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Low-cost multi-function diaphragm is expected to significantly increase lithium battery life

For the true popularity of electric vehicles, the service life of lithium-ion batteries is very high, and it is almost maintained for about 10 years. In this article, Israeli scientist Doron Aurbach et al. reviewed the progress in the inhibition of the dissolution of manganese in batteries by multi-functional membrane materials since 2016, as well as the latest understanding of the performance degradation of lithium-ion batteries. In general, multi-functional membranes (MFSs) capture manganese ions, remove acidic components from the battery, and even provide alkali metal ions, thereby increasing the capacity retention and rate performance of the battery during cycling. In most of the multi-functional separator articles, the positive electrode is generally LiMn2O4 (LMO), LiNi0.6Mn0.2Co0.2O2 (NCM622) or LiNi0.5Mn1.5O4 (LNMO), the negative electrode is graphite, and the electrolyte is LiPF6/mixed organic carbon. Acid salts, common characterization methods are XRD, ICP-OES, XANES, HR-SEM, FIB-SEM and MAS-NMR. This article not only reviews the current challenges, opportunities, and futures of multi-functional diaphragms, but also releases some newer data as a support.

Automotive electrification is one of the revolutionary changes in today's automotive transportation industry. To accommodate this change, lithium-ion batteries (LIBs) will become the main technology choice for automotive electrification in the foreseeable future. Compared with lithium batteries in the portable electronics and grid storage battery markets, electric vehicles require lithium batteries with higher duty cycles (ie wide operating temperature and high charge and discharge rates) and durability (expired lifespan of 10 years). . Therefore, although steady progress has been made in the development of various positive and negative active materials. Lithium-ion batteries still have considerable challenges in terms of mileage and fast charging capability. Spinel-type LMO cathodes have the advantages of a wide temperature range, acceptable energy density, high thermal stability, low cost, and non-toxicity, but manganese in the material is easily dissolved in the electrolyte.

There are three common attenuation modes for batteries: (1) DMDCR attenuation, generally transition metal (especially manganese ions) is dissolved from the positive electrode, then migrated through the electrolyte and deposited on the negative SEI; (2) electrochemical activity and transportability of the material Reduction of lithium ions; (3) decomposition of the electrolyte, reduction in the anode, oxidation in the cathode, and accompanying gas production. In real batteries, three attenuation modes are often present, and mechanisms 2 and 3 exist in all lithium ion batteries regardless of the structure or composition of the positive active material.

In recent years, scientists have proposed several methods to delay the degradation of lithium-ion battery performance: (1) anion-cation substitution in the LMO lattice, (2) manganese ion barrier layer, and (3) ALD on the active material or electrode. Coating, (4) electrolyte additive, (5) lower SOC range. However, it turns out that none of the above methods are 100% effective. Over the past decade, the author's group has designed and studied a new approach to delaying performance degradation, namely multi-functional membrane modification. Based on this, the work reviewed in this paper is the progress of the membrane component in the design of lithium-ion batteries. Compared with other parts, the characterization of the membrane is easier, and it can be clearly characterized after the battery is disassembled. Moreover, the materials used by the authors and others are all Low cost materials (less than $3/kg) so they can be easily processed and integrated into existing diaphragm lines.

The functionalized membrane obtained by chemical modification can effectively prevent manganese dissolution and acid scavenging, wherein (a) is 18-crown-6, (b) is aza-15-crown-5, and (c) is iminodiacetic acid Sodium (IDANa2), (d) is dilithium maleate (MALi2), (e) is lithium acrylate (LiAA); (f) is ethylenediamine tetralithium (Li4EDTA), (g) is bipyridine (BPA) (h) is polyethyleneimine (PEI) and (i) 4-vinylpyridine (4VP). Although crown ether polymers can capture manganese ions well, their use in actual batteries is prohibitively expensive, and commercial ion exchange resins and compounds are relatively inexpensive, and are expected to replace crown ether polymers.

