Edinburgh Research Explorer Titanium migration driven by Li vacancies in Li1xTi2O4 spinel

Gentle oxidation of lithium titanate spinel (LiTi 2 O 4 ) with water at room temperature gives Li-deficient Li 0.33 Ti 2 O 4 . Combined X-ray and neutron Rietveld analysis shows that 28% of the Ti cations are displaced to alternative octahedral sites, in keeping with a proposed model based on Ti-migration limited by Li-vacancy concentration.

Lithium transition metal oxide spinels have important physical and electrochemical properties. LiTi 2 O 4 spinel has been extensively studied as it displays superconductivity with a critical temperature of 11 K. [1][2][3][4][5][6] Lithium titanate spinels also have attractive properties as negative electrodes for lithium ion batteries such as a very flat potential and a relatively low volume change upon chargingdischarging and with a good rate performance. [7][8][9][10][11][12][13] The cubic spinel structure of LiTi 2 O 4 consists of cubic-closepacked (ccp) oxide layers with Li cations occupying tetrahedral sites (with symmetry label 8a in the Fd% 3m space group) and Ti cations in 16d octahedral sites, and is written here as (Li) 8a [Ti 2 ] 16d O 4 . Alternative 16c octahedral cation sites (marked 'X' on Fig. 1) close to the 8a sites are empty and define a three-dimensional network of channels through which lithium ions diffuse during electrochemical reactions. Intercalation of additional lithium into LiTi 2 O 4 results in migration of Li cations from 8a to 16c sites, giving an ordered rocksalt type product denoted as {Li 2 } 16c [Ti 2 ] 16d O 4 . 5,8,14 The structural consequences of delithiating LiTi 2 O 4 to form Li 1Àx Ti 2 O 4 are less well understood. Electrochemical delithiation was found to be limited to a deficiency of x B 0.3 due to oxidative decomposition of the electrolyte 3 so chemical methods have mainly been used. An early study showed delithiation up to x = 0.8 could be achieved with iodine in acetonitrile. The product Li 0.2 Ti 2 O 4 was reported to be a cation-disordered defect rocksalt material (cubic, a = 4.116 Å) without spinel superstructure peaks, indicating that 16c and 16d sites have equal cation occupancies. 8 A wide two-phase region between x E 0.15 and x E 0.7 has subsequently been reported. 6 Samples in the small-x region maintain the spinel cation arrangement although a small amount of Ti migration was observed, e.g. 1.8% Ti migration from 16d to 16c sites at x = 0.14. 4 The x E 0.7 product was reported to have substantial Ti occupancy of the 16c octahedral sites and also the tetrahedral 8a sites. 6 However both oxidation methods used gave secondary phase impurities; oxidation by air introduced an amorphous carbonate impurity and Br 2 or I 2 treatments led to a non-spinel Li 1Ày Ti 2Ày O 3 secondary phase. A simple oxidation method to generate high purity material is thus needed to determine the structure and degree of cation migration in Li 1Àx Ti 2 O 4 accurately.
Here we report the preparation of lithium-deficient disordered spinel Li 0.33 Ti 2 O 4 by mild oxidation through immersion in water at room temperature (RT). The sample has been characterized by chemical and thermogravimetric (TGA) analysis, and combined powder X-ray and neutron structure refinement has been used to establish the cation distribution from which a likely migration mechanism is proposed.
The LiTi 2 O 4 spinel precursor was prepared by a reported solid state method. 2 Immersion of LiTi 2 O 4 in water for short periods of a few days was found to produce two-phase spinel samples like those reported previously. 6 To obtain the x E 0.7 phase, 0.1608 g of LiTi 2 O 4 precursor was delithiated chemically by reaction with distilled water (100.0 ml) in a capped flask for 2 weeks at RT. Bubbles were seen when the precursor was immersed due to evolution of H 2 and the solution became strongly basic (pH = 12). The reaction is; and the Li concentration in the solution after delithiation was measured using ICP-AES giving x = 0.667. The blue-black sample was filtered and dried at RT in a desiccator for a few days. TGA was performed by heating the product up to 1000 1C in air at 2 1C min À1 . Oxidation of Li 1Àx Ti 2 O 4 gave a 1.6% mass gain corresponding to x = 0.67, in excellent agreement with the ICP-AES result. The final TGA product was white, consistent with full oxidation of Ti 3+ to Ti 4+ . Combustion analysis (by the Elemental Analysis Service of London Metropolitan University) revealed that the water-treated Li 1Àx Ti 2 O 4 sample contained o0.1% C and H. This rules out possible Li + /H + ion exchange or water incorporation during deintercalation, and shows that LiOH or Li 2 CO 3 impurities are not present. Hence, although many previous deintercalation studies have used dry non-aqueous solvents, water oxidation is found to be a useful method for preparation of Li 1Àx Ti 2 O 4 .
Powder X-ray diffraction (PXRD) data from a Bruker D2 instrument using CuKa radiation displayed in Fig. 2 show only spinel peaks from the Li 0.33 Ti 2 O 4 sample with no secondary phases, unlike products in ref. 6. Substantial diffuse scattering in the background evidences disorder of the heavy Ti atoms within the spinel lattice. Rietveld fits showed that Ti is disordered over octahedral 16c and 16d sites, but did not give a significant improvement (R wp decreased slightly to 5.71 from 5.76%) when Ti was also allowed to occupy the tetrahedral 8a Li sites as proposed elsewhere, 6 so this possibility was not considered further.
