Bismuthene nanosheet anchored on planar Cu foil enabled a stable lithium metal anode

Lithium metal as an anode is high-profile in rechargeable batteries, owing to its small gravimetric density, lowest redox potential, and incomparable theoretical specific capacity. Unfortunately, severe dendrite growth and fluctuant solid electrolyte interphase severely hampered its application. Herein, a hierarchical three-dimensional Bi/Cu skeleton is prepared by replacement reaction. The porous Bi/Cu can restrain the random deposition of Li through distribute the current evenly across the Bi/Cu electrodes. Meanwhile, the lithiophilicity of Bi reduces the nucleation overpotential. In contrast to the Cu skeleton, the 3D Bi/Cu skeleton significantly enhances the electrochemical performance. Furthermore, the prepared Bi/Cu anode owns highly stable coulombic efficiency, and symmetrical cells can maintain for more than 800 h (current density is 0.5 mA cm-2).


INTRODUCTION
In order to meet the rapid development of electric vehicles, higher requirements of energy density and power density are put forward for portable storage facilities.Lithium metal has been long dubbed with "holy grail", benefiting from its low density (0.59 g cm -3 ), lowest redox potential (-3.04 V vs HER), meanwhile larger theoretical specific capacity (3840 mAh g -1 ) [1].With this concern, lithium metal batteries are supposed to be the most impressive candidate for the next secondary rechargeable batteries [2,3].Nevertheless, the inevitable dendrite growth in lithium metal batteries severely hampers its practical application Dendrites growth is a complex process resulting from multiple factors, which not only cause huge volume expansion, electrolyte consumption, electrode pulverization, but also induce security risks [4].Many strategies have been conducted to regulate the disordered growth of lithium metal, for example, engineering solid-electrolyte interface (SEI), exploring the versatility of electrolyte or electrolyte additive, and constructing anode-free three-dimensional skeleton as Li host [5][6][7].However, pristine Li foil with a "host-less" nature is difficult to eliminate potential security problems when used as the anode.In contrast, a 3D conductive skeleton reduces the Li consumption, lowers the local current density, hence, moderates the irregular growth of metallic Li, and realizes a higher coulombic efficiency (CE) than traditional Li anode.
In this work, a galvanic replacement reaction is proposed to produce a 3D net structure.Here, uniform and dense Bi nanosheet arrays on Cu foam, such a hierarchical structure can directly use as a scaffold for Li striping/platting.Compared with the Cu foam, the obtained electrochemical performance of newly composite has been vastly enhanced.

Preparation of Bi/Cu
Cu foil is cut into 5*5 cm -2 pieces, and ultrasonic in acetone, ethanol, and water in sequence for 15 minutes, to remove surface impurities.Then, 0.02 M of BiCl 3 was added to 100 mL of dimethyl sulfoxide (DMSO), stirring until the powder is completely dissolved.Subsequently, the treated Cu was dropped into the above solution for galvanic replacement reaction for 24 h at 25 0 C. The obtained product was washed with ethanol and water three times, and donated as Bi/Cu.

Structure Characterization
The phase composition of the prepared electrode was measured by X-ray diffraction (XRD) of Rigaku Ultima IV.The surface morphology of Bi/Cu was conducted by scanning electron microscopy (SEM, FEI Quanta 250 field-emission).

Electrochemical measurements
The coin cell was assembled for electrochemical measurements.The battery case is CR2025-type.The half cells were assembled with a Bi/Cu skeleton, a Celgard 2400 separator, and the Li foil, of which Bi/Cu is the working electrode and the counter/reference electrode is Li foil.In this article, the electrolyte is a lithium-sulfur electrolyte (40 μL 1.0 M LiTFSI in DOL/DME = 1:1 Vol% with 1.0% LiNO 3 ).The half-cell was used to conduct a galvanostatic charge-discharge test, electrochemical impedance spectroscopy, and deposit Li on Cu and Bi/Cu skeleton.
The assembly of the symmetrical cell is similar to that of a half-cell.Two electrodes are Bi/Cu skeleton, and we inject 100 mL electrolyte into the coin cell.Before assembly, the Bi/Cu electrode was plated with fix amount of Li metal at a fixed current density of 0.5 mA cm -2 .

