Polar-group functionalized polyetherimide separator with accelerated Li-ion transport for stable lithium metal batteries

INTRODUCTION

Lithium energy storage devices have been widely applied in portable electronic equipment, electric automobiles, wearable electronics, etc., owing to their high operating voltage, long cycling stability and low self-discharge rate^[1,2]. Among them, lithium metal batteries (LMBs) are regarded as hopeful next-generation lithium energy storage devices due to their low electrochemical potential and super high theoretical specific capacity (3,860 mAh·g^-1) which is ten times that of commercial graphite anodes^[3,4]. However, the practical application of LMBs faces challenges such as irregular lithium dendrites and unstable solid electrolyte interphases (SEI) caused by the side reactions between lithium metal and electrolyte, resulting in rapid capacity decline and safety issues^[5,6]. Therefore, many efforts, such as anode modifications^[7-9], electrolyte additives^[10-12], functional separators^[13-15] and solid electrolytes^[16-18], have been made to solve the problem of dendrites growth.

As one of the most essential components of LMBs, separators act as the physical barrier, preventing electrode contact and providing porous channels for ion transport^[19-21]. As commercially available separators, polyolefin separators are extensively used in lithium-ion batteries owing to their stability and excellent mechanical strength. However, because of their hydrophobic nature and low porosity, there exist high energy barriers for the entry and passage of Li⁺. Moreover, the slit-shaped pore structure leads to anisotropic mechanical properties^[22], which are unfavorable to stable lithium plating/stripping. Therefore, improving the performance of the separator to prevent dendrites and increase ion transport rate is regarded as a promising method for increasing the electrochemical performance of batteries. In the past decade or more, many research works have focused on modifying polyolefin-based separators or designing new separators beyond polyolefin^[23-25]. Guo et al. designed a modified polyacrylonitrile separator with graphene oxide by regulating the interfacial groups, and found that increasing the amount and types of oxygen-containing polar groups can boost the number of active sites for bonding and transferring sodium ions and accelerate the transmission of Na^+[26].

The porous structure of the separators affects ion transport and the interface of the batteries. Generally, the high porosity of separators can facilitate ion transport^[27,28], but having too large pores can give rise to some problems, such as self-discharge, open circuit voltage drops and the growth of lithium dendrites along the pores^[29,30]. Chemical modification and introducing polar functional groups are effective ways to adjust the pore structure and improve the separator performance. Recently, polyetherimides (PEIs) with attractive characteristics such as good chemical stability, high thermal stability, excellent insulation properties and good electrolyte wettability have been used as separators^[31,32]. Additionally, PEIs are easy to process and modify, and the C–N bond of the cyclic imide can be opened in special circumstances^[33,34], while exposing more polar groups. Notably, the chemical modification method can not only regulate the outer surface property of PEIs without changing the structural stability, but also improve the inherent properties. Some studies have shown that the nitrogen atoms in the amino or amide groups can reduce the binding energy of anion and cation, promote ion dissociation, and increase the free ion concentration^[35,36], which will improve the Li⁺ transfer kinetics and enhance the battery performance.

Herein, an ethylenediamine (EDA) grafting PEI [ethylenediamine grafting polyetherimide (PEI-EDA)] separator with spongy-like porous structure was fabricated through phase-inversion method. The PEI-EDA separator presents excellent flame retardancy, high ionic conductivity, superior lithium ion transference number, and exceptional electrochemical performance. Additional polar functional groups on PEI-EDA separators improve the transport dynamics of lithium ions. When combined with the evenly connected pore structures, they can efficiently accelerate the transport of Li⁺, induce stable plating/stripping, prevent dendritic Li growth, and form a homogeneous interface. Therefore, the Li||Li cells show a longer cycle-life and the Li||Cu cells get a higher coulombic efficiency even without electrolyte additives. Through the generation of fast ion transport and fine interface layers, the cells with PEI-EDA separators exhibit greatly improved cycling stability and high-rate capability.

