An Integrated, Flexible Lithium Metal Battery with Improved

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An Integrated, Flexible Lithium Metal Battery with Improved Mechanical and Electrochemical Cycling Stability Shaowen Li, Yue Ma, Jin Ren, Huanyan Liu, Kun Zhang, Yuanyuan Zhang, Xiaoyu Tang, Weiwei Wu, Changchun Sun, and Bingqing Wei ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00369 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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ACS Applied Energy Materials

An Integrated, Flexible Lithium Metal Battery with Improved Mechanical and Electrochemical Cycling Stability Shaowen Li, a Yue Ma,*,a Jin Ren, a Huanyan Liu, a Kun Zhang, a Yuanyuan Zhang, a Xiaoyu Tang, a Weiwei Wu, a Changchun Sun, a Bingqing Wei,*,b a. State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China. b. Department of Mechanical Engineering, University of Delaware, Newark, DE19716, USA. * E-mail: [email protected]; * E-mail: [email protected]

ABSTRACT The success of beyond lithium-ion batteries (LIBs) technologies, i.e., Li oxygen and Li–S cells, depends in large part on the use of metallic anode materials. Metallic lithium, however, tends to grow parasitic dendrites, which would penetrate the separator to cause safety concerns. Moreover, the dendrites are highly reactive to carbonate electrolytes and invariably render a reduced Coulombic efficiency. Here, we propose our design of a dendrite blocking layer with the size-tunable Ag nanoparticles (NPs) decorated on the mesoporous SiO2 nanospheres. The synergistic coupling of these complementary components enables the effective regulation of the

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lithium nucleation behavior via a protected Li-Ag alloying process. The Li plating/stripping process of a symmetric Ag NPs@SiO2-coated Li-metal cell exhibits a reduced voltage hysteresis, facile lithium nucleation process and enhanced Coulombic efficiencies at various current densities. When the Ag NPs@SiO2-coated copper foil anode was integrated with lithium manganese oxide (LMO) or sulfur (S) cathodes in full-cell configurations, both the Li-LMO and the Li-S cells exhibit the high energy densities with a Coulombic efficiency of ~98.5 % for 150 cycles. Furthermore, a flexible metallic cell model with satisfactory mechanical and electrochemical cycling stability (up to 1750 h) was established. The prototype directly integrates the modified polyethylene (PE) separator with the Ag NPs@SiO2 dendrite blocking layer. Our approach could lead to the simultaneous realization of high energy density, mechanical flexibility and safe operation of the Li-metal-based batteries.

KEYWORDS: Flexible devices; Lithium metal battery; Integrated electrode; Dendrite-free; Lithiophilic alloy

TOC GRAPHICS


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Introduction To accommodate increasing renewable energy forms and transportation electrification, the development of alternative battery chemistries is of paramount importance to unlock the performance limits of higher energy/power densities and longer lifetime at a lower cost.1,2 Metallic lithium is considered as an ideal anode in the rechargeable batteries due to the highest theoretical specific capacity (3860 mAh g-1), relatively low density (0.53 g cm-3) and the lowest redox potential (-3.040 V vs. the standard hydrogen electrode).3 Unfortunately, the metallic Li would plate on the current collector with uncontrolled dendritic morphology as the lithium ions meet electrons. This parasitic process tends to pierce the solid electrolyte interphase (SEI) layer and the separator, which eventually shorts the cell and causes the fatal failure of the batteries. These safety concerns thus plague the practical deployment of rechargeable Li metal batteries.4-7 Sand’s time model has been proposed to describe the nucleation process for the metallic dendrite.8 As showed by the Equation (1) depicted as follows, the initial concentration of electrolyte (C0) is proportional to the initiation time of dendrite formation (τ), while the effective current density (J) and anionic transference number (ta) inversely correlate with τ. 𝜏 = 𝜋𝐷

( ) 𝑒𝐶0

2𝐽𝑡𝑎

(1)


