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A Biobased Composite Gel Polymer Electrolyte with Functions of Lithium Dendrites Suppressing and Manganese Ions Trapping Ming Zhu, Jiaxin Wu, Wei-Hong Zhong, Jinle Lan, Gang Sui,* and Xiaoping Yang* guaranteeing a high working voltage of a cell.[3] Besides, the total weight of the finished batteries significantly deceases because utilization of lithium (Li) metal anode can exclude the current collectors in traditional anodes. However, the uncontrollable Li dendrites growth in the repeated Li deposition/stripping processes has restricted the development of rechargeable LMBs for a long time.[2,4–6] When the dendrites grow through the separator and then connect the cathode and anode during the charge–discharge cycling, it can cause a danger of catching fire due to the resulting short-circuit.[7] Furthermore, the growth of Li dendrites can lead to a continuous consumption of the Li metal in the process of cycling, while the continuous breakdown/reconstruction of solid electrolyte interphase (SEI) will result in a low coulombic efficiency of LMBs.[8] To develop high performance energy storage devices, the researchers also pay much attention on some promising cathode materials by addressing the known “shuttle” effect of soluble polysulfides for the sulfide cathodes[9] and dissolution issues of transition metal (TM) ions for the mixed TM oxide cathodes.[10] Among all of TM oxide cathodes, the spinel LiMn2O4 (LMO) is one of the most promising positive active materials for large-format applications compared with other commercial cathode materials (layered LiCoO2, LiNiO2, LiNixMnyCo1-x-yO2, etc.) because of its excellent electrochemical performance, high thermal stability, low cost, and low toxicity.[11] However, according to the existing reports, the LMO in Li-ion batteries (LIBs) with lithium hexafluorophosphate (LiPF6) electrolyte solutions could experience an appreciable capacity fading and a reduced lifetime, resulting from Mn ions dissolution, migration, and deposition on the negative electrode, especially during exposure to a high temperature.[12] The irreversible loss of Mn disrupts structural integrity of active material in the cathode and the dissolved Mn ions migrate to the anode and damage the SEI layer followed by causing serious degradation of capacity during charge/discharge cycles.[13,14] In the past few years, various strategies have been undertaken to suppress or even block the growth of Li dendrites, such as optimizing SEI with selected solvents,[15,16] solid-state electrolytes,[4,17] separators,[18] and delicate engineering of

Lithium (Li) dendrites in Li anodes, and dissolution and migration of manganese (Mn) ions in LiMn2O4 (LMO) cathodes, have hampered these extraordinary electrode materials from being efficiently applied in high performance Li batteries. Here, a novel, bifunctional, biobased composite gel polymer electrolyte (c-GPE) is created to simultaneously deal with the two critical issues. The skeleton of c-GPE is constructed from a sandwich structure composed of porous polydopamine spheres and two layers of the environmentally friendly soy protein isolate-based nanofiber membranes, and the carbonized polydopamine spheres are coated without any binder on the surface of the membranes. After a facile and innocuous preparation process, the skeleton material displays excellent thermal stability and good affinity to liquid electrolyte, which endows c-GPE with significant functions of effective mitigation of the dissolution of Mn ions, and chelation of the fleeing Mn ions, as well as the dramatic suppression of Li dendrite growth. Consequently, the LMO/ Li batteries involving c-GPE show a great improvement in the cycling stability and rate performance compared with those of the cells based on commercial Celgard 2400. This work will be quite promising to meet the distinct requirements from Li batteries and provide a high-efficiency and safe biobased GPE for next generation energy storage systems.

1. Introduction With the development of electric vehicles and consumer electronics, there are growing demands for high-energy-density storage systems. Among them, lithium metal-based batteries (LMBs) are an ideal candidate and have received considerable attention in recent years.[1,2] As the anode for LMBs, metallic lithium delivers high theoretical specific capacity of 3860 mA h g−1, low density of 0.59 g cm−3, and extremely negative potential of −3.04 V versus a standard hydrogen electrode, Dr. M. Zhu, Dr. J. Wu, Dr. J. Lan, Prof. G. Sui, Prof. X. Yang State Key Laboratory of Organic-Inorganic Composites College of Materials Science and Engineering Beijing University of Chemical Technology Beijing 100029, China E-mail: [email protected]; [email protected] Prof. W.-H. Zhong School of Mechanical and Materials Engineering Washington State University Pullman, WA 99164, USA The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201702561.

