Throughout the history of lithium battery development, a great deal of research and development work has focused on the improvement of cathode, anode and electrolyte. While the diaphragm is often considered a passive battery component, it also plays a key role in determining the cost, reversibility, power density and overall safety of Li-ion batteries. New generation LIB diaphragms should be able to significantly improve battery performance compared to conventional diaphragms (microporous polyolefin membranes), while being easy to prepare, low cost, and able to easily tune physicochemical properties.
Figure 1. Sketch of porous septa prepared using the monomer 1,4-butanediol diacrylate (BDDA) and the porogenic agent ethylene carbonate (EC). UV polymerization caused microphase separation to obtain porous membranes, which were used in electrochemical cells.
In the ideal PIPS process, the initial homogeneous solution of monomer and porogenic agent undergoes sub-stable phase separation as the monomer is converted to a polymer, reducing its miscibility with the porogenic agent. Since this diaphragm fabrication concept utilizes EC as the porogenic agent and electrolyte component in the assembled electrochemical cell, the key point is whether EC is involved in the polymerization reaction that immobilizes and/or changes its properties.The IR spectra of the precursor solution and polymeric material during the PIPS process are shown in Figure 2a. The spectra were normalized to the intensity of the carbonyl stretching mode of the acrylate group around 1720-1730 cm-1 and the relative changes of the other chemical functional groups and components were observed. The C=C stretching modes of BDDA present in the precursor solution (BDDA/EC) at 1615 cm-1 and 1645 cm-1 disappeared completely after photopolymerization, while the other peaks in the fingerprint region of the spectrum remained at the same wave number, indicating that only the conversion of the acrylate double bond took place. The conversion fraction of the double bonds was not quantified, but the mechanical strength of the obtained solid films was optimized to be insoluble in organic solvents, which indicates the formation of networks. As can be seen in Figure 2b, the high strength stretching pattern of the C=O carbonyl bonds in EC before and after polymerization did not change. These peaks at 1775 cm-1 and 1805 cm-1 are split by Fermi resonance, which is caused by the short-range ordering of the strong carbonyl dipoles. These characteristic peaks remain constant throughout the polymer synthesis, indicating that EC is inert and unreactive throughout the PIPS process. The carbonyl stretching peaks were predicted to merge into a single peak at 1740 cm-1 if the EC underwent a ring-opening polymerization reaction, but this was not actually observed. After washing with acetone, the carbonyl stretching peak of EC disappeared completely, proving that EC did not enter or become immobilized in the chemical structure of the polymer. Therefore, EC is a suitable porogenic agent.
Figure 2. FTIR-ATR spectra normalized to the monomeric carbonyl peak at 1720 cm-1, shown as (a) offset full spectrum and (b) overlapping spectra in the critical range, illustrating the aggregation of BDDA and subsequent removal of EC.
An important feature of the PIPS process is the ability to adjust the membrane structure compositionally through precursors. The effect of the porogenic concentration on the membrane properties can be clearly seen in Figure 3, where the lower the EC concentration, the lower the porosity and the smaller the pore size, while the membrane is translucent. As the EC concentration in the solution increases, both the porosity and pore size increase, leading to an increase in light scattering, which makes the membrane less and less transparent. Image analysis of the scanning electron micrographs shown in Figures 3b-e was used to quantify the apparent porosity percentage and pore size of the membranes. It was found that small pores dominate the collection of pores in all formulations, with small pores shifting toward larger size radii in the more porous diaphragms. Typically, the pore size of LIB diaphragms must be less than 1 μm to prevent small particles of active material from passing through the membrane. However, smaller pores are beneficial to the cell because they help the lithium-ion cell to operate at higher current conditions while also inhibiting the growth of lithium dendrites on the graphite cathode by eliminating excess ion diffusion gradients. Therefore, pBDDA60 with suitable hole size is an ideal material for lithium-ion battery applications. The authors also evaluated the surface energy of the film by measuring the static water contact angles (WCAs). With a constant polymer composition, the WCA decreased from 57.8° (pBDDA30) to 33.8° (pBDDA60) due to the porous nature of the membrane. In the water contact angle experiments, pBDDA30 and pBDDA40 do not absorb water, while pBDDA50 can absorb water and pBDDA60 absorbs more water, which is related to the porosity of the membrane. The decrease in the contact angle is due to the transition from the Cassie-Baxter state to the Wenzel state as the pore size increases.
