Stable Ultrathin Lithium Metal Anode Enabled by Self-Adapting Electrochemical Regulating Strategy

12 Ultrathin lithium metal foils with controllable capacity could realize high-energy-density batteries, 13 however, the pulverization of Li metal foils due to its extreme volume change results in rapid active Li loss 14 and capacity fading. Herein, we report a strategy to stabilize ultrathin Li metal anode by in-situ transferring 15 Li from ultrathin Li foil into a well-designed 3D gradient host during plating process. A 3D carbon fiber 16 with gradient distribution of Ag nanoparticles is placed on the ultrathin Li foil in advance and acts as a Li 17 reservoir, guiding Li deposition into its interior and thus alleviating the volume change of ultrathin Li foil 18


INTRODUCTION
[7][8] However, due to the high reactivity of Li metal, metallic Li will spontaneously react with non-aqueous electrolytes to form solid electrolyte interphase (SEI) layer, contributing to low Li plating/stripping efficiency and low Li utilization rate. [9,10] n addition, because of the conversion reaction and the hostless Li plating/stripping characteristic, the Li metal anode suffers from infinite volume changes during cycling, which are highly responsible for the pulverization of Li metal, active Li loss, and the blocking of ionic transport. [11,12] [15][16][17] Thus, numerous artificial SEI layers, including organic layers [18,19] , inorganic layers [20][21][22] , and organicinorganic hybrid layers [23,24] , are designed and adopted to protect the ultrathin Li metal anodes.
However, the constant volume fluctuation from the intrinsic hostless characteristic of Li metal anode challenges the structure stability in the long running.Studies have shown that encapsulating active Li metal into a host is efficacious to address the volume change issue. [25,26] herefore, various micro/nanostructured hosts have been widely studied, including three-dimensional (3D) Cu current collectors [27,28] , reduced graphene oxide [29][30][31] , carbon fibers [32][33][34] , porous carbon granules [35][36][37] , and so on.Combing with the high lithiophilic sites, the plating/stripping behaviors and the cycling performances of the composite Li metal anode have been greatly improved.However, the synthesis of composite Li metal anode with controllable capacity is usually time-consuming and high-cost. [38]ww.energymaterj.com

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Therefore, it is urgent to develop a simple, highly efficient and inexpensive method to protect ultrathin Li metal anode.
Herein, a self-adapting electrochemical regulating strategy, that is, in-situ self-migrating of Li metal from ultrathin Li foil into a well-designed 3D host by electrochemical cycling is used to extend the cycling life of ultrathin Li anode.The key-enabling technique is to design a lithiophilic host with gradient distribution of Ag nanoparticles.During Li plating process, Li metal gradually migrates into the prestaging host with the aid of Ag nanoparticles.During the following cycling, Li nucleates on the Ag nanoparticles and grows from bottom to up in the host, which helps stabilize the ultrathin Li anode volume and avoid internal short-circuit arising from the electrical connection of Li dendrite between Li anode and cathode (Scheme 1).As a result, the hybrid Li anode with dendrite-free morphology demonstrates a high reversibility with a high Li plating/stripping efficiency of 99% and superior cycling performance with a lifetime for more than 2000 h under 0.5 mA cm −2 with a plating capacity of 2 mA h cm −2 in symmetric cells.The Li anode also demonstrates its application potential by presenting a long lifespan of 180 cycles at a very low N/P ratio of 1.5 under a high cathode areal capacity in full cells.

Materials characterizations
Field emission scanning electron microscopy (FESEM, Sigma 500) was used to characterize the microscopic morphology of the materials and the electrode during cycling, while Energy Dispersive Spectrometer (EDS) was combined to qualitatively analyze the local elemental distribution of the samples.To observe the morphology of the electrode clearly, the cycled electrode was washed by 1,2-dimethoxyethane (DME) solvent repeatedly.The component of the CC and CC-Ag electrode was characterized using XRD (Bruker D8 Advance) with Cu (Kα-rays, λ=1.54 Å) as the target material at a current and voltage of 40 mA and 40 kV, respectively.The existence form of the Ag on the CC electrode was characterized by X-ray photoelectron spectroscopy (XPS).

