New porous polyimide copper foil separator for safe lithium-ion batteries

As rechargeable lithium-ion batteries (LIBs) gradually penetrate into various aspects of our daily lives, fire and explosion related safety issues of LIBs have become very important. In fact, the diaphragm plays an important role in the safety of LIB. The separator used between the cathode and anode avoids electronic short circuits and provides a transport path for lithium ions in the electrolyte. Basically, the ideal membrane for LIB should have high porosity and exhibit excellent electrolyte wettability to achieve rapid ion transport, while also having mechanical strength for ease of manufacturing. For battery safety, the separator should have thermal stability. Otherwise, it may shrink or melt at high temperatures around or inside the battery, causing damage or even explosion. Finally, it is important that the separator has electrochemical stability to withstand strong reduction and oxidation reaction environments during battery cycling. Developing advanced membranes with these excellent properties remains a huge challenge. Currently, microporous polyolefin membranes are used forLIB diaphragmThe serious disadvantage of membranes such as polyethylene (PE), polypropylene (PP) and their double-layer composite materials (PE-PP) and three-layer (PP-PE-PP) is poor thermal stability due to their low melting points. In recent years, researchers have been committed to developing LIB membrane replacement materials with excellent thermal stability. Polyimide (PI) is a new type of insulation material that has been widely used in various fields due to its excellent thermal and chemical properties. PI almost meets all the requirements of LIB membranes and is expected to become an ideal membrane for safe, high-pressure, and high-power LIBs. Cao et al. prepared PI nanofiber membranes by electrospinning from phthalic anhydride (PMDA) and 4,4-oxadiamine (ODA). Liang et al. introduced Al2O3 and SiO2 layers onto electrospun PI membranes through immersion coating. The electrospun PI film coated with SiO2/Al2O3 exhibits better electrochemical performance than Celgard 2400. Wang et al. synthesized organic soluble PI and prepared porous PI membranes through wet phase transformation process. manufacturePI diaphragmThe method includes two steps: (1) preparing a solution of polyamic acid (PAA) and processing the PAA precursor into the desired form (such as membrane, film, and fiber), and (2) converting it into PI through imidization treatment.

In this article, as shown in Figure 1, the PI membrane is prepared by non solvent induced phase separation (NIPS) using two pore forming agents: dibutyl phthalate (DBP) and glycerol (Gly). It was found that using two types of pore forming agents is easier to obtain a uniform porous PI membrane than using only one DBP or Gly. Compared with commercially available PE membranes, PI membranes exhibit significant thermal stability, better ion conductivity, and wettability in the carbonate and ether electrolytes of LIBs. The obtained PI separator was tested in the battery cell, and even after heating at 140 ° C for 1 hour, the battery cell remained significantly sturdy.

2.1 Experimental content

Mix PMDA (Aladdin, ≥ 99%) and ODA (Aladdin, ≥ 98%) at room temperature and mechanically stir for 20 hours to obtain a transparent and uniform PAA solution for further processing.

DBP (1g, ≥ 99.5%) and Gly (Aladdin, ≥ 99.7%) were added to 3.5g PAA solution (12wt%), and then stirred at room temperature for 2 hours to form a uniform casting solution. The solution was then scraped onto a glass plate using a scraper with a thickness of 100 μ m. Finally, immerse the obtained membrane in an ethanol coagulation bath at 40 ℃ for phase exchange, and repeat the process two or three times to remove solvents and additives. Afterwards, dry and imidize the wet film in air circulating ovens at 100 ℃, 200 ℃, and 300 ℃ for 1 hour, and then use the obtained opaque yellow PI film as the next experiment.

2.2 Morphology and Structure of PI Membrane

Figure 2. SEM image of PI membrane, (a) without pore forming agent, (b) only Gly, (c) only DBP, (d, e) two pore forming agents Gly and DBP at different magnifications, and (f) SEM image of PI membrane cross-section with two pore forming agents.