EXPERIMENTAL

PEI (ULTEM 1000) was provided by SABIC Innovative Plastic. Polyvinylpyrrolidone (PVP, K88-96, Mw = 1,300,000) and Lithium bis (oxalate) borate (LiBOB, ≥) were purchased from Aladdin Biochemical Technology Co. Analytical-grade N, N-dimethylacetamide (DMAc), methanol and EDA were purchased from Sinopharm Chemical Reagent Co., Ltd. Propylene carbonate, diethyl carbonate and liquid electrolyte consisting of 1 M LiPF₆ dissolved in ethylene carbonate (EC): dimethyl carbonate (DMC): ethylmethyl carbonate (EMC) = 1:1:1 (vol%) were purchased from Dodochem, Suzhou, China. A commercial 25 μm polypropylene (PP) separator was provided by Guangdong Canrd New Energy Technology Co Ltd.

The PEI separator was prepared by phase inversion method. First, the 1.12 g PEIs and 0.84 g PVP were dissolved in DMAc at 70 °C to form a uniform solution. After standing for 12 h, the mixture was cast evenly onto a clean glass plate, soaked in water vapor, and then immersed in deionized water. The obtained PEI separator was dried at room temperature. Second, the PEI separator was immersed in a methanol solution containing 5% EDA for 10 h. After being grafted by EDA, the separator was used distilled water to wash 3 times. Third, the PEI-EDA separator was dried and preserved in a 60 °C oven for 6 h before use.

The cell assembly, electrochemical measurement and characterizations were introduced in Supplementary Materials in detail.

RESULTS AND DISCUSSION

The PEI and PEI-EDA porous separators were prepared by phase inversion method [Figure 1A], which is simple and can be used to prepare the isotropic separators on a large scale. Meanwhile, the PEI-EDA separator can be prepared on a large area and rolled into rolls according to the size of the coated glass plate [Figure 1B]. By optimizing and regulating the content of pore-forming agents [Supplementary Figure 1], both the PEI and PEI-EDA separators have interconnected sponge-like pore structures from the top-surface and cross-section scanning electron microscopy (SEM) morphology images [Figures 1C and Supplementary Figure 2], which proved the chemical grafting of EDA hardly breaks the original pore structure of PEI separators. This morphology of PEI-EDA separators will be favorable for Li⁺ transport and induce the homogeneous deposition of lithium ions owing to the uniformly distributed pores and good affinity with electrolytes^[32]. However, the commercialized PP separator exhibits a typical slit-like pore structure through melt spinning and cold stretching [Supplementary Figure 3]. This will lead to low porosity and poor wettability of separators, and then causes poor compatibility with interfaces, restricted ion migration and inhomogeneous stripping/plating^[37,38]. Fourier transform infrared (FT-IR) spectra were used to characterize the structure of PEI and PEI-EDA separators [Figure 1D]. The absorbance peaks at 3,070 and 2,969 cm^-1 are ascribed to the stretching vibration of the unsaturated and saturated C−H bonds, respectively. The characteristic peaks of benzene rings and carbonyl bonds are represented by the absorption peaks at 2,000-1,550 cm^-1. In addition, the absorptions at 1,276 and 1,102 cm^-1 are attributed to the stretching vibration of C−O and C−O−C single bonds in PEI chains^[39]. The PEI-EDA separator modified with EDA has a stretching vibration of N−H bonds of −NH₂ and −CONH− at 3,300 cm^-1, indicating the existence of the exposed groups of −NH₂ and −CONH−. The characteristic peaks of −C=O bonds in the range of 2,000-1,550 cm^-1 are partially enhanced and weakened. Specifically, the original characteristic peaks of imide at 1,737 and 1,360 cm^-1 have weakened obviously, and the new peaks of amide appear at 1,652 and 1,543 cm^-1. Although the values of stress and strain of PEI-EDA are lower than PEI due to the grafting and destruction of the rigid structure of cyclic imides, its mechanical property is sufficient to support the use of a separator [Figure 1E]. Specifically, a PEI-EDA separator maintains flexible and good mechanical performance. As shown in Supplementary Figure 4, upon multiple folding and repeated folding, the PEI-EDA separator recovered completely without any cracks.

Figure 1. Preparation and physical characterization of separators. (A) Fabrication process of PEI-EDA separator; (B) Rolls of the PEI-EDA separator; (C) SEM morphology images of PEI-EDA separator; (D) FT-IR spectra of PEI and PEI-EDA separators; (E) Strain-stress behavior of the PEI and PEI-EDA separators; (F) Burning images; (G) Photos of separators after different temperature heat treatment for 30 min; (H) DSC analysis. PEI-EDA: Ethylenediamine grafting polyetherimide; PEI: polyetherimide; SEM: scanning electron microscopy; FT-IR: fourier transform infrared; DMAc: N, N-dimethylacetamide; PP: polypropylene.