Electrochemical tests
The CR2032 type coin cells were assembled in an argon-filled glove box where the gas contained less than 0.1 ppm O2 and H2O, and were tested on a NEWARE test system.The microporous polypropylene fiber (Celgard, 2400) was used as separator while 1 M lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) that dissolved in a mixture of 1,3-dioxolane (DOL)/1,2dimethoxyethane (DME) (V/V = 1:1) containing 2 wt% lithium nitrates was used as the electrolyte.

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For symmetric cells, one side was ultrathin Li and CC or CC-Ag (top and bottom stack) hybrid anode (denote as Li/CC, Li/CC-Ag), and the other side was lithium foil.LiFePO4 (LFP), polyvinylidene fluoride (PVDF), and super P at a mass ratio of 8:1:1 was grinded to prepare the LFP cathode slurry.The full cells were assembled using LiFePO4 as the cathode and the Li foil, Li/CC, or Li/CC-Ag as anodes.

RESULTS AND DISCUSSION
A simple method, involving carbonization of natural cottons followed by thermal evaporation of Ag nanoparticles, was developed to prepare a 3D carbon host with gradient lithiophilicity (CC-Ag) (Fig. 1a).The CC derived from the carbonized cottons exhibits a 3D interconnected network with uniform distribution of C and N elements (Fig. S1).After the thermal evaporation process, abundant nanoparticles with an average diameter of 50-60 nm are homogeneously distributed on the CC (Fig. S2).The nanoparticles are identified as Ag element evidenced by the typical Ag characteristic peaks in the X-ray diffraction (XRD) patterns and X-ray photoelectron spectroscopy (XPS), as well as the signal of Ag in the EDS elemental mapping (Fig. 1b, Fig. S2, 3).The Ag nanoparticles build a lithiophilic layer with an average thickness of about 30 μm, endowing a lithiophilic-lithiophobic gradient characteristic of the CC scaffold (Fig. 1c, d).The Ag nanoparticles enable improved electrical conductivity of 18.1 S cm −1 for the CC-Ag, which is superior to that of 3.5 S cm −1 for CC (Fig. S4).

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Li foil serves as the counter electrode.Due to the good affinity of Ag to Li and the lithiophilic difference between the bottom and top of the CC, Li prefers to deposit on the bottom rather than on the top of the CC-Ag (Fig. 2b, d).Upon the cycling, more Li can be observed on the bottom of the CC-Ag, indicating ultrathin Li gradually self-migrates into the interconnected network of CC-Ag from bottom to up (Fig. 2c, e, Fig. S5).Due to the high porosity of the CC-Ag, the areal capacity around 8.1 mA h cm −2 (Fig. S6) of the ultrathin Li anode can be well accommodated into the CC-Ag.For comparison, the Li morphological evolution on the bare CC without Ag was also investigated to demonstrate the importance of lithiophilic gradient structure in the Li self-inducing deposition.Due to the absence of lithiophilic gradient structure, the Li metal directly deposits on the surface of CC after the initial cycle (Fig. 2g, i).As the cycle goes on, Li metal continuously plates on the CC surface and gradually emerges into Li dendrites, increasing the risk of short circuit (Fig. 2h, j).

