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

A novel anion electrode Cu3(PO4)2 is proposed at the first time. The reaction mechanism of Cu3(PO4)2 electrode is investigated. The dual-ion cell is constructed by using pretreated Cu3(PO4)2 and Na0.44MnO2. A novel anion electrode Cu3(PO4)2 is proposed at the first time. The reaction mechanism of Cu3(PO4)2 electrode is investigated. The dual-ion cell is constructed by using pretreated Cu3(PO4)2 and Na0.44MnO2. Electrode materials which can reversibly react with anions are of interest for aqueous dual-ion batteries. Herein, we propose a novel anion electrode, Cu3(PO4)2, for constructing an aqueous dual-ion cell. The Cu3(PO4)2 electrode can operate in a quasi-neutral condition and deliver a reversible capacity of 115.6 mAh g−1 with a well-defined plateau at −0.17 V versus Ag/AgCl. Its reaction mechanism shows that Cu3(PO4)2 decomposes into Cu2O and subsequently is converted into Cu during the initial discharge process. In the following charge process, Cu is oxidized into Cu2O. It suggests Cu3(PO4)2 reacts with OH− ions instead of PO43− ions after the initial discharge process and its potential thereby depends upon the OH− ions concentration in electrolyte. Additionally, an aqueous dual-ion cell is built by using pretreated Cu3(PO4)2 and Na0.44MnO2 as anode and cathode, respectively. During cycling, OH− ions and Na+ ions in electrolyte can be stored and released. Such a cell can provide a discharge capacity of 52.6 mAh g−1 with plateaus at 0.70 and 0.45 V, exhibiting the potential of application. This work presents an available aqueous dual-ion cell and provides new insights into renewable energy storage and adjustment of the OH− ions concentration in aqueous buffer solution.


Introduction
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 ecofriendliness [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 dualion battery which can cycle in a quasi-neutral condition is of the great importance and desired.
The update of aqueous dual-ion battery depends on the selection of electrode materials which acts as its key components. Many literatures have reported the electrode materials for releasing/storing the cations [27,28]. Yet studies for investigating the anion containers are relatively less. Hence, we herein demonstrate a novel anion container, Cu 3 (PO 4 ) 2 , for constructing an aqueous dual-ion cell. This material can operate in a quasi-neutral condition with welldefined plateaus and good performance, and its price is lower than that of Ag, although its reaction mechanism is far different from our original vision. We also use the pretreated Cu 3 (PO 4 ) 2 as anode to assemble the aqueous dual-ion cell coupled with Na 0.44 MnO 2 as cathode. It presents welldefined operating plateaus and good cycling performance.

