Solid State Ionics 188 (2011) 148–155
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Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Magnetic analysis of lamellar oxides for Li-ions batteries X. Zhang a, C.M. Julien b, A. Mauger a,⁎, F. Gendron c a b c
Institut de minéralogie et de physique des milieux condensés, Université Pierre et Marie Curie, 140 rue de Lourmel, 750015 Paris (France) Laboratoire de Physicochimie des Electrolytes, Colloïdes et Sciences Analytiques, Université Pierre et Marie Curie, 4 place Jussieu, 75005 Paris (France) Institut des Nanosciences de Paris, Université Pierre et Marie Curie, 140 rue de Lourmel, 750015 Paris (France)
a r t i c l e
i n f o
Article history: Received 29 May 2010 Received in revised form 27 October 2010 Accepted 3 November 2010 Available online 8 December 2010 Keywords: Lamellar intercalation compounds Lithium-ion batteries Magnetic properties
a b s t r a c t We show that the investigation of magnetic properties is the best tool to identify and quantify the impurities and defects that limit the ability of the lamellar intercalation compounds for use as cathode for Li-ion batteries. The results are illustrated for LiNiO2, LiNi1-yCoyO2, LiCoO2, LiNi0.5Mn0.5O2, and LiNi1/3Mn1/3Co1/3O2 (LNMCO). Despite the extensive studies of these ionic compounds in the past, not only for practical use, but also for themselves, the present work reveals that the magnetic properties of these lamellar compounds have been largely misunderstood, and that, at contrast with the common belief, they do not belong to the family of two dimensional frustrated antiferromagnets. The misunderstanding comes from confusion between extrinsic and intrinsic effects. This distinction allows for an overall understanding of the intrinsic properties of these materials, and opens the route to their optimization, in particular for LNMCO that is the most promising element of this family. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The cathode is a key element that limits the performance of Li-ion batteries in terms of energy and power. Its electrochemical properties depend critically on the purity of the materials that are used, and on the defects that can poison them. This can be easily understood, if we note that these materials are intercalation compounds implying the presence of paths for the Li+ ions, which can be easily blocked by any impurity or defect. That is why a tremendous effort is made to characterize the samples, with all the tools available from the materials science technology. Many insertion materials have been investigated (see, for instance ref. [1] for a review). Since a decade, however, attention has been focused on these compounds that include oxygen and a transition element is their chemical formula, since they are best suited to applications. The family includes lamellar compounds of chemical formula LiMO2, spinel compounds of formula LiM2O4, or olivine materials LiMPO4, where M stands for a transition element of the first series, or a combination of them, in case of partial substitution of the transition element by other ones. The transition elements are magnetic (with only few exceptions like Fe2+ of Co3+ in the lowspin state). In addition, the materials are often semiconductors with small electron (or hole) concentration, so that the magnetic exchange interactions are essentially superexchange interactions that are short-
⁎ Corresponding author. E-mail address:
[email protected] (A. Mauger). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.11.003
range. The magnetic properties of the cathode material will then be dominated by the interaction between each magnetic ion and the other ions close to it. It gives us the opportunity to use the magnetic ion as a probe of the atoms in its vicinity at the atomic scale. That is why the study of the magnetic properties is an important tool to characterize the sample, by detecting impurity phases or local defects. It comes in complement to other more conventional tools. XRD, SEM, TEM are probes of the crystallinity of the samples at the nanometer scale; the IR optical spectroscopy is a probe at the molecular scale; the magnetism is a probe at the atomic scale, and it thus nicely complementary to the other more conventional tools. In most cases, it is even the most sensitive tool, since it detects impurities and defects that are impossible to detect by any other means. We can still find too often in the literature many works were the samples under investigation are claimed free of any impurity and defect just because they are not detected in the X-ray diffraction pattern. Indeed, we found that a fraction as small as 0.1% of the transition metal ions that is involved in impurities is sufficient to modify dramatically the electrochemical properties in olivines [1], and we show in the present work that it holds true also in lamellar compounds. In that case, the investigation of magnetic properties is unavoidable. This combination of different techniques to probe the quality of the samples is currently used for the different types of materials cited above. It is the purpose of this paper to review the magnetic properties and their analysis in this context of materials science. In the short space available for this review paper, however, we restrict ourselves to the lamellar compounds. For the olivines, which are also quite promising cathode elements, we refer to a recent work devoted to LiFePO4, where this magnetic analysis has been reviewed [2].