In order to compare the effects of pore forming agents on morphology, several PI membrane samples were prepared using the NIPS method, without any pore forming agents, containing DBP, Gly, and DBP and Gly, respectively. The morphology of the porous PI membrane was studied by SEM, as shown in Figure 2. The PI membrane prepared without any pore forming agent had very few pores (Figure a). When a small amount of Gly pore forming agent was added, a small number of pores appeared (Figures b and c). As the content of pore forming agent increases, the porosity only increases on one side, resulting in uneven pore distribution (Figure f). When using a single DBP pore forming agent, the same phenomenon can be observed (Figure 2c and). As shown in Figures d and e, when DBP and Gly are used, more pores are found in the PI membrane and evenly distributed. The porous PI membrane can be made into 10.5 μ m, as shown in Figure 2f. The sponge like and interconnected pore structure facilitates the rapid shuttle of lithium ions, thereby helping to suppress the growth of lithium dendrites. The formation of a uniform sponge like structure is related to the use of two pore forming agents. One possible reason could be the network formed by hydrogen bonds between Gly and DBP.

2.3 Thermal properties of PI film

Figure 3. Thermal properties of PE and PI films, (a) Digital photos of the films after half an hour of heat treatment at different temperatures, (b) DSC curves between 50 and 250 ° C

The thermal shrinkage of the separator plays an important role in lithium-ion batteries (LIBs). Polyolefin membranes typically shrink and wrinkle at high temperatures, which can lead to serious safety accidents. To avoid internal electrical short circuits, it is required to have no thermal shrinkage or minimum thermal shrinkage (<5%). Figure 3a shows digital photos of PI and PE membranes before and after heat treatment at 120, 140, and 180 ° C in hot ovens for half an hour at each temperature. The PE membrane continuously shrinks until it completely melts at high temperatures and exhibits significant area shrinkage and morphological changes at -140 ° C. However, even at a high temperature of 180 ° C, there was no dimensional change in the PI diaphragm. This indicates that the dimensional stability of PI membranes is much better than that of PE membranes, and batteries assembled with PI membranes can avoid internal short circuits caused by thermal shrinkage.

The thermal stability of the membrane was further analyzed by DSC and TGA. As shown in Figure 3b, the curve of the PE membrane exhibits a melting endothermic peak at 135 ° C, corresponding to the melting point of the PE membrane. For the curve of PI diaphragm, no obvious melting peak appeared until 250 ° C at 28 ° C, indicating that PI diaphragm has better thermal stability than PE diaphragm and can better maintain its shape at higher temperatures. Therefore, the excellent thermal stability of PI separators can meet the practical safety requirements of lithium-ion batteries and is expected to be used in power batteries.

2.4 Ionic conductivity and electrochemical stability

Figure 4 shows the impedance and linear sweep voltammetry of PE and PI separators with different electrolytes (a, c) for LiTFSI electrolyte and (b, d) for LiPF6 electrolyte.

Usually, the ionic conductivity is mainly influenced by the amount and mobility of lithium ions, and the higher the absorption of liquid electrolytes, the higher the amount of lithium ions. The migration rate of lithium ions is related to the porosity. The liquid electrolyte absorption of PI membranes in LiTFSI and LiPF6 electrolytes is 200% and 220%, respectively, significantly higher than that of PE membranes (132% and 129%) in LiTFSI and LiPF4 electrolytes. This may help improve the porosity of PI membranes (80% and 76% in LiTFSI and LiPF6 electrolytes, respectively). Calculate the ion conductivity of the membrane containing an appropriate amount of liquid electrolyte based on EIS, as shown in Figures 4a and 4b. According to the ion conductivity formula, the ion conductivity of PI membrane in LiTFSI and LiPF6 electrolytes is 0.54 and 0.55 mS/cm, respectively, and 1 at 25 ° C, both higher than PE membrane (0.43 and 0.49 mS/cm) In Celgard PE membranes, insufficient interaction between the polymer host and the liquid electrolyte results in electrolyte level activation energy for ion movement. The increase in ion conductivity may help lithium ions to conduct along the surface of the PI pore wall, which can increase the transport of lithium ions in the PI membrane.

In order to ensure the charging/discharging voltage, the electrochemical stability window is crucial in LIB and is tested through LSV experiments. Figures 4c and 4d show the LSV curves of different batteries (stainless steel | separator | lithium) at a scan rate of 5 mV/s, with potential windows between 0 and 6 V. For PE and PI separators with the same LiTFSI electrolyte, the current starts to rapidly increase at around 4.5 and 4.7 V (vs Li/Li), respectively, and then continues to increase with the increase of voltage. In addition, the stability of the two membranes is similar in LiPF6 electrolyte. These results indicate that the PI separator is compatible with both electrolytes, thus fully meeting the requirements of high-energy lithium-ion batteries