Hydrolysis of Solid Buffer Enables High‐Performance Aqueous Zinc Ion Battery

Abstract Aqueous zinc (Zn) ion batteries (AZIBs) have not yet fulfilled their talent of high safety and low cost since the anode/electrolyte interface (AEI) has long been impeded by hydrogen evolution, surface corrosion, dendritic growth, and by‐product accumulation. Here, the hydrolysis of solid buffers is elaborately proposed to comprehensively and enduringly handle these issues. Take 2D layered black phosphorus (BP) as a hydrolytic subject. It is reported that the phosphoric acid generated by hydrolysis in an aqueous electrolyte produces a zinc phosphate (ZPO) rich solid electrolyte interphase (SEI) layer, which largely inhibits the dendrite growth, surface corrosion, and hydrogen evolution. Meanwhile, the hydrolytic phosphoric acid stabilizes the pH value near AEI, avoiding the accumulation of alkaline by‐products. Notably, compared with the disposable ZPO engineerings of anodic SEI pre‐construction and electrolyte additive, the hydrolysis strategy of BP can realize a dramatically prolonged protective effect. As a result, these multiple merits endow BP modified separator to achieve improved stripping/plating stability toward Zn anode with more than ten times lifespan enhancement in Zn||Zn symmetrical cell. More encouragingly, when coupled with a V2O5·nH2O cathode with ultra‐high loadings (34.1 and 28.7 mg cm−2), the cumulative capacities are remarkably promoted for both coin and pouch cells.

The ampule was sealed and then placed horizontally into a muffle furnace.
Subsequently, the furnace was heated to 873 K at a rate of 2 K min −1 , kept at this temperature for 2 hours, cooled to 738 K after 490 min, maintained for another 2 hours, and finally cooled down to room temperature naturally.

Preparation of BP@GF
A certain amount of BP (0.5 g) was added into the N-methylpyrrolidone (NMP) solvent (50 mL).The mixture was ultrasonically treated for 1 hour.Subsequently, the purchased GF separators were added to the mixture and ultrasonicated for 20 minutes.
In this work, the GF separators were purchased from Whatman including GF/A (260 μm in thickness, 5.2 mg cm -2 ) and GF/D (675 μm in thickness, 11.5 mg cm -2 ).The BPloaded GF (BP@GF) separators were dried in a vacuum oven at 60°C for 12 hours.
Then the separators were cut into disks with a diameter of 16 mm or square pieces (45*45 mm).The mass loadings of BP on GF/A and GF/D were 1.78 mg and 2.32 mg cm -2 , respectively.

Preparation of electrodes
Phosphoric acid aqueous solution (H3PO4, 85 wt.%) and zinc sulfate (ZnSO4•7H2O) powder used in the experiment were purchased from Aladdin.Zn foil (>99.9%) with a thickness of 100 μm was used as the anode.The ZPO@Zn anode was prepared by immersing the Zn foil into the 20 mM H3PO4 for 2 hours.The V2O5•nH2O cathode was synthesized by the reaction of V2O5 powder with H2O2 at room temperature.V2O5 powder (3.64 g, Aladdin) and 30% H2O2 (16 mL, Aladdin) were added into 200 mL of deionized water.After aging for 24 hours, the obtained V2O5•nH2O sediment was collected, washed and freeze-dried for 12 h.The V2O5•nH2O powder was mixed with polyvinylidene fluoride (PVDF) with a mass ratio of 9:1.The mixture was then dispersed in NMP solvent.For the low-mass loading cathode, the slurry was cast on the carbon paper.For the high-loading cathode, the slurry was casted into the polytetrafluoroethylene groove that was filled with foam carbon paper (FCP).
The FCP used in this work has a self-mass loading of 16.5 mg cm -2 , with a thickness of 2 mm, and it can be customized as per requirements.Then, the electrode and groove were dried in a vacuum oven at 60°C for 12 hours.The obtained electrodes were cut into disks with a diameter of 12 mm or square pieces (40*40 mm).The cathodes with different loadings (11.7, 19.5 and 34.1 mg cm -2 ) were obtained by adjusting the ingestion of slurry.The actual capacity of Zn (100 μm) is 58.5 mAh cm -2 , [1]   corresponding to a negative/positive (N/P) ratio of 11 in pouch cell.

Battery assembly and electrochemical measurements
The 2025-type button cell was used for electrochemical performance analysis.Zn or ZPO@Zn were used as an anode.V2O5•nH2O based electrode with different mass loadings were used as cathodes.GF (GF/A and GF/D) or BP@GF was used as a separator.2 M ZnSO4 or 20 mM H3PO4/2 M ZnSO4 aqueous solution was applied as electrolytes.In the Zn||Cu half cell and Zn||Zn symmetrical cell, the amount of electrolyte was 50 and 100 μL for GF/A or GF/D based separators.In this work, GF/A based membrane was used as a separator when the areal capacity was less than 2 mAh cm -2 , and GF/D based membrane was used as a separator when the deposition surface capacity was more than 2 mAh cm -2 .The galvanostatic charge-discharge (GCD) tests were performed on a Land system.Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and Tafel curves were performed on an electrochemical workstation (Chenhua, chi760d).The Tafel test was conducted by scanning from -0.6 to -1.3 V.

