All-solid-state Li-ion batteries promise safer electrochemical energy storage with larger volumetric and gravimetric energy densities. A major concern is the limited electrochemical stability of solid electrolytes and related detrimental electrochemical reactions, especially because of our restricted understanding. Here we demonstrate for the argyrodite, garnet and NASICON type solid electrolytes, that the favourable decomposition pathway is indirect rather than direct, via (de)lithiated states of the solid electrolyte, into the thermodynamically stable decomposition products. The consequence is that the electrochemical stability window of the solid electrolyte is significantly larger than predicted for direct decomposition, rationalizing the observed stability window. The observed argyrodite metastable (de)lithiated solid electrolyte phases contribute to the (ir)reversible cycling capacity of all-solid-state batteries, in addition to the contribution of the decomposition products, comprehensively explaining solid electrolyte redox activity. The fundamental nature of the proposed mechanism suggests this is a key aspect for solid electrolytes in general, guiding interface and material design for all-solid-state batteries.3 All-solid-state-batteries (ASSBs) are attracting ever increasing attention due to their high intrinsic safety, achieved by replacing the flammable and reactive liquid electrolyte by a solid electrolyte 1 . In addition, a higher energy density in ASSBs may be achieved through; (a) bipolar stacking of the electrodes, which reduces the weight of the non-active battery parts and (b) by potentially enabling the use of a Li-metal anode, which possesses the maximum theoretical Li capacity and lowest electrochemical potential (3860 mAhg -1 and -3.04 V vs. SHE). First of all, the success of ASSBs relies on solid electrolytes with a high Li-ion conductivity 2-5 . A second prerequisite, is the electrochemical stability at the interfaces of the solid electrolyte with the electrode materials in the range of their working potentials. Any electrochemical decomposition of the solid electrolyte may lead to decomposition products with poor ionic conductivity that increase the internal battery resistance 2-4,6 . Third, ASSBs require mechanical stability as the changes in volume of the electrode materials upon (de)lithiation, as well as decomposition reactions at the electrode-electrolyte interface may lead to contact loss, also increasing the internal resistance and lowering the capacity 2-4 .

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 Li 4 -Ti 5 O 12 [1][2][3][4][5] is very interesting because it has the unusual combination of fast (dis)charge rates [6] and an extremely flat potential, [3,7] the latter being due to the two-phase reaction be- [8] 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 Li 4+x Ti 5 O 12 proceeds through a two-phase equilibrium, [3,8] 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 [9] and at elevated temperatures [10] indicate that solid-solution behavior could occur close to room temperature. The aim of this contribution is to study the Li 4+x Ti 5 O 12 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, Li 4+x Ti 5 O 12 as a two-phase system (consisting of the end members Li 4 Ti 5 O 12 and Li 7 Ti 5 O 12 ) 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 Li 4 Ti 5 O 12 . 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 Li 4+x Ti 5 O 12 .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 reflectio...

Li-ion battery, Neutron Diffraction, NanosizingIn this work we report enhanced Li storage in nano sized Li 4 Ti 5 O 12 spinel. The near surface environment of the nano sized particles allows easier accommodation of Li, leading to a larger storage capacity and also explaining the curved nature of the equilibrium cell voltage. At low voltages the increasing lithium storage within the surface layer triggers an opposing mechanism leading to irreversible capacity loss, most likely due to surface reconstruction. Such mechanism rationalizes the existence of an optimal particle size. The general nature of these results suggests similar surface storage effects and an optimal particle size in terms of Li storage to exist for transition metal oxides electrode in general.216th ECS Meeting, Abstract #405, © The Electrochemical Society

Amorphous titanium oxide nanoparticles were prepared from titanium isopropoxide. In situ measurements reveal an extraordinary high capacity of 810 mAh/g on the first discharge. Upon cycling at a charge/discharge rate of 33.5 mA/g, this capacity gradually decreases to 200 mAh/g after 50 cycles. The origin of this fading was investigated using X-ray absorption spectroscopy and solid-state nuclear magnetic resonance. These measurements reveal that a large fraction of the total amount of the consumed Li atoms is due to the reaction of H 2 O/OH species adsorbed at the surface to Li 2 O, explaining the irreversible capacity loss. The reversible capacity of the bulk, leading to the Li 0.5 TiO 2 composition, does not explain the relatively large reversible capacity, implying that part of Li 2 O at the TiO 2 surface may be reversible. The high reversible capacity, also at large ͑dis͒charge rates up to 3.35 A/g ͑10C͒, makes this amorphous titanium oxide material suitable as a low cost electrode material in a high power battery. © 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3332806͔ All rights reserved. Electrochemical storage devices based upon lithium-ion technology have replaced earlier battery types in numerous applications, e.g., portable devices, mainly due to their high energy density, long cycle life, and their relatively low impact on the environment. If materials that support higher current densities during discharging and satisfy the safety issues concerned, Li-ion batteries would become available for heavy duty applications such as ͑hybrid͒ electrical cars.A high power density requires both good ionic and electronic transport properties of the electrode materials. In many cases, the solid-state diffusion of Li ions through the electrode materials is several orders of magnitude smaller than in the electrolyte. Therefore, if the power density is to be improved, the electrode performance is to be investigated. In commercially available Li-ion batteries, the electrode material is dispersed in the electrolyte as microsized crystallites, which are capable of hosting the lithium ions inside their crystalline voids. By simply decreasing the size of these crystallites, the electrode-electrolyte interface is increased, whereas the diffusion length inside the electrode crystallite decreases. However, recent studies reveal a more complex behavior of nanosized Li insertion compounds in, e.g., TiO 2 anatase, 1-3 TiO 2 rutile, 4,5 or Li x FePO 4 , 6 showing distinct changes in electronic structure and ionic mobility upon downsizing to the nanodomain. 7 Usually, these differences in electronic structure and ionic mobility between bulk and nanosized crystallites are ascribed to the relatively increased impact of surface phenomena. [8][9][10] Between the crystalline structures anatase and rutile TiO 2 , similarities were observed in the physical behavior of the nanoscale compounds. Both reveal an increased Liion capacity compared to their microscale counterparts, which appears to be facilitated by an anomalous phase behavi...