Synthesis And Properties Of Silicon Based Anode Materials For Lithium-ion Batteries

Lithium- ion batteries(LIBs) are the power source of cho ice not only for portable electronics devices but a lso ever- growing electr ic ve hic les. Among a ll anode mater ia ls, Si is regarded as one of most promis ing candidates due to its abundance in nature, low d ischarge potentia l, and highest know n theoretic d ischarge capacit y(~ 4200 mA h g-1 vs 372 mA h g-1 of commerc ial graphite). However, the large volume expansio n of Si(300%) upon lit hiat ion/de lit hiat ion processes leads to serious capacit y perfor mance fad ing due to the pulverizat ion and thus ind uced electr ical contact loss of Si w ith curre nt collector. Cons idering the high conduct ivit y, excellent fle xibility and mecha nical strength of graphe ne as well as high theoretic capacit y of Si, in this thesis, two kinds of 3D S i/ graphene compounds were successfully synt hesized. The ir structura l characterizat ions and e lectrochemica l perfor mances were syste mat ically analyzed and evaluated. The main contents are listed as follows:A three- dimens io nal macroporous s ilicon/ graphene compos ite has b een synt hes ized via a s imple but cost-effect ive one- step hydrotherma l met hod by us ing graphene oxide and dopamine decorated silicon na nopartic les as raw materia ls fo llowed by a therma l annealing process. O wing to the good dispersib ilit y of dopamine decorated Si nanopartic les in water, the result ing 3D macroporous silicon/ graphene compos ite shows the unifor m d istr ibut io n of S i na nopartic les along the backbone of the 3D graphe ne oxide after hydrother ma l process, superior to that of silicon/ graphene composite w it h prist ine Si na nopartic les as raw materia ls w here serious S i aggregat ions were c learly observed. The porous struct ure of 3D graphene can e ffective ly buffer t he volume cha nge o f s ilicon d ur ing t he lit hiat ion and delithiat io n process, and simultaneous ly reduce the charge transfer resista nce. As a consequence, the prepared 3D macroporous Si@ a-C @ GO e lectrode exhib ited an init ia l high d ischarge capacit y of 1703.2 mA h g- 1 at 500 m A g-1 wit h good cyc ling stabil ity.2. Inspired by bean pod structure, we synthes ized a three- dimens io nal void- conta ining Si@ amorphous carbon@ graphene nanosheet scrolls(Si@ a- C@ GNSs) by freeze- drying the mixture suspens ion o f mildly reduced graphene oxide and dopamine decorated Si na nopart ic les, fo llowed by a ther mal annea ling process. In this structure, Si@ a-C nanopartic les homo geneous ly disperse w ithin the ga ller y of graphene na noscroll, lea ving large a mounts of voids between graphe ne and Si@ a-C. In terms o f s ilicon anode electrode mater ials, t he prepared pea pod- like Si@ a-C@ GNS nanostr ucture has several adva ntages as compared wit h other Si/graphe ne composites. First, t he tub ular shape of 3D GN S geometry configurat ion provides an effect ive access for ions/e lectron transportation. Second, the large amounts of vo ids in Si@ a- C@ GNS structure can effect ive ly buffer the tre mendous volume change of silico n nanopartic les dur ing the lithiat io n/delit hat ion process and guarantee the integr it y of the electrode dur ing the cycling. Third, the flexib le tubular shaped GN S outer shell layer and amorpho us inner carbon layer provides a dua l protection for ins ide Si nanopartic les, and thus effective ly prevent the repeat deposit ion of so lid electrolyte inter face layer on sur face of ins ide Si na nopartic les. Thanks to these extraordinary advantages, the prepar ed Si@ a-C@ GNS- based electrode exhibited a high capacity about 2243 mA h g-1 at 0.3A g-1, excellent rate capabilit y(947.8 mA h g-1 at 3A g-1) and good cyc ling stabilit y(91.5% capacit y retention after 110 cycles at 2A g-1).