Sodium-Ion Hybrid Capacitor of High Power and Energy Density.

Sodium-ion hybrid capacitors (NHCs) have been attracting research interest in recent years. However, NHCs suffer from slower redox reaction kinetics of electrodes as compared to non-Faradaic capacitive counterparts. Herein, a high-performance NHC using porous NaBi as anode, activated carbon (AC) as cathode, and 1.5 M of NaPF6 in diglyme as electrolyte is reported. In a charging process, Na+ is inserted into NaBi to form Na3Bi, and PF6 - is stored in the electric double layers of the AC cathode; in a reverse process, the Na3Bi is desodiated to NaBi and eventually Bi, and the adsorbed PF6 - is released into the electrolyte in the first cycle. The NHC exhibits a capacity of ∼298 mA h gBi -1, capacity retention of 98.6% after 1000 cycles at 2 A gBi -1, and Coulombic efficiency of >99.4%. The achievable power and energy density are as high as 11.1 kW kgtotal -1 and 106.5 W h kgtotal -1, respectively. The superior electrochemical performance is ascribed to the gradually formed three-dimensional (3D) porous and stable networks of the anode, ensuring its comparable fast reaction kinetics and cycle stability to the AC cathode.

Introduction

Batteries are based on bulky redox reactions of electrodes to produce high energy but low power density, while supercapacitors are featured as high power with low energy density via ion storage in electric double layers (EDLs) at the interface of electrode and electrolyte.[1−3] For a combination of the advantages of batteries and supercapacitors, hybrid capacitors (HCs) have attracted extensive research interest. In particular, sodium-ion hybrid capacitors (NHCs) are promising for large-scale electric energy storage benefiting from the high abundance and low cost of sodium resources.[4] NHCs are generally composed of two electrodes for redox reactions in the battery anode and ion sorption in the EDL cathode, respectively.[5] However, the sluggish redox reaction kinetics of the battery-type electrode fails to match with the fast capacitive sorption in the capacitor-type electrode.[6,7] NHCs are faced with a material challenge for both high power and energy density, especially lacking suitable battery-type electrodes with fast redox response.

Up to now, various materials, such as carbonaceous materials, metal oxides, transition-metal oxides, sulfides, and alloys, have been widely investigated as battery-type anodes.[8−18] Among them, alloys have attracted increasing attention due to their large theoretical capacities, high electronic conductivity, and safe potentials for (de)alloying with Na+.[19−23] Unfortunately, they undergo severe pulverization and quick decaying performance in commonly used carbonate-based electrolytes, which are induced by a dramatic volume change upon sodiation/desodiation.[24,25] How to tune the nanostructure of alloys to enhance their cycle stability and redox kinetics is critically important for high-performance NHCs. Cui et al. prepared Si nanotubes to increase accessible surface area to electrolytes and thus allow ions to intercalate at the interior and exterior of the nanotubes.[26] Zhu’s group has reported nanosized bismuth embedded within carbon nanofibers for enhanced cycle stability.[27] Three-dimensional Si/Ge nanorod array anodes buffered by a TiN/Ti interlayer were reported by Yue et al., displaying improved capacities and cycle stability because of their favorable 3D nanostructures.[28] Notably, the 3D porous networks can effectively shorten diffusion distance of ions, mitigate volume change, and favor reaction kinetics.[29,30] Therefore, it is desirable to design and synthesize a 3D porous alloy anode for NHCs.

Herein, we construct a novel NHC with high power and energy density by using porous NaBi as anode, commercial activated carbon (AC) as cathode, and 1.5 M of NaPF6 in diglyme as electrolyte. The porous NaBi anode was obtained via electrochemical presodiation of bulk Bi in diglyme-based electrolyte. Upon charging, Na+ is intercalated into NaBi to form Na3Bi, and the PF6– anions in the electrolyte are adsorbed into the AC cathode. When discharging, the Na3Bi is desodiated to NaBi and then Bi, and the adsorbed PF6– is released into the electrolyte. Favored by the 3D porous network of the anode, this NHC exhibits superior cycle stability and rate capability. A high reversible capacity of ∼298 mA h gBi–1 with a capacity retention of 98.6% was achieved at 2 A gBi–1 after 1000 cycles. It also presents a high power density of 11.1 kW kgtotal–1 at 58.2 W h kgtotal–1 and a high energy density of 106.3 W h kgtotal–1 at 105.0 W kgtotal–1. The superior performance of NHCs will provide insights into the rational design of electrodes with fast kinetics and cycle stability.