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Two-dimensional materials are particularly suitable to intercalation reactions, because the weak Van der Waals interactions between the layers allow for the dilatation-contraction of the inter-slab space to adjust the lithiation-delithiation process. Among them, the two cathode materials that have been considered as most promising (and thus the most investigated ones) are LiCoO2 and LiNiO2, and their derivates obtained by partial substitution for the transition metal element that will also been reviewed. Nevertheless, as we shall see, even for such cases that are studies since decades, some results and analyses published recently must be revisited. It is also the purpose of this work to make an up-to-date analysis of the magnetic properties and make a clear distinction between intrinsic effects and extrinsic effects, since there has been confusion between the two of them in different instances. 2. Magnetic properties The layered intercalation compounds LiMO2 (e.g. LiCoO2, LiNiO2, LiNi0.5Mn0.5O2) crystallize in the α-NaFeO2 type lattice illustrated in Fig. 1. The structure is built with alternating layers of trigonally distorted MO6 and LiO6 octahedra sharing edges. The unit cell is rhombohedral (R 3m symmetry group). In the ideal case, the transition metal ions M are located in the octahedral 3a (000) sites. The oxygen anions are in a cubic close packing (ccp) occupying the 6c (00z) sites. Li cations reside at Wyckoff 3b (0,0,1/2) sites. The M ions and the Li+ ions occupy the alternating (111) planes. The magnetic properties and the electrochemical properties, however, are dependent of the choice of M, so the cases must be explored separately. 2.1. LiNiO2 Many papers have been devoted to this material, since sixty years [3], not only for the point of view of the electrochemical properties, but also for its magnetic properties. It is very difficult, if not impossible, to reach stoichiometry, since there is a tendency for Li and Ni to exchange their position so that the final composition of the samples is always Li1-zNi1+zO2. The magnetic properties depend very much on z, so that the material has been considered to be a ferromagnet [4], a ferrimagnet [3,5], a frustrated (triangular) spin ½ antiferromagnet [6–8]. At the end, the magnetic properties of this controversial material must be revisited. We have reported in Fig. 2 the temperature dependence of the inverse of the magnetic susceptibility χ of a Li1-zNi1+zO2 sample taken from [9]. Above T = 50 K, the magnetization M is a linear function of
Fig. 2. Temperature dependence of the inverse of the magnetic susceptibility χ=M/H (H = 10 kOe) of a Li1-zNi1 + zO2 sample taken from [9].
the magnetic field H: M = χH up to the highest magnetic field available in the experiments (30 kOe). In addition the Curie-Weiss law χ = C/(T-θ) is satisfied in this whole range of temperature 300 ≥ T ≥ 50 K. This large range of temperature allows for a determination of the slope of χ-1(T), and thus of the Curie constant C = Nμ2eff/ (3kB), with μeff the effective magnetic moment of the nickel ions in concentration N. The result is C = 4.26 × 10-3 emu/g, from which we deduce the experimental value of μeff, namely μexp eff = 1.81 μB. It is larger than expected, because in stoichiometric LiNiO2, the charge neutrality implies that all the nickel ions are in the Ni3+ state of charge and in the low-spin state. Now remember that Ni3+ in the low spin state has 1 unpaired electron, so S = 1/2, and the effective magnetic moment associated to it is μeff = [S(S+1)]1/2μB = 1.73 μB. The experimental value μeff = 1.81 μB is then evidence that all the nickel ions are not in the Ni3+ configuration: a fraction x of them is in the Ni3+ state, the other fraction 1-x is the Ni2+ state due to some deviation from stoichiometry. According to the chemical formula Li1-zNi1+zO2, the charge neutrality equation implies that each antisite defect amounts to a substitution of Li+-Ni3+ by Ni2+-Ni2+, so that the material contains on average 2z Ni2+ ions and (1-z) Ni3+ per chemical formula, so that the relative concentrations of nickel in the divalent and trivalent state are 2z/(1+z) ≈ 2z and (1-z)/(1+z) ≈ 1-2z to second order in z, respectively. Therefore: exp
μeff =
Fig. 1. Illustration of the α-NaFeO2 type lattice of LiNiO2.
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3+ 2+ 2 2 ð1−2zÞμeff Ni Ni + 2zμeff
ð1Þ
Taking into account that μeff(Ni3+) = 1.73, μeff(Ni2+) = 2.83, μexp eff = 1.81 μB, Eq. (1) gives the value of z = 0.027 (rather than z = 0.01 reported in [1]). Note the slope of Curie constant is proportional to (μeff)2, so the difference between 1.81 μB with respect to 1.73 means a change in the slope of the χ-1(T) curve by a factor (1.81/1.73)2, i.e a 9% change in the slope that is easily detected. That is why magnetic properties are so sensitive to the deviation from stoichiometry. Let us now investigate the properties at low temperature. At the blocking temperature TB = 8 K a cusp of the susceptibility curve is observed, as it can be seen in Fig. 3. In that case it is important to use two experimental procedures, i.e. field cooled (FC) and zero-field cooled (ZFC) procedures. The ZFC data are obtained by first cooling the sample down to the lowest temperature (4.2 K) without any magnetic field. Then a magnetic field (H = 10 kOe in the present case) is applied, and the magnetization is recorded upon increasing T. In the
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τ is the Néel relaxation time, and the value of the pre-factor τ0 simply comes from the fact that it must be an ‘atomic time’, i.e. a time characteristic of the atomic reactions. In practice, we measure the magnetization at a time t after application of the magnetic field. The time t = τ defines a temperature TB called the blocking temperature. The ‘laboratory time’ t being the order of half an hour, t = τ is achieved for an argument of the exponential equal to 25 [14]: KV = 25 kB TB :
Fig. 3. Field cooled (FC) and zero-field cooled (ZFC) magnetic susceptibility curves in the vicinity of the blocking temperature in Li1-zNi1 + zO2 (same sample as in Fig. 2, ref. [9]).