RESULTS & DISCUSSION
The XRD spectrum of Bi/Cu is exhibited in Figure 1.Obviously, Bi nanosheet is successfully obtained.The intense diffraction peak at 40-45 is the Cu signal contributed by Cu foam.Moreover, a slight oxide signal also can be detected, which is derived from the fast surface oxidation of Bi nanosheets during and after the galvanic replacement reaction.
Figure 1 The XRD pattern of Bi/Cu Figure 2 is the morphology of the Bi/Cu skeleton.Bi nanosheet is uniform and dense and anchored on the Cu foam.From the Figure 2b, the Bi nanosheet formed a porous and hierarchical 3D structure, which provided a large amount of space to accommodate Li metal.Meanwhile, the large specific surface area lowered the local current density.Meanwhile, it is beneficial to induce uniform deposition of lithium.Moreover, the three-dimensional interworking structure shortens the paths of lithium transmission and is more conducive to the realization of a dendrite-free anode.

Figure 2 Morphological characterization of Bi/Cu
From the first charge/discharge profile of Cu and Bi/Cu (Figure 3), the nucleation overpotential of Cu and Bi/Cu are 11.6 mv and 26.3 mv respectively.Different from the Cu skeleton, the superior lithiophilic of Bi makes it easier for lithium to be deposited on the Bi/Cu surface from LiBi 3 alloy.Furthermore, the lower nucleation overpotential increases the lithium nucleus at the initial charge/discharge process, and it is conductive to the plat lithium deposit.As is shown in Figure 3b, there is a slightly slanted discharge curve with Bi/Cu, demonstrating the lithium storage properties of Bi/Cu.The EIS (Electrochemical impedance spectroscopy) of the Cu foil and Bi/Cu electrode before and after cycles and corresponding equivalent circuits are displayed in Figure 5.In the high-frequency region, Cu has a larger semicircle than Bi/Cu has, no matter before or after the cycle.It is proved that Li + has greater interfacial transfer resistance at the Cu electrode than at the Bi/Cu electrode, which means the Bi/Cu electrode forms a stable SEI (solid electrolyte interface), which can induce Li plating on the Bi/Cu skeleton uniformly.In order to further confirm the effectiveness of the Bi/Cu electrodes, galvanostatic charge-discharge tests in symmetrical cells were carried out.From Figure 6, the original voltage hysteresis of the Bi/Cu electrode is larger than that of Cu foil, after several cycles, the corresponding potential hysteresis of Bi/Cu in the symmetrical cell is decreased and stable.This phenomenon demonstrates that a stable SEI is formed on the surface of the Bi/Cu electrode.Meanwhile, Bi/Cu symmetrical cells can stable cycle for more than 800 h.In contrast, Cu symmetrical cell has higher voltage hysteresis after a cycle for 50 h, which means there is a short circuit inside the Cu foil cell.From the inset of Figure 6, the voltage curve is zigzag, which means fluctuate electrode surface.In a word, the excellent morphology of the Bi/Cu electrode can induce uniform dendrite growth and is conductive to prolong cycle life.

CONCLUSION
In summary, the Bi/Cu skeleton with a hierarchical 3D porous network structure was prepared by galvanic replacement reaction.The Bi/Cu composite anode extended the Li nucleation time, lowered the Li metal nucleation barrier, and was conductive to form a flat and dendrite-free surface.Profiting from the distinct characteristics of the Bi/Cu skeleton, the stable CEs and ultra-long cycle life span (over 800 h) in symmetric cells are realized.

Figure 3 4 Figure 4
Figure 3 Nucleation overpotentials of Li on Cu foil and Bi/CuFigure4is the CEs of Cu and Bi/Cu skeleton in half-cell at 0.5 mA cm -2 , 1 mAh cm -2 .It is worth noting that the half-cell with Bi/Cu as an anode has a more stable cycle, however, the half-cell with Cu as anode has a fluctuant CE.This phenomenon demonstrates that the Bi/Cu electrode possesses a more reversible Li plating/striping process.This result is consistent with the nucleation overpotential.

Figure 5
Figure 5 Nyquist plots of Cu foil and Bi/Cu before and after 50 cycles.Inset is the equivalent circuit.

5 Figure 6
Figure 6 Long-term cycling performance of Cu foil and Bi/Cu in symmetric cells