D. Larcher

Beyond Intercalation‐Based Li‐Ion Batteries: The State of the Art and Challenges of Electrode Materials Reacting Through Conversion Reactions

Despite the imminent commercial introduction of Li-ion batteries in electric drive vehicles and their proposed use as enablers of smart grids based on renewable energy technologies, an intensive quest for new electrode materials that bring about improvements in energy density, cycle life, cost, and safety is still underway. This Progress Report highlights the recent developments and the future prospects of the use of phases that react through conversion reactions as both positive and negative electrode materials in Li-ion batteries. By moving beyond classical intercalation reactions, a variety of low cost compounds with gravimetric specific capacities that are two-to-five times larger than those attained with currently used materials, such as graphite and LiCoO(2), can be achieved. Nonetheless, several factors currently handicap the applicability of electrode materials entailing conversion reactions. These factors, together with the scientific breakthroughs that are necessary to fully assess the practicality of this concept, are reviewed in this report.

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.

Recent findings and prospects in the field of pure metals as negative electrodes for Li-ion batteries{

In the race for better Li-ion batteries, research on anode materials is very intensive as there is a strong desire to find alternatives to carbonaceous negative electrodes. A large part of these studies is devoted to alloying reactions, which have been known for more than thirty years but that have regained great interest by downsizing particle sizes, moving to nano-textured/nanostructured composites, or designing new electrode concepts. It is not the scope of this review to retrace twenty-five years of research, but rather to highlight recent advances that have been made in the use of Sn or Si-based electrodes together with the remaining challenges to be addressed and issues to be solved prior to such electrodes being commercially implemented in Li-ion cells.

The Electrochemical Reduction of Co3 O 4 in a Lithium Cell

The first stage of the electrochemical reduction of crystallized spinel in lithium cells was investigated by means of in situ X-ray diffraction. Through the use of tailor-made materials prepared from Co-alkoxide precursors, we observed that the formation of the intermediate phase previously evidenced by several authors was highly dependent on the discharge rate, the texture of the active material (i.e., crystallite size, specific surface area), and the cycling temperature. When starting from a highly divided oxide and/or using a low current, we found that this plateau was actually associated with the formation of α-CoO, subsequently leading to metallic cobalt upon further reduction. Alternatively, was formed when using materials with a large crystallite size or/and applying a high discharge rate, later on similarly decomposing into divided metal. These findings and the related competition between different reaction paths represent an explanation for numerous electrochemical observations, and for the need of fast insertion in such host materials to stabilize intermediate lithiated compounds. This work illustrates the major influence of the initial texture as well as the temperature on the reactivity of the 3d-metal based oxides recently reinvestigated for their electrochemical performances as negative electrode materials. Also, it emphasizes the implications of the reactive grain size evolution upon cycling.

Mechanism for limited 55 C storage performance of Li{sub 1.05}Mn{sub 1.95}O{sub 4} electrodes

A survey of the chemical stability of high-surface area LiMn{sub 2}O{sub 4} in various Li-based electrolytes was performed as a function of temperature. The evidence for an acidic-induced Mn dissolution was confirmed, but more importantly the authors identified, by means of combined infrared spectroscopy, thermogravimetric analysis, and X-ray diffraction measurements, the growth, upon storage of LiMn{sub 2}O{sub 4} in the electrolyte at 100 C, of a protonated {lambda}-MnO{sub 2} phase partially inactive with respect to lithium intercalation. This results sheds light on how the mechanism of high temperature irreversible capacity loss proceeds. Mn dissolution first occurs, leading to a deficient spinel having all the Mn in the +4 oxidation state. Once this composition is reached, Mn cannot be oxidized further, and a protonic ion-exchange reaction takes place at the expense of the delithiation reaction. The resulting protonated {lambda}-Mn{sub 2{minus}y}O{sub 4} phase has a reduced capacity with respect to lithium, thereby accounting for some of the irreversible capacity loss experienced at 55 C for such a material.

Si electrodes for Li-ion batteries-A new way to look at an old problem

High-capacity Si-based electrodes could replace carbon-based electrodes in the next generation of Li-ion batteries. Although Si-based electrodes have large gravimetric capacities, they typically suffer from poor cyclability. One reason for the poor cyclability is large volume expansion associated with 3.75 mol of Li reacting with 1 mol of Si. A theoretical approach to design electrodes that can accommodate this large volume expansion is discussed. It is shown that experimental results agree well with the theoretical approach. We show that Si-based electrodes with a relatively low Si content (<33 wt %) and high binder content (33-56 wt %) cycle at large capacities (∼ 660 mAh/g) for hundreds of cycles. No special electrode processing or cycling procedures are required to achieve high capacities with good cyclability.