The thermal stability and flame retardancy of the separator are essential to the safety performance of the batteries. During the burning tests, the PEI-EDA separator self-extinguished quickly, whereas the commercial PP and PEI separators burn out entirely [Figure 1F and Supplementary Videos]. The better flame retardancy of PEI-EDA separators may be due to the increase of nitrogen element content. As presented in Figure 1G, 3 separators had the same size at 25 °C, and after staying in a muffle furnace at 100 °C for half an hour, PEI and PEI-EDA separators had no obvious shrinkage, but the PP separator curls a little. Moreover, the PP separator showed significant shrinking when heating to 150 °C, and then it shrunk entirely at 180 °C. However, the PEI and PEI-EDA retained their initial size well, even when heated to 200 °C, indicating they have excellent thermal stability. Differential scanning calorimetry (DSC) was used to characterize the thermal properties [Figure 1H]. In the DSC curve of the PP separator, the endothermic peak appears between 129 °C and 162 °C, indicating that the PP is beginning to melt and does not keep its shape intact. However, there are no obvious heat absorption peaks of the PEI-EDA separator at a temperature below 400 °C. The results show that a PEI-EDA separator is safer than PP and PEI separators.

The porosity and electrolyte uptake of different separators were tested and calculated according to Supplementary Equations 1 and 2, and the results were shown in Figure 2A. The PEI-EDA separator can maintain shape integrity and does not dissolve when immersed in different electrolyte solvents, even for 15 h [Supplementary Figure 5]. The porosities of PP, PEI and PEI-EDA separators are 54.0%, 87.2% and 87.6%, and their electrolyte uptake rates are 91.0%, 451.0%, and 456.0%, respectively. Because of the similar porous structures, the porosities and electrolyte uptake rates of PEI and PEI-EDA are similar and far higher than that of the PP membrane. Notably, a PEI-EDA separator has the highest porosity and electrolyte uptake, owing to the extra polar groups of −NH₂ and −CONH−, which improve the affinity of the PEI-EDA separator with electrolyte and facilitate capillary intrusion of the liquid electrolyte into the separator^[40]. The affinity between electrolyte and separator could be evaluated with contact angle [Figure 2B], and the contact angle with a PEI-EDA separator is 11.9°, obviously lower than with PEI (16.9°) and PP (42.5°) separators, which verifies its good affinity with carbonate solvents.

Figure 2. Evaluation of physiochemical parameters for PP, PEI and PEI-EDA separators. (A) Porosity and electrolyte-uptake rate; (B) Contact angle; (C) LSV curves (inset: the magnified LSV curves); (D) EIS analysis (inset: the magnified EIS analysis); (E) Li⁺ ion transference number (inset: Nyquist plots before and after polarization); (F) Radar chart of key properties indicators. PEI-EDA: Ethylenediamine grafting polyetherimide; PEI: polyetherimide; PP: polypropylene; LSV: linear sweep voltammetry; EIS: electrochemical impedance spectroscopy.

The linear sweep voltammetry (LSV) measurement is used to evaluate the electrochemical stability of separators. The initial decomposition voltage of PEI-EDA is up to 5.04 V, indicating its excellent oxidation stability [Figure 2C]. The ion conduction behaviors of three types of separators were investigated via electrochemical impedance spectroscopy (EIS) with symmetric cells with stainless steel (SS||SS) plates as electrodes [Figure 2D], and the ionic conductivities and ion transference numbers of 3 separators were calculated according to Supplementary Equations 3 and 4, respectively. After being modified by EDA, the bulk resistance (R_b) of the SS||PEI-EDA||SS cell gradually decreases from 0.89 to 0.78 Ω, and the ion conductivity increases from 1.72 to 1.96 mS·cm^-1, both exceeding that of PP (0.96 mS·cm^-1). Furthermore, the lithium-ion transference numbers ($$ t_{Li^{+} } $$) of 3 separators were calculated by the results of Nyquist plots and I-t curves of Li||Li symmetric cells [Figure 2E and Supplementary Figure 6]. The $$ t_{Li^{+} } $$ of a PEI-EDA separator is 0.74, higher than that of PP (0.45) and PEI (0.55) separators. It is suggested that the PEI-EDA separator facilitates the lithium ion transfer due to its interconnected sponge-like pore structure and exposed polar groups of −NH₂ and −CONH−. Overall, the PEI-EDA separator was made with a simple and scalable fabrication procedure, and provided superior wettability, ion transference number, porosity and ion conductivity for safe-credible LMBs [Figure 2F].

