An efficient molten‐salt electro‐deoxidation strategy enabling fast‐kinetics and long‐life aluminum–selenium batteries

Aluminum–selenium (Al–Se) batteries have been considered as one of the most promising energy storage systems owing to their high capacity, energy density, and cost effectiveness, but Se falls challenges in addressing the shuttle effect of soluble intermediate product and sluggish reaction kinetics in the solid–solid conversion process during cycling. Herein, we propose an unprecedented design concept for fabricating uniform Se/C hollow microspheres with controllable morphologies through low‐temperature electro‐deoxidation in neutral NaCl–AlCl3 molten salt system. Such Se/C hollow microspheres are demonstrated to hold a favorable hollow structure for hosting Se, which can not only suppress the dissolution of soluble intermediate products into the electrolyte, thereby maintaining the structural integrity and maximizing Se utilization of the active material, but also promote the electrical/ionic conductivity, thus facilitating the rapid reaction kinetics during cycling. Accordingly, the as‐prepared Se/C hollow microspheres exhibit a high reversible capacity of 720.1 mAh g−1 at 500 mA g−1. Even at the high current density of 1000 mA g−1, Se/C delivers a high discharge capacity of 564.0 mAh g−1, long‐term stability over 1100 cycles and high Coulombic efficiency of 98.6%. This present work provides valuable insights into short‐process recovery of advanced Se‐containing materials and value‐added utilization for energy storage.


INTRODUCTION
5][6][7][8][9] However, the charge storage performance of carbonaceous materials is limited by their inherent intercalation mechanism, resulting in a relatively low specific capacity of about 100 mAh g −1 .1][12][13] However, similar to the S-based positive materials, the batteries utilizing Se-based materials experience rapid capacity decline, poor cycle stability, and low Coulombic efficiency due to the shuttle effect and obvious volume change during charging and discharging. 14,15oreover, the pure Se material exhibits low reaction activity and weak interface reaction, leading to the actual specific capacity far from the theoretical value of Al-Se battery (1357 mA h g −1 , based on four-electron conversion of Se to SeCl 4 ). 16n order to mitigate the unexpected shuttle effect caused by soluble intermediate product in the charge and discharge processes, three kinds of methods are widely concerned and researched to suppress the shuttle effect for Al-Se batteries, including architecture regulation of positive materials, optimal design of electrolyte, and modification of separator.(1) Architecture regulation of Se positive materials, such as coating or compositing with high surface area materials, has been successfully applied to adsorb the soluble intermediate Se 2 Cl 2 .Se nanowires and mesoporous carbon (CMK-3) nanorods composite was designed to prevent the generated Se 2 Cl 2 from entering the electrolyte and weaken the volume change of the positive materials. 17The discharge capacity was retained at 178 mAh g −1 , with the discharge voltage of 1.5 V. Similarly, porous nanospheres TiO 2 @Se was combined with reduced graphene oxide (RGO) to form TiO 2 @Se-RGO, which increased the electronic and ionic diffusion ability. 18iO 2 @Se-RGO delivered the initial discharge specific capacity of 1127.3 and 208.7 mAh g −1 at the current density of 200 and 1000 mA g −1 , respectively.(2) Optimization and design of electrolyte.Due to stable and strongly integrated electrode/electrolyte interface, gel-polymer electrolyte has been designed to inhibit the diffusion of Se 2 Cl 2 , further enhancing the capacity and cycling stability. 19Additionally, Pang et al. 20 recently developed a NaCl-AlCl 3 molten salt electrolyte, which facilitated facile Al 3+ desolvation kinetics, and resulted in high faradaic exchange cur-rents and rate capability for Al-Se batteries.(3) Another approach involves the application of a modified separator.It was reported that CMK-3 porous carbon-modified separator would prevent selenide from dissolving in the electrolyte, greatly improving its reversible capacity and cycling stability. 16,21Through employing these strategies, researchers aim to mitigate the shuttle effect and enhance the electrochemical performance of Al-Se batteries, paving the way for their practical application in energy storage systems.
3][24][25][26] In the preparation of Se-containing materials, molten-salt electrochemistry method has attracted more and more attention due to wide liquid temperature range, large electrochemical window, and low cost. 27,28Meanwhile, it can effectively control the particle size and uniformity of the products.Because of multivalence states of Se, when the melt is in neutral or alkaline conditions, the reduction process of tetravalent Se is multi-step reduction. 29,30In the pioneering work, we have systematically investigated the specific redox process of SeO 2 working electrode with metal Mo as counter electrode in the neutral NaCl-AlCl 3 molten salt system. 31The reduction mechanism has been clarified as Se 4+ → Se 2+ → Se.The morphology evolution processes of SeO 2 through direct electro-deoxidation was revealed along with the change of electrolysis potentials, and the high-crystallinity pure Se materials were obtained.
Herein, for inhibiting the unexpected shuttle effects induced by the dissolution and improving the diffusion kinetics of the Se positive electrode, a novel strategy through a two-electrode configuration with SeO 2 pellet as cathode and graphite rod as anode is applied to the controllable electrolytic formation of hierarchical hollow and graphite confined Se/C microspheres, and to achieve excellent Al storage capability.Specifically, the unique interactions between graphite anode and NaCl-AlCl 3 molten salt result in self-template formation of hierarchical structured Se/C hollow microspheres in the cathode.The relationship between the morphology of electrolytic products and electrolytic conditions is clarified to synthesize the uniform Se/C hollow microspheres.When evaluated as the positive electrode for AIBs, the as-prepared Se/C hollow microspheres are demonstrated to hold a favorable hollow structure for hosting Se, which can not only suppress the dissolution of soluble intermediate products into the electrolyte, thereby maintaining the structural integrity and maximizing Se utilization of the active material, but also promote the electrical/ionic conductivity, thus facilitating the rapid reaction kinetics during cycling.