DOI: 10.1002/aenm.201702561

Adv. Energy Mater. 2018, 1702561

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electrode.[6,19,20] For example, Cui and co-workers[16] added lithium polysulfide (Li2S8) and lithium nitrate (LiNO3) into the ether-based electrolyte to stabilize and reinforce SEI layer on the Li metal anodes. However, the mechanical strength of the SEI layer was not sufficient to suppress the growth of Li dendrites. Guo and co-workers[21] showed that a 3D current collector with a submicrometer-sized skeleton and porous structure could significantly improve the electrochemical deposition behavior of Li, but the process of fabricating 3D-Cu current collector was difficult to implement and very costly. Yang and co-workers[22] prepared a tough and compact 3D-network gel polymer electrolytes (GPEs), which exhibited a combination of mechanical strength, ionic conductivity, and effective suppression of Li dendrites growth. However, preparation of the GPEs through ring-opening polymerization reaction was complex and not environmentally friendly, and the electrochemical properties of the GPEs they obtained were not very satisfying because of the limited uptake ability of the casting membranes for liquid electrolytes. Hu and co-workers[7] fabricated a thermally conductive separator coated with boron-nitride (BN) nanosheets to improve the stability of the Li metal anodes, while the insufficient wettability between BN and liquid electrolytes may blemish the ion transport and BN nanosheets were easy to detach from the polymeric separators owing to the poor adhesion. Therefore, more applicable and efficient strategies to supress Li dendrites are highly needed. In the meantime, some measures for preventing the TM ion dissolution and migration have been also proposed and investigated, including selected electrolytes,[23] modified separators,[13,24] and new synthetic cathodes.[25] Chen and co-workers[26] proposed a novel rigid-flexible GPE, which can significantly suppress the dissolution of Mn ions from surface of LMO because of strong interaction energy of Mn ions with the soft segments of poly(ethyl α-cyanoacry-late) incorporated, while the efficiency in Mn ions trapping was not desirable. Aurbach and co-workers[24] prepared a separator to chelate Mn ions and scavenge trace hydrofluoric acid, which was made by embedding the poly(ethylene-alternate-maleic acid) di-lithium salt polymer into a poly(vinylidene fluoride-hexafluoropropylene) copolymer matrix. However, the synthesis of the composite separator was tortuous and the separator could not ensure a safe and efficient operation of Li batteries for a long time. Although tremendous efforts have been devoted to improve the electrolytes or separators, there is rarely work on synchronously solving the issues of Li dendrites and TM ions dissolution and migration. Here, we report a novel bifunctional biobased composite GPE (c-GPE) that was designed to concurrently deal with the two issues of Li dendrites and Mn ions dissolution and migration to extend the lifetime of Li-based rechargeable batteries. The skeleton of the c-GPE was consisted of a layer of mesoporous polydopamine spheres (PDSs), two layers of nanofiber membranes containing biodegradable soy protein isolate (SPI), and a layer of mesoporous carbonized polydopamine spheres (CPDSs). In addition, no toxic or polluting chemicals were involved in the preparation of the composite skeleton material. The musselinspired PDS and CPDS involved in the skeleton materials cannot only improve thermal stability of the c-GPE because polydopamine can not only maintain its physical strength up to 200 °C[27] but also contribute to the electrochemical


performance because these nanospheres had high porosity and plenty of polar functional groups (CN, CN, CO, OH, and NH) to increase the uptake amount of liquid electrolyte.[28] Besides, PDS with catechol moieties exhibited extraordinarily strong adhesion, which was wet-resistant and thus even effective in liquid environments.[27] Therefore, PDS had been demonstrated to adhere and trap Mn ions. At the same time, the CPDS have excellent chemically stability, good electronic conductivity, high porosity, and abundant surface lithiophilic groups, such as pyridinic and pyrrolic nitrogen, and in consequence, the c-GPE with CPDSs could suppress Li dendrites growth due to the homogeneous mass and charge transfers across the Li/electrolyte interface. The study results indicated that the electrochemical properties of Li batteries involving the novel c-GPE were significantly improved on account of successfully achieving Li dendrites suppressing and Mn ions trapping simultaneously.

