up to becoming Co0-terminated. To increase the reaction extent, the Li and P atoms lying in the subsurface of LiCoP have to migrate toward the external surface. As illustrated in the bottom portion of Figure 9a, this can be achieved by the shortening of the Co− Co bonds at the Co-terminated surface of LiCoP to facilitate the opening of new Li,P-migration paths. The Co0 nanoparticles then result from a surface reconstruction (contraction of the Co planes initially present in the LiCoP bulk) rather than from the migration of Co within the electrode. This could explain why the metallic nanoparticles generally formed through a conversion reaction never exceed a few nanometer sizes, regardless of the MX starting material or the cycle number. The fully discharged electrode should then consist of nanosized Co0 particles homogeneously spread into a matrix made of Li3P particle agglomerates. Upon charge, the Li3P + Co0 electrode is shown to reconvert through the formation of more or less lithiated (LiξCoP)−Co0 and (CoP)−Co0 interfaces (see Figure 9b). According to Table 2, the former (O4) should form first, prior to competing with the latter (O3 + O4). To confirm this, we have realized a GITT measurement in slightly less exigent conditions than the sevenday relaxation procedure presented in Figure 5. As shown in Figure 10, two different voltage slopes are clearly observed in the first part of charge, in very good agreement with the voltages computed for O3 and O4. The (LiCoP)−Co0 interface

Figure 10. GITT measurements carried out on the CoP/Li half-cell with steps of 1 h at C/10 (charge or discharge) and rest to open circuit voltage until the potential slope is less than 30 mV·h−1. The experimental values are compared with the theoretical predictions.

should be preferentially formed at low voltages from 0.19 to 0.56 V, where the (CoP)−Co0 interface should start growing at higher voltages from 0.56 to 0.75 V. This competition should end at the state of charge which depends on the relative weight of these two interfaces. Thus, in contrast with the discharge where thin interfaces were created to facilitate/activate the nucleation of LixCoP particles (therefore acting as a buffering zone), here the interfaces are expected to grow to the detriment of Li3P. This means that the oxidation process to reconvert the Li3P + Co0 electrode is associated with a different electrochemical equilibrium than the conversion mechanism in reduction. In other words, the phases involved in the charge and discharge processes are different, so as the grand potentials of the associated electrochemical reactions. In the second part of charge, lithium should be extracted from the remaining Li x CoP particles through the formation of Li-poor (Lix−ξCoP)−LixCoP interfaces (x = 0.5 or 1), until CoP is fully recovered. This last process is computed from 0.90 to 1.20 V, again in good agreement with the GITT results of Figures 5 and 10. Whether this process occurs in one or two steps (as for the discharge) is difficult to say here. It should depend on the lithium composition of the interfaces formed from the O4 process. The mechanistic study presented above allows us to estimate the voltage hysteresis expected for the insertion and conversion processes. As expected, the insertion mechanism is much less altered by the surface reactivity of CoP and LiCoP than the conversion mechanism. In the very beginning of the discharge, the first-step insertion from CoP to Li0.5CoP should be progressively replaced by an interface solid solution mechanism as long as the active material surface area increases (V ∼ 1 V). The nucleation of Li0.5CoP should nevertheless compete with an additional lithiation of the CoP surface up to the nucleation of LiCoP (V = 0.86 V), depending on the particle size and/or scan rate. The second-step insertion process corresponds to rapid lithium diffusion into the Li0.5CoP particles, through a bulk solid solution mechanism to form LiCoP particles (V ∼ 0.70 V). These two insertion mechanisms were shown to be almost perfectly reversible. Following eq 3, it is now possible to estimate the voltage hysteresis at different states of charge/ discharge. Assuming fully nanosized particles (ns ≫ nb), our calculations predict that the very beginning of discharge occurs at V = 1.12 V, while the very end of charge occurs at V = 1.20 V, leading to a voltage hysteresis less than 0.1 V. Around x = 1 847



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in charge and discharge. The voltage hysteresis associated with the global insertion process from CoP to LiCoP is computed to be very close to the experimental one, ΔVth ins = 0.04 V. In contrast, the conversion reaction is strongly affected by the electrode nanosizing. Different interfaces are shown to grow in charge and discharge. The asymmetric response of the system upon reducing and oxidizing conditions implies different reaction equilibriums in charge and discharge which directly affects the electrochemical grand potential. The voltage hysteresis computed for the conversion of LiCoP into Li3P + Co0 is computed to be ΔVth conv = 0.41 V. This result not only matches very well the experimental value but also validates the methodology presently used to quantify the impact of microscopic mechanisms onto the macroscopic behavior of the electrode. This study should then open new routes to rationalizing the limiting factors affecting the performances of nanosized electrode materials in Li-ion batteries.

