EPR studies of Li1−x(NiyCo1−y)1+xO2 solid solutions

Solid.State Communications, Vol. 102, No. 6, pp. 457-462, 1997 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 003&1098/97 $17.00+.00

PII: SOO38-1098(97)0

EPR STUDIES R. Stoyanova,”

OF Li I_~Nii,Coi-y)l+X02

E. Zhecheva,”

R. Alcintara,b

SOLID SOLUTIONS P. Lavelab and J.L. Tiradob

“Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria blaboratorio de Quimica Inorg&tica, Facultad de Ciencias, Universidad de CXrdoba, Avda. San Albert0 Magno s/n, 14004 Cordoba, Spain (Received 27 November

1996; accepted 9 January 1997 by hf. Cardona)

Ni3+ EPR measurements were used to investigate the electronic structure and ion distribution of Li1_~Ni,CoI-Y)l+X02 (y = O.I3 and 0.8 5 y 5 1~01 solid solutions prepared between 450 and 800°C. The data on the Ni3 distribution were obtained from the analysis of the EPR line width. The results show that a transition from non-random to a random Ni/Co distribution in the Ni,Co1_,02 slabs occurs between 650 and 750°C. 6Li solid state MAS NMR and magnetic susceptibility measurements are in good correspondence with this interpretation. 0 1997 Elsevier Science Ltd Keywords: Electron paramagnetic nickel oxide.

resonance,

A substantial improvement of the performance of lithium rechargeable batteries was recently achieved by the application of the lithium-ion concept. In this type of cell, both active electrode materials are chosen among the numerous solids that intercalate reversibly lithium. Due to their good cycling performance and enhanced energy density, three mixed oxide systems have deserved particular interest as the cathode: LiMn204, LiNiOz and LiCoO*. For LiCoOz, two structural varieties are known: a high-temperature modification I-IT-LiCo02 characterized by a layered structure (R 3 m, e.g. c/u = 4.98 [l, 21) and a low-temperature modification LT-LiCoO* with intermediate structure between the ideal layered and the spine1 structure (e.g. R 3 m and Fd 3 m, respectively, c/u = 4.90 [3-111). The nickel analogue of HT-LiCo02 has a trigonal crystal structure in which, on the contrary to LiCoOz, internal disorder reactions cause a partial mixing of Li and Ni into the “alien” layers [12, 131. In addition, non-stoichiometric Li 1_,Ni 1+XO2 (0.0 < x < 0.4) oxides are usually obtained, due to the difficult oxidation of Ni2+ to Ni3+. The impurity Ni2+ ions disturb further the long-range ordering of Li and Ni, which, in turn, deteriorate the electrochemical properties of Li1_XNiI+X02 [ll-141. The nickel analogue of LT-LiCo02 has not been obtained up to now owing to the low reactivity of NiO towards the lithium salts. However, small additives of Co to NiO improve the 457

lithium cobalt oxide, lithium

reactivity of the latter. As a result, nonstoichiometric LT-Li1_X(Ni,Co1_,.)1+X02 with x = 0.85 and with a structure closer to the trigonal modification is obtained at 450°C [15]. Solid solutions of general formula Li ,_#$Col_,.) ,+,O, have electrochemical properties better than those of Lil-XNil+x02 and LiCo02 [16, 171. It is established that, in the case of the high-temperature modification, small cobalt additives to Li,_XNi1+X02 stabilize both the Ni3+ ions and the layered crystal structure [18]. With increasing cobalt content, the trigonal distortion of the crystal lattice increases and the deviation from the stoichiometry of Li &$Col_Y) 1+XO2decreases (x - 0) [18]. On the contrary, with the low-temperature modification of LiCo02, the nickel is dissolved up to 40 at.% [4, 151, the trigonal distortion of the crystal lattice being independent of the Ni/Co ratio [4]. Recently, 6,7Li MAS NMR [19, 201 was found to be a valuable tool for studying the local environment of the lithium ions in respect to Ni/Co in two adjacent layers of the mixed oxides. The electronic structure of Ni3+ and co3+ ions in these oxides (low-spin, t&e: and t&e~, respectively) makes Ni3+ an interesting probe of the electron-electron and electron-nuclei interactions in the solid solutions. Here we show the results of the EPR characterization of these solids obtained under different experimental conditions. A comparison with

