aNational Key Laboratory of Science and Technology on Advanced Composites in Special Environments, and Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080, China
bMIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China
cChongqing Research Institute, Harbin Institute of Technology, Chongqing 401151, China
dSchool of Mechanical Engineering, Chengdu University, Chengdu 610106, China
Nickel-rich layered Li transition metal oxides are the most promising cathode materials for high-energy-density Li-ion batteries. However, they exhibit rapid capacity degradation induced by transition metal dissolution and structural reconstruction, which are associated with hydrofluoric acid (HF) generation from lithium hexafluorophosphate decomposition. The potential for thermal runaway during the working process poses another challenge. Separators are promising components to alleviate the aforementioned obstacles. Herein, an ultrathin double-layered separator with a polyimide (PI) basement and a polyvinylidene difluoride (PVDF) coating layer is designed and fabricated by combining a nonsolvent induced phase inversion process and coating method. The PI skeleton provides good stability against potential thermal shrinkage, and the strong PI-PVDF bonding endows the composite separator with robust structural integrity; these characteristics jointly contribute to the extraordinary mechanical tolerance of the separator at elevated temperatures. Additionally, unique HF-scavenging effects are achieved with the formation of hydrogen bonds for the abundant coordination sites provided by the imide ring; hence, the layered Ni-rich cathodes are protected from HF attack, which ultimately reduces transition metal dissolution and facilitates long-term cyclability of the Ni-rich cathodes. Li||NCM811 batteries (where "NCM" indicates ) with the proposed composite separator exhibit a capacity retention after 400 cycles at room temperature and remain sustainable at with a capacity retention after 200 cycles. By adopting a new perspective on separators, this study presents a feasible and promising strategy for suppressing capacity degradation and enabling the safe operation of Ni-rich cathode materials.
Conventional lithium-ion batteries (LIBs) with or cathode and graphite anode have been increasingly incapable of meeting the rapid-growing demands for high-energy-density electric vehicles [1⇓-3]. The Li metal anode, with a theoretical specific capacity of and an ultralow electrochemical potential of versus the standard hydrogen electrode, is considered the most promising candidate to replace the graphite anode (theoretical specific capacity: ) and enhance LIB energy density [4,5]. The use of novel positive active materials such as layer-structured transition metal (TM) oxides , generally referred to as NCM, where Ni and Mn are nickel and manganese, respectively) at a higher upper cut-off voltage is another prospective strategy to overcome the energy density limit for higher theoretical capacities. In particular, the Ni-rich NCM811 has drawn attention [6⇓⇓-9].
However, NCM811 commercialization has been hindered by the severe capacity fading of this material at elevated battery voltages, via a phenomenon widely known as "rollover" failure [10⇓-12]. Extensive research has recently been conducted on two major types of degradation mechanisms for Ni-rich cathode materials. One mechanism originates from the irreversible loss of active materials induced by cation mixing and structural reconstruction of the layered oxides during the cycling process, with the formation of an electrochemically inactive rock-salt phase [8,13]. The other mechanism originates from the significant dissolution of TMs from the cathode, which induces continuous degradation of the solid-electrolyte interface (SEI) [14⇓-16]. Large amounts of active materials are consumed to generate a new SEI, which further increases the internal impedance and irreversible capacity. Both degradation mechanisms are associated with hydrofluoric acid (HF) and other acidic species originating from Li hexafluorophos-phate decomposition [14,17⇓-19]. With the continuous cathode deterioration caused by HF erosion, TM dissolution is promoted and, ultimately, undesired electrolyte decomposition occurs [20⇓-22]. Therefore, HF and similar acidic species can be regarded as the root cause of NCM811 performance degradation. Moreover, thermal instability remains a significant obstacle to the practical implementation of Ni-rich cathodes [23⇓⇓-26]. To date, in-depth studies have been conducted to improve the electrochemical performance of NCM811, targeting the failure mechanisms discussed above. Most studies have considered modification of the Ni-rich cathode materials, for example, via ion doping [27⇓-29] and surface modification [30⇓⇓-33]; however, the suppressing effects of separators on NCM811 capacity degradation have not yet been thoroughly investigated.
