Vacuum~volume 41/numbers 7-9/pages 1739 to 1742/1990 O042-207X/9083.00 + .00 Printed in Great Britain © 1990 Pergamon Press plc

Surface segregation and sodium transport Na=CoOa

in

J P K e m p , D J Beal, P A Cox and J S F o o r d , Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, UK

The sodium cobalt bronzes, Na, Co02, have been synthesised and studied by XPS, UPS, EELS and EDAX. It is found that heating in oxygen gives rise to a surface layer of Na20, up to several hundred ~ in thickness, and that this is normally present as a result of synthetic conditions. The formation of this layer is investigated as a function of temperature, oxygen pressure, and the value of x, and comparisons with lithium analogues made. We discuss reasons for this behaviour, and implications for sodium transport in this material, and thus its use as a battery cathode.

Introduction

In many tertiary and quaternary oxide systems surface compo- sition may be dramatically different from the bulk of the solid 1-2. In simple systems (e.g. oxides of A-metals), progress has been made towards quantitative prediction of behavior 3" 4, but in transition metal oxides, the situation is complicated by the possibilities of variable valence and gross non-stoichiome- try, which broadens the range of surface phases available. In this work, we study the surface composition of the phases Na~CoO2, as a function of temperature and oxygen pressure. The structure of these compounds is based on close packed oxygen layers, with variable stacking order, and alternate layers of octahedral interstices occupied by cobalt and sodium 5'6. They have been intensively researched as possible cathodes for a secondary cell 7 9. Surface composition is of importance here, since it affects rates of sodium transport into the material, and thus current density available.

Experimental

Sodium cobalt oxides were synthesised by heating intimately ground mixtures of Na202 and CoO in pure, dry oxygen at 550C for periods of up to 30 h ~° and characterised by analysis for metals, and also by X-ray powder diffraction, which gave patterns in agreement with those previously reported 5, with no trace of starting materials. As these phases are highly moisture sensitive, all handling operations were performed in a dry argon atmosphere. LiCoO2 was prepared by heating Li2CO 3 and CoO in air at 900°C for 24 h.

The spectrometer used was a VG ESCALAB 5, with a preparation chamber where samples may be heated by rf induc- tion in gas pressures of up to 1000 mbar. It is, however, not possible to measure the sample temperature directly with this arrangement. Electron microscopy was performed using a JEOL 2000FX analytical electron microscope. Mapping of Na and Co concentrations over single grains was done via energy dispersive analysis of X-rays (EDAX), at 200keV incident

beam energy, using a spot size of approximately 30 nm and a dwell time of one second per point.

Results

XPS of ground, pelletised samples revealed some carbon con- tamination, and ionisations attributable to cobalt were rather lower in intensity than expected, especially for larger values of x. Mild argon ion etching (typically 500 #C cm -2 at 1 keV) removed the carbon, and gave an increase in surface Co. The resulting XPE spectrum for NaCoO2 is shown in Figure l(A), and shows ionisations for all elements present. Figure I(B) shows the same material after being heated to 500°C for one hour under 100 mbar 02. Following such treatment, the surface

Na ls

Na KLL Auger

0 Is J i 0 KLL

I. ~x..~ Co 2p

Co LPIH Auger

200 L~O0 600 8 O0 1000 Binding Energy (eV)

Figure 1. Widescan XPE spectra (MgK~ radiation) for an NaCoO 2 sample, (A) after argon ion etching, and (B), after heating in 100 mbar 0 2 .

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J P Kemp et al." Surface segregation and sodium transport in NaxCoO 2

~i ~ ~ ~

.... ~ ~ ~ ~ iii ~ ~ i )

Figure 2. Elemental maps for two grains of NaCoO2, from EDAX, showing Na concentrations (left pair), Co concentrations (centre) and composite maps (right) showing areas where threshold concentrations are exceeded. For upper right, white areas show Na and Co above threshold, grey areas Na only. For lower right, white area shows Na above threshold, other areas have been omitted for clarity.

becomes totally deficient in cobalt, for a depth of at least 40 A, given an escape depth in XPS in the region of 1 0 / ~ . Peak integration ~t, 12 gave a composition for the oxidised surface of Na20, within the error incurred in this procedure. UVPE and EEL spectra of this surface were characteristic of a wide band-gap insulator, as would be expected for this material.

