Jiechen Lu

A layer-structured Na2CoP2O7 pyrophosphate cathode for sodium-ion batteries

We introduce Na2CoP2O7 pyrophosphate as a new potential cathode for sodium-ion batteries. Based on a layered structure, it offers a two-dimensional Na-diffusion pathway delivering a reversible capacity close to 80 mA h g−1 involving Co3+/Co2+ redox activity with an average potential of 3 V.

Magnetic structures of NaFePO4 maricite and triphylite polymorphs for sodium-ion batteries.

The magnetic structure and properties of polycrystalline NaFePO4 polymorphs, maricite and triphylite, both derived from the olivine structure type, have been investigated using magnetic susceptibility, heat capacity, and low-temperature neutron powder diffraction. These NaFePO4 polymorphs assume orthorhombic frameworks (space group No. 62, Pnma), built from FeO6 octahedral and PO4 tetrahedral units having corner-sharing and edge-sharing arrangements. Both polymorphs demonstrate antiferromagnetic ordering below 13 K for maricite and 50 K for triphylite. The magnetic structure and properties are discussed considering super- and supersuperexchange interactions in comparison to those of triphylite-LiFePO4.

Magnetic structure and properties of the Na2CoP2O7 pyrophosphate cathode for sodium-ion batteries: a supersuperexchange-driven non-collinear antiferromagnet.

The crystal and magnetic structure and properties of the Na(2)CoP(2)O(7) Na(+)-ion battery cathode material have been characterized by magnetic susceptibility, specific heat, and variable-temperature neutron powder diffraction measurements. Na(2)CoP(2)O(7) crystallizes in the orthorhombic space group Pna2(1) with a = 15.4061(3) Å, b = 10.28854(9) Å, and c = 7.70316(15) Å, having a layered structure with slabs of [CoP(2)O(7)](∞) separated by Na cations. The magnetic property measurements and neutron diffraction data analysis reveal that the material undergoes long-range ordering to a noncollinear antiferromagnetic G-type structure below T(N) ≈ 6.5 K. The magnetic structure is rationalized as a result of supersuperexchange between Co(2+) atoms linked by phosphate groups.

Rhombohedral NASICON-type NaxFe2(SO4)3 for sodium ion batteries: comparison with phosphate and alluaudite phases

Sulfate has attracted considerable interest as an anion for cathode materials in lithium and sodium ion batteries because they induce high voltages. Herein, Fe2(SO4)3 in the rhombohedral NASICON phase was examined as a Na-ion cathode. Electrochemical, structural, and theoretical methods were employed. Starting from the charged state, Fe2(SO4)3 can be discharged to near theoretical capacity under very slow GITT experimental conditions. However, the available capacity rapidly dropped under high current. First-principles calculations found that one of the alkali metal sites was metastable and a high diffusion barrier of 0.9 eV for Na+ ion migration was found. The NASICON-type structure, with its open framework, is generally considered a good ionic conductor. However, the small size of SO42− anions limit the site availability and the mobility of Na+ ions in the NASICON phase of Fe2(SO4)3, which accounted for their inferior properties compared with the recently discovered alluaudite polymorph.