(when the electrode is made of LiCoP particles), it is even lower, since the discharge voltage is found to approach V = 0.86 V, while charging these LiCoP particles begins at V = 0.90 V. In contrast, the conversion mechanism is much more affected by the surface reactivity than the insertion process. In discharge, the LiCoP surface should first decompose to form Li3P at V = 0.14 V, while the concomitant formation of CoP and LiCoP interfaces in charge should start at V = 0.56 V. For a state of charge/discharge close to the end of the insertion mechanism (x ∼ 1) and close to the middle of the conversion process (x ∼ 2), the hystereses are then computed to be ΔVth ins = 0.04 V and ΔVth conv = 0.41 V, respectively. This result is in excellent agreement with the experimental values extracted from the GITT of Figure 5, although the states of charge/discharge where the seven-day relaxation procedure was applied cannot be rigorously determined. More thorough comparisons of the computed vs measured hystereses all along the reaction extent would require very long GITT measurements. Nevertheless, the present results are already very encouraging. They not only shed light onto the conversion mechanism but also allow validating the methodology presently developed to account for surface/interface effects in nanosized and multiphased electrodes.

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ASSOCIATED CONTENT

* Supporting Information S

In situ/in operando XRD patterns of the CoP(C)-electrode upon discharge/charge, EPR measurements of the fully discharged electrode, as well as bulk calculations of the phases involved in the reaction. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSION The electrochemical activity of the CoP conversion electrode was investigated through the combination of computational and experimental techniques. Better performances were obtained for the carbon-free CoP electrodes compared to carbon-coated CoP(C) electrodes, showing that the carbon coating does not benefit to the material surface reactivity vs lithium. This is in sharp contrast with the beneficial role of carbon coating in many insertion materials.41 The experimental voltage profile of the carbon-free CoP/Li half-cell is complex. It shows three different processes in discharge and only two in charge that could not be rationalized with experimental techniques, in particular due to the nanosized and multiphased feature of the electrodes. This inability to characterize the intermediate electrodes led us to resort to atomistic modeling. The bulk vs surface reactivity of the CoP vs Li was then investigated through first-principles DFT calculations. The phase stability diagram performed on the bulk structures predicts a two-step insertion mechanism from CoP to Li0.5CoP and from Li0.5CoP to LiCoP, followed by a conversion mechanism from LiCoP to Li3P + Co0. The ternary phases involved in the insertion processes are structurally closely related, suggesting a two-phase process for the non-topotactic transformation of CoP into Li0.5CoP and a single-phase process for the topotactic transformation of Li0.5CoP into LiCoP. This theoretical prediction is fully consistent with the galvanostatic measurements performed at different scan regimes over 10 cycles and showing that the first electrochemical process is kinetically more limited than the second process. A seven-day relaxation procedure (GITT) was further carried out to quantify the voltage hysteresis for the insertion and conversion processes. These voltage hystereses are shown to be 1 order of magnitude higher for the conversion process (ΔVexp conv = 0.44 V) compared to the insertion process (ΔVexp ins = 0.08 V). The leading elementary reactions susceptible to occur at the surface of the electrode were then investigated through surface calculations. This mechanistic study confirms that the insertion mechanism is not significantly affected by the electrode nanosizing. Nearly equivalent bulk and surface voltages are indeed obtained, both

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +33(0)467143681. Fax: +33(0)467144839. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS ́ Sánchez The authors gratefully thank Luis Enrique Diaz (Institut für Theoretische Physik, Universität Kassel, Germany) for providing us its basin hopping code for structure optimization of nanostructures, Denis Arčon (Jozef Stefan Institute, Ljubljana) for preliminary EPR and magnetic measurements on the fully discharged electrode, and Cyril Marino and Laure Monconduit (ICG Montpellier 2) for their help in in situ XRD characterization. The ALISTORE-ERI European Institute is acknowledged for the RK PhD’s funding and for fruitful discussions.

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REFERENCES

(1) Advances in Lithium-ion Batteries; van Schalkwijk, W., Scrosati, B., Eds.; Kluwer Academic/Plenum: New York, 2002. (2) Lithium Batteries Science and Technology; Nazri, G.-A., Pistoia, G., Eds.; Kluwer Academic/Plenum: Boston, MA, 2004. (3) Whittingham, M. S. Chem. Rev. 2004, 104, 4271−4302. (4) Bruce, P. G.; Scrosati, B.; Tarascon, J.-M. Angew. Chem., Int. Ed. 2008, 47, 2930−2946. (5) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1188−1194. (6) Padhi, A. K.; Nanjundaswamy, K. S.; Masquelier, C.; Okada, S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1609−1613. (7) Combelles, C.; Ben Yahia, M.; Pedesseau, L.; Doublet, M.-L. J. Phys. Chem. C 2010, 114, 9518−9527. (8) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J.-M. Nature 2000, 407, 496−499. (9) Cabana, J.; Monconduit, L.; Larcher, D.; Palacín, R. Adv. Mater. 2010, 22, E170−E192 (and the references therein). 848



