Preparation of flower-like iron phosphate materials as a novel anode for dual-ion batteries

Dual-ion batteries (DIBs) have exceptional advantages over other electrochemical energy storage devices.


Introduction
With the large-scale application on electric vehicle field, the blooming lithium-ion batteries (LIBs) industry has been developed to meet the continuous requirements of mass production. Currently, transition metals like nickel, cobalt and manganese become important components in cathodes of LIBs. However, these metal resources employed for cathodes are constantly depleting, severely affecting the future industrial development of LIBs. [1][2][3][4][5] Or, from another perspective, unstablility of cathode matrix structures might induce crystalline failure, including overcharging, over-heat, manufacture problem and various misusing. [6][7][8][9][10] This, in turn, would lead to thermal runaway and security issues. [11,12] Hence, resources limitation and potential social safety are the main bottlenecks for wide application of LIBs in electric vehicle field.
Faced with the large-scale application of LIBs, alternative dual-ion batteries (DIBs) are increasing explored in recent years. DIBs are promising energy-storage systems owing to their special storage mechanisms and material characteristics. [13][14][15][16][17][18] Unlike "rocking-chair structure" of LIBs, the anions like PF 6 -in electrolyte are inserted into layered-graphite cathodes of DIBs while lithium ions migrate towards anode electrode during charge. [19,20] These ions will then return to the electrolyte after discharge owing to transfer of anions and cations from the electrolyte. At voltages of 5.4 or 5.5 V (vs lithium reference), graphene layer of cathode could store large amounts of anions to form anion intercalation compounds (C(PF6)n). [21][22][23][24] This yields reversible capacities ranging from 130 to 150 mAh g -1 at n=24. [25,26] It is almost the same as other cathode materials, such as LiFePO4, LiCoO2 or Li2Mn2O4.
And, the graphite based-on anion intercalation can get rid of the limitation of metal matrix. Hence, usage of graphite can become a promising cathode electrode in DIBs. Currently, the development of DIBs is still limited by several technical issues, such as hard paring between graphite cathode and general anode electrode, such as graphite anode, [27][28][29] soft carbon or hard carbon. [30,31] The elevated voltage windows lead to oxidative decomposition and deterioration of the electrochemical performances during charge-discharge processes. Note that most current DIBs are based on ionic liquids (ILs) electrolyte. [32][33][34][35] Moreover, electrochemical process of DIBs can consume large amounts of anion and cation (Li-salt) from the electrolyte owing to the work mechanism. And, the formation-SEI consumes lithium resources at low anode potentials (≤1 V) during initial charging processes. [36,37] During reversible discharge, ion imbalances in electrolyte might yield incomplete extraction from graphite cathode (C(PF6)n). Therefore, the use of common anode might limit their wide applications in DIBs.
Therefore, finding suitable anode materials is important for better performance DIBs. Herein, we propose a possible anode with the promising voltage window, that is the iron phosphate (FePO4) material, to tentatively study its performance in DIBs. The FePO4 is a precursor of LiFePO4 cathode material commonly produced in industry with extremely wide range of applications and low-cost. [38][39][40][41] Lithium iron phosphate (LiFePO4) with specific capacity of 140-150 mAh g-1 and excellent reversibility widely used in vehicles batteries and large-scale energy storage systems is employed as cathode material. The stability of LiFePO4 material is mainly due to its stable structure and inertness of Fe 2+ /Fe 3+ . [42,43] During delithiation, lithium ions in the material are extracted from the olivine crystals, changing the active material to FePO4.
During reverse reaction, FePO4 gradually transforms into LiFePO4 as lithium storage precursor. Meanwhile, volume of the material will not change at this process. In addition, LiFePO4 with redox pair of 3.3/3.4 V is commercialized, and this material can be obtained by calcination of precursor FePO4 and lithium hydroxide at high temperature. [44,45] Some recent studies have synthesized amorphous iron phosphate with higher specific capacity as cathode material of LIBs. [46][47][48] Compared to olivine-LiFePO4, amorphous LiFePO4 possesses an amorphous structure with wide voltage window at 1.5-4 V and lithiation-delithiation curve of certain slope voltage value.
Here, the possibility of FePO4 anode material is studied to explore in application of DIBs. The particular potential plateau of the redox pair in FePO4 anode prevents formation of SEI consuming lithium sources during first electrochemical processes. Owing to the low electronic conductivity of FePO 4 , we applied the combination methods of sol-gel and hydrothermal to obtain flower-like FePO 4 materials. Through this structure, the FePO 4 anode ultra-thick electrode was reached to pair the graphite cathode in dual-ion batteries. [49] These yields first reversible efficiencies close to 100% to protect the stable lithium resources in electrolyte. By conversion of LiFePO4 cathode, stable electrochemical performance and high voltage are obtained by pairing graphite cathode, confirming that FePO4 could apply in the DIBs to obtain stable performance.