Li-stainless steel (Li||SS) cells were used to evaluate the deposition/stripping kinetics with different separators. Figure 3A showed the cyclic voltammetry (CV) profiles of Li||SS cells with three membranes in the voltage range from -0.2 to 2.5 V (vs. Li/Li⁺) at a scan rate of 0.5 mV·s^-1. Three predominant redox peaks appeared close to 0 V, which could be attributed to Li⁺ plating and stripping processes on the stainless-steel electrode. Remarkably, at the turning point of the curve with a negative sweep, the lithium deposition overpotential of the Li||SS cells assembled with the PEI-EDA separator was 38.9 mV, which was smaller than with PP (56.6 mV) and PEI separators (61.4 mV). In the subsequent plating and stripping process, the SS||PEI-EDA||Li cell exhibited a higher peak current density of 1.15 mA·cm^-2 compared with the SS||PEI||Li (0.36 mA·cm^-2) and SS||PP||Li cells (0.61 mA·cm^-2), which agreed with the improved Li⁺ conductivity and Li⁺ ion transference number when used with PEI-EDA separators. That showed that the PEI-EDA separator had a higher plating/stripping efficiency and remarkable reversibility. Furthermore, the Tafel curves in Figure 3B displayed the exchange current density of 0.065, 0.059 and 0.104 mA·cm^-2 for PP, PEI and PEI-EDA, respectively, indicating that the PEI-EDA separator had increased Li⁺ transfer kinetics. Then, the activation energies were further calculated by plotting Arrhenius equation [Supplementary Equation 5] in Figure 3C. The PEI-EDA separator presented the lowest activation energy (E_a) of 3.20 kJ mol^-1, meaning that it can reduce the energy barrier for Li⁺ and is favorable for Li⁺ transport.

Figure 3. Li⁺ transport of separators and interaction with electrolyte. (A) CV profiles of Li||SS cells with 3 separators; (B) Tafel curves of 3 separators; (C) E_a obtained from linear fitting with Arrhenius equation; (D) ³¹P NMR spectra and (E) ¹⁹F NMR spectra of electrolyte and different separators with electrolyte; (F) ⁷Li NMR spectra of electrolyte and PEI-EDA separator with electrolyte; (G) FT-IR spectra between 3,400-3,100 cm^-2 of PEI-EDA separator with and without electrolytes; (H) Schematic representation of the Li⁺ transfer mechanism in the PEI-EDA separator. CV: Cyclic voltammetry; NMR: nuclear magnetic resonance; PEI-EDA: ethylenediamine grafting polyetherimide; FT-IR: fourier transform infrared; PP: polypropylene; PEI: polyetherimide.

In order to explain the active role of the polar groups of −NH₂− and −CONH−, the ³¹P nuclear magnetic resonance (NMR), ¹⁹F NMR and ⁷Li NMR spectra of the separators with electrolytes were tested and shown in Figure 3D-F. As shown in Figure 3D and E, there was a new split peak showing in ³¹P NMR and ¹⁹F NMR of PEI-EDA separators; however, this phenomenon was not observed in other separators. It suggested that PEI-EDA separators provided different environments and had a weak interaction with PF₆^-. The ⁷Li NMR spectrum was measured to investigate the interaction between PEI-EDA separators and Li⁺ [Figure 3F]; it could be found that the peak becomes wider and transforms from double peaks to multiple peaks. Additionally, the main peak at -0.19 ppm in the ⁷Li NMR spectrum shifts upfield compared to the pure electrolyte at -0.17 ppm. This result was attributed to the lone-pair electrons on nitrogen atoms from the groups of −NH₂ and −CONH− of PEI-EDA separator grafted with EDA, which exhibited strong interaction with Li⁺. The high Li⁺ affinity of PEI-EDA separators could stimulate the high dissociation of lithium ions from their counterparts, thereby enhancing the mobility of lithium ions^[41]. The ionic affinity of the PEI-EDA separator to LiPF₆ was also examined by FT-IR spectra. A blue shift of −NH₂ and −CONH− in PEI-EDA separators was observed [Figure 3G]. The effect of the −NH₂ and −CONH− groups on the PEI-EDA separator for Li⁺ transport properties was further demonstrated in Figure 3H. The PEI-EDA separator contains uniformly distributed terminal amino groups (−NH₂) and the exposed amide groups (−CONH−), which interact with Li⁺ and PF₆^-. These polar groups facilitate Li⁺ transport and promote uniform lithium-ion flux.