Structural characterizations of electrolytic products
To comprehensively assess the electrochemistry behaviors of Se for battery design, theoretical calculations on the pristine chain-like Se 6 , possible Se reduction product Al 2 Se 3 , and Se oxidation products (Se 2 Cl 2 and SeCl 4 ) have been conducted through HSC software, as depicted in Figure 1A.Theoretical potentials of Se 2 Cl 2 and SeCl 4 are calculated to be 1.83 and 1.94 V versus Al 3+ /Al, respectively.The results emphasize the significance of achieving the desired deep oxidation from Se to Se 4+ in order to maximize the energy storage capacity.It is imperative to preserve the structural stability of the pristine Se and intermediate Se 2 Cl 2 , which is generated during the charge process.It is worth noting, however, that the soluble Se 2 Cl 2 can lead to the structural collapse and significant loss of active material, further resulting in the rapid capacity decay.Accordingly, for inhibiting the unexpected shuttle effects and overcoming the diffusion kinetics limitation of Se, the hollow and graphite confined Se/C microspheres were designed to achieve excellent Al storage capability.
Se/C hollow microspheres was electrochemically prepared in a NaCl-AlCl 3 molten salt at 190 • C through a two-electrode configuration, in which pressed SeO 2 pellet wrapped with Mo mesh was used as cathode and graphite rod was used as anode.Schematic formation mechanism illustration of Se/C hollow microspheres is represented in Figure 1B.The elementary Se can be obtained by a two-step two-electron electro-deoxidation process of SeO 2 in the cathode. 31O 2-ions generated from SeO 2 electrodeoxidation migrate into the molten salt and transfer to the surface of graphite anode, which can combine with graphite electrode to generate CO gas.Meanwhile, it has been proved that AlCl 4 -anions can intercalate into graphitic layers of graphite electrode. 32As the electrolysis voltage increases, AlCl 4 -anions are continuously intercalated into graphite, making it be exfoliated into thin graphite nanosheets (Figure S2).In this process, CO gas also accelerates the stripping of graphite.Thereafter, under the dual action of the AlCl 4 -anions intercalation and CO microbubble effect, these exfoliated graphite nanosheets are continuously shifted to the surface of SeO 2 cathode.Finally, at the side of cathode, the resulted elementary Se combines with graphite nanosheets to form hierarchical structured Se/C hollow microspheres.On one hand, this molten-salt electro-deoxidation approach provides a new insight for the extraction and recovery of advanced scattered metal-containing materials in a short process, such as Se and Te.On the other hand, considering the simul-taneous exfoliation of graphite and reduction of SeO 2 in the molten-salt electrolysis process, which may lead to the deposition of Se at the surface of the Se/C hollow microspheres.Therefore, it is necessary to regulate the electrolysis parameters to obtain Se-containing materials with better crystallinity and morphology structure.
In order to investigate the evolution of morphology and composition of the products at different electrolysis conditions, the electro-deoxidation products were systematically detected.Initially, the morphology change of the products was studied at 190 • C for 4 h with increasing the electrolysis voltages from 1.0 to 2.0 V, as shown in Figure S3.Under the voltage of 1.0-1.2V, small pieces of Se/C composite are agglomerated together.The current-time curve of SeO 2 cathode after constant voltage electrolysis of 1.2 V for 4 h is shown in Figure S4.The current shows a tendency to fall steadily after a rapid decline.When the voltage is further raised to 1.4-1.8V, spherical Se/C composite appears and becomes more and evener.As the voltage reaches 2.0 V, spherical Se/C composite tends to decrease instead.Moreover, the influence of the electrolysis time and temperature on the morphology was further studied, as shown in Figures S5 and S6.With the increase of electrolysis time (2-6 h), the diameter of the spherical Se/C composite increases significantly (Figure S5), implying that SeO 2 wrapped on the surface of the initially formed spherical Se/C composite is further reduced and attached to the surface of the spheres, resulting in an increase in the diameter of the spheres.The change of temperature also affects the morphology of the products (Figure S6).It can be found that increasing the temperature is conducive to raising the proportion of spherical morphology.Moreover, the exfoliated graphite always exists in the electrolyzed products, unless at a very low voltage (Figure S7).Besides, when applying TiB 2 inert anode, pure Se products were also obtained and the crystallinity of the products increases with increasing voltage, from X-ray diffraction (XRD) patterns in Figure S8.Therefore, in order to obtain spherical Se/C composite with uniform morphology, the electrolysis parameters of voltage of 1.8 V, temperature of 190 • C and electrolysis time of 4 h should be considered with graphite anode.
The electro-deoxidation products electrolyzed at 1.8 V, 190 • C for 4 h are further taken as a typical illustration to elaborate the morphology, composition, conductivity, and size distribution.Scanning electron microscopy (SEM) image in Figure 1C shows that the spherical surface of Se/C is flat without holes, and the elemental mapping images indicate that Se and C elements are evenly distributed.It is observed from transmission electron microscopy (TEM) and SEM images in Figure 1D that Se/C possesses a hollow microsphere structure with a wall thickness of ∼120 nm, which further clarifies the formation process of spherical Se/C composite.Selected area electron diffraction (SAED) image in Figure 1E exhibits that Se/C is composed of single crystal Se and polycrystalline graphite.The presence of graphite will greatly improve the conductivity of Se.The sharp XRD patterns in Figure 1F also verify the formation of trigonal-phase Se with high crystallization.The conductivity of Se/C hollow microspheres tested by the four-probe method under the pressure of 5 MPa is 0.5 S cm −1 , which is significantly higher than the conductivity of elementary Se (1 × 10 −3 S cm −1 ). 33The elemental composition and the valence status of Se/C hollow microspheres were further carried out by X-ray photoelectron spectroscopy (XPS) analysis.It is clear to find from Figure 1G that the binding energies of Se are assigned to Se 3d 5/2 (55.0 eV) and Se 3d 3/2 (55.9 eV), 31 indicating the presence of elementary Se.Moreover, according to the C 1s XPS spectra in Figure S9, significant ordered sp 2 graphitic carbon (284.8 eV) and slight disordered sp 3 bonded carbon (285.5 eV) characteristics are observed.Meanwhile, a large number of oxygen-containing groups (C-O, 286.4 eV and O C O, 288.8 eV) imply the structural defects of graphite material. 34,35e/C hollow microspheres were characterized by Raman spectroscopy to further determine the structure of Se and C in the composite (Figure 1H).For comparison, gray Se was also measured.It is found from Figure S10 that gray Se with fine crystallinity is also trigonal phase, which is composed of particles with the size of about 2 µm.Pure Se shows obvious Raman characteristic peak at 233 cm −1 , which is attributed to chain-structured Se molecules.[36][37][38] For Se/C hollow microspheres, the peak at 237 cm −1 is assigned to Se with slight blue shift.Moreover, the D peak of at 1342 cm −1 and G peak at 1586 cm −1 are observed, representing disordered aromatic structure of sp 3 bonded carbon and the bond stretching of sp 2 atoms in the two-dimensional hexagonal lattice, respectively.6,[39][40][41] The D peak in Se/C hollow microspheres is slightly higher than the G peak, indicating that there is a certain amount of disordered and defective carbon.41 Thermogravimetric analysis measurement of Se/C hollow microspheres in Ar atmosphere was performed from room temperature to 800 • C, as presented in Figure S11.The mass loss at 100 • C (0.82%) is caused by the evaporation of moisture adsorbed on the material surface, while the mass loss around 380 • C (78.72%) is related to the sublimation of Se. 42 Therefore, the composition of this Se/C hollow microspheres is calculated to be 79.37 wt.% Se and 20.63 wt.% C. Furthermore, particle size distribution of Se/C hollow microspheres was measured and shown in Figure S12.It can be seen that the particle size presents a normal distribution, and the volume within the particle size ranging from 0.675 to 1.65 µm accounts for 80.9% of the total volume.Therefore, the size distribution of obtained Se/C hollow microspheres is relatively uniform, with an average diameter of 1 µm.