Synthesis of electrochemically active liCoO2 and liNiO2 at 100°c

Abstract Presently, fabrication of LiCoO2 requires calcination and annealing at temperatures ranging from 850 °C to 1000 °C from one to two days. Herein we show that LiCoO2 powders can be prepared at temperatures as low as 100 °C through the use of a two days ion exchange reaction between CoOOH and an excess of LiOH · H2O. The resultant LiCoO2 powders were studied through the use of X-ray diffraction, thermal gravimetry, and infrared analysis. Low-temperature fabricated LiCoO2 powders were well crystallized with lattice parameters agreeing with 850 °C prepared samples. The samples had an infrared spectrum similar to that of LiCoO2 with, however, weak extra lines indicating the presence of both carbonate and hydroxyl species. The presence of these species was found to severely affect both the capacity and reversibility of the electrochemical intercalation reactions. Heating these powders at moderate temperatures, around 250 °C, was effective in removing the species and drastically improving their electrochemical cycling properties. The same ion exchange reaction was successfully applied to the synthesis of LiNiO2.

In Situ Observation and Long-Term Reactivity of Si/C/CMC Composites Electrodes for Li-Ion Batteries

Si/C/CMC composite for electrodes (Na-Carboxy-Methyl-Cellulose) appear today as the most promising strategy in view of substituting carbonaceous materials for silicon as negative active material in Li-ion batteries, hence the need to understand their reaction mechanism. By means of solid state Nuclear Magnetic Resonance spectroscopy, we confirmed that CMC chains can bind to Si via covalent or hydrogen bonding depending upon the pH of the mother suspension. Through coupled in situ Scanning Electron Microscopy and Electrochemical Impedance Spectroscopy observations of such electrodes reacting with Li, we demonstrated the ability of their porosity to buffer the Si swelling up to 1.7―2 Li/Si, further lithiation resulting in internal reorganization with either a definitive break of the covalent CMC-Si bond, or preservation of both the texture and electric wiring in the case of weaker Si-CMC hydrogen bonding thanks to a self-healing process. A relationship between the nature of the Si-CMC bonding and the electrode performance was found with a very positive impact of hydrogen interaction as 100 cycles could be achieved with preservation of the initial texture and excellent retention (3000 mAh/g Si after 100 cycles). Besides, we demonstrated that an alteration in the electrode texture/porosity, by a freeze-drying process, also impacts the electrode reversibility.

Electrochemically Active LiCoO2 and LiNiO2 Made by Cationic Exchange under Hydrothermal Conditions

The layered LiMO{sub 2} (M = Co, Ni) compounds, which are of potential interest for Li-ion batteries, were synthesized at low temperatures by treatment under hydrothermal conditions of LiOH{center_dot}H{sub 2}O aqueous solutions containing powdered H{sub x}MO{sub 2} phases. The authors studied the reaction mechanism and the influence of temperature, pressure, water dilution, and precursor ratio on the degree of progress of the ion exchange process. Single-phase LiMO{sub 2} can be obtained in 48 h at 160 C under an air pressure of 60 bars from an MOOH/LiOH{center_dot}H{sub 2}O/H{sub 2}O mixture. The degree of advancement of the exchange reaction for M = Co was monitored in situ using an autoclave which allows the withdrawal of samples in the course of the reaction. From transmission electron microscopy coupled with x-ray diffraction studies the authors conclude that the reaction occurs by surface H{sup +}/Li{sup +} exchange and is accompanied by a progressive breaking of the particles due to an interfacial collapse phenomenon. Infrared studies indicate that the LiCoO{sub 2} and LiNiO{sub 2} phases obtained are contaminated by carbonates that can more easily be eliminated in the case of LiCoO{sub 2} by water washing and post-heating treatments under primary vacuum at 200 C formorexa0» 2 days. Once the ion-exchange parameters are controlled, the LiMO{sub 2} products exhibit electrochemical performances comparable to those of high-temperature made phases.«xa0less

On the electrochemical reactivity mechanism of CoSb3 vs. lithium

The electrochemical reactivity of CoSb 3 vs. lithium has been studied. This phase reacts with more than 9.5 lithium in a two-step process, consisting of the uptake of 9 Li at a constant voltage close to 0.6 V, and of about one lithium over the final voltage decay to 0.01 V. Upon recharge, only 8 lithium can be extracted. From in situ X-ray diffraction, microscopy, and magnetic measurements, we provide evidence that the constant voltage process is rooted in the decomposition of CoSb 3 , leading to the formation of a composite made of Co and Li 3 Sb nanograins. We also illustrate that the mechanism by which the internal nanostructured (Co + Li 3 Sb) electrode, formed during reduction, converts back to CoSb 3 , is quite unusual. It involves, concomitant with the Li 3 Sb → Li 2 Sb → Sb dealloying reaction, a chemical reaction between Co and Sb nanograins. The extra capacity, measured at low potential, appears to be nested in a decomposition-type reaction catalyzed by the cobalt nanoparticles, in a manner similar to that previously reported for CoO. Although these materials can reversibly uptake about 8 lithium, they are of negligible value, since their capacity rapidly decays with cycling, independent of the electrode processing.