Lithium intercalation in KxTi8O16 compounds

International Journal of Inorganic Materials 1 (1999) 117–121

Lithium intercalation in K x Ti 8 O 16 compounds ´ ´ *, A. Kuhn, F. Garcıa-Alvarado ´ M.T. Gutierrez-Florez ´ ´ ´ ´ Departamento de Quımica Inorganica y Materiales, Facultad de Ciencias Experimentales y Tecnicas , Universidad San Pablo-CEU, Urbanizacion ´ , 28668 Boadilla del Monte, Madrid, Spain Monteprıncipe

Abstract Several bronzes K x Ti 8 O 16 have been prepared from K 1.64( 1 ) Ti 8 O 16 by means of oxidative reactions. These bronzes have been intercalated with lithium by electrochemical methods in order to test their performances as electrode materials in lithium rechargeable batteries. The main result is that although the maximum capacity (260 Ah kg 21 ) is reached in the compound with the lowest potassium content, a low reversibility and high polarisation are obtained. Bronzes with intermediate potassium content are more attractive for applications in rechargeable lithium batteries. These compounds maintain an acceptable capacity (200 Ah kg 21 ) while having high cyclability and low polarisation. The application should be addressed to anode materials rather than to cathode due to the low voltage (1.75 V) obtained during the intercalation reaction.  1999 Elsevier Science Ltd. All rights reserved. Keywords: A. inorganic compounds; B. chemical synthesis; B. intercalation reactions; C. electrochemical measurements; D. energy storage

1. Introduction It has been recently claimed that the 21st century will become a battery-based society, where economic growth will be compatible with conservation of the environment; and that lithium batteries, ‘jumping’ from electronic to power market are very promising options for this close future [1]. Then it can be easily understood that a lot of works are devoted to the optimisation of very well-known cathode materials (LiCoO 2 , LiNiO 2 , LiMn 2 O 4 , V2 O 5 , MnO 2 , etc.). The use of the first listed oxide as the cathode, coupled with carbon as the anode, now dominates lithium ion technology [2]. However there is a need for new anode materials that may improve the capacity of the negative side, thus facilitating their use in powering applications (the electric vehicle for example). In this sense, some recent works claim to have obtained materials that may substitute carbon as the negative electrode (see, for example, Refs. [3–5]). Among the compounds that have proven their abilities to accept lithium ions through an intercalation reaction, hollandites are very well known mainly due to the performances of a-MnO 2 . The intercalation potential of this

*Corresponding author. Fax: 191-553-8610.

latter compound is too high to be considered as a candidate for anode material, and for that reason its use is claimed as positive. A change of the skeleton cation while maintaining the structure has to produce a change in this intercalation potential, since this depends on the Madelung energy [6]. In our last works several titanium oxides, where Ti 41 [7] was in a very similar environment to oxygen, were studied. In each of these materials intercalation proceeds at a quite low voltage (,1.5 V). The aim of the present work is the study of the intercalation reaction of lithium in some titanium hollandites. The polymorph TiO 2 (H) has been known since 1989 [8]. It was prepared by potassium extraction from the potassium bronze Kx TiO 2 (0.13#x#0.25) which also has a hollandite structure. In the present study we have performed intercalation reactions in these types of potassium bronzes, in order to find the potassium content that makes both capacity and reversibility of the corresponding lithium cell as high as possible.

2. Experimental The synthesis of the bronze K x Ti 8 O 16 was made as previously described [9]. Once the compound was char-

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´ ´ et al. / International Journal of Inorganic Materials 1 (1999) 117 – 121 M.T. Gutierrez-Florez

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Table 1 Experimental conditions and compositions obtained for the oxidised samples (standard deviations are given in parentheses) Oxidising agent

Temperature (8C)

Time (h)

Composition

H 2 O / H 2 SO 4 H 2 O / H 2 SO 4 Br 2 / CH 3 CN I 2 / CH 3 CN –

80 80 80 80 –

66 50 66 66 –

K 0.03( 2 ) Ti 8 O 16 K 0.18( 8 ) Ti 8 O 16 K 0.23( 6 ) Ti 8 O 16 K 0.63( 2 ) Ti 8 O 16 K 1.64( 1 ) Ti 8 O 16

