Deeply Cyclable and Ultrahigh‐Rate Lithium Metal Anodes Enabled by Coaxial Nanochamber Heterojunction on Carbon Nanofibers

Abstract Lithium metal anodes (LMAs) are the most promising candidates for high‐energy‐density batteries due to the high theoretical specific capacity and lowest potential. However, the practical application of LMAs is hampered by the short lifespan and unsatisfactory lithium utilization (<50%). An oxide–oxide heterojunction enhanced with nanochamber structure design is proposed to improve lithium utilization and cycling performance of LMA under ultrahigh rates. Typically, a MnO2–ZnO heterojunction provides high binding energy for strong absorption of Li‐ions and intimately bonded interfaces for fast transfer of electrons. Under the guidance of the smooth Li‐ion migration and rapid electron flow, the Li metal can be restricted as thin layers within submicro scale in nanochambers with constrain boundary and stress dissipation, inhibiting the local agglomeration and blocking. Thus, the lithiophilic active sites can be effectively exposed to the Li‐ions within submicro scale, improving the reversible conversion for high lithium utilization during long‐term cycling. As such, the Li@MnZnO/CNF electrode achieves a high lithium utilization of 70% at a record‐high current density of 50 mA cm−2 with areal capacity of 10 mAh cm−2. This work offers an avenue to improve lithium utilization for long‐lifespan LMAs working under high current densities and capacities.

Figure S11. Coulombic efficiency of Li deposition on the MnO 2 /CNF, ZnO/CNF and MnZnO/CNF substrates (a) at a current density of 1.0 mA cm -2 with a capacity of 1.0 mAh cm -2 ; and at a current density of 5 mA cm -2 with a capacity of (b) 2 mAh cm -2 and (c) 4 mAh cm -2 . Figure S12. (a) Rate performance of the symmetrical cells with Li@MnZnO/CNF electrode; Cycling performance of symmetrical cells using Li@MnZnO/CNF composite anode under a current density of 50 mA cm -2 with a capacity of (b) 3 mAh cm -2 , (c) 5 mAh cm -2 and (d) 10 mAh cm -2 ; (e) Electrochemical performance of symmetrical cells with Li@MnZnO/CNF electrode compared with other reported Li-metal composite anodes. Figure S13. SEM images of 10th plating morphology of MnZnO/CNF with a fixed capacity of 1.0 mAh cm -2 at a current density of (a) 1.0 mA cm -2 and (b)10 mA cm -2 . SEM images of 10th plating morphology of ZnO/CNF with a fixed capacity of 1.0 mAh cm -2 at a current density of (c) 1.0 mA cm -2 and (d) 10 mA cm -2 . SEM images of 10th plating morphology of MnO 2 /CNF with a fixed capacity of 1.0 mAh cm -2 at a current density of (c) 1.0 mA cm -2 and (d) 10 mA cm -2 . Figure S14. SEM images of 10th plating morphology of MnZnO/CNF with a capacity of 5 mAh cm -2 at a current density of 50 mA cm -2 . Figure S15. Cross-sectional SEM images demonstrating thicknesses and inner structures of (a) Li foil before cycling and b) Li foil after 100 cycles; c) Li@MnZnO/CNF electrode before cycling and d) and Li@MnZnO/CNF electrode after 300 cycles at a current density of 50 mA cm -2 under areal capacity of 1 mAh cm -2 .   (d) charge/discharge profiles when cycling at 5 C of LFP cells.
The EIS tests show that LFP//Li@MnZnO/CNF cell has much lower interfacial resistance (~90 Ω) than that of LFP//Li foil cell (460 Ω), resulting in improved cycling performance and stability at high rates ( Figure S18b). Moreover, the LFP//Li@MnZnO/CNF cell exhibits stable voltage profiles and reversible charge/discharge capacities under different rates ( Figure S18c). It also displays stable charge/discharge platforms with slight capacity degradation or polarization increase under a high rate of 5 C ( Figure S18d).

