|Nanoscoops for high-power anodes. Credit: ACS, Krishnan et al.Click to enlarge.|
A team at Rensselaer Polytechnic Institute (RPI) has developed a functionally strain-graded carbon-aluminum-silicon anode architecture that overcomes the normally poor performance of Li-ion batteries for high power applications involving ultrafast charging/discharging rates.
The new anode materials consist of an array of nanostructures each comprising an amorphous carbon nanorod with an intermediate layer of aluminum that is finally capped by a silicon nanoscoop on the very top. The gradation in strain arises from graded levels of volumetric expansion in these three materials on alloying with lithium.
A paper on their work is published in the ACS journal Nano Letters.
Batteries designed for electric vehicles should be able to provide high energy and power densities. Lithium (Li)-ion batteries are known to deliver very high energy densities in comparison to other battery systems. However, they suffer from low power densities. In contrast, supercapacitors provide very high power densities due to their surface-based reactions.
To replace a traditional combustion engine, it is highly desirable to combine the advantages of Li-ion batteries and supercapacitors into one single battery system. Earlier reports have shown the development of high rate cathode materials. This has also led to an equal effort in the development of high-rate capable anode architectures. Silicon (Si) has been envisioned as a promising anode material because of its high theoretical capacity of ~4200 mAh/g based on the stoichiometry of the alloy Li22Si5. The main limitation of this high capacity is an accompanying volumetric expansion of ~400% for crystalline Si (or ~280% for amorphous Si) which results in pulverization and delamination of the electrode structure.
Pulverization results in more capacity losses due to increased solid electrolyte interphase (SEI) formation while delamination results in loss of electrical contact with the substrate. At higher charge/discharge rates (C-rates), these failure mechanisms are severely exacerbated and thus it is important to design architectures that perform well at fast C-rates to enable high power Li-ion rechargeable batteries.—Krishnan et al.
By introducing materials between Si and C that have intermediate volumetric strains, the authors note, a natural strain gradation can be developed in a multilayer architecture. In the specific case of the work reported in the paper, the team used aluminum to gradually transition the strain from the least strained material (C) to the most strained material (Si).
This minimizes the mismatch at interfaces between differentially strained materials and enables stable operation of the electrode under high-rate charge/discharge conditions, they explain.
At an accelerated current density of ~51.2 A/g (i.e., charge/discharge rate of ~40C), they found that the strain-graded carbon-aluminum-silicon nanoscoop anode provides average capacities of ~412 mAh/g with a power output of ~100 kW/kgelectrode continuously over 100 charge/discharge cycles.
They also show that the C-Al-Si composite can yield power densities as high as ~250 kW/kgelectrode (current density of ~128 A/g) continuously over 100 cycles with an average capacity of ~90 mAh/g.
The C-Al-Si architecture has the potential for mass scalability by increased deposition time as well as the possibility of stacking C-Al-Si nanostructure films on intermediate carbon thin film supports. When the mechanism of charge generation involves alloying with the host material and the demand for current is high, the electrode architecture is put through large strain rates accompanied by rapid volume changes. In such a situation, a functionally strain-graded structure could potentially undergo rapid volume changes with reduced possibility of interfacial cracking or delamination. By building a strain-graded structure, it is possible to eliminate interfaces between materials that have a large strain difference during lithiation.
Low strain difference between adjacent materials in the composite leads to highly efficient alloy-dealloy reactions preserving the overall structural integrity of the electrode. To further improve the strain gradient, we could potentially insert materials such as Sb (strain of ~147%) or As (strain of ~201%) between Al and Si. This will also help in increasing the area mass density while still improving the performance. Such strain-graded multilayer anode architectures show significant potential for the design of high power and high capacity Li-ion rechargeable batteries.—Krishnan et al.
Rahul Krishnan, Toh-Ming Lu, and Nikhil Koratkar; Functionally Strain-Graded Nanoscoops for High Power Li-Ion Battery Anodes Nano Lett., Article ASAP doi: 10.1021/nl102981d