FC process, the field H is applied at room temperature and the magnetization is recorded during the cooling. If the peak of susceptibility is associated to a long-range antiferromagnetic ordering at the Néel temperature, there should be no difference, because thermal equilibrium is reached at the time-scale of the experiments. The measurement of FC and ZFC susceptibility curves in Li1-zNi1+zO2 reveals that the physical meaning of the susceptibility cusp is actually different, since this is the temperature where magnetic irreversibility occurs, evidenced by a departure between FC and ZFC curves. Indeed, no long-range ordering is expected in a lamellar material. Geometric frustration on a triangular lattice has been invoked for this particular material, but actually, this is not even needed. The 2D spin-1/2 Heisenberg system cannot order at finite temperature, because of zero-point quantum fluctuations, whether the system is ferromagnetic [10] or antiferromagnetic [11]. In the case of large values for the spin S, the quantum fluctuations become less important, and the magnetic ordering becomes possible, since the series development in 1/S of the spin waves after a Bogoliubov transformation of the Hamiltonian becomes convergent [12], but we are dealing here with spin systems where S is small (1/2 for Ni2+, 1 for Ni3+) so that magnetic ordering inside the uncoupled layers is not possible. On another hand, the lattice dimension D = 2 is the marginal dimension for the formation of self-trapped magnetic polarons [12], so that any defect can generate a ferromagnetic cluster in its vicinity [13]. These ferromagnetic clusters, here, are due to the ferromagnetic coupling of the Ni on site Li (noted NiLi), with the neighboring Ni ions in the Ni layers. At T N 8 K, these clusters are in the superparamagnetic state, i.e. each cluster acts as an effective spin that does not interact with the other ones, since the concentration of such clusters is far below the percolation threshold between them. To align the magnetic moment of the clusters along the external magnetic field, the magnetic moments must quit their polarization along the easy axis of magnetization, which costs energy proportional to the number of magnetic moments, i.e. to the volume V of the particle: Eanis = KV:
ð4Þ
If t b τ, i.e. T b TB, the particle has not enough time to reach equilibrium, hence the difference between FC and ZFC data in Fig. 3. At high temperature where the ferromagnetic ordering between NiLi and the neighboring Ni ions is broken by thermal fluctuations, in which case the magnetic response associated to NiLi is paramagnetic and just adds to the intrinsic contribution to the magnetic susceptibility. The paramagnetic Curie temperature is θ = +30 K for this sample (see Fig. 2). In the literature, θ is usually associated to the ferromagnetic coupling between NiLi and neighboring nickel ions in the adjacent atomic layers (leading to the formation of the ferromagnetic clusters at lower temperature). This, however, is unlikely. The reason is that the value of C shows that ALL the nickel ions contribute efficiently to the magnetic susceptibility in the paramagnetic regime, not only the few NiLi defects. Actually, the contribution of NiLi defects is about two orders of magnitude smaller than the experimental value of χ(T) after Eq. (1). For this reason, the value of θ should be more an intrinsic property, only affected marginally by the defects. Actually, if the value of θ were dictated by the defects, we would have the linear law θ∝z. Some authors have even proposed to use this law to determine z [15]. However, it does not hold true. We find that θ is quite a robust parameter, since it is in quantitative agreement with the value found by Yamaura et al. [16] for their sample closer to stoichiometry (μeff = 1.91 μB), and Chappel et al. reported θ = 26 K for a sample quasi stoichiometric (z = 0.004, μeff = 2 μB) [17]. From this feature, we can conclude that this value of θ is basically intrinsic. Note that this positive value implies that the magnetic interactions are dominantly ferromagnetic, so that the material cannot be considered as a frustrated antiferromagnet, as it has been proposed on several occasions already mentioned. The NiLi defect is not the only problem met with this material. Some impurity phase can also poison the material, depending on the synthesis process. The magnetization curves in Fig. 4 have been obtained for a sample prepared by a low temperature sol-gel method [18], while the sample in ref. [9] was prepared by the synthesis process described in [19]. Now Fig. 4 shows the onset of a ferromagnetic component at about T = 200 K, which is then the
ð2Þ
K is the anisotropy coefficient. Thermal equilibrium requires jumping over this energy barrier by thermal activation process, so that the Arrhenius law gives the time necessary to respond to H: KV −13 ; τ0 ≈10 τ = τ0 exp s: kB T
ð3Þ
Fig. 4. Magnetization curves for another Li1-zNi1 + zO2 sample from ref. [17], different from the sample of ref. [9] analyzed in Figs. 2 and 3 (see text).
