Marnix Wagemaker

Equilibrium lithium transport between nanocrystalline phases in intercalated TiO2 anatase

Microcrystalline TiO2 with an anatase crystal structure is used as an anode material for lithium rechargeable batteries, and also as a material for electrochromic and solar-cell devices. When intercalated with lithium, as required for battery applications, TiO2 anatase undergoes spontaneous phase separation into lithium-poor (Li0.01TiO2) and lithium-rich (Li0.6TiO2) domains on a scale of several tens of nanometres. During discharge, batteries need to maintain a constant electrical potential between their electrodes over a range of lithium concentrations. The two-phase equilibrium system in the electrodes provides such a plateau in potential, as only the relative phase fractions vary on charging (or discharging) of the lithium. Just as the equilibrium between a liquid and a vapour is maintained by a continuous exchange of particles between the two phases, a similar exchange is required to maintain equilibrium in the solid state. But the time and length scales over which this exchange takes place are unclear. Here we report the direct observation by solid-state nuclear magnetic resonance of the continuous lithium-ion exchange between the intermixed crystallographic phases of lithium-intercalated TiO2. We find that, at room temperature, the continuous flux of lithium ions across the phase boundaries is as high as 1.2 × 1020 s-1 m-2.

Size Effects in the Li4+xTi5O12 Spinel

The nanosized Li(4+x)Ti(5)O(12) spinel is investigated by electrochemical (dis)charging and neutron diffraction. The near-surface environment of the nanosized particles allows higher Li ion occupancies, leading to a larger storage capacity. However, too high surface lithium storage leads to irreversible capacity loss, most likely due to surface reconstruction or mechanical failure. A mechanism where the large near-surface capacity ultimately leads to surface reconstruction rationalizes the existence of an optimal particle size. Recent literature attributes the curved voltage profiles, leading to a reduced length of the voltage plateau, of nanosized electrode particles to strain and interface energy from the coexisting end members. However, the unique zero-strain property of the Li(4+x)Ti(5)O(12) spinel implies a different origin of the curved voltage profiles observed for its nanosized crystallites. It is proposed to be the consequence of different structural environments in the near-surface region, depending on the distance from the surface and surface orientation, leading to a distribution of redox potentials in the near-surface area. This phenomenon may be expected to play a significant role in all nanoinsertion materials displaying the typical curved voltage curves with reduced length of the constant-voltage plateau.

Lithium Storage in Amorphous TiO2 Nanoparticles

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 H2O/OH species adsorbed at the surface to Li2O, explaining the irreversible capacity loss. The reversible capacity of the bulk, leading to the Li0.5TiO2 composition, does not explain the relatively large reversible capacity, implying that part of Li2O at the TiO2 surface may be reversible. The high reversible capacity, also at large (dis)charge rates up to 3.35 A/g (10C), makes this amorphous titanium oxide material suitable as a low cost electrode material in a high power battery.

Nature of Li2O2 Oxidation in a Li–O2 Battery Revealed by Operando X-ray Diffraction

Fundamental research into the Li-O2 battery system has gone into high gear, gaining momentum because of its very high theoretical specific energy. Much progress has been made toward understanding the discharge mechanism, but the mechanism of the oxygen evolution reaction (OER) on charge (i.e., oxidation) remains less understood. Here, using operando X-ray diffraction, we show that oxidation of electrochemically generated Li2O2 occurs in two stages, but in one step for bulk crystalline (commercial) Li2O2, revealing a fundamental difference in the OER process depending on the nature of the peroxide. For electrochemically generated Li2O2, oxidation proceeds first through a noncrystalline lithium peroxide component, followed at higher potential by the crystalline peroxide via a Li deficient solid solution (Li(2-x)O2) phase. Anisotropic broadening of the X-ray Li2O2 reflections confirms a platelet crystallite shape. On the basis of the evolution of the broadening during charge, we speculate that the toroid particles are deconstructed one platelet at a time, starting with the smallest sizes that expose more peroxide surface. In the case of in situ charged bulk crystalline Li2O2, the Li vacancies preferentially form on the interlayer position (Li1), which is supported by first-principle calculations and consistent with their lower energy compared to those located next to oxygen (Li2). The small actively oxidizing fraction results in a gradual reduction of the Li2O2 crystallites. The fundamental insight gained in the OER charge mechanism and its relation to the nature of the Li2O2 particles is essential for the design of future electrodes with lower overpotentials, one of the key challenges for high performance Li-air batteries.

Dynamic solubility limits in nanosized olivine LiFePO4.

Because of its stability, nanosized olivine LiFePO(4) opens the door toward high-power Li-ion battery technology for large-scale applications as required for plug-in hybrid vehicles. Here, we reveal that the thermodynamics of first-order phase transitions in nanoinsertion materials is distinctly different from bulk materials as demonstrated by the decreasing miscibility gap that appears to be strongly dependent on the overall composition in LiFePO(4). In contrast to our common thermodynamic knowledge, that dictates solubility limits to be independent of the overall composition, combined neutron and X-ray diffraction reveals strongly varying solubility limits below particle sizes of 35 nm. A rationale is found based on modeling of the diffuse interface. Size confinement of the lithium concentration gradient, which exists at the phase boundary, competes with the in bulk energetically favorable compositions. Consequently, temperature and size diagrams of nanomaterials require complete reconsideration, being strongly dependent on the overall composition. This is vital knowledge for the future nanoarchitecturing of superior energy storage devices as the performance will heavily depend on the disclosed nanoionic properties.

