A Kinetic Two‐Phase and Equilibrium Solid Solution in Spinel Li4+xTi5O12

Advances in Li-ion batteries for energy storage have facilitated the success of mobile electronic equipment. In particular, high power densities in combination with low-price materials may also make Li-ion batteries attractive for more heavy-duty automotive applications. To facilitate such developments it is essential to understand the material properties that are responsible for the kinetic performance of Li-ion-battery electrodes. In general it is believed that two-phase reactions in electrode materials, responsible for the flat potential upon (dis)charging, lead to relatively low (dis)charge rates, hence limiting the power density. In this context, spinel Li4Ti5O12 [1–5] is very interesting because it has the unusual combination of fast (dis)charge rates and an extremely flat potential, the latter being due to the two-phase reaction between the end members Li4Ti5O12 and Li7Ti5O12. [8] In Li4+xTi5O12 a fraction (1/6) of the Ti on the 16d sites is replaced by Li, and for x = 0 the electrochemically active Li ions fully occupy the tetrahedral 8a sites. Lithiation leads to 16c occupation and, in addition, causes Li ions to move from 8a towards 16c positions, most likely because of Coulomb repulsion between nearest Li ions occupying the 8a–16c positions (separated by 1.81 A). Finally, at x = 3 all the electrochemically active Li ions occupy the octahedral 16c sites. Both end members, Li4Ti5O12 and Li7Ti5O12, are described by the cubic space group Fd–3m and have very similar lattice parameters (8.3595 and 8.3538 A, respectively). As a result the twophase reaction will not lead to substantial structural strain, a favorable property because lattice strains upon cycling are among the main causes of capacity loss in lithium battery electrodes. Although it is established in the literature that at room temperature the (dis)charging in Li4+xTi5O12 proceeds through a two-phase equilibrium, which is responsible for the very flat potential for 0.09 < x < 2.91, the absence of strain and the observation of partial 16c occupation at room temperature and at elevated temperatures indicate that solid-solution behavior could occur close to room temperature. The aim of this contribution is to study the Li4+xTi5O12 structure in detail to gain more understanding of its performance as a battery electrode. The unexpected results completely change our understanding of this material. In contrast to common knowledge, Li4+xTi5O12 as a two-phase system (consisting of the end members Li4Ti5O12 and Li7Ti5O12) appears to be unstable at room temperature, and relaxes to a homogeneous solid-solution phase for the whole concentration range. True two-phase separation in equilibrium is only observed below 100 K. The relaxation towards equilibrium takes place on the timescale of spontaneous Li-ion diffusion (in absence of an applied gradient), and reveals that faster Li insertion will lead to a kinetically induced effective two-phase reaction, which is commonly observed for Li4Ti5O12. However, unlike previous assumptions, the present results demonstrate that this is actually a nonequilibrium situation. The solid-solution-induced disorder, resulting from the mixed 8a/16c occupation, is most likely responsible for the high rate-capabilities in Li4+xTi5O12. Room-temperature neutron diffraction of chemically lithiated materials, given in Figure 1a, show that only subtle changes take place in the spinel structure upon lithiation. Although hardly visible in Figure 1a, the high intensity and large d-spacing range probed by neutron diffraction on the General Materials Diffractometer (GEM, ISIS, Didcot, UK), lead to a large number of resolved reflections with excellent statistics. As a result of this data quality unusually subtle structural details can be extracted, demonstrated, for instance, by the 1 % occupancy of split Li-ion positions in anatase TiO2. [11] Being an established two-phase system, an attempt was made to fit the intermediate compositions with a combination of the end members Li4Ti5O12 and Li7Ti5O12. The end members have the same symmetry and the difference between the lattice parameters is very small (Li4Ti5O12: 8.3595 A [1] Li7Ti5O12: 8.3538 A). This should result in overlap of the X-ray and C O M M U N IC A IO N