The Li||Li symmetric cells were assembled to demonstrate the effect of the separators with liquid electrolytes for the suppression of lithium dendrites. When the current density was 0.2 mA·cm^-2, the polarization voltages of the PP, PEI and PEI-EDA separators were 34.3, 31.4 and 27.6 mV after cycling 200 h [Figure 4A], respectively. As the cycle time increased to 1,800 h, the polarization voltage of PP, PEI and PEI-EDA separators increased to 51.9, 44.0 and 36.3 mV, respectively. It is worth noting that the PEI-EDA separator has superior performance with the smallest polarization compared with PP and PEI separators. Under the condition of low current density, a PEI separator had a slightly smaller polarization than a PP separator owing to its connected and uniform pores. When the current density rose to 1 mA·cm^-2(1 mAh·cm^-2) [Figure 4B], the large pore size of the PEI separator can easily promote the growth of lithium dendrites along the pores^[29,30]. Therefore, the Li||PEI||Li cell showed a shortened lifespan at the beginning of cycling. The low porosity and wide pore size distribution of a PP separator leads to the nonuniform distribution of lithium-ion flux^[25,42], and would cause the dendritic growth of lithium metal, especially at higher current densities. Therefore, the PP separator suffered from increased polarization voltage as the cycle time increased to 250 h due to the unstable interface layer. Impressively, the Li||PEI-EDA||Li cell could cycle consistently for 300 h with a relatively lower polarization voltage at 300 h. This advantage was also reflected in the rate performance [Figure 4C], and the PEI-EDA separator showed the smallest polarization at different current densities, demonstrating its stable interface layer. To comprehend the mechanism for the lower plating/stripping overpotential in the different separators, the EIS tests were used to evaluate the Li||Li symmetric cells at different conditions [Figures 4D and Supplementary Figure 7], and the Nyquist plots fitted from the equivalent circuit elements were summarized in Supplementary Table 1. When compared with PP and PEI separators, the R_b and interfacial resistance (R_i) of PEI-EDA separators showed little change at the pristine state, after 7 days of rest and after 50 cycles. This stability was attributed to the ability of the PEI-EDA separator to induce uniform Li plating and form the stable SEI layer. Moreover, the Li||PEI-EDA||Li symmetric cell showed minimum interfacial impedance, mainly due to the fewer insulating by-products formed on the Li anode and the lower diffusion resistance of Li^+[43].

Figure 4. Performance of the Li||Li and Li||Cu cells with PP, PEI and PEI-EDA separators. Constant current cycle curves of Li||Li symmetric cells at 0.2 mA·cm^-2, 0.2 mAh·cm^-2 (A), 1 mA·cm^-2, 1 mAh·cm^-2 (B), and at stepped-increased current densities from 0.5 to 5 mA·cm^-2 (C); (D) EIS spectra of the Li||Li cells after 50 cycles; (E) Galvanostatic voltage profiles of Li deposition; (F) Performance of the Li||Cu cells; (G) Charge and discharge profiles of the Li||Cu cells; (H-J) SEM image of the Li-metal on Cu foil after five cycles at the density current of 0.5 mA·cm^-2: (H) PP, (I) PEI, (J) PEI-EDA. PEI-EDA: Ethylenediamine grafting polyetherimide; PEI: polyetherimide; PP: polypropylene; SEM: scanning electron microscopy; EIS: electrochemical impedance spectroscopy.