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To decrease the anionic transference (ta) and delay the dendrite nucleation, additives such as fluorinated compounds (FEC, Fluoroethylene carbonate) or LiNO3 were dissolved into the carbonate electrolytes to stabilize the SEI layer and thus to depress the anionic transference.9,10 However, the polymeric nature of the SEI layer exhibited the insufficient mechanical strength to accommodate the infinite volume propagation during the Li plating process. As a result, the anode pulverization would compromise the Coulombic efficiency (CE) and cyclability during the battery operation. To further decrease ta via the interfacial layer with mechanical strength and chemical/electrochemical stabilities, artificial protection layers composed of,11-16 for instance the functionalized carbon layer,11 the boron nitride layer,14 the Al2O3 layer15,16 and the Li3PO4 layer12 were developed to prohibit the vertical growth of lithium dendrites. However, these protective layers loaded on the rigid metallic foils would crack upon the long-term cycling, and thus fail to balance the protection functionality with the mechanical strength at the flexing states. Another effective strategy was to distribute the effective current density (J) by enlarging the specific surface area of the electrode substrate. For instance, three-dimensional (3D) current collectors, namely the porous copper foil,17 submicron metallic fibers,18 3D graphene monolith19 were employed to delay the Li nucleation process and thus mediated the morphology of plated lithium. Besides tuning key parameters in Sand’s equations for suppressing the dendrites, Cui’s group also reported the strategy of introducing the lithium-metallic alloys.20 For instance, the substrates which exhibit a definite solubility in lithium, such as Au, Ag, Zn, Mg, Al and Pt metallic foils, tend to reduce the nucleation barriers. It was concluded that metallic lithium alloys induced the preferential metallic growth by mediating the Li nucleation pattern. However, the alloy formation still involved huge volume expansion and the electrical disintegration from the substrates, and thus to deteriorate the CE upon the long-term cycling. In this sense, incorporation


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of these Li-metal alloys as the nucleation seeds in a volumetric buffer matrix could be a viable strategy to render a more mechanically robust protective layer with compliant lithium nucleation behaviors. In this report, we proposed a structural design of a lithium dendrite blocking layer which composed of the hollow SiO2 nanospheres with size-tunable Ag nanoparticles (NPs) decorated on the surface. Acting as the “artificial protective interfacial films” for lithium dendrites, the SiO2 nanosphere layer not only afforded the porosity for accommodating the guest metallic species, but also demonstrated the good mechanical strength to dissipate the volume expansion of the deposited lithium.21-23 Additionally, size-tunable Ag NPs were in-situ decorated on the SiO2 hollow spheres via a facile precipitation process. Alloying with lithium ions, the lithiophilic Ag NPs seeded the preferential lithium nucleation and, more importantly, regulated the lithium growth into round dots instead of dendrites within the mesopores. These complementary components collectively rendered the excellent electrochemical performances of Ag NPs@SiO2coated Li-metal anode in terms of the high CE, long cycle life, and low plating overpotential (40 mV at 1 mA cm–2) with the reactive carbonate electrolytes. Two full-cell prototypes were constructed by pairing the Ag NP@SiO2-coated Cu foil anodes with the lithium manganese oxide (LMO) or sulphur (S) cathodes, respectively. The specific energy density of 359.6 Wh kg-1 was estimated for the Li-LMO full-cell model. When the Li-S battery was galvanostatically cycled at 0.82 C, the energy density of 816.7 Wh kg−1 and power density of 408.4 W kg−1 were simultaneously realized. Furthermore, the Ag NP@SiO2 blocking layer was integrated with a SiO2-modified polyethylene (PE) separator to establish a flexible lithium metal battery configuration, in which the usage of metallic current collector could be eliminated. Stable cycle