Separators are important for the electrochemical performance and safety of LIBs [34,35]. The key to enabling stable cyclability of LIBs with Ni-rich cathodes is the development of thermostable separators that can alleviate the observed capacity decay via specific mechanisms. However, commercial polyolefin separators cannot prevent "rollover" failure and exhibit remarkable thermal shrinkage at high temperatures [36,37]. Accordingly, heat-resistant polymers such as polyimide (PI), cellulose, and polybenzimidazole have been widely used to fabricate novel separators via phase inversion or electrospinning [26,38]. Notably, Zhao et al. [39] developed a core-shell PI@fluorinated poly-m-phenyleneisophthalamide (F-PMIA) nanofiber separator via coaxial electrospinning. The thermally stable PI core helped maintain the structural integrity of the composite separator up to and the gel F-PMIA shell endowed it with an enhanced electrolyte affinity, synchronously extending the service lives of Li-metal cells. Zhu et al. [40] utilized a facile papermaking process to fabricate a novel composite separator using heat-resistant polyphenylene sulfide fibers and cellulose fibers; this separator retained its original size after a heat treatment. Despite their favorable thermal stabilities, practical applications of these separators are impeded by their insufficient mechanical strengths, which may be attributed to the unique pore-forming mechanisms of the preparation methods. This inadequate mechanical strength does not support the higher separator thickness required for further enhancement of LIB energy density.
Functional coating is an efficient and low-cost method that can enhance the mechanical robustness of separators. The coating materials can be classified into two major categories: polymers and inorganic ceramics [41⇓⇓-44]. A functional ceramic coating requires the use of binders, thereby increasing costs. However, ceramic materials tend to agglomerate and block the separator pores, even forming a dense layer, which is detrimental to the electrochemical performance of LIBs. Therefore, porous polymer coatings are preferable.
However, the previous literature has rarely emphasized the effect of coating thickness on the energy density of the battery, and few literatures have used separators to indirectly solve the problem of high nickel cathodes. In this study, we developed an ultrathin composite separator with a PI basement and a polyvinylidene difluoride (PVDF) coating layer, which was fabricated using a combination of the non-solvent induced phase inversion (NIPI) process and coating method. The PI skeleton provided good stability against potential thermal shrinkage, and the strong interfacial adhesive force owing to the high viscosities of both the large-molecular-weight PI and PVDF endowed the composite separator with robust structural integrity. These properties jointly contributed to the extraordinary mechanical tolerance of the proposed composite separator at elevated temperatures. The PVDF coating layer also provided additional electrolyte retention, which was beneficial for long-term battery cycling. In addition, special HF-scavenging effects could be achieved with the formation of hydrogen bonds for the abundant coordination sites provided by the imide ring. These effects protected the layered Ni-rich cathodes from HF attack. Thus, TM dissolution was suppressed, which alleviated cation mixing and electrolyte decomposition such that improved interfacial stabilities were achieved on both electrodes. Thus, the internal resistance did not increase sharply and, ultimately, the capacity degradation of the Ni-rich cathodes was prevented. When the functionalized composite separator was applied in Li||NCM811 batteries, the batteries exhibited competitive capacity decays of 0.0235% and 0.0430% per cycle at room temperature and , respectively. Thus, the proposed approach constitutes a feasible strategy for achieving long-term cyclability of high-energy-density batteries with Ni-rich cathodes.
