Novel Materials for Performance Enhancement in Lithium Batteries – Electrolytes and Additives

Abstract

One of the main challenges for the coming decades is the development of new technologies for storage of electrochemical energy. The supply and management of energy are particularly at the centre of our daily concerns and represent a socio-economic priority. The depletion of oil reserves and necessity to reduce carbon dioxide emissions, stimulate the development of electric vehicles. Lithium-ion battery (LIB) seems to be the best choice for electric vehicles, and perhaps for the storage of electricity from wind turbines or photovoltaic cells. Although lithium-ion technology has known remarkable improvements over the last two decades in terms of enhanced energy density, a technological breakthrough seems to be necessary to further increase the energy density, charge rate, safety and longevity. The performance of LIB’s can be improved either by optimizing the electrolyte, or by developing electrode materials. Lithium batteries use organic electrolytes due to their wide operating voltage. For lithium ion rechargeable batteries, these electrolytes are almost universally based on combinations of linear and cyclic alkyl carbonates. These electrolytes make possible the use of Li as the anodic active component and results in an already high power and energy density characteristics of the Li ion chemistries. This presentation will include major highlights of innovative routes (seed ideas of Prof. Michel Armand) to overcome the shortcomings in lithium batteries. Ethers have been sought after as an alternative electrolyte solvent in order to get rid of the alkyl carbonate, fragile to reduction (RO-, RCO2 •) with low flash points (dimethyl carbonate, DMC, Fp = 17 °C). Although high conductivities could be achieved they co-intercalate in the graphite negative electrode at the same time as the cation. A new class of glymes called “Hindered glymes” (patent pending) developed in order to prevent solvent co-intercalation in cells employing graphite anodes will be presented. Moreover previous glyme solvates (1:1) when used in an operating battery with lithium-exchanging electrodes have problems of solvent balance at the electrodes. At the anode (negative electrode on discharge, positive on charge) when lithium is injected in the electrolyte, there is no free glyme available to solvate the corresponding salt, and this will lead to its precipitation. Conversely, at the cathode the desolvation process frees some glyme, not coordinated to Li+ and this lowers the potential at which the electrolyte get oxidized. In order to address both these concerns redesigned PEG ethers with unsymmetrical end groups, ethyl and butyl that overcomes the above mentioned problems will be discussed. Electrolyte additives like Boron Esters (a new family of Boron complexes-patent granted), that forms adducts with anions bound in a dynamic manner and exchange easily with the same species in a different environment will be introduced as additives for controlling the thickness of SEI layers formed in lithium batteries. Lastly cathode additives “Sacrificial Salts” (patent granted), a relatively different approach towards overlithiation for compensating initial charge irreversibility in lithium batteries would also be highlighted.