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behaviour up to 1750 h and the structural robustness under the different bending states demonstrated the potential use of this integrated battery design in flexible electronics. Results and discussion To elucidate the size-dependent effect of Ag NPs on the lithium nucleation pattern, various Ag NP@SiO2 composites were fabricated with the precise control over the in-situ precipitation process of Ag. When the reaction time was set as 10 min, 20 min, 30 min, and 4 h, the composite electrodes were designated as Ag NP@SiO2-10min, Ag NP@SiO2-20min, Ag NP@SiO2-30min, and Ag NP@SiO2-4h, respectively. The scanning electron microscope (SEM) image of the Ag NP@SiO2-20min composite, shown in Figure 1a as an example, exhibited the aggregates composed of SiO2 nanospheres (SiO2 NS) with the diameter of ~ 80-100 nm. Transmission electron microscope（TEM）images of the Ag NP@SiO2-20min composite (Figure 1b and 1c) exhibited the uniform distribution of Ag NPs with the diameter of ~ 4-5 nm both within the interior (Figure 1b) and on the surface (Figure 1c) of the hollow SiO2 NSs. The enlarged image of the white square (inset Figure 1b) revealed the estimated shell thickness of SiO2 NS of ~ 10 nm. Based on the representative region marked by a white square in Figure 1c, the lattice fringes spaced by 0.24 nm apart match to (111) planes of the hexagonal phase of Ag（PDF#41-1402） (Figure 1d). Figure S1 also exhibited the morphology and microstructure of the as-fabricated Ag NP@SiO2 composites from the control experiments. The average size of Ag NPs measured as ~ 3 nm for Ag NP@SiO2-10min (Figure S1a-c), largely hinged on the duration of precipitation process, while Ag NPs with the diameter range of 6-9 nm were uniformly distributed on the SiO2 NSs as observed in the electron microscopic images of Ag NP@SiO2-30min (Figure S1d-g). Noted the as-reduced Ag NPs were evenly distributed on the silica shell for all the Ag NP@SiO2 composites without particle agglomeration, this characteristic would favor the uniform


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lithiophilic nucleation across the electrode. Based on the SEM and TEM images of SiO2 NS precursor (Figure S1h and S1i), the diameter of the hollow silica was estimated as ~ 80 nm with the shell thickness measured as ~ 10 nm. These parameters were consistent with other Ag NP@SiO2 products, suggesting the preservation of silica shell upon the in-situ precipitation of Ag NPs on the SiO2 NS. To explore the crystal growth limit of Ag NPs, the chemistry precipitation process was extended to 4h. It was observed that Ag NPs had agglomerated into the particles of a few hundreds of nanometers (Figure S2a-f). The elemental maps of Ag, Si, and O in Energy-dispersive X-ray spectroscopy (EDX) tests showed the encapsulation of the Ag particles within the silica sheath. However, the distribution of the Ag particles was quite random, which would render the non-uniform current density across the electrode. As shown in Figure 1e, the photo-emission spectrum for the core-level Ag 3d revealed the peaks situated at the binding energy (BE) of 371.1 and 365.1 eV, which indicated the reduction of the AgNO3 precursor into the zero-valent Ag NPs in the composite. The nitrogen adsorption-desorption isotherm curve of the Ag NP@SiO2-20min (Figure 1f) exhibited a type IV characteristic with a distinct hysteresis loop within the P/P0 range of 0.45-1.0,24,25 suggesting the typical mesoporous silica structures. Based on the Barret-Joyner-Halender model, the pore size was measured within the range of 2 to 4 nm. The mesoporosity contributed to a Brunauer-Emmett-Teller (BET) surface area of 215.6 m2 g−1 and a pore volume of 0.65 cm3 g−1. Incorporation of the mesopores and hollow cavities within the SiO2 NS could not only buffer the significant volumetric expansion of the deposited lithium, but also sustain an evenly distributed Li+ flux across the electrode and thus the reduced effective current density (J).


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Figure 1. (a) SEM image of Ag NP@SiO2-20min. TEM image of (b) Ag NP@SiO2-20min and high-resolution TEM (inset image) of the shell of SiO2 NS. HRTEM image of (c) the decorated Ag NPs on the Ag NPs@SiO2-20min composite. (d) HRTEM image of a representative Ag NP and the spacing lattice marked by the white arrows. (e) The Ag 3d core-level XPS spectrum of Ag NP@SiO2-20min. (f) N2-sorption isotherm and BJH pore size distribution (shown in the inset) of the Ag NP@SiO2-20min composite. Figure 2 schematically illustrates the protected Li-Ag alloying mechanism of the Ag NP@SiO2 composite layer to regulate the lithium nucleation behavior. The electrochemical reduction of Li+ enables its preferential alloying process with Ag (process I) in consideration of the relatively higher potential of Li-Ag alloying process (~0.1 V vs. Li+/Li) as compared to the


