In this project we aim to develop an innovative and safe Li-ion cell that overcomes the state-of-the-art analogues. Our focus will be the exploitation of novel formulations by combining a high potential positive electrode (i.e. the spinel LiMn1.5Ni0.5O4, LMNO), a safe nanostructured negative electrode (i.e. Li4Ti5O12, LTO) and innovative advanced electrolytes (based on sulfone solvents, post-LiPF6 salts and additives, such as ionic liquids) able to safely operate at high cell voltage. To this aim, the optimization of the positive electrode/electrolyte interface at high potential is the major challenge and will be the core-innovation of this project.
In the first part of the project, based on the previous knowledge and experience of the Research Team concerning materials for Li-cells (e.g. high potential cathodes and novel non-aqueous electrolytes based on ionic liquids) and surface investigations, the study of the nature and stability of electrode interface at high potential will be addressed. The final goal is to suppress or mitigate irreversible capacity losses upon cycling, due to the parasitic reactivity of the electrolyte above 4.5 V vs. Li. To this aim, the electrolyte (formed by sulfone-ionic liquid mixtures with boron-salt additives) will be designed and optimized in order to drive the spontaneous formation of an effective solid-electrolyte-interface layer at the positive electrode/electrolyte interface.
In the second part of the project, the innovations developed will be implemented in full Li-ion configurations. The Li-ion cells, adopting the most promising home-made electrolytes and commercial LTO anode and LMNO cathode, will be tested to assess their performance retention upon cycling and stability of the electrode/electrolyte interfaces. The final target of this project is to develop a prototypal coin-cell, at lab-scale demonstrator, able to overcome the state-of-the-art performances in terms of specific energy, cycling life and safety.
A breakthrough progress in Li-ion batteries (LIBs) can be achieved in terms of higher energy densities, longer cycle life, improved safety and sustainability [1] by the development of anode, cathode and electrolyte materials relying on innovative chemistries [2, 3].
In recent years we have proposed in the literature a number of possible alternative formulations for next generation lithium-ion cells based on a variety of different chemistries at the cathode, anode and electrolyte sides [4,5]. Among them we demonstrated also the concept of a 3-3.5 V Li-ion cell made by coupling LiNi0.5Mn1.5O4 spinel (LNMO) and TiO2-based anodes [6,7]. Titanium -based anodes have relevant advantages compared to graphite and conversion/alloying materials: the working potential falls within the thermodynamic stability window of the standard organic carbonate electrolytes (> 0.8 V vs. Li); their density is two times larger than graphite and therefore the volumetric performances can doubled compared to a standard graphite-based Li-ion cells [8]. Unfortunately their high operating potential (1.5 V vs Li) is also an important setback for the full cell energy density. Thus, the need for their coupling with high-potential cathodes, e.g. manganese spinel oxide like LMNO [3], to achieve competitive performances in respect to the state-of-the-art formulations [1].
However it has been demonstrated that the use of high potential LMNO materials in combination with commercial carbonate-based electrolytes, clashes with a massive increase of parasitic reactivity upon cycling [7, 9-10]. This effect negatively reflects on long-cycling performances, self-discharge and pave the way to battery failure.
So far, no solution for stable liquid electrolytes above 4.2 ¿ 4.5 V vs. Li (high potentials, HPs) has been found [11]. Indeed, developing a stable and safe electrolyte that works at cell voltages as high as 5V vs. Li is a formidable challenge in the present Li-ion-battery research, since high voltages are beyond the electrochemical stability of the conventional carbonate-based solvents available. In the past few years, HP film-forming additives and new solvents, such as sulfones, ionic liquids, nitriles and fluorinated carbonates, have been investigated [12-16]. Also, the use of standard LiPF6 as conducting salt is questionable, due to its toxicity, decomposability and high voltage reactivity [10], even though its most balanced properties compared to other lithium salts. Novel boron-based materials have been reported as lithium salts or electrolyte additives in Li batteries with excellent properties in terms of solid electrolyte interface, cycle and temperature stability [17,18]. It appears to be very difficult to find a good HP electrolyte with a single-component solvent at the present stage. Using mixed solvents and additives are two realistic solutions for advanced practical applications.
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