Synthesis of Polyimide (PI)
p-phenylenediamine (PPD, >99.0%; Mw = 108.14 g mol -1 ) and 3,3',4,4'-Biphenyl tetracarboxylic diandhydride (BPDA, >99.5%; Mw = 294.22 g mol -1 ) with the molar ratio of 1:1 were dissolved in N,N-dimethylformamide (DMF) . Afterwards, the mixture was stirred at 0°C for 24h to obtain the electrospinning solution containing 10wt% PPD and BPDA. The electrospinning process was operated with applied voltage of ~20 kV, tip-to-collector distance of 20 cm and flow rate of 1.0 mL h -1 . Then the PI membranes were obtained by the thermal imidization of the as-prepared electrospun fibrous membranes in a horizontal tubular furnace at 350 °C for 30 min under air atmosphere, with a heating rate of 3 °C min -1 .

Synthesis of ZnO/CNF
The PI membranes were then treated by plasma in the Schwarze Plasma Cleaning Machine for 2 min. 0.1 g PI was soaked in 10 g Zn(CH 3 COO) 2 water solution (10 wt%) at 60 °C for 4 h and naturally dried for 20 h. Afterwards, PI membranes loaded with acetate precursor were carbonized in 800 °C for 1 h under Ar atmosphere with a heating rate of 10 °C/min. Finally, ZnO/CNF films were obtained after naturally cooling to room temperature with protection of Ar flow. The pure PI membranes were carbonized with same procedure to obtain CNF for comparison.

Synthesis of MnZnO/CNF
120 mg ZnO/CNF membrane was immersed in 400 mL KMnO 4 solution (containing 20 mg KMnO 4 ). The reaction was kept at 70 °C until the solution color changed from purple to light brown. The membrane was rinsed with deionized water several times, and then treated by vacuum drying at 100 °C to obtain MnZnO/CNF.

Characterization
The morphologies and elemental distributions of samples were investigated using scanning electron microscope (SEM, SU-8010) coupled with an Energy dispersive spectrometer (EDS) attachment.
Transmission electron microscope (TEM, JEOL 2100F) tests were matched with SEM results to provide more details about the structure and composition of the samples. X-ray diffraction (XRD) patterns of samples were examined using D/max-2500/PC with filtered Cu Kα radiation (scanning rate: 5° min -1 , 2θ range: 10° ~ 80°). The specific surface areas, pore size distributions and pore volumes of the samples were calculated from the adsorption/desorption isotherms of N 2 at 77K by an automatic adsorption system (Bellsorp-mini) using the Brunauer-Emmett-Teller (BET) method. According to the mass of Li@MnZnO-CNF, reversible areal capacity of 10 mAh cm -2 is equal to lithium utilization of 70% and specific capacity of 1736 mAh g -1 .

Assembly of cells
Half cells use the skeleton (MnZnO/CNF, ZnO/CNF) with a diameter of 10 mm as the working electrode and Li foil as the counter electrode. Symmetric cells were assembled by two Li foils or two composite Li metal electrodes with a diameter of 10 mm.
For the full cells, the LiFePO 4 (LFP) or LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NCM811) was employed as cathode while the fabricated composite Li metal electrode or Li foil was used as anode. The areal mass loading of LFP was about 3.5 mg cm -2 and the areal mass loading of NCM811 was ~ 5.0 mg cm -2 .
The cathode diameter was 12 mm. Li-ion capacitors (LIC) were assembled utilizing PCNF as cathode and fabricated composite Li metal electrode or Li foil as anode.
In half cells and symmetrical cells, 1.0 M lithium bistrifluoro-methanesulfonylimide (LiTFSI) in DOL and DME (volume ratio 1:1) with 2% lithium nitrate was employed as the electrolyte. The

Electrochemical measurements
To evaluate the Coulombic efficiency in half cells, a certain capacity of Li was deposited on the working electrode, and then Li was stripped until the voltage rose to 1.5 V.
In symmetrical cell tests, different and specific current densities and areal capacities of Li were set for measuring the electrochemical properties of the as-prepared composite Li metal electrodes.