It is very meaningful to study the inhibition of the dissolution of transition metal ions by the separator. Various evidences indicate that the manganese ions in the electrolyte are mainly trivalent rather than divalent. This result is in the LMO//graphite battery (J. Am. Chem. Soc., 139, 1738 (2017).) or in the LNMO//graphite battery (Anal. Chem., 88, 4440 (2016). ), there is sufficient evidence to prove that the ratio of ions is different. In addition, the ability of different materials to capture various transition metal ions is also very different. In principle, the choice of the capture membrane material can be adjusted according to the specific positive electrode used in the lithium ion battery. It is worth noting that in addition to the TM ion trapping capability, the actual production of chemically modified separator materials requires consideration of other factors such as ease of processing, mechanical properties, chemical resistance and cost in the battery environment, and the like.

The lithium batteries tested by Doron Aurbach are generally 2023 type button batteries (2–3 mAh) and single-layer soft pack batteries (50 mAh). The positive electrode is LMO, NCM622 or LNMO, the negative electrode is graphite, and the electrolyte is 1M LiPF6. EC/DMC/EMC, the ratio is generally 1:1:1 or 2:2:6 (v/v). Using a commercial PP membrane or PVDF-HFP membrane as a control, NMR characterization was performed before and after the cycle. As shown in the above figure, the SEM image of the separator with poly(4VP) filler shows that the membrane consists of a large pore network (Fig. a). These macropores are surrounded by PVDF-HFP (Fig. b), which is filled with agglomerated poly(4VP) (Fig. c), and the fillers agglomerate to form nanopores (Fig. d).

The above table compares the electrochemical characterization and manganese ion capture performance of different membranes in LMO//graphite and NCM622//graphite batteries. Some obvious trends can be seen from the table, such as the manganese capture membrane pair during the cycle. The increase in LMO//graphite battery capacity retention depends on the battery test temperature. At 100 ° C and 30 days at 30 ° C, the capacity of the lithium trapping membrane battery is 10% to 17% higher than that of the commercial diaphragm; at 100 ° C and 30 days at 55 ° C, the capacity of the lithium trapping diaphragm battery is higher than that of the commercial diaphragm. 20% to 38%. At the same time, when circulating at 30 ° C, the manganese content in the graphite electrode is reduced by 5 to 24 times, and when it is cycled at 55 ° C, it is reduced by 4 to 21 times.

Figure a above shows the cycle data of LMO//graphite and NCM622//graphite batteries (with commercial diaphragm and poly(4VP) diaphragm) at 55 °C. It can be clearly seen that the battery with poly(4VP) diaphragm is in circulation. The capacity retention of the LMO//graphite battery was increased by a factor of 2, while the amount of manganese deposited on the graphite electrode was also reduced by a factor of 4.8. Since the membrane does not capture manganese ions but removes acidic species, this data provides direct evidence of acid removal using a membrane to inhibit manganese dissolution in LMO//graphite batteries. In addition, in NCM622//graphite batteries that are not affected by manganese dissolution, the poly(4VP) separator reduces the amount of manganese deposited on the graphite negative electrode from only 167 ppm to 102 ppm, a 39% reduction.

Figure b further illustrates the importance of the acid scavenging membrane to inhibit manganese dissolution. In this figure, a conventional membrane (PP), an acid scavenging membrane (poly(4VP)) and two manganese trapping membranes (poly(IDANa2) and Na4EDTA) was compared in a NCM622//graphite coin cell battery. The capacity retention rates of the four separators were ranked as: PP< Na4EDTA < poly(IDANa2) << poly(4VP). Figures c and d test poly(4VP) and PP separators in LNMO//graphite batteries. The authors found that the average capacity loss rate of three cells with poly(4VP) separators ranged from 0.01% to 120 cycles. 0.11% is different, and the amount of transition metal nickel on the graphite electrode is reduced by 25 times. That is to say, the stability of the capacity retention rate is not due to the manganese dissolution being greatly suppressed, but by maintaining the manganese dissolution amount at the performance attenuation threshold. the following.