Compared to commercial Celgard 2500-type diaphragms, pBDDA consistently exhibits a lower WCA (87.5°) (Table 1). Despite the higher apparent porosity of Celgard, the pBDDA surface is more polar and can form stronger and more durable interactions with polar solvents (and also hydrogen bonding with water). pBDDA membranes are more polar and are expected to be advantageous in LIB applications to promote electrolyte uptake and Li+ migration. In fact, the liquid electrolyte content of pBDDA60 (127%) was found to be higher than that of Celgard 2500 (98%). As expected, electrolyte uptake was directly related to porosity. The observations show that the apparent porosity increases by 27% from pBDDA50 to pBDDA60, while electrolyte uptake increases by 90%
Figure 3. (a) Images of microporous pBDDA septa with different concentrations of microporous pBDDA septa after removal of porogenic agents (from left to right, pBDDA30, pBDDA40, pBDDA50, pBDDA60, respectively). (b,f) SEM images and static water contact angle experiments of pBDDA membranes prepared with (b,f) 30% w/w EC, (c,g) 40% w/w EC, (d,h) 50% w/w EC and (e,i) 60% w/w EC.
Table 1. Physical properties of pBDDA films and commercial polypropylene diaphragm Celgard 2500 prepared with different amounts of porogenic agents.
The ionic conductivity of the liquid electrolyte in pure solution and in the microporous septum was studied (Figure 4). In this case, the activation energy of the liquid electrolyte is 13.8 ± 0.3 kJ/mol, and the activation energies of the Celgard 2500 and pBDDA60 samples are 13.4 + 1.1 kJ/mol and 13.4 ± 1.6 kJ/mol, respectively. this implies that (1) the pathway affecting the ionic conductivity is through the diffusion of the liquid electrolyte phase within the pores of the microporous septum, and (2) the diaphragm does not introduce significant additional energy barriers to prevent electrolyte diffusion. Furthermore, given the lower porosity pBDDA membranes have a slightly increased activation energy. This may be due to the fact that these membranes contain a small fraction of closed pores that may require diffusion through an additional transport mechanism such as the gel phase between polymer chains.
Fig. 4. Variation of conductivity with temperature for Celgard 2500 and pBDDA samples.
NMC532/Li half-cells were used to evaluate the electrochemical performance of pBDDA membranes and were compared with Celgard 2500 (Figure 5). The pBDDA60 was chosen for testing because it has the highest apparent porosity and conductivity of all the diaphragms. Figure 5 provides charge and discharge capacities and coulombic efficiencies for over 100 cycles. The battery has an average discharge capacity of 142 mAh/g at 3.0 ~ 4.2V cycles and a capacity retention rate of 98.4% after 100 cycles. Considering the similar ion transport characteristics and diaphragm thickness of pBDDA60 and Celgard 2500, the cycling performance of both diaphragm materials is expected to be close to the same. The stability over 100 cycles is a strong evidence that pBDDA can be adapted to advanced LIBs.
Figure 5. NMC532/Lithium half-cell cycled at C/3. (a) Discharge capacity (circles) and Coulomb efficiency (diamonds) after two initial cycles at C/10, cycled for more than 100 cycles at C/3. Voltage vs. discharge capacity curves are plotted at (b) the first cycle, (c) the 50th cycle and (d) the 100th cycle; the inset shows the knee region of the curves, indicating almost identical performance of the two diaphragms.
In order to exclude the effect of EC on the thermal stability of porous diaphragms, the authors characterized the thermal stability of porous diaphragms using thermogravimetric analysis. Figure 6a shows that the onset of BDDA decomposition is 374°C, while the onset of Celgard 2500 decomposition is 375°C. The thermal stability of both materials is sufficient for LIB applications because the alkyl carbonate electrolyte starts to decompose at 190°C. Another important thermal stability factor is the phase change at lower temperatures. The differential scanning thermal analysis plot in Figure 6b shows that the PP of Celgard 2500 starts to undergo a primary melting transition at around 150°C. The pBDDA, on the other hand, does not undergo thermal transformation at up to 200°C, indicating that the diaphragm retains its mechanical integrity at high temperatures and can effectively eliminate diaphragm failure.
Figure 6. Thermal characterization of pBDDA and Celgard 2500. (a) Thermogravimetric analysis from 50 °C to 700 °C with a gradient rate of 5 °C/min. (b) Differential scanning calorimetry from 0 °C to 200 °C with a rate of 10 °C/min and nitrogen purge. (c) Photographs of Celgard 2500 (25 μm) and pBDDA60 (19 μm) after heating at room temperature for 30 min to 90 °C, 125 °C and 150 °C.
In summary, the authors present a fast, UV-initiated PIPS technique to fabricate microporous septa. This technique is used to obtain microporous diaphragms with good performance (electrolyte absorption, ionic conductivity, thermal stability, etc.) by adjusting the concentration of the reactive multifunctional monomer precursor to a porogenic agent (EC in this paper) in a single step process. The diaphragm was applied to a LIB half-cell (NMC532 vs. Li) and tested, and the results showed that the diaphragm could be reversible for more than 100 cycles with 98.4% capacity retention compared to commercial polypropylene diaphragms. The assembled cells were successfully cycled without removing the EC porogenic agent, highlighting the potential of PIPS technology to enable fast, inexpensive, one-step manufacturing of lithium-ion batteries. Finally, PIPS is expected to serve as a platform technology through which the chemistry and performance of the diaphragm can be easily tuned and modified to meet current and future challenges facing lithium-ion batteries.