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Curie temperature TC for the ferromagnetic impurity responsible for this effect. Moreover, the plot of the FC and ZFC curves M/H measured at H = 10 kOe as a function of the temperature in Fig. 5 shows that the onset of irreversibility also takes place at Tc. Therefore, the impurity particles are big enough to be “blocked” as soon as they become ferromagnetic. At least, the magnetization is linear in H at room temperature, which excludes the presence of nanoparticles of nickel, since the Curie temperature for Ni is much larger than 300 K. The fact that TB = 200 K also excludes the possibility that the magnetic particles take their origin from NiLi. In effect, we have seen in section I that, in presence of NiLi, the system remains paramagnetic, and the Curie-Weiss law remains valid down to 50 K. We are thus in presence of ferromagnetic clusters of a secondary phase. To identify it, we look in the literature for some Ni compounds with a magnetic ordering at about the same temperature. The answer is provided in Fig. 6, which illustrates the magnetic properties of Li0.05Fe0.02Ni0.93O [20]. The analogy is obvious, even if the Curie temperature is 250 K rather than 200 K. This shift can easily be understood if we note that the Curie temperature is very sensitive to the actual composition of the NiObased material. Therefore, the nature of the clusters of size larger than 1 nm evidenced in the Li1+zNi1-zO2 of ref. [18] is attributable to nonstoichiometric NiO with some impurities, the main one being Li (note surprising since there is Li in the chemical formula of the host). 2.2. LiNi1-yCoyO2 LiNiO2 has two problems. We have just shown that Ni has a tendency to occupy Li sites and generate a deviation from stoichiometry that is damaging to the electronic performance. The second problem is due to the fact that Ni3+ is a Jahn-Teller ion, which means that the d-electrons in this configuration create a local lattice distortion. The accumulation of such distortions upon cycling favor the formations of cracks, grain boundaries and other extended defects that reduce the lifetime of the battery when this material is used as the active element of the cathode. Since both problems come from the Ni3+ ions, many attempts have been made to reduce their concentration by substituting them for another transition metal, in particular Co, since the end the series, LiCoO2, is a well known cathode material of the first generation of Li-ion batteries. The conclusion that can be found in the literature is usually that the introduction of Co in LiNi1-yCoyO2 is indeed benefit to the reduction of the stoichiometry parameter z. Let us show, however, that this conclusion is not necessarily correct. We return to the results of ref. [18] where LiNi1-yCoyO2 has been prepared for different values of y, using the same synthesis process [19,20] as the one used for the sample y = 0 analyzed in the previous section. The magnetization curves are illustrated in Fig. 7. They are quite similar to the magnetization curves observed for the Li1-zNi1+zO2 (z = 0.027) sample taken from [9] that we have investigated before through the analysis of Figs. 2 and 3. In addition, for this sample, we
Fig. 5. Field cooled and zero-field cooled magnetic susceptibility of the same Li1-zNi1 + zO2 sample as in Fig. 4 from ref. [17].
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Fig. 6. Magnetization of Li0.05Fe0.02Ni0.93O, after ref. [19].
recover the value TB ~ 8 K that we have associated to NiLi. This result shows that the introduction of a concentration y = 0.2 of Co has not cured the departure from stoichiometry, but has suppressed the impurity phase. This conclusion is confirmed by the investigation of the mean size d of the ferromagnetic clusters of volume V = d3. In case TB is the same as the Curie temperature TC of the magnetic clusters (the case we have
Fig. 7. Field-cooled (FC) and zero-field cooled (ZFC) susceptibility curves (a) and magnetization curves (b) for LiNi1-yCoyO2 for y = 0.3 after ref. [17].
