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.

The constant increase in global energy demand, together with the awareness of the finite supply of fossil fuels, has brought about an imperious need to take advantage of renewable energy sources. At the same time, concern over CO(2) emissions and future rises in the cost of gasoline has boosted technological efforts to make hybrid and electric vehicles available to the general public. Energy storage is a vital issue to be addressed within this scenario, and batteries are certainly a key player. In this tutorial review, the most recent and significant scientific advances in the field of rechargeable batteries, whose performance is dependent on their underlying chemistry, are covered. In view of its utmost current significance and future prospects, special emphasis is given to progress in lithium-based technologies.

Electrolytes are essential for the proper functioning of any battery technology and the emerging Na-ion technology is no exception. Hence, a major focus on battery research is to identify the most appropriate formulation so as to minimize interface reactions and enhance both cell performances and safety aspects. In order to identify suitable electrolyte formulations for Na-ion chemistry we benchmarked various electrolytes containing diverse solvent mixtures (cyclic, acyclic carbonates, glymes) and Nabased salts having either F-based or perchlorate anions and measured viscosity, ionic conductivity, and thermal and electrochemical stability. The binary EC:PC solvent mixture has emerged as the best solvent formulation and has been used to test the performance of Na/hard carbon cells with both NaClO 4 and NaPF 6 as dissolved salts. Hard carbon electrodes having reversible capacities of 200 mA h g À1 with decent rate capability and excellent capacity retention (>180 cycles) were demonstrated. Moreover, DSC heating curves demonstrated that fully sodiated hard carbon cycled in NaPF 6 -EC:PC exhibits the highest exothermic onset temperature and nearly the lowest enthalpy of reaction, thus making this electrolyte most attractive for the development of Na-ion batteries.

The development of a rechargeable battery technology using light electropositive metal anodes would result in a breakthrough in energy density. For multivalent charge carriers (M(n+)), the number of ions that must react to achieve a certain electrochemical capacity is diminished by two (n = 2) or three (n = 3) when compared with Li(+) (ref. ). Whereas proof of concept has been achieved for magnesium, the electrodeposition of calcium has so far been thought to be impossible and research has been restricted to non-rechargeable systems. Here we demonstrate the feasibility of calcium plating at moderate temperatures using conventional organic electrolytes, such as those used for the Li-ion technology. The reversibility of the process on cycling has been ascertained and thus the results presented here constitute the first step towards the development of a new rechargeable battery technology using calcium anodes.

International audienceDue to its success in the domain of power electronics, the lithium ion battery technology is currently being considered for electric vehicle propulsion and even electric grid storage.1,2 However, the implementation of a lithium based technology on a large scale faces controversial debates on lithium availability and cost. Alternative lower cost and sustainable chemistries would be specially suited for large scale applications even if they involve a penalty in energy density. The most appealing alternative is to use sodium, instead of lithium. Indeed, it exhibits a rich intercalation chemistry3,4 and its resources are in principle unlimited (high concentrations in seawater) being very easy to recuperate. Sodium technology has already been successfully implemented in today's commercialized high-temperature Na/S cells for MW storage and for Na/NiCl2 ZEBRA-type systems for electric vehicles, both of which take advantage of the highly conducting solid beta-alumina ceramics at temperatures of ca. 300 C. Mindful of these considerations, and within the current knowledge gained in Li-ion technology, a room temperature analogous Na-ion cell is a realistic target. If achieved, it would bring about a radical decrease in cost with respect to lithium ion technology while ensuring sustainability