Materials characterizations
The scanning electron microscopy (SEM, SIRION-100) equipped with an energy dispersive spectrometer (EDS, Oxford) attachment and the transmission electron microscopy (TEM, JEM-2100) were applied to detect the morphology and structure.
The contact angle measurements were performed in a Dataphysics OCA20 system.The operando-pH and in situ QCM characterization were performed by a home-made system including an electrochemical workstation (Chenhua, 760), an electrochemical quartz crystal microbalance (Chenhua, CHI-440) and a pH probe.The crystal structure was identified by X-ray diffractometer (XRD, SHIMADZU-7000).Raman test was carried out on a Horiba JY LabRAM HR Evolution instrument (532 nm).The surface morphology of Zn anode was observed by a laser confocal microscope (KEYENCE, VK-X150).The cross section of Zn anode was analyzed by dual-beam SEM (FEI Strata 400S).Ion information was detected on the ion chromatography.(Thermo Scientific ICS-5000+).In-situ optical tests were realized under an optical microscope (Giorgione).
The X-ray photoelectron spectroscopy (XPS, Escalab 250Xi) with Al Kα radiation was used to identify the surface chemistry.The surface and depth distributions of Zn anode were identified by a time-of-flight secondary ion mass spectrometry (TOF-SIMS, 5-100/ION).The sputtering and analysis areas were 250×250 and 50×50 μm 2 , respectively.In situ time-resolved GC test was realized in a breather valve with a volume of 20 mL, in which a 2025-type button cell was placed.The Φ2 mm hole was opened in negative shell to release hydrogen.Argon was used as carrier with a flow rate of 25 sccm (standard cubic centimeter per minute).

DFT calculations
The density functional theory (DFT) calculations were performed with the CASTEP package in Materials Studio.Generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional was selected to describe the electronic exchange correlation energy. [2]The Zn (002), ZPO (101) and ZPO (100) surfaces were modeled as the substrates.The thickness of vacuum layer was set as 15 Å.The Kohn-sham orbits on the basis of plane wave was expanded with a cut-off energy of 600 eV.For geometry optimization, the energy and force convergence were less than 10 −5 eV and 0.05 eV Å −1 , respectively.
The Gibbs free-energy (ΔGH*) was defined as: ΔGH* =ΔEH* +ΔEZPE − TΔS, where ΔEH* represents the adsorption energy of hydrogen species on the substrate surface, ΔEZPE represents the zero-point energy difference of H in adsorbed state and gas phase state.ΔS represents the entropy change of H* adsorption.Due to the entropy of hydrogen in absorbed state was negligible, ΔS can be calculated as -1/2 S0, where S0 was the entropy of H2 in the gas phase at standard conditions.Therefore, the free energy of the adsorbed state can be simplified as: ΔGH* =ΔEH* +0.24 eV. [3]The H adsorption energy was defined as:

Fig. S4
Fig. S4 Ex situ QCM results of BP in 2M ZnSO4 electrolyte after aging (a) 20 and (b)

Fig. S6
Fig. S6 (a) SEM image of the hexagonal nanosheet on the Zn surface and (b, c) the corresponding EDS results.The actual ratio of Zn to S elements is 4.8, slightly higher

Fig. S7
Fig. S7 In situ optical observations of Zn deposition on Cu foil with (a-c) BP@GF and (d-f) GF separators.The separators are tightly attached to the Zn anode.

Fig. S10
Fig. S10 (a) The container (20 mV) used for GC test, inside is a Zn||Zn symmetrical cells coin cell.(b) A hole (2 mm) is opened on the positive shell to release hydrogen.

Fig. S13
Fig.S13GCD curve of BP@GF cell at the 1800 th cycle.

Fig
Fig. S14 (a, d) Top-view, (b, e) 2D contour map and (c, f) 3D view confocal images of the pristine Zn and the Zn foil after immersing in 20 mM zinc phosphate aqueous solution (ZPO@Zn), respectively.

Fig. S15
Fig. S15 Top view SEM images of (a) pristine Zn and (b) ZPO@Zn.(c) Cross sectional SEM images of ZPO@Zn and (d) the corresponding linear EDS results.

Fig. S16
Fig. S16 Top view SEM image and the corresponding EDS mapping results of ZPO@Zn.

Fig
Fig. S20 (a, c) Top view and (b, d) 2D contour maps of confocal images of the cycled Zn (100 h) in the Zn||Zn symmetrical cells with BP@GF and GF separators at 5 mA cm −2 & 1 mAh cm −2 , respectively.

Fig. S26
Fig. S26 (a) Optical photos of FCP current collector at (a) natural state and (b) bending state, highlighting the good flexibility.

Fig. S27
Fig. S27 Optical photos of manufacture of V2O5•nH2O cathode with high mass loadings.(a) PTFE groove filled with FCP.(b) After the slurry injection.(c) After drying.

Fig. S29
Fig. S29 Digital photos showing the pouch cells with BP@GF as the separator to power an electronic timer at different states, highlighting the reliability and safety.

Table S1
Comparison of the CPC and areal capacity in symmetrical Zn||Zn battery between our work and very recent reports.
Table S2 Comparison of the CPC and areal capacity in full battery between our work and very recent reports.