Article

(10) Silva, D. C. C.; Crosnier, O.; Ouvrard, G.; Greedan, J.; SafaSefat, A.; Nazar, L. F. Electrochem. Solid-State Lett. 2003, 6, A162− A165. (11) Boyanov, S.; Bernardi, J.; Gillot, F.; Dupont, L.; Womes, M.; Tarascon, J.-M.; Monconduit, L.; Doublet, M.-L. Chem. Mater. 2006, 18, 3531−3538. (12) Gillot, F.; Boyanov, S.; Dupont, L.; Doublet, M.-L.; Morcrette, M.; Monconduit, L.; Tarascon, J.-M. Chem. Mater. 2005, 17, 6327− 6337. (13) Boyanov, S.; Bernardi, J.; Beckaert, E.; Ménétrier, M.; Doublet, M.-L.; Monconduit, L. Chem. Mater. 2009, 21, 298−308. (14) Dreyer, W.; Jamnik, J.; Guhlke, C.; Huth, R.; Moskon, J.; Gaberscek, M. Nat. Mater. 2010, 9, 448−453. (15) Dalverny, A.-L.; Filhol, J. S.; Doublet, M.-L. J. Mater. Chem. 2011, 21, 10134−10142. (16) Dalverny, A.-L. Ph.D. Thesis, Montpellier 2 Université, 2011. (17) Rundqvist, S. Acta Chem. Scand. 1962, 16, 287−292. (18) Fjellvag, H.; Kjekshus, A.; Andersen, A. F. Acta Chem. Scand. 1986, 40, 227−229. (19) Bekaert, E.; Bernardi, J.; Boyanov, S.; Monconduit, L.; Doublet, M.-L.; Ménétrier, M. J. Phys. Chem. C 2008, 112 (51), 20481−20490. (20) Yang, Z.; Liu, L.; Wang, X.; Yang, S.; Su, X. J. Alloys Compd. 2011, 509, 165−171. (21) Jin, S.; Tiefel, Th.; McCormarck, M.; Fastnacht, R.; Rchen, R. Science 1994, 264, 413−416. (22) Dalverny, A.-L.; Filhol, J.-S.; Doublet, M.-L. J. Phys. Chem. C 2010, 114, 21750−21756. (23) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 47, 558−561. (24) Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169−11186. (25) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (26) Blöchl, P. E. Phys. Rev. B 1994, 50, 17953−17979. (27) Bridel, J.-S.; Grugeon, S.; Laruelle, S.; Hassoun, J.; Reale, P.; Scrosati, B.; Tarascon, J.-M. J. Power Sources 2010, 195, 2036−2043. (28) Bichat, M.-P.; Politova, T.; Pfeiffer, H.; Tancret, F.; Monconduit, L.; Pascal, J.-L.; Brousse, T.; Favier, F. J. Power Sources 2004, 136 (1), 80−84. (29) Private communication with Denis Arčon (Ljubljana). (30) Filhol, J.-S.; Combelles, C.; Yazami, R.; Doublet, M.-L. J. Phys. Chem. C 2008, 112, 3982−3988. (31) Wales, D. J.; Doye, J. P. K. J. Phys. Chem. A 1997, 101, 5111− 5116. (32) Wales, D. J.; Scheraga, H. A. Science 1999, 285, 1368−1372. (33) Gehrke, R.; Reuter, K. Phys. Rev. B 2009, 79, 085412−1− 085412−10. (34) Metropolis, N.; Rosenbluth, A. W.; Rosenbluth, M. N.; Teller, A. H.; Teller, E. J. Chem. Phys. 1953, 21, 1087−1092. (35) Rundqvist, S. Acta Chem. Scand. 1960, 14, 1961−1979. (36) Juza, R.; Bohmann, T. Z. Anorg. Allg. Chem. 1961, 308, 159− 178. (37) Bichat, M.-P.; Gillot, F.; Monconduit, L.; Favier, F.; Morcrette, M.; Lemoigno, F.; Doublet, M.-L. Chem. Mater. 2004, 16, 1002−1013. (38) Gillot, F.; Monconduit, L.; Morcrette, M.; Doublet, M.-L.; Dupont, L.; Tarascon, J.-M. Chem. Mater. 2005, 17, 6327−6337. (39) Souza, D. C. S.; Pralong, V.; Jacobson, A. J.; Nazar, L. F. Science 2002, 296, 2012−2015. (40) Bichat, M.-P.; Politova, T.; Pfeiffer, H.; Tancret, F.; Monconduit, L.; Pascal, J.-L.; Brousse, T.; Favier, F. J. Power Sources 2004, 136 (1), 80−87. (41) Gibot, P.; Casas-Cabanas, M.; Laffont, L.; Levasseur, S.; Carlach, P.; Hamelet, S.; Tarascon, J.-M.; Masquelier, C. Nat. Mater. 2008, 7, 741−747.

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Origin of the Voltage Hysteresis in the CoP Conversion Material for

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