Results and Discussion

Characterization of Electrodes

Figure A schematically illustrates the structure and operation process of the assembled NHC with a configuration of NaBi//NaPF6-diglyme//AC. In the charging process, the porous NaBi anode is alloyed with Na+ in electrolyte, and simultaneously PF6– is adsorbed onto the porous AC. In a reverse process, the Na+ is extracted to form Bi, and the adsorbed PF6– in the AC cathode is released into the electrolyte. The scanning electron microscopy (SEM) image in Figure B reveals that the commercial AC is tens of micrometers in size. Its specific surface area is high, up to 1735.5 m2 g–1 from N2 sorption measurement, as displayed in Figure S1, which is beneficial for adsorption of PF6– anions. The Raman spectrum of AC is presented in the inset of Figure B, in which two characteristic Raman bands at 1350 and 1590 cm–1 are attributed to the D and G band, respectively. The intensity ratio of IG/ID is ca. 0.26, indicative of its disordered and amorphous nature.[31,32] This is further confirmed by the X-ray diffraction (XRD) pattern and high-resolution transmission electron microscopy (HRTEM) image in Figure S1. The NaBi anode is obtained by presodiation of Bi in a half battery, which presents 3D porous networks in the SEM image. The adsorption/desorption isotherm of the NaBi in Figure S2 also indicates its porous structure, and the specific surface area is 19.0 m2 g–1. The NaBi anode has high purity and crystallinity from the XRD pattern in the inset of Figure C. The energy-dispersive X-ray spectroscopy (EDX) also confirms the atomic ratio of Na/Bi of 1:1 as listed in Table S1.

Figure 1

Schematic illustration of the NHC and characterization. (A) Schematic illustration of an NHC (SS: stainless steel). (B) SEM image and Raman spectrum (inset) of commercial AC. (C) SEM image and XRD pattern (inset) of a porous NaBi anode. (D) Charge/discharge profiles of AC (orange) at 100 mA gAC–1 and Bi (blue) at 400 mA gBi–1 in half batteries.

Electrochemical properties of the two electrodes are separately assessed by galvanostatic charge/discharge and cyclic voltammetry (CV) in half batteries, as shown in Figure D and Figure S3. The two typical flat plateaus in the charge/discharge profiles beyond the first cycle in Figure D and Figure S3 suggest two distinct two-phase reactions in the cathode. It is consistent with the two reversible couples of redox peaks in the CV curves of the Bi electrode, which delivers a large specific capacity of 382.2 mA h gBi–1 at 2 A gBi–1 with retention of 98.3% after 1000 cycles in Figure S3. Simultaneously, the AC cathode in Figure D presents typical linear charge/discharge profiles with 59.8 mA h gAC–1 at 100 mA gAC–1, which originates from its capacitive sorption of PF6–, as depicted in Figure A. It is in agreement with the rectangular shape of the CV curve in Figure S4. After 500 cycles, it retains a reversible capacity of 59.3 mA h gAC–1 with capacity retention of 99.1% and Coulombic efficiency (CE) of approaching 100%, suggesting excellent cycle stability. Such a high performance of the anode and cathode will favor construction of NHCs.