Powder neutron diffraction (PND) data were collected from 0.1 g of the Li 0.33 Ti 2 O 4 sample on instrument D20 at ILL Grenoble with a neutron wavelength of 1.3029(4) Å. Combined fits to the PXRD and PND profiles using the GSAS package 15 were used to test further Li/Ti disorder models. The total lithium content was fixed to 0.33 in keeping with the ICP-AES and TGA results. Ti was found to be disordered over 16c and 16 d sites. Models where Li was allowed to occupy alternative 8b tetrahedral sites, or co-occupy octahedral 16c or 16d positions with Ti gave insignificant or negative Li occupancies at those sites. Hence we conclude that the average structure of the Li-deficient Li 1Àx Ti 2 O 4 spinel phase is best described by the model shown in Table 1   We note that a straightforward explanation for the migration of octahedral cations in delithiated Li 1Àx Ti 2 O 4 spinels can be given on the basis of the occupancy of octahedral sites between close-packed oxide layers. Several kinds of stacking are observed among stable TiO 2 polymorphs; rutile, ramsdellite and the a-PbO 2 form are close to hexagonal close-packing (hcp), anatase has ccp, and brookite has a mixed ccp-hcp sequence. 5 Ti 4+ ions occupy half of the octahedral sites between each pair of oxide layers in all of these structures. This provides a key distinction from the hypothetical [Ti 2 ] 16d O 4 spinel polymorph. The LiTi 2 O 4 spinel structure has two alternating types of layer in terms of cation occupancy as shown in Fig. 1. One type (labelled Layer 1) has 3/4 of the octahedral sites filled by Ti, while the other (Layer 2) has 1/4-filling of octahedral Ti sites plus half the tetrahedral sites occupied by Li. Complete delithiation to give [Ti 2 ] 16d O 4 would leave a 3 : 1 Layer 1 : Layer 2 ratio of occupied octahedral sites which is highly disfavoured due to cation-cation repulsions with respect to 1 : 1 distributions. The alternative 16c sites in spinel have a 1 : 3 Layer 1 : Layer 2 ratio, so migration from 16d to 16c sites is a mechanism for equalizing the filling of octahedral sites in the two layer types. The observed 16d and 16c Ti occupancies of 72 and 28% in our refinement of Each 16c site is only 1.79 Å from two 8a Li sites, so both of these tetrahedral sites have to be vacant for the 16c site to be occupied by Ti (see Fig. 1). Hence we propose that for small x, local defect clusters of two Li vacancies surrounding an interstitial Ti at 16c are formed. This assumption gives y = x/2 at small x. Combining this with the above boundary conditions gives a simple quadratic variation y = (x/2)(1 + x), which provides an approximate description of greater degrees of clustering at high x. For our highly deficient x = 0.67 sample this equation predicts y = 0.56 (16c Ti occupancy = 0.28), in perfect agreement with the observed populations in Table 1. This indicates that Ti migration from 16d to 16c sites in highly Li-deficient Li 1Àx Ti 2 O 4 is limited by the distribution of remaining Li cations, with Li vacancies clustering around Ti cations on 16c sites.
For Li 0.86 Ti 2 O 4 which represents the distinct small Li-deficiency (x o 0.15) phase, the y = 0.08 value predicted by the clustering model is somewhat larger than the reported value of 0.04. 4 This suggests that defect clustering may drive the phase separation observed between x E 0.15 and x E 0.7 in the Li 1Àx Ti 2 O 4 system, 6 with very little Ti migration in the small-x region, and a cascade effect above the x E 0.15 limit where Li vacancies and 16c Ti cations cluster in small areas with large x E 0.7.
Clustering of vacancies and cations is reported in many other solids and may be associated with electronic effects such as charge order or metal-metal bonding. For example, in Na 1Àx CoO 2 , redistribution of Na + into two prismatic sites between CoO 2 layers is accompanied by Co 3+ /Co 4+ charge ordering, 22 while in KNi 2 Se 2 Ni-Ni bonding is an electronic driving force for vacancy formation and Ni migration into adjacent K layers. 23 Hence Ti-Ti bonding 8 and local Ti 3+ /Ti 4+ charge order may also play a part in the Ti migration in LiTi 2 O 4 .
In conclusion, water oxidation is demonstrated to be a useful method for obtaining Li-deficient Li 1Àx Ti 2 O 4 spinels without decomposition to other Ti oxide phases. Combined PXRD and PND refinement shows that Li 0.33 Ti 2 O 4 has Li at tetrahedral sites only, but Ti is disordered over two octahedral positions. This Ti migration acts as a mechanism for equalizing the filling of octahedral sites between close-packed oxide layers to minimize cation-cation repulsions. The wide immiscibility gap between x E 0.15 and 0.7 is driven by clustering of Li vacancies and 16c Ti cations. Further microstructural and computational investigations of defect clustering and phase segregation will be useful to add further insights.
We thank EPSRC, STFC and the Kyoto University Global Frontier Project for Young Professionals ( John-Mung Advanced Program) for support, and C. Ritter for assistance with data collection at ILL.