EPR STUDIES OF Lil-#?i,CZol_Y)l+~O~ SOLID SOLUTIONS

458

6Li MAS NMR and magnetic susceptibility measurements is also included. The LiNi,Col_,,Oz solid solutions, 0 I y 5 1, were prepared by solid state reaction between LiN03 and Ni~Co~-~(O~~O~)H*O at 850°C for 30 h under air (for cobalt-rich compositions) and under oxygen (for nickel-rich compositions). Special attention was paid to the preparation of cobalt-rich and nickel-rich oxides at low- and high-temperatures. In this case, the solid reaction proceeds between LiOH and (Ni~Co~-~)~O~spine1 oxides for cobalt-rich samples (y = 0.13) and between LiOH and Ni,Cor_,O with a rock-salt structure for nickel-rich samples 0, = 0.88). The nickel-cobalt spine1 and rock-salt oxides were prepared by thermal decomposition at 400°C of nickel-cobalt hydroxide-~rbonates co-precipitated at pH = 8.5 and 10.5, respectively. The solid state reaction proceeded in air at 450, 650 and 750°C and 800°C for 100,30 and 24 h, respectively. The lowest synthesis temperature was chosen on the basis of the melting point of LiOH (T = 450°C). The chemical impositions dete~ined from atomic absorption spectroscopy and iodometric titration data and the unit cell parameters calculated for a trigonal unit cell, a and c, are given in Table 1. For cobalt-rich composition, the changes in the trigonal distortion of the cubic lattice (c/a ratio) are not concomitant with significant changes in oxide stoichiomet~, while for nickel rich ~m~sition there is a close relationship between the trigonal distortion of the cubic lattice and the oxide stoichiometry (Table 1). The distribution of Ni3+ ions in the (Ni,Co1_,,)02layers was monitored by EPR. Recently, the relationship between the EPR parameters of low-spin Ni3+ ions and the structural peculiarities of HT-solid solutions Lil_x(NiyCol-y)l+XO~ was reported [21-241. The results obtained can be summarized as follows: (i) The Ni3+ ions (up to 5%) substituting isomorphitally the Co3+ ions in the CoOz-layers give at T > 30 K an isotropic Lorentzian line due to the dynamic JahnTeller effect 1211. The Jahn-Teller effect and the spin lattice relaxation determine an “U”-shaped dependence of the EPR line width on the registration temperature. In

Vol. 102, No. 6

the vicinity of 200 K, the line width reaches its minimal value. The g-factor of the low-spin Ni3+ ions depends on the nearest metal shell: g = 2.142 for Ni3+ in a Co3+su~ounding and g = 2.137 for Ni3+ in a Ni3+, Co3+surrounding [21, 221. (ii) For the trigonal LiNiOr, the two-dimensional ferromagnetic Ni3+- O-Ni3+ interactions in the NiOzlayers cause the appearance of an exchange narrowed Lorentxian with g = 2.137 ]22]. The competition between the fe~oma~etic iteration and the Jahn-Teller effect results in the complex temperature behaviour of the EPR line width of a “narrowing up to 100 K-broadening between 50 and 100 K-narrowing up to 10 K” type [22]. An essential parameter derived from the EPR of Ni3+ in Ni-doped LiCoOz and LiNiO* is the EPR line width in the temperature range where it retains a constant value: this is the line width determined between 180-220 K and 85-100 K for both types of samples, respectively. Figure 1 shows the “temperature independent” line width, AHpr,,as a function of the Ni3+ content in LiNi,Co1_,,02 obtained at 850°C. Three regions can be divided: up to 10% Ni3’, the line width increases slightly; between 10 and 60% Ni3+, there is a strong linear broadening; and above 60% Ni3+, the line width decreases. In the same sequence, the line shape is changed: the Lorentzian function fits the EPR line for compositions with 0 < y < 0.10 and 0.6 < y ZG1.0, while the fitting with a Gaussian is better for compositions with 0.1 5 y 5 0.6. The observed dependence of the line width and the line shape on the nickel content can be considered as a result from the magnetic dipole-dipole and exchange interactions between the Ni3+ ions. The di~le-di~le coupling between the spins causes positive and negative shifts of the EPR line as compared to the EPR line of the isolated spins, these shifts being proportional to the distance between the spins, l/r3. In the case of a random distribution of spins, the convolution of the lines gives a complex

Table 1. Composition and c~?$tal data of the Lir_, (Ni,CoI_Y)r+XO~solids No.

