A Superior Î´-MnO2 Cathode and a Self-Healing Zn-Î´-MnO2 Battery

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A Superior #-MnO2 Cathode and a Self-Healing Zn-#-MnO2 Battery Donghong Wang, Lufeng Wang, Guojin Liang, Hongfei Li, Zhuoxin Liu, Zijie Tang, Jianbo Liang, and Chunyi Zhi ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b04916 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 17, 2019

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A Superior δ-MnO2 Cathode and a SelfHealing Zn-δ-MnO2 Battery Donghong Wang,† Lufeng Wang,‡ Guojin Liang,† Hongfei Li,† Zhuoxin Liu,† Zijie Tang,† Jianbo Liang,*, ‡ Chunyi Zhi*, †, § †

Department of Materials Science and Engineering, City University of Hong Kong, 83

Tat Chee Avenue, Kowloon, Hong Kong 999077, China ‡

Department of Chemistry, Capital Normal University, 105 West Third Ring Road

North, Haidian District, Beijing 100048, PR China. §

Shenzhen Research Institute, City University of Hong Kong, Shenzhen, PR China

ABSTRACT: While ɑ-MnO2 has been intensively studied for zinc batteries, δ-MnO2 is usually believed to be more suitable for ion storage with its layered structure. Unfortunately, the extraordinary Zn ion storage performance that δ-MnO2 should exhibit has yet not been achieved due to the frustrating structural degradation during charge-discharge cycles. Here, we found the Na ion and water molecules preintercalation can effectively activate stable Zn ion storage of δ-MnO2. Our results reveal that the resulted Zn//pre-intercalated δ-MnO2 battery delivers an extraordinarily highrate performance, with a high capacity of 278 mAh g-1 at 1 C and up to 20 C, a high capacity of 106 mAh g-1 can still be measured. The capacity retention is as high as 98 % after charged-discharged up to 10000 cycles benefiting from smooth Zn ion diffusion 1 ACS Paragon Plus Environment

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in the pre-intercalated structure. Further in-situ/ex-situ characterization confirms the superfast Zn ion diffusion in the pre-intercalated structure at room temperature. In addition, utilizing the well-chosen electrode material and modified polyurethane shell, we fabricated a quasi-solid-state healable Zn-δ-MnO2, which can be self-healed after multiple catastrophic damages, emphasizing the advanced features of aqueous Zn ion battery for wearable applications.

KEYWORDS: δ-MnO2, zinc ion batteries, fast diffusion, high rate, self-healable Large-scale renewable energy storage devices are triggered and promoted vigorously due to the increasing energy crisis and environmental pollution.1-3 Among numerous candidates, rechargeable zinc ion batteries (ZIBs) with the electrolyte of mild aqueous solution are receiving wide attention owing to the environmental friendliness and high energy density.4-7 Among the various types of cathode materials for ZIBs, vanadiumbased materials 8-13 and MnO2 nanomaterials4-7,14,15 stand out for their easy fabrication, high theoretical capacity and low cost. Especially, the higher discharge voltage and lower toxicity of MnO2 contribute its widely research in ZIBs. While MnO2 with many crystalline structures has been reported (Scheme 1) for ZIBs, α-MnO2 with 2*2 tunnels of 4.6 Å is a hot topic in studies of mild Zn-MnO2 batteries due to their superior performance.5-7,15,16 However, compared with this tunnel structure, δ-MnO2, is theoretically much more suitable for Zn ion storage and release due to its layered structure with much larger interspacing channels, around 0.7 nm, as shown in Scheme 1d.17-20 Unfortunately, until now there is no outstanding electrochemical performances 2 ACS Paragon Plus Environment

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obtained for the Zn//δ-MnO2. All the reported works are all troubled by the quick capacity fading and poor rate capability.17-19,21,22 Detailed research reveals that in a typical Zn//δ-MnO2 battery system, the layered structure of δ-MnO2 was detected to transform into other polymorphs, this transformation causes a large volume change and structure collapse, which is the key reason for the poor cycling stability.17-19,21,22 Another problem concerning with δ-MnO2 on the poor rate capability is its sluggish ion conductivity.18 Due to these intrinsic limits, the discharge capacity of Zn//δ-MnO2 battery was only 7 mAh g-1 at a current rate of 5.5 C,17 and the retention was only 50 % after tested for 100 cycles. The latest work about δ-MnO2 reported a cycling stability of 79.6 % retention after 2000 cycles for Zn ion storage.18 Such a disappointing performance is not desired for δ-MnO2 with a perfect layered channel for Zn ion ingress. Furthermore, these performances are far behind that of α-MnO215. Achieving high performance that δ-MnO2 should exhibit with its layered structure is highly expected. At device-level, the aqueous electrolyte ZIB possesses a variety of advantages as wearable power system with its absolute safety. Endowing self-healing feature to a device is an effective approach to enhance its durability, but it remains a challenge for batteries due to multi-component structure. Up to now, the reported self-healable zinc batteries are both based on a self-healable electrolyte,8,23 while healable electrodes have not been reported. In fact, electrode healing directly affects the impedance of the whole devices and determines the overall performance of a battery. Endowing the electrodes with self-healing ability is on urgently demand. 3 ACS Paragon Plus Environment

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Here, we used a pre-intercalation method with Na ions and water molecules to stabilize the layered structure and activate the intrinsic high-performance of δ-MnO2 which approaching the theoretical value. With the Na ions and water as pillars, the interlayer possesses a gap of 0.72 nm. When used as a host for ZIBs, they displayed an extraordinary Zn ion storage performance which addressed the bottlenecks of the previous work about Zn//δ-MnO2. An ultralong-stable cycle life of 10000 cycles was achieved with 98% retention. Even at a rate as high as 20 C, a large usage capacity of 106 mAh g-1 is discharged. Based on the superior performance of this Zn//δ-MnO2 battery and high safety of ZIBs, a self-healable Zn-MnO2 battery with high performance was designed and achieved through using self-healing carboxylated-polyurethane (CPU) as the substrates for electrodes. The discharge-charge capacities at 10 C kept well after several times of cut and healing.

