Towards better lithium batteries
During the past three decades, lithium ion batteries have become the leading technology for powering consumer electronics.
Energy is stored in the battery when positively charged lithium ions fl ow through an electrolyte from a positive lithium-containing electrode to a more negative electrode; electrons then pass through an external circuit to balance the charge. Power is generated from the battery when the process is reversed.
More recently, the use of lithium batteries has been spreading to the automotive market. However, the required improvement in their effi ciency has been slow, and has somewhat held up the development of the electric car.
One of the key factors to improving their performance is to understand how the lithium ions are exchanged between the electrode materials during the charge/discharge cycle. This requires being able to ‘visualise’ the changes in the crystal structure of the electrodes. Neutron diffraction is an excellent technique for seeing lithium ions moving through the electrodes, because neutrons are readily scattered by light elements such as lithium (unlike X-rays).
For this reason, we developed a novel battery to study what happens in the electrode materials while it is actually operating. The aim was obtain a high-quality, real-time movie of the entire process. The cell had larger-than-normal electrodes so as to obtain the optimum quality neutron-diffraction patterns for analysis. The most important feature was the use, in the fabrication of the cell, of a titanium–zirconium alloy, which is known for being neutrontransparent.
In this way, only the signal from the electrode of interest was collected, while avoiding other unwanted contributions from the cell. Moreover, the use of a deuterated version of the electrolyte further reduced the incoherent scattering and improved the signal obtained.
The studies were carried out on the D20 high flux diffractometer – the high neutron flux being an extremely important feature, given that the samples were small in terms of what is normally required for neutron-diffraction experiments.
The working of the cell, the methodology and the quality of the diffraction patterns were first evaluated using relatively well-understood electrode materials such as lithium iron phosphate (LiFePO4). We were then able to study some newer electrode materials based on lithium manganese oxide (LiMn2O4). This is an interesting positive-electrode material, with a high capacity and charge/discharge rate.
However, its capacity quickly fades upon cycling, thus thwarting commercial application. The problem can be solved, however, by incorporating more lithium into the structure at the expense of manganese during synthesis, to give rise to compositions Li1+xMn2-xO4, where x is the amount of extra lithium. This reduces the usable capacity but eliminates the problem of capacity-fading. We synthesised three compounds where x is 0, 0.05 and 0.10, and studied how their structure is modifi ed as the lithium ions flow.
The study showed that not only is the volume change induced by the loss of lithium ions reduced as the proportion of lithium in the electrode material is increased, but also that the mechanism by which this happens is modified. As a result, the materials with the highest amount of lithium is a much better batteryelectrode material.
Given these promising results, the study of this material type is expanding to different compositions such as those containing additional nickel (LiMn1.6Ni0.4O4), operating at higher voltages and thus having more energy.
Instrument: D20 - High-intensity two-axis diffractometer
REFERENCE: M. Bianchini et al., J. Phys. Chem. C, 2014, 118(45), 25947; M. Bianchini et al., J. Electrochem. Soc., 2013, 160 (11), A2176.