title: Tetragonal phase germanium nanocrystals in lithium ion batteries.
abstract: Various germanium-based nanostructures have recently demonstrated outstanding lithium ion storage ability and are being considered as the most promising candidates to substitute current carbonaceous anodes of lithium ion batteries. However, there is limited understanding of their structure and phase evolution during discharge/charge cycles. Furthermore, the theoretical model of lithium insertion still remains a challenging issue. Herein, we performed comparative studies on the cycle-dependent lithiation/delithiation processes of germanium (Ge), germanium sulfide (GeS), and germanium oxide (GeO2) nanocrystals (NCs). We synthesized the NCs using a convenient gas phase laser photolysis reaction and attained an excellent reversible capacity: 1100-1220 mAh/g after 100 cycles. Remarkably, metastable tetragonal (ST12) phase Ge NCs were constantly produced upon lithiation and became the dominant phase after a few cycles, completely replacing the original phase. The crystalline ST12 phase persisted through 100 cycles. First-principles calculations on polymorphic lithium-intercalated structures proposed that the ST12 phase Ge12Lix structures at x ≥ 4 become more thermodynamically stable than the cubic phase Ge8Lix structures with the same stoichiometry. The production and persistence of the ST12 phase can be attributed to a stronger binding interaction of the lithium atoms compared to the cubic phase, which enhanced the cycling performance.
authors.affiliation: Department of Chemistry, Korea University , Jochiwon 339-700, Korea.
authors.forename: Yong Jae
authors.initials: YJ
authors.lastname: Cho
authors.forename: Hyung Soon
authors.initials: HS
authors.lastname: Im
authors.forename: Han Sung
authors.initials: HS
authors.lastname: Kim
authors.forename: Yoon
authors.initials: Y
authors.lastname: Myung
authors.forename: Seung Hyuk
authors.initials: SH
authors.lastname: Back
authors.forename: Young Rok
authors.initials: YR
authors.lastname: Lim
authors.forename: Chan Su
authors.initials: CS
authors.lastname: Jung
authors.forename: Dong Myung
authors.initials: DM
authors.lastname: Jang
authors.forename: Jeunghee
authors.initials: J
authors.lastname: Park
authors.forename: Eun Hee
authors.initials: EH
authors.lastname: Cha
authors.forename: Won Il
authors.initials: WI
authors.lastname: Cho
authors.forename: Fazel
authors.initials: F
authors.lastname: Shojaei
authors.forename: Hong Seok
authors.initials: HS
authors.lastname: Kang