RESULTS AND DISCUSSION The δ-MnO2 sample was fabricated through oxidation of Mn(OH)2 nanoplatelets with NaClO in aqueous solution. During this reaction, Mn2+ was oxidized to be Mn4+, with the Na+ ions and H2O intercalated into the structure. The crystallography and micromorphology of the Mn(OH)2 precursors are exhibited in Figure S1, the X-ray diffraction (XRD) patterns reveal that the as-synthesized precursor is a high crystalline Mn(OH)2 (JCPDS No. 73-1133). The field electron emission scanning electron microscope (FESEM) image (Figure S1b) shows the sample is composed of hexagonal nanoplatelets with a thickness of about 50 nm and a lateral dimension of about hundreds 4 ACS Paragon Plus Environment

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of nanometers to micro-size. The as-obtained MnO2 sample was also studied by XRD, as displayed in Figure 1a, the characteristic diffraction peaks match well to a pure δphase MnO2 with a birnessite framework and layered structure (JCPDS No. 80-1098), and no obvious impurity peaks are detected. The (001) and (002) peaks are intense and symmetric, corresponding to the layered structure. The sharp feature of the two peaks illustrates the high crystallinity of the MnO2 product. The detailed composition and valence state of Mn were investigated through X-ray photoelectron spectroscopy (XPS), as shown in Figure 1b and 1c, the splitting energy between the two Mn 2p and 3s peaks are 11.8 eV and 4.58 eV, respectively. According to the splitting energy of Mn 3s peak, the average oxidation state of the obtained δ-MnO2 sample was calculated to be 3.77 (Figure 1c).24,25 The presence of Na is confirmed by the appearance peak around 1070 eV (Figure 1d), and the ratio of Na/Mn was calculated to be 0.22. Considering the presence of Na ion and the valence balance of the MnO2, the oxidation state of Mn should be around 3.78, which is very close to the calculated value. The deconvoluted O 1s spectrum (Figure 1e) can be fitted with the presence of Mn-O-Mn bond (centered at 529.7 eV), Mn-O-H bond (531.5 eV) and H-O-H bond (532.9 eV).25,26 Subsequently, thermogravimetric (TG) analysis was used to identify the water content. As shown in Figure 1f, there is about 4.2 % weight loss up to 100 oC, this is mainly due to the physically adsorbed water. Another weight loss of 11.6 % appears from 100 to 450 oC, confirming the presence of the structure water with a content of 12.1 wt%. Thus, the molecular formula of the as-prepared product is then determined to be Na0.44Mn2O4·1.5H2O (δ-NMOH), which is close to the composition of standard layered 5 ACS Paragon Plus Environment

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Na0.46Mn2O4·1.4H2O (JCPDS No. 80-1098). The layered structure is clearly illustrated in Figure 1g, the layer is constructed by Mn-O sheet, which is connected by MnO6 octahedra sharing with the edges, and the Na+ and crystal water accommodate in the interlayer space, which act as the stabilizer of the layered structure.7,17,27 The suitable amount of intercalated Na ions and water molecules for sample δ-NMOH are expected to result in a stable layered structure.28 In addition, the large amount of water not only stabilize the intercalated Na ions in the structure,28 but also promote the intercalation of Zn ions during discharge in ZIBs.12,13 FESEM, energy dispersive X-ray (EDX) element mapping and transmission electron microscope (TEM) were characterized to confirm the morphology and microstructures of δ-NMOH. The FESEM image (Figure 1h) shows that the δ-NMOH sample consists of massive hexagonal nanoplates with irregular shapes. These nanoplatelets possess clean and smooth surfaces. EDX element mappings (Figure 1i) of the δ-NMOH sample show that the Mn, O and Na elements distribute in the sample homogeneously. TEM images in Figure 1j and Figure S2 further exhibit the irregular hexagonal nanoplate morphology of the δ-NMOH, with a side-length from 100 nm to 300 nm. The lattice fringe can be clearly observed in the high-resolution transmission electron microscopy (HRTEM) image (Figure 1k), with a spacing of 0.23 nm and 0.22 nm, which corresponds to the (-111) and (201) planes of the δ-NMOH, regardless of the orientations of the crystalline domains. However, the lattice fringes with a larger spacing around 0.72 nm and 0.36 nm, corresponding to (001) and (002) planes are not observed in the HRTEM image. It indicates that the crystalline (001) direction might be perpendicular to the basal surfaces of the 6 ACS Paragon Plus Environment

