CHEMICAL SCIENCE

An ultra-high-capacity lithium-oxygen battery using an functionalized molecular sieve diaphragm


On November 9, 2022, Professor Xu Jijing’s team of Jilin University published a research result entitled “In situ construction of glass fiber-directed zeolite microtube woven separator for ultra-high-capacity lithium–oxygen batteries” in the journal Matter.

Inspired by the self-assembly of biomineral tissues, this work successfully developed a molecular sieve diaphragm with a customized surface, three-dimensional network microtube structure, using the precursor scaffold-solid phase crystallization method. The separator can combine the two steps of capturing and converting the discharge intermediate, thereby expanding the discharge reaction area from the positive electrode to the separator, which greatly improves the discharge capacity of the lithium-oxygen battery. The lithium-oxygen battery with molecular sieve separator with customized surface shows ultra-high discharge capacity, excellent cycle life and good safety, showing its broad application prospects in the field of energy storage batteries in the future.

The first authors of the article are doctoral students Zheng Lijun and Dr. Bai Pu of Jilin University, and the corresponding authors are Professor Xu Jijing of Jilin University.

Short cruising range and slow charging speed are the core challenges limiting the development of electric vehicles. The theoretical energy density of rechargeable lithium-oxygen batteries is much higher than that of state-of-the-art lithium-ion batteries (~3 kWh kg−1, based on O2 + 2Li+ + 2e−↔Li2O2), which is expected to provide an effective solution to this problem. At present, the main challenge facing the development of lithium-oxygen batteries is that the irregularly accumulated insulating discharge product Li2O2 on the positive electrode blocks the mass transfer channels of oxygen, ions and electrons, which ultimately limits the specific capacity of the battery. In order to make the cathode can accommodate more discharge products and improve the actual discharge capacity, many researchers have focused on the microstructure design of cathode materials and electrolyte additives to adjust the growth behavior and morphology of Li2O2 to achieve high capacity under high-power discharge. However, the accumulation of excessive discharge products on the surface of the positive electrode can lead to excessive charging overpotential, which in turn leads to low battery energy efficiency and poor cycling performance. Therefore, how to ensure the number of discharge products without negatively affecting the passivation of the cathode is a seemingly contradictory but urgent problem that needs to be solved. If a functionalized separator is designed to expand the battery discharge reaction area from the positive electrode to the separator, this contradiction can be well solved. Molecular sieves are a class of microporous aluminosilicates that fully meet all the requirements of a functional diaphragm design due to its ordered micropores, excellent thermal stability, and surface designability. Traditional molecular sieve diaphragms usually use molecular sieves grown on a diaphragm substrate or introduced into the composite diaphragm as an inorganic additive, which will block the ion transport channel of the diaphragm, and the diaphragm itself will not participate in the process of positive electrode discharge reaction. Therefore, it is important to prepare a molecular sieve diaphragm with the function of guiding the deposition behavior of discharge products and having its own self-supporting structure.

Figure 1: Synthesis method and working mechanism of molecular sieve diaphragm

In view of this, the team of Professor Xu Jijing of Jilin University successfully prepared a self-supporting molecular sieve (ZMT WF) separator with a unique three-dimensional network micron tube non-woven structure by using glass fiber as the scaffold template and using a new strategy of precursor scaffold-solid phase crystallization synthesis, realizing a lithium-oxygen battery with ultra-high capacity. The diaphragm exhibits ion conductivities of up to 2.2× 10−2 S cm−1 thanks to the abundant natural angstrom-level pores in the molecular sieve and the ordered ion transport channels formed by the microtube nonwoven structure (Figure 1). In particular, the molecular sieve separator can combine the two steps of capturing and converting the discharge intermediate, thereby expanding the discharge reaction area from the positive electrode to the separator, which greatly improves the discharge capacity of the lithium-oxygen battery. In addition, the excellent thermal stability of the separator can effectively prevent fire and explosion caused by short circuit of the battery, and improve the safety of the battery. The lithium-oxygen battery with molecular sieve separator with customized surface shows ultra-high discharge capacity, excellent cycle life and good safety, showing its broad application prospects in the field of energy storage batteries in the future.

Figure 2: Characteristics of a molecular sieve microtube nonwoven diaphragm synthesized in situ

Characterization techniques such as scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, and Fourier infrared spectroscopy all demonstrate the successful preparation of ZMT WF diaphragms with microtube scaffolds and three-dimensional network structures (Figure 2). Subsequently, the ionic conductivity of the ZMT WF separator at 25 °C was evaluated using AC electrochemical impedance spectroscopy, and its ionic conductivity was as high as 2.2× 10−2 S cm−1, thanks to the abundant natural angstrom-level pores of the molecular sieve, which can preferentially control the migration of large-sized TFSI−. In addition, the voids between molecular sieves in the microtube structure also provide a fast transport channel for the diffusion of Li+ ions.

Figure 3: Surface modification of the ZMT WF diaphragm

The molecular sieve has excellent cation exchange ability and surface designability, and the ZMT WF diaphragm is modified by cation exchange to reverse its surface charge properties. The functionalized customized surface of the ammonium bromide-modified zeolite microtube nonwoven diaphragm is rich in positively charged NH4+, and the reaction intermediate O2− can be trapped on the surface of the zeolite diaphragm by interacting with NH4+ to form an ion-pair NH4+-O2−CIP, and then disproportionation occurs during discharge and is converted into the final discharge product Li2O2 (Figure 3).

Figure 4: Discharge reaction process on the surface of the ZMT WF diaphragm

In situ electrochemical Raman spectroscopy showed that evidence of NH4+-O2−CIP (1125 cm−1) and Li2O2 (790 cm−1) could be clearly observed on the surface of the zeolite diaphragm, indicating that O2− dissolved in the electrolyte was adsorbed and NH4+-O2−CIP was formed, followed by a rapid disproportionation reaction to rapidly generate Li2O2 on the surface of the zeolite diaphragm. The surface-functionalized zeolite separator combines the two steps of capture, fixation and conversion, so that the discharge reaction area extends from the air cathode to the separator, greatly increasing the discharge capacity of the battery (Figure 4). The discharge capacity of lithium-oxygen batteries based on this separator is as high as 25100 mAh g−1, which is much higher than the 5500 mAh g−1 of commercial fiberglass separators. In addition, by reasonably selecting the size, structure and charge properties of the modified group, the discharge reaction process can be artificially controlled, and then the composition and morphology of the final discharge product can be controlled and adjusted, which can further improve the discharge capacity of the battery.

Figure 5: Electrochemical performance of lithium-oxygen batteries

Thanks to the unique three-dimensional network microtube structure and designable surface properties of ZMT WF diaphragm, lithium-oxygen batteries using ZMT WF separator show an ultra-long cycle life of 4000 times while maintaining ultra-high discharge capacity (Figure 5), showing the broad application prospects of molecular sieve diaphragm with customized surface functionalization in future energy storage systems. (Source: Science Network)

Related paper information:https://doi.org/10.1016/j.matt.2022.10.013



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