Electrochemical measurements of Se/C hollow microspheres
The unique hollow microsphere-like structure of electrochemically generated Se/C composite can benefit the diffusion of ions and provide an effective shield against the shuttle of the soluble intermediate Se 2 Cl 2 in Al-Se batteries.Herein, Se/C hollow microspheres and gray Se were applied as the positive electrode for Al-Se batteries, in order to evaluate the role of hollow microsphere-like structure.Cyclic voltammetry (CV) curves at 1 mV s −1 in Figure 2A exhibit two pairs of redox peaks at 1.78 V/1.38 V (O 2 /R 2 ) and 2.10 V/1.51 V (O 1 /R 1 ), which is ascribed to conversion from Se to Se 2 Cl 2 and conversion from Se 2 Cl 2 to SeCl 4 , 16,19,43 respectively.It is also found that Se/C hollow microspheres present smaller polarization.The chargedischarge behaviors and cycling stability of Se/C hollow microspheres and gray Se were measured at a high current density of 1000 mA g −1 , as exhibited in Figure 2B-D.As a contrast, the first capacity of gray Se is only 240.5 mAh g −1 due to the low electronic and ionic transport behaviors, with a Coulombic efficiency of 46.3%.As expected, Se/C hollow microspheres display higher and longer potential plateau of 1.65 V, with the initial discharge capacity of 564.0 mAh g −1 and Coulombic efficiency of 54.5%.Coulombic efficiency for the first few dozen cycles is relatively low in comparison with the previous reports, 13,15,20,44 which is mainly due to the irreversible capacity resulted from the soluble intermediate Se 2 Cl 2 during the charging process, leading to the loss of the active material that corresponds to the capacity loss.Meanwhile, the higher surface area of Se/C hollow microspheres can cause the increase of the adsorption capacity of chloroaluminate anions, thus resulting in the increase of the irreversible capacity.Moreover, both Se-based electrodes show the similar tendency of rapid decline followed by stabilization.After cycling for 1100 times, the capacity of gray Se is as low as 35.6 mAh g −1 , with a Coulombic efficiency of 95.5%.In contrast, Se/C hollow microspheres demonstrate remarkable performance, delivering a reversible capacity of 175.0 mAh g −1 and maintaining a high Coulombic efficiency of 98.6%.This significant improvement in capacity and Coulombic efficiency highlights the superior electrochemical performance of Se/C hollow microspheres.
Electrochemical impedance spectroscopy (EIS) measurements were employed to determine the ion transport kinetics of the Se-based electrodes before cycling.Compared with gray Se electrode, Se/C hollow microspheres display lower charge transfer resistance (R ct ) at highmedium frequency region (Figure 2E).R ct of Se/C hollow microspheres is 281 Ω, far less than that of gray Se (515 Ω).The corresponding distribution of relaxation times (DRT) results from EIS measurements are shown in Figure 2F.matrix as continuous electron conductor, thus resulting in reduced polarization and enhanced current transportation.This unique hollow microsphere-like structure can effectively confine the soluble intermediate products into the electrolyte, and suppress the dissolution of intermediate products and structural collapse during repeated ion conversion processes.Furthermore, the reduced ion diffusion pathway can facilitate the ion diffusion capability, thereby accelerating the redox reaction kinetics of the active material.

