3D Hierarchical Sunflower‐Shaped MoS2/SnO2 Photocathodes for Photo‐Rechargeable Zinc Ion Batteries

Abstract Photo‐rechargeable zinc‐ion batteries (PRZIBs) have attracted much attention in the field of energy storage due to their high safety and dexterity compared with currently integrated lithium‐ion batteries and solar cells. However, challenges remain toward their practical applications, originating from the unsatisfactory structural design of photocathodes, which results in low photoelectric conversion efficiency (PCE). Herein, a flexible MoS2/SnO2‐based photocathode is developed via constructing a sunflower‐shaped light‐trapping nanostructure with 3D hierarchical and self‐supporting properties, enabled by the hierarchical embellishment of MoS2 nanosheets and SnO2 quantum dots on carbon cloth (MoS2/SnO2 QDs@CC). This structural design provides a favorable pathway for the effective separation of photogenerated electron‐hole pairs and the efficient storage of Zn2+ on photocathodes. Consequently, the PRZIB assembled with MoS2/SnO2 QDs@CC delivers a desirable capacity of 366 mAh g−1 under a light intensity of 100 mW cm−2, and achieves an ultra‐high PCE of 2.7% at a current density of 0.125 mA cm−2. In practice, an integrated battery system consisting of four series‐connected quasi‐solid‐state PRZIBs is successfully applied as a wearable wristband of smartwatches, which opens a new door for the application of PRZIBs in next‐generation flexible energy storage devices.


Material preparation
Preparation of SnO2 QDs@CC substrates: 1.2 g of stannous chloride dihydrate (SnCl2•2H2O, Sigma-Aldrich) and 0.4 g of thiourea (CH4N2S, Sigma-Aldrich) were dissolved in 40 mL deionized water and stirred vigorously for 48 hours at room temperature, and then a clear and light-yellow solution containing SnO2 QDs was obtained.After that, a piece of acid-treated CC (4 × 4 × 0.036 cm 3 ) was totally immersed into the SnO2 QDs solution and finally annealed at 200 °C for 4 h under a vacuum condition.

Preparation of MoS2/SnO2 QDs@CC photocathodes: The as-obtained SnO2
QDs@CC substrates were transferred to 30 mL aqueous MoS2 precursor solution consisting of ammonium molybdate tetrahydrate (0.076 g, Sigma-Aldrich) and thiourea (1 g, Sigma-Aldrich).Subsequently, the precursor solution with substrates was transferred to an autoclave reactor and then kept at 180 °C for 15 h.Finally, the substrates were cleaned with deionized water and afterward dried at 80 °C.

Material characterization
Morphologies and crystal structures of photocathodes were characterized by SEM (ZEISS Ultra 55), energy dispersive X-ray spectroscopy (EDS) and TEM (FEI Talos F200X).The composition and crystal structure of photocathode were determined by X-ray powder diffraction XRD (Rigaku, Cu Kα) with diffraction angles (2θ) ranging from 5 to 80°.The XPS experiments were performed by the surface analysis system (AXIS SUPRA) with Al Kα radiation to determine the elemental valence and bonding of the materials.The Raman spectra were recorded by using Alpha 300R Raman system with an excitation laser wavelength of 488 nm.The optical absorbance of the photocathodes was tested by UV-vis (UV-759).The water contact angles were tested by Optical contact angle measuring instrument (JC2000D3P).The level of heat absorption of the material in illumination is evaluated by an infrared thermal imager (FOTRIC 226s).Absorption and transmittance of visible light from the material is measured using an Ocean Optics (USB4000).

Manufacturing and electrochemical testing of photodetectors
The electrical photo-responses of MoS2/SnO2 QDs@CC were studied by planar metal-semiconductor-metal (MSM) and stacked PDs, respectively.Herein, the slurry was composed of 5 mg active materials (MoS2 powder dissolved in NMP solvent).
Similarly, the SnO2 QDs solution and MoS2 slurry were sequentially spin-coated onto fluorine-doped tin oxide (FTO) glass substrates, then waited for drying and vaporcoated with silver metal on the top contacts to obtain stacked PDs.The current-time response was measured with and without bias voltage under dark and illuminated conditions, respectively.The I-t tests were performed with a Keithley 2440 test system for photoelectric response from -1 V to +1 V in both dark and illuminated conditions.