2. Results and discussions
2.1. Design concept and physical properties of PI/PVDF composite separator
is the preferred salt for state-of-the-art electrolytes because of its unique advantages of high ionic conductivity and favorable electrochemical stability [45]. However, it has poor thermal stability and exhibits autocatalytic decomposition, generating phosphorus pentafluoride . In the presence of only trace amounts of water, HF is subsequently generated, as follows [47]:
In the case of an NCM811 cathode, this mechanism continuously erodes the cathode and catalyzes a series of undesired reactions originating from deteriorative TM dissolution. Commercial mainstream polyethylene (PE) separators do not alleviate this phenomenon and have no effect on the migration of the dissolved TM from the cathode to anode, which poisons the SEI. This continuous SEI degradation consumes additional active , which, in turn, induces pernicious SEI generation and reversible capacity loss. Thicker and less-conductive passive layers, as well as significant Li-dendrite growth, are generated on the anode surface, ultimately triggering "rollover" failure. Fig. 1 illustrates the design concept of the PI/PVDF composite separator and demonstrates its physical properties. A PI-based separator can target the root cause of the capacity degradation by acting as an HF scavenger or cathode protector, owing to the strong adsorption effect between PI and HF induced by the significant interaction between the atom of the imide ring and atom of the , as displayed in Fig. 1(a). The adverse reactions initiated by HF attack can be effectively hindered, preventing significant consumption of active and a remarkable impedance increase. Thus, for Li||NCM811 batteries with such PI-based separators, the chain effects of this HF scavenging contributes to stable cycling.
Moreover, as greater separator thickness is required to achieve higher energy density, a porous PVDF membrane was used as a functional coating to improve the mechanical robustness of the proposed composite separator. A PVDF membrane was selected for the following three reasons:
(1) High-molecular-weight PVDF has high viscosity; thus, it ensures a strong interfacial adhesive force between the two layers and enhances the integral mechanical strength of the composite separator
(2) PVDF exhibits favorable affinity with electrolytes and electrodes owing to its strong polarity and stability [48].
(3) The solvent used to fabricate the PVDF membrane has no influence on the PI separator. A large-scale ultraporous PVDF-coated PI separator (hereinafter referred to as the "PI/PVDF separator") is shown in Fig. 1(b); the composite structure demonstrates flexibility and scalability.
PI is a polymer with an imide group in the main chain of its molecular structure. In imides, nitrogen atoms are attached to two carbonyl groups. In high-performance PI, an aromatic ring and a heterocyclic ring constitute the main structural units of the main chain. The primary chemical structures of the PI used in this study, which was prepared through the polymerization reaction between 1,2,4,5-benzenetetracarboxylic anhydride (PMDA) and 4,4’-oxydianiline (ODA), are shown in Fig. S1 (Appendix A). Fig. S2(a) (Appendix A) shows the Fourier-transform infrared (FTIR) spectra of PVDF, PI, and PI/PVDF separators. Four absorption peaks at 1780,1720,1380, and are apparent in the PI spectrum, corresponding to the asymmetric stretching vibration of , symmetric stretching vibration of , stretching vibration of , and deformation vibration of , respectively. Notably, polyamic acid (PAA) absorption peaks at 1710,1660, and are not apparent in the spectrum, indicating complete conversion of the PAA precursor into PI following thermal imidization. After the coating process of PVDF, new peaks, such as those at 1400, 1180, and , which correspond to PVDF, emerge in the PI/PVDF FTIR spectrum, indicating successful fabrication of the PI-based composite separator. The X-ray diffraction (XRD) results shown in Fig. S2(b) reveal co-occurrence of the characteristic peaks attributed to PVDF and PI, further confirming formation of the PI/PVDF composite structure. Scanning electron microscopy (SEM; Carl Zeiss, Germany) was used to observe the PI/PVDF-separator surface and cross-sectional morphologies. As displayed in Fig. 1(d) and Fig. S3 (Appendix A), the upper surface of the PI layer of the composite separator exhibited a sponge-like nanoporous morphology, whereas the bottom surface was relatively dense. This difference originated from the different phase inversion rates near and off the substrate during the NIPI process. Note that micropores of hundreds of nanometers and a uniform pore size distribution inhibit small-particle penetration and ensure a smooth current during charging/discharging. In contrast to the uniformly distributed nanopores of the PI layer, the PVDF layer presented a surface morphology with a larger pore size and a less-uniform pore distribution (Fig. 1(e)), which was attributed to the high rate of acetone volatilization. Both the PVDF and PI layers exhibited good electrolyte wettability, with contact angles of and , respectively; these values are significantly smaller than that for a PE separator (Fig. S4 in Appendix A). The superior wettability of the PI/PVDF separator can be ascribed to its higher surface polarity, which is attributed to the abundant polar groups of both PI and PVDF. Correspondingly, a high electrolyte uptake of was achieved for the PI/PVDF separator, whereas that of the PE separator was only , as shown in Fig. S5(a) in Appendix A. This difference in electrolyte uptake again reflects the porosity differences of the different separators (Fig. S5(b)).
