Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Ronggui Yang

Second Advisor

Se-Hee Lee

Third Advisor

Yung-Cheng Lee

Fourth Advisor

Kurt Maute

Fifth Advisor

Steven George

Abstract

There have been growing interests in developing high-capacity, high-power, and long-cycle-life lithium-ion (Li-ion) batteries due to the increasing power requirements of portable electronics and electrical vehicles. Various efforts have been made to utilize nano-structured electrodes since they can improve the performance of Li-ion batteries compared to bulk materials in many ways: fast electrode reaction due to the large surface area, efficient volume-change accommodation due to the small size, and fast Li-ion transport along the nanoscale gaps. Among various nanostructures, nanowire arrays present an excellent candidate for high performance lithium-ion battery electrodes, which have attracted intensive research over the past few years. However, problems such as the strain mismatch at the active/inactive material interface (A/I interface) and the agglomeration of nanowires hinder nanowire array-based electrodes from delivering superior battery performance. We found that the formation of a continuous Ni-Sn film at the base of the nanowires results in quick loss of electrical contact between the active material and the current collector because of the large strain mismatch at the large continuous A/I interface. By growing short Cu nanorods as a buffer layer before Ni-Sn nanowire growth, the formation of Ni-Sn film was inhibited and the A/I interface was scaled down to nanoscale islands. The strain mismatch is thus significantly reduced, which results in enhanced structural stability and the battery performance.

Another problem with nanowire array electrodes is that agglomeration in long nanowire arrays impedes them from delivering high areal capacity, by degrading the nanoscale wires to micron-sized bundles and reducing the mechanical stability. We develop a simple way to fabricate three-dimensional (3D) Ni-Sn nanowire networks by using 3D porous anodic alumina (PAA) templates synthesized from low-cost impure aluminum foils. By eliminating agglomeration, stable high-areal-capacity anodes are demonstrated with 3D self-supporting Ni-Sn nanowire network structures. With a nanowire length of 40 μm, the 3D Ni-Sn nanowire networks can deliver an areal capacity as high as 4.3 mAh cm-2 with a cycle life longer than 50 cycles when used as an electrode.

The 3D network has been envisioned as a superior electrode architecture of Li-ion batteries that can significantly enhance both ion and electron transport to improve battery performance. A 3D carbon nano-network is fabricated through chemical vapor deposition of carbon on a 3D PAA template, which serves as the conducting framework in Li-ion battery electrodes. The low conductivity active material, TiO2, is then uniformly coated on the surfaces of the 3D carbon nano-network using atomic layer deposition. A large areal capacity of ~ 0.37 mAh·cm-2 is achieved due to the large areal mass loading of the 3D C/TiO2 electrodes. The electrodes also deliver a high gravimetric capacity of ~ 240 mAh·g-1 based on the whole electrode at the test rate of C/5 and a long cycle life of over 1000 cycles at 1C. The effects of the electrical conductivity of carbon nano-network, ion transport in the active material, and the electrolyte permeability on the rate performance of these 3D C/TiO2 electrodes are also systematically studied.

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