Effect of monomer structure on properties of polyimide as LIB separator and its mechanism study

Grafting Polyethyleneimine-Poly(ethylene glycol) Gel onto a Heat-Resistant Polyimide Nanofiber Separator for Improving Lithium-Ion Transporting Ability in Lithium-Ion Batteries

To improve the lithium-ion transporting ability in lithium-ion batteries, a high-performance polyimide-based lithium-ion battery separator (PI-mod) was prepared by chemically grafting poly(ethylene glycol) (PEG) onto the surface of a heat-resistant polyimide nanofiber matrix with the assistance of amino-rich polyethyleneimine (PEI). The resulted PEI-PEG polymer coating exhibited unique gel-like properties with an electrolyte uptake rate of 168%, an area resistance as low as 2.60 Ω·cm², and an ionic conductivity up to 2.33 mS·cm^-1, which are 3.5, 0.10, and 12.3 times that of the commercial separator Celgard 2320, respectively. Meanwhile, the heat-resistant polyimide skeleton can effectively avoid thermal shrinkage of the modified separator even after 200 °C treatment for 0.5 h, which ensures the safety of the battery working under extreme conditions. The modified PI separator possessed a high electrochemical stability window of 4.5 V. Compared with the batteries from the commercial separator Celgard 2320 and the pure polyimide matrix, the assembled coin cell with the PI-mod separator showed much better rate capabilities and capacity retention due to the high electrolyte affinity of the PEI-PEG polymer coating. The developed strategy of using the electrolyte-swollen polymer to modify the thermal-resistant separator network provides an efficient way for establishing high-power lithium-ion batteries with good safety performance.

Polyimides as Promising Materials for Lithium-Ion Batteries: A Review

Lithium-ion batteries (LIBs) have helped revolutionize the modern world and are now advancing the alternative energy field. Several technical challenges are associated with LIBs, such as increasing their energy density, improving their safety, and prolonging their lifespan. Pressed by these issues, researchers are striving to find effective solutions and new materials for next-generation LIBs. Polymers play a more and more important role in satisfying the ever-increasing requirements for LIBs. Polyimides (PIs), a special functional polymer, possess unparalleled advantages, such as excellent mechanical strength, extremely high thermal stability, and excellent chemical inertness; they are a promising material for LIBs. Herein, we discuss the current applications of PIs in LIBs, including coatings, separators, binders, solid-state polymer electrolytes, and active storage materials, to improve high-voltage performance, safety, cyclability, flexibility, and sustainability. Existing technical challenges are described, and strategies for solving current issues are proposed. Finally, potential directions for implementing PIs in LIBs are outlined.

Studies on the Application of Polyimidobenzimidazole Based Nanofiber Material as the Separation Membrane of Lithium-Ion Battery

Aromatic polyimide has good mechanical properties and high-temperature resistance. Based on this, benzimidazole is introduced into the main chain, and its intermolecular (internal) hydrogen bond can increase mechanical and thermal properties and electrolyte wettability. Aromatic dianhydride 4,4'-oxydiphthalic anhydride (ODPA) and benzimidazole-containing diamine 6,6'-bis [2-(4-aminophenyl)benzimidazole] (BAPBI) were synthesized by means of a two-step method. Imidazole polyimide (BI-PI) was used to make a nanofiber membrane separator (NFMS) by electrospinning process, using its high porosity and continuous pore characteristics to reduce the ion diffusion resistance of the NFMS, enhancing the rapid charge and discharge performance. BI-PI has good thermal properties, with a Td5% of 527 °C and a dynamic mechanical analysis Tg of 395 °C. The tensile strength of the NFMS increased from 10.92MPa to 51.15MPa after being hot-pressed. BI-PI has good miscibility with LIB electrolyte, the porosity of the film is 73%, and the electrolyte absorption rate reaches 1454%. That explains the higher ion conductivity (2.02 mS cm^-1) of NFMS than commercial one (0.105 mS cm^-1). When applied to LIB, it is found that it has high cyclic stability and excellent rate performance at high current density (2 C). BI-PI (120 Ω) has a lower charge transfer resistance than the commercial separator Celgard H1612 (143 Ω).