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plating of lithium metal (below 0 V vs. Li+/Li). Afterwards, the Ag-Li alloy tends to seed the continuous nucleation of deposited lithium (process II). Furthermore, the hollow cavities of SiO2 NS accommodate the volume expansion of the deposited lithium, preventing the vertical propagation of dendrites from piercing the separator. Theoretically, this structure regulates the nucleation process and growth model of metallic lithium, and thus inhibits the detrimental dendrite growth.

Figure 2. Schematic illustration of the protected Li-Ag alloying mechanism of the Ag NP@SiO2 composite layer in prohibiting the dendrite formation. (Ag NPs act as lithiophilic materials to seed the formation of Li-Ag alloys (the process I), and silica shell provides the mechanical strength to inhibit the dendrite penetration (the process II)) To validate the effectiveness of this structural configuration for reversible Li plating/stripping process, the Ag NP@SiO2 composite layers were directly assembled versus Li foils for galvanostatic measurements in half-cells. Figure 3a compared the voltage profiles of the Ag NP@SiO2-20min electrode and the bare Li foil counterparts in the symmetric Li cells at the current density of 1 mA cm–2.26-31 The Li||Ag NP@SiO2-20min half-cell exhibited stable voltage profiles with small hysteresis of about 40 mV for 420 h, whereas the Li||Li symmetric cell


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displayed a more pronounced and fluctuating hysteresis, which gradually decreased from 100 mV to 60 mV upon the continued cycling for 420 h. As shown in Figure 3b, the cycle performance of the symmetric cells during the period from 100 to 200 h were further amplified. A flat voltage plateau of the Li||Ag NP@SiO2-20min, upon the lithium plating/stripping cycles was retained without fluctuation in voltage hysteresis. In stark contrast, the bare Li||Li electrode exhibited the fluctuating voltage plateau with the higher overpotential at both the initial and final stages of stripping/plating process. When the upper cut-off voltage was set as 1.0 V and the total deposited lithium amount was fixed at 4 mA h, as illustrated in Figure 3c, the overlapped 5th, 15th, and 20th plating/stripping cycle curves demonstrated stable capacity retention of the Ag NP@SiO2-20min electrode after activation of 5 cycles. The Ag NP@SiO2-20min coated on Cu foil electrode showed the best reversibility of Li deposition/stripping with a rather stable overpotential hysteresis of 15 mV (Inset image of Figure 3c). Despite the good cyclability of both the Ag NP@SiO2 NS-10min and Ag NP@SiO2 NS-30min electrodes, the overpotential gaps were measured to be 30 mV for the Ag NP@SiO2-10min electrode (Figure S3a) and 22 mV for the Ag NP@SiO2-30min electrode (Figure S3b). As compared in Figure 3d, the initial CE of the Ag NP@SiO2-20min coated Cu foil was about 89% (8h plating/stripping at 0.5 mA cm-2), while the bare Cu foil exhibited a similar CE of ~85%. Upon the continuous 150 repeated plating/stripping cycles, the CE of the Ag NP@SiO2-20min coated Cu foil remained above 95% due to the dendrite-free Li deposition. However, the CE of the bare Cu foil dropped to ~75% after 15 cycles and below 50% after 150 cycles. When the current density was further increased up to 1.0 mA cm-2 and 2.0 mA cm-2, the CE of the bare Cu foil showed drastic fluctuations upon 150 plating/stripping cycles. Specifically, the value dramatically decreased from 85% to 17% at 1.0 mA cm-2, and from 83% to 7% at 2.0 mA cm-2, respectively. The CE fading might be


