Solid State Ionics 40/41 (1990) 585-588 North-Holland

LAYERED P O T A S S I U M V A N A D I U M O X I D E S AS H O S T M A T E R I A L S FOR L I T H I U M AND S O D I U M I N S E R T I O N

K. WEST, B. ZACHAU-CHRISTIANSEN, T. JACOBSEN, AND S. SKAARUP a Institute of Physical Chemistry, Physics Laboratory II1 a, The Technical University of Denmark, DK-2800 Lyngby, Denmark

Lithium and sodium insertion into the layered host materials KV308 and K3VsOI4 has been studied. KV308 has a very high stoichiometric capacity for lithium insertion and good cycling properties, which make it an interesting material for use in second- ary lithium cells. K3VsOt4, which has a very open structure, inserts lithium and sodium ions at unusually low potentials, probably due to inadequate shielding of coulombic guest-guest interactions.

1. Introduction

Host lattices o f reduced dimensionality can be de- formed extensively during intercalation reactions without rearrangements o f strong bonds in the host matrix. Low-dimensional lattices are thus often more versatile host materials for intercalation than lattices bonded in three dimensions. Vanadium oxides gen- erally offer high capacities for electrochemical in- sertion, and it is reasonable to expect that new host materials with high capacity for both lithium and so- dium intercalation can be found among the vana- dium oxides with layer structures.

Simple oxides analogous to TiS2 with planar, van der Waals-bonded layers are not stable due to Cou- lombic repulsion between adjacent layers o f oxide ions. There are, however, a number o f more complex oxides with stable layer structures as exemplified by the Ml+xV308 ( M = L i , Na) compounds. In these materials alkali metal ions are situated between puckered layers o f VO6 octahedra, with one alkali metal ion per formula unit in the fully oxidized com- pound stabilizing the structure. Reversible lithium and sodium insertion have been demonstrated up to the limiting compositions Li4VaO 8 and Na3V308 with good capacity retention when used in rechargeable electrochemical cells [ 1,2 ].

In this work two additional layer structures, KV308 and K3V5OI4 - both bonded by interlayer potassium ions, have been studied as electrode materials for re- chargeable solid state sodium or lithium cells.

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2. Experimental

Preparation of KV30 s involved the digestion con- stant volume of an acidified metavanadate solution at elevated temperature [ 3 ]:

3VOw- + K + + 2 H + ~ K V 3 0 8 + H 2 0 . ( 1 )

Immediately after the addition o f acid a voluminous dark red precipitate was formed. The resulting slurry was stirred under reflux for 24 h at 80-85°C. During this period the precipitate slowly turned orange. The precipitate was then vacuum filtered, washed with distilled water and dried at 110°C to remove oc- cluded water. The resulting product, a voluminous, opalescent orange powder, was analyzed for potas- sium and vanadium, and the mean oxidation state o f vanadium was determined. The overall compo- sition was found to be KI.o4V308.02, with a purity of 99.7%. The potassium excess (or vanadium deficit) is dependent on the excess of acid used.

The X-ray powder diffraction pattern of the prod- uct (41 lines with I / Io> 1% and d > 1.4 A) could be indexed in a monoclinic unit cell (P2~/m) with lat- tice constants: a = 7 . 6 2 9 ( 2 ) A, b = 8 . 3 4 9 ( 4 ) A, c = 4.968 (2) A and [3 = 96.90 (3) ° in good agreement with literature values [4 ].

K3VsOI4 was prepared by melting KVO3 and V205 in 3:1 molar ratio at 850°C [5]. After cooling the resulting product was pulverized and analyzed. The overall composit ion was found to be K3,2VsO,45 with a purity better than 96%.

586 K. West et al. /Layeredpotassium vanadium oxides

A agonal unit cell (P31m) with a=8.682 (2) /k and c= 4.995 ( 3 ) A showing fair agreement both with the relative intensities reported by Kelmers [ 5 ], and with the unit cell reported by BystriSm [6], a=8 .679(2 ) A and c=4.9914(8) A.

The methods for preparation of electrolytes and electrode films have been described previously [2].

3. Results and discussion

B

Fig. I. Structure of (A) LiVsOs, and (B) KV30 s. The figures show the coordination polyhedra around vanadium atoms and the alkali metal ions (filled circles) of the unit cell.

X-ray powder diffraction patterns of the product showed 21 lines with a relative intensity exceeding 5% (d> 1.4 A). All lines could be indexed in a hex-

The crystal structure of K V 3 0 8 is different from that of the trivanadates of Li and Na mentioned above, see fig. 1. It consists of corrugated sheets of distorted, edge sharing VO6 octahedra. The K-ions are situated between these layers, and the oxygen packing density is 75% relative to the close packed futile modification of VO2 indicating a rather open structure with ample room for inserted ions.

Upon constant current discharge in solid-state lithium cells with PEO-based electrolytes more than 3.5 Li is inserted per formula unit (discharge to 1.75 V versus Li), fig. 2. In the corresponding sodium cells nearly 2.5 Na is inserted per formula unit upon dis- charge to 1.0 V versus Na. The calculated stoichio- metric energy densities 645 Wh/kg, 1555 Wh/~ for Li intercalation and 310 Wh/kg, 765 Wh/~ for Na intercalation are among the highest values for va-

4.0

3.0

2.0

1.0

E / (V vs M) i i i

X in MxKV308 0.0 I i L

0 1 2 3 4

Fig. 2. First cycle of the cell: LilPEOy-LiCFsSOs ( y = 12 ) I LixKVsOs, C, PEO. Temperature: 120 ° C, current density: 50 laA/cm 2. The first discharge of a corresponding sodium cell is also shown (temperature: 80 ° C, current density: 25 laA/cm 2 ).