2. Results and Discussion In this study, a kind of original skeleton structure for c-GPE was designed to specifically deal with the two common and critical technical issues, Li dendrites and the dissolution and migration of Mn ions, occurring in the battery with Li metal as anode and LMO as the cathode. The schematic diagram for the fabrication process of the skeleton materials is shown in Figure 1a. A layer of PDSs was sandwiched by two layers of the environmental and porous SPI/polyvinyl alcohol (PVA) nanofiber membranes prepared by electrospinning, and a layer of CPDSs dispersed into ethanol was filtrated on the top of membranes. The PDSs and PDCSs dispersed into ethanol were filtrated on the environmental and porous SPI/PVA nanofiber membranes prepared by electrospinning. The packing of the CPDS nanoparticles would provide good mechanical protective layer for Li dendrites suppression and the packing of the PDS could adhere and trap Mn ions and form another physical barrier to block Li dendrites. The PDSs and CPDSs filtrated on the nanofiber host exhibited uniform and dense configuration, as shown in Figure S1 (Supporting Information), which was beneficial for their adhesion with gel-nanofiber to form the integrated c-GPE. After the gelation of nanofiber membranes, they can strongly stick to PDSs and CPDSs with their polar functional groups to block free migration of nanoparticles (see their polar groups in Figure S2, Supporting Information). The optical photographs of the skeleton c-GPE before and after absorbing liquid electrolytes are shown in Figure S3a–c and Figure S3d,e of the Supporting Information, respectively. It can be observed that the nanofiber membranes were keeping integrity of its original morphology and the CPDSs were well distributed on the surface of c-GPE. Moreover, the c-GPE was folding and flexible. This result suggested the c-GPE could be desired to accommodate the volumetric changes of Li deposition. To further investigate the morphology of SPI/PVA nanofiber membranes, PDS layers and CPDS layers, the magnified scanning electron microscopy (SEM) analysis and transmission electron microscopy (TEM) analysis were performed and the results are shown in Figure 1b–g. It can be seen that the nanofibers were interlaced with each other randomly and formed a 3D network structure,

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Figure 1. a) A schematic diagram for the fabrication process of skeleton materials of composite biobased gel polymer electrolyte (c-GPE). The SEM images of b) SPI/PVA nanofiber membranes, c) polydopamine sphere (PDS) layer, and d) carbonized polydopamine sphere (CPDS) layer. TEM images of e) SPI/PVA nanofiber, f) PDSs, and g) CPDSs.

and the average diameter of SPI/PVA nanofibers was about 200 nm, which was also revealed in TEM images, as shown in Figure 1e. As can be seen from Figure 1c,d, before and after carbonization the mesoporous microspheres were quite uniform with an average diameter of ≈210 nm. Under observation at a high magnification by using a TEM (Figure 1g,h), the pore size was estimated to be ≈7 nm and mesochannels of CPDS were more distinct than that of PDS. Thus, the ordered mesoporous carbon spheres can deliver homogeneous mass and charge transfers across the Li/electrolyte interface and the formation of dendritic Li and “dead” Li would be mitigated. A schematic diagram of a normal secondary battery with separator is shown in Figure 2. There are two major drawbacks occurring in the battery with Li metal as anode and LMO as the cathode, Li dendrites and the dissolution and migration of Mn ions (see in Figure 2a). By contrast, in the battery with c-GPE, the growth of Li dendrites and the dissolution and migration of Mn ions were suppressed, as shown in Figure 2b. For the commercial Celgard 2400, it was difficult to form the stable SEI layers because the separator cannot withstand the mechanical deformation during Li plating/stripping process. The Li ionic flux was locally enhanced at the defects of the SEI layers and the Li dendrites grew in an uncontrollable way and eventually would penetrate SEI layer. A lot of fresh Li metal surfaces would be exposed to the electrolyte and thus new SEI layers can continuously be generated over cycling. Moreover, the resulting Li dendrites would pierce through the separator and lead to internal short circuits for the batteries. By contrast, the ordered mesoporous carbon layer of the c-GPE had a good electronic conductivity, high specific surface area, and some surface