T”C

y

l-x

a (;i>

c t;i>

da

1 2 3 4 5 6 7

450 650 750 800 450 650 750

0.13 0.13 0.13 0.33 0.88 0.88 0.88

0.975 0.971 0.991 0.994 0.842 0.905 0.961

2.8219 2.8221 2.8208 2.8210 2.8700 2.8603 2.8663

13.839 13.936 13.996 14.004 14.070 14.087 14.128

4.904 4.938 4.962 4.964 4.903 4.925 4.929

Fig. 1. Temperature independent EPR line width, AH; vs Ni3+ content for LiNi,Cor_,,OZ solid solutions obtained at 850°C.

EPR STUDIES

Vol. 102, No. 6

OF Li 1_,(Ni,Co I-,,) 1+X02 SOLID SOLUTIONS

envelope where the resulting line shape and line width depend on the amount of the spins and the distance between them. For magnetically diluted systems, the dipole-dipole interactions perturb only slightly the line shape of the isolated spins. The drastic changes in the line shape and line width take place when the magnitude of the dipole-dipole interactions is higher than the line width of the isolated spins. According to the method of moments, the contribution of the dipole-dipole interactions to the EPR line width is expressed by AH& = const g2S(S + 1) x

l/r:

(1)

i,k

and the line shape of the resulting envelope is close to a Gaussian function [23]. As one can see, the magnetic dipolar broadening explains well the changes observed in the line shape and line width for the samples with O 2a as isolated Ni”+ ions. When the distance between the Ni.3+ ions is lower than 2a, r 5 2a, the line width is strongly increased (Table 2). In this case, the convolution of the individual lines gives a resulting Gaussian envelope as it was observed for the samples with 0.1 < y < 0.6 (region II in Fig. 1). We shall denoted the ions located at a distance lower than 2a as nonThus, the analysis of the EPR line isolated Ni3+ ions. . Table 2. Contribution of the nth neighbours in a triangular lattice of Ni3+ ions to the dipolar broadening C.S. = coordination sphere, Nn = No. of neighbours C.S.

Distance

Nn

(AH$$2, %

AHdd pp’ mT

1 2 3 4 5 6 7 8

a

6 6 6 12 6 12 6 6

94.20 3.49 1.47 0.55 0.13 0.08 0.05 0.02

117.7 22.6 14.7 6.4 4.4 2.4 2.7 1.7

3 ll’a 2a

7 1na 3a

13’“a 2a3 “’ 4a

459

width enables to distinguish the contribution of Ni3+ at r > 2a and Ni3+ at I I 2a to the EPR line width. For the samples with a higher Ni content 0, > 0.6, region III in Fig. l), the line narrowing observed is a result from the developing, in addition to the magnetic dipolar coupling, of the ferromagnetic Ni3+--O-Ni3+ interactions AHpp = const

g*S(S + 1) Ci.k l/r$ J8/3S(S

+ 1)~ J ’

(2)

