Christian Masquelier

Room-temperature single-phase Li insertion/extraction in nanoscale Li(x)FePO4.

Classical electrodes for Li-ion technology operate by either single-phase or two-phase Li insertion/de-insertion processes, with single-phase mechanisms presenting some intrinsic advantages with respect to various storage applications. We report the feasibility to drive the well-established two-phase room-temperature insertion process in LiFePO4 electrodes into a single-phase one by modifying the materials particle size and ion ordering. Electrodes made of LiFePO4 nanoparticles (40 nm) formed by a low-temperature precipitation process exhibit sloping voltage charge/discharge curves, characteristic of a single-phase behaviour. The presence of defects and cation vacancies, as deduced by chemical/physical analytical techniques, is crucial in accounting for our results. Whereas the interdependency of particle size, composition and structure complicate the theorists attempts to model phase stability in nanoscale materials, it provides new opportunities for chemists and electrochemists because numerous electrode materials could exhibit a similar behaviour at the nanoscale once their syntheses have been correctly worked out.

Effect of Particle Size on Lithium Intercalation into α ­ Fe2 O 3

The electrochemical reaction of lithium with crystallized -Fe2O3 (hematite) has been studied by means of in situ X-ray diffraction. When reacting large particles (~0.5 µm), we observed the well-known transformation of the close-packed anionic array from hexagonal (hc) to cubic (ccp) stacking. At the early stage of the reduction, a very small amount of lithium (xc<0.1 Li/Fe2O3) can be inserted before this structural transformation occurs. Nanosize -Fe2O3 made of fine monolithic particles (200 A) behaves very different, since up to one Li per formula unit (-Li1Fe2O3,xc = 1) can be inserted in the corundum structure without phase transformation. To our knowledge, this is the first time this phase is maintained for such large xc values. This cationic insertion was found to come with a small cell volume expansion evaluated to 1%. Unsuccessful attempts to increase the xc values on large particles by decreasing the applied discharge current density suggest that the particle size is the only parameter involved. The better structural reversibility of this monophasic process compared to the biphasic one was confirmed by electrochemical cycling tests conducted with hematite samples of various particle sizes. Therefore, by using nanosize particles, we can drastically increase the critical Li concentration required to observe the hcccp transition. This work demonstrates that a careful control of the texture/particle size of electrochemically active oxide particles is likely an important variable that has been largely disregarded for such properties. ©2002 The Electrochemical Society. All rights reserved.

Toward Understanding of Electrical Limitations (Electronic, Ionic) in LiMPO4 (M = Fe , Mn) Electrode Materials

To better understand the factors responsible for the poor electrochemical performances of the olivine-type LiMnPO 4 , various experiments such as chemical delithiation, galvanostatic charge and discharge, cyclic voltamperometry, and impedance conductivity, were carried out on both LiFePO 4 and LiMnPO 4 . Chemical delithiation experiments confirmed a topotactic two-phase electrochemical mechanism between LiMnPO 4 and the fully delithiated phase MnPO 4 (a = 5.909(5) A, b = 9.64(1) A, and c = 4.768(6) A). We conclude that the limiting factor in the MnPO 4 /LiMnPO 4 electrochemical reaction is nested mostly in the ionic and/or electronic transport within the LiMnPO 4 particles themselves rather than in charge transfer kinetics or structural instability of the MnPO 4 phase. For instance, the electrical conductivity of LiMnPO 4 (σ ∼ 3.10 - 9 S cm - 1 at 573 K, ΔE ∼ 1.1 eV) was found to be several orders of magnitude lower than that of LiFePO 4 (σ ∼ 10 - 9 S cm - 1 at 298 K, ΔE ∼ 0.6 eV).

Silicate cathodes for lithium batteries: alternatives to phosphates?

Polyoxyanion compounds, particularly the olivine-phosphate LiFePO4, are receiving considerable attention as alternative cathodes for rechargeable lithium batteries. More recently, an entirely new class of polyoxyanion cathodes based on the orthosilicates, Li2MSiO4 (where M = Mn, Fe, and Co), has been attracting growing interest. In the case of Li2FeSiO4, iron and silicon are among the most abundant and lowest cost elements, and hence offer the tantalising prospect of preparing cheap and safe cathodes from rust and sand! This Highlight presents an overview of recent developments and future challenges of silicate cathode materials focusing on their structural polymorphs, electrochemical behaviour and nanomaterials chemistry.

A comparative structural and electrochemical study of monoclinic Li3Fe2(PO4)3 and Li3V2(PO4)3

