Conceptually, lithium-ion batteries are simple devices comprising the anode, cathode and electrolyte. The anode’s job is to grab and stash lithium ions as fast as it can and to give them up speedily when current is drawn off through the cathode. The anode needs to be able to withstand repeated charge-discharge cycles, or to just hold onto the charge without leaking until it’s needed, like in a car battery.
Even though lithium ions are tiny, lots of them are stored, which can eventually swell the anode and lead to failure. Thus, the search for anodes with large-charge storage capacities and the ability to discharge quickly is a high priority.
Two papers recently published in Advanced Materials investigate new anode materials from very different materials categories and with completely different charge storage mechanisms.
First, out of Oak Ridge National Lab, a team led by Hansan Liu, Gilbert Brown and Parans Paranthaman investigated TiO2-B (or TiO2(B)) the nomenclature for so-called titania bronze, which is the monoclinic polymorph of titania. (The common polymorphs of anatase and rutile are tetragonal.)
The group synthesized micrometer-sized spherical particles with mesoporous morphology. The open network of channels and pores with sizes in the 10-15 nm range, allows lithium to intercalate in the TiO2-B structure by a pseudocapacitve process, rather than solid state diffusion one. The mesoporous structure is evenly distributed on the surface as well as through the bulk of the particles, which means that the electrolyte has good contact with the anode. Also, the grains in the microspheres are nanosize, which allows for easy electronic transport along the grain boundaries.
The microsphere morphology is good for fabricating compact, uniform electrode layers because of the spheres’ high packing density and particle mobility. However, the microsphere synthesis process is complicated, so challenges remain to be resolved in scaling up the process.
Electrochemical tests showed that the TiO2-B, at low current rates, displays a high discharge capacity: It’s a whopping 93 percent of theoretical capacity, compared to only about 70 percent for anatase nanopowders. The authors remark in the paper, “At high current rates, the difference of reversible capacity between the two materials is even more remarkable.” They report that during high rates of charge-discharge, the capacity of anatase nanopowders is determined by double layer capacitance. The pseudocapacitance behavior of TiO2-B, however, allows the material to maintain large capacities at high charge-discharge rates.
In an ORNL press release, Liu says, “We can charge our battery to 50 percent of full capacity in six minutes while the traditional graphite-based lithium-ion battery would be just 10 percent charged at the same current.” This improved charging and discharging, according to the release, “combined with the fact oxide materials are extremely safe and long-lasting alternatives to commercial graphite make it well-suited for hybrid electric vehicles and other high-power applications.”
In a test of 5,000 charge-discharge cycles, TiO2-B demonstrated a capacity loss of only 10 percent. According to the paper, “The superior cycling performance can be attributed to the structure stability of TiO2-B polymorph and the good accommodation to volume/strain changes of mesoporous structure during lithium insertion-extraction.”
The paper proposes that a TiO2-B microsphere anode coupled with a cathode capable of handling high charge rages, such as some LiFePO4 materials, could provide the basis for a long lifetime, rechargeable battery for high power applications.
The paper is “Mesoporous TiO2-B Microspheres with Superior Rate Performance for Lithium-Ion Batteries,” Liu et al., Advanced Materials (doi: 10:1002/adma20110599).
Coincidentally, a group at Lawrence Berkeley National Lab also published an article about a new material for lithium battery anodes.
This group studied a particulate composite composed of a polyfluorene-based conducting polymer matrix and silicon. They found that incorporating a carbonyl functional group in the PF improved the performance of the anode, increasing the electrical conductivity and assisting with electron and ion transport to the silicon particles. According to the press release, “the polymer is itself conductive and continues to bind tightly to the silicon particles despite repeated swelling and shrinking.”
The group also says that the polymer composite anodes are economical and “the manufacturing process is … compatible with established manufacturing technologies.”
See “Polymers with Tailored Electronic Structure for High-Capacity Lithium Battery Electrodes,” Liu et al., Advanced Materials (doi:10.1002/adma.201102421).