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nanoplatelets. The corresponding selected area electron diffraction (SEAD) is displayed in Figure 1l and it confirms that the orientation of the nanoplate is perpendicular to [001].21 Due to the spacious layered channels offered by this specific structure with Na+ and crystal water intercalated, this sample is expected to perform as an excellent host for Zn ion insertion. Firstly, coin cells were assembled and tested using the obtained δNMOH sample as the cathode, Zn plate as the anode, with the aqueous electrolyte consisting of 2 M ZnSO4 and 0.2 M MnSO4. The electrochemical performance was first assessed through cyclic voltammetry (CV), with the result shown in Figure 2a. Two reduction peaks were measured separately around 1.36 V and 1.17 V in the initial cycle, with the oxidation peaks overlapped at 1.62/1.64 V. In the following cycles, the reduction peak located around 1.2 V shifts positively while the oxidation peak around 1.60 V shifts negatively, this difference is ascribed to the large overpotential need to be activated in the first cycle.29 These shifts narrow the potential window between the oxidation and reduction, which indicates a more reversible electrochemical cycling process.30 Furthermore, as the cycling proceeds, the intensity of the reduction peak around 1.35 V and oxidation peak at 1.6 V increase obviously, demonstrating an activation process of discharge and charge. The rate performance of the designed aqueous battery was evaluated from 1 C to 20 C with the voltage range from 0.95 V to 1.85 V. As exhibited in Figure 2b, at 1 C (1 C = 0.380 A·g-1), the δ-NMOH electrode delivers a reversible capacity of average 278 mAh·g-1, which is close to 90 % of its theoretical capacity (308 mAh·g-1). As the rate increases, values of 232, 187, 161, 145, 7 ACS Paragon Plus Environment

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134, 125, 111 and 103 mAh g-1 are measured as the reversible discharge capacity at 2, 4, 6, 8, 10, 12, 16 and 20 C, respectively. The average capacity at 20 C is 37.1 % of that at 1 C because the favorable layered structure benefits the fast insert/extract of electrolyte ions. In addition, impressively, when the rate returns from 20 C to 1 C, the discharging capacity recovers to the 91 % of its initial capacity (253 mAh g-1). All these demonstrate a superior rate capability of the δ-NMOH materials. The superior rate capability outperforms all the δ-phased MnO2,17,18,21,31 and CNT/α-MnO25,16. The discharge-charge curves corresponding to different rates are exhibited in Figure 2c, at 1 C, the δ-NMOH cathode delivers a safe voltage of 1.45 V versus Zn2+/Zn and a sloping plateau at around 1.4 V followed with a flat plateau at ca. 1.2 V. As reported in previous rechargeable aqueous Zn-MnO2 battery, the first discharge plateau is dominated by the insertion of H+ and the second one corresponds to the Zn2+ insertion. 29,31

In addition, as the rate increases, the H ion insertion contribution to discharge

capacity is enhanced. This is a reasonable observation since H ion has a much higher diffusion coefficient than Zn ion resulted from the apparent ion radii difference.7 Furthermore, the characteristic plateaus can still be easily distinguished in chargedischarge curves even at the high current density of 20 C (6.16 A·g-1). As a decent rate capability endows a high energy and power density for Zn-δ-NMOH battery systems, the high energy and high power densities of δ-NMOH are further evident in the Ragone plot, in comparison to VS2,32 CuHCF,33 Zn0.25V2O5·nH2O,34 Zn3V2O7(OH)2·2H2O,35 Na0.9MnO236 and δ-MnO218. δ-NMOH exhibits the highest energy density while limiting the discharge time within several minutes. As presented 8 ACS Paragon Plus Environment

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in Figure 3d, δ-NMOH can achieve an energy density of 374 Wh kg−1 at 1 C and still possess a relatively high value of 130 Wh kg−1 with a large power density of 7775 W kg−1. The power density and energy density of δ-NMOH cathode are eventually better than those of the α-MnO215 and β-MnO237 cathodes, delivering a performance it should have. Actually, the δ-NMOH cathode provides the best performance for mild Zn-MnO2 battery reported so far. The stability of the δ-NMOH electrode at various rates of 8 C, 10 C, 12 C, 16 C, and 20 C is further evaluated. As shown in Figure 2e, nearly 100 % retention are observed after 2000 cycles for 8 C and 10 C, respectively. When the current is raised to a higher rate of 12 C and 16 C, the cell still maintains a highly stable performance. The Zn-δNMOH battery was further evaluated for long cycles at 20 C. As exhibited in Figure 2f, the discharge capacity in the first cycle is 108.8 mAh g-1. An enhancement was observed at all rates as revealed in Fig. 2e and 2f. We attribute this capacity rise to the gradual activation of electrodes, which is very common for Zn-MnO2 batteries in the ZnSO4+MnSO4 electrolyte.14,30 The activation process can be monitored through EIS measurements, as shown in Figure S3, the electrochemical activation must be caused by the reduction of the charge transfer impedance (Rct).30 After 10000 cycles, the discharge capacity maintains at 106.5 mAh g-1, as exhibited in Figure 2f. To further clarify the mechanism of the extraordinary cycling stability, the δ-NMOH sample was thermally treated at 500 oC to remove the structural water, which is named as δ-NMOH-500, it displays a much lower capacity and a poorer retention than those of δ-NMOH. As presented in the discharge-charge profile at 20 C in Figure 2g and 9 ACS Paragon Plus Environment