Such a surface layer would also be expected to arise during the synthesis of Na~CoO 2, since this involves heating in oxy- gen, but would probably be removed to some extent by grind- ing prior to pelletisation, and then further by etching. Though the thickness must be several times the escape depths for XPS, it must also be small compared to the size of the grains formed in synthesis, since no trace of Na2 O is detect- able by powder diffraction. Figure 2 shows elemental maps (Na and Co) of two grains of NaCoO 2. These were performed on a freshly synthesised sample, and grinding was avoided in preparing the sample for microscopy, to prevent break-up of any surface layer present. The 200 keV electron beam used penetrates through the whole of the grain, so the maps represent a projection of elemental concentrations onto a plane. The data has also been processed to show regions where concentrations of Na and Co exceed threshold values. It is noticeable that there is a region around the outside of both grains which is depleted in cobalt. On one grain this takes the form of a shell, with a thickness in the region

of 50-100 nm. On the other grain, however, a larger region is involved. This may possibly represent some unreacted starting material.

High resolution XPE spectra were also recorded. Both the oxygen ls and sodium KLL Auger region of the spectrum for NaCoO2 after etching are found to be doublets. In each case the highest binding energy component occurs at the same energy as the corresponding peak in the spectrum of the oxidised surface, Figure i(B). Na KLL Auger energies vary due to screening of the interaction between the two holes in the final state by the environment. In typically ionic compounds (e.g. simple halides, Na20), the kinetic energy falls in the region of 989 eV, generally increasing with anion polarisabil- ity 13. For sodium in many transition metal oxides, screening of the interaction is more efficient and the kinetic energy up to 5 eV greater. Thus the components of the doublet correspond to the two phases present. Similar arguments presumably apply to the O is region, except there the core-hole is stabilised by polarisation of the environment.

The cobalt 2p region of the spectrum showed considerable satellite intensity. This tends to be characteristic of the +2 oxidation state ~4, indicating that some reduction has occured in the course of etching. The main lines are also quite broad (FWHM 3.5 eV). After annealing at approximately 500°C in 0.07 mbar of oxygen, the satellite intensity, and linewidth were

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J P Kemp et ah Surface segregat ion and sodium transport in Na~CoO2

gs 2~ 2~5 2~0 Binding

sb s~s s~0 Energy (eV)

5i5

Figure 3. Structure in Na KLL Auger (left) and O Is (right) regions of the XPE spectrum after heating at various temperatures in 0.07 mbar O~,

much reduced, and the spectrum is very similar to that of the analogue LiCoO2 ~4

A sample of NaCoO 2 was also heated to equilibrium under various conditions of temperature and oxygen pressure. Exact measurement of sample temperature is not possible in our equipment, so all values quoted are approximate with an expected error of at least _+ 50°C. Figure 3 shows the effect on the O ls and Na KLL Auger spectra of heating to progressively higher temperatures in the range 350-650°C, under a pressure of 0.07 mbar of 02. As the temperature increases, so does the intensity of the peak assigned to the cobalt containing phase, at the expense of the peak due to NaEO. The intensity of the cobalt 2p ionisation also increases. Similar, spectra recorded at constant temperature and increasing oxygen pressure showed a decrease in cobalt intensity, and an increase in the amount of surface Na20.