lithiophilic groups, like pyridinic and pyrrolic nitrogen, as illustrated in Figure 2c, which can guide the metallic Li nucleation incurring the metal dendrites to distribute uniformly on the anode surface.[29] The conductivity of CPDSs can reduce the effective current density for lithium dissolution and deposition, and the lithium dendrite growth can be therefore inhibited.[20,30] Besides, the inner empty space of ordered mesoporous CPDSs can be able to constrain dendrite nucleate sizes below critical dimensions for stabilizing electrodeposition at the Li-metal/ electrolyte interface and offer the space for the electrolyte permeating to facilitate a relatively homogeneous Li ionic flux.[31] In addition, nanofiber membranes with high flexibility also may be desired to accommodate the volumetric expansion of Li deposition and PDS can form the another physical barrier to block Li dendrites. Further, as depicted in Figure 2c, the amino and carbonyl groups of CPDS have good affinity to the electrolyte and form hydrogen bond with PF6− to release a lot of free Li ions. The free Li ions also can intercalate into the graphite to uniformize the Li+ flux and improve the wettability of Li metal on the CPDS.[32] Consequently, the transport rate of lithium ions was increased and the consuming lithium ions near the electrode surface can be supplied, thus, guiding the Li ions to form uniform Li metal deposition. In short, the lithium dendrite can be suppressed and the electrochemical properties will be significantly improved. Figure 2d shows illustration of the interactions between c-GPE and LMO cathode, which was used to explain the reason for the reduction of Mn ions. The polar functional groups of SPI/PVA nanofiber and PDSs exhibited strong interactions with LMO to inhibit dissolution of Mn ions from cathode. In Figure 2e, the trapping mechanism of Mn ion

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Figure 2. A schematic diagram of a normal secondary battery a) with separator and b) with c-GPE. c) Schematics showing the mechanism c) for the Li dendrites suppressing and d) for the dissolution of Mn ions reducing from the LMO cathode, as well as e) for the Mn ions trapping.

is shown. It can be seen that the amount of dissolved Mn ions understandably decreased when they passed c-GPE. In c-GPE, the PDS with multiple polar groups can effectively capture the dissolved Mn ions from electrolytes via coordination interactions and form Mn ions chelate complexes. The specific surface area and pore structure of PDSs and CPDSs were investigated through N2 adsorption–desorption isotherms measurements. As shown in Figure 3a, the isotherm profiles of PDSs and CPDSs exhibited a hysteresis loop, suggesting the existence of mesopores. The specific surface areas of PDSs and CPDSs were 24.3 and 39.1 m2 g−1, respectively. The pore size distribution calculated using the Barrett–Joyner– Halenda (BJH) model showed that the size of the majority of the pores fell in the range of 2–9 nm (see inset in Figure 3a), which agreed well with that estimated from the TEM images. These mesopores would increase uptake ability for liquid electrolytes to achieve high ion conductivity as described later. Significantly, these mesopores structures would lead to uniform Li deposition and the Li dendrite growth can be efficient restrained. The X-ray photoelectron spectroscopy (XPS) measurements were used to carry out the chemical identification of the heteroatoms in all these samples. The XPS survey spectra are shown in Figure 3b, revealing the presence of C, O, and N elements. The atomic contents of PDSs, CPDSs, and SPI/PVA nanofiber


membranes were quantitatively analyzed in the XPS test, as shown in Table S1 (Supporting Information), which confirmed the existence of a variety of functional groups on the surfaces of PDSs, CPDSs, and nanofiber membranes. The Fourier transform infrared spectroscopy (FTIR) spectroscopy also verified this conclusion, as shown in Figure S2 (Supporting Information). A plenty of polar groups in these samples would increase the affinity with electrolytes and the uptake amount of liquid electrolyte, or even inhibit dissolution of Mn ions from cathode and capture the Mn ions. It is significant to achieve excellent thermal shrinkage of separator for ensuring the safety performance of Li batteries because thermal shrinkage will result in internal short circuits between anodes and cathodes and eventually cause cell explosion. The photographs of modified SPI/PVA nanofiber membrane, SPI/PVA nanofiber membrane, and a commercial separator before and after exposure under 150 °C for 2 min are presented in Figure 3c. It can be seen that the Celgard 2400 membrane shrank severely accompanied by the color change from white to transparent after exposure under the high temperature, whereas the SPI/PVA nanofiber membrane only exhibited a little shrinkage (the thermogravimetric analysis (TGA) profile of the SPI/PVA nanofiber membrane is shown in Figure S4 in the Supporting Information). Intriguingly, the SPI/PVA nanofiber membrane combined with PDS and CPDS