Insights into the accelerated kinetics
Optical changes of gray Se and Se/C hollow microspheres immersed in electrolyte were conducted to verify the struc-tural stability of materials, as illustrated in Figure 3A.The color change of Se/C hollow microspheres is less than that of gray Se over time, indicating that Se/C hollow microspheres prepared by the molten-salt electrolysis possess the relatively more stable structure.XPS, TEM, SAED, SEM, and energy dispersive spectroscopy (EDS) characterizations of Se/C hollow microspheres were carried out to elucidate the energy storage mechanism and structure stability in the charge and discharge process.Se 3d XPS spectra of Se/C hollow microspheres at different states are presented in Figure 3B-E.After charging to 2.1 V, in addition to Se 3d 5/2 (55.0 eV) and Se 3d 3/2 (55.9 eV), a pair of peaks located at 55.7 and 56.6 eV are observed, indicating the transformation of chain-like Se molecules into Se 2 2+ species. 16When charging to 2.3 V, a new peak at 59.6 eV appears, indicating the further oxidation to Se 4+ . 16,18Upon discharging to 1.3 V, the peak representing Se 4+ disappears and the pair of peaks on behalf of Se 2 2+ appear.When discharging to 0.1 V, peak of elementary Se is only observed, implying the reversible reduction of Se 4+ → Se 2 2+ → Se 0 .Moreover, after charging to 2.3 V and discharging to 0.1 V, the obvious Cl 2p and Al 2p peaks (Figure S13) indicate the successful incorporation of AlCl 4 -and Al 2 Cl 7 -anions.TEM and SAED images of Se/C hollow microspheres after fully charging state that the spherical morphology retains largely intact, but crystal structure changes from single crystal to amorphous phase (Figure 3F).After fully discharging, the corresponding TEM and SAED images further imply that the crystal structure can return to single crystal from amorphous phase (Figure 3G).Additionally, SEM and element mapping images of the charged and discharged Se/C hollow microspheres were investigated with EDS element analysis, as shown in Figures S14 and S15.It is noteworthy that the charged sample exhibits a significantly stronger Cl signal than that of the discharged sample.Meanwhile, it is important to note that Se/C hollow microspheres maintain their basic morphology after charging or discharging, implying the great durable structural stability.Moreover, SEM measurement of Se/C hollow microspheres after multiple cycles at 1000 mA g −1 was conducted, as shown in Figure S16.Clearly, Se/C microspheres have no structural collapse after multiple cycles, further identifying the structural stability.Meanwhile, this durable hollow microsphere-like structure is beneficial for suppressing the soluble intermediate products into the electrolyte, thereby enhancing the cycling stability.
The density functional theory (DFT) calculations were carried out to further demonstrate the improved reaction kinetics of Se/C hollow microspheres.The structures of chain-like Se, AlCl -anions on Se/C material.Moreover, the electron density differences and charge distribution were simulated to evaluate the charge transfer process, as shown in Figures 4C,D, S21, and S22.The net electronic charge in the intermediate region between Se 2 Cl 2 /SeCl 4 and graphite matrix suggests the charge transfer from the adsorbed polyselenide to adjacent carbon atoms.Furthermore, compared to the Bader charge for pure Se, Se 2 Cl 2 , and SeCl 4 , the Bader charge for AlCl 4 -adsorbed on Se and Se 2 Cl 2 significantly increases, suggesting strong chemical binding phenomenon between AlCl 4 -anions and Se/Se 2 Cl 2 .Accordingly, benefiting from the high-conductive graphite host, Se/C hollow microspheres provide effective electron transport channels, deliver the fast charge transfer, and guaranty the material structure integrity, thus ensuring the high reversibility of Se in the repeated cycling processes.