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electrochemical impedance spectroscopy (EIS) curves in Figure 2h, the discharge plateau negatively shifts, and the charge transfer resistance increases by half after the loss of structure water. In addition, the interlayer distance shrinks after calcination, as shown in Figure S4, after removing the water molecules, the low amount of small Na ions is not enough to sustain the previous δ structure,28 so the crystallinity also decreases significantly. Furthermore, after the thermal treatment, removal of structural water leads the δ-NMOH-500 sample lose the magic power of lubricant for the intercalation of foreign ions.12,13,38 The obvious performance degradation after water loss demonstrates the important role of water molecules in stabilizing the layered structure and as a promoter in δ-MnO2 for Zn ion storage. The capacity retention and cycle life of typical cathodes for ZIB are collected and compared with the δ-NMOH cathode, which is displayed in Figure 2i, where a striking advancement is noted in the cycle life for δ-NMOH sample. The decent cycling performance of δ-NMOH surpasses traditional MnO2 in the ZnSO4 electrolyte.4,5,14-18,21,29-31,37,39-42 The popular α-MnO215 was also chose and prepared as a control sample, with the structure and morphology shown in Figure S5a and b. As shown in Figure S5c, α-MnO2 cathode delivers a much poorer cycling stability than δ-NMOH, with a discharge capacity of only 54.6 mAh g-1 remained after 9000 cycles at 20 C. This outstanding performance of δ-NMOH suggests the high stability of this layered skeleton with Na and water intercalated, which provides a large channel for the fast kinetics of H+ and Zn2+ intake and release. Thus, fast transport and a long-term insertion/extraction for H and Zn ion are allowed. The high stability of the layered structure was also tested using XRD measurement, as 10 ACS Paragon Plus Environment

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presented in Figure S6, the two characteristically symmetric peaks of (001) and (002) persist well after 2000 cycles and even 10000 cycles. The change of morphology during discharge and charge process was also studied, with the results shown in Figure S7. The shape of nanoplate maintains after discharge and charge process. The high rate performance and long-term stability of Zn-δ-NMOH batteries is significantly controlled by their kinetics process, which was analyzed based on the scan rate test CV. The CV curves of Zn-δ-NMOH batteries are displayed in Figure 3a, with two discharge peaks and two charge peaks presented. The scan rate and peak current have been proved to obey a relationship as follows: 12,32 (1),

𝑖 = 𝑎𝑣𝑏 it can be rewritten as: log (𝑖) = 𝑏log(𝑣) + log (𝑎)

(2),

where i refers to current, v stands for scan rate, and the corresponding adjustable parameters are a and b. Through fitting log(i) versus log(v), the coefficient b for peaks 1, 3 and 4 can be determined according to the slope of the linear regression lines, with values of 0.70, 0.84, and 0.78, respectively, as displayed in Figure 3b. (Because the peak 2 became a shoulder at high scan rates, so the peak intensity cannot be measured.) The result implies that the electrochemical reaction of δ-NMOH is little influenced by diffusion process, instead the kinetic reaction mainly relies on the pseudocapacitance process within the scanning rate ranging from 0.1 to 1 mV s−1. This characteristic is in charge of the high rate performance of the Zn-δ-NMOH batteries.43

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As mentioned above, a two-step discharge process was detected in both CV and charge-discharge curves. Ex-situ XRD measurements were performed on δ-NMOH cathode at different stages to determine the crystal structure evolution during the discharge-charge process. Figure 3c and Figure S8a show the XRD patterns of different products collected at given discharge-charge stages with the corresponding dischargecharge profiles of δ-NMOH cathode at 0.3 C exhibited in Figure 3d. Through carefully examining the peak patterns, the layered structure kept well during discharge and charge process, as exhibited in Figure 3c, no other peaks are attributed to spinel ZnMn2O4 or spinel MnOOH.4,29 We also found that after firstly discharged to 0.95 V, the (001) peak shifted to a smaller angle and recovered back to the original position after charging. During the second cycle, the (001) peak shifted positively at the state of 1.35 V while shifted to a much smaller angle after discharged to 0.95 V, as revealed in Figure 3c. Due to the significant difference between the size of H+, Na+, and Zn2+, after H+ insertion, the interlayer shrinks, this can be confirmed by comparing the peak positions of δ-MnOOH with δ-MnO2 in Figure S9. Whereas, after ingress of Zn ion, the interlayer expanded due to the much larger radii of Zn ion. The result further confirms the first H and second Zn insertion during discharge. Interestingly, no obvious peaks change of layered δ-NMOH was measured, it means no crystal structure change during the discharge-charge process. This result is consistent with some other studies for δMnO2.19,31,44 Galvanostatic intermittence titration techniques (GITT) was applied to investigate the kinetic behaviors of the Zn-δ-NMOH battery.13,29 Figure 3e shows the voltage change upon a pulse of 0.5 C for 120 s followed with a rest of 30 min until the 12 ACS Paragon Plus Environment