Reaction Mechanism of the NHC

An NHC with a configuration of NaBi//1.5 M of NaPF6 in diglyme//AC was assembled, as depicted in Figure A. In situ XRD was employed to investigate phase transitions of the NaBi anode in a charge/discharge cycle of the NHC in Figure A–C. The characteristic diffraction peaks of NaBi are clearly observed at 25.58° and 31.78°, indexed to the facets of (110) and (111), respectively. Upon charging in Figure B, their intensities gradually decrease with the appearance of Na3Bi, which is made evident by the diffraction peaks at 20.88°, 32.89°, and 33.70°. However, a small quantity of NaBi exists when charged to the cutoff voltage of 3.5 V, because of the incomplete conversion of NaBi to Na3Bi and considerable delay of the crystal structure transformation in this hybrid system.[33] At Stage II of discharge, the diffraction peaks of the hexagonal Na3Bi gradually weaken and vanish in Figure C when discharged to 2.8 V. At the same time, the tetragonal NaBi is formed continuously. Subsequently, at Stage III, rhombohedral Bi begins to appear via desodiation of the NaBi, and eventually dominates after being discharged to 1.5 V with its only diffraction peak at 27.3°. Furthermore, selected area electron diffraction (SAED) patterns of the pristine, charged, and discharged NaBi electrodes are obtained in Figure D–F. Obvious diffraction spots are consistent with the in situ XRD patterns. More evidence can be found in the HRTEM images in Figure S5. The thin solid–electrolyte interphase (SEI) films of several nanometers are stable in the whole charge/discharge cycle, which ensure the structural stability and fast Na transport in the porous framework, and thus provide fast kinetics to match with the AC cathode. It is clear that, in the first charge, one electrochemical reaction occurs, NaBi → Na3Bi; in the following discharge, two two-phase reactions exist, Na3Bi → NaBi → Bi. This agrees well with one oxidation and two reduction peaks of the CV curve in the first cycle of the NHC at 1 mV s–1 in Figure S6. Beyond the first cycle, the reversible reactions in the anode are Bi ↔ NaBi ↔ Na3Bi.

Figure 2

Characterization of anode in a charge/discharge cycle. In situ XRD patterns during (A) charge and (C) discharge, and (B) the corresponding charge/discharge profile of the NHC at 40 mA gBi–1. SAED patterns at different states: (D) the pristine state, (E) charged to 3.5 V, and (F) discharged to 1.5 V.

On the other hand, capacitive behaviors of the AC cathode in a charge/discharge cycle of an NHC were examined by X-ray photoelectron spectroscopy (XPS). All AC cathodes were taken out from NHCs, and then washed with diglyme to remove residual electrolytes in an argon-filled glovebox. As exhibited in Figure , the binder polytetrafluoroethylene (PTFE) is evident by the C–F and C–F2 bonds in the XPS C 1s and F 1s spectra of Figure A,B, respectively. After charging, new peaks present at 686.5 eV in the spectra of F 1s in Figure B and at 135.2 eV in the spectra of P 2p in Figure C correspond to the P–F bond, indicative of capacitive adsorption of PF6– on the AC cathode. Upon discharging, the signals of P–F decrease apparently because of the release of PF6– into the electrolyte. The reversible behavior of adsorption and desorption of PF6– is also captured in elemental mappings and energy-dispersive X-ray spectroscopy (EDX) patterns in Figure S7.

Figure 3

XPS spectra of the AC electrode at the pristine, charged, and discharged states in the first cycle: (A) C 1s, (B) F 1s, and (C) P 2p.

Electrochemical Performance of NHCs

Figure A displays CV curves of the NHC beyond the first cycle at various scan rates from 1 to 10 mV s–1. All of them present two couples of redox peaks, consistent with the two distinct plateaus of charge/discharge profiles in Figure B. The two redox couples are assigned to the two-phase reactions of Na3Bi ↔ NaBi and NaBi ↔ Bi, as confirmed by XRD patterns in Figure . The NHC can deliver a reversible capacity of 298 mA h gBi–1 at 2 A gBi–1 in Figure B. Almost overlapped charge/discharge profiles over 1000 cycles suggest excellent cycle stability of the NHC. As shown in Figure C,D, the NHC presents high capacity retentions of 96.7% and 98.6% at 0.4 and 2 A gBi–1 for 1000 cycles, respectively, and the Coulombic efficiencies are approaching 100% in each cycle. They correspond to a capacity loss of 0.0033% and 0.0014% per cycle over the long cycle, respectively. During cycles, the XRD patterns of the anode indicate reversible formation of Bi without other impurities as revealed in Tables S2 and S3 and Figure S8. The superior cycle stability of the NHC is ascribed to the NaBi electrode, and its kinetics is comparable to the AC cathode.

