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Researchers call for integration of materials sustainability into battery research; the need for in situ monitoring

18 January 2017

In a review paper in the journal Nature Materials, Jean-Marie Tarascon (Professor at College de France and Director of RS2E, French Network on Electrochemical Energy Storage) and Clare Gray (Professor at the University of Cambridge), call for integrating the sustainability of battery materials into the R&D efforts to improve rechargeable batteries. The pair argue for the selection of chemistries that have a minimum footprint in nature and that are more readily recycled or integrated into a full circular economy.

Concerns over sustainability as well as cost directs that battery lifetimes must be greatly improved, and second-life applications considered during the development phase. As part of this, Gray and Tarascon suggest, the state of health of batteries must be monitored continuously during operation to minimize their degradation. This requirement, in turn, pushed the boundaries of operando techniques to monitor increasingly complex processes.

In their paper, they discuss key advances in both more sustainable chemistries and operando techniques, along with some of the remaining challenges and possible solutions, as we personally perceive them.

Substantial progress in battery technology is essential if we are to succeed in an energy transition towards a more carbon-neutral society. We need new storage technologies if more renewables are to be used on the electrical grid; similarly, the electrification of transport requires much cheaper and longer-lasting batteries. The size of these batteries (in comparison to those used for portable electronics) places severe pressure on materials resources. Although estimates vary widely, the predicted penetration of lithium-ion technology into these large-volume markets could result in as much as a threefold increase of production for the cathode material, reaching nearly 400,000 tonnes per year by 2020.

We need to rethink the processes, and move towards more sustainable energy harvesting and storage technologies that become part of a circular economy. Materials sustainability will become an overriding factor in the years to come, conjointly with consumer demands that are very real and ever-increasing. They include the desire to increase the driving range of more modestly priced electric cars from 150 to well over 500 km, and the need to lower the cost and provide maintenance-free technologies for grid applications. The issue is thus straightforward: can we increase battery sustainability, while lowering cost and improving battery performance?

… Because of Tesla and other less vocal companies, the field of electrochemical energy storage is potentially undergoing a real revolution. A nearly threefold decrease of the stored kWh price to €200 per kWh at the pack level is predicted in the next 10 years, driven at least in part by both the large volume production and the realistic prospects of a second life for electric vehicle batteries. Under such a scenario, the production of Li-ion batteries should expand hugely over the years to come, hence reviving the issue of finite Li reserves. These reserves are indeed limited, but Li can be recycled by hydrometallurgy, although the economics of such a process has yet to be worked out. This concern has driven researchers to explore new, potentially more sustainable chemistries, including Na-ion, metal–air chemistries Li(Na)–O2, Li–S, multivalent (Mg, Ca), redox flow batteries (RFBs) and aqueous-based technologies.

—Grey and Tarascon

Grey1
Elemental resources. The abundance of the different chemical elements currently used in the systems considered in the paper, reported on a log scale as a rectangle, with the values in ppm given on top. Note the trend in abundances of Al > Ca > Mg > Na > Li, and Fe > Mn > Ni > Co. The standard redox potentials of metal anodes together with their capacities are given (in the colored rectangles). Note that the values for O and S correspond to their use as positive electrodes. Abundances for alkali and alkali earth metals are shown in blue, transition metals in red and main group elements in green. Source: Grey and Tarascon. Click to enlarge.

A medium-term strategy would be to gradually move away from Li-ion technology to technologies such as Li/S, Na-ion, Mg-ion, Ca-ion, Li-air. All these technologies have advantages in terms of abundance (sodium is 1000 times more abundant than lithium, and calcium 3000 times more) or in terms of recycling as with organic electrodes or binders obtained from natural resources such as CMC (carboxymethylcellulose).

Admittedly, these “batteries of the future” face numerous hurdles. For example, in the case of Mg-ion batteries there is difficulty to identify materials capable of inserting Mg2+ ions beyond 1.3V and finding compatible electrolytes. However, significant advances are being made. In the case of Na-ion batteries, the CNRS (National Center for Scientific Research) and the CEA (Atomic Energy Commission) in France have developed a viable prototype currently being transferred to the industry through the RS2E.

Grey2
Schematics of different rechargeable batteries. Cells based on monovalent (Li+, Na+) and divalent (Ca2+, Mg2+) cations generally consist of intercalation materials, separated by a membrane, which are immersed in a liquid electrolyte and serve as positive and negative electrodes. When the electrodes are externally connected, redox reactions proceed in tandem at both electrodes to deliver energy.

When moving to a Li–air cell, the positive electrode is replaced by a porous carbon electrode over which the oxygen reduction/oxidation takes place.

For Li–S, the positive sulfur electrode consists of a composite made of carbon, sulfur and, in some batteries, oxides.

Such configurations change for the redox flow cell which comprises two electrolyte flow compartments separated by an ion-selective membrane; the electrolyte solutions are pumped continuously from external tanks that contain the soluble redox species and thus the stored energy. Source: Grey and Tarascon. Click to enlarge.

The continued push for cheaper, higher-energy-density and more sustainable battery technology has led to a blossoming of research activities centered on new chemistries such as Na-ion, metal–air (Li, Na, Zn), Li–S, multivalent ions and redox flow, to name but a few. Although some of these … are in the very early stages of commercialization, there is no clear-cut winner; several advances have, however, been made, and so optimism must prevail, motivating continued research and development of all of these technologies.

—Grey and Tarascon

In situ monitoring. At the laboratory level, methods such as NMR, MRI, EPR, TEM, GC/MS, TGA, XPS have experienced dramatic advances in recent years and enable observation for reactions such as outgassing happening in the electrolyte, lithiation/delithiation fronts, or even to enter into the secret of interfaces such as the SEI (solid electrolyte interface).

Unfortunately, these spectacular advances in the lab do not necessarily translate into real-time analysis capability in the field. Indeed, they are either too low resolution, require very large equipment (from a diffractometer to a full-blown synchrotron...) or even “customized” batteries (therefore not representative of the real products), such as those using a Teflon casing.

The two researchers therefore propose to take inspiration from individualized medicine with the use of optical fiber sensors directly inserted inside 18650 cells to have a live and easy access to parameters such as temperature and pressure. They call for the launch of significant research efforts in the field in order to develop such passive, non-destructive and usable in-industry methods.

Further, once faults are identified, the authors say, there needs to be the ability either to switch on repair mechanisms or to build in self-healing processes into the original battery design.

Resources

  • C. P. Grey & J. M. Tarascon (2017) “Sustainability and in situ monitoring in battery development” Nature Materials 16, 45–56 doi: 10.1038/nmat4777

January 18, 2017 in Batteries, Materials, Sustainability | Permalink | Comments (0)

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