Battery technologies based on the use of monovalent ions (Na⁺) or multivalent ions (Mg⁺⁺, Ca⁺⁺...), or even both

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Recently, Li-ion batteries have emerged as the best technology for electric vehicle applications, and as a serious option for stationary applications. However, the growing development of these markets raises the question of potential lithium reserves. Fears of a significant increase, particularly in price, due to limited resources located in a few producer countries, have led to the study of alternatives, among which the most interesting and mature is undoubtedly the use of sodium. Sodium resources are unlimited, distributed throughout the world and accessible at very low cost. What's more, while some criteria, notably mass, may appear penalizing, they are counterbalanced by the possibility of replacing the copper imposed as a current collector at the negative electrode in the case of lithium, by the lighter and cheaper aluminum, which does not form an alloy with sodium. Efforts in this field are divided between two systems, which differ in that one uses polyanionic compounds _Na3V2(_PO4₎_2F3, and the other lamellar compounds _Na2/3Mn0_.5Fe0_._5O2 as the positive electrode respectively. The advantages and disadvantages of each of these systems, which currently use a negative carbon electrode but may in the longer term use NaxSb or even NaxPb intermetallic phases, were presented. To date, the _Na3V2(_PO4₎_2F3/C Na-ion system is the most attractive, with cycling and power performance on a par with Li-ion technology. Only high-temperature performance remains an issue.

Both Mg-ion and Ca-ion technologies are part of the same drive towards environmentally-friendly systems, and both rely on highly abundant elements. However, for both technologies, the difficulties lie in i) the choice of materials capable of reversibly intercalating ^Mg2+ or ^Ca2+ and ii) the absence of suitable electrolytes to minimize interfaces. However, this has not prevented the emergence of SMEs (such as Pellion Technologies) for Mg systems - a technology developed by D. Aurbarch in 2002.

In conclusion, we have also mentioned the latest advances in monovalent/multivalent ion batteries, based on the use of electrolytes containing a mixture of two salts. This hybrid concept will be explained for the _LiFePO4/Mg system containing a salt of ^Li+ and ^Mg2+, where the _LiFePO4 electrode is only active towards ^Li+. In simple terms, the use of hybrid electrolytes in combination with an electrode specific to a single ion means combining two electrolytes separated by a membrane, as in the Daniell cell.