First author: Nyalaliska W. Utomo
Corresponding author: Lynden A. Archer
Corresponding unit: Cornell University, USA
It is well known that solid-state electrolytes (SSEs) formed in electrochemical cells by polymerization of liquid precursors initiated by Lewis acid salts, It provides a promising strategy to solve the problems of electrolyte wetting and contact with active electrodes in solid-state batteries . Among them, composite solid-state polymer electrolytes (HSPEs) produced by in-situ polymerization of traditional liquid precursors can provide a selectively enhanced solid-state electrolyte because the liquid precursor contains well-dispersed electrochemically inert nanostructures. , while achieving ideal electrolyte properties without the need for high polymerization degrees and long polymerization times. Here, Professor Lynden A. Archer of Cornell University in the United States synthesized a 1,3-dioxopentane containing hairy nanosilica particles initiated by Lewis acid salt Al(OTf)3 ( DOL) polymerized HSPEs, and their structure, chemical kinetics, and electrochemical properties were studied. Among them, small-angle X-ray scattering analysis showed that the particles were well dispersed in DOL. The polymerization kinetics were studied through time-dependent mechanical property measurements, revealing macroscopic kinetic changes in the ring-opening reaction upon the addition of hairy nanoparticles (HNPs). Strong interactions between poly(ethylene glycol) (PEG) molecules bound to silica particles and poly(DOL) lead to co-crystallization and anchor nanoparticles in their hosts and Enables the polymerization-depolymerization process in DOL to be studied and controlled. The reason why
chose nanoparticles composed of high dielectric constant silica nanocores and PEG chains as candidate materials for in-situ formation in this study is that: must mix the liquid electrolyte in DOL/PEG-SiO2 before DOL ring-opening polymerization. Maintain simple, liquid-like flow characteristics in the battery to promote complete wetting of all interfaces within the battery. Among them, polyethylene glycol chains may have at least three advantages: First, , the densely grafted polyethylene glycol chain layer on each silica nanoparticle will enhance the dispersion of particles to prevent the formation of particle aggregation. . Second, by co-crystallizing with the host SPE molecule, the PEG chains will introduce disorder into the host's crystal domain, reducing the crystal size and potentially lowering the SPE's melting temperature. Third , polyethylene glycol oligomer itself can conduct lithium ions.
To demonstrate the role of in situ formed HSPE, Li|HSPE|Cu half-cells were assembled based on HSPE formed from HNP, LiNO3 and LiTFSI via ring-opening polymerization of DOL, which exhibited nearly 99% Coulomb efficiency (CE) and Long cycle performance. In addition, solid-state lithium-sulfur full batteries based on in-situ formed Li|HSPE|SPAN (sulfided polyacrylonitrile) show good cycle stability, and their in-situ formed high mechanical strength SPE provides a promising path for the preparation of all-solid-state batteries. way.
Related research results were published on Adv. Mater. under the title "Structure and evolution of quasi-solid-state hybrid electrolytes formed inside electrochemical cells".
[Research Background]
In recent years, solid-state rechargeable batteries have attracted widespread attention from researchers and global investors. Solid-state batteries eliminate basic safety and performance barriers in batteries using lithium metal as the negative electrode. The past decade has seen the emergence of many solid-state electrolyte options, including inorganic solids (e.g., ceramics), organic polymers, and organic-inorganic hybrids. Although solid-state electrolytes with high room-temperature ionic conductivity and good mechanical properties have been proposed, lithium dendrites can still grow through defects within them. An emerging approach is to fabricate solid-state battery electrodes with built-in ion conduction pathways to achieve uniform ion transport, but this reduces the volume and specific capacity of the electrode and may also increase battery cost.Solid polymer electrolytes (SPEs) are able to overcome the wettability challenges they face, but PEO-based polymer electrolytes are semi-crystalline polymers that crystallize in the temperature range of 60-65°C. Therefore, SPEs based on PEO polymers can only penetrate the pores of battery electrodes at temperatures slightly higher than the target temperatures for most applications.
In previous research, a solid electrolyte formed based on the ring-opening polymerization of 1,3-dioxopentane (DOL) can effectively promote the reversible deposition and stripping of lithium. However, the ring-opening polymerization reaction is reversible, which This means that at any given time, there is a broad distribution of macromolecules in the battery, and while it is beneficial to achieve high room temperature ionic conductivity, it is not possible to produce high enough molecular polymers to achieve the mechanical properties required for solid-state batteries. of electrolytes .
【Core content】
1. Ion conductivity and activation energy of mixed Poly(DOL)/SiO2-PEG
Figure 1 reports the hybrid composed of Poly(DOL), PEG-SiO2 and Poly(DOL)/SiO2-PEG Ionic conductivity of SPEs. At relatively modest silica loadings, the introduction of PEG-SiO2 increased the room temperature conductivity by 1.5 mS/cm (more than a twofold increase), demonstrating that further improvements can be achieved by adding PEG-SiO2 nanoparticles to the material. At the same time, we would like to further understand whether the increase in room temperature ionic conductivity of Poly(DOL) is the result of changes (such as slowing down) in the ring-opening polymerization kinetics of DOL in the presence of PEG-SiO2 nanoparticles. Therefore, the change in ionic conductivity during the mixing process during the polymerization reaction was evaluated at a fixed concentration of Al(OTf)3 initiator. At the same time, the evolution of dynamic elasticity/storage modulus with time of hybrid SPE was also studied. The results reported in Figure 1b show that within 1 h of start-up, the modulus increases by nearly four orders of magnitude, reaching a stable steady-state value.