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voltage was meet. As revealed in Figure 3c, the total overvoltage in the region I is only 0.068 V, while in region II, the total overvoltage (0.508 V) is about ten times the value in region I. The big overvoltage in the region II is considered to result from both high voltage jump and slow ion diffusion.29 Considered the smaller size of H+ than Zn2+ and the fact that the two foreign ions H+ and Zn2+ can insert into MnO2 structures as reported before,4,15,19,25,29,31 it is believed that the first discharge plateau is caused by the H+ insertion, whereas the second voltage plateau mainly results from the Zn2+ insertion.29,31 The corresponding ion diffusion coefficient was also calculated according to equation (4), as displayed in Figure 3f, the coefficient decreases as the ions insert, and the value in the first region (below 50 % ion insertion state) (10-5-10-7 cm2 s-1) is much larger than the second one (10-8 cm2 s-1), which further confirms that the discharge process is consist of a first H+ dominated insertion and a second Zn2+ insertion. The high performance of aqueous Zn-δ-NMOH battery motivated us to flexible Znδ-NMOH batteries. Therefore, we prepared a quasi-solid-state Zn-δ-NMOH battery based on polyacrylamide (PAM) hydrogel electrolyte, the electrochemical properties were evaluated, as shown in Figure S10 and 11. With the hydrogel electrolyte, the Znδ-NMOH cell can still exhibit its characteristic two-step discharge process, as exhibited in the CV curves and discharge-charge curves in Figure S10a and 10b. The rate capability matches that in the liquid electrolyte within 1 C and 12 C, as displayed in Figure S11. Based on this, a self-healable Zn-δ-NMOH battery was designed, using the CPU as the self-healing substrates.


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The CPU sheet was firstly applied to test and verify the excellent self-healing property, with the demonstration exhibited in Figure 4a. Firstly, the CPU was cut into two half parts, then we rejoined the two cut ends and pressed gently within tens of seconds. After 5 mins, the wounds were healed successfully with only a liner surface scar. The similar stress-strain curves before cut and after 4 times of self-healing further confirmed the amazing heal ability of CPU, as shown in Figure S12. The tensile strength was well maintained at about 66 % and Young’s modulus was slightly raised. This efficient mechanical self-healing is derived from the reversible hydrogen bonds in CPU.45-47 As schematically illustrated in Figure 4b, once a CPU sheet was cut, the strong intermolecular hydrogen bonds were broken. When the broken hydrogen fragments were put in contact, hydrogen bonding could re-establish. Owing to its decent self-healing power, CPU can be used as a substrate for our battery device. A self-healing Zn-δ-NMOH energy device was fabricated using the free-standing CPU as a self-healing polymer substrate, the assembly structure of the self-healing device and the self-healing process are illustrated in Figure 4c. From the topmost layer to

the

bottom,

CPU

sheet/carbon

cloth/δ-NMOH

paste,

PAM

hydrogel,

electrodeposited Zn sheet/carbon cloth/CPU sheet are sequentially assembled into a total self-healing battery device. The restoration ability of the CPU on the electrical conductivity of the electrodes was researched based on a fresh planer battery. A multimeter was used to trace the resistance of the anode and cathode, as shown in Figure 4d. Apparently, after cut and healing, the resistance of anode and cathode increased slightly to 5.3 and 3.5 Ω, respectively. And the open voltage of the fresh cell 14 ACS Paragon Plus Environment

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reduced from 1.379 V to 1.355 V. The corresponding EIS and open circuit voltage change relating to healing times were recorded in Figure 4e and f, a rise is observed in both interface resistance (Ri) and charge transfer resistance (Rct) as manifested in Figure 4e, it was caused by the uncompacted contact between the cut ends of the electrodes. Consequently, the open circuit voltage for charged devices exhibited a tiny drop from original 1.51V to 1.48 V after experienced 4 times of cut and healing. The restoration of electrochemical performance on the self-healing aqueous battery was investigated. Figure 4g exhibits the cycling stabilities before cut and after healing, after each time of cut and healing, a small decrease was observed compared with the origin flexible battery. Impressively, capacity of 100 mAh g-1 can still be remained even at 10 C, with a considerate retention of 81.2 % after 4 times of cut and recovery. The discharge-charge profile at 10 C in Figure 4g can give a further certification as no significant change was observed, it still delivered a voltage of ca. 1.4 V and the characteristic two-step discharge process. Just a small reduction in the discharge capacity was observed from 122 mAh g-1 to 108 mAh g-1. Figure 4j displays an effective application of a flexible self-healing Zn-δ-NMOH battery as an energy device to power an electric watch in a circuit. A single device successfully enlighted the electric watch, after breaking of the flexible battery, the working watch quenched. Then, the two halves were brought into together, the planer device self-healed with the electric watch re-run.

CONCLUSION 15 ACS Paragon Plus Environment

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δ-MnO2, with its layered structure, is theoretically more suitable as a host for Zn ions than all α-, β-, γ-manganese oxides. However, so far it failed to deliver its potentially high performance owing to the inherent unstable structure and slow ion diffusion. Through pre-intercalating δ-MnO2 with Na ions and water molecules, we successfully realized the full usage of δ-MnO2 as cathodes for ZIBs, endowing it with the electrochemical performances it should exhibit (close to its theoretical capacity). The δ-NMOH cathode provides the best energy density and cycling stability for mild ZIBs in comparison to all currently reported α-, β-, γ- and δ-MnO2 cathodes. δ-NMOH can achieve an energy density of 374 Wh kg−1 at 1 C and maintains at ≈130 Wh kg−1 with a power density of 7775 W kg−1. Ultrafast charge-discharge is also realized for the designed Zn-δ-NMOH battery, up to 20 C, a high capacity of 106 mAh g-1 can still be discharged. It can also endure 10000 charging cycles with a capacity retention of 98%. To demonstrate its application, furthermore, we successfully fabricated a self-healable Zn-δ-NMOH battery with stable restorations after repeated detrimental cutting through decorating self-healing ability on the electrodes. Our research evolves the old δ-MnO2 to be a well-chosen battery material providing an outstanding electrochemical performance it should exhibit and boost the development for better zinc ion batteries.

