Invited Presentation: On the Frontier of Li Ion Batteries and the Horizon Beyond Them

In this presentation we will review first, the frontier in Li ion battery technology. Starting with current state-of-the-art systems which include graphite anodes and catode materials such as LiCoO₂, LiFePO₄ or Li[MnNiCo]O₂ (around 150-160 mAh/gr), we intend to move further to increase as much as possible the energy density of Li ion batteries, not on the account of safety. If we mention electric-vehicle applications as the most challenging direction of Li ion batteries , indeed in term of safety and cycle life considerations, we can use today only systems with a moderate performance, such as graphite-Li[MnNiCo]O₂ or graphite-LiFePO₄ batteries. Conventional full EVs powered by battery modules based on these chemistries can drive no more than 150 km between charges. The eectrodes materials that can take us further are silicon for the anode side and 3 types of cathode materials: high voltage LiMn_1.5Ni_0.5O₄ and LiCoPO₄ or high capacity Li & Mn rich layered compounds with an initial structure - xLi₂MnO₃-(1-x)LiMn_1/3Ni_1/3Co_1/3O₂. We will discuss which type of silicon electrodes can provide prolonged cycle life. Monolithic Si electrodes with columnar or nano-wire morphology are advantageous. The choice of potential regime and the nature of the electrolyte solutions are critically important in order to obtain really stable reversible Si anodes. LiCoPO₄ suffers from severe instability problems in standard electrolyte solutions (based on LiPF₆). This can be overcome. However a more severe problem is the practical capacity of these electrodes, that cannot reach more than 120-130 mAh/g (out of 165 mAh/g theoretical). LiMn_1.5Ni_0.5O₄ maybe an excellent electrode material, even in standard electrolyte solutions. A main problem arises in full cells, vs. graphite anodes at elevated temperatures (even at 45 ⁰C), due to transition metal ions dissolution. This phenomenon, even when occuring at a low scale, affects very badely the passivation of graphite electrodes. We will discuss approaches to mitigate these detrimental situations.  xLi₂MnO₃-(1-x)LiMn_1/3Ni_1/3Co_1/3O₂ cathodes can really demonstrate very high capacity, approaching 300 mAh/g. It is possible to ensure their reversibility and capacity retention during prolonged cycling. Their main problem is a gradual voltage shift towards lower values during cycling, due to an irreversible phase transition to a spinel type structure. It is hard to see how this everage voltage decay, which is a bulk effect, can be avoided. Being realistic: considering the most advanced electrodes materials, the energy density of Li ion battery technology  can be increased beyond the state of the art level by no more than 50%. The use of sulfur as a main cathode material brings with it a better chance to increase the energy density of Li batteries, due to the high theoretical capacity of Li-S electrodes (1675 mAh/g for a full conversion to Li₂S). We will discuss the options to develop highly stable Li-S cathodes. However, what may be not less important is to develop anodes suitable for Li-S athodes. Li metal cannot really serve as an anode material in practical rechargeable batteries. The most promising option to close the gap between Li batteries and internal combustion engines is by development of Li-oxygen batteries. The problem is that the moieties formed by reduction of oxygen in the presence of Li ions are too basic and nucleophilic, thus attacking any polar-aprotic solvents that might be relevant for electrolyte solutions  in Li-O₂ cells. The feith of these systems and their chance to emerge as secondary battery systems will be thoroughly discussed. Another direction beyond the horizon on Li batteries are magnesium batteries. Work in this direction became intensive in recent years. We will review progress, but the way to practicality is still very long. Finally, we will mention the chance of Li ion battery technology to contribute for load leveling applications.