Figure 4

Electrochemical performance. (A) CV curves of NHCs beyond the first cycle at different scan rates in the voltage window 1.5–3.5 V. (B) Selected charge/discharge curves at 2 A gBi–1. Cycle performance of NHCs at (C) 400 mA gBi–1 and (D) 2 A gBi–1.

Rate capabilities of the NHC are evaluated and presented in Figure A and Figure S9. The NHC can deliver specific capacities of 312.4, 305.8, 296.6, 278.7, 239.6, and 204.4 mA h gBi–1. Even at a high current of 40 A gBi–1, the NHC can still retain a reversible capacity of 172.2 mA h gBi–1, meaning a fast charge within 15.5 s, as shown in Figure S9. This is related to the synergy between the electrolyte and anode and the capacitive sorption mechanism of the AC cathode. Such a superior rate performance has never been reported before, as compared with those in references in the Ragone plots of Figure S10. As shown in Figure B, a high energy density of 106.3 W h kgtotal–1 is achieved at 105.0 W kgtotal–1; even at a high power density of 11.1 kW kgtotal–1, 58.2 W h kgtotal–1 is obtained. This NHC of NaBi//AC shows advantages of both high energy density of batteries and power density of supercapacitors at the same time. It bridges the performance gap between batteries and supercapacitors.

Figure 5

Rate and power capability of the NHC and morphology evolution of the anode. (A) Rate capability of the NHC. (B) Energy and power densities of the NHC in comparison with other energy-storage systems based on total mass of active materials. (C–E) SEM images of discharged anodes at different cycles.

Morphology Evolution

The high-rate capability of the NaBi anode, comparable to that of the capacitive AC, is further understood by monitoring its morphology evolution during cycles in the SEM images of Figure C–E. The commercial Bi is bulky with tens of micrometers in size (Figure S3). After presodiation, the obtained NaBi electrode evolves with porous integrity with a number of holes on its surface in Figure A. The pores become larger, and this porous network is more obvious, in contrast with typical pulverization or cracks in commonly used carbonate-based electrolytes. This is related to the strong chemical adsorption of solvent molecules on the Bi surface and the movement of its surface atoms.[30] An attractive hierarchical porous network is captured after 1000 cycles, as depicted in Figure E and Figure S11. Namely, pinholes emerge on the network of this porous integrity in the magnified SEM image in Figure E. The volume expansion from Bi to NaBi and from Bi to Na3Bi is 65.3% and 256.0%, respectively. Such a unique architecture can not only mitigate this dramatic volume change of the anode during sodiation and desodiation, but also favor electron/Na+ transportation and hence ensure fast kinetics. This porous anode combined with capacitive AC cathode contributes to high stability, long lifespan, and fast charge/discharge rates of the NHC.

Conclusions

In summary, a high-performance NHC was assembled with the presodiated NaBi anode, commercial AC cathode, and diglyme-based electrolyte. The NaBi anode undergoes phase transformation of NaBi → Na3Bi and Na3Bi → NaBi → Bi in the first discharging and charging process, and in the following cycles, the anode involves two distinctive two-phase reactions of Bi ↔ NaBi ↔ Na3Bi. On the cathode, PF6– is reversibly adsorbed/desorbed into/from the EDLs of AC for charge compensation. This NHC exhibits a large specific capacity of 298 mA h gBi–1 at 2 A gBi–1 and stable cycle performance with capacity retention of 98.6% after 1000 cycles. It shows a high energy density of 106.5 W h kgtotal–1 and power density of 11.1 kW kgtotal–1 for an advantage combination of batteries and capacitors. The excellent electrochemical performance is attributed to the 3D porous integrity of the anode evolved upon cycling and its comparable kinetics to the capacitive AC cathode. This NHC is promising for large-scale electric energy storage, and provides new insights into the rational design of high-performance energy-storage devices.