The Li||Cu half cells were assembled with 3 separators to test the efficiency of the deposition and stripping. As shown in Figures 4E, the nucleation potentials of PP, PEI and PEI-EDA separators were 74.4, 230.9 and 33.4 mV, respectively, suggesting that the energy barrier for Li⁺ deposition with PEI-EDA was lower than that of PP and PEI separators. In addition, the coulombic efficiency of Li||PEI-EDA||Cu cells remained around 81.0% after 100 cycles, which was significantly higher than the cells with the other 2 separators [Figure 4F and G]. After the Li||Cu half cells cycling at 0.5 mA·cm^-2 (0.5 mAh·cm^-2) for 5 cycles, the lithium metal on Cu foil with the PEI-EDA separator exhibited relatively smooth and regulated lithium dendrites growth [Figure 4H-J]; meanwhile, the surface morphology of the lithium metal with the PP and PEI separators displayed rough and filamentary lithium dendrites growth. Although the PEI separator had uniform pores, its large pore size made it easy for lithium dendrites to grow along the pores. However, the PEI-EDA separator had additional groups of −NH₂ and −CONH− that can interact with Li⁺ and facilitate Li⁺ transfer. Moreover, the uniform and interconnected pores of PEI-EDA separators help promote uniform lithium-ion flux and induce stable plating/stripping. The synergistic effects of these factors efficiently inhibit the growth of lithium dendrites.

To further reveal the chemical composition of SEI films of the Li||Cu half cells assembled with 3 separators, X-ray photoelectron spectroscopy (XPS) was performed [Figure 5A]. The SEI on Cu foil substrates with 3 separators is mainly composed of LiF, Li₂CO₃, ROCO₂Li, and Li_xPO_yF_z. Specifically, in the C 1s spectra, the peaks of C=O–C, C=O, C–O and C–C derive from the decomposition of the liquid electrolyte. The F 1s spectra exhibited 2 peaks of LiF and Li_xPO_yF_z species, located at 684.8 and 688.6 eV, respectively^[44]. The Li 1s spectra displayed 3 peaks belonging to the LiF, Li₂CO₃ and ROCO₂Li, and the LiF content of the PEI-EDA separator had a high percentage of 37.8% due to the interaction of the PEI-EDA separator with PF₆^-. In comparison, the percentages of LiF were only 21.8% and 27.7% when equipped with PP and PEI separators, respectively [Supplementary Figure 8]. It is generally believed that the higher the LiF content in the SEI film, the more significant the lithium dendrite suppression effect, and the longer the cycle life of symmetric batteries^[45]. A transmission electron microscope (TEM) was employed to measure the characteristics of SEI on Cu foil and cathode electrolyte interphases (CEI) of LiFePO₄ (LFP) cathodes during the cycle process with different separators. After five cycles at the current of 0.5 mA·cm^-2, the SEI on Cu foil with the PEI-EDA separator was more uniform [Figure 5B-D]. On the side of the cathode [Figure 5E-G], after the first cycle, the CEI on the LFP cathode with PEI-EDA was thin and uniform, which is conducive to the Li⁺ transport.

Figure 5. Interfacial chemistry with different separators. (A) Compositions of the SEI on Cu foil substrates after five cycles at the current of 0.5 mAh·cm^-2 and corresponding XPS patterns by Li 1s, F 1s and C 1s. TEM characterization of the SEI on Cu foil after five cycles at the current of 0.5 mAh·cm^-2 with different separators: (B) PP, (C) PEI, (D) PEI-EDA. TEM characterization of the LFP cathode after one cycle with different separators: (E) PP, (F) PEI, (G) PEI-EDA. OLEMS of Li|| LFP cells with different separators: (H) PP, (I) PEI, (J) PEI-EDA. (K) Li stripping/plating behavior with PP, PEI and PEI-EDA separators. SEI: Solid electrolyte interphases; XPS: X-ray photoelectron spectroscopy; PEI-EDA: ethylenediamine grafting polyetherimide; PEI: polyetherimide; PP: polypropylene; TEM: transmission electron microscope; OLEMS: online electrochemical mass spectroscopy; LFP: LiFePO_4.

