De nable Two-Dimensional Mesoporous Polydopamine-Graphene Heterostructure as Multi- Functional Ion Redistributor for Ultrastable Sodium Metal Anodes

Jieqiong Qin Dalian Institute of Chemical Physics Haodong Shi Dalian Institute of Chemical Physics Kai Huang Dalian Institute of Chemical Physics Pengfei Lu Dalian Institute of Chemical Physics Pengchao Wen Dalian Institute of Chemical Physics Feifei Xing Dalian Institute of Chemical Physics Bing Yang Dalian Institute of Chemical Physics https://orcid.org/0000-0003-3515-0642 Mao Ye Dalian Institute of Chemical Physics https://orcid.org/0000-0002-7078-2402 Yan Yu University of Science & Technology of China Zhong-Shuai Wu (  wuzs@dicp.ac.cn ) Dalian Institute of Chemical Physics https://orcid.org/0000-0003-1851-4803


Introduction
The rapid growth of portable electronics, electric vehicle, autonomous aircraft and smart grid has intensively stimulated the urgent requirements for low-cost, high-energy-density batteries [1][2][3][4] . Due to high theoretical capacity (1166 mAh g − 1 ), low redox potential (-2.714 V vs. standard hydrogen electrode), natural abundance and low price, metal sodium (Na) has been regarded as a highly competitive anode for next-generation rechargeable battery [5][6][7][8][9] . Unfortunately, its high reactive activity, large volume change, unstable solid electrolyte interface (SEI) and uncontrollable dendritic growth bring about low Coulombic e ciency, limited cyclability, and even safety risk for high-energy-density Na metal batteries, such as Na-S 10 and Na-O 2 batteries 11 , substantially inhibiting their actual applications 5,[12][13][14][15] . To overcome the issues, various strategies, including tailoring electrolyte formulation (e.g., highly concentrated electrolyte, uoroethylene carbonate additive) 16,17 , using solid-state electrolytes (gel polymer with boron nitride, Na 3 Zr 2 Si 2 PO 12 ) 18,19 , creating arti cial SEI (e.g., Al 2 O 3 , sodium benzenedithiolate, graphene) [20][21][22] , and designing nanostructured Na anodes (e.g., Na@O-functionalized carbon nanotube networks, Na@porous Al, Na@carbonized wood) 14,23,24 , have been developed to suppress the growth of Na dendrites and realize stable and safe Na metal anodes. Nevertheless, these designs usually revealed single chemical or physical function for regulating Na dendrites, and faced high processing cost and limited scalability. Besides, constructing functional separators is considered as a more reliable and cost-effective way to realize uniform Na deposition from chemical molecule and physical structure levels [25][26][27] . From the viewpoint of molecular level, polymer brushes with abundant polar functional groups (e.g., C = O, -OH, -COOH and N-H) can enhance the electrolyte wettability, provide robust SEI interface, and thus easily homogenize the alkali-metal ion distribution and nucleation 21,[28][29][30][31][32] . In particular, two-dimensional (2D) graphene-like polymer materials (e.g., poly(N-isopropylacrylamide), polyacrylamide grafted graphene oxide (GO) and polypyrrole-GO heterostructure) with sheet-like structure, high speci c surface area (SSA), abundant surface chemistry and outstanding mechanical exibility, show tremendous advantage to regulate alkali-metal deposition and physically restrain dendrite puncture 22,28,29,33 . From the perspective of structure design, 2D defective GO and ordered mesoporous structure can serve as nanoporous buffer and ion channels to homogenize alkali-metal ions distribution and deposition 33 . Therefore, the reasonable construction of de nable 2D mesoporous functional polymer heterostructure for realizing ultrastable, dendrite-free Na metal anodes from chemical molecule and physical structure levels is highly competitive yet remains elusive.

Results
Fabrication and characterization of 2D mPG heterostructures. The synthesis process of 2D mPG heterostructures is schematically shown in Fig. 1a. First, the GO nanosheets modi ed by polydiallyldimethylammonium chloride are employed as 2D free-standing substrates. Then, monodispersed SiO 2 nanospheres with negative charge serve as mesoporous templates to orderly assemble on the surface of GO substrates through electrostatic adsorption. Afterwards, homogeneous polydopamine (Supplementary Scheme 1) layer is patterned on the surface of as-prepared SiO 2 -GO nanosheets to form SiO 2 -PGO nanosheets. Finally, 2D mPG heterostructures with de ned mesopore size and sheet thickness are generated after hydrothermal treatment (de ned as SiO 2 -PG) and SiO 2 etching. It is worth noting that this template strategy can e ciently realize the strong coupling of sodiophilic polydopamine, defective graphene (Supplementary Fig. 1) and adjustable mesopores, which is highly conducive to construct multi-functional ion redistributors for dendrite-free Na metal anodes (Fig. 1b), as discussed below.
