title: Role of intermediate phase for stable cycling of Na7V4(P2O7)4PO4 in sodium ion battery.
abstract: Sodium ion batteries offer promising opportunities in emerging utility grid applications because of the low cost of raw materials, yet low energy density and limited cycle life remain critical drawbacks in their electrochemical operations. Herein, we report a vanadium-based ortho-diphosphate, Na7V4(P2O7)4PO4, or VODP, that significantly reduces all these drawbacks. Indeed, VODP exhibits single-valued voltage plateaus at 3.88 V vs. Na/Na(+) while retaining substantial capacity (>78%) over 1,000 cycles. Electronic structure calculations reveal that the remarkable single plateau and cycle life originate from an intermediate phase (a very shallow voltage step) that is similar both in the energy level and lattice parameters to those of fully intercalated and deintercalated states. We propose a theoretical scheme in which the reaction barrier that arises from lattice mismatches can be evaluated by using a simple energetic consideration, suggesting that the presence of intermediate phases is beneficial for cell kinetics by buffering the differences in lattice parameters between initial and final phases. We expect these insights into the role of intermediate phases found for VODP hold in general and thus provide a helpful guideline in the further understanding and design of battery materials.
authors.affiliation: Graduate School of Energy, Environment, Water, and Sustainability, World Class University, Korea Advanced Institute of Science and Technology, Yuseong-gu, Daejeon 305-701, Korea.
authors.forename: Soo Yeon
authors.initials: SY
authors.lastname: Lim
authors.forename: Heejin
authors.initials: H
authors.lastname: Kim
authors.forename: Jaehoon
authors.initials: J
authors.lastname: Chung
authors.forename: Ji Hoon
authors.initials: JH
authors.lastname: Lee
authors.forename: Byung Gon
authors.initials: BG
authors.lastname: Kim
authors.forename: Jeon-Jin
authors.initials: JJ
authors.lastname: Choi
authors.forename: Kyung Yoon
authors.initials: KY
authors.lastname: Chung
authors.forename: Woosuk
authors.initials: W
authors.lastname: Cho
authors.forename: Seung-Joo
authors.initials: SJ
authors.lastname: Kim
authors.forename: William A
authors.initials: WA
authors.lastname: Goddard
authors.forename: Yousung
authors.initials: Y
authors.lastname: Jung
authors.forename: Jang Wook
authors.initials: JW
authors.lastname: Choi
mesh_terms: D003461:Crystallography; D011756:Diphosphates / Q000737:chemistry*; D011211:Electric Power Supplies; D004563:Electrochemistry; D007700:Kinetics; D008962:Models, Theoretical; D017968:Vanadium Compounds / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D011756:Diphosphates; D017968:Vanadium Compounds; C003319:sodium pyrophosphate
keywords: ab initio calculation; atomic reorganization; cathode; single voltage