EXPERIMENTAL SECTION Preparation of Mn(OH)2 nanoplate precursor: Typically, 4 mmol of Mn(Ac)2·4H2O was dissolved in deionized water with a volume of 40 mL. Then, aqueous solution of hydrazine hydrate (N2H4·H2O, 50%, 3 mL) was added into the 16 ACS Paragon Plus Environment

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Mn(Ac)2 solution dropwise under the stirring condition for 5 min. The resulting mixture was transferred and sealed into a 100 mL Teflon-lined stainless-steel autoclave, and heated in an oven to 180 oC at which temperature kept for 12 h. Afterwards, the oven was naturally cooled down to room temperature. The solid sample was harvested by filtration, and then through washing with deionized water and anhydrous ethanol. Finally, white products were collected after dried at 60 oC for 12 h. Preparation of layered MnO2 nanoplates with Na ion and water molecule intercalated: Firstly, a dispersion solution of Mn(OH)2 nanoplate precursors was obtained through sonicating 0.2 g of the as-obtained Mn(OH)2 in 50 mL deionized water, and then added 10 mL NaClO solution (active chlorine>10%) under stirring, the white suspension turned black immediately. The mixture was magnetically stirred for another 24 h at room temperature. Afterwards, the solid sample was collected through filtering and washing with deionized water. Finally, the desired products were prepared after dried under vacuum at 60 oC overnight. Preparation of MnO2 cathode: To prepare the cathode, Na ion and water molecule intercalated MnO2 nanoplates (δ-NMOH, 70 wt%) were mixed with conductive additive (acetylene blacks, 20 wt%) and binder (PVDF, 10 wt%) in a solvent of NMethyl-2-pyrrolidone. The mixtures were first ground in a mortar for 30 mins, then stirred for 3 h to form a paste. Afterwards, the obtained paste was painted on a piece of carbon cloth paper. Finally, the carbon cloth coated with δ-NMOH active materials was dried at 60 oC in an oven. The effective amount of δ-NMOH loaded on the carbon cloth is controlled at 2.0−3.0 mg cm-2. 17 ACS Paragon Plus Environment

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Preparation of flexible Zn anode: The fabrication of flexible Zn electrode was conducted with a two-electrode setup (electrochemical workstation, CHI 760D) based on electrochemical deposition. A piece of Ni/Cu alloy cloth was chosen as the working electrode, with a zinc plate as the counter and reference electrode. The electrolyte was ZnSO4 aqueous solution (1 mol·L-1). The parameters for electroplating were set at a constant current density of 10 mA cm−2 with the time of 1 hour. The mass of the electrodeposited zinc was controlled around 3.0-5.0 mg cm-2. Preparation of PAM Electrolyte: Typically, the monomer mixture was composed of monomer (3 g acrylamide), initiator (30 mg K2S2O8) and cross-linker (4 mg N,N'methylenebisacrylamide) in 20 ml deionized water. The above solution was stirred until transparent and injected into a mold with a designed thickness. After a polymerization reaction at 60 °C for 2~3 h, thin film of PAM hydrogel formed. PAM electrolyte was prepared through soaking the as-prepared PAM film in aqueous electrolyte (Solution containing 2 M ZnSO4 and 0.2 M MnSO4) for more than 12 hours. Preparation of free-standing CPU sheet: Free-standing PU sheet was prepared by drying PU solution in a mould at room temperature in air. Characterization methods: The XRD patterns of the powder materials were collected under radiation of Cu Kα (λ = 1.54 Å) with a diffractometer (Bruker D2 Phaser). The morphologies and the corresponding EDX mappings were recorded on an FESEM equipment (FEI/Philips XL30). The TEM and HRTEM images were obtained from a field-emission TEM (JEOL-2001F). The XPS was performed on a Physical Electronics. PHI 5802 equipped with a monochromatic Al Kα source. 18 ACS Paragon Plus Environment

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The average oxidation state of Mn in the obtained MnO2 is calculated according to the following equation:25,48 AOS = 8.95 − 1.13ΔE (eV)

(3)

where ∆E stands for the energy difference between the main Mn 3s peak and its satellite peak. Electrochemical measurement: Coin cells can be assembled in open air environment, based on the obtained MnO2 cathode, Zn anode, using 2 M ZnSO4 and 0.2 M MnSO4 aqueous solution as electrolyte, non-woven filter paper as the separator. The self-healable quasi-solid Zn-MnO2 can be assembled on basis of the obtained MnO2 cathode, Zn anode, using PAM electrolyte as the separator. Self-healable PU sheets are coated on the two surfaces of the battery. CV curves and EIS (100 kHz to 0. 01 Hz) were tested using the three-electrode system on an electrochemical workstation (CHI 760D). Galvanostatic discharge/charge and cycling performance were examined by a battery testing system (Land 2001A) with the voltage ranging from 0.95V to 1.85V at 24 oC. The diffusion coefficient of electrolyte ions was measured by using Galvanostatic Intermittent Titration Technique (GITT), as the E versus τ1/2 shows a straight line behavior over the entire time period of current flux (as shown in Fig. S12), then the diffusion coefficient can be calculated based on the following equation:14,49,50


