Thin film carbon for lithium ion batteries

The charge capacity, lithium ion diffusion constant, and the voltage profile (open circuit voltage (V{dollar}sb{lcub}rm oc{rcub}){dollar} versus molar concentration of inserted lithium ions) of thin carbon films were investigated to study their performance as anodes for lithium ion batteries. Films were deposited by dc anodic vacuum arc, electron beam (e-beam) evaporation, and pulse laser deposition (PLD) techniques using microcrystalline graphite source material. X-ray studies indicated that they had a disordered nanocrystalline graphitic nature.; The films had a maximum reversible charge capacity of 900 mAh/g or x {dollar}approx{dollar} 2.5 in {dollar}rm Lisb{lcub}x{rcub}Csb6{dollar} which is 2.5 times higher than that of the lithium graphite intercalation compound LiC{dollar}sb6.{dollar} Such a remarkable storage capacity can be explained by simultaneous presence of three storage mechanisms: (i) Li ion intercalation between graphite planes, (ii) lithium adsorption on outer surfaces of crystallites, and (iii) formation of lithium metal in pores between crystallites and/or on the carbon/current collector interface.; The lithium ion diffusion constant was in the range of {dollar}rm 10sp{lcub}-10{rcub} to 2times10sp{lcub}-9{rcub} cmsp2/s.{dollar} It increased with lithium concentration probably because of separation of graphite layers and possibly enhanced hopping rate at surfaces due to the low binding energy of lithium at surface.; Heavily lithiated films had potentials practically identical to those of lithium metal. Their average open circuit voltages were about 0.7 V when averaged over the entire stoichiometry range of x = 0 to 2.5. A density of states modeling study of single layer graphene molecules {dollar}rm Csb{lcub}n{rcub}(n=24,{dollar} 96, 216) suggests that {dollar}rm {dollar} may possibly be brought closer to the lithium potential by decreasing the molecule diameter.; A thin film (LiCoO{dollar}sb2{dollar}/inorganic electrolyte/carbon) battery exhibited highly reversible charging/discharging for more than 400 cycles at rates of 10 and 25 {dollar}mu{dollar}A/cm{dollar}sp2.{dollar}; Carbon films deposited by the dc arc technique had a slightly higher lithium diffusion constant compared to PLD and e-beam films. The arc deposited films also exhibited higher reversible capacities compared to films deposited by e-beam evaporation.; Our carbon films did not contain any hydrogen (they were fabricated from pure graphite) and they had higher reversible charge capacity (up to approximately x = 2.5) compared to that reported for both "pure" (with no hydrogen) and "impure" (hydrogen containing) carbon powder materials.; Our investigation demonstrated that thin carbon films deposited from graphite sources, when used as anodes in lithium ion batteries, produced very useful charge capacity, discharge voltage, and cycling performance.