Overall Al-Se battery performance
To evaluate the contribution ratio of capacitive-and diffusion-controlled process, CV curves of as-prepared Se/C hollow microspheres was performed at various scan rates ranging from 0.2 to 1 mV s −1 , as shown in Figure 5A.
As the scan rate increases, the oxidation peaks shift toward higher potential and reduction peaks shift toward lower potential.The capacitive contribution of Se/C hollow microspheres can be quantified as 16.4% at a scan rate of 1 mV s −1 , as presented in Figure 5B.The contribution ratios of both processes at different scan rates are also determined in Figure 5C.With the increase of the scan rate, the capacitive contribution increases gradually, while the diffusion contribution decreases as expected.Figure 5D is the typical charge-discharge curves at various current densities ranging from 500 to 1000 mA g −1 .The reversible capacity reaches up to 720.1 mAh g −1 at 500 mA g −1 .Even at a higher current density of 600, 800, and 1000 mA g −1 , the first capacity is as high as 678.5, 607.6, and 564.0 mAh g −1 , respectively.As a key feature, the rate capability of Se/C hollow microspheres was tested under various current densities, as shown in Figure 5E.The reversible capacity decreases from 706.3 to 284.0 mAh g −1 over 10 cycles at a current density of 500 mA g −1 .Upon increasing the current density, Se/C hollow microspheres retain a stable capacity of 241.3, 185.5, and 167.5 mAh g −1 at a current density of 600, 800, and 1000 mA g −1 , respectively.Coulombic efficiency increases upon the higher current densities, which is likely because shorter contact time between the electrode and electrolyte reduces the entry of soluble intermediate products into the electrolyte.Meanwhile, under higher current density, the reduction in the self-discharge time is evident, which is beneficial for improving the Coulombic efficiency.When current density gradually decreases from 1000 to 500 mA g −1 , the discharge capacity still remains stable and reaches above 171 mAh g −1 , indicating that the carbon compositing and hollow microsphere-like structure is conducive to improving rate capability.
Compared to the original Se/C electrode, the cycled Se/C electrode exhibits smaller R ct in the high-medium frequency region (Figure 5F).After five cycles, R ct decreases to 64 Ω, far less than the initial R ct of 281 Ω.The related DRT results in Figure 5G indicate that the cycled Se/C electrode shows reduced response time for ion diffusion, thus resulting in notable decrease of electrochemical reaction resistance.The galvanostatic intermittent titration technique (GITT) measurement was conducted to determine the ionic transportation kinetics of the Se/C hollow microspheres upon cycling, as presented in Figure S23.During charging, the diffusion coefficient delivers an initial high diffusion coefficient (1.48 × 10 −11 cm 2 s −1 ), then gradually decreases and stabilizes at 10 −13 to 10 −16 cm 2 s −1 .Compared to the reported sulfide electrodes, [45][46][47] the diffusion coefficient of Se/C hollow microspheres possesses fast kinetic process.Moreover, the high discharge voltage and large capacity of Se/C hollow microspheres are superior to most previously reported Se-based materials applied in AIBs (Figure 5H). 13,15,17,20,44,48,49More detailed electrochemical data on Se-based materials are compared in Table S1.Furthermore, the long-term cycling stability for 1100 times enables Se/C hollow microspheres as one of the most competitive positive materials for AIBs.
Overall speaking, the molten-salt electro-deoxidation strategy is employed to achieve short-process recovery of advanced Se-containing materials in an electrolysis cell.As illustrated in Figure 6A, Se/C hollow microspheres can be /Se 4+ on the positive electrode.Importantly, the enhanced electrochemical performance can be ascribed to the unique hollow microsphere-like structure and highly continuous and electrically conductive carbon networks, which not only inhibits the soluble intermediate products from dissolving into the electrolyte, but also maintains the structural integrity of the active materials and facilitates the reaction kinetics during cycling.Moreover, the modified separator also plays a positive role by adsorbing the intermediate products and suppressing the shuttle effect, further achieving the superior electrochemical performance.