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encountered when the magnetic cluster is the non-stoichiometric NiO impurity cluster), we can only write that d N dm with dm = (25 kBTB/K)1/3, because the only reason for the absence of magnetic irreversibility at larger temperature comes from the fact that the impurity particles are not ferromagnetic, irrespective of their size. On another hand, we can write d = dm like in Eq. (4), when we are in the situation where the blocking temperature is smaller than TC because the onset of magnetic irreversibility is now a size effect. The determination of d or dm after Eq. (4) still requires the determination of K, which can be achieved by the analysis of the magnetic response of the impurity clusters to the application of the magnetic field, since the anisotropy opposes the alignment of the magnetic moment along the applied field H. The magnetization can be decomposed under the form: MðHÞ = Mcl + χ int H;
ð5Þ
where χintH is the intrinsic part that is linear in H, and Mcl the extrinsic contribution of the magnetic impurities. Mcl saturates easily upon application of the external field, according to the law [18]: 2 2 Mcl ðH Þ = MS 1−a = H−b = H ; b = βðK =Ms Þ :
ð6Þ
The term a/H is the thermal effect arising from the series development of the Langevin function; and the b-term is the anisotropy effect. The coefficient β only depends on the nature of the material, and is β = 0.0762 in the present case [18]. The fit of the magnetization curves in the high field domain can thus be used to determine MS and b after the first equation in Eq. (6), then K can be deduced form the second equation. Therefore, we can deduce from Eq. (4): 3
V = dm =
25kB TB Ms
rffiffiffiffi β b
ð7Þ
The result is displayed in Fig. 8. The decrease of d(y) illustrates the shrinking of the impurity clusters that virtually disappear at y = 0.2. At y N 0.2, however, we are left with the small magnetic clusters formed by NiLi. The small size (about 0.4 nm) shows that the cluster results essentially from the ferromagnetic pairs formed by NiLi and its nearest neighbors on adjacent planes. This size is a characteristic of the defect and only depends on the topology of the lattice, so that is independent of y, at least in first approximation, since a small variation of the exchange coupling may result from the variation of the lattice parameter as a function of y. It confirms that the introduction of Co has been efficient to get rid of the impurity
Fig. 8. Size d of the magnetic clusters in LiNi1-yCoyO2. For small values of y, only a lower limit dm can be determined, as mentioned in the figure.
phase, but failed to remove NiLi defects, contrary to what was expected (and reported in the literature). 2.3. LiCoO2:B LiCoO2 is in continuity with the solid solution of the previous section, since it corresponds to the end-member y = 1 of the family. It has been the most widely used commercial cathode material for Li-ion batteries for thirty years [21,22]. Due to the stronger and more covalent bonding in the CoO2 slabs, LiCoO2 is less sensitive to synthesis condition than LiNiO2. Single-phased samples free of any impurity are normally obtained through solid-state reaction of lithium salts and cobalt oxides. The strong covalent bond in LiCoO2, with reduced Co-O bond distance, results in stabilization of Co3+ in low-spin ground state, i.e., d6 = (t2g)6(eg)0, S = 0 [21]. Three decades of intensive studies of all the physical properties, including magnetic, of this material would suggest that there is no need to make any further investigations. However, this is not true. There are recent works to claim that LiCoO2 is a realization of a frustrated two-dimensional triangular antiferromagnet (2D-AF) [23]. We have already mentioned in the previous section that this concept had been wrongly applied to LiNiO2. Of course this is not compatible with the spin state S = 0 of Co3+, so different hypothesis have been made to explain this 2D-AF behavior, including a disproportionation 2Co3+ → Co2+ + Co3+ [23]. Recently, the claim that stoichiometric LiCoO2 would be antiferromagnetic with a Néel temperature TN = 30 K by the same group [24] must also be revised, since this is in contradiction not only with the low spin state S = 0 of Co3+ but also with the hypothesis of two-dimensional triangular AF: we have already mentioned in the discussion on LiNiO2 that the 2D-AF does not order at finite temperature because of quantum fluctuations. To clarify this situation, we have investigated the magnetic properties of LiCoO2 doped with boron. Alcantara et al. [25] have studied the effect of boron doping on LiCoO2. Galvanostatic cycling of LiCo0.95B0.05O2 revealed that B dopant improves the reversibility of the Li de-intercalation/intercalation process and favors lattice adaptation to Li order-disorder in the depleted LiO2 layers. The boron doping is thus useful to improve the electrochemical properties, and Nazri et al. [26] have shown that the solubility limit for the formation of solid solution upon substitution of B in LiCoO2 was around 25%. We took this advantage to prepare LiCo1-yByO2 up to this composition y = 0.25. We shall report in a forthcoming paper that the boron doping improves the electrochemical properties by preventing the Verwey transition upon delithiation. The Verwey transition, originally discovered by Verwey in Fe3O4 is a generic name for the charge ordering that happens when the magnetic ion is 50% in one state of charge and 50% in another one. This situation is met in Li0.5CoO2 (Co3+ and Co4+), and the Verwey transition implies that it is not possible to decrease the delithiation process beyond this composition. The present work being devoted to magnetic properties, we present here the results of magnetic measurements for this composition, but the same analysis of the magnetic properties applies to any sample with smaller values of y, including y = 0. The magnetization curves are shown in Fig. 9. The magnetization curves are not linear in H at small field, which is evidence of the existence of small ferromagnetic clusters. The magnetization in this case is just the addition of the contribution of this impurity, which saturates easily in field to a value Ms, and the intrinsic part that is linear in H like in Eq. (4). Following the procedure described in prior works [27,28], we determine Ms by the intercept of the linear law M(H) at H N 10 kOe with the ordinate axis H = 0. Since the value of Ms does not depend on temperature, the Curie temperature of the impurity phase is much larger than room temperature, but remains unknown. The consequence is that we cannot identify the origin of the impurity from the magnetic properties alone, and electron spin resonance (ESR) experiments are needed. The ESR spectrum (not
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MS despite the very small amount of nickel. It is expected, because Co is trivalent and its ground state is non-magnetic (S = 0), so that the intrinsic signal is very small. On one hand, χ depends on temperature, which reveals the existence of a Curie-Weiss contribution. On another hand, 1/χ is not linear in temperature, which shows that the constant term χ0 of the so–called modified Curie-Weiss law: χðT Þ = χ0 + C = ðT + θÞ
Fig. 9. Magnetization curves of LiCo0.25B0.75O2. The straight lines are the intrinsic components (drawn at two temperatures only for clarity). Their extrapolation to H = 0 gives the magnetic moment Ms associated with nickel (in concentration 60 ppm) that has precipitated under the form of ferromagnetic nanoparticles.