Notably, the composite-separator thickness was only (PVDF- and PI-layer thicknesses are 2 and , respectively), as verified by the cross-sectional morphology in Fig. 1(c) and element mapping in Fig. S6 in Appendix A, consistent with the current application trend for thinner separators. The effects of thickness reduction on the energy density were intensively investigated via practical calculations in our previous study [49]. As illustrated in Fig. 1(i), impressive increases of 12.01% and 16.68% in volumetric energy density can be realized for batteries when the separator thickness is reduced from 25 to 10 and , respectively. Despite its small thickness, the PI/PVDF-separator tensile strength was enhanced to via the PVDF coating. This is in contrast to the tensile strength of the PI separator without PVDF coating, a value comparable to those of commercial separators (Fig. S7(a) in Appendix A). In addition, as shown in Fig. S7(b), the puncture strength of the PI/PVDF-separator was considerably improved when the polymer coating was applied, indicating superior inhibition of short circuits during assembly and blocking of dendrite penetration during the working process. Fig. 1(j) and Table S1 (Appendix A) present a comparison of the tensile strengths and thicknesses of the PI-based separators reported in this study and in other published studies [50⇓⇓⇓⇓⇓⇓⇓⇓⇓-60]. Clearly, the proposed PI/PVDF separator has greater potential for commercial applications than the other separators, owing to its ultrathin and mechanically robust characteristics.
In recent years, PI, which has excellent thermal stability, has been widely applied as a heat-resistant material [61⇓⇓-64]. To investigate whether the PVDF coating adversely affected the thermal properties of the PI layer, thermogravimetric (TG) and forward-looking infrared radiometry (FLIR) analyses were performed. Thus, the thermal stability of the composite separator was evaluated. The TG and derivative thermogravimetry (DTG) profiles are shown in Fig. S8 in Appendix A. The commercial PE separator underwent an initial mass loss at no more than , and no evident thermal decomposition occurred for the PI separator until . After the coating process, the PI/PVDF separator continued to inhibit internal short circuits induced by thermal shrinkage during thermal decomposition in response to a subtle temperature decrease of approximately . Furthermore, the thermal shrinkages and distributions of various separators were analyzed after prolonged heat-treatment of up to , as shown in Figs. 1(f)-(h). For temperatures exceeding , severe thermal shrinkage and visible melting deformation occurred in the commercial PE separator; these behaviors can be ascribed to the relatively low melting point of this material and, accordingly, its inability to withstand excessively high temperatures. In contrast, the PI separator exhibited almost no shrinkage and a relatively uniform thermal distribution, even at a high temperature of ; this result indicates that the proposed separator can effectively resist thermal shrinkage in high-temperature environments and markedly reduce the risk of battery short circuits. Finally, the PI/PVDF composite separator maintained favorable thermal stability without local heat accumulation, demonstrating the ability of the proposed separator to ensure battery security when exposed to thermal runaway incidents.