Electrospun Benzimidazole-Based Polyimide Membrane for Supercapacitor Applications

A benzimidazole-containing diamine monomer was prepared via a simple one-step synthesis process. A two-step procedure involving polycondensation in the presence of aromatic dianhydrides (4,4'-oxydiphthalic anhydride, ODPA) followed by thermal imidization was then performed to prepare a benzimidazole-based polyimide (BI-PI). BI-PI membranes were fabricated using an electrospinning technique and were hot pressed for 30 min at 200 °C under a pressure of 50 kgf /cm². Finally, the hot-pressed membranes were assembled into supercapacitors, utilizing high-porosity-activated water chestnut shell biochar as the active material. The TGA results showed that the BI-PI polymer produced in the two-step synthesis process had a high thermal stability (T_d5% = 527 °C). Moreover, the hot-press process reduced the pore size in the BI-PI membrane and improved the pore-size uniformity. The hot-press procedure additionally improved the mechanical properties of the BI-PI membrane, resulting in a high tensile modulus of 783 MPa and a tensile strength of 34.8 MPa. The cyclic voltammetry test results showed that the membrane had a specific capacitance of 121 F/g and a capacitance retention of 77%. By contrast, a commercial cellulose separator showed a specific capacitance value of 107 F/g and a capacitance retention of 49% under the same scanning conditions. Finally, the membrane showed both a small equivalent series resistance (R_s) and a small interfacial resistance (R_ct). Overall, the results showed that the BI-PI membrane has significant potential as a separator for high-performance supercapacitor applications.

Effects of the Separator MOF-Al2O3 Coating on Battery Rate Performance and Solid-Electrolyte Interphase Formation

Metal organic frameworks (MOFs) have unique advantages in optimizing the ionic conductivity of battery separators because of their rich cavity structure and highly ordered and connected pores. In this study, we used a hydrothermal method to synthesize a functional material, Ag-MOF crystal, as a separator coating content, and then studied the properties and application effect of the MOF-Al₂O₃-blended coating applying to a polyethylene (PE) separator (MOF_xAl_1-x/PE). Results show that MOF_0.08Al_0.92/PE (MOF/Al₂O₃ = 0.08:0.92) used in NCM811||Li cells significantly not only improves the fast charge-discharge performance of the cells but also inhibits the growth of lithium dendrites during long-term charge-discharge cycling; the Li⁺ transference number (t_Li⁺) of the MOF_0.08Al_0.92/PE composite separator is 0.61; the Li||separator||Li half-cell circulates stably for 1000 h at varying current density from 0.5 to 10 mA cm^-2 and only produces low overpotentials, indicating that MOF_0.08Al_0.92 stabilizes lithium. The initial capacity of the NCM811||Li cell using the MOF_0.08Al_0.92/PE separator is 165.0 mA h g^-1, its capacity retention is 70.67% after 300 cycles at 5 C, and the interface resistance of the cells only increases from 13.8 to 31.5 Ω, whereas the capacity retention of Al₂O₃/PE separator batteries is only 40.41% (62.2 mA h g^-1) under the same conditions. During the charge-discharge cycling, the MOF-Al₂O₃ coating induces the lithium anode to quickly form a stable and dense solid-electrolyte interphase layer, promotes the uniform deposition of Li⁺, and inhibits the growth of lithium dendrites as well.

Reduced graphene oxide promoted assembly of graphene@polyimide film as a flexible cathode for high-performance lithium-ion battery

Organic carbonyl polymers have been gradually used as the cathode in lithium-ion batteries (LIB). However, there are some limits in most organic polymers, such as low reversible capacity, poor rate performance, cycle instability, etc., due to low electrochemical conductivity. To mitigate the limits, we propose a strategy based on polyimide (PI)/graphene electroactive materials coated with reduced graphene oxide to prepare a flexible film (G@PI/RGO) by solvothermal and vacuum filtration processes. As a flexible cathode for LIB, it provides a reversible capacity of 198 mA h g⁻¹ at 30 mA g⁻¹ and excellent rate performance of 100 mA h g⁻¹ at high current densities of 6000 mA g⁻¹, and even a super long cycle performance (2500 cycles, 70% capacity retention). The excellent performance results in a special layer structure in which the electroactive PI was anchored and coated by the graphene. The present synthetic method can be further applied to construct other high-performance organic electrodes in energy storage.

G@PI/RGO is prepared by a combination of solvothermal reaction and carbonization. With good mechanical flexibility and high conductivity, it shows excellent performance when directly used as the cathode for LIB.