High Capacity Metal Oxides for Li-ion Battery Anodes
Graphite materials have exclusively been used at the Li-ion battery anode since their commercialization in the 1990s. Until now, graphite materials have been sufficient since the energy density of these batteries has been more than sufficient. However, the power demand of mobile devices is rapidly increasing as the public’s demand for enhanced functionality shows no sign of stopping. The automotive industry is also putting considerable pressure on traditional materials as increased energy storage is needed to enable an acceptable vehicle range. Both these applications demand lightweight, small footprint solutions that can only be accomplished by transitioning from graphite to higher capacity materials.
Transition metal oxides are a promising class of materials because they allow for multiple electron transfer steps per mol of active material. Unfortunately, most metal oxides undergo alloying or chemical conversion reactions that change the bonding of active materials, and sometimes even forming electrochemically inactive materials, limiting both cycle retention and rate capability.
Our primary work in this area has revolved around understanding the roles of structure and conductivity in the reaction reversibility during charge and discharge of metal oxides, with NiO being our primary probing compound.
New Methods Development for Li-ion Batteries
The behavior of metal oxide Li-ion anodes is very complex. During charge and discharge, the cell is cycling between (at least) two completely distinct chemical phases, which impacts electronic conductivity, Li+ diffusion, etc. Additionally, Li is invisible to x-rays techniques, which makes it very difficult to use high energy synchrotron techniques to make in-situ measurements. Therefore, a new toolbox is needed to understand the behavior of metal oxide anodes in Li-ion batteries. We have developed two new techniques that we widely employ: i) multiphase diffusion decoupling using the results of current-pulse relaxation (CPR) measurements and a novel model developed in our lab; and ii) identical location TEM imaging (a sample image is shown below for how the structure of ordered mesoporous NiO changes with cycle number – note the structure is completely lost, which shadows the capacity loss). The IL-TEM work has provide some very interesting insights in combination with charge/discharge testing, which suggests that the ionic conductivity is the primary variable in determining capacity retention. We have used this to improve our cell construction and can now have NiO anodes with > 700 mAh/g capacity after 100 cycles.
High Performance Supercapacitors
Historically, electrochemical capacitors have offered very high power discharges, but have been limited in terms of energy density. This has left a large void between the operating ranges of batteries (high energy density, low-to moderate power) and capacitors. Bridging this gap with next-generation capacitor materials has the potential to open up significant new markets that are currently unapproachable without complex electronic architectures and multiple devices.
Our work in this area has focused on using doping to control the electrostatic interaction between the capacitor electrode and the electrolyte ions. Increasing the attraction between the two has the potential to shrink the double layer thickness, allowing the Helmholtz capacitance to increase. Several carbon-based materials have been tested, including amorphous and ordered mesoporous N-functionalized carbon.
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