2.2. Electrochemical performance of Li||NCM811 batteries with PI/ PVDF separators
To determine the effects of PI-based composite separators on the "rollover" failure of NCM811 batteries, PE, PI, and PI/PVDF separators were electrochemically tested in half-cells between 3.0 and 4.3 V. A high-temperature electrolyte prepared by Guangdong Canrd New Energy Technology Co., Ltd., (China) was used, which included in ethylene carbonate diethyl carbonate (DEC) + propylene carbonate + phenyl sulfone + vinylene carbonate + butyronitrile (30:15:35:15:3:2 vt.%). When the PI/ PVDF separator was employed, the PI side was placed on the NCM811 cathode and the PVDF side was placed on the Li anode to maximize the cathode-preserving effects of the HF-scavenging PI. As shown in Fig. 2(a), the discharge capacities of PI- and PI/ PVDF-separator were superior to those of commercial PE separator as the current density increased. The capacity retentions of the battery with the PE separator were , and at 2,5, and , respectively, whereas those of the PI-separator battery reached , and , respectively, second only to the values for the PI/PVDF-separator battery of , and , respectively. All cells exhibited favorable reversibility as the rate returned to . The higher capacities of the PI-separator battery at high rates originated from the higher porosity and better wettability between the separator and electrolyte compared with those of the PE-separator battery. The PVDF coating further improved the separator affinity for the electrolytes and electrodes, facilitating the electrode interfacial reaction and contributing to a reduced charge-transfer resistance, as shown in Fig. S9 (Appendix A). From Fig. 2(b), the cells with the PI-based separators had more stable room-temperature cycla-bility than those with the PE separators. After 400 cycles, the capacity decays of the PI/PVDF- and PI-separator batteries were only and , respectively, whereas that of the PE-separator cell was severe at . In addition, near the end of the cycling process, attenuation of the Coulombic efficiency occurred for the PE-separator battery; however, this property remained stable above for the cells with PI/PVDF and PI separators. As the temperature was increased to , more prominent differences became apparent, owing to enhanced reaction activities and rates (Fig. 2(c)). The PE-separator cell underwent "rollover" failure after only 110 cycles, whereas the capacity retentions of the PI/PVDF and PI separators were and , respectively. The room-temperature and capacity decays of and per cycle in PI/PVDF-separator batteries, respectively, are impressive improvements compared with those previously reported for Li||NCM811 batteries with other separators, as detailed in Table S2 in Appendix A. Such stable cyclability at room temperature can be attributed to the reduced resistance growth owing to effective HF scavenging.
In addition to proper operation at , the batteries with the three separators were subjected to instantaneous operation at extreme temperatures. As shown in Fig. S10(a) in Appendix A, no short circuiting was observed for any of the three batteries. As PI has good thermal performance, its structural integrity was maintained at high temperatures. Further, as PVDF has a high melting point, the PVDF coating layer remained intact at . No short circuit occurred for the PE separator, even above its melting point, as the original pore structure was irreversibly closed. As the temperature returned to room temperature, even after treatment at for , the batteries with PI/PVDF and PI separators maintained normal operations despite slight capacity degradations, which were attributed to electrolyte and electrode degradation (Fig. S10(b)). This outcome indicates that the pore-structure integrity was ensured by the favorable thermal stabilities of the PI and PVDF. In contrast, severe battery failure was observed for the PE separator with its closed pore structure, with a specific discharge capacity of (Fig. S10(c)). The cycling performance of the flipped double-layer separator (PI layer-to-anode and PVDF layer-to-cathode structure) was also explored, as shown in Fig. S11 in Appendix A. No significant difference was observed between the cycling performance results obtained for the two separator orientations. This was because, regardless of whether the PI layer faced the anode or cathode, its highly porous structure and strong HF attraction ensured effective HF absorption in the electrolyte. However, placing the PI layer on the cathode can increase the contact area with the electrolyte near the cathode, enabling preferential adsorption of the HF around the cathode; hence, the cathode-preserving effect is maximized, which improves the cycling performance. In addition to the outstanding capacity retentions of the PI-based separators, as apparent from the battery charge-discharge curves (Figs. 2(d)-(f)), the voltage-plateau variation was also markedly lower compared with that of the PE-separator battery, demonstrating effective alleviation of the detrimental structural transformation from layered to rock salt or mixed induced owing to HF attacks. To explore the changes in internal impedance of the cells with different separators, electrochemical impedance spectroscopy (EIS) was conducted after the first and 100th cycles at for comparison (Figs. 2(g)-(i)). The interfacial resistance of the PE-separator cell increased significantly after 100 cycles, almost doubling, demonstrating the continuous thickening of the less-conductive layers (such as , ) on the electrode surface. For the cells with the PI/PVDF and PI separators, the impedance increased to a lesser extent, which is consistent with the HF-scavenging effects protecting the cathode materials, which alleviated the TM dissolution from the cathode to anode and retarded the growth of the interfacial passive layer (as discussed in Section 2.3).