CONCLUSION
In summary, through taking advantage of stripped graphite nanosheets generated at the interface between graphite anode and molten salt, Se/C hollow microspheres have been successfully prepared by electrolysis in low-temperature neutral NaCl-AlCl 3 molten salt.The electrolyzed product at 1.8 V, 190 • C for 4 h is composed of the uniform hollow microspheres of average diameter of 1 µm with a wall thickness of ∼120 nm, which possesses high Se content of 79.37 wt.% and high conductivity of 0.5 S cm −1 .Benefiting from the hollow microsphere-like structure, highly continuous and electrically conductive carbon matrix, the as-prepared Se/C hollow microspheres for Al-Se batteries deliver high and long discharge voltage (1.65 V), high capacity (720.1 mAh g −1 at 500 mA g −1 ), long-term cycling stability (1100 cycles), and superior rate capability (564.0 mAh g −1 at 1000 mA g −1 ).The design concept in this present work would be of great significance for the further development of highly stable Se-containing materials, and provide a feasible strategy for improving the energy density of multivalent metal-Se batteries.

Electrochemical preparation of Se/C hollow microspheres
The reagent grade SeO 2 was pressed into a cylindrical pellet with a diameter of 15 mm under the pressure of 10 MPa and wrapped with Mo (99.9%, 30 mesh) mesh, which was then put in the glove box with high-purity Ar.The chloride composite NaCl-AlCl 3 (molar ratio 1:1) was mixed and put into a sealed quartz glass electrolyzer in the glove box.The electrolyzer was put into the oven at 190 • C in Ar atmosphere.Using the electrochemical workstation (PARSTAT MC), constant voltage electrolysis was carried out in the molten salt at various temperatures and times ranging from 1.2 to 2.0 V, with pressed SeO 2 pellet as cathode and graphite rod (Φ 15 mm) as anode.The electrodes are connected by stainless steel rods with a diameter of 6 mm.The electrolyzed products were subsequently soaked in deionized water three times, soaked in alcohol twice, and then dried at 60 • C. The structure and morphol-ogy of the products were analyzed by X-ray diffractometer (Rigaku, D/max-RB), X-ray photoelectron spectrometer (Kratos AXIS Ultra DLD), Raman spectrometer, laser particle sizer, four-probe electrical conductivity instrument, thermogravimetric analyzer, filed emission scanning electron microscope (JEOL, JSM-6701F) with energy dispersive spectroscope and transmission electron microscope.

Fabrication of Se/C positive electrode for Al-ion batteries
The assembly processes of the as-prepared Se/C hollow microspheres for Al-ion batteries were as follows: (1) Production of [EMIm]Al x Cl y ionic liquid electrolyte: the mixture of anhydrous AlCl 3 and 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) with a molar ratio of 1.3:1 was stirred in a flask in the glove box, yielding a clear yellow liquid that served as the [EMIm]Al x Cl y ionic liquid electrolyte.(2) Preparation of Se/C positive electrode: a slurry containing 60 wt.%Se/C hollow microspheres, 30 wt.% acetylene black (AB) as conducting agent, and 10 wt.% poly(vinylidene difluoride) (PVDF) as binder was mixed in N-methyl-2-pyrrolidone (NMP) solvent.The resulting mixture was then coated onto a tantalum (Ta) foil (15 mm × 15 mm) current collector.After dried at 80 • C for 12 h, the coating thickness of the electrode was rolled to approximately 90 µm, and the areal loading content of Se was about 1.0 mg cm −2 .(3) Preparation of modified separator: AB and PVDF were mixed with a mass ratio of 10:1 and dissolved in NMP.After continuously stirring for 2 h, a certain volume of as-obtained dispersion liquid was vacuum-filtered using glass fiber (GF/A) separator (Φ 10 cm) as the filter membrane, followed by washing with NMP and drying at 70 • C for 12 h.(4) Fabrication of pouch cells: Se/C positive electrode, modified GF/A separator, and Al negative electrode (25 mm × 25 mm × 50 µm) were assembled within Al plastic film.For comparison, gray Se was also prepared as positive electrode under the same conditions.The fabricated cells were dried in the bake oven at 70 • C and then dripped with the as-prepared [EMIm]Al x Cl y electrolyte in the glove box.The diagram from electrode fabrication to battery assembly processes is presented in Figure S1.