The microstructure and chemical composition of 2D mPG heterostructures are presented in Fig. 2. Taking mPG with 12 nm mesopores (mPG-12) nanosheets as example, scanning electron microscopy (SEM) image displays at and uniform 2D morphology with lateral size ranging from 0.5 to 3 µm (Fig. 2a). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images clearly indicate homogeneous 2D mesoporous structure of mPG-12 nanosheets with uniformly ordered mesopores of ~ thickness of ~ 20 nm (Fig. 2d). More importantly, the accurate patterning of mesoporous polydopamine on graphene surface with de nable pore size and thickness can be precisely controlled, e.g., by changing the size of SiO 2 template (Fig. 2e-h and Supplementary Fig. 2). Specially, mPG-7 nanosheets show uniform mesopores of ~ 7 nm and an average thickness of 14 nm, while mPG-22 nanosheets present ordered mesopores of ~ 22 nm and sheet thickness of ~ 28 nm. Meanwhile, non-mesoporous polydopamine-graphene (nPG) nanosheets are also synthesized ( Supplementary Fig. 3) to highlight the importance of structure-directing SiO 2 templates. Moreover, N 2 adsorption and desorption isotherms of mPG-7, mPG-12, mPG-22, and nPG nanosheets represent type IV curves with H2-type hysteresis loop ( Supplementary Fig. 4). Notably, mPG-12 nanosheets display larger SSA of 157 m 2 g − 1 than mPG-7 (144 m 2 g − 1 ), mPG-22 (114 m 2 g − 1 ) and nPG (54 m 2 g − 1 ) nanosheets (Supplementary Table 1). Further, the dominated mesopore sizes of 6.8 nm for mPG-7, 12.2 nm for mPG-12, and 18.6 nm for mPG-22 nanosheets are well validated by the pore size distribution (Fig. 2i), nearly consistent with TEM observation. Fourier transform infrared spectroscopy (FT-IR) spectrum of mPG-12 reveals the existence of double peaks at 2930 and 2863 cm − 1 featuring the indole structure, the characteristic vibration peak at 1712 cm − 1 assigning to quinone (C = O) groups, and two distinct peaks at 1572 and 1462 cm − 1 attributing to C = C resonance vibration and N-H bending vibration ( Supplementary Fig. 5), demonstrative of the hybridization of polydopamine and graphene in mPG-12 30,34,35 . Furthermore, X-ray photoelectron spectroscopy (XPS) of mPG-12 con rms three strong signals of C1s, N1s and O1s (Supplementary  34,36,37 . Moreover, the N1s spectrum reveals three peaks at 401.7 ± 0.1 for -NH 2 , 400.0 ± 0.1 eV for -NH-, and 398.5 ± 0.1 eV for -N = functional groups, respectively 34,36,37 . It is evidenced that the mesoporous polydopamine layer is effectively grafted with graphene, and the abundant polar functional groups, e.g., C = O, -OH and -NH-, are existed in 2D mPG heterostructure. Calculation of sodiophilic nature and Na deposition behavior for mPG. To examine the effect of polar functional groups and 2D mesoporous structure on Na metal deposition, density functional theory (DFT) calculation and nite volume method (FVM) simulation were conducted. Figure 3a-c and Supplementary  Fig. 7 display the binding energies and charge densities between Na atom and polydopmine, graphene, PP and Cu. It can be seen that the interaction between Na and Cu (-1.20 eV) or PP (-0.41 eV) is much weaker than that between Na and polydopmine, owing to abundant polar groups (C = O of -4.24 eV, -OH of -3.47 eV, -NH-of -3.37 eV). Thus, the sodiophilic nature of mPG coupled polydopmine and graphene with a high binding energy of -2.08 eV can serve as active site to realize more uniform ion transport and Na deposition. Furthermore, the effect of mesopore size of 2D mPG on Na ions distribution is investigated by FVM simulation. As shown in Fig. 3d and Supplementary Fig. 8, the migration of Na ions driven by electric eld and diffusion ow through 20 layers of mPG nanosheets is taken into account for the simulation. With the shunting of 2D mesoporous structure in mPG, the distribution of Na ions becomes increasingly uniform as ions migrate along the Y-axis, and achieves almost stability after four layers of mPG ( Fig. 3e and Supplementary Fig. 9). The 2D mPG-7 reveals the lowest standard deviation whether with same amplitude and period or same uctuation and period of Na ion distribution at the entrance, implying the smaller mesopores are more bene cial to the redistribution and uniformity of Na ions (Fig. 3f, g). However, electrochemical impedance spectroscopy (EIS) shows that mPG-12 possess lower charge transfer resistance (14.9 Ω) and higher Na ion diffusion coe cient (5.7 × 10 − 12 cm 2 s − 1 ), compared with mPG-7 (36.2 Ω and 2.7 × 10 − 13 cm 2 s − 1 ) and mPG-22 (32.9 Ω and 2.6 × 10 − 12 cm 2 s − 1 ), possibly because of larger SSA, appropriate meopore size and thickness for faster ion diffusion transport [38][39][40] (Supplementary Fig. 10). Taken the above into consideration, 2D mPG heterostructures (especially mPG-12) possess huge merits of serving as multi-functional redistributor to homogenize distribution of Na ions and prevent "tip effects" of Na deposition 33,41 .