2.3. Capacity degradation relief of NCM811 with PI/PVDF separators during high-temperature cycling
To determine the beneficial impacts of PI-based separators on NCM811-battery capacity degradation, the cycled cathodes with different separators were subjected to XRD (Bruker, Germany), SEM, X-ray photoelectron spectroscopy (XPS; Thermo-Fisher, USA), and high-resolution transmission electron microscopy (HRTEM) analyses (FEI Talos F200x, USA). XRD patterns of the cathode materials before and after high-temperature cycling were obtained to explore the potential structural changes in the NCM811 (Fig. S12 in Appendix A). A diffraction peak was observed at , which was attributed to the aluminum foil that carried the cathode material. No new peaks or obvious shifts appeared in the patterns of the cathodes with the PI and PI/PVDF separators, demonstrating that the NCM811 layer structure was well preserved during high-temperature cycling owing to the HF-scavenging effect. However, for the (101), (012), (015), (110), and (113) peaks of the cathode with the PE separator, some shifts toward higher diffraction angles and broader full widths at half maximum were observed. These behaviors can be attributed to NCM811 lattice shrinkage induced by a significant loss of elec-troactive or an increased degree of mixing [16,47]. Fig. S13 in Appendix A shows the surface morphologies of the NCM811 cathodes with different separators after 50 cycles. The NCM811 particles in batteries with the PE separators were severely covered with by-products generated during the high-temperature operation compared with the pristine NCM811 (Fig. S13(d)). In contrast, their counterparts with PI and PI/PVDF separators exhibited relatively smooth surfaces with fewer by-products, indicating appropriate preservation of the NCM811 structure when PI-based separators were used. These results are consistent with those of the XRD analysis.
XPS was employed to analyze the chemical constituents of the decomposed layers on the surface of the cycled NCM811. As apparent from the spectra of the cycled cathodes with different separators (Figs. 3(a)-(c)), , and peaks (the first three assigned species corresponding to the typical constituents of electrolyte decomposition products) were observed for the PI/PVDF and PI separators. For the PE separator, the O-M peak in the spectrum , which corresponds to the O lattice of the active material, disappeared. Laboratory XPS has a penetration depth of only several nanometers; thus, the formation of a thicker layer on the surface of the PE-separator cathode can be hypothesized [65,66]. Similarly, higher F-C peaks in the spectra (Figs. 3(d)-(f)), corresponding to the PVDF binder for PI/PVDF separators (17.12%) and PI separators compared to that of the PE separator . This outcome reflects the lower surface deposition from electrolyte decomposition and lesser rock-salt phase generation (which is catalyzed by HF attacks) for the former separators. Moreover, abundant LiF was observed on the surface of the cathode with the PI/PVDF separator; this material is conducive to cathode-electrolyte interphase stabilization owing to its sufficient mechanical robustness and outstanding oxidation resistance [67⇓⇓-70]. In the P 2p spectra in Figs. 3(g)-(i), considerably higher content, where this material is a major component of the electrolyte decomposition products, is observed for the PE separator than for the PI/PVDF and PI separators, in agreement with the results at in the spectra. This finding further confirms a larger decomposed-layer thickness on the cathode surface for the PE separator than for the PI-based separators. Thick, electrochemically inactive films tend to increase the interfacial impedance and hamper transport, ultimately causing NCM811-battery capacity degradation. These observations suggest that electrolyte decomposition, which can be catalyzed by acidic species such as HF, is significantly suppressed in NCM811 batteries with PI and PI/PVDF separators. In other words, cathode-material preservation and improved interfacial stability can be achieved by exploiting the HF-scavenging effects of PI-based separators.
