The Interfacial Properties Of Graphite Anodes/Hybrid Electrolytes And Li₄Ti₅O₁₂ Coating Modification

The nonflammable lithium salt/ionic liquid electrolytes for theapplication of lithium-ion battery have some problems, such as“poor compatibility with solid electrode”,“co-intercalation oforganic cation” and “low mobility of Li-ion” etc. However, theintroduction of organic solvents will not only keep thenon-flammability and safety, but also alleviate the above-mentionedproblems. Therefore, we choose LiPF6as lithium salt, a range ofcompositions of PP13TFSI and EC–DEC as hybrid solvents, andinvestigate the effects of EC-DEC and graphite structures on thegraphite anodes/LiPF6-PP13TFSI/EC-DEC interfacial properties. Inorder to further improve the electrochemical performances ofgraphite anodes in hybrid electrolytes, we synthesizeLi4Ti5O12-coated CMB composites (denoted as LTO-CMB) andcompare their electrochemical performances in hybrid electrolytes.Then, the possible mechanisms related to the electrochemicalenhancement are also discussed.1. The SEI properties of CMB anodes/hybrid electrolytes interfaceThe typical CMB anodes/hybrid electrolytes interfacial propertiesare evaluated using X-ray diffraction, transmission electronmicroscopy, infrared and raman spectra. It is found that, after the addition of lithium salt to the pure PP13TFSI, more PP13+cationsdecompose on the CMB surface and tend to form a “blocking” solidelectrolyte interphase (SEI) film. By contrast, the EC-DEC componentin the hybrid electrolytes could effectively passivate the CMB surface,by impeding the interfacial reaction of PP13+, and forming a relativelythin SEI film.2. Effect of carbon structure on SEI filmThe electrochemical behaviors of different carbon materialsusing cyclic voltammograms are compared. The surface andstructure properties of cycled carbon anodes are characterized byscanning electron microscope and X-ray diffraction.The results show that the interfacial reactions are closelyrelated with edge planes. Benefiting from its larger specific surfacearea and more dispersed insertion point, the spherical CMB shows amore positive Li+de-intercalation behavior, and more stable internalstructure than NG-198and AG-360in the hybrid electrolytes.Therefore, CMB has more advantages in the application ofPP13TFSI-based electrolyte.3. The application of Li4Ti5O12-coated CMB anode in the hybridelectrolyteThe Li4Ti5O12coated CMB compositions have been prepared byhydrothermal method (denoted as2wt.%LTO-CMB,5wt.%LTO-CMB).The electrochemical performances and surface properties arecharacterized by meanings of cyclic voltammograms, galvanostaticcharge-dischage method, electrochemical impedance spectroscopy,X-ray diffraction and scanning electron microscopy. We find that theLTO coating can greatly reduce the interfacial reactions of PP13+and EC-DEC, and decrease initial irreversible capacity. At0.05C, the initialcoulombic efficiency could be improved from62.7%of pristine CMB to63.5%of2wt.%LTO-CMB and72.5%of5wt.%LTO-CMB respectively,when the0.7mol/kg40%PP13TFSI-EC/DEC electrolyte was used.Especially, the cycled5wt%LTO-CMB covered by a stable and densegranular SEI film,shows better rate-capability and cycle life. Thedischarge capacity can retain about290mAh/g during the first20cycles at0.1C. Even at1C or2C, the reversible capacity is about50-100mAh/g higher than the pristine CMB. Unfortunately, due to itsincomplete surface coating, the discharge capacities of2wt%LTO-CMB vary with cycles as a wave shape, and show a largecapacity decline.