New lead germanate-graphene nanosheet composite as high capacity Li-ion anode material

17 November 2014

Researchers at the University of Wollongong (Australia) have synthesized lead germanate-graphene nanosheets (PbGeO₃-GNS) composites for use as anode materials for Li-ion batteries (LIBs). In the voltage window of 0.01–1.50 V, the composite anode with 20 wt.% GNS delivered a discharge capacity of 607 mAh g⁻¹ at 100 mA g⁻¹ after 50 cycles. Even at a high current density of 1600 mA g⁻¹, a capacity of 406 mAh g⁻¹ can be achieved.

In an open access paper in the Nature journal Scientific Reports, the team suggests that the PbGeO₃-GNS composite can thus be considered as a potential anode material for higher performing lithium-ion batteries.

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Ge has been intensively researched as an alternative anode material, owing to its high theoretical capacity (1600 mAh g⁻¹), low working potential, and high lithium ion diffusivity (400 times higher than that of the well-studied Si). This makes Ge a promising anode material for both high energy and high power applications. Pure Ge anode suffers from rapid capacity loss, however, accompanied by a huge irreversible capacity. The main reason is that the dramatic Ge volume changes and agglomeration during Li insertion/extraction processes lead to the pulverization and exfoliation of the active material, electrically isolating the particles from the current collector and degrading their cycling performance.

Tremendous efforts have been made to address this issue, including reducing the anode material to the nanoscale, construction of porous architectures, and amorphization of the anode material. Recently, it has been proposed that preparing metal germanate nanowires or nanobelts could be a strategy to mitigate these problems. The metal oxide matrix could provide an elastic buffer to accommodate the volume changes and prevent the agglomerations of nanosized Ge particles formed in-situ in the matrix after the initial discharge process, which could be helpful for improving the electrochemical performance of this material. Pb is a highly abundant element around the world, and its compounds exhibit good electrochemical performance as anode for LIBs, making PbGeO₃ an anode candidate to satisfy the growing demand for various energy-storage technologies.

Since PbGeO₃ is actually an alkaline earth metal oxide, it is low in electrical conductivity, and its electrochemical performance is limited…it is still necessary to explore simple synthesis methods and an effective matrix for the formation of PbGeO₃ nanocomposite anode materials.

Graphene nanosheets (GNS)—two-dimensional macromolecular sheets of carbon atoms with a strongly bonded carbon network—are of interest as substrates for the growth of functional nanomaterials for lithium storage due to their superior electrical conductivity, large theoretical specific surface area, and chemical tolerance, as well as their structural flexibility.

The Wollongong team devised a simple one-step hydrothermal method to synthesize PbGeO₃-GNS composites as a novel anode material for LIBs in which PbGeO₃ nanowires with diameters in the range of 100–200 nm are embedded in conductive and interconnected GN networks.

They found that the composite anode exhibits superior electrochemical properties in terms of specific capacity, cycling stability, and rate capability compared to the pure PbGeO₃ anode.

(a) Cycling stability of the pure PbGeO3 and the PbGeO3-GNS anodes, and (b) rate capability of the pure PbGeO3 and the PbGeO3-GNS2(20 wt.%) anode. Wang et al. Click to enlarge.

The metal oxides formed in situ after the initial discharge could help to alleviate the volume expansions during the lithium ion uptake/release. Further, they suggested, the reversible reaction between Ge and Li₂O is presumed to improve the Li storage performance.

The combination with the GNS enables fast electron migration for the Li-ion uptake/release in PbGeO₃, contributing to enhanced Li storage kinetics. In addition, embedding PbGeO₃ nanowires in the graphene (GN) matrix could maintain the structural integrity of the composite anode by preventing large volume changes and particle agglomerations during cycling.

Resources

• Jun Wang, Chuan-qi Feng, Zi-qi Sun, Shu-lei Chou, Hua-Kun Liu Jia-zhao Wang (2014) “In-situ One-step Hydrothermal Synthesis of a Lead Germanate-Graphene Composite as a Novel Anode Material for Lithium-Ion Batteries” Scientific Reports 4, Article number: 7030 doi: 10.1038/srep07030

• Jinkui Feng, Lijie Ci, Yongxin Qi, Ning Lun, Shenglin Xiong, Yitai Qian (2014) “Low temperature synthesis of lead germanate (PbGeO—)/polypyrrole (PPy) nanocomposites and their lithium storage performance,” Materials Research Bulletin, Volume 57, Pages 238-242 doi: 10.1016/j.materresbull.2014.06.011