Figure 1. Conductivity testing of pure and mixed samples.
2. Structure and nanoparticle dispersion in hybrid SPE
At the same time, small-angle X-ray scattering (SAXS) was used to study the dispersion problem of PEG-SiO2 nanoparticles in hybrid poly(DOL)/SiO2-PEG. Figure 2 illustrates that in different Research results on HSPEs at Al(OTf)3 initiator concentration. The results in Figure 2a show that both features are well-dispersed and unaggregated spherical scattering features for all initiator contents. According to previous experimental and theoretical studies, the peak with the lowest value of q=q1 reflects the repulsion between hairy nanoparticles, while the lowest peak with q=q2 reflects the entropic attraction between polyPEG chains. Figure 2c shows that both distances increase with increasing initiator content. This trend is generally observed in dilutions of uniform particle suspensions , where less correlated nuclei lead to larger interparticle distances.
Figure 2. Small-angle X-ray scattering (SAXS) curves to determine the structure of PEG-SiO2HNPs/poly(DOL) hybrid SPE
3. Ring-opening polymerization kinetics
Considering PEG-SiO2 nanoparticles at room temperature for hybrid SPE beneficial effects on ionic conductivity, but do the nanoparticles achieve this effect by somehow interfering with the ring-opening polymerization of DOL. The authors studied the polymerization reaction kinetics of DOL at room temperature and illustrated the above issues by measuring the relationship between mechanical loss and time.
Figure 3. The polymerization kinetic changes caused by PEG-SiO2 HNPs in Poly(DOL) SPE were studied based on the time-varying dynamic shear flow measurement method.
4. Electrochemical cycle based on Poly(DOL)/SiO2-PEG system
The results in Figure 4a show that after adding LiNO3 to the Li||Cu battery based on Poly(DOL)/SiO2-PEG system, the Coulomb of the battery The efficiency is increased to 99%, and a similar effect can be seen in liquid DOL electrolyte, where the addition of LiNO3 also increases the CE value to as high as 97%. It has been reported that the addition of LiNO3 can reduce DOL reactivity over a wide potential range, which is partly responsible for the observed CE enhancement.Meanwhile, to illustrate the potential applications of HSPEs, in full cells based on sulfurized polyacrylonitrile (sPAN) cathode, metallic lithium anode, and HSPE with and without 0.5 M LiNO3 as electrolyte, the CE of HSPE in the first cycle was high and stable. , after an initial period of attenuation, the discharge capacity shows good stability under constant current density. It is worth noting that when cycling from a higher rate down to the original 0.1C, it has a 90% capacity retention rate compared with the stable value of the first few cycles.
Figure 4. Electrochemical properties of HNPs/Poly(DOL) hybrid electrolyte.
[Conclusion Outlook]
In summary, DOL ring-opening polymerization containing PEG-SiO2 hairy nanoparticles can be used to synthesize mixed solid-state Poly (DOL) electrolytes with different initiator contents in batteries. The PEG-SiO2 structure hinders the crystallization of Poly(DOL), which results in a significant increase in the room temperature ionic conductivity of the hybrid electrolyte. Compared with SPEs composed entirely of PEG-SiO2, the hybrid electrolytes also show an increase in values from the nS/cm scale to the mS/cm scale. At the same time, the structure of the dispersion was studied by small-angle X-ray scattering (SAXS), and it was observed that the PEG-SiO2 particles were well dispersed in its Poly(DOL) host. Analysis of structure factors derived by SAXS showed that the distance between nanoparticles and PEG tethers increases with increasing initiator content. Therefore, increasing the initiator content is thought to have a diluting effect similar to that of nanoparticles in suspension. Time-dependent mechanical shear analysis showed that the PEG-SiO2 particles changed the polymerization kinetics, which was enhanced by the addition of LiNO3 additive. The addition of LiNO3 hinders ring-opening polymerization, manifested by a longer induction time, but otherwise has no effect on the polymerization process. The enhancement in room temperature ionic conductivity relative to SPEs composed of self-suspended PEG-SiO2 is due to a transition from hindered soft glass dynamics to polymer-like behavior. Compared with PEG-SiO2 Poly(DOL) electrolyte with the same initiator content and self-suspended, hybrid SPE exhibits lower energy. The resulting Poly(DOL) electrolyte was finally evaluated in Li||SPAN full cells and demonstrated its improved electrochemical performance.
【Literature information】
Nyalaliska W. Utomo, Yue Deng, Qing Zhao, Xiaotun Liu, Lynden A. Archer*, Structure and evolution of quasi-solid-state hybrid electrolytes formed inside electrochemical cells, 2022, Adv. Mater., https://onlinelibrary.wiley.com/doi/10.1002/adma.202110333
