Regarding the anode side, I am not sure that we can replace carbonaceous materials (as main components) by other alternatives such as conversion reactions or alloying (e.g. LixSi, SnxSi) if high volumetric energy density and very prolonged cycling are important (as is the case for EV applications). Turning to electrolyte solutions, using fluorinated solvents can help to extend anodic stability and enhence passivation of the anodes. In recent years, we discovered the importance of binders in determining the performance of the composite electrodes we used. We will show that the choice of separators can also play an important role in detemining peformance. The inevitable energy density limitation of Li ion batteries for propulsion by full EVs, promotes intensive work on Li-sulfur and Li-oxygen batteries, which theoretical energy density may approach that of propulsion by ICE. However, it is not clear that there are non-aqueous sovents available, that are stable in the presence of superoxide or peroxide moieties formed by ORR, in solution phase and in the presence of Li ions. Work on these systems promotes renaissance of Li metal anodes. It is important to mention intensive earlier work (15 years ago) that demonstrated the inevitable limitations of Li metal anodes upon cycling at current densities required for practical, high rate batteries. In any event, energy density calculations show that the volumetric energy density of practical Li-S and Li-oxygen systems designed for prolonged cycle life, may not rival that of Li ion batteries. Recent work on both systems demonstrate success in developing sulfur and oxygen cathodes for prolonged cycling. However, replacing Li anodes in these systems by more stable and reversible alternatives without paying too high penalty re: energy density, remains a key challenge.
Sodium ion batteries is an emerging field in recent years. While sodium is much more abundant than lithium on earth crust, the most expansive components are the same transition metals used in Li ion batteries. Despite advantages found for several sodium intercalation cathodes in terms of rate capabilities (higher than that of their Li counter-parts), there is no chance that Na ion systems can rival Li ion batteries in term of energy density. Hence, it is hard to see sodium ion batteries being relevant for electro-mobility. Thus, Na ion batteries can be addressed for load leveling application. Consequently, the greatest challenge in this field is development of Na ion batteries with very prolonged cycle life and relatively low hysteresis between charge and discharge.
Another topic that attracts many researchers in the batteries field in recent years is rechargeable magnesium batteries. We can point out on impressive progress in development of non-complex electrolyte solutions, possessing wide electrochemical windows and fully reversible Mg anodes behavior in them. We will discuss this progress showing some examples. A great challenge remains the positive side. It is interesting that the first generation of rechargeable Mg batteries which comprises Mg metal anodes, complex electrolyte solutions and MgxMo6S8 Chevrel phase cathodes exhibits surprisingly high volumetric energy density with a main advantage which is prolonged cycle life.
Our opinion is based on recent work that is summarized in the references below.
1.Li ion batteries: Erickson, E. et Al;“Horizons for Conventional Lithium Ion Battery Technology“. J. Phys. Chem. Lett.,5, 3313, (2014).
2.Li-sulfur batteries: Markevich, E. et Al; “Review on Li-Sulfur Battery Systems, an Integral Perspective“, Advanced Energy Materials,5, article number 1500212 (2015).
3.Li-oxygen batteries: Sharon, D. et Al; “Lithium-Oxygen Electrochemistry in Nonaqueous Solutions“, Israel Journal of Chemistry, 55, 1 (2015).
4.Electrolyte solutions: Erickson, E. et Al;“Development of advanced rechargeable batteries: A continuous challenge in the choice of suitable electrolyte solutions – Review“, J. Electrochem. Soc., 162 ,A2424 (2015).
5.Mg batteries: Shterenberg, I. et Al;Evaluation of (CF3SO2)2N− (TFSI) Based Electrolyte Solutions for Mg Batteries. Journal of the Electrochemical Society 162, A7118 (2015).
6.Na ion batteries: De la Llave, E. et Al; “Comparison between Na-ion & Li-ion Cells: Understanding the Critical Roles of the Cathodes Stability and the Anodes Pretreatment on the Cells Behavior” , ACS Applied Materials & Interfaces (2016) In press.