acterised and its composition calculated, it was treated with different oxidising agents (I 2 in acetonitrile, Br 2 in acetonitrile or a mixture of H 2 O 2 / H 2 SO 4 ) at 808C and different times (see Table 1). The latter strong oxidant has been reported to be useful to achieve complete potassium extraction [8]. After filtration, the reaction products were washed with acetonitrile or water and dried under vacuum at room temperature. The potassium content was determined by inductively coupling plasma (ICP-AES), using a model JY-701 sequential-multichannel. Thermogravimetric analysis (Seiko-II Instrument) was used to determine the presence of water in the prepared sample, as well as to determine the temperature for water removal. These experiments allowed the preparation of almost water-free samples whose performances were also tested in electrochemical cells. Structural characterisation was made by means of X-ray diffraction. Data were collected from a Siemens D-501 powder diffractometer with Cu Ka radiation. Electrochemical behaviour of both ‘hydrated’ and ‘dehydrated’ compounds was studied by means of both galvanostatic and potentiostatic experiments using a MacPile system [10]. Electrochemical experiments were carried out in Swagelok test cells [11]. A commercial electrolyte (Selectipur) which consisted of a 1 M solution of LiPF 6 in dimethoxyethane (DME) and ethylencarbonate (EC) (1:1), was used to soak a glass paper separator. Lithium was used as both reference and counter electrode, while a mixture of active material (K x Ti 8 O 16 ), carbon black (Super S) and a binder (ethylene propylene diene terpolymer EPDTP) was used as the working electrode. The mixture was pressed in either 5- or 8-mm diameter pellets. Cells were assembled under an argon atmosphere.

the sintering time under forming gas, the bronze can have different potassium contents. In our case the starting bronze has the composition K 1.64(1) Ti 8 O 16 . As expected using mild oxidants, such as iodine, the quantity of extracted potassium is lower than when a stronger oxidant, such as bromine or a mixture of hydrogen peroxide and sulfuric acid, is used. In this latter case, after 66 h reaction time at 808C we got an almost potassium-free compound that corresponds to the polymorph TiO 2 (H) according to Latroche et al. [8]. The presence of intercalated water in all samples has been detected by TGA. Fig. 1 shows the weight loss with temperature of the sample K 1.64(1) Ti 8 O 16 (curve a) which is quite representative of the general thermal behaviour of both parent and extracted bronzes. It can be seen that between room temperature to 4008C a loss of 1.8% occurs. Since the major loss occurs at relatively high temperature (close to 2008C), we ascribe this weight loss to intercalated water. However, a small fraction of water is removed below 1008C, indicating the presence of adsorbed water. For the case shown in Fig. 1, the weight loss corresponds to almost 1 mol of water per formula, this means that the starting compound has to be formulated as K 1.64( 1) Ti 8 O 16 ? 0.96 H 2 O. The water molecules likely come from both the solvent used and the atmosphere. At temperatures higher than 4008C, we observed a weight increase due to the up taking of oxygen. To confirm this point, a similar experiment was carried out under argon. As expected, weight gain was not observed over the temperature range from 30 to 6008C. Fig. 2 shows the different reductive–oxidative processes, detected in a potentiostatic experiment, that occur during a complete lithium intercalation–deintercalation cycle in a sample with composition K 1.64( 1) Ti 8 O 16 ?0.96

3. Results and discussion Table 1 shows the experimental conditions used to prepare the oxidised samples together with the determined potassium content. The starting bronze has been also included in the analysis since we have observed that, depending on the specific conditions of preparation, mainly

Fig. 1. TG curves obtained for (a) K 1.64( 1 ) Ti 8 O 16 ?0.96 H 2 O, and (b) the sample after treatment at 2008C for 100 h under vacuum. The water content corresponds to 0.19 H 2 O per formula (K 1.64( 1 ) Ti 8 O 16 ?0.19 H 2 O).

´ ´ et al. / International Journal of Inorganic Materials 1 (1999) 117 – 121 M.T. Gutierrez-Florez

Fig. 2. Variation of current with potential in the range 2.5–0.5 V for a Li / EC:DME (1:1), 1 M LiPF 6 / K 1.64( 1 ) Ti 8 O 16 cell having water contents of 0.96 and 0.19 molecules per formula.

H 2 O. It can be seen that on oxidation and above 1.7 V, two peaks appeared although only one is detected during reduction. If the same potentiostatic experiment is made with a sample treated at 2008C for 100 h under vacuum conditions we observe that a better reversibility is obtained, since the processes observed in the voltage region E.1.7 have almost disappeared. We deduce that these processes may be due to the presence of water, since the TGA experiment for this sample shows that the heat

Fig. 3. Voltage composition plot for several K x Ti 8 O 16 compounds (0.03,x,1.64) down to 0.5 V (i50.25 mA cm 22 ).

