NMR gets inside lithium-ion batteries

A simple and precise method for "seeing" the chemistry taking place in a rechargeable lithium-ion battery using lithium-7 NMR spectroscopy has been developed by UK and US scientists. The work might help improve battery design to remedy flaws in this kind of power supply, namely the tendency of these batteries to form Li metal particles (dendrites) when the batteries are charged rapidly.

In extreme cases, the formation of Li dendrites can cause short circuits in a battery, which can lead to spontaneous combustion, potentially a very serious problem for laptop users. It is also a problem that if left unsolved could seriously affect the uptake of next-generation electric vehicles, such as the Nissan Leaf, which will be built in the UK from 2013. The approach will allow scientists to determine under what conditions the dendrites grow, before a short-circuit of the battery occurs, and to rapidly screen potential methods for prevent these dendrites from forming.

The rechargeable lithium ion batteries commonly used in portable devices, such as media players and digital cameras, deliver power only for a short time before they need recharging. More frustrating is that with continued use, these batteries also lose the ability to accept a full recharge.

In rechargeable lithium batteries, the positive electrode is commonly made of lithium cobalt oxide, the negative electrode of carbon, and the electrolyte is an organic solvent. In use, lithium ions migrate from the negative electrode to the positive electrode, producing an electric current as they do so. In the reverse process, to recharge the battery, positive lithium ions are now pushed through the electrolyte from the positive to the negative electrode by an external current. Rapid charging can sometimes cause The bbuild-up of lithium metal deposits on the negative electrode, the Li ions plating on the electrodes rather than inserting inside the graphic carbon. According to Clare Grey of the University of Cambridge, "Fire safety is a major problem that must be solved before we can safely use batteries in a wider range of transportation applications."

Grey and colleagues Rangeet Bhattacharyya, Baris Key, and Hailong Chen in the Chemistry Department, at Stony Brook University, New York, working with Adam Best and Anthony Hollenkamp of the Commonwealth Scientific and Industrial Research Organization, Division of Energy Technology, in Clayton, Victoria, Australia, have turned to NMR spectroscopy to get an insider perspective on the chemistry of active lithium-ion batteries.

Writing in the journal Nature Materials, Grey and colleagues explain that the formation of "dead" lithium fibres within a lithium-ion battery, so-called dendrites known as "moss", have so far been a significant impediment to the commercialisation of higher capacity, and so more powerful and longer-lasting rechargeable batteries based on lithium metal as an anode rather than the carbon used today.

Previously, researchers have used theoretical models and optical and scanning electron microscopy to study dendrite formation, but finding a way of quantifying the amount of dendrites formed has proved impossible until now.

The team has used in situ lithium NMR spectroscopy to observe the chemistry that takes places as the dendrites form in a tiny, 10mm-long, battery. Highly sensitive ⁷Li NMR spectroscopy, when carried out in situ during electrochemical cycling, is a non-invasive method for investigating the structural changes that can occur in electrode materials," the team explains.

They add that because the ⁷Li signal (spin D 3/2 92:5% abundance) can be acquired on a timescale that is much faster than the typical charge-discharge cycle any structural changes that occur in the active material at various states-of-charge can be detected and quantified by recording spectral snapshots at suitable time intervals."

⁷Li NMR also relies on the finite ability of the radio frequency field used to penetrate the bulk lithium metal electrode beyond the tens of micrometres "skin depth". "Using simple calculations based on the skin depth of metallic structures under radiofrequency excitations, we show that it is possible to quantify the amount of mossy/dendritic lithium formed," the team explains.

"Now that we can monitor dendrite formation inside intact batteries, we can identify when they are formed and under what conditions," adds Grey.

The team adds that, "In the development of batteries with metallic lithium anodes, this [work] means that the whole range of factors that might be used to control or eliminate the formation of dendrites can now be readily assessed and implemented with much more rigour than previously possible."

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.