STEIN RESEARCH GROUP

POROUS SOLIDS — NANOCOMPOSITES — SELF-ASSEMBLED FRAMEWORKS

 Monolithic Macroporous Carbon Electrodes
 for Lithium-Ion Secondary Batteries

As consumers and scientists alike become more heavily dependent on portable electronics, it will be necessary to improve existing battery designs to meet future needs. Progressively smaller microelectronic and microelectromechanical systems, in particular, can benefit from miniaturization of quickly rechargeable power sources that can deliver currents on demand. Nanotechnological advances are permitting shifts in design paradigm for lithium battery electrodes, as demonstrated by recent work from Professor Stein’s group. Graduate students Justin Lytle and Nick Ergang, together with Kyu Tae Lee, a visiting graduate student from Professor Seung Oh’s group at Seoul National University, fabricated electrode materials with a novel architecture composed of nanometer-sized walls that surround interconnected close-packed spherical voids with sub-micrometer diameters. They demonstrated that the architecture can lead to improved rate performance of the electrodes. The electrodes were prepared by infiltrating colloidal crystals of uniformly sized poly(methyl methacrylate) (PMMA) spheres with a resorcinol-formaldehyde (RF) sol, polymerizing the RF precursor and carbonizing it by heating at high temperature in a nitrogen atmosphere. PMMA spheres were removed via thermal depolymerization during this last step, leaving a three-dimensionally ordered macroporous (3DOM) carbon skeleton. An example of the resulting porous monolith is shown in the top left figure and a scanning electron micrograph of the pore structure in the top right figure.

In an application as an anode for lithium ion secondary batteries, 3DOM carbon has several advantages that would permit high rate performance of lithium ion secondary batteries: 1) nano-sized solid-state diffusion lengths, 2) high ionic conductivity of the electrolyte in the porous matrix, 3) reasonable electric conductivity and 4) no need for a binder and/or a conducting agent. These advantages arise from the morphology of 3DOM materials which have well-interconnected wall and pore structures and nanoscale wall thicknesses. As a result, rate performance was significantly enhanced compared to similarly prepared non-templated carbon (figure at right) and was also improved compared to spherical carbon electrodes mixed with binder. It was, however, not affected by the pore spacing in the range from approximately 285 to 340 nm, mainly because wall thicknesses were similar in these materials and the pore sizes were significantly larger than mobile species in the electrochemical system. 3DOM carbon retained three orders of magnitude greater discharge capacity than bulk carbon at specific currents greater than 12 mA g^-1, and possessed good cycling stability with 83 % reversible capacity retention after 30 cycles. The specific capacity and rate performance of 3DOM carbon electrodes could be increased by coating the carbon surface with tin oxide nanoparticles. The ability to prepare monolithic pieces of 3DOM carbon facilitates its handling and testing as an anode material, so that it can be prepared as a stand-alone electrode or as a platform for three-dimensionally microstructured batteries covering only a few square centimeters. The open architecture allows further modification of the electrode, which can be advantageous for battery applications, as well as sensing applications. This work is described in an article in Advanced Functional Materials 2005, vol. 15, pp. 547-556.