Further evidence of the NCM811 preservation effects achieved with the PI/PVDF separators was acquired via TEM characterization. As shown in Figs. 3(j) and (k), after 50 cycles at , well-defined layered structures with 0.245 and interplanar spacings were obtained for the PI/PVDF and PI separators, which matched the (101) and (012) planes of NCM811, respectively. This outcome indicates appropriate preservation of the pristine cathode structure during high-temperature cycling when the PI-based separators were used. For the PI/PVDF separator, the average thickness of the layer on the cathode surface was , slightly lesser than that obtained for the PI separator(2.61nm). This indicates that the PVDF coating helps alleviate electrolyte decomposition via improved interfacial affinity between the separator and electrolytes. For the cycled cathode with the PE separator, a vast region of rock-salt phase with an average thickness of was observed (Fig. 3(l) and Fig. S14 in Appendix A), which may be attributed to detrimental interfacial side reactions catalyzed by HF corrosion. In the NCM811 batteries with PI/PVDF separators, the thinner decomposed layer on the cathode surface and the absence of irreversible transformation from the layer structure to the rock salt phase supported the cathode-preserving effects achieved via HF scavenging, further preventing the electrolyte decomposition and enabling stable battery cyclability.
Cycled Li metal anodes were also studied to verify the capacity degradation behaviors of the NCM811 cells with PI/PVDF separators during high-temperature cycling. Inductively coupled plasma optical emission spectrometry was performed for the cycled anodes to measure the degrees of TM dissolution in the batteries with different separators, as shown in Fig. S15 in Appendix A. Effective TM-dissolution inhibition was achieved in the cells with PI/PVDF and PI separators, especially for Ni (only half the Ni content of the PE-separator cell). These results further verify the HF-scavenging effects of PI-based separators. As mentioned previously, a series of undesired reactions originating from deteriorative TM dissolution is catalyzed, and it continuously poisons the SEI layer if the cathodes are vulnerable to HF attack. This continuous SEI degradation inevitably consumes more electroactive , which causes pernicious SEI generation. Thicker and less-conductive passive layers, as well as significant Li dendrite growth, are then generated on the anode surface, ultimately triggering a "rollover" failure. Figs. 4(a)-(f) display the cross-sectional and surface morphologies, respectively, of Li metal anodes with the three different separators following 50 cycles at . The Li anode with the PE separator exhibited severe surface corrosion with a thick layer of (Fig. 4(c)). Furthermore, mossy Li dendrites were observed on the Li-foil surface (Fig. 4(f)). These outcomes indicate that the PE separator is incapable of reducing the continuous HF-induced cathode corrosion and, therefore, the TM dissolution is multiplied and reactions detrimental to anode stability are catalyzed. By contrast, the anode surfaces for the PI/PVDF and PI separators are flatter and denser, without obvious "dead" Li deposition (Figs. 4(d) and (e)). The deposited layers of the PI/PVDF and PI separators are 51.6 and thick (Figs. 4(a) and (b)), respectively; thus, they are significantly thinner than that for the PE separator. These results indicate suppressed reactions at the electrolyte-anode interphase, benefiting from the HF-scavenging effects of the PI-based separators.
To confirm the HF-scavenging capability of the composite separator, HF concentration tests were conducted by mixing the three different separators with HF-bearing electrolytes ( HF concentration) and monitoring the HF-concentrations after three days. As expected, the PI/PVDF and PI separators exhibited effective HF scavenging with HF-concentration reductions of and , respectively. The PE separator exhibited no such effect (Fig. S16 in Appendix A).
In summary, the advantageous HF-scavenging effects of a PI-based separator protect the cathode from HF erosion and reduce the dissolution of TMs, which may migrate into the Li layer to cause a layered-to-rock-salt transformation and catalyze the electrolyte decomposition. Hence, improved interfacial stability is achieved on both the cathode and anode, preventing extreme growth of the interface impedance due to continuous thickening of the electrolyte decomposition layer and Li-dendrite deposition layer. Thus, the NCM811-battery capacity degradation is mitigated.