where z is the coordination number of Ni3+ in the NiO*layers and J is the exchange integral between the Ni3+ ions [24, 251. Substituting the extrapolated value of the magnetic dipolar broadening in LiNi02, from equation (2) we can estimate the exchange integral: J - 0.2 cm-‘. This value seems to be reasonable if we consider that the spin correlations in stoichiometric LiNiOI occur below 9 K [26]. For LiNi,Co ,-YO2 solid solutions, the effect of the exchange narrowing appears at y 2 0.62 (region III in Fig. 1). Having in mind that exchange interactions are developed in an infinite cluster of bonds, this value corresponds to the percolation threshold for Ni3+-Ni3+ bonds in LiNi,Co,-YOz solid solutions. However, for a triangular lattice, the percolation threshold for bonds is 0.5 which indicates that the Co3+-Co3+ interatomic interactions are higher than the Ni3+ -Ni3+ interatomic interactions. Applying the analysis of the EPR line width, we can extract information on Ni3+ distribution in cobalt-rich samples prepared at low- and high-temperatures (450, 650, 750 and 800°C). The EPR spectra of these samples contain two lines: one narrower Lorentzian with g = 2.142 and other broader Gaussian with g = 2.137 (Fig. 2). For the sample prepared at 800°C only the broader signal could be resolved (Fig. 2 and Table 3). For both signals, the line widths have an “U’‘-shaped temperature dependence (Fig. 3): from 400 to 250 K the line becomes narrower, between 250 and 160 Kit remains constant and below 160 K, line broadening is observed again. According to our previous investigations [21, 221, these two signals correspond to two kinds of low-spin Ni3+ ions characterized with different Ni-0 bond covalency and metal ion surrounding: isolated and non-isolated Ni3+ ions, respectively. Using the experimentally determined relationship Al$, = F(y) (Fig. l), the amounts of isolated and non-isolated Ni3+ ions are given in Table 3. It should be noted that the total content of isolated and nonisolated Ni3+ ions ’ calculated from EPR data is comparable to the total nickel content determined from chemical analysis. For comparison, the ratio of the isolated and non-isolated Ni‘3+ ions can also be determined from the intensities of both signals (Table 3). Irrespective of the method of the estimation, for all samples studied the amount of non-isolated Ni3+ ions exceeds considerably the amount of isolated Ni3+ ions. When the preparation

Vol. 102, No. 6

EPR STUDIES OF Li r_x(Ni,Cot_,),+,Oz SOLID SOLUTIONS

460

T=213

K

11

4.5[’ 8 ’ 100 200

3

’ 300

3

’1 400

I,

SOI’ 100

T Wf

_.

_..... _.

I

/

1

I

I

I

300

400

500

’ 200

’

’ 300

I’ 400

1’(K)

Fig. 3. Temperature variation in the EPR line width of the signals with g = 2.142 (A) and g = 2.137 (B) for Li1_x(Ni0.13C00.87)1+x0~ obtained at 450 (a), 650 (b), 750°C (c) and 800°C (d).

a + IO0 200

’

B (mT)

for a random distribution and thus the signal ratio Fig. 2. EPR spectra at 213 K of Li1_~(Ni0.13CO0.S7$1+*02 S-l/(S - 1 + S - Z), a value of 0.08 can be calculated. obtained at 450 (a), 650 (b), 750 (c) and 800°C (d). If this value is compared with those obtained from the Dotted curve in each case shows the experimental specEPR line width (Table 3) the deviations in the trum, full curves mean the Lcrentzian line with g = 2.142 and the residual spectrum obtained after subtrac- amount of isolated Ni3+ from the calculated values tion of the Lorentzian line from the experimental spec- show unambiguously a non-random Ni/Co distributrum. For spectrum (d), the residuals were negligible. tion in the MO*-layer, With increasing the preparation temperature, the amount of isolated Ni3+ tends to temperature increases, the content of non-isolated Ni3’ also the lower value corresponding to the random distribuincreases whereas the content of isolated Ni3+ decreases, tion. For the sample prepared at 800°C the correwhile at 800°C only non-isolated ions are observed. These sponding narrow signal and the S-l/(S - 1 + S - 2) changes in the ratio between isolated and non-isolated Ni3+ ratio are particularly low and thus S-l cannot be ions reveal the changes with tem~rature of the Ni/Co deconvoluted from the experimental EPR spectra distribution in the Ni/CoOz-iayers. [Fig. l(d)]. In the case of random Ni/Co distribution, the probFor HT-Li(Ni,Cor_,)02 solid solutions with y < 0.7, ability for a given Ni3+ ion to have only diamagnetic it has been recently shown that 6Li high-speed magicCo3+ ions at a distance 2a (18 Co3+ ions, Fig. 4) can be angle-spinning (MAS) NMR is a valuable tool for calculated as: P, = (f)( 1 - y)” -*y*, where (A*)is the studying the local environment of the lithium ions in binomial coefficient and (1 - y) and y correspond to Co respect to Ni/Co in two adjacent layers (upper and lower, and Ni amount, respectively. If the probability of the Fig. 4) [19, 201. The RT spectra for cobalt-rich oxide appearance of such NiCots-cluster for y = 0.13 is used compositions obtained at different temperatures are colto estimate the theoretical amount of isolated Ni3+ lected in Fig. 5. For all samples studied, the spectra Table 3. Preparation temperature (T “C) of samples with Ni/(Ni + Co) ratio y = 0.13, line width of the EPR signals with g = 2.142 (S-l) and g = 2.137 (S-Z), amounts of isolated Ni3+ and non-isolated Ni3+ ions (calculated from signal 1 and signal Z), total Nii3 content from EPR data, intensity ratio between the signal at 0 ppm and the signal at - 15 ppm determined from 6Li MAS NMR, 1&u + I_ t5)