To test the feasibility of multi-functional membranes in actual battery production, the authors created three poly(IDANa2) membranes (MFS 1-1, MFS 1-2, MFS 1-4) with thickness and commercial membranes (10 and 25). Μm) is similar. As shown in Figure a above, the capacity retention of the poly(IDANa2) membrane in the LMO//graphite battery is significantly higher than that of the commercial membrane, and the manganese capture capacity of the MFS 1-1 membrane is saturated even during cycling. In Figures b and c, it can be seen that after the cycle of the battery containing the MFS 1-1 membrane, there is no manganese on the graphite anode, which clearly shows that in the battery, the use of the multi-function membrane can increase the capacity retention rate and extend the battery life. .

As can be seen from the above figure, after 12 cycles of the battery, the battery containing the MFSs separator has a graphite electrode impedance 4 to 6 times smaller than that of a commercial PP separator. Similarly, after 12 cycles, it contains The cell of the MFSs separator has a LMO electrode impedance that is about 3 times smaller than that of a battery containing a commercial PP separator. The reduction in impedance clearly demonstrates that the multifunctional membrane effectively inhibits the growth of SEI, thereby improving power retention and capacity utilization during cycling, or improving battery health and durability.

Scanning electron microscopy SEM and electron spectroscopy EDX were used to detect the cross-section of the fiber on the graphite electrode after the cycle, and the main cause of the impedance reduction was found. As can be clearly seen from the above figures a and b, when the cycle is over, the SEI on the graphite electrode in the battery using the commercial membrane is much thicker than the SEI on the graphite electrode of the battery using the MFS 1-1 membrane, and The thickness of the SEI can be seen in c, which is 18 μm and 30 μm, respectively. In addition, Figure c also reveals two other important differences in the SEI morphology on the two electrodes. First, in the presence of the MFS 1-1 membrane, the SEI thickness distribution is much smaller than in the presence of the PP membrane. : less than 250 nm and greater than 350 nm respectively; secondly, after recycling, the SEI of the graphite electrode is small, and the SEI thickness of about 1% is less than 50 nm, which indicates that there is very small or very narrow SEI. Areas (pinholes or trenches) that are problematic for battery performance because they are prone to lithium plating and produce uneven current distribution, local overcharge and overdischarge during battery cycling .
 
In contrast, SEI is more compact and uniform on graphite electrodes that use the MFS 1-1 diaphragm cycle. In addition to the physical differences in the SEI of the battery containing two membranes, the chemical compositions revealed by EDX, 19F-MASNMR and FTIR also differ. In Figures e and f, the EDX spectra of the red dot regions marked in Figure d are shown. It can be seen that the f:p ratios of the two sampling regions are completely different. In addition, in addition to the internal heterogeneity of the sample, there is also a significant difference in the f:p ratio between the two electrodes SEI. The f:p ratio inside the SEI of the MFS 1-1 diaphragm battery is close to 6, which indicates that in mfs 1- In the presence of a membrane, the PF6-anion is more stable than in the presence of a PP membrane.
 
【Conclusion and Outlook】

In summary, the authors have demonstrated in the paper that multi-functional chemically active separators can improve the rate performance, capacity utilization and retention of different transition metal oxide positive and graphite negative batteries, and the membranes used are low-cost, physical. Properties (thickness and porosity) are also consistent with commercial membranes. The improvement in performance can be demonstrated by the observed reduction in impedance. In addition, the multi-functional separator suppresses the dissolution of transition metal ions at both high and low temperatures, but some membranes can capture ions and some use acid to achieve inhibition. In addition to improving the performance of LMO//graphite batteries, the acid scavenging diaphragm can also increase the cycle capacity retention of NMC622//graphite and LNMO//graphite batteries because they truly address the root cause of electrolyte-induced battery decay: electrolyte solutions The acidification leads to parasitic reactions unrelated to the dissolution of the transition metal ions. Therefore, acid scavenging appears to be a viable and more widely applicable strategy to improve the performance and durability of lithium-ion batteries.

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