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Figure 3. a) N2-sorption isotherm and BJH pore size distribution (shown in inset) of the PDS and CPDS. b) The total XPS spectra of SPI/PVA nanofiber, PDS, and CPDS. Comparison photographs of the thermal shrinkage of skeletons of n-GPE and c-GPE and Celgard 2400 c) before and d) after exposure to 150 °C.

showed a more negligible change in its dimensions comparing to the pristine SPI/PVA nanofiber membrane under the same condition, which implied that the excellent thermal stability of the skeleton materials existed in the c-GPE. The remarkable low thermal shrinkage of the modified SPI/PVA nanofiber membrane was mainly because the impacted PDS and CPDS layers provided resistance against thermal shrinkage. The polydopamine possessed a high decomposition temperature of more than 250 °C, as shown in Figure S4 (Supporting Information). Thus, the composition of modified SPI/PVA nanofiber membrane was beneficial for enhancing the safety performance of Li batteries. In this study, the mechanical properties of the c-GPE and neat biobased GPE (n-GPE) wre also tested. The stress– strain curves are shown in Figure S5 (Supporting Information). The tensile strength of the skeleton of c-GPE was 16.3 MPa, which was higher than that of n-GPE and good enough for use in lithium ion batteries. The ionic conductivity of the GPEs or separator is a crucial property for the application in a LMB. The electrolytes are characterized by the impedance responses wherein the bulk resistance (Rb) is used to calculate the ionic conductivity of the GPEs or separator. The impedance spectra of the stainless steel (SS)/GPEs/SS cells and its magnified image are shown in Figure S6a,b of the Supporting Information, respectively. The ionic conductivity of Celgard 2400, n-GPE, and c-GPE can be obtained from Figure S6b (Supporting Information). The


n-GPE involving SPI/PVA nanofiber displayed an ionic conductivity of 1.5 × 10−3 S cm−1, which was much higher than that of the Celgard 2400 (0.36 × 10−3 S cm−1). Moreover, the ionic conductivity of c-GPE based on SPI/PVA nanofiber embedded with PDS and CPDS can reach 2.2 × 10−3 S cm−1. The high ionic conductivity was produced from the high uptake of liquid electrolyte resulting from the fully interconnected porous structure in the nanofiber membranes and the mesoporous structures of PDS and CPDS (porosity and saturated electrolyte uptakes of these samples are listed in Table S2, Supporting Information). This c-GPE with high ionic conductivity was expected to improve the electrochemical performance of Li batteries. Good electrochemical stability was necessary to the GPEs for the practical application in Li batteries. A linear sweep voltammetry was used to characterize the electrochemical stability of GPEs in the potential range from 2.0 to 8.0 V (vs Li+/Li) at a scan rate of 10 mV s−1, as shown in Figure S6c (Supporting Information). The oxidative degradation of the prepared n-GPE just took place over 5.5 V corresponding to electrochemical oxidation limit for LiPF6 (5.1 V), which was higher than the decomposition voltage of commercial separator Celgard 2400 (5.2 V), deriving from some interactions between the skeleton materials with special molecular structure of SPI in n-GPE and the absorbed electrolytes. Furthermore, the c-GPE involving PDS and PDCS exhibited a stable electrochemical working window of 5.6 V. The improvement for the electrochemical

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stability was also testified by incorporating other inorganic fillers into the GPE.[33] Here, the high electrochemistry stability will be sufficient to support the suitability for the application in high-energy-density Li batteries with different types of cathodes, such as LMO, LiFePO4, and sulfur-based composites. The good compatibility of the electrolyte with Li electrode is also important for obtaining good cycle performance of Li cells. The initial interface impedance spectra of Li/GPEs/Li cells are shown in Figure S6d (Supporting Information), and the semicircles in the high frequency region and a straight linear line at low frequency region can be observed. The c-GPE and n-GPE displayed a lower interfacial resistance than the commercial Celgard 2400 separator (390 Ω), which can be obtained from the diameter of the semicircle on the real axis called the charge-transfer resistance (Rct) values. This result indicated that the SEI film formed on the Li surface for the c-GPE was more stable and conductive than that for the liquid electrolyte, leading to a lower and more stable voltage polarization of the Li/c-GPE/Li. To provide comprehensive information on Mn ions chelation of the c-GPE, the amount of Mn ions trapped by PDSs