Electrochemical measurement and characterization of Se/C hollow microspheres
Galvanostatic charge-discharge measurements were executed in the voltage range between 0.1 and 2.3 V, with respect to the Al 3+ /Al reference.The measurements were conducted using Neware BTS-53 tester.CV measurements were performed in the voltage range between 0.1 and 2.4 V at a scan rate of 0.5 mV s −1 , with respect to the Al 3+ /Al reference.The EIS tests were carried out using a perturbation amplitude of 5 mV in an open-circuit potential.The frequency range varied from 100 kHz to 0.1 Hz.The GITT measurement was measured with pulses of 30 mA g −1 and interruption time for 0.5 h.XPS was used to study the valence bond changes of Se, Al, and Cl elements of Se/C electrodes at different charged and discharged states.SEM with EDS and TEM tests were employed to determine the morphologies and microstructures of the Se/C electrodes at different charged and discharged states.

Computational methods and models
All calculations were carried out using the DMol3 module, which relies on DFT. 50The generalized gradient approximation approach parameterized by Perdew-Burke-Ernzerh function, was used to calculate the exchange correction function. 51A double numerical polarization basis set was used to calculate the exchange correction function.The initial value of vertical distance of all adsorbates from a 5 × 5 supercell of graphene substrate was set to 2 Å.Structural optimization parameters included a maximum energy difference of adjacent ionic steps of 1.0 × 10 −5 eV atom -1 , the maximum force of 0.02 eV Å -1 , and the maximum displacement of 0.5 Å.
To eliminate the interactions between adjacent layers, a vacuum layer with a size of about 20 Å was established.The effect of van der Waals force on the results of anchor material adsorption was also considered to enhance the accuracy.The lattice parameters for all structures were calculated on a 5 × 5 × 1 k-point grid, while a 15 × 15 × 1 was employed to calculate the electron density of states and band gap.The adsorption energy E a was employed to evaluate the anchoring effect of the material on AlCl 4 -, and the adsorption energy was defined as:

F I G U R E 1
Schematics of Se species conversion and low-temperature electrolytic preparation of Se/C hollow microspheres.(A) Theoretically calculated potentials and specific capacity of different Se species.(B) Schematic illustration of Se/C hollow microspheres conversed in the NaCl-AlCl 3 molten salt, with SeO 2 pellet as cathode and graphite rod as anode.(C) Scanning electron microscopy (SEM) and Se, C elemental mapping images of the Se/C product at 1.8 V, 190 • C for 4 h.(D) Transmission electron microscopy (TEM) image of Se/C hollow microspheres, and inset is SEM image.(E) Selected area electron diffraction (SAED) image of Se/C hollow microspheres.(F) X-ray diffraction (XRD) pattern of Se/C hollow microspheres.(G) High-resolution X-ray photoelectron spectroscopy (XPS) spectra of Se 3d of Se/C hollow microspheres.(H) Raman spectra of Se/C hollow microspheres and gray Se.
The response time of ion diffusion in Se/C hollow microspheres has been obviously reduced in comparison with that of pure Se, leading to notable decrease in electrochemical reaction resistance, which further reveals the improved ion diffusion of Se/C electrode.The enhancement on the ion diffusion of Se/C hollow microspheres is attributed to the highly electrically conductive carbon and unique hollow microsphere-like structure, both of which can effectively enhance the kinetic behaviors of Al-Se batteries.As shown in Figure2G, compared with gray Se, Se/C hollow microspheres possess continuous carbon F I G U R E 2 Electrochemical performance of gray Se and Se/C hollow microspheres as positive electrodes for aluminum-ion batteries (AIBs).(A) Cyclic voltammetry (CV) curves of gray Se and Se/C hollow microspheres at 1 mV s −1 .(B and C) Galvanostatic charge-discharge curves of gray Se and Se/C hollow microspheres at a current density of 1000 mA g −1 .(D) Long-term cycling stability of gray Se and Se/C hollow microspheres at 1000 mA g −1 .(E and F) Nyquist plots and corresponding distribution of relaxation times (DRT) curves of gray Se and Se/C hollow microspheres before cycling.(G) Schematic representation of gray Se and Se/C hollow microspheres during cycling.

