Xin Zhao, Cary Hayner, Mayfair Kung, and Harold Kung
Northwestern University and CEES
High energy density, rapid charging/discharging, and long cycle life are critical performance parameters for electrical energy storage devices, such as Li-ion batteries. In today’s Li-ion batteries, graphite is the anode material of choice. The large aspect ratio of the graphitic sheets limits the speed at which lithium can be inserted into the structure and therefore access to the theoretical charge density (capacity) of the electrode (372 mAh/g) at high rate.
Researchers at Northwestern University, a partner in the Center for Electrochemical Energy Science, have discovered a method to overcome this limitation by introducing nanometer-size vacancy defects in the graphene sheets to generate cross-plane diffusion paths for the Li ions to percolate through the graphene stack. These holey graphene sheets can be charged with lithium, up to 200 mAh/g, in less than 2 minutes. The researchers designed a high-capacity composite Si-graphene architecture capable of operating at high charge/discharge rates by sandwiching Si nanoparticles (20–50 nm) between these graphene sheets in a three-dimensional, electronically conducting network. A self-supporting, flexible electrode was shown to deliver a reversible capacity of ~1100 mAh/g at 8 A/g, equivalent to a full discharge in 8 minutes, or ~3200 mAh/g at the 1 A/g rate, repeatable up to 99.9% between cycles for over 150 cycles. The results have been reported in Advanced Energy Materials and ACS Nano.
(a) Schematic of a Si-defect graphene composite electrode showing Si nanoparticles (green) sandwiched between graphene sheets with in-plane vacancy defects to form a 3-D host structure for lithium (blue).
- “In-Plane Vacancy-Enabled High-Power Si-Graphene Composite Electrode for Lithium-Ion Batteries,” Xin Zhao, Cary M. Hayner, Mayfair C. Kung, Harold H. Kung, Advanced Energy Materials 2, 1079–1084 (2011).
- “Flexible Holey Graphene Paper Electrodes with Enhanced Rate Capability for Energy Storage Applications,” Xin Zhao, Cary M. Hayner, Mayfair C. Kung, Harold H. Kung, ACS Nano 5(11), 8739–8749 (2011).