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The reversibility and electrochemical activity of Li from the ultrathin Li foil are investigated by electrochemical performances.The Li/CC-Ag delivers a high Li plating/stripping efficiency of 99.5% under a current density of 0.5 mA cm −2 with an areal capacity of 8 mA h cm −2 , indicating a high Li reversibility can be achieved (Fig. S7).In addition, Li||Li symmetric cells were assembled to evaluate the Li/CC-Ag electrode stability.As shown in Fig. 3a, the Li||Li/CC-Ag symmetric cells present longer cycle life for over 2000 h and lower polarization voltage less than 20 mV at 0.5 mA cm −2 with a fixed areal capacity of 2 mA h cm −2 than Li||Li/CC (1000 h/30 mV) and Li||Li cells (800 h/50 mV), confirming that the Li/CC-Ag has better cycling reversibility and volume stability.
After cycling for 800 h, the voltage profiles of the Li/CC-Ag and Li/CC remain a typical plateau characteristic while the Li foil exhibits an arching voltage trace, indicating the interfacial mass transport of ultrathin Li is blocked by the generated dead Li (Fig. 3b).Electrochemical impedance spectrum was further performed on the cycled Li/CC-Ag to evaluate the interfacial stability and charge transfer ability.As shown in Fig. 3c, the first semicircle at the high-frequency region is in connection with Li + transport through the SEI interfacial layer.The Li||Li/CC-Ag symmetric cells show smaller interfacial resistance, which is mainly due to the uniform Li morphology and suppressed Li/electrolyte interfacial side reactions.The second semicircle at the low-frequency region refers to the charge transfer resistance.The charge transfer resistance of Li||Li/CC-Ag symmetric cells is lower than that of Li||Li/CC-Ag symmetric cells, indicating the improved electronic conductivity and Li + transport ability of the Li/CC-Ag electrode.After 1920 h (240th) cycling, the charge transfer resistance of the Li||Li/CC-Ag symmetric cells decreases from 2.6 Ω to 1.5 Ω, which can be attributed to the generated mixed Li + /e -conductor of Li/CC-Ag anode (Fig. 3d).The transport kinetic differences for the two symmetric cells were investigated using the galvanostatic intermittent titration technique (GITT).The Li||Li/CC-Ag symmetric cells www.energymaterj.com

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demonstrate lower overpotential than the Li||Li/CC symmetric cells, confirming faster mass transport kinetics related to the less accumulation of pulverized Li or dead Li for the Li/CC-Ag (Fig. S8).Mossy Li or dendritic Li is easy to be detached from the bulk Li foil and form electrically isolated dead Li at high areal capacity or high current density.Hence, the symmetric cells were cycled at a high current density of 1 mA cm −2 for a high areal capacity of 8 mA h cm −2 per charge/discharge cycle.As shown in Fig. 3e, the Li/CC-Ag symmetric cells remain a long cycling for more than 600 h without obvious overpotential increasement, indicating the pulverization of Li is effectively alleviated by Li/CC-Ag.When the symmetric cells were cycled under increasing current densities and deposition capacities, increasing overpotentials for the two types of symmetric cells are clearly observed in the voltage profiles.Wherein, the height of the voltage arcing regions for the Li/CC-Ag is lower than that of Li/CC, demonstrating the accumulation of dead Li is delayed by CC-Ag (Fig. 3f).

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4a), indicating the highest reversibility and transport kinetics of Li in the CC-Ag scaffold.After 250 cycles, the discharge capacity of the full cell with Li/CC-Ag anode is maintained at 136 mA h g −1 , which corresponds to a high-capacity retention of 92%.On the contrary, the full cells with Li/CC and Li foil anode exhibit rapid capacity decay (Fig. 4b).The superior transport kinetics of Li in the CC-Ag scaffold can be also seen from the smallest charge transfer resistance (Fig. 4c).
The possible reasons for the improved kinetics using the Li/CC-Ag anode can be attributed to the less accumulation of dead Li.Hence, further increasing the rate to1.0 C (Fig. 4d), the cell still maintains a reversible capacity of 134 mA h g −1 for 300 cycles with the Coulombic efficiency approaching 100% in each cycle.Upon increasing the areal capacity of LFP cathode from 1.7 mA h cm −2 to over 5 mA h cm −2 , the full cells pairing with Li/CC-Ag anode still demonstrate superior electrochemical performances.The full cells deliver high reversible capacities of 5.3 and 4.8 mA h cm −2 at 0.1 C and 0.5 C at a very low N/P ratio of 1.5, respectively, and maintain for 180 cycles without significant capacity attenuation (Fig. 4e).With further reducing the N/P ratio to 1.2, the full cells deliver a high reversible capacity of 5.1 mA h cm −2 and 4.2 mA h cm −2 at 0.1 and 0.5 C, demonstrating its application potential of ultra-thin Li anode with the self-adapting strategy (Fig. 4f).