in the skeleton of the c-GPE was systematically investigated using inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis (see in Figure 4a). The PDSs were soaked in manganese perchlorate (Mn(ClO4)2) and manganese acetate (Mn(CH3COOH)3) electrolyte solution (15.0 × 10−3 m Mn(ClO4)2 and 7.5 × 10−3 m Mn(CH3COOH)3 containing 1 m LiPF6/ethylene carbonate:dimethyl carbonate (EC:DMC) (1:1, v/v)). After adding PDSs, the samples measured by ICP-OES showed more transparency. The PDSs showed excellent Mn ions chelating ability and the amount of Mn ions captured by PDSs was 40.8 (mg per mg PDSs). Moreover, Figure 4b shows that the PDSs chelating Mn ions exhibited some small aggregation and became irregularity. Besides, the strong signal of Mn element was displayed in the X-ray spectroscopy (EDS) scanning images in the inset of Figure 4b. These results verified that PDSs in c-GPE played crucial roles in trapping the Mn ions dissolved from LMO cathode, and the hydroxy and amino groups of PDSs can significantly promote the formation of Mn ions chelate complexes, as shown in Figure 2d. A further analysis was undertaken to elucidate that advantageous effects of c-GPE on the restraint for dissolution of Mn ions from surface of LMO.

Figure 4. a) ICP-OES analysis showing the amount of Mn ions (in mg per mg PDSs) before and after adding PDSs. The inset is the photograph of the samples measured by ICP-OES. b) SEM image of PDSs chelating Mn ions and its EDS image of Mn element. c) XRD patterns for the LMO from the disassembled cells with c-GPE, n-GPE, and Celgard 2400 after 200 cycles and quantitative data for the lattice constant α of the LMO from XRD patterns. SEM images of d) the pristine LMO cathode surface and e) the cycled LMO cathode surface assembled with Celgard 2400, f) with n-GPE, and g) with c-GPE. The dotted circles represent byproducts.


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After 200 cycles, the cell was disassembled and its components and surface morphology of LMO were characterized by using X-ray diffraction (XRD) and SEM, as shown in Figure 4c–g. A visible shift of peaks was presented in the XRD patterns of LMO taken from cycled cells involving Celgard 2400 separator compared to the pristine LMO, especially pronounced peak shifts at higher diffraction angles. By comparison, the shift of peaks in the XRD patterns of LMO derived from cells containing n-GPE and c-GPE was very little. The quantitative data for the lattice constant, α, of the LMO in the various cells are listed in Table S3 (Supporting Information). The large reduction of α was 0.56% for LMO taken from the cell involving Celgard 2400, whereas the reduction of α was 0.26% and 0.10% for LMO taken from the cells containing n-GPE and c-GPE, respectively. These results indicated that a lot of polar groups in nanofiber membranes and PDSs exhibited strong interaction with LMO cathode, as shown in Figure 2c. Moreover, Figure 4d–g shows the surface morphology of the pristine LMO and the cycled LMO cathode assembled with Celgard 2400, n-GPE, and c-GPE, respectively. The LMO cathode involving n-GPE and c-GPE exhibited a relatively clean surface similar to surface of the pristine LMO, whereas the LMO cathode combined with Celgard 2400 was contaminated with a significant amount of byproducts. Therefore, the notorious structural disintegration of LMO cathodes during cycling was largely avoided when the cells were loaded with c-GPE developed in the present work. Also, the changes in the microstructure of the Celgard 2400, n-GPE, and c-GPE adjacent to the LMO cathode after 200 cycles were investigated (Figure S7a–f, Supporting Information). No appreciable disruptions or defects and dense gel film were observed for the n-GPE and c-GPE samples. By contrast, the porous structure of the Celgard 2400 was partially contaminated with byproducts. This morphological comparison directly demonstrated the long-term structural and electrochemical stability of the n-GPE and c-GPE. A galvanostatic Li deposit/strip electrochemical cycling measurement was performed to investigate the dynamical stability of the Li/electrolyte interface. A symmetric Li/c-GPE/Li cell was assembled based on c-GPE involving two CPDS layers coupled with Li metal sheets as the electrodes (see inset in Figure 5b) and was measured at a current density of 1 mA cm−2 over a discharging and charging process (4 h for each process). The voltage profile was plotted versus cycling time, as shown in Figure 5a,b. Negative voltage value referred to the Li deposition, while positive voltage value referred to the Li stripping. It can be seen that the cell with n-GPE and Celgard 2400 exhibited the high over potential for whether deposition or stripping and the voltage hysteresis kept rising with the increase of cycle time, mainly due to the accumulated thick SEI and deteriorated electrolyte/electrode interface caused by nonuniform Li deposition and dendrites growth, which resulted in the cracking of SEI and eventually induced the consumption of electrolyte, formation of excessive SEI (see Figure S8a–f, Supporting Information), and increase of overpotential. By contrast, apart from the relatively high-voltage polarization in the first few cycles for the electrode activation and formation of a stable SEI layer, the Li/c-GPE/Li cell displayed a much small over-potential around 5 mV, which was related to larger amount of electrolyte uptake and a better electrode–electrolyte contact. Moreover, in Figure 5b, a much