2.4. Theoretical calculations for structural stability and HF-scavenging effect of PI/PVDF separator
Density functional theory calculations were conducted to further analyze the structure and HF-scavenging mechanism of the PI/PVDF separators. As indicated by the electron density map in Fig. 4(g), the PVDF molecules shared an intensive and uniform electron distribution with the PI molecules, indicating favorable physicochemical compatibility and strong interactions between the PVDF and PI. Therefore, by incorporating the PVDF coating, higher mechanical intensity can be achieved without sacrificing the remarkable thermal stability of the PI, ensuring safe and steady battery operation at elevated temperatures. In addition, the special HF-scavenging effects of the PI/PVDF separator were verified via calculation of the binding energies between the separator constituents and HF. As shown in Fig. 4(h), no interaction occurred between the commercial PE separator and pernicious HF, as indicated by a minimal binding energy of . By contrast, the PI-HF binding energy value was , demonstrating the strong attraction effect of the PI layer on the remaining HF in the electrolyte. This attraction can be attributed to the formation of hydrogen bonds for the abundant coordination sites provided by the imide ring. Furthermore, adsorption occurred between the PVDF and HF, with a binding energy of . Therefore, the PI/PVDF composite separator effectively anchored the free HF molecules in the electrolyte, thereby mitigating HF attack-induced TM dissolution. Thus, harmful reactions at the electrode-electrolyte interphase were prevented, which improved the electrochemical performance of the NCM811 batteries with PI/ PVDF separators (Fig. 4(i)).
Owing to its unique HF-scavenging effects, the PI/PVDF composite separator can protect high-Ni cathodes from severe capacity fading at elevated battery voltages and, therefore, has significant application potential in the context of high-energy-density Li-based battery fabrication. Notably, this functionalized composite separator can be simply integrated with other optimized electrodes as a substitute for conventional separators, provided the hexafluorophosphate-containing electrolyte is compatible. This proposed approach is expected to profoundly affect future research and development related to high-energy-density batteries.
3. Conclusions
In summary, we demonstrated a strategy to effectively alleviate the capacity degradation of Li||NCM batteries using a novel HF-scavenging separator with a PI basement and PVDF coating. At thickness, the proposed composite separator is ultrathin and exhibits unprecedented mechanical robustness and favorable thermal stability. In this study, the advantageous HF-scavenging effects of this functional separator adequately preserved the NCM811 cathode and reduced TM dissolution. Parasitic reactions at the electrode/electrolyte interphase and undesirable cation mixing were minimized, yielding improved interfacial stability on both electrodes; this stability prevented unlimited growth of the interface impedance and continuous thickening of the interfacial passive layers and, ultimately, mitigated the NCM811-battery capacity degradation. Impressive capacity retentions of and after 400 cycles at room temperature and 200 cycles at , respectively, were obtained with the PI/PVDF separator. From a novel perspective of separators, the fundamental findings in this work extend the feasible strategies to optimize NCM811 cathodes and achieve practical high-energy-density batteries.
Acknowledgments
This study was supported by the Science Foundation of the National Key Laboratory of Science and Technology on Advanced Composites in Special Environments. This work was sponsored by the Natural Science Foundation of Chongqing, China (CSTC2021jcyj-msxmX10305, CSTB2022NSCQ-MSX0246, CSTB2022NSCQ-MSX0242, CSTB2022NSCQ-MSX1244, CSTB2022NSCQ-MSX0441, CSTB2022NSCQ-MSX1356, CSTB2022NSCQ-MSX1572, CSTB2022 NSCQ-MSX1583, CSTB2022NSCQMSX0487, CSTB2022TFII-OFX0034, and CSTB2023TIAD-KPX0010), and the Chongqing Technology Innovation and Application Development Special Key Project (CSTB2023TIAD-KPX0010).
Compliance with ethics guidelines
Shijie Zhong, Liwei Dong, Botao Yuan, Yueyao Dong, Qun Li, Yuanpeng Ji, Yuanpeng Liu, Jiecai Han, and Weidong He declare that they have no conflict of interest or financial conflicts to disclose.
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