Start Date: March 2007
Yi Cui and Friedrich B. Prinz, Department of Materials Science and Engineering, Stanford University
The objective of this project is to develop high-energy-density electrodes for lithium ion batteries for electric vehicles. Specifically, close-packed, core-shell nanowire electrode architectures will be investigated as a means for using low-cost and high specific energy electrode materials that would suffer from poor stability in other configurations.
CO2 emissions from the combustion of carbon-based fuels in transportation applications account for a quarter of global energy-related emissions, and the percent of global greenhouse gas emissions from transportation sources is expected to approach one third of all emissions in the coming decades. The impact of transportation on the environment may be reduced through the development of carbon-based fuels synthesized from low-carbon energy, the reduction of the amount of energy required for transportation, or by decoupling energy end-use from CO2 production. This last option encompasses the use of electric vehicles for ground transportation in combination with stationary electricity generation using centralized carbon management technologies. With this goal in mind, the development of electric vehicles faces several significant technical challenges, including the development of batteries with high energy density and stability.
This project studies novel electrode architectures based on close-packed nanowire arrangements (see Figure 1). Compared to existing technologies using bulk electrodes, this approach has the potential to achieve five to seven times the energy density of lithium batteries. Nanowire devices provide a better ionic transport pathway because each nanowire is in direct contact with the electrolyte, forming a continuous pathway for electrons from the electrode collectors to the tips of the nanowires.
This feature, along with the large surface-to-volume ratio of the nanowires, enables full use of the theoretical charge capacity of the electrode material and increased speed of charging and discharging, resulting in higher energy and power density. Additionally, the proposed device architecture allows the use of high energy-density electrode materials that would be unstable in bulk configurations.
For example, Li4,4Si is a potential anode material with a very high specific energy capacity of 43,200kJ/kg, but it is unstable in bulk configurations due to its large volume change (300%) during charging/discharging cycles. Structural stability can be provided by the proposed nanowire geometry and core/shell structure, where multiple thin shells are used to stabilize the Li4,4Si core. In addition, the use of an ionically conducting but electronically insulating shell such as SiO2 may prevent interfacial chemical reactions between the electrode and the electrolyte, resulting in longer device lifetime.
This project seeks to optimize the performance of lithium batteries by studying high-energy density materials and core/shell nanowire structures for both the cathode and the anode. Proposed cathode materials include LiMn2O4, Li(Ni0.5Mn0.5)O2, and vanadium pentoxide (V2O5). Besides showing good electrical and ionic conductivity, LiMn2O4 also has a crystalline structure that will theoretically allow the insertion/extraction of two Li+ ions into/out of one molecular formula of Mn2O4, resulting in a theoretical specific energy of 3,629kJ/kg. Problems related to volume change and the dissolution of Mn into the electrolyte can be overcome by using the core/shell nanowire architecture. Ab initio computational modeling shows that lithium-ion diffusion, and hence charging/discharging rates, in the layered structure of Li(Ni0.5Mn0.5)O2 can be enhanced by producing Li(Ni0.5Mn0.5)O2 from Na(Ni0.5Mn0.5)O2 through ion exchange. This synthesis approach combined with the nanowire architecture should allow use of the full energy capacity of this compound (~4,000kJ/kg). Lithium insertion into V2O5 can cause significant structural changes, but this material is chemically stable and its good reversibility suggests that it may have a long lifetime. In addition, the core/shell nanowire architecture is well suited for this material in order to improve fatigue resistance and operation rate. Li4,4Si is proposed as a replacement for commonly used graphite-based anodes for lithium batteries, with a potential tenfold gain in energy density. The nanowire geometry and the core/shell structure make it possible to deal with the large volume changes occurring in this material during charging/discharging cycles. Additional alternative materials for both the cathode and the anode will be identified through computational design.
Nanowires are grown through a vapor-liquid-solid (VLS) growth technique, using a liquid mixture of dissolved reactants and catalysts on the surface of a crystalline substrate. Control of the nanowire orientation, density, and diameter is achieved through selecting lattice-matching crystalline substrates and altering the catalyst particle size.
The process to achieve project goals includes the following tasks:
- Investigate ionic and electronic transport and structural changes in single nanowires in order to understand the fundamental processes taking place during charging/discharging, and to optimize the core/shell nanowire design.
- Build and measure single nanowire devices in various operation conditions.
- Study the uniformity of lithium intercalation into nanowires using ionic and electronic impedance imaging techniques.
- Monitor structural and chemical transformations during charging/discharging through electrical testing using in-situ transmission electron microscopy (TEM) with atomic-scale resolution and electron diffraction.
- Use computational modeling to optimize device design parameters, including the nanowire diameter, length, spacing, and shell thickness.