Scheme 1 .
Scheme 1. Schematics of Li/CC-Ag hybrid anode before and after cycling.(a) CC-Ag stacked with ultrathin lithium before Li/CC-Ag cycle.(b) After a few turns of Li/CC-Ag cycles, ultrathin lithium is gradually transferred to CC-Ag.(c) Complete transfer of ultrathin lithium to CC-Ag after Li/CC-Ag cycles.

Fig. 1
Fig. 1 (a) Optical photographs of cotton, AC, CC, and CC-Ag.(b) XRD patterns of CC and CC-Ag.(c) SEM cross-sectional image of CC-Ag and corresponding EDS element mapping images of C, N, and Ag elements.(d) Linear elemental mapping of Ag element along the CC-Ag.Due to the inevitable parasitic reaction and large electrode volume change, ultrathin lithium

Fig. 2
Fig. 2 Schematic diagram of Li/CC-Ag (a) and Li/CC (f).Top (b) and bottom (d) SEM images of Li/CC-Ag anode after 1 cycle.Top (g) and bottom (i) SEM images of Li/CC anode after 1 cycle.Top (c) and bottom (e) SEM images of Li/CC-Ag anode after 50 cycles.Top (h) and bottom (j) SEM images of Li/CC anode after 50 cycles.The Li|Li symmetric cells were performed under a current density of 0.5 mA cm −2 with a plating areal capacity of 2 mA cm −2 .

Fig. 3
Fig. 3 (a) Voltage profiles of Li/CC-Ag, Li/CC and Li foil symmetric cells performed at 0.5 mA cm −2 and an areal plating capacity of 2 mA h cm −2 and (b) the enlarged polarization voltage comparison for the three electrodes.(c) Comparison of the electrochemical impedance diagram of the Li/CC-Ag and Li/CC symmetric cells after 50 cycles.(d) EIS of the Li/CC-Ag symmetric cells after 240 cycles.(e) Voltage profiles of Li/CC-Ag and Li/CC symmetric cells performed at 1 mA cm −2 and with a high areal capacity of 8 mA h cm −2 .(f) Voltage profiles of Li/CC-Ag and Li/CC symmetric cells performed at various current densities and capacities.Full cells pairing with a high areal capacity LiFePO4 (LFP) cathode (>1.7 mA h cm −2 ) and

Fig. 4
Fig. 4 (a) Charge/discharge voltage profiles for the 50 th cycle of Li/CC-Ag||LFP, Li/CC||LFP, and Li foil||LFP full cells at 0.5 C and (b) corresponding cycling performances.(c) Nyquist impedance plots of Li/CC-Ag||LFP and Li/CC||LFP full cells after 50 cycles at 0.5 C. (d) Cycling performances of the LFP full cell with the Li/CC-Ag at 1.0 C. (e) Cycling performances of the high capacity LFP full cell with the Li/CC-Ag at 0.5 C. (d) Cycling performance of Li/CC-Ag||LFP full cells with a low N/P ratio at 0.5 C.

Fig. S1
Fig. S1 Elemental mapping images of the conductive carbon cotton (CC).

Fig. S2
Fig. S2 SEM image of CC-Ag and its corresponding EDS element mapping images.

Fig. S3
Fig. S3 XPS spectrum of the CC-Ag and CC.

Fig.
Fig. S4 I-V curves of CC-Ag and CC.

Fig. S5
Fig. S5 Top (a) and bottom (b) SEM images of Li/CC-Ag anode after 10 cycles.Top (c) and bottom (d) SEM images of Li/CC anode after 10 cycles.

Fig. S7
Fig. S7 Coulombic efficiency of Li plating/stripping on CC-Ag with an areal capacity 8 mA h cm −2 at the current density of 0.5 mA cm −2 (a) and Charge/discharge voltage profiles (b).