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attributed to the irreversible lithium consumption of the plating process and the separator failure under partial short-circuit scenarios. However, the Ag NP@SiO2-20min coated Cu foil showed a rather satisfactory CE of higher than 93% at 1.0 mA cm-2 and 91% at 2.0 mA cm-2 from the second cycle onwards. As indicated in Figure 3e, there was an apparent gentle voltage drop for the initial Li plating process with nucleation overpotential of ~ 5 mV for the Ag NP@SiO220min electrode, whereas the abrupt voltage drops at the initial plating process were observed for both the Ag NP@SiO2-10min and the Ag NP@SiO2-30min electrodes.32-35 The nucleation overpotential of Li plating was increased to 32 mV for the Ag NP@SiO2-10min electrode since the Ag-Li alloying effect weakened due to the smaller Ag NPs of ~ 3 nm. This trend prevailed in terms of the nucleation overpotential of 240 mV for the Cu foil without the formation of Ag-Li alloy (Figure 3f), which indicated the crucial role of Ag NPs in the composite layer in facilitating the lithium nucleation process and inhibiting the nucleation overpotential via the lithiophilic LiAg alloy intermediates. As compared to the Ag NP@SiO2-30min electrode, the increased diameter of Ag NP of ~ 6-9 nm further increased the nucleation overpotential of Li plating to 27 mV. As a result, we concluded that the Ag NP@SiO2-20min composite layer afforded enough space for Li plating and the uniform distribution of Ag NPs of ~ 4-5 nm favored the formation of Ag-Li alloy with the smallest voltage hysteresis and nucleation overpotential. These observations suggested that a range of factors had to be optimized, e.g., the appropriate size range and spatial distribution of Ag NPs, adequate buffering space for volume expansion, uniform current density distribution as well as the compact component arrangement, all of which collectively harmonized into a mechanically durable and dendrite-free structure. In our study, this Ag NP@SiO2-20min composite layer was the closest to the sweet spot.


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Figure 3. Electrochemical characterization of various Ag NP@SiO2 composite electrodes. (a) Galvanostatic cycling of a symmetric Li||Ag NP@SiO2-20min electrode (red) and Li||Li metal (black) at the current density of 1mA cm−2. (b) The enlarged view of the voltage-time curve of the symmetric Li||Ag NP@SiO2-20min cell during 100 h to 200 h. (c) Voltage profiles of the Ag


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NP@SiO2-20min electrode for the 5th, 15th and 20th cycles with the upper cut-off voltage of 1.0 V. Image in Figure 3c is an enlarged view of the bottom plot which indicated the voltage hysteresis of 15 mV. (d) Comparison of the CE of the Ag NP@SiO2-10min coated Cu foil and the bare Cu foil for 150 cycles at 0.5, 1.0, and 2.0 mA cm-2, respectively. The predetermined deposition capacity of Li is fixed at 1 mA h cm-2. (e) Comparison of the overpotential hysteresis of voltage profiles of Ag NP@SiO2-10min, Ag NP@SiO2-20min, and Ag NP@SiO2-30min electrodes during the 1st plating/stripping cycle. The nucleation overpotential of Ag NP@SiO210min, Ag NP@SiO2-20min, and Ag NP@SiO2-30min are 32 mV, 5 mV and 27 mV. (f) Voltage profiles for Li plating and stripping process on a copper-foil for the 1st, 2nd, 5th, 25th.

Figure 4. (a) The initial four CV cycles for the Ag NP@SiO2-20min electrode (A Solartron electrochemical workstation is employed for cyclic voltammetry (CV) tests at a scan rate of 0.1 mV s-1 between -0.1 to 0.55 V). (b) The schematic illustrates the transport mechanism of Li+ between two electrodes in the half-cell models and electron in the anode. We further investigated the electrochemical process of the Ag NP@SiO2-20min half-cell versus Li foil as the reference/counter electrode through cyclic voltammetry (CV) analysis. As shown in the 1st cycle CV curve in Figure 4a, the notable board cathodic peak ranges from 65 mV to -5 mV could be attributed to the alloying process of Ag NPs with Li+ to form the Li-Ag