shown here) reveals a resonance at a magnetic field corresponding to the gyromagnetic factor g = 2.2, which is characteristics of nickel metal [29,30]. Therefore, the ferromagnetic particles are identified as Ni nanoparticles due to the presence of Ni as a residual impurity in the commercial cobalt used as a precursor to prepare the samples. Of course the quantity of nickel is not large, its amount deduced from the experimental value of MS, is 60 ppm of Ni in the sample. It is, however, sufficient to give rise to this parasitic magnetic signal, another illustration of the sensitivity of the magnetic properties. Note the value of MS does not depend on temperature. This is simply because the Curie temperature of Ni is huge, so that the magnetic moment of the Ni clusters is saturated even at room temperature. Also the Ni particles are very small (not surprising since the amount of Ni is so small), so that he blocking temperature is too small to be detected in the experiments. The second step is the study of the intrinsic magnetic susceptibility defined as χ = (M-Ms)/H. It is illustrated in Fig. 10 where we have reported 1/χ rather than χ. Note the importance of the subtraction of
Fig. 10. Plot of the inverse of the magnetic susceptibility of LiCo0.25B0.75O2 as a function of temperature. Only half of the experimental data H/M measured in H = 10 kOe have been reported for clarity (circles). The triangle symbols are obtained after subtraction of the spurious contribution of ferromagnetic cobalt clusters: χ-1 = H/(M-Ms), where Ms is defined in Fig. 6.
ð8Þ
is not negligible. χ0 is the intrinsic contribution due to Co3+ ions. Co3+ is in low-spin state, so that its ground state is the tg6 configuration that is non-magnetic (S = 0). However, an exited state is magnetic, giving rise to Van-Vleck paramagnetic contribution χp. More precisely, χp is associated to the coupling between the non-degenerate (i.e. nonmagnetic) ground state and the magnetic excited state of these ions by the spin-orbit operator. χp is thus the unquenched orbital moment contribution to the magnetic susceptibility. Since χp is larger than the diamagnetic contribution χd of the core, χ0 = χp + χd is positive [31]. The fit is excellent and illustrated in Fig. 11. Let us now discuss the values of the fitting parameters. The value of the Curie constant is very small. We presume it is due to the presence of Co4+ generated by Li vacancies. We know from earlier studies on LixCoO2 that, in the range 0.95 ≤ x ≤ 1, the Co4+ ions are in the high-spin state [32], so that the Li-vacancy concentration that fits our experimental data is 0.09%. The other fitting parameter entering the Curie-Weiss term is θ = 2 K. Such a small value is expected, because the concentration of Co4+ spins is so small that they do not interact significantly so that their contribution to the magnetic properties is close to the Curie law (case θ = 0). The last fitting parameter is χ0 = 5x10-5 emu/mole. This value of χ0 compares well with the experimental results observed for instance in CoO [33,34] Co3O4 [35] and in Na1-xCoO2 [36]. In this last case that is close to our material, the diamagnetic contribution is about 3.5× 10-5 emu/mole [36]. If we retain this value for our material, the value of χ0 leads to an estimation of χp = 8.5 × 10-5 emu/mole in agreement with the theoretical value that we have calculated in the past for this tg6 orbital configuration appropriate to Co3+ and Fe2+ [31]. Therefore, the magnetic properties have been quantitatively described. We have illustrated the study for a Boron concentration of 25 at.%, but we have found the same results at any (smaller) boron concentration. As a consequence, LiCoO2 is not a two-dimensional frustrated AF lattice, there is no disproportionation of the cobalt in it, and the cobalt is in the Co3+ state, except for a very small concentration (b0.1%) of Co4+ ions due to a residual concentration of Li vacancies. Note this property is a general property of colbatates, since it also applies to NaCoO2 for instance [37]. The misunderstanding in the
Fig. 11. Magnetic susceptibility curve of LiCo0.75B0.25O2 and its theoretical fit.