What is particularly noteworthy is the gassing problem for many carbonate electrolytes in LMBs. OLEMS was used to detect the components of gases produced in the 3 separators during the first charge and discharge process [Figure 5H-J]. The result demonstrated that the main gases produced from 3 cells with different separators were hydrogen (H₂) and carbon dioxide (CO₂). H₂ mainly comes from the reaction of moisture water in salts with lithium metal, side reactions between binder PVDF and lithium metal, and so on^[46,47]. The cell production of H₂ with PP, PEI and PEI-EDA separators was 0.438, 0.408, and 0.153 μmol, respectively. CO₂ mainly comes from the decomposition of EC in electrolytes^[48]. It could be seen that PP and PEI separators had gas production of 0.166 and 0.298 μmol, much higher than that of a PEI-EDA separator (0.029 μmol). From these results, it is clear that using a PEI-EDA separator can reduce the production of gas effectively by generating a stable and unique interface layer during the cycle process. Thanks to the flame retardancy and low production of gas, the safety of LMBs will be enhanced.

Figure 5K shows the Li stripping/plating behavior on the Li anode or Cu foil systems with commercial PP, PEI and PEI-EDA separators. The low porosity and wide pore size distribution of a PP separator leads to the nonuniform concentrated lithium-ion flux and then induces unmanageable dendrite growth during Li plating process. When paired with the PEI separator, its large pore size can easily lead to the growth of lithium dendrites along the pores. However, when employing the PEI-EDA separator, because of the uniform and interconnected pores and the homogeneously extra distributed polar −NH₂ and −CONH− groups, lithium-ion flux is uniformed and then the following electro-deposition is well regulated accordingly. Furthermore, the high $$ t_{Li^{+} } $$ and porosity of a PEI-EDA separator can reduce the removal of lithium ions near the electrode surfaces and effectively suppress dendritic depositions by reducing the space charge effects^[49]. Meanwhile, this also leads to the uniform formation of the fluorine-rich SEI.

The advantage of the PEI-EDA separator had also been reflected by assembling into the Li||LFP cells. The CV profiles of Li||LFP cells with three different separators were measured and shown in Supplementary Figure 9. The CV profiles all display a pair of reversible oxidation-reduction peaks, and the Li||PEI-EDA||LFP cell has faster reaction kinetics with higher redox peaks. The cycling performance of the cells had been evaluated at a current density of 5 C (1 C = 170 mAh·g^-1) [Figure 6A]. The initial capacity of cells with PP and PEI separators were 84.5 and 77.7 mAh·g^-1, and the remaining capacities were 70.7 and 46.5 mAh·g^-1 after 800 cycles with a capacity retention rate of 83.7% and 60.0%, respectively. However, the initial capacity of Li||PEI-EDA||LFP cell was 90.5 mAh·g^-1, and the remaining capacity was 83.5 mAh·g^-1 with a capacity retention rate of 92.3% after 800 cycles, which was obviously higher than the cells with other 2 separators. The detailed voltage-capacity curves to describe the capacity retention after 800 cycles at 5 C were shown in Supplementary Figure 10. Among all cells, the PEI-EDA separator displayed the highest capacity retention with a flat and stable voltage platform. Figure 6B showed the rate performance of 3 cells assembled by using 3 separators at different rates of 0.1, 0.2, 0.5, 1, 2, 3, 4 and 5 C, respectively. The Li||PEI-EDA||LFP cell showed slightly higher capacities at different current densities, especially at high current densities. The corresponding galvanostatic charge and discharge curves at different rates further confirmed the better capacities of Li||PEI-EDA||LFP cells. Moreover, the Li||PEI-EDA||LFP cell exhibited significantly lower polarization voltage at each rate than PP and PEI separators [Supplementary Figure 11], indicating more efficient reaction kinetics and better rate performance. Furthermore, the Li||PEI-EDA||LFP cell had a smaller R_i compared with the other cells [Figure 6C].

Figure 6. Electrochemical performance of Li||LFP cells with PP, PEI and PEI-EDA separators. (A) Cycling performance at the current density of 5 C; (B) Rate performance; (C) EIS at different times; (D) Cycling performance at 60 °C; (E) Cycling performance of cells with high loading; (F) Cycling performance of Li||PEI-EDA||LFP pouch cell; (G) Comparison of electrochemical performances between this work and other reported separators. LFP: LiFePO₄; PEI-EDA: ethylenediamine grafting polyetherimide; PEI: polyetherimide; PP: polypropylene; EIS: electrochemical impedance spectroscopy.