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alloy intermediates. The cathodic peak ranging from -30 mV to -100 mV indicated that the Ag-Li alloy tended to seed the continuous nucleation of depositing lithium. For the anodic sweep, the pronounced delithiation peaks at 82 mV suggested the stripping process, while the anodic peaks situated at 179 mV and 360 mV corresponded to the continuous dealloying process of the Ag-Li intermediates.36-40 From the second cycle onwards, the CV curves well overlapped, indicating excellent reversibility of the cell. Figure 4b schematically illustrates the Li+ diffusion and electron transfer paths in the cells. Conductive carbon additives (Super P) wrap Ag NP@SiO2 nanospheres to establish a 3D electron pathway to facilitate the electron transfer from the current collector to Ag NPs. On the other hand, the electrolyte percolates the Ag NP@SiO2 composite layer and enables the combination of the Li+ with the electron upon the alloying process with Ag NPs. The formation of the Li-Ag alloy intermediates was reflected by the cathodic peak ranging from 65 mV to -5 mV in the CV analysis. To further examine the post-mortem morphology, the SEM images of the Ag NP@SiO2-20min electrode after the 1st and 100th cycle were compared in Figure 5. After the 1st Li deposition, the top-view (Figure 5a) and cross-sectional (Figure 5b) SEM images of the Ag NP@SiO2-20min electrode showed the effective suppression of the Li dendritic growth. After 100 cycles of severe Li plating/stripping process, a densely-compacted smooth texture was well-maintained for the composite electrode with a relatively flat surface (Figure 5c). The dendrite-free deposition process was further supported by the cross-sectional SEM analysis (Figure 5d), in which no cracks or localized dendrite formation were observed. In sharp contrast, the metallic dendrites with the typical randomly arrayed arms were marked on the copper foil after the 1st Li deposition, as shown in the top-view (Figure 5e) and cross-sectional SEM images (Figure 5f). After 100 cycles of Li deposition/dissolution process, top-view (Figure 5g) and cross-sectional (Figure 5h) SEM images demonstrated the ramified morphology of more


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pronounced protruding branches, suggesting the development of the metallic dendrites (indicated by the white arrows).

Figure 5. (a) Cross-sectional and (b) top-view SEM images of the Ag NP@SiO2-20min supported on Cu foil after the 1st plating process. (c) Cross-sectional and (d) top-view SEM images of the Ag NP@SiO2-20min supported on Cu foil after 100 cycles of Li plating/stripping process, and (e) Cross-sectional SEM image and (f) top-view SEM image of the Cu foil anode


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after the 1st lithium plating process. (g) Cross-sectional SEM image and (h) top-view SEM image of the Cu foil after 100 cycles of Li plating/stripping process. (The default lithium plating process was controlled to be 8 h at 0.5 mA cm-2) To deliver a more informative performance demonstration, two prototype full cells were constructed by employing the Ag NP@SiO2-20min supported on Cu foil as the anode.41 Firstly, we fabricated a modified LMO cathode, and the preparation procedures were described in the experimental section. Figure S4 recorded the galvanostatic charge-discharge cycles for the modified LiMn2O4 cathode measured at 0.8 C (1 C = 120 mA g-1), and the electrochemical tests demonstrated the reversible capacity of ~ 110 mA h g-1 upon the continued cycling and the discharge-charge curves almost overlapped. When the modified LMO cathode was paired with the Ag NP@SiO2-20min anode, the full-cell prototype demonstrated an impressive cycling efficiency for 50 cycles at 0.82 C. As shown in Figure 6a, the first discharge and charge capacities of the LiMn2O4||Ag NP@SiO2-20min full cell were recorded as 113.7 mAh g-1 and 117.6 mAh g-1, respectively, resulting in a CE of 98.9%. From the second cycle onwards, the discharge and charge curves revealed a rather robust structural stability and cycle reversibility. Figure 6b showed the cycling performance of the LiMn2O4||Ag NP@SiO2-20min full cell at 0.82 C. The full cell maintained a high reversible capacity of ~ 112 mAh g-1 after 50 cycles with CE higher than 99.0 %. According to the equations as described in the Supporting information, we calculated the specific energy and power density of this full cell model as 359.6 Wh kg-1 and 294.9 W kg−1, respectively. Additionally, we constructed a Li–S full-cell prototype by pairing the Ag NP@SiO2-20min anode with a modified sulphur cathode. Based on the discharge-charge curves shown in Figure 6c, the first discharge and charge capacities of the S||Ag NP@SiO220min cell model were 1104.5 mAh g-1 and 1173.1 mAh g-1, respectively, with a CE value of