The graphs in Figure 4 compare relative amounts of Na, Co and O in each phase. For the surface phase, attributed to sodium oxide, the mean composition (solid line) in Na2.330, which is reasonably close to Na20, given the errors involved in this procedure. The second graph in Figure 4 shows the compo- sition from the Co 2p3¢2 and bulk-phase O ls emission. All the data lie very close to a cobalt : oxygen ratio of 1 : 2.4. The third graph similarly shows bulk Na KLL and O 1 s intensities, all of which closely fit a sodium : oxygen ratio of 1.3 : I. This suggests an overall composition of Na3ACoO2. 4. In the presence of a surface layer of thickness D, photoelectrons with mean free path ,~, will appear with intensity 2, reduced by a factor of exp(-D/2) . This would account for the oxygen excess over C002, but certainly not for all of the excess sodium. It seems likely, therefore, that this phase is an intermediate involved in the oxidation of NaCoO2. The N a - C o - O phase diagram has been shown to contain sodium-rich phases, such as Na4CoO45, and thus the PES experiment is probably detecting an interfa- cial layer between NaCoO2 and N%O, which consists of a mixture of NaCoO 2 and more oxidised, sodium-rich phases.

Similar experiments were also performed on the sodium- depleted phase, Nao.75C002, and also on the lithium analogue LiCoO 2. In the latter compound, it is not possible to monitor the alkali metal concentration by XPS, but the oxygen:cobalt

20

18

16

12

4

(A) 2

1 5

0.5.

/ o is

I 2 3 /* 5 0 1 2 3 /* 5 0 ls 0 ls

Figure 4. Relative cross-section and mean-free path weighted intensities for (A), surface phase components of Na KLL and O ls regions (left), (B), Co 2Psi2 and bulk phase component of O Is (centre), and (C), bulk phase components of Na KLL and O ls (right).

ratio lies in the range of 2.0 2.5:1 on being heated in oxygen pressures ranging from 10 6 to 100 mbar. The former com- pound, however, shows surface segregation in a manner similar to NaCoO 2, but in order to achieve similar intensity ratios for surface and bulk emission, lower temperatures and/or higher oxygen pressures are necessary.

Discussion

Many segregation phenomena are driven by relaxation of the lattice parameters at the surface, or changes in Madelung potential. This is clearly not the case in this system, since such effects generally are only effective in the top monolayer or two of the solid, and would not lead to any dependence on oxygen pressure. The pressure dependence suggests an oxidative de- intercalation of the sort:

Nax COO,, + y/4 O2~Na x _ y CoO 2 + y/2 Na20.

This would explain the temperature dependence seen in the XPES results, as the oxidised cobalt phase would become increasingly unstable with respect to oxygen loss (i.e. the re- verse of the above) at higher temperatures. Also, as x is reduced, electrochemical measurements show that it is more difficult (in terms of free energy) to remove more sodium 7 9, and is harder still to remove Li from LiCoO 2. This would account for the fact that the oxidation occurs less readily as x falls, and hardly at all for lithium. At 298 K, the free energy changes for the reaction (per mole of M20) lie in the ranges 30-80 kJ (Na, x = 1.0), 70 120 kJ (Na, x = 0.75) and 160- 180kJ (Li, x = 1.0) 8. However, since the products appear to form separate phases, the position of the equilibrium should be independent of their relative amounts, and, as the free energies are positive, the reaction should not therefore occur at all.

If the reaction were homogeneous, the extent of reaction could be arbitrarily limited. At high temperatures, it is possible that the material is a single phase and the interlayer space contains both sodium and oxide ions. The equilibrium then would be between oxygen gas, and the additional lattice oxide ions. These then separate out on cooling as a surface layer of N%O, whose thickness is determined by the concentration of lattice oxide. The concentration of the latter would be small (since the interlayer space is bounded by oxygen which will bear some degree of negative charge), and would be a function of the ease of oxidation of the host.

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J P Kemp et 81; Surface segregation and sodium transport in NaxCoO 2

The existence of such a surface phase has implications for the use of Nax CoO2 phases as cathodes in a sodium-based battery. The discharge cycle for such a cell involves sodium transport into the oxide up to x = 1.0. For a sufficiently thick surface film of Na20, this process could easily prove to be rate-limiting, and so lead to a reduction in available energy density at high (practical) discharge currents. Some observations of large over-voltage effects at high currents have in fact been reported, but the exact cause is difficult to ascertain, as other factors, such as changes in the oxgyen stacking order with increasing x may be involved 7.

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