Assembly of PRZIBs
First, the commercial positive caps of coin cell (CR2032) were processed by cutting a 10 mm hole and then sealing the glass window with EVO-STIK epoxy resin, so that the PRZIBs could get illumination for operation.Thereafter, Zn foil anode, photocathode and glass microfiber separator (Whatman GF/B) were assembled in CR2032-type PRZIBs with 3 M Zn (CF3SO3) 2 aqueous electrolyte.

Assembly of flexible pouch cells
The poly (vinyl alcohol) (PVA) polymer film was prepared by completely dissolving 3.5 g of PVA (Mw: ~89000-98000) in 50 mL DI water at 80 °C and naturally drying for 1 day.Subsequently, a saturated PVA/ Zn (CF3SO3) 2 gel electrolyte was obtained via immersing the as-prepared PVA film in 3 M Zn (CF3SO3) 2electrolyte for 30 s.

Electrochemical test of PRZIBs
All PRZIB cyclic voltammetry (CV) cycles, voltage-time (I-t) tests, and AC impedance (EIS) measurements are measured using the CHIE660 electrochemical station.The impedance tests were performed over a frequency range of 100 kHz to 0.01 Hz with a voltage amplitude of 0.01 mV.The galvanostatic charge/discharge (GDC) profiles were performed on the Neware battery testing system.For the illuminated condition, Xenon lamp light source with wavelengths of 400−1100 nm, and intensity of 1 sun (100 mW cm −2 ) was used.

Wearable demonstration
The commercial smart-watch with Android system was used to construct the integrated self-powered smart watch devices.Before the operations, the battery in the smart watch has been removed.The four QSSPZs (size 1.0 × 4.0 cm ×0.2 cm) are charged to 1.3 V and connected in series.Finally, the QSSPZs are attached to drive the smart watch.All the heart rate information was sent to the mobile phone and cloud storage through Bluetooth in real time.The experimenter is informed and agrees to the public release of these experimental data.

DFT calculation
Theoretical calculations were performed by Quantum ESPRESSO package [1] .We employed the ultrasoft pseudopotentials [2] with the Perdew−Burke−Ernzerhof (PBE) exchange−correlation functional [3] to optimize all the structures.The plane-wave kinetic energy cutoffs for the wavefunctions and the augmented charge density were set to 25 and 250 Ry, respectively.To reduce computational cost, the MoS2/SnO2 heterojunction is created by placing a (2 × 5) five atomic layers stoichiometric SnO2 (001) surface onto a ( 3 2 4 2  ) MoS2 bilayer (lattice mismatch: ~ 4%).There are 294 atoms in the heterojunction.A vacuum layer of ∼15 Å was applied to eliminate the spurious interaction between the periodic images.The Grimme's DFT-D2 method [4] was applied to consider the weak van der Waals interactions between MoS2 bilayer and SnO2.Due to the large size of the MoS2/SnO2 heterojunction, only the Gamma point was sampled in the Brillouin zone in the calculations.The activation energy of ion diffusion was computed by the climbing image nudged elastic band (CI-NEB) method [5] .We calculate the binding energy of the MoS2/SnO2 by the following expressions: Eb = E(MoS2/SnO2) + E(SnO2) -E(MoS2), where E(MoS2/SnO2), E(SnO2) and E(MoS2) represents the total energy of the MoS2/SnO2 heterojunction, the isolated MoS2 bilayer, the isolated SnO2 surface, respectively.The Zn 2+ adsorption energy is calculated by the by the following expressions: ΔE = E(Zn/slab) + E(Zn) -E(slab), where E(Zn/slab), E(Zn) and E(slab) represents the total energy of the Zn 2+ on the heterojunction (or MoS2), Zn 2+ , the heterojunction (or MoS2), respectively.

Figure S6 .
Figure S6.CV plots of MoS2/SnO2 QDs@CC (a) and CC (b) collected at different scanning rates in 1 M Na2SO4; Relations of the current density and the scanning rate (c).

