Lithium-metal anodes are favored for use in next-generation rechargeable Li-air or Li-sulfur batteries due to a tenfold higher theoretical specific capacity than graphite (3,860 mAh/g vs. 372 mAh/g); light weight and lowest anode potential. However, safety issues resulting from dendrite formation and instability caused by volume expansion have hampered development and deployment of commercially viable solutions.
A team at Stanford led by Prof. Yi Cui has now introduced a simple approach to address both issues by effectively encapsulating lithium inside a porous host scaffold using a facile melt-infusion approach. Uniformly confined within the matrix, the lithium creates a material that can deliver a high capacity of around 2,000 mAh/g (gravimetric) or 1,900 mAh/cm3 (volumetric) as stable anodes for Li-metal batteries. A paper on their work is published in Proceedings of the National Academy of Sciences (PNAS).
This novel design affords remarkable battery performance with a low interfacial impedance, stable voltage profile and long cycle life, due to its high conductive surface area, stable electrolyte/electrode interface, and negligible volume fluctuation. Compared with a hostless Li metal electrode, this Li/C composite electrode has multiple advantages and therefore can open a new avenue for solving the intrinsic problems of Li metal-based batteries.—Liang et al.
In their search for an appropriate scaffold to serve as the Li host, the researchers looked for the following attributes:
- mechanical and chemical stability toward electrochemical cycling;
- low gravimetric density to achieve high-energy density of the composite anode;
- good electrical and ionic conductivity to provide unblocked electron/ion pathway, enabling fast electron/ion transport; and
- relatively large surface area for Li deposition, lowering the effective electrode current density and the possibility of dendrite formation.
The team chose carbon-based porous materials to provide the required features; for the PNAS study, they used electrospun carbon fiber network as an example. The team coated the carbon fiber network with a lithiophilic coating, then infused the molten lithium (which has a melting point of 180 ˚C). The Li flowed easily into the scaffold and occupied the empty spaces between each single fiber.
|Schematic and optical images of Li encapsulation by melt infusion. Liang et al. Click to enlarge.|
… in contrary to the hostless Li metal, the as-proposed Li/C composite anode is able to accommodate the volume variation and therefore mitigate the potential safety hazard; moreover, the reduced current density, rooted to larger surface area, also triggers a greatly improved electrochemical performance, with stable cycling of over 2,000 mAh/g for more than 80 cycles at a high current density of 3 mA/cm2.
The gravimetric specific capacity of a standard Li-metal anode is 3,860 mAh/g. The weight percentage of Li in the composite is ∼60%; the corresponding gravimetric specific capacity is 3,860 mAh/g × 60% = 2,316 mAh/g. The calculated specific capacity (2,316 mAh/g) is close to the measured specific capacity (2,061 mAh/cm3) obtained through a simple electrochemical stripping, the researchers noted.
Zheng Liang, Dingchang Lin, Jie Zhao, Zhenda Lu, Yayuan Liu, Chong Liu, Yingying Lu, Haotian Wang, Kai Yan, Xinyong Tao, and Yi Cui (2016) “Composite lithium metal anode by melt infusion of lithium into a 3D conducting scaffold with lithiophilic coating” PNAS 113 (11) 2862-2867; doi: 10.1073/pnas.1518188113