F I G U R E 3
Energy storage mechanisms of Se/C hollow microspheres.(A) Optical changes of gray Se and Se/C hollow microspheres immersed in electrolyte.(B-E) High-resolution X-ray photoelectron spectroscopy (XPS) spectra of Se 3d of Se/C hollow microspheres at different states.(F) Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) images of Se/C hollow microspheres after fully charging.(G) TEM and SAED images of Se/C hollow microspheres after fully discharging.
4 , Se 2 Cl 2 , SeCl 4 , and graphite are illustrated in Figure S17.The adsorption structures of Se, Se 2 Cl 2 , and SeCl 4 on graphite surface are indicated in Figures 4A and S18-S20.The adsorption energy (E a ) of the optimized Se/C, Se 2 Cl 2 /C, and SeCl 4 /C structure is F I G U R E 4 Mechanisms of boosted energy storage behaviors of Se/C hollow microspheres.(A) Optimized adsorption configurations for Se, Se 2 Cl 2 , and SeCl 4 on graphite surfaces.(B) Energy profiles for the oxidation process of Se. (C) Calculated electron density differences and charge distribution of pure Se, Se 2 Cl 2 , and SeCl 4 .(D) Calculated electron density differences and charge distribution of Se, Se 2 Cl 2 , and SeCl 4 adsorbed on graphite surface.estimated to be −0.25,−2.63, and −0.69 eV, respectively.This implies that graphite exhibits a considerable adsorption toward the soluble intermediate Se 2 Cl 2 , thus achieving the potential inhibition and contributing to stable cycling performance.To confirm how the graphite accelerate the reaction kinetics, the energy barrier of AlCl 4 anions diffusion on Se and Se/C is presented in Figure 4B.It is noted that during the conversion process of Se 6 to Se 2 Cl 2 to SeCl 4 , the energy barrier on Se/C is much lower than that on Se, indicating faster diffusion kinetics of AlCl 4

F I G U R E 5
Superior Al storage performance of Se/C hollow microspheres.(A) Cyclic voltammetry (CV) curves of Se/C hollow microspheres at various scan rates.(B) Separation of the capacitive-and diffusion-controlled contributions at a scan rate of 1 mV s −1 .(C) Contribution ratio of the capacitive-and diffusion-controlled process at different scan rates.(D) Galvanostatic charge-discharge profiles of Se/C hollow microspheres at different current densities.(E) Rate capability of Se/C hollow microspheres.(F and G) Nyquist plots and corresponding distribution of relaxation times (DRT) curves of Se/C hollow microspheres before and after cycling.(H) Comparison between Se/C hollow microspheres and previously reported Se-based materials for Al-ion batteries.

F I G U R E 6
Schematic diagram of electrolysis cell and rechargeable Al-Se battery involved in this work.(A) Schematic diagram of electrolysis cell for preparing Se/C hollow microspheres in a low-temperature molten salt.(B) Schematic diagram of rechargeable Al-Se battery with Se/C hollow microspheres as positive electrode.
obtained in the cathode by regulating electrolysis parameters in a low-temperature molten salt, with purpose of realizing the value-added utilization for energy storage.The results clearly demonstrate that Se/C hollow microspheres present a very excellent long-term cycling stability, high capacity, and superior rate capability.As illustrated in Figure 6B, Al 2 Cl 7 − anions decompose into AlCl 4 − anions and metal Al on the negative electrode during charging.Meanwhile, the insertion of AlCl 4 − anions can cause the oxidation reaction between Se 0 /Se 2 2+ and Se 2 2+ a =  G+Se or Se 2 Cl 2 or SeCl 4 −  G −  Se or Se 2 Cl 2 or SeCl 4 where  G+Se or Se 2 Cl 2 or SeCl 4 ,  G ,  Se or Se 2 Cl 2 or SeCl 4 are the total energy of anchoring material, graphene/(graphene + AlCl 4 -), and Se/Se 2 Cl 2 /SeCl 4 , respectively.The smaller the energy of the  a , the stronger the adsorption of the material to the adsorbate; vice versa, larger  a indicates weaker adsorption.A C K N O W L E D G M E N T SThis work was supported by the National Natural Science Foundation of China (51874019), the Fundamental Research Funds for the Central Universities (FRF-TP-19-079A1), and the State Scholarship Fund.C O N F L I C T O F I N T E R E S T S TAT E M E N TThe authors declare no conflict of interest.O R C I DJiguo Tu https://orcid.org/0000-0003-1118-7897Cheng Chang https://orcid.org/0000-0002-5187-3819RE F E R E N C E S