Figure S8 .
Figure S8.Comparison of transmission (a) and absorption (b) for the CC and MoS2/SnO2 QDs @CC.

Figure S10 .
Figure S10.Cycling performance of the battery at 500 mA g -1 without optical window under alternative dark and illuminated states (a) and the corresponding GDC profiles (b).

Figure S11 .
Figure S11.CV curves of MoS2/SnO2 QDs@CC PRZIBs recorded at different rates of 0.2-1.0 mV s -1 under dark (a) and illuminated (b) conditions; Linear relations of anodic peak currents (ip) versus the square roots of scanning rate (c).

Figure S12 .
Figure S12.Determination of b values for cathodic and anodic peaks in dark (a) and illuminated (b) conditions; Comparison of capacitance capacity and diffusion-limited capacity contributions to anode current peak in photocathodes (c); CV profiles at 1.0 mV s -1 showing the capacitive contribution (shaded area) to the total current under illuminated (d, top) and dark (d, bottom).

Figure S15 .
Figure S15.The DFT-optimized structures of the isolated MoS2 bilayer and the isolated SnO2 surface (a); Zn 2+ ion migration on the MoS2/SnO2 and the isolated MoS2 bilayer (b).

Figure S17 .
Figure S17.Image showing the open-circuit voltage of a quasi-solid-state PRZIB assembled with MoS2/SnO2 QDs@CC (left); The GCD curves of a flexible quasisolid-state Zn ion battery at 0.5 mA cm -2 under dark and illuminated conditions (right).

Figure S18 .
Figure S18.Images showing the smart watch powered by a wearable wristband consisting of four series-connected QSSPZs in different bending radius of (a) 4, (b) 5, (c) 6 cm.

Figure S1 .
Figure S1.(a) Digital graph of a laser beam irradiated SnO2 QDs solution; (b) HRTEM image and (c) particle size distribution of SnO2 QDs.

Figure S6 .
Figure S6.CV plots of (a) MoS2/SnO2 QDs@CC and (b) CC collected at different scanning rates in 1 M Na2SO4; (c) Relations of the current density and the scanning rate.

Figure S10 .
Figure S10.Cycling performance of the battery at 500 mA g -1 without optical window under alternative dark and illuminated states (a) and the corresponding GDC profiles (b).

Figure S11 .
Figure S11.CV curves of MoS2/SnO2 QDs@CC PRZIBs recorded at different rates of 0.2-1.0 mV s -1 under (a) dark and (b) illuminated conditions; Linear relations of anodic peak currents (ip) versus the square roots of scanning rate (c).

Figure S12 .
Figure S12.Determination of b values for cathodic and anodic peaks in dark (a) and illuminated (b) conditions; Comparison of capacitance capacity and diffusion-limited capacity contributions to anode current peak in photocathodes (c); CV profiles at 1.0 mV s -1 showing the capacitive contribution (shaded area) to the total current under illuminated (d, top) and dark (d, bottom).

Figure S15 .
Figure S15.(a) DFT-optimized structures of the isolated MoS2 bilayer and the isolated SnO2 surface; (b) Absorption and migration energy with Zn 2+ on the MoS2/SnO2 and the isolated MoS2 bilayer.

Figure S17 .
Figure S17.Image showing open-circuit voltage of a QSSPZ assembled with MoS2/SnO2 QDs@CC (a), and GCD curves of the QSSPZ at a current density of 0.5 mA cm -2 under dark and illuminated conditions (b).

Figure S18 .
Figure S18.Images showing the smart watch powered by a wearable wristband consisting of four series-connected QSSPZs in different bending radius of (a) 4, (b) 5, (c) 6 cm.

Table S1 .
Comparison of current variations of photocathode in different device configurations.

Table S2 .
Comparison of electrochemical performance of different photorechargeable battery systems under dark and illuminated conditions.

Table S3 .
Comparison of photoconversion efficiency of different photocathodes indifferent photo-rechargeable batteries system.

Table S1 .
Comparison of current variations of photocathode in different device configurations.

Table S2 .
Comparison of electrochemical performance of different photo-