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analysis of LiCoO2 showing AF behavior was to consider it as an intrinsic effect. Instead, it gives evidence of an impurity. In particular, the magnetic ordering in [23] is the signature of Co3O4 used as a precursor, which still remains under the form of a nano-structured impurity phase in the sample of ref. [23]. Indeed, bulk Co3O4 orders antiferromagnetically at 40 K, and the ordering temperature of nanoparticles of Co3O4 depends on the size of particles, hence the width of the transition that has been observed.
2.4. LiNi0.5Mn0.5O2 To improve the thermal and the structural stability of LiCoO2, various attempts have been made to replace Co by other transition elements. Among the most investigated combinations is LiNi0.5Mn0.5O2. The manganese is in the Mn4+ state and is here only to improve the structural stability, while the nickel insures the electrochemical properties, as it is in the Ni2+ state, and can change valence upon delithiation in LixNi0.5Mn0.5O2 up to Ni4+ in the fully delithiated state x = 0 [38–40]. The magnetic properties have been explored in [41] and can be summarized as follows. At high temperature (T N 200 K) where the Curie-Weiss law is satisfied (see Fig. 12), the effective magnetic moment deduced from the Curie constant is in agreement with the theoretical value deduced from the moments of Ni2+ (S = 1, μeff = 2.83 μB) and Mn4+ (S = 3/2, μeff = 3.87 μB). The value of the Curie-Weiss temperature is θ = -86 K. The negative sign means that the magnetic interactions are dominantly antiferromagnetic. However, a ferromagnetic component takes place at 140 K, which again is attributable to the presence of Ni2+(3a) (or NiLi in the Kröger-Vink notation), since this defect generates a 180° interlayer Mn4+(3b)-ONi2+(3a) superexchange interaction that is ferromagnetic after the Goodenough rules [42]. Indeed, this interaction is responsible for the ferromagnetic ordering of Bi2NiMnO6 [43] at the Curie temperature 140 K, and the onset of the ferromagnetic component LiNi0.5Mn0.5O2 is due to the freezing of the spins of the Mn4+(3b)-Ni2+(3a) in the ferromagnetic arrangement that takes place at the same temperature. The magnetic moment associated to this ferromagnetic pair is thus gμB[S(Ni2+) + S(Mn4+)] = 2 μB(1 + 3/2) = 5 μB. The amount of such pairs is then readily deduced from the magnetization at saturation of the extrinsic part of the magnetization at low temperature (i.e. Mcl in Eq. (5)). The procedure will be illustrated in the next section on the example of LiNi1/3Mn1/3Co1/3O2. The lowest value obtained for the best sample is that 7% of the nickel is on the Li sites. This value is in agreement with the value deduced form the Rietveld refinement of the X-ray diffraction pattern, and shows that the cation disorder is even worse than in LiNiO2.
Fig. 12. Inverse of the magnetic susceptibility of LixNi0.5Mn0.5O2 as a function of temperature at x = 1 and x = 0, after ref. [40].