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94.2%. The stable electrochemical performance of the full cell configuration was attributed to the effective sulfur utilization as well as inhibition of the dendrite growths on the anode side. Figure 6d presented the cycling life of the S||Ag NP@SiO2-20min cell measured at 0.5 C, delivering a specific capacity (concerning the mass of sulfur) of ~997.5 mAh g−1 and capacity retention of 91% after 150 cycles. This corresponded to an average capacity decay of 0.06% per cycle with an average CE maintained higher than ~ 98.5 %. Based on the data presented in Figure 6d, the specific energy and power densities of the full cells were 816.7 Wh kg

−1

and

408.4 W kg−1 (as calculated in Supporting information), respectively.42 The postmortem characterizations as shown in Figure S5 exhibited the cross-sectional and top-view morphologies of the Ag NP@SiO2-20min protective layer coated on the Cu foil, as the composite anode was disassembled from the S||Ag NP@SiO2-20min full cell after 150 cycles. Similar to the dendritefree morphology as shown in the half-cell (Figure 5c and 5d), the cycled Ag NP@SiO2-20min composite electrode maintained the densely-compacted structure with a relatively steady, flat surface. These results validate that our composite anode design could effectively avoid the formation of the protruding dendrites and the rugged surface upon the long-term cycling, which increases the uniformity of electrodeposited lithium on the electrode surface.


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Figure 6. Electrochemical performance of various cathode||Ag NP@SiO2-20min full cells. (a) Galvanostatic discharge-charge curves of LiMn2O4||Ag NP@SiO2-20min full cell during the 1st, 10th, 20th and 50th cycles; (b) Cycling stability of LiMn2O4||Ag NP@SiO2-20min at 0.82 C for 50 cycles; (c) Galvanostatic discharge-charge curves of S||Ag NP@SiO2-20min full cell during the 1st, 2nd, 10th, 50th and 75th, 100th, 150th cycles. (d) Cycling stability of S||Ag NP@SiO2-20min full cell at 0.5 C for 150 cycles. Inset image in Figure 6d: The schematic illustrates the transport mechanism of Li+ between two electrodes in the full-cell models.


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To meet the requirements of the emerging applications such as implantable biomedical devices or wearable electronic devices, tremendous research interests have been focused on the integrated, current-collector-free electrodes which could sustain the mechanical strain during flexing. The metallic battery configuration of such type has not been investigated thus far, since the precise control over mechanical stress and structural robustness under flexing states was rather challenging due to the infinite lithium plating/stripping process. We thus demonstrated a flexible lithium metal battery design by integrating the Ag NPs@SiO2 dendrite blocking layer onto a robust, yet flexible modified polyethylene (MPE) separator. The nanosilica was cast onto the commercial PE membrane to reinforce the separator, and the modification procedures were described in the experimental section. MPE exhibits the improved mechanical strength limit of 99.6 MPa, which is 12% higher than the unmodified PE separator (89.4 MPa) (Figure S7). The Ag NP@SiO2-20min composite layers of different thickness, i.e. 5, 10 and 15 m, were cast onto the MPE separators to form the integrated electrodes (MPE-Ag NP@SiO2-20min-5, MPE-Ag NP@SiO2-20min-10 and MPE-Ag NP@SiO2-20min-15 respectively) (Figure S6a). The crosssectional morphology of the MPE-Ag NP@SiO2-20min-5 and elemental distributions of Ag, Si, and O (Figure S6b) demonstrate a close packed layer-by-layer structure with the Ag NP@SiO220min composite directly integrated on the MPE separator. In the flexible symmetric cell configurations, the MPE-Ag NP@SiO2-20min-5 was assembled as integrated anode versus Li foil for galvanostatic measurements. Figure 7a records the voltage profiles of MPE-Ag NP@SiO2-20min-5 under the flat and the bending state at 1 mA cm–2. The stable voltage profiles with a small hysteresis of ~20 mV maintained for 1750 h. The voltage profile of the symmetric cells during the period from 1580 to 1650 h were further amplified to show the electrochemical performance under the bend state (marked by the black dot squre). Obviously, flat voltage