K. West et aL /Layeredpotassium vanadium oxides 587

Fig. 3. Structure of K3V~OI4. The figure shows the coordination polyhedra around vanadium atoms and the potassium ions (filled circles) of the unit cell.

nadium oxides reported so far. The lithium insertion reaction is seen to proceed through two two-phase regions, appearing as ranges where the potential is virtually independent on the composition.

A fraction ( < 10%) of the lithium inserted during the first discharge cannot be extracted at potentials below the practical stability limit o f the electrolyte, 3.5 V. This reduces the stoichiometric capacity o f subsequent discharges, but as these discharges pro- ceed at slightly higher potentials without the voltage plateaux of the first discharge, the reduction in stoi- chiometric energy density is relatively small.

Multi-phase behaviour associated with an inser- tion reaction is the indication o f strong guest-host interactions [7]. The resulting lattice strains will

often lead to loss o f the long-range coherency and crystallinity o f the host lattice that is necessary to sustain guest-host interactions. For these reasons re- peated cycling through two-phase regions will gen- erally only be attainable with hosts strongly bound in three dimensions, and even in these cases asso- ciated with gradual losses of crystallinity.

The layers of the K3VsO]4 structure consist of nearly ideal VO5 square pyramids and VO4 trigonal pyramids (tetrahedra). The pyramids are joined by basal corners with their apices all pointing in the same direction. The V - O polyhedra thus form a purely two-dimensional network kept together by interfol- iated K-ions, fig. 3. The oxygen packing density of K3V5OI4 is low, only 63% relative to VO2(R ), mak- ing this the most open structure of a vanadium oxide yet studied as host material for alkali metal inser- tion. The V - O coordination could however be ex- pected to limit the insertion interval or the reversi- bility of this material, as V tetrahedrally coordinated to oxygen is only seen for vanadium in the highest oxidation state, V(5) . Vanadium ions in lower ox- idation states are larger, requiring a higher oxygen coordination.

The discharge curves for solid-state lithium and sodium cells with K3VsOI4 are shown on fig. 4. It is seen that the insertion reactions proceed at signifi- cantly lower potentials than in other vanadium ox- ides of the same state o f oxidation, e.g. KV308. This

4.0

3.0

2.0

i.O

E / (V v s M) l i l

M = Li

M = Na

MxK3VsO i, x in o.o I l I

o ~ 2 3 4

Fig. 4. First cycle of the cell: Li I PEOy-LiCF3SO3 (y= 12 ) I LixK3VsOl4, C, PEO. Temperature: 120 ° C, current density: 50 ~A/cm 2. The first discharge of a corresponding sodium cell is also shown (temperature: 80 ° C, current density: 50 p.A/cm2).

588 K. West et al. /Layeredpotass ium vanadium oxides

could be due to the fact that the very open in ter layer space does no t offer the same shie lding of Co u l o m- bic in te rac t ions as does K V 3 0 8. Discharge to 1.5 V versus Li results in the inse r t ion of a r o u n d 3.5 Li per fo rmula un i t - or ~ 2 Na u p o n discharge to 1.0 V versus Na. Higher alkali meta l uptakes can be real- ized by fur ther discharge. The large hysteresis be- tween the discharge an d recharge curves shown on

fig. 4 is typical o f e lectrodes using this mater ia l , p robably caused by a s t ruc tura l b r eak -down induced by the inse r t ion react ion.

4. Conclusions

High capaci t ies for bo th l i t h ium an d s o d i u m in- ser t ion have been d e m o n s t r a t e d for the two layered

po tas s ium v a n a d i u m oxides K V 3 0 8 and K3V5014. For bo th mate r ia l s repeated cycl ing leads to loss of crys ta l l in i ty of the host lattice. Apparen t ly the cy-

cl ing proper t ies of electrodes us ing K V 3 0 8 is not l im- i ted by this proper ty , and good cycling pe r fo rmance has been observed.

References

[ I ] K. West, B. Zachau-Christiansen, M.J.L. Osterghrd and T. Jacobsen, J. Power Sources 20 ( 1987 ) 165.

[ 2 ] K. West, B. Zachau-Christiansen, T. Jacobsen and S. Skaarup, Solid State lonics 28-30 (1988) 1128.

[3] A.D. Kelmers, J. lnorg. Nucl. Chem. 21 ( 1961 ) 45. [4 ] H.T. Evans and S. Block, Inorg. Chem. 5 ( 1966 ) 1808. [5] A.D. Kelmers, J. Inorg. Nucl. Chem. 23 ( 1961 ) 279. [ 6 ] A.M. Bystr6m and H.T. Evans, Acta Chem. Scand. 13 ( 1959 )

377. [ 7 ] K. West, in: High conductivity solid ionic conductors: Recent

trends and applications, ed. T. Takahashi (World Scientific, Singapore, 1989 ).