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2

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2

( )( )

4 𝑚 𝐵𝑉 𝑀 𝐷= 𝜋𝜏 𝑀𝐵𝑠 4 = 𝜋𝜏

𝛥𝐸𝑠 𝛥𝐸𝜏

( )( 2

𝑉𝑀

𝑀𝐵 (

(

𝑠 ) 𝑚𝐵

𝑉𝑀 4 = 𝜋𝜏 𝑀𝐵𝑆𝐵𝐸𝑇

2

2

)

𝛥𝐸𝑠 𝛥𝐸𝜏

2

)( ) 𝛥𝐸𝑠 𝛥𝐸𝜏

(4) Where 𝜏 represents the duration of the current pulse (s). VM is the molar volume of δ-NMOH, the value is 103.2cm3 mol-1 based on the standard crystallographic data (JCPDS: 80-1098). MB is the molecular weight, which is calculated for the molecular formula (Na0.44Mn2O4·1.4H2O), with a value of 209.3 g mol-1. The active surface area (S) of the cathode was calculated using the BET area (SBET), which is 20.2 m2 g-1 based on BJH adsorption. The ∆𝐸s and ∆𝐸τ are steady-state voltage change (V) by the current pulse and voltage change (V) during the constant current pulse. The measurement method is displayed in Figure S13. Ex-situ XRD measurement: The positive electrodes obtained at designated discharge-charge stages were washed with deionized water, diluted HCl (0.5M) once and deionized water, and then rinsed with water and ethyl alcohol for several times. After vacuum dried at 60 oC, these electrodes were measured by XRD using a SmartLab X-ray diffractometer. For comparison, XRD measurements were also performed on the cathodes only washed with water.


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Scheme 1. Polyhedral representations of several manganese oxide crystal structures: a) α-MnO2; b) β-MnO2; c) γ-MnO2; d) δ-MnO2. 6,7,20,39,48


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Figure 1. (a) XRD of as prepared MnO2 sample and the standard pattern of δ-MnO2 (JCPDS: No. 80-1098); (b-e), XPS of Mn 2p (b), Mn 3s (c), Na 1s (d) and O 1s (e) of the obtained MnO2 sample. The value of 11.8 eV in b and 4.58 eV in c refer to the splitting energy of Mn 2p and Mn 3s peak, respectively. Both the experimental and fitted spectra of Mn 2p and 3s, Na 1s and O 1s are exhibited in b, c, d, and e, respectively. (f) TGA curve of the as-prepared MnO2 sample from room temperature to 22 ACS Paragon Plus Environment

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500 oC; (g) Structural illustration of the as-prepared Na ion and water molecule intercalated layered δ-MnO2; (h-l), SEM (h), EDX (i), TEM (j), HRTEM (k), SEAD (l) images of the generated Na ion and water molecule intercalated δ-MnO2.

Figure 2. Electrochemical performance of δ-NMOH cathode. (a) CV curve of Zn-δNMOH battery in aqueous electrolyte (2M ZnSO4+0.2M MnSO4) at a scan of 1mV s23 ACS Paragon Plus Environment

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1;

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(b) Rate capability (1 C-20 C); (c) Discharge/charge profiles with C-rate from 1 C to

20 C; (d) The Ragone plot of Zn-δ-NMOH battery, in contrast with other aqueous ZIBs (Zn-VS2 (ref.32), Zn-CuHCF (ref.33), Zn0.25V2O5 nH2O (ref.34), Zn3V2O7(OH)2 2H2O (ref.35), Zn-Na0.95MnO2 (ref.36 ), α-MnO2 (ref.15), β-MnO2 (ref.37) and δ-MnO2 (ref.18); (e) Evaluation of the long-term cycling stability of Zn-δ-NMOH battery at different Crate (8 C-12 C); (f) Evaluation of the long-term cycling stability of Zn-δ-NMOH battery and Zn-δ-NMOH-500 battery at 20 C, with the inset showing the activation process within 50 cycles; (g) Discharge-charge voltage profiles at 20 C at 10th cycles for Zn-δNMOH battery and Zn-δ-NMOH-500 battery; (h) EIS curves of fresh Zn-δ-NMOH battery and Zn-δ-NMOH-500 battery within 0.01-100K Hz; (i) A comparison in capacity retention and cycle life for several typical ZIBs. Zn-CNT/MnO2 (ref.42), ZnMnO2/graphene(ref.14), Zn-CuHCF (ref.33), Zn-β-MnO2 (ref.37), Zn-α-MnO2 (ref.

15),

Tpdorokite MnO2 (ref.40), Zn-ZnHCF (ref.51), Zn-Birnessite MnO2 (ref.18), Zn0.25V2O5·nH2O (ref.34), Zn-V2O5·H2O (ref.12), Zn-LiV2O5 (ref.13), Zn-Birnessite MnO2 (ref.41) and Zn-δ-MnO2 (ref.17).


