Electrochemical insertion of lithium ions into disordered carbons derived from reduced graphite fluoride

摘要

Both covalent (obtained by direct fluorination at high temperature) and semi-ionic carbon fluorides (synthesized at room temperature) were reduced inorder to obtain disordered carbons containing very samll content of fluorine and different physical properties according to the reduction treatment (chemical,thermal or electrochemical).After a physical characterization (X-ray diffraction,electron spin resonance and FT-IR spectroscopies),the electrochemical behaviours of the pristine carbon fluorides and of the treated samples were investigated during the insertion of lithium using liquid carbonate-based electrolytes (LiClO_4-EC/PC,50:50%,v/v).Both galvanostatic and voltammetric modes were performed and revealed that the voltage profiles and the capacities differed according to the starting material and the reduction treatment.Semi-ionic carbon fluoride treated in F_2 atmosphere for 2 h at 150 degC and then chemically reduced in KOH exhibits high reversible capacities (the reversible capacity is 530 mAh g~(-1) in the second cycle);in the case,the voltage profiles show a large flat portion at potentials lower than 0.3V which is attributed to the insertion/deinsertion of lithium ions between the small graphene sheets and/or the absorption of pseudo metallic lithium into the microporosity of the sample.Nevertheless,a part of the lithium ions are removed at potentials higher than 0.5 V versus Li~+/Li limiting the useful capacity.