Nuclear Magnetic Resonance Study Of Solid-State Cyclodextrin/Polyethylene-Oxide Alkali Metal Salt Polymer Electrolytes

With the popularity of mobile phone, iPad, wearable devices, tesla motors and other consumer electronics, the Li-ion rechargeable batteries have been commonly used and been considered as the most promising chemical power. However, because of the using of liquid electrolyte and gel electrolyte, the lithium-ion batteries have the risk of leakage, flammability, low impact tolerance and lithium dendrite growth. To avoid these problems, researchers have tried to use the solid electrolytes instead of the liquid electrolyte. As a potential tpye of solid electrolyte, solid-state polymer electrolytes(SPE) have been intensively investigated because of its high stability, safety, and energy density. Compared to liquid electrolytes, the main shortcoming of SPE is the low conductivity in the room temperature range. In this thesis, we prepare a new category and high conductivity of SPE via the supramolecular self-assembly method with Poly(ethyl oxide) PEO, cyclodextrin (CD) and alkali metal salts. The new type of SPE, named as the CD-PEO/Li+complexes, consists of the nanochannels formed by CDs. The nanochannels provide the pathway for the directional motion of the Li+ions. By employing the state-of-art Nuclear Magnetic Resonance (NMR) technology, we investigate the structure, ion motions and polymer chain mobility. Based on these experimental results, the conductivity mechanism was analyzed. Meanwhile, several conductivity influential factors, including the channel radius, molecular weight of PEO and the cation type were also studied. The main results and conclusions are summarized as the following:1. Firstly, we prepared a new category of polymer electrolyte, i.e. a-CD-PEO/Li+. By applying the NMR technology, we studied the channel structure, lithium ion’s distribution, dynamics of Li+ions and the PEO segments systematically. The obtained results show that the PEO chain threads into the channel formed by a-CD. The 7Li NMR reveals the existence of 5 different kinds of Li+ ions. According to the state-of-art solid state NMR spectroscopy, we successfully achieved the 7Li signal assignment and quantitative analysis. Meanwhile, the dynamics of Li+ions in the sample was studied by using 7Li-7Li 2D exchange NMR and first-order reaction model. In addition,2H NMR shows a discrete 4-site jump motion present in the trans-trans-gauche (ttg) conformational sequences of PEO chain. The strong motion of PEO and lithium ion indicates the high conductivity of this kind of channel structure polymer electrolytes.2. The conductivity difference between a-CD-PEO/Li+and PEO/Li+complex were studied by electrochemical impedance spectroscopy (EIS). Results reveal that the conductivity of a-CD-PEO/Li+is 1.5 orders higher than that of the PEO/Li+complex at room temperature. The higher mobility of PEO chain and Li+ion’s directional motion are believed to be the main causes. In order to improve the conductivity and clarify the conductive mechanism, we prepared the CD-PEO/Li+with different radius of cyclodextrins (β-CD and y-CD). The polymer electrolytes samples are β-CD-PEO/Li+ and y-CD-PEO/Li+. Results show the radius of CD can greatly affect the conductivity of the complex. For single PEO chain system, the conductivity increase with the CD’s size. Meanwhile, the PEO chain motion will also increase with the CD’s size at single-PEO chain system. However, if the CD’s size is big enough for taking double-PEO chains, the conductivity drops sharply.3. Different molecular weight polymer electrolytes (3-CD-PEO/Li+have been prepared by using 4k,6k,10k PEO. The EIS results reveal that the conductivity decrease with the molecular weight’s increase. Because of the low resolution of 7Li NMR under MAS in this sample, we apply the static state 7Li NMR and 19F NMR to investigate the cation (Li+) and anion ([AsF6]-) motion at different temperature. NMR results show the cation (Li+) motion of these sample decrease with the molecular weight’s increase. Interestingly, the rate of cations (Li+) movement can be greatly increased by rising temperature. However, the anions ([AsF6]-) remain almost stationary state in the whole range of temperature. These results suggest that the ion transport in crystalline polymer electrolytes is dominated by the cations. Moreover, the PEO chain mobility is also decrease with the molecular weight’s increase.4. By using NaAsF6 instead of LiAsF6, we successfully prepared the β-CD-PEO/Na+ polymer electrolytes. This SPE has the similar channel structure with the a-CD-PEO/Li+ and β-CD-PEO/Li+ sample. Interestingly, the conductivity of β-CD-PEO/Na+ SPE is 10 times higher than that in lithium polymer electrolytes. Why this β-CD-PEO/Na+ SPE has the highest conduvtivity? 23Na NMR results reveal the strong Na+ ion motion at very low temperature (250 K). Meanwhile, the PEO chain motion is also stronger than the other lithium polymer electrolytes. Comparing with different EO:Na molar ratio, we found the EO:Na=6:1 (mole ratio) sample has the highest conductivity.The work of this thesis shows that, compared with the traditional PEO/Li+ SPE, the new channel structure SPE has higher conductivity and better application prospect. In this thesis, NMR technique plays an important role for the research of the structure of polymer electrolytes, ion distribution, ion motion and polymer chain motion. Meanwhile, for different samples, we used different selective NMR technology to explain the conductive mechanism at the molecular level. All of these results reveal the NMR can be powerful and general tool for all these polymer electrolytes and other samples.