stable and low voltage fluctuation was observed when the cell involving c-GPE was cycled for over 500 h, which indicated a uniform Li growth along with stable Li deposition–stripping process, validating the inhibiting effect of CPDS layer against dendrites growth and ensuring an enhanced cycle life of a Li metal battery. The electrochemical performance of the c-GPEs was further evaluated by assembling a cell using LMO as the cathode and Li metal as the anode, as illustrated in Figure 5c–e. In Figure 5c, the curves are plotted to characterize cycling stability performance of the cells with c-GPEs. The LMO/c-GPE/Li cell still exhibited high reversible discharge capacities (69.5 mA h g−1) at 0.2 C rate and capacity retention (71.3%) after 200 cycles. By contrast, the cell with Celgard 2400 presented only a capacity retention ratio of 58.5%. In addition, the initial charge and discharge capacities as a function of cycle number (up to 200 cycles) at 0.2 C are shown in Figure 5d. The initial charge–discharge curves showed potential plateaus of LMO and revealed a reversible cycling process. The cell with c-GPE delivered a remarkable initial discharge capacity of 97.5 mA h g−1, which was higher than that of the cell involving n-GPE (96.6 mA h g−1) and Celgard 2400 (92.2 mA h g−1). Moreover, Figure 5e shows the rate performance of the cell based on Celgard 2400, n-GPE, and c-GPE at various rates from 0.1 to 2 C. The discharge capacity of Li batteries continuously decreased with the increasing current densities. A satisfactory discharge capacity of 65.5 mA h g−1 was achieved even at a rate of 2 C from the cell with c-GPE (67.2% of the capacity at 0.1 C), while the corresponding capacity of the LMO/Celgard 2400/Li cell was only 9.8 mA h g−1. These superior performances of the cells with c-GPE can be mainly attributed to high saturated electrolyte uptake caused by abundant pore structure and suppression of Li dendrites growth arising from good electrochemically interface stability, as well as efficient capture of Mn ions and reducing dissolution of Mn ions from the LMO cathode in the presence of the polar functional groups of PDSs. Therefore, the c-GPE displayed promising potential application for high-performance rechargeable Li batteries. The electrochemical impedance spectrum (EIS) plots of the cell with c-GPE after 1, 100, and 200 cycles at 0.2 C are shown in Figure 5f, which can be used to evaluate the interfacial behavior of LMO/c-GPE/Li batteries and further understand excellent cycling performance of the cell with c-GPE. It can be clearly seen that the interface resistances (Rf and Rct) with the LMO/c-GPE/Li cell displayed small change from the first cycle to 200th cycle, while that of LMO/Celgard 2400/Li cell increased distinctly. This suggested stable SEI layer had been formed on the electrodes and agreed well with the galvanostatic cycling results in Figure 5c. Figure 5g–i shows the surface morphologies of the pristine Li anode and Li anode harvested from the coin cell after 200 cycles at 0.2 C. The morphology changes of Li metal can visually confirm the remarkable Li dendrites inhibition. The pristine Li anode exhibited a smooth and dense surface (Figure 5g), whereas the irregular dendritic Li was visible on the surface of electrode disassembled from cell with Celgard 2400 after 200 cycles at 0.2 C. Similarly, some dendrites were observed on the surface of the Li anode dissembled from LMO/n-GPE/Li cell. However, for the cell based on c-GPE, the surface of the Li metal anode still remained smooth after 200 cycles, as shown

