Dr. Yi Cui and colleagues at Stanford University—including Dr. Steven Chu, Nobel Laureate and the former Secretary of Energy, now a professor in the Physics department at Stanford—report progress toward a stable lithium metal anode for use in high-energy-density batteries such as Li-sulfur or Li-air systems.
Lithium metal is a very promising anode material for rechargeable batteries due to its theoretical high capacity (3,860 mAh g−1—i.e., ~10x that of the 372 mAh g−1 of graphite anodes in Li-ion batteries), but it fails to meet cycle life and safety requirements due to electrolyte decomposition and dendrite formation on the surfaces of the lithium metal anodes during cycling. Thus, numerous efforts are being made to develop a safe, extended cycling lithium-metal electrode and/or supporting electrolyte (Earlier post, earlier post.)
In paper in the journal Nature Nanotechnology, the Stanford researchers report that coating a lithium metal anode with a monolayer of interconnected amorphous hollow carbon nanospheres helps isolate the lithium metal depositions and facilitates the formation of a stable solid electrolyte interphase. They showed that lithium dendrites do not form up to a practical current density of 1 mA cm–2, and that the Coulombic efficiency improves to ∼99% for more than 150 cycles.
… to make viable Li metal anodes, two fundamental challenges would need to be resolved: (1) accommodating the large change in electrode volume during cycling (unlike graphite and silicon anodes, where lithiation produces volume changes of ∼10% and 400%, respectively, Li metal is ‘hostless’ and its relative volumetric change is virtually infinite); and (2) controlling the reactivity towards the electrolyte (Li is one of the most electropositive elements). … One problem lies in the fact that the SEI layer cannot withstand mechanical deformation and continuously breaks and repairs during cycling. As a result, Li metal batteries have low Coulombic efficiency (80–90% for carbonate solvents and 90–95% for ether solvents) and low cycle life due to the rapid loss of Li and electrolyte in the continuous formation/dissolution of the SEI. A second problem is that Li deposition is not uniform across the electrode surface and can form large dendrites that cause short circuiting of the battery. Third, reactions between the Li metal and the electrolytes are exothermic and large surface areas can pose risks of overheating (thermal runaway).
… Stabilizing the interface between the Li metal and the electrolyte is therefore key in improving the cycling performance of Li metal batteries. The ideal interfacial layer for the Li metal anode needs to be chemically stable in a highly reducing environment, and also mechanically strong. High flexibility is desired to accommodate the volumetric expansion of Li deposition without mechanical damage. In addition, the ability to control the flow of Li ions with the SEI inhomogeneities is essential to ensure uniform Li deposition. Here, we describe a flexible, interconnected, hollow amorphous carbon nanosphere coating with the aim of realizing such an ideal interfacial layer.—Zheng et al.
The Stanford team’s nanosphere layer resembles a honeycomb: it creates a flexible, uniform and non-reactive film that protects the unstable lithium from the drawbacks that have made it such a challenge. The carbon nanosphere wall is just 20 nanometers thick.
The Stanford team says that their interfacial nanoscale engineering approach offers three main benefits:
Amorphous carbon is chemically stable when in contact with Li metal;
The thin amorphous carbon layer does not increase the impedance to charge transfer, but has a Young’s modulus of ∼200 GPa, high enough to suppress Li dendrite growth; and
The hollow nanosphere layer is weakly bound to the metal current collector and can move up and down to adjust the availability of empty space during cycling.
Future research is needed to develop the application of this approach to practical batteries (the Coulombic efficiency needs to be improved to >99.9% for practical batteries, and alternative electrolyte combinations need to be developed to meet different battery chemistries). A viable route to this end could be to combine the nanoscale engineering approach described here with electrolyte additives. Anodes with interfacial layers on the current collector could be combined with cathodes with preloaded Li ions, such as the existing Li metal oxides and Li2S. Our work demonstrates that the interfacial nanoscale engineering approach can improve Li metal cycling performance. We believe that the nanoengineering concepts we have described may be a viable route towards Li metal anode batteries and, more specifically, to high-energy-density batteries, such as Li–S and Li–O2.—Zheng et al.
Guangyuan Zheng, Seok Woo Lee, Zheng Liang, Hyun-Wook Lee, Kai Yan, Hongbin Yao, Haotian Wang, Weiyang Li, Steven Chu & Yi Cui (2014) “Interconnected hollow carbon nanospheres for stable lithium metal anodes,” Nature Nanotechnology doi: 10.1038/nnano.2014.152