...the huge volumetric expansion/shrinkage due to the alloying/dealloying reactions of Sn with lithium (Li) causes severe mechanical disintegration (such as cracking and pulverization), breakdown of the electrical conduction pathways in the electrodes, and even the loss of physical and electronic integrities of the active material.

These enormous volume and structural changes lead to severe degradation of the electrodes upon cycling and dramatically shorten the cycle life of the electrode. As a result, practically durable high-rate and high cycle life Sn-based electrodes have not yet been achieved.

—Ji et al.

Schematic illustration of the graphene/Sn-nanopillar nanostructure preparation procedures. Click to enlarge.

To address this, researchers have tried a variety of tin and carbon composites with different structures and forms, seeking to have the carbon matrices buffer and accommodate the mechanical stress induced by volume expansion and shrinkage.

The team, led by Yuegang Zhang, a staff scientist with Berkeley Lab’s Molecular Foundry, in the Inorganic Nanostructures Facility, assembled the graphene/Sn-nanopillar multilayered nanocomposite anodes by employing both self-assembly and conventional film processing approaches. The rationally designed nanoarchitecture offers a number of mechanical and electrical benefits, they concluded:

• The unique geometry of the self-assembled Sn nanopillar arrays with large Li-storage capacity can provide the most freedom for dimension changes and alleviate the mechanical stress/strain induced by the volume change during alloying/dealloying reactions.

• The nanopillars can also enhance Li-ion insertion by reducing the diffusion/migration barrier and allow easy penetration of the electrolyte between neighboring nanopillars and hence reduce internal resistance, which is particularly helpful for high energy/power applications.

• The addition of flexible and conductive graphene layers to the Sn nanopillar arrays can provide extra “cushion” for the structure to accommodate large volume change induced by Li–Sn alloying/dealloying reactions.>

• High electrical conductivity of both constituent materials and their distinctive structures in the nanocomposites can palliate the problems of the slow electrochemical kinetics and sluggish transport rate by offering high surface area and short diffusion pathway for more efficient transport of both electrons and Li-ions.

• Graphene sheets also have considerable reversible Li-storage capacity because lithium could be stored not only on both sides of graphene, but also on their significant disorder, edges, vacancies, and covalent sites.

As a result, these assembled graphene/Sn-nanopillar multilayered nanostructures may exhibit synergic properties and display superior electrochemical performance with large reversible capacity, excellent rate capability and cyclic performance, when used as anodes for rechargeable LIBs. Furthermore, polymer binders and conductive additives which are commonly used for other electrode materials are not needed in such integrated electrodes, which will improve the overall energy density of the batteries.

—Ji et al.

To create the composite material, a thin film of tin is deposited onto graphene. Next, another sheet of graphene is transferred on top of the tin film. This process is repeated to create a composite material, which is then heated to 300° Celsius (572° Fahrenheit) in a hydrogen and argon environment. During this heat treatment, the tin film transforms into a series of pillars, increasing the height of the tin layer.

The obtained graphene/Sn multilayered nanocomposites, contained about 70 wt% Sn and 30 wt% graphene.

At a current density of 0.05 A g^-1, a half-cell using the multilayered graphene/Sn-nanopillar nanocomposite electrode showed an initial discharge capacity of 734 mA h g^-1. At the second charge/discharge cycle, the multilayer anode still had a large reversible capacity of about 714 mA h g^-1, which indicates a high capacity retention rate of 97.3% from the first cycle, the authors said. After 15^th and 30^th cycles, the reversible capacities are preserved at about 723 and 679 mA h g^-1 (98.4 and 92.5% retention rates from the first cycle), respectively, indicating very slow capacity decay.

Further cycling tests showed that the graphene/Sn-nanopillar nanocomposite electrodes also exhibited excellent cycle life and rate capability at higher current densities.

Portions of this work at the Molecular Foundry were supported by DOE’s Office of Science.

Resources

• Liwen Ji, Zhongkui Tan, Tevye Kuykendall, Eun Ji An, Yanbao Fu, Vincent Battaglia, and Yuegang Zhang (2011) Multilayer nanoassembly of Sn-nanopillar arrays sandwiched between graphene layers for high-capacity lithium storage. Energy & Environmental Science doi: 10.1039/c1ee01592c