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Figure 5. a) Galvanostatic cycling curves of Li/c-GPE/Li, Li/n-GPE/Li, and Li/Celgard 2400/Li symmetrical cells, and b) the enlarged image of galvanostatic cycling curves of Li/c-GPE/Li. c) Cycling performances of LMO/c-GPE/Li, LMO/n-GPE/Li, and LMO/Celgard 2400/Li cells up to 200 cycles at a charge/discharge current density of 0.2 C. d) Initial charge and discharge curves of the cell based on Celgard 2400, n-GPE, and c-GPE. e) The rate performance of the cell based on Celgard 2400, n-GPE, and c-GPE. f) EIS of the LMO/c-GPE/Li, LMO/n-GPE/Li, and LMO/Celgard 2400/Li cells after 1 and 200 cycles at 0.2 C. SEM images of the surface of the Li electrode g) pristine Li, h) the Li electrode obtained from a LMO/Celgard 2400/Li cell, i) from a LMO/n-GPE/Li cell, j) from a LMO/c-GPE/Li cell after 200 cycles at 0.2 C. The dotted circles represent dead Li or Li dendrites.

in Figure 5j. These results supported the experimental analyses of Li symmetrical cell as shown in Figure 5a. Furthermore, the SEM images of the Celgard 2400, n-GPE, and c-GPE adjacent to Li anode were obtained to indirectly understand the formation of Li dendrites. As depicted in Figure S7g–l (Supporting Information), massive dead Li had covered on the surface of Celgard 2400, while some Li dendrites were also formed on the surface of n-GPE. As a vivid contrast, a compact layer with CPDS was still kept on the surface of c-GPE, which possessed higher mechanical modulus and was beneficial to inhibition


of Li dendrites. These morphological characterizations directly proved that c-GPE were able to successfully depress Li dendrites during charge–discharge cycles (see in Figure 2b). Thus, an enhanced cycle performance and good rate performance can be achieved by adopting the c-GPEs.

3. Conclusions A novel bifunctional c-GPE was designed and fabricated via a facile and effective technique route. In order to build the

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skeleton of c-GPE, a layer of PDSs was sandwiched by two layers of the environmental and porous SPI/PVA nanofiber membranes, and a layer of CPDSs was coated without any binder on the surface of membranes. After gelation with liquid electrolyte, the microspheres were firmly trapped in the c-GPE structure. The resulting c-GPE involving the skeleton was able to solve two crucial problems successfully in LMO/Li cell, i.e., growth of Li dendrites and the dissolution and migration of Mn ions. Consequently, the serious structural disintegration of LMO cathode during cycling was largely avoided, and flat voltage profiles and stable cycling of more than 500 h were achieved at a high current density of 1 mA cm−2. Moreover, the c-GPE exhibited high ionic conductivity of 2.2 × 10−3 S cm−1, and the cell based on the c-GPE displayed a higher capacity retention (71.3%) after 200 cycles and reversible capacity (65.5 mA h g−1) at 2 C than those of LMO/Celgard 2400/Li (58.5%, 9.8 mA h g−1). These results were considered to benefit from PDS with polar surface functional groups, which can strongly chelate the dissolution of Mn ions from surface of LMO. In addition, the N-doped CPDS adhering the nanofiber membranes can effectively suppress the Li dendrites growth. This work provides a successful strategy for development of an efficient and safe biobased GPE for high performance energy storage devices with Li metal electrode.

4. Experimental Section Materials: Polyethylene–polypropylene glycol (F127, average Mn = 13 000 g mol−1) was purchased from Macklin Biochemical Co., China. Dopamine hydrochloride, 1,3,5-trimethyl benzene (98%), ammonia hydroxide (28–30% NH3 basis), and ethanol (99%) were purchased from Aladdin Industrial Co. Ltd., China. The commercial SPI powder (protein content, >90%; fat content,