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plateau of the lithium plating/stripping cycles was retained without fluctuation in the voltage hysteresis upon flexing (Figure 7b). Figure 7c demonstrated the current-potential response of the MPE-Ag NP@SiO2-20min composite films (5 cm×1 cm) with various loading thickness. When the potential was swept between -0.3 and 0.3 V with a scan rate of 10 mV s

−1,

no difference

could be observed in the current response for all the integrated composite films under flat, bent and twisted states, indicating that the electrical continuity of the integrated configuration (MPEAg NP@SiO2-20min) upon mechanical flexing. Notably, the resistance or conductivity of the MPE-Ag NP@SiO2-20min-5 and MPE-Ag NP@SiO2-20min-10 was almost unaffected by the deformation, suggesting the good mechanical flexibility of the integrated electrode (Figure 7d). The good mechanical flexibility can be ascribed to the intimate coupling of mechanically robust Ag NP@SiO2 composite layer and MPE. In comparison, the integrated electrodes with relative thick loading (MPE-Ag NP@SiO2-20min-15) or composite layer without the decorated Ag NPs (MPE-SiO2-5) demonstrated the significant conductivity change upon flexing. As shown in Figure S7, the mechanical strength limit of the integrated MPE-Ag NP@SiO2-20min-5 electrode was evaluated as 98.7 MPa, which exhibited the similar value as MPE. Additionally, the tensile response of the MPE-Ag NP@SiO2-20min-5 demonstrated the ductility of 163.3% before the failure point. This elasto-plastic response of the integrated electrode renders the structural robustness of the MPE-Ag NP@SiO2-20min-5 on the device level.43


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Figure 7. (a) Galvanostatic cycling of a symmetric Li||MPE-Ag NP@SiO2-20min-5 separator electrode at the current density of 1mA cm−2. (b) The enlarged view of the voltage-time curve of the symmetric Li||MPE-Ag NP@SiO2-20min-5 separator cell during 1580 h to 1650 h. (c) Conductivity of the MPE-Ag NP@SiO2-20min film at various strained states. (d) Resistance of the flexible MPE-Ag NP@SiO2-20min film at various bending states. Conclusion In summary, we designed a dendrite blocking layer composed of the size-tunable Ag NPs uniformly decorated on the mesoporous SiO2 NSs. The mechanical strength of SiO2 hollow spheres synergized with the decorated lithiophilic Ag NPs, which collectively regulated the


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lithium nucleation and deposition behaviors via a protected Li-alloying process. When the symmetric cell of Li||Ag NP@SiO2-20min was operated at a current density of 1 mA cm−2, a stable plating/stripping process was maintained for over 400 h with a low voltage hysteresis of ~ 40 mV. Furthermore, we paired the Ag NPs@SiO2-20min composite anode with the LiMn2O4 or the modified sulphur cathodes in the full-cell prototypes, and high specific energy densities with satisfactory CE were realized for both models. Finally, we integrated the modified polyethylene (PE) separator with the Ag NPs@SiO2 dendrite blocking layer to develop a flexible lithium metal battery. Electrochemical cycling stability under different flexing states was maintained up to 1750 h. Our approach experimentally elucidated the dependence of uniform Li+ flux, deposited lithium nuclei morphology, cycling stability as well as the nucleation overpotential on the rational structural design of the Ag NPs@SiO2 protective layer, and thus enabled the feasible utilization of metallic lithium in the flexible electronics.

ASSOCIATED CONTENT Supporting Information Experimental section, additional characterization of materials (SEM images, TEM images, Stress-Strain curves,etc.), additional electrochemical performance (charge-discharge curves) AUTHOR INFORMATION Corresponding Author: * E-mail: [email protected]; * E-mail: [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT We acknowledge the financial support of this work by the National Natural Science Foundation of China (51602261 and 51711530037), the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No.160-QP-2016); and Young Talent fund of University Association for Science and Technology in Shaanxi, China.

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An Integrated, Flexible Lithium Metal Battery with Improved

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