(Invited) Improve Li-ion Cells Performances: From Materials and Electrolyte Chemistry to Cell Design via Electrodes Process

Abstract

The development of a new Li-ion cell to cover a specific application is a complex task starting with the choice of positive and negative active materials: this is only the first step. Then, the electrolyte must be finely tuned to the electrochemistry and to the performances requests. Electrode formulation and process also have a direct influence on electrodes structure which in turn directly impacts cells performances. Finally, cell design is also of great importance. All these aspects will be covered during the talk through the presentation of dedicated studies. A study on phosphate-based Li-ion cells will come as a first illustration. Phosphate material as positive active material allows high intrinsic safety at cell level. LiFePO4 was the first phosphate active material commercially available. It displays very good performances overall, but with 15% energy less than lamellar oxides due to lower voltage plateau. Moving from LiFePO4 to Li(Fe,Mn)PO4 allows a 15% energy gain thanks to higher voltage plateau of Mn3+/Mn2+ redox couple vs. Fe3+/Fe2+. This is the first way to address high energy request with phosphate chemistry. A second approach consists in considering increase of electrode thickness. This allows decreasing weight and volume of non active components in the cell, such as foils and separator. This approach is not a trivial one and requires a whole study of electrode formulation and process that will be discussed during the presentation. The choice of electrolyte is also of uttermost importance. Electrolytes are real solution cocktails with blends of solvents, salts and additives, opening the space to an infinity of solutions. Starting with a fundamental understanding of the structure of bulk electrolyte and mechanisms of reactions at the two electrodes interfaces is imperative to focus experimental tests for an efficient optimization of electrolyte. Improved electrolytes dedicated to specific chemistries and requirements were developed, is it for very long life graphite-based cells, for high voltage spinels LiNi0.5Mn1.5O4 positive materials for energy/safety/power, or for Li4Ti5O12 as fast charge negative active material. Thermal management of battery during cycling is key to tailor cells life performances. It will be shown that it can be controlled at very different levels, such as electrode design and choice of electrolyte at cell level and use of advanced materials for improved thermal management at module level. Thermal modelling of cell and module heating will illustrate the benefit of those solutions. Last, but not least, life cycle analysis has to be considered right from the beginning in the development of a cell. State of the art manufacturing of positive electrodes requires the use of N-Methyl Pyrrolidone NMP. Replacing NMP by water brings a great progress in the life cycle analysis, but it is technically far from obvious for layered oxides with multiples issues. Promising solutions have been found and will be shown to conclude the presentation.