Pure monoclinic Li 3 M 2 (PO 4 ) 3 (M: Fe, V) powders (<1 μm in diameter) were obtained by an original route that involved initial homogenization of precursors in aqueous solution followed by slow evaporation and annealing under controlled atmosphere at moderate temperatures. The crystal structure of Li 3 V 2 (PO 4 ) 3 was determined for the first time through Rietveld refinements of neutron diffraction data. As for Li 3 Fe 2 (PO 4)3 , Li is distributed within three crystallographic sites, fully occupied at room temperature. The values of the temperature factors on Li(2) and Li(3) sites (five-fold coordination) were found significantly higher than that of Li(1) (four-fold coordination). Li 3 V 2 (PO 4 ) 3 shows four reversible redox phenomena upon insertion of two Li + (V 3+ /V 2+ couple), at 1.98, 1.88, 1.73 and 1.70 V vs. Li. By comparison, Li 3 Fe 2 (PO 4 ) 3 shows two reversible redox phenomena upon insertion of two Li(Fe 3+ /Fe 2+ couple), at 2.88 and 2.73 V vs. Li. Experimental capacities close to the theoretical ones were obtained after optimal composite electrode preparation through ball-milling. In situ X-ray diffraction showed very minor changes from Li 3 M 2 (PO 4 ) 3 to Li 5 M 2 (PO 4 ) 3 Additionally, Li is extracted from Li 3 V 2 (PO 4 ) 3 towards V 2 (PO 4 ) 3 (V 4+ /V 3+ and V 5+ /V 4+ couples) through four redox phenomena at 3.59, 3.67, 4.06 and 4.35 V vs. Li. Despite all these phase transitions, the [M 2 (PO 4 ) 3 ] framework is remarkably stable on cycling, particularly for M: Fe, while partial vanadium dissolution into the electrolyte occurs either on deep reduction to 1.5 V or deep oxidation to 4.6 V vs. Li.

An Electrochemical Cell for Operando Study of Lithium Batteries Using Synchrotron Radiation

A new electrochemical cell has been specially designed for operando experiments at synchrotron facilities both for X-ray diffraction and X-ray absorption. It allows the investigation of insertion materials under high current densities (up to 5C rate) and hence to study complex phenomena of structural and electronic changes out of equilibrium. The LiFePO 4 -FePO 4 system has been chosen as a case study to validate this cell, and tricky phenomena, with apparent delays in phase formation compared with the number of electrons exchanged, have been spotted.

In situ X-ray diffraction techniques as a powerful tool to study battery electrode materials

The performances of rechargeable Li-based batteries depend on many factors amongst which is the structural evolution of the electrode materials upon cycling. To address these issues, efforts have been devoted towards reliable, rapid, and facile ways to perform in situ measurements. We show how recent advances in both cell design (e.g. the emergence of plastic cells) and instrumentation have boosted the implementation of in situ X-ray characterisation methods to the field of energy storage. The benefits of such measurements are discussed and commented through descriptive examples of a new insertion electrode material (PNb9O25) and an existing one of commercial interest LiCoO2. The link between the fundamental findings and their relevance to practical applications is highlighted. The rapidly growing field of in situ characterisation techniques in the field of battery materials extending beyond X-rays and involving XANES, Mossbauer, Raman, RMN and microscopy measurements, is also commented on.

On the origin of the 3.3 and 4.5 V steps observed in LiMn{sub 2}O{sub 4}-based spinels

Different series of LiMn{sub 2}O{sub 4}-based spinels were studied, all presenting two reduction steps at 4.5 and 3.3 V in addition to the normal spinel plateaus around 4 V. A correlation was found between the capacity recovered on these additional steps, the manganese oxidation degree, and the cell parameter of a given spinel. By means of in situ synchrotron diffraction the authors were able to detect the appearance of a new set of diffraction peaks upon oxidation of the 3.3 V step at 3.95 V, that disappeared on subsequent reduction at 3.3 V. Electron diffraction and high resolution electron microscopy studies on partially delithiated samples revealed the formation of double hexagonal layers upon oxidation, consistent with the additional peaks observed in the in situ experiments. Finally, a model to explain the existence of the redox steps at 4.5 and 3.95/3.3 V based on the creation of these double hexagonal layers is proposed.

Solid electrolytes: Lithium ions on the fast track

The solvent-based electrolytes used at present in lithium-ion batteries can be unsafe for large-scale applications. A crystalline electrolyte with high ionic conductivity could soon enable all-solid energy storage systems.

Structural and Mechanistic Insights into Fast Lithium-Ion Conduction in Li4SiO4–Li3PO4 Solid Electrolytes

Solid electrolytes that are chemically stable and have a high ionic conductivity would dramatically enhance the safety and operating lifespan of rechargeable lithium batteries. Here, we apply a multi-technique approach to the Li-ion conducting system (1-z)Li4SiO4-(z)Li3PO4 with the aim of developing a solid electrolyte with enhanced ionic conductivity. Previously unidentified superstructure and immiscibility features in high-purity samples are characterized by X-ray and neutron diffraction across a range of compositions (z = 0.0-1.0). Ionic conductivities from AC impedance measurements and large-scale molecular dynamics (MD) simulations are in good agreement, showing very low values in the parent phases (Li4SiO4 and Li3PO4) but orders of magnitude higher conductivities (10(-3) S/cm at 573 K) in the mixed compositions. The MD simulations reveal new mechanistic insights into the mixed Si/P compositions in which Li-ion conduction occurs through 3D pathways and a cooperative interstitial mechanism; such correlated motion is a key factor in promoting high ionic conductivity. Solid-state (6)Li, (7)Li, and (31)P NMR experiments reveal enhanced local Li-ion dynamics and atomic disorder in the solid solutions, which are correlated to the ionic diffusivity. These unique insights will be valuable in developing strategies to optimize the ionic conductivity in this system and to identify next-generation solid electrolytes.