Cu3(PO4)2: Novel Anion Convertor for Aqueous Dual-Ion Battery

For the storage of energy coming from renewables such as solar and wind, numerous efforts have been dedicated to the development of rechargeable battery over past several decades [ 1 , 2 ]. Among the multitudinous explored rechargeable batteries, aqueous dual-ion battery as the novel energy storage device has attracted intensive attention recently because of its availability, low cost, high safety and eco-friendliness [ 3 , 4 , 5 , 6 ]. Its concept is different from that of tradition rocking-chair battery in which anions or cations migrate across electrolyte and then react with anode and cathode [ 7 , 8 ]. For aqueous dual-ion battery, anions react reversibly with the electrode, whereas cations do the same way in the other electrode. It is developed from dual-carbon batteries or dual-graphite batteries as scientists find that anions can be inserted into graphite [ 9 , 10 ]. The first prototype of dual-ion batteries used nonaqueous electrolytes and carbonaceous electrodes are proposed by McCullough et al. [ 11 ]. In that patent, the electrochemical behavior of this battery is described according to the “dual-intercalation” mechanism. Thereafter, continuous progress is made to the development of dual-ion batteries [ 12 , 13 , 14 ]. Although traditional dual-ion batteries using organic electrolytes (including ionic liquid electrolytes) exhibit high safety, high working voltages (normally > 3 V), and reasonable specific capacity (~ 80 mAh g −1 ), the flammability and toxicity of organic electrolytes make them suffer from the safety issues [ 13 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ]. These problems hinder their wide application. To solve these problems, dual-ion batteries with nonflammable and low toxicity aqueous electrolytes have been proposed, and several configurations such as Ag/MnO 2 [ 23 ], NaTi 2 (PO 4 ) 3 /Bi [ 24 ], and NaTi 2 (PO 4 ) 3 /Ag [ 25 , 26 ] have been demonstrated and fabricated so far. Notably, these reported systems use silver (Ag) and bismuth (Bi) as the electrodes to capture the anions. Although the performance of these materials shows decent, they possess several drawbacks which need to be conquered. Ag is a little bit expensive in price, whereas Bi can hardly react with anions in a mild solution. Thus, constructing an available aqueous dual-ion battery which can cycle in a quasi-neutral condition is of the great importance and desired.