T"C

450 650 750 800

.3+

Ni3’

AHpp

%R

S-l

s-2

S-l

s-2

6.05 5.00 5.00

69.0 71.0 96.0 107.0

0.037 0.023 0.023 -

0.157 0.161 0.210 0.225

IQ

0.194 0.184 0.233 0.225

S-l!

S-l/

(S-l + S-2)”

(S-l + s-2)b

0.19 0.12 0.10 0.08”

0.23 0.19 0.05 0.08’

r&o + I-

15)

0.83 0.80 0.62 0.60 0.53*

a Calculated from the EPR line width. b Calculated from the intensity of the EPR signals. ’ Probability for the appearance of a NiCota-cluster at y = 0.13. d Ratio between the probabilities for the appearance of {Li(Co3C03)‘(Co3C03)“} and {Li(CogCo~)‘(NilCos)‘t} configurations.

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EPR STUDIES

OF Li t_XNi,Co r-,,) 1+XO2SOLID SOLUTIONS

461

Fig. 5. 6Li MAS NMR spectra of Li1_,(Ni0.13C00.87)1+X02 obtained at 450 (a), 650 (b), 750 (c) and 800°C (d).

Fig. 4. Schematic representation of the Li02-slabs (B) and Ni,Co r_,Ozslabs (A and C). For simplicity the oxygen sublattice has been omitted. 1 and 2 denote Co and/or Ni as first and second neighbours in respect to Li. The NiCo rs-cluster, which corresponds to isolated Ni3+ ions, is represented by bold lines. consist of a set of sharp lines in the -200 to 200 ppm region as referred to 1 M LiCl and the corresponding spinning sidebands (Fig. 5). The intense line always present at cu. 0.0 ppm was the only signal observed in the spectra of pure LiCoOZ. Thus it can be ascribed to lithium ions surrounded only by Co3+ ions: 3 Co3+ in the upper plane and 3 Co3+ in the lower plane ({Li(Co3Co~)‘(Co3c03>“) configuration, Fig. 4). Besides, other lines at - 15, - 30, . . . ppm have to be ascribed according to [25] to the presence of one, two or more Ni3+ ions as second neighbours ({Li(Co3C03)1(NiZCo6-z)11} configurations with t = 1, 2, . . . . Fig. 4). Since the NMR peak intensity is proportional to the amount of Li having a fixed number of Ni and Co neighbours, the changes in the ratio between the first (0 ppm) and the negatively shifted peaks (-15, -30, . . . ppm) reveal that a significant Ni/Co redistribution takes place between 650 and 750°C. Table 3 shows the intensity ratio between the signal at 0 ppm and the signal at -15 ppm, which corresponds to the ratio of {Li(Co3C03)1(Co3CJo3)11} and {Li(Co$o3)‘(Ni ,Co~)“} configurations. Using the binomial probability for a random Co/Ni distribution, the ratio of {Li(Co3C03)1(CogCo3)11} and {Li(CogCo3)1(Ni1Cog)11} configurations is 0.53. As one can see, with increasing the preparation temperature, the experimentally determined value tends to the theoretical one for statistical distribution. This trend towards a random Ni/Co distribution with increasing preparation temperature agrees well with the results obtained by EPR of Ni3+ (Table 3). It must be