Nanosize Storage Properties in Spinel Li4Ti5O12 Explained by Anisotropic Surface Lithium Insertion

Nanosizing is a frequently applied strategy in recent years to improve storage properties of Li-ion electrodes and facilitate novel storage mechanisms. Due to particle size reduction, surface effects increasingly dominate, which can drastically change the storage properties. Using density functional theory calculations we investigate the impact of the surface environment on the Li-ion insertion properties in defective spinel Li(4+x)Ti(5)O(12), a highly promising negative electrode material. The calculations reveal that the storage properties strongly depend on the surface orientation. The lowest energy (1 1 0) surface is predicted to be energetically favorable for Li-ion insertion into the vacant 16c sites. The (1 1 1) surface allows capacities that significantly exceed the bulk capacity Li(7)Ti(5)O(12) at voltages greater than 0 V by occupation of 8a sites in addition to the fully occupied 16c sites. One of the key findings is that the surface environment extends nanometers into the storage material, leading to a distribution of voltages responsible for the curved voltage profile commonly observed in nanosized insertion electrode materials. Both the calculated surface-specific voltage profiles and the calculated particle size dependent voltage profiles are in good agreement with the experimental voltage profiles reported in literature. These results give a unique insight into the impact of nanostructuring and further possibilities of tailoring the Li-ion voltage profiles and capacities in lithium insertion materials.

Rate-Induced Solubility and Suppression of the First-Order Phase Transition in Olivine LiFePO4

The impact of ultrahigh (dis)charge rates on the phase transition mechanism in LiFePO4 Li-ion electrodes is revealed by in situ synchrotron diffraction. At high rates the solubility limits in both phases increase dramatically, causing a fraction of the electrode to bypass the first-order phase transition. The small transforming fraction demonstrates that nucleation rates are consequently not limiting the transformation rate. In combination with the small fraction of the electrode that transforms at high rates, this indicates that higher performances may be achieved by further optimizing the ionic/electronic transport in LiFePO4 electrodes.

The Role of Coherency Strains on Phase Stability in LixFePO4: Needle Crystallites Minimize Coherency Strain and Overpotential

We investigate the role of coherency strains on the thermodynamics of two-phase coexistence during Li (de)intercalation of LixFePO4. We explicitly account for the anisotropy of the elastic moduli and analytically derive coupled chemical and mechanical equilibrium criteria for two-phase morphologies observed experimentally. Coherent two-phase equilibrium leads to a variable voltage profile of individual crystallites within the two-phase region as the dimensions of the crystallite parallel to the interface depend on the phase fractions of the coexisting phases. With a model free energy for LixFePO4, we illustrate the effect of coherency strains on the compositions of the coexisting phases and on the voltage profile. We also show how coherency strains can stabilize intermediate solid solutions at low temperatures if phase separation is restricted to Li diffusion along the b-axis of olivine LixFePO4. A finite element analysis shows that long needlelike crystallites with the long axis parallel to the a lattice vector of LixFePO4 minimize coherency strain energy. Hence, needlelike crystallites of LiFePO4 reduce the overpotential needed for Li insertion and removal and minimize mechanical damage, such as dislocation nucleation and crack formation, resulting from large coherency strain energies.

The electronic structure and ionic diffusion of nanoscale LiTiO2 anatase

Upon lithium insertion in the pristine TiO2 anatase phase the theoretical maximum of LiTiO2 can be reached in crystallite sizes less than approximately 10 nm, whereas bulk compositions appear limited to Li(x) approximately 0.6TiO2 at room temperature. Both X-ray absorption spectroscopy (XAS) and ab initio calculations have been applied to probe the electronic structure of the newly formed LiTiO2 phase. These results indicate that a large majority of the Li-2s electrons reside at the Ti-3d(t2g)/4s hybridized site. About 10% of these electrons are transferred to non-localized states which makes this compound a good electronic conductor. Ionic conductivity is probed by nuclear magnetic resonance (NMR) relaxation experiments indicating relatively small hopping rates between the Li-ion sites in LiTiO2. Formation of the poor ionic-conducting LiTiO2 at the surface of the particles explains why micro-anatase Li(x)TiO2 is not able to reach the theoretical maximum capacity at room temperature, and why this theoretical maximum capacity reached in nano-sized materials cannot be (dis)charged at high rates.

High-voltage LiMgδNi0.5−δMn1.5O4 spinels for Li-ion batteries

New high-voltage cathode materials for lithium and Li-ion batteries, with the general formula LiMgδNi0.5−δMn1.5O4 (δ=0.00, 0.05 and 0.10), have been synthesized and characterized. The crystal structure of these cubic spinel materials has been refined with space group P4332 with the following site occupation: Li+ on 8c, Mg2+ on 4b, Ni2+ on 4b/12d, Mn4+ on 12d/4b and O2− on 24e and 8c. Refinement with space group Fd3m was not possible. As a function of the Mg content, it was found that: (I) the cubic lattice constant increases from 8.1685 A (δ=0.00) and 8.1703 A (δ=0.05) to 8.1733 A (δ=0.10); (II) the flat potential profile at 4.7 V vs. Li/Li+ (δ=0.00) changes to a slightly sloping profile with an increased average potential of 4.75 V (δ=0.10); (III) the cyclability and the conductivity of the materials improve. It is concluded that LiMgδNi0.50−δMn1.5O4 (δ<0.10) are promising cathode materials that, when combined with a low-voltage anode material like LiCrTiO4, can result in ∼3.25 V rechargeable Li-ion battery with spinel electrodes.