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treatment leads to an important decrease of the water content (see Fig. 1, curve b). All electrochemical data shown hereafter correspond to samples treated at 2008C for 100 h in order to minimise the irreversible effects due to water. The influence of potassium content in the quantity of lithium that the bronzes can accept is very clear. Fig. 3 shows the typical voltage composition plot of the lithium intercalation reaction in bronzes K x Ti 8 O 16 with 0.03#x# 1.64. While for the starting bronze K 1.64( 1) Ti 8 O 16 ?0.19 H 2 O the quantity of lithium is very small (2 Li / formula) the extracted samples are able to accept very high quantities of this ion. This quantity reaches a maximum for the sample K 0.03 Ti 8 O 16 ?0.07 H 2 O which can be considered as the hollandite polymorph of TiO 2 . On the other hand, the removal of lithium is expected to be accompanied by a gradual loss of electronic conductivity that is reflected in the observed change of colour, from dark blue in the initial bronze to white in the most extracted sample. The consequence is that, although ‘potassium-free’ compounds have the maximum theoretical specific capacity, this can only be reached at very low intercalation rate (C / 35 for the experiment shown in Fig. 3). Even in this case the observed polarisation is quite large making TiO 2 (H) a very poor electrode material. Another effect caused by different potassium contents is that a fading of capacity is observed upon cycling for samples with very low potassium content, as can be seen in Fig. 4 where the variation of the specific capacity of the samples K x Ti 8 O 16 ?n H 2 O (0.03#x#0.63, 0.07#n#0.19) is plotted over different cycle numbers. One can deduce that, although the initial capacity is higher in the most extracted samples, it falls to values close to those of the less extracted samples after a few cycles. If one of the above referred materials should be selected as electrode material for lithium rechargeable batteries, both the potassium richest and poorest should be dis-

Fig. 4. Variation of the specific capacity of extracted bronzes versus cycle number.

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´ ´ et al. / International Journal of Inorganic Materials 1 (1999) 117 – 121 M.T. Gutierrez-Florez

intercalate lithium at even lower voltage. When using the titanium oxides as anodes, instead of carbon for example, the loss of energy due to the smaller output voltage may be alleviated by using the recently discovered 5 V materials [12] as the positive electrode.

4. Conclusions

Fig. 5. Cycling of K 0.18 Ti 8 O 16 ?0.10 H 2 O down to 1V (i50.25 mA cm 22 ).

carded. The choice would be those samples with an intermediate potassium content, enough to keep a high capacity while having a low polarisation. As an example of an optimum material, the performance of K 0.18( 8) Ti 8 O 16 ? 21 0.10 H 2 O is shown in Fig. 5. A capacity of 200 Ah kg is developed at C / 10 rate, while a relatively low polarisation and an excellent cycling behaviour is maintained. For comparison, see the cycling behaviour of K 0.03( 6) Ti 8 O 16 ? 0.07 H 2 O shown in Fig. 6. Since the intercalation reaction proceeds at low voltage, the described materials may be used as the negative electrode material for rocking chair batteries. The situation of this material when compared with the presently used carbonaceous materials is quite similar to that of another titanium oxide, Li 2 Ti 3 O 7 [5]. Both have to compete with carbonaceous material that

The extraction reactions performed on K 1.64( 1) Ti 8 O 16 ? 0.96 H 2 O allowed the preparation of bronzes with different potassium content. These materials, as well as the starting bronze, are able to take water from either the solvent or the atmosphere. The quantity of water can be reduced by treating the samples at 2008C under vacuum. The intercalation reaction of lithium in the almost water-free samples is strongly dependent on the potassium content. Although the maximum capacity is obtained for the potassium-poorest sample, the lower reversibility and higher polarisation that they present makes bronzes with intermediate potassium content the most attractive for applications in rechargeable lithium batteries. As an example the compound 21 K 0.18( 8) Ti 8 O 16 ?0.10 H 2 O can develop 200 Ah kg at C / 10 rate, while keeping a very good cyclability. This application should be addressed to anode materials rather than to cathode due to the low intercalation voltage.

Acknowledgements The authors thank Universidad San Pablo CEU for financial support through the project 8 / 97. We also thank CICYT for supporting the project MAT 98-1053-C04-04. The ICP measurements were carried out in the Atomic Spectrometry Centre of Universidad Complutense de Madrid.

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Fig. 6. Cycling of K 0.03 Ti 8 O 16 ?0.07 H 2 O down to 1V (i50.25 mA cm 22 ).

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