mentioned that Ni/Co segregation in HT-LiNi,Co_Y02 solid solution was detected by 6,7Li MAS NMR in [20]. The EPR spectra of nickel-rich samples exhibit a broad Lorentzian line with g = 2.137. For LT- and HT-samples, the line width of this signal depends differently on registration temperature (Fig. 6). The line width for the LT-samples decreases linearly from 350 to 200 K and reaches a minimum value of about llO120 mT at 130-150 K (Tsra), after which the isotropic Lorentzian line is split. For HT-samples, the line width possesses a linear temperature dependence between 400-100 K (Fig. 6). The slope, dAHJdT, is shown in Table 4. Obviously, the dAHJdT coefficients decrease with increasing oxidation degree of the Ni,Cor, ions (Table 4). Owing to the sensitivity of the dAH,+JdTcoefficient towards the Ni2+ and Ni3+ segregation in Li1_,Ni1,02 [27, 281, this result shows that only for the high-temperature samples the EPR spectrum reflect the intralayer ferromagnetic Ni3+-0-Ni3+ interactions. In addition, the dAHJdT-coefficients for nickel-rich HT samples are lower than that for pure nearly stoichiometric LiNiOz (0.39 mT K-l), indicat-

Fig. 6. Temperature variation in the EPR line width of the signal with g = 2.137 for LiI-,(Nio.ssCoo.lz)1+,02 obtained at 450 (l), 650 (2) and 750°C (3).

EPR STUDIES

462

OF Li i,(Ni,Co

Table 4. Preparation temperature (T “C) of samples with Ni(Ni + Co) ratio y = 0.88, dLW,ddT-coefficient, experimental and calculated Curie constants (C, and Cc=,,respectively) and Weiss constants (0) for Lii,(Ni, - Co 1-Y)ifX02 samples No.

T”C

dAH,,ldT

C,

C,I

5 6 7

450 650 750

0.80 0.35 0.17

-

-

0.450 0.403

0.545 0.445

0

i-,,) 1+XO2SOLID SOLUTIONS REFERENCES 1. 2. 3. 4.

20 62

5. 6.

ing Co3+-population in the Ni-layers. The EPR peculiarities for the LT-samples can be explained by the effect of antiferromagnetic interactions between Ni3+ and Ni2+ ions segregated in two adjacent planes. It must be emphasized that pure Li0.s3Ni1.i702 obtained at high temperatures and the low-temperature Co-doped oxide with the same lithium content have similar EPR parameters: dM,,ldT = 1.11 mT K-’ and TEPR = 163 K for pure Li0.s3Ni1.t702 and dM,,ldT = 0.80 mT K-’ and TEpR = 153 K for Lio.84~io.88cOo.12)1.160~.The lower values of the EPR parameters of the cobalt-doped oxide suggest that diamagnetic Co3+ ions reside into the magnetic Ni2+ -Ni3+ configurations. In addition, the lack of a sharp split of the EPR line in the vicinity of the critical temperature is an indication of composition microinhomogenities in the LT-samples. This result is consistent with magnetic susceptibility measurements which evidence composition microinhomogenities: above the critical temperature, the Curie-Weiss law is not obeyed for LT-oxides. Contrary to the LT-samples, the CurieWeiss law describes well the temperature variation in magnetic susceptibility of HT-samples (Table 4). For nickel-rich samples obtained at 650, 750 and 800°C the Curie-Weiss constants give evidence of ferromagnetically interacting Ni3+ ions. This result is consistent with EPR data (Fig. 6 and Table 4) and chemical composition of the samples (Table 1). In conclusion, EPR of Ni3+ ions can be used as an experimental tool for the estimation of Ni3+-distribution in LiNi,Co1_,02 solid solutions obtained at low- and high-temperatures. The results show that random Ni, Co distribution appears above 650°C. Acknowledgements-The authors acknowledge financial support from EU (contracts JOU2-CT93-0326 and CIPD-CT94-0501). The authors also express their gratitude towards the NMR Service of Univ. Cordoba. E.Zh. and R.S. are indebted for financial support from National Research Foundation of Bulgaria (Ch 463/94).

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