Results and Discussion
Cu 3 (PO 4 ) 2 is an inorganic compound which is composed of copper cations and phosphate anions. Due to its insolubility in water, Cu 3 (PO 4 ) 2 can be prepared by the facile precipitation method. The typical synthesis is described in supporting information. The as-obtained powder is sky blue material as shown in Fig. 1a. X-ray diffraction (XRD) pattern (Fig. 1b) suggests that two phases exist in this powder, which are Cu 3 (PO 4 ) 2 and Cu 3 (PO 4 ) 2 ·3H 2 O, respectively, according to the two reference patterns. Besides, most of diffraction peaks are found to show the large full width at half maximum, indicative of its small crystallite size. The scanning electron microscope (SEM) images prove this result. As observed in Fig. S1a, b, the Cu 3 (PO 4 ) 2 powder consists of countless nanosheets with thickness around 25 nm, providing large surface area to contact with the electrolyte. Additionally, the water content in this powder is measured by thermogravimetric (TG) analysis (Fig. S2). About 6% of mass is lost below 200 °C, corresponding to the elimination of the physically absorbed and zeolitic water [29].
Cu 3 (PO 4 ) 2 selected as the electrode material in this work is based on its low thermodynamic solubility product [30]. We think when the Cu 3 (PO 4 ) 2 electrode is discharged, Cu will be produced and subsequently a metal-sparingly soluble salt electrode is constructed until Cu 3 (PO 4 ) 2 is vanished. The half reaction should be as follows: Its potential thus can be given by: Finally, the potential of Cu 3 (PO 4 ) 2 electrode is calculated to be −0.22 V versus Ag/AgCl.
To verify the aforementioned half reaction and corresponding potential, the Cu 3 (PO 4 ) 2 electrodes are fabricated and tested in three-electrode cells. Figure 1c, d exhibits the galvanostatic discharge/charge profiles of Cu 3 (PO 4 ) 2 electrode and its corresponding cycling performance. Two distinct plateaus around −0.14 and −0.40 V versus Ag/AgCl is observed upon the initial discharge process, whereas only one plateau at −0.17 V versus Ag/AgCl appears in the recharge process. This case leads to that the initial discharge capacity (265.1 mAh g −1 ) is much higher than the following recharge capacity (115.9 mAh g −1 ). We consider that the large irreversible capacity loss during initial cycle is attributed to the formation of several intermediates as some reported metal oxides [31,32], which can be reacted with lithium ions in the first discharge process. Additionally, the difference between calculated potential and the experimental one is slight. In the following second and third cycles, the large irreversible capacity losses almost disappear, and the charge capacities of Cu 3 (PO 4 ) 2 electrode reach to 132.6 and 129.9 mAh g −1 , respectively. The differential dQ/dV plots of Fig. 1c are displayed in Fig. S3. An increase in charge capacity may be owing to the fact that electrolyte does not contact well with Cu 3 (PO 4 ) 2 electrode before cycling. After 45 cycles, the Cu 3 (PO 4 ) 2 electrode can deliver a reversible capacity of 115.6 mAh g −1 with 87.2% of its second capacity. Even after 145 cycles, the reversible capacity can still be maintained at 96 mAh g −1 . These results suggest the good cycling performance. If any defects could be introduced into this active material, the cycling performance may be better [33,34].
What are the intermediates during discharging and the corresponding mechanism? To answer these two questions, we have characterized the Cu 3 (PO 4 ) 2 electrodes at various states of discharge by using XRD measurement. The obtained results depicted in Fig. 2 are far different The first plateau ∶ 2Cu 3 ( The corresponding mechanism is schematically illustrated in Fig. 2d. According to the electrochemical mechanism mentioned above, the Cu 3 (PO 4 ) 2 electrode reacts with OH − ions instead of PO 4 3− ions after initial discharge process, and its potential depends upon the concentration of OH − ions in electrolyte. The voltage profiles of Cu 3 (PO 4 ) 2 electrodes in electrolytes with different pH validate this result (Fig. S4). It should be noted that although Cu 3 (PO 4 ) 2 electrode provides larger specific capacity and lower plateau in 0.75 M NaH 2 PO 4 electrolyte and 0.75 M Na 3 PO 4 electrolyte, respectively, their cycling performances (Fig. S5) are inferior to that of Cu 3 (PO 4 ) 2 electrode in 0.75 M Na 2 HPO 4 electrolyte (Fig. 1d). Nevertheless, an aqueous dual-ion cell can still be constructed and the corresponding schematic is depicted in Fig. 3a. As viewed, we select Na 0.44 MnO 2 as cathode due to its low cost and eco-friendliness [35][36][37]  During charging, Na + ions and OH − ions are released by the Na 0.44 MnO 2 and pretreated Cu 3 (PO 4 ) 2 electrodes, respectively. Meanwhile, this cell can increase the concentration of NaOH in electrolyte. Upon discharging, these two ions are captured by the cathode and anode, respectively, leading to the reduction in the concentration of NaOH. As a result, this aqueous dual-ion cell can not only modify the concentration of OH − ions in electrolyte, but also provide electrical energy.
The reaction of this cell can be written as follows: Figure 3b displays galvanostatic discharge/charge profiles of pretreated Cu 3 (PO 4 ) 2 /Na 0.44 MnO 2 dual-ion cell. Due to the presence of irreversible capacity loss in the initial cycle, the Cu 3 (PO 4 ) 2 electrode needs to be pretreated before the dual-ion cell assembly. For the pretreatment, the Cu 3 (PO 4 ) 2 electrode is discharged and subsequently recharged for 1 cycle. As observed in Fig. 3b, this as-fabricated dual-ion cell can provide a discharge capacity of 52.6 mAh g −1 at 0.5 C based on the mass of Na 0.44 MnO 2 . Thus, the x value in Na 0.44-x MnO 2 can be calculated, being 0.19. Two well-defined plateaus are observed at 0.70 and 0.45 V. By contrast, recently reported desalination batteries, such as Na 2 Mn 5 O 10 //AgCl [23], TiS 2 //K20 [38], and BiOCl//Na 0.44 MnO 2 [39], displayed the operating plateaus only at ~ 0.3, ~ 0.4, and ~ 0.1 V, respectively. The detailed comparison for operating voltage of pretreated Cu 3 (PO 4 ) 2 / Na 0.44 MnO 2 dual-ion cell in this work with other cells in the literatures [23,[38][39][40][41][42][43] is plotted in Fig. 3c and Table S1. It is worth noting that the pH value changes during cycling as shown in Fig. S8, indicating that this system can adjust the OH − ions concentration in aqueous electrolyte. Besides, XRD patterns of Na 0.44 MnO 2 in dual-ion cell during cycling are characterized as displayed in Fig. S9. The diffraction MnO 2 //Ag [40] S//Fe [41] Na 0.44 MnO 2 //BiOCl [39] Na 2 Mn 5 O 10 /AgCl [23] FePO 4 //Ag [40] FePO 4 //Ag [42] S//Cu [43] Na peaks of Na 0.44 MnO 2 show a quasi-regular change, suggesting that the variation of Na 0.44 MnO 2 during cycling is quasireversible. The result is also in good agreement with previous study [44]. Figure 3d presents the cycling performance of this cell. It can retain the capacity of 43.8 mAh g −1 after 15 cycles. When cycled to 31 cycles, the dual-ion cell still provides 31.5 mAh g −1 , showing its potential of application.

Conclusions
In summary, we propose a novel electrode material Cu 3 (PO 4 ) 2 as an anion container for aqueous dual-ion cell.  4 3− ions after the initial discharge process. Eventually, an available aqueous dual-ion cell has been successfully constructed by applying pretreated Cu 3 (PO 4 ) 2 and Na 0.44 MnO 2 as anode and cathode. It can provide a discharge capacity of 52.6 mAh g −1 with plateaus at 0.70 and 0.45 V, exhibiting its potential of application. On the basis of this work, our next study shall focus on adjustment of the OH − ions concentration in electrolyte by using this dual-ion cell.
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