Material preparation
The FePO4 was synthesized by mixing FeSO4 with (NH4)H2PO4 at stoichiometric ratio of Fe/P=1. The two aqueous solutions were then mixed to obtain colloid solution. The ascorbic and PEG-400 were added into the solution. And, after addition of H2O2, yellow precipitate hydration precursor was obtained. The suspension solution was put into the microwave reactor at 30 min. Then, the solution was transferred to the polytetrafluoroethylene reactor in hydration hotbox overnight at 180 °C.
After hydrothermal reaction, the compound was washed several times. At last, the precipitation with H 2 O was calcined at 380 °C for 6 h in muffle furnace to obtain well-dispersed FePO4. FePO4 was employed as anode

Characterization
The electrode thickness was mainly controlled by that of control

Results and discussion
In this study, a flower-like FePO4 material was synthesized by the solgel and hydrothermal reaction. Owing to the low electronic conductivity of Herein, such FePO 4 material was used as the anode electrode to show some properties, as shown in Fig. 3a  The FePO 4 can as host to store the dissociated Li-ions from electrolyte, mainly providing an appropriate voltage platform over 1 V (vs Li 0 /Li + ). To further analyze the voltage platforms, the charge-discharge processes of three electrode types in full batteries were recorded and the results are displayed in Fig. 7a might have to do with the better reversibility of lithiation at electrodes with high loading mass owing to partial lithium intercalation. Moreover, the depth of lithium intercalation of a-FePO4 anode reduced capacity retention ratio of the batteries. In Fig. 7b, the variation trend of discharge voltage platform remained similar to that of charging voltage. The enhancement in loading mass of anode could not only improve cycling performance of materials but also ensure batteries with high discharge voltages and elevated energy densities. The electrochemical impedance spectroscopy (EIS) was conducted to verify the changes at interface of electrode for DIBs in Fig. 7c and 7d. The electrode (m-/m+=1.5/1) shows more stable interface impedance in relative to another electrodes (3:1 and 5:1). And, the changes of electrode (3/1) can be also accepted owing to without obvious increasing of impedance. This reason is attributed to the excellent structure of flower-like to ensure enough pathway of electron and ions.
Based on batteries with different loading masses, the best ratio of anode:cathode was determined as 3:1. about 96%. This suggested that irreversible anion in intercalation compounds can be extracted into the electrolyte. During following cycles, the charge and discharge capacities remained stable without obvious decrease. After 500 cycles, high capacity still can be to obtain above 100 mAh g-1 with CE around 96%. The charge-discharge curves at different cycles (1, 5, 50, 100, and 500 cycles) are presented in Fig. 8b, corresponding to cycling of Fig. 7a. The capacity kept stable and charge capacity slightly reduced. The voltage plateau decreased from 1 to 500 cycles. A plateau at about 3 V appeared with obvious depression from 3 V (1 cycle) to 2.9 V (500 cycles). Over 0.1 V voltage platform fade was reflected in electrochemical performances after cycling. The voltage changes obtained during charging were larger than those recorded during discharge. This was attributed to high resistance of anion intercalation in layered-graphite. However, the capacity was not affected owing to the relatively wider charge-discharge voltage window. Fig. 8c gathers the rate capacities of full batteries under different current densities of 50, 100, 200, 500, 1000, and back to 50 mA g -1 . The specific capacity was kept ideal under higher current densities. The capacity retention reached 92%, 88%, 84% and 76% at 100, 200, 500 and 1000 cycles, respectively. Note that the value of 50 mA g-1 at starting stage, and showed remarkable recovery after

Conclusions
Based on graphite cathode, the dual-ion batteries exhibit excellent advantages in terms of high-safety and low-cost for next generation energy storage devices. However, the electrolyte is difficult to withstand oxidative decomposition above 5 V. Here, a new flower-like FePO4 anode was proposed by pairing graphite-based DIBs. As precursor of LiFePO4 material, FePO4 showed obvious advantages in terms of structural stability and low cost, and exhibited an extended electrochemical window of 1-3.5 V. In full batteries, suitable voltage window was obtained owing to the wide 1-3.5 V voltage range. For further improving full batteries voltages, different loading mass ratios of anode/cathode were explored and 3 V was determined as best value. In addition, the electrolyte of PC-EMC solvent was selected. Overall, these findings look promising for industrial application of DIBs with low-cost and high energy density.