Magnetic properties of LixNi0.5Mn0.5O2 at different stages of delithiation have also been investigated. In the case x = 0 (see Fig. 12), the change of slope of χ-1(T) at high temperature is due to the change of μeff that is in agreement with the theoretical value 4.41 μB expected for the combination of the spin-only value of Mn4+(S = 3/2) and Ni4+ in the high-spin state (S = 2). This high-spin configuration for nickel in the charged state contrasts with the low-spin configuration of Ni3+ (S = 1/2) found from the Curie-Weiss law at x = 0.5 [44]. The low spin configuration violates the Hund's rule that applies in absence of crystal field. Therefore, the low-spin state is the signature of large crystal field, which is due to the fact that Ni3+ is a Jahn-Teller ion. The local lattice distortion and strain field associated with the presence of Ni3+ in the intermediate delithiation rates evidenced by this low spin state of Ni3+ is damaging to the cycling life of the battery and is then a inconvenience revealed by the magnetic properties. The other parameter of the Curie -Weiss law is θ that has shifted from θ(x = 1) = -89 K to θ(x = 0) = -235 K so that the antiferromagnetic interaction has been enhanced. This feature is attributable to the fact that the Mn4+(3b)-O-Ni2+(3b), which are only weakly antiferromagnetic, have been replaced by Mn4+(3b)-O-Ni4+(3b) that are strongly antiferromagnetic upon the Goodenough rules [42]. The results in Fig. 12 also show that the ferromagnetic spin freezing remains unchanged, and the magnetic properties are almost independent of x below 200 K, i.e. in the temperature range where the magnetization is dominated by the ferromagnetic spin freezing of the Mn4+(3b)-O-Ni2+(3a) pairs. This feature gives evidence that the nickel on the (3a) sites remain in the in the divalent state upon delithiation. Indeed, the Mn4+(3b)-O-Ni2+(3a) pairs is unaffected by the delithiation process that only results in a change of valence of the nickel on (3b) sites. 2.5. LiNi1/3Mn1/3Co1/3O2 We have seen in the previous sections that both the structural stability and the cation disorder are problems in LiNiO2; the introduction of Mn in LiNi0.5Mn0.5O2 insures a better stability, but has worsened the cationic disorder. To reduce the cationic disorder, Co has to be added, hence the efforts made to investigate LiNi1/3Mn1/3Co1/3O2 (named LNMCO hereafter). Ohzuku's group has first introduced it in 2001 as a candidate of cathode materials [45,46]. The whole magnetic properties have been detailed in [47], and we focus attention here on the investigation of the cation disorder than can be determined from their analysis. In continuity of the lamellar compounds studied in the previous section, the charge compensation is achieved by stabilization of Ni2+, Mn4+ and Co3+ ions. Just like in LiNi0.5Mn0.5O2, the Ni2+(3a) defect
Fig. 13. Magnetization curves of LNMCO. The concentration of Ni(3b) defects is estimated from MS.
X. Zhang et al. / Solid State Ionics 188 (2011) 148–155
generates a Mn4+(3b)- Ni2+(3a) ferromagnetic pair at low temperature. This pairing is responsible for the deviation of the magnetization curves at low temperature as shown in Fig. 13. At high filed and low temperature, the magnetization due to the Mn4+(3b)- Ni2+(3a) ferromagnetic pairs is almost saturate to a value Mcl = MS in Eq. (5), so that MS can be determined by extrapolation of the linear portion of the magnetization curves to H = 0 (see the Fig. 13). Taking into account that the magnetic moment of each pair is 5 μB (see section on LiNi0.5Mn0.5O2), we find for this sample that the substitution rate is 1.8%, in quantitative agreement with the Rietveld refinement of the XRD spectra [47]. This is so far the sample with the smallest cationic disorder that has been synthesized. 3. Concluding remarks We have reported the magnetic properties of various lamellar samples: LiNiO2, LiNi1-yCoyO2, LiCoO2, LiNi0.5Mn0.5O2, and LiNi1/3Mn1/ 3Co 1/3O2. They have been chosen because they are the most investigated ones and the most promising for applications as cathode elements of Li-ion batteries. The analysis has allowed us to revise some ideas that have been published even in the recent past, concerning for instance the idea that these materials were examples of frustrated 2D triangular antiferromagnetic lattice; actually, none of them belong to this family. Of course different composition have been made and it is not possible to review all of them. However, their magnetic properties can be readily extrapolated from the cases we have analyzed here for these materials, using the same procedures and analysis used in the present work. In particular, the investigation of the magnetic properties prove to be the most sensitive tool to detect the impurity and defects that control the electrochemical properties. In all the cases, it was possible to identify the defects and the impurities when they exist, and also to determine their concentrations, even in concentration much to small to be detected by XRD or any other technique. These analyses also prove that the LMCO is the most promising element of the family, since it can be synthesized with a cationic disorder that is very small, and free from impurity. This has been confirmed by the electrochemical properties recently reported for this sample [46]. References [1] C. Julien, in Materials for Lithium-Ion Batteries, Nato Science Series 3. High Technology – Vol. 85, edited by C. Julien and Z. Stoynov, Kluver Academic Publishers (Dordrecht, Boston, London), p 1 (2000). [2] K. Zaghib, A. Mauger, J.B. Goodenough, F. Gendron, C.M. Julien, in: J. Garche (Ed.), Encyclopedia of Electrochemical Power Sources, Five-volume set, Elsevier Science, 2009. [3] J.B. Goodenough, D.G. Wickham, W.J. Croft, J. Phys. Chem. Solids 5 (1958) 107. [4] R. Stoyanova, E. Zhecheva, C. Friebel, J. Phys. Chem. Solids 54 (1993) 9. [5] K. Hirikawa, H. Kadowaki, K. Ubukoshi, J. Phys. Soc. Jpn 54 (1985) 3526. [6] M. Itoh, I. Yamada, K. Ubukoshi, K. Hirikawa, H. Yasuoka, J. Phys. Soc. Jpn 55 (1986) 2125.
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