Characterization and performance of mPG-12 redistributor for Na metal anode. Considering the unique advantages of 2D mPG heterostructures, we fabricated mPG-12 based PP (mPG-12@PP) hybrid separators for Na metal anodes and Na metal batteries. Compared with bare PP separator with plenty of large pores (~ 500 nm) and 25 µm thickness, mPG-12@PP separators exhibit uniform, layer-stacked lm (only ~ 9 µm thickness) of 2D mPG-12 nanosheets on the surface of PP, while retain outstanding exibility and well-de ned pores (Fig. 4a-c and Supplementary Fig. 11, 12). Further, the contact angle of electrolyte on mPG-12@PP separator is ~ 8° (Fig. 4d), which is much lower than that on PP separator (31°), indicative of enhanced a nity of mPG-12@PP with electrolyte that is bene cial for Na ion diffusion 28,42 .
To highlight the importance of mesopores in mPG-12, nPG based PP (nPG@PP) separator was also assembled. The electrochemical performance of Na/Cu half cells with mPG-12@PP, nPG@PP and PP separators was rst evaluated to illustrate Na deposition behavior. At a current density of 0.5 mA cm − 2 with a cycling capacity of 0.5 mAh cm − 2 , the Na/Cu cells with mPG-12@PP separator exhibit ultrastable plating/stripping process for 550 cycles (~ 1100 h) with a steady Coulombic e ciency of ~ 99.8%, which is better than that of nPG@PP (400 cycles with ~ 99.5% Coulombic e ciency) and PP separators (200 cycles with < 90.0% Coulombic e ciency) (Fig. 4e). It can be well explained by the enlarged platingstripping curves (Fig. 4f), in which the Na/Cu cells with mPG-12@PP separator show the lowest nucleation overpotential of only 20 mV than those of Na/Cu cells with nPG@PP (60 mV) and PP (170 mV) separators. The reduced nucleation overpotential veri es the low interface resistance and enhanced Na ion transport kinetics through mPG-12@PP separator. Even at higher current density and areal capacity (4.0 mA cm − 2 and 4.0 mAh cm − 2 ; 8.0 mA cm − 2 and 8.0 mAh cm − 2 ), the Coulombic e ciency and cycling performance of Na/Cu cells with mPG-12@PP separator (~ 99.9% for 450 cycles; 99.7% for 350 cycles) still outperform PP separator (~ 78.5% for 90 cycles; ~51.7% for 50 cycles), further demonstrating the superiority of mPG-12@PP separator for protecting Na metal anodes ( Fig. 4g and Supplementary Fig. 13). To visually understand the impact of mPG-12@PP separator on Na plating/stripping process, the morphologies of Na metal anodes were traced by SEM ( Supplementary  Fig. 14). For the cell with bare PP separator, the anode exhibits obvious Na dendrites with loosely moss-like structure at a Na loading of 1.0 mAh cm − 2 . In a sharp contrast, mPG-12@PP separator endows Na metal anode with a rather uniform and smooth surface without obvious dendrites, and this morphology becomes denser upon an increased Na loading of 2.0 mAh cm − 2 . The distinctive morphological transition of Na metal anodes with different separators demonstrates the key role of mPG-12 on e cient regulation of the Na ions distribution and Na deposition.
