Decades-old aqueous technologies (Zn-MnO2 and others) are making a comeback in rechargeable versions : what can we expect ?

Abstract

Combined with the constant demand for greater durability, safety and lower battery costs, a legitimate question is resurfacing : could aqueous systems be the replacement solution by 2022, given the predicted cost of €100 per kWh of stored energy for Li-ion ? This is what this lecture will attempt to answer. It will begin with a quick review of conventional aqueous batteries (Pb-acid, Ni-Cd, Ni-MeH and even Li(Na)-ion aqueous batteries), before focusing on _Zn-MnO2 technology. The history of the Zn electrode, from the Volta battery to Leclanché's saline _Zn-MnO2 and then alkaline batteries, will be retraced to put current research into perspective. Rechargeable Zn-MnO2 batteries_,first brought to market by RAYOVAC in 1986, are enjoying a resurgence in popularity, with numerous papers in high-impact journals. But what's the real story ? What's new about ? The problem with this technology lies in mastering the Zn electrode, since whatever the pH, Zn reduces water →_{H2 (gas)}. This means we're in a situation where we need to avoid passivation and limit dissolution. Initially, low pH was favored (Lechanché-saline batteries), albeit with problems of high self-discharge, limited power and poor low-temperature performance. To get around these difficulties, researchers turned to basic pH (the alkaline batteries we use today), whose conversion to rechargeable batteries (RAYOVAC) was not very successful, due to limited cycling performance (< 20).

However, studies on electrolytes continued with the discovery of a _ZnSO4 (2M)-based electrolyte that increased the coulombic and energy efficiency of these systems. Subsequent studies have shown the benefit of adding _MnSO4 as an additive to electrolytes with a pH approaching 4.6, thus returning to acid electrolytes. Numerous reaction mechanisms with different _MnO2 polymorphs were proposed. The first mechanism was based on the reversible insertion of Zn into _MnO2. A new line of aqueous Zn-ion batteries was thus born. To date, this Zn insertion has been strongly disputed, to the detriment of other mechanisms involving the insertion of ^H+ ; the formation of a pH-dependent _Zn4SO4(OH)₆._4H2Ophase, and so on. As we have seen, reaction mechanisms are still far from a consensus and remain to be elucidated. The electrochemical insertion of Zn is, by contrast, much better documented for vanadium oxides _AxVOy, Chevrel phases and the recently studied Prussian blues. A comparison of these different families shows that Prussian blues offer the highest potential, but limited energy and power performance. Vanadium oxides, on the other hand, have low potentials, but high capacities and powers. Mn oxides offer the best compromise between potential, capacity and power. To date, it is very difficult to know the real performance of these systems, as the race is on for sensation rather than practicality. For example, the capacity performances reported at high currents in order to mask self-discharge reactions are nothing more than a sham. We'll have to wait a few more years to decide whether these aqueous Zn-ion systems are viable.