The Study Of Battery Materials Via Nuclear Magnetic Resonance And Electron Paramagnetic Resonance

The energy transition is picking up speed around the world,for which the research and development of the energy storage materials is one of the critical centerpieces.The research approaches have been developed rapidly over the past decade.Many intrinsic mechanisms were unraveled through a number of emerging characterization methods.Nuclear magnetic resonance(NMR)and electron paramagnetic resonance(EPR)can probe the local environments around the nuclei and the unpaired electrons,respectively,which play irreplaceable roles in battery researches.Several research groups around the world have carried out a lot of studies on the battery systems via NMR or EPR,and obtained many important results.The magnetic resonance techniques are niche fields,while it is still in its infancy for those battery material studies using magnetic resonance techniques,which deserves to be further explored and developed.To this end,this dissertation focused on the application of NMR and EPR to the battery material research.In particular,several typical battery materials which have the practical application value were studied as model systems.I hope this will contribute fundamentally to the interdisciplinary research between magnetic resonance field and battery material field.In the first part of this dissertation,a relatively economical ¹⁷O-labeling protocol was established,and then the ¹⁷O-labeled Li Co O₂ was studied by NMR.Li Co O₂ is one of the most important cathode materials,of which the cycle stability at high voltage is in urgent need of improvement at present.A quadrupolar lineshape arised in the ¹⁷O NMR spectra of Li Co O₂ at high state of charge,indicating a covalent bonding between Co and O atoms.The structural instability during the phase transition process at high voltage may be related to the increased covalency of the Co–O bond.Li Co O₂ is a representative layered cathode material.Thus,this part of work can help provide a reference for the other NMR studies of layered cathode materials.In the second part of this dissertation,a ¹⁷O-labeled cation-disordered cathode material Li_1.2Ti_0.4Mn_0.4O₂ was studied by NMR and EPR.The cation-disordered cathode material was a newfound class of cathode material only a few years ago,which shows high capacity thanks to the participation of the anionic redox reactions.The diamagnetic and paramagnetic Li environments in the material were distinguished through ⁷Li NMR.It was found that Li ions were preferentially extracted from those Li-rich regions,which is in good agreement with the 0-TM percolation theory.By analyzing the ¹⁷O NMR spectra and EPR spectra,the anionic redox reactions were found to be involved with two different mechanisms—the formation of the peroxo-like species(O₂)^n-and the electron holes on O 2p non-bonding state around Mn ions—the former leading to the large voltage hysteresis between charge and discharge.This part of work can share some thoughts on those cathode materials based on anionic redox mechanism and disordered materials.In the third part of this dissertation,an operando EPR cell was designed and the sodium-ion cathode material Na Cr O₂ was studied by operando EPR and EPR imaging.The results showed that an insulator-metal transition occurred during Na Cr O₂ charging,which was reversible between voltage range 2.2-3.6 V.When charging above 3.75 V,a new sharp signal arised in the EPR spectra,which was localized in the electrolyte through EPR imaging,and attributed to the dissolved Cr⁵⁺ions.The dissolved Cr ratio was calculated from the operando EPR results,by which then a surface Cr disproportionation mechanism was proposed.The trace Cr dissolution in the battery material was first identified in this part of work,which demonstrated the high sensitivity of EPR technology.In the fourth part of this dissertation,an EPR imaging approach targeting at the front view of the anode current collector has been developed.Then the process of Li plating and striping on the copper foil was investigated.The results indicated that the initial Li plating position determined the Li plating position in the subsequent cycles.In the plating process,Li tended to plate on several individual sites,where the dendrite Li would form after cycling,resulting in the loss of electrochemical activity.EPR imaging has very high resolution,and the method developed in this work can be applied to those non-invasive researches of the Li metal anodes and the anode-free batteries.There is deep physics behind the magnetic resonance technologies,which makes many chemists and material scientists feel confused about it.The fundamental principles of NMR and EPR and the various internal interactions in the research samples were clarified in brief in the introduction part of this dissertation.I hope it will be helpful to those who try to figure out the NMR and EPR spectra.At last,with the ever-increasing investment in fundamental science,I believe that the magnetic resonance technologies will be further developed,thereby driving other disciplines to make greater breakthroughs together.