Furthermore, symmetric Na/Na cells were fabricated to assess the voltage hysteresis and cycling stability. Figure 4h exhibits the voltage-time curves of Na/Na cells with mPG-12@PP, nPG@PP and bare PP separators at 1 mA cm − 2 , 1 mAh cm − 2 . Notably, the cells with mPG-12@PP separator delivers virtually at voltage plateaus over 2000 h with small overpotential (4 mV), illustrating their excellent interfacial stability and outstanding cyclability. In a sharp contrast, the cells with nPG@PP display higher voltage hysteresis after 500 h (> 14 mV), and the cells with bare PP separators appear micro-short circuiting after dozens of hours (inset of Fig. 4h and Supplementary Fig. 15). When increasing the areal capacity to 5 mAh cm − 2 or even 10 mAh cm − 2 , the cells with mPG-12@PP separator still show stable voltage plateaus over 1700 h and 1400 h, respectively ( Supplementary Fig. 16, 17).  Table 3).
Electrochemical performance of Na/NVP full batteries. To highlight the applicability of mPG based separator, the Na/NVP full batteries were assembled ( Fig. 5a and Supplementary Fig. 20), in which asdeposited Na metal foil (10 mAh cm − 2 ) with PG-12@PP or bare PP separator was used as the metal anode. Figure 5b exhibits the comparison of cycling stability of Na/NVP full cell with mPG-12@PP and PP separators at 1 C. Signi cantly, the Na/NVP cell with mPG-12@PP separator demonstrates exceptionally stable cycling performance with 90% retention of the initial capacity over 500 cycles (Fig. 5c). While the speci c capacity of Na/NVP full cell with bare PP separator decays rapidly after 120 cycles caused by the formation of Na dendrites and "dead Na". Moreover, the full battery with mPG-12@PP separator reveals superior rate capability to that with PP separator (Fig. 5d). Speci cally, the discharge capacities of the full cell with mPG-12@PP separator are 103, 96, 86 and 75 mAh g − 1 at 1 C, 5 C, 15 C and 30 C, much higher than those of cell with PP separator (99, 91, 76 and 55 mAh g − 1 ; Fig. 5e and Supplementary Fig. 21). These results further con rm that 2D mPG based separators enable Na metal batteries with signi cantly enhanced Coulombic e ciency, cycling stability and rate performance.

Discussion
The outstanding performance of Na metal anodes is attributed to the reasonable design of 2D multifunctional mPG heterostructure synergistically coupled the sodiophilic polydopamine surface, 2D defective graphene and de nable mesoporous structure. First, the polydopamine layer features abundant polar groups, e.g., C = O, -OH and -NH-, endowing it with excellent electrolyte wettability and strong substrate adhesion to facilitate homogeneous Na ion transport and suppress the growth of Na dendrites at molecular level. Second, 2D GO with intrinsic defects, atomic thickness and excellent exibility, offers a homogeneous Na ions delivery by this nanoporous buffer layer and enhances the mechanical strength of mPG and corresponding functional separators 25,33 . Third, the exposed mesoporous structure not only increases the accessible contact area between active polydopamine and electrolyte, but also provides de nable Na ion pathways to regulate local current density and terminate the "tip effect". Consequently, 2D mPG heterostructures as multi-functional ion redistributors substantially realize dendrite-free uniform Na deposition from chemical molecule to physical structure levels.
In summary, 2D mPG heterostructures with de nable pore size and sheet thickness have been demonstrated for ultrastable and high-capacity Na anodes. Owing to the abundant sodiophilic groups, nanoporous graphene and exposed mesoporous structure, 2D mPG heterostructures endow uniform Na ion transport and dendrite-free Na deposition, as con rmed by both theoretical and experimental analysis.
Notably, the Na metal anodes with mPG-12@PP separator exhibited high Coulombic e ciency (> 99.5%), outstanding cycling stability (~ 2000 h) and landmark rate performance (25 mA cm − 2 and 25 mAh cm − 2 ). Further, 2D mPG based separator realized high-performance Na/NVP metal batteries with exceptionally enhanced cyclability and rate capability. Therefore, this strategy paves a new avenue for the design of 2D mesoporous polymer materials towards next-generation safe, rechargeable sodium metal batteries.

Declarations
Data Availability The authors declare that all the data supporting the ndings of this study are available within the article and its Supplementary Information or from the corresponding author upon reasonable request.