Under extreme conditions, such as at high temperatures, lithium metal is more likely to react with electrolytes to produce inert lithium, ultimately leading to a sharp decline in battery performance. The cycling performance of three Li||LFP cells with different separators was tested at 60 °C. As shown in Figure 6D, the Li||PEI-EDA||LFP cell still circulated perfectly, and there was almost no capacity attenuation after 100 cycles. Meanwhile, the capacity retention of PP separator-based cells was 93.1%. However, the Li||PEI||LFP cell showed a short circuit after seven cycles owing to the intense growth of lithium dendrites under a higher temperature. Furthermore, under the high loading of 8.87 mg·cm^-2, the Li||PEI-EDA||LFP cell was running well, and had an obviously higher capacity retention of 88.1% than a PP separator of 77.6% after 200 cycles [Figure 6E]. Not surprisingly, PEI separator-based cell short-circuited after 38 cycles.

In addition, the pouch cell with a size of 5.5 cm × 4.3 cm was assembled with a PEI-EDA separator, lithium metal and LFP, and it operated smoothly at a current density of 0.2 C [Figure 6F]. The Li||PEI-EDA||LFP pouch cell exhibited an initial discharge capacity of 25.6 mAh and a capacity retention of 98.3% after 70 cycles with an active material loading of 9.20 mg·cm^-2. Even after folding 90° and 180°, the pouch cell could still light up the device. This meant that PEI-EDA separator-based cells had good safety and promising commercialization potential. Furthermore, PEI-EDA separators could be matched to many other high-voltage cathode materials (e.g., LiCoO₂ and LiNiCoAlO₂) and displayed commendable performance, as clearly shown in Supplementary Figure 12. All these results coincided with the conclusion from the Li||Li symmetric cells that the PEI-EDA was a preferable separator material to PP and PEI separators in LMB application. This was because the exposed groups of −NH₂ and −CONH− after grafting give the separator a good ability to transport ions and inhibit lithium dendrites growth. Furthermore, the cycle numbers and capacity retention of cells using the PEI-EDA separator were compared with other reported separators [Figure 6G and Supplementary Table2]. It is worth noting that, in terms of the long-cycle operation of the cells, the cells with PEI-EDA separators exhibit superior capacity retention compared with other separators reported in the literature^[50-56].

CONCLUSION

In conclusion, a PEI-EDA separator was prepared through phase-inversion and chemical grafting methods. By using this separator, we demonstrated a strategy to accelerate ion transport and regulate lithium dendrites growth for constructing stable LMBs. After grafting with EDA, the affinity with electrolyte, ionic conductivity (1.96 mS·cm^-1), Li⁺ ion transference number (0.74), flame retardance and thermal stability of the separators were significantly enhanced. Moreover, the lone-pair electrons in the nitrogen from the groups of −NH₂ and −CONH− on PEI-EDA separators interacted with Li⁺, facilitating rapid Li⁺ transfer, uniform lithium-ion flux, and stable Li⁺ plating/stripping to suppress lithium dendrites growth. The functional groups were H₂-containing and had the electrostatic interaction with PF₆^-, realizing the formation of stable interphase fluorine-rich layers. During the cycling of Li||PEI-EDA||LFP cells, a stable interface layer could form to reduce gas production. Therefore, when using the PEI-EDA separator, the performances of both the Li||Li symmetric cell and Li||LFP cell were better than those of the PP and PEI separators. The Li||PEI-EDA||Li symmetric cells delivered a good cycling stability of over 300 h at a current density of 1 mA·cm^-2 and a capacity of 1 mAh·cm^-2. Uniform and dense lithium deposition not only provides the Li||Cu and Li||Li cells with long cycle life, but also improves the rate performance and cell lifespan. The Li||PEI-EDA||LFP cell exhibited superior rate performance and cycling stability, even with high cathode loading and without any electrolyte additives, and presented a capacity retention of 92.3 % after 800 cycles at 5 C. Furthermore, the Li||PEI-EDA||LFP pouch cells exhibited a capacity retention of 98.3% after 70 cycles at 0.2 C. Consequently, PEI-EDA separators have a promising application prospect that can be prepared on a large scale and provide remarkable performance for secure dendrite-free metal batteries.