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Figure 3. (a) CV curves of Zn-δ-NMOH batteries with the scan rate from 0.1 to 1.0 mV/s; (b) Log(i) vs. log (v) plots of three peaks in CV curves; (c, d) The ex-situ XRD patterns of δ-NMOH electrode (c) and the corresponding discharge-charge profiles of the first and second cycle at 0.3 C (d); (e, f) The discharge GITT profiles of the Zn-δNMOH cell (e) (0.5 C for 120 s followed by a 0.5 h rest) and the corresponding diffusivity coefficient (D) of H+ and Zn2+ in discharge process (f); (g) Diagram showing the process of the sequential intercalation of H+ and Zn2+ into the layered channel of δNMOH.


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Figure 4. (a) Demonstration of the self-healing process of CPU: the original CPU, after the cut and after self-healing; (b) Schematic diagram of the self-healing mechanism for CPU; (c) Illustration of the cutting and self-healing process and the assembly structure of CPU-Zn-PAM-δ-NMOH-CPU device; (e) Demonstration of electrical resistance of CPU loaded anode and cathode, and open circuit voltage of a fresh CPU-Zn-PAM-δNMOH-CPU device before cut and after healing. The resistance displayed on the 26 ACS Paragon Plus Environment

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multimeter is in the unit of Ω., and the voltage is in the unit of V; (e) Experimental and fitted EIS curves (100 kHz to 0.01 Hz) of the healable Zn-δ-NMOH battery before healing and after healing, with the inset showing the corresponding circuit for fitting. The Rs represents the series resistance of the cell; Ri refers to the interface resistance associated with the hydrogel electrolyte and electrode layer, Rct is the resistance reflecting the faradic charge-transfer, which locates at the medium frequencies in a form of semicircle; the resistance (RT) is relative to the phase change, and the diffusion process is in the form of Warburg impedance (Zw);16,37; (f) Open circuit voltage change under different healing times after charging; (g) Capacity change verse healing times at 10 C before healing and after 4th healing; (h) Discharge-charge profiles at 10 C before healing and after 4th healing ; (i) Cycling performance of the obtained device before healing and after 4th healing; (j) Demonstration of a self-healing device powering an electric watch before cut, after cut, and after healing.

ASSOCIATED CONTENT The authors declare no competing financial interest. Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.


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XRD and SEM of Mn(OH)2 precursors; TEM images of δ-NMOH sample; Activation process; XRD patterns of δ-NMOH and δ-NMOH-500; SEM and XRD patterns of αMnO2; Comparison of cycling performance of α-MnO2 and δ-NMOH; XRD patterns of the δ-NMOH cathode after long cycles at 20 C; SEM images of δ-NMOH cathode at different discharge-charge stages; The ex-situ XRD patterns of δ-NMOH electrode; SEM and the corresponding EDS spectra of the cathode discharged to 0.95 V; Standard XRD patterns of layered ZnxMnO2, layered MnOOH, layered MnO2 and ZnMn2O4; Electrochemical performance of Zn-δ-NMOH battery in PAM hydrogel; Tensile measurements of pure CPU before cut and after healing; Methods for calculating Diffusion coefficient based on GITT.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Donghong WANG: 0000-0002-8397-2053 Zhuoxin Liu: 0000-0003-2643-3835 Chunyi Zhi: 0000-0001-6766-5953 ACKNOWLEDGMENTS 28 ACS Paragon Plus Environment

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This research was supported by GRF under Project CityU 11305218 and the work was also partially sponsored by the Science Technology and Innovation Committee of Shenzhen Municipality (the Grant No. JCYJ20170818103435068). Furthermore, the National Natural Science Foundation of China (Grant No. 21771127) is acknowledged. REFERENCES (1) Whittingham, M. S., Lithium Batteries and Cathode Materials. Chem. Rev. 2004, 104, 4271−4301. (2) Liu, J., Addressing the Grand Challenges in Energy Storage. Adv. Funct. Mater. 2013, 23, 924−928. (3) Dunn, B.; Kamath, H.; Tarascon, J.-M., Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928−935. (4) Xu, C.; Li, B.; Du, H.; Kang, F., Energetic Zinc Ion Chemistry: The Rechargeable Zinc Ion battery. Angew. Chem. Int. Ed. 2012, 51, 933−935. (5) Li, H.; Han, C.; Huang, Y.; Huang, Y.; Zhu, M.; Pei, Z.; Xue, Q.; Wang, Z.; Liu, Z.; Tang, Z.; Wang, Y.; Kang, F.; Li, B.; Zhi, C., An Extremely Safe and Wearable Solid-State Zinc Ion Battery Based on a Hierarchical Structured Polymer Electrolyte. Energy Environ. Sci. 2018, 11, 941−951. (6) Konarov, A.; Voronina, N.; Jo, J. H.; Bakenov, Z.; Sun, Y.-K.; Myung, S.-T., Present and Future Perspective on Electrode Materials for Rechargeable Zinc-Ion Batteries. ACS Energy Lett. 2018, 3, 2620−2640. 29 ACS Paragon Plus Environment

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TOC

Through pre-intercalating δ-MnO2 with suitable amount of Na ions and water molecules, stable and spacious layered channels were established, thus fast diffusion for intercalated ions and high performance in zinc ion batteries were realized.


A Superior Î´-MnO2 Cathode and a Self-Healing Zn-Î´-MnO2 Battery

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