Battery : the environmental issue
In recent years, batteries have entered their golden age and have gained the trust of consumers and industrials alike. Colossal investments are underway to meet a growing need (Tesla’s Gigafactory...). In order not to repeat the mistakes made by some industrials, Jean-Marie Tarascon (Professor at College de France and Director of RS2E, French Network on Electrochemical Energy Storage) addresses the most promising paths to push further the environmental compatibility of batteries (resources consumed, lifespan...). In collaboration with a British colleague, Clare Gray (Professor at the University of Cambridge), he published an article in Nature Materials, in which he proposes to open the debate beyond the question of mere autonomy. What materials should be used to produce tomorrow's batteries? How can we ensure a longer lifespan? How to identify a defective cell to quickly replace or repair it?
BEYOND AUTONOMY: THE ENVIRONMENTAL ISSUE
The growing need for rechargeable batteries, especially for the automobile industry and the stationary storage of renewable energies, is based on the lithium-ion technology and places heavy pressure on lithium resources. Even if a shortage is not yet to be feared for the next few years, the researchers are working on technologies using materials containing little to no lithium such as the Na-ion technology, which is also more easily recyclable.
The two researchers give the example of LiFePO4 (a positive electrode material for Li-ion batteries that is widely marketed and synthesizable at low temperature, containing only iron and phosphate, two very abundant compounds on Earth). Even if it suffers from a limitation in its energy density (autonomy) it is increasingly used in electric vehicles. This is the opposite of compounds like LiCoO2, widely used in smartphones, but containing rare, expensive and toxic cobalt.
Another 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, see fig.1) or in terms of recycling as with organic electrodes or binders obtained from natural resources such as CMC (carboxymethylcellulose). However, in order to develop these “batteries of the future” numerous hurdles need to be overcome. 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 made. In the case of Na-ion batteries, the CNRS (National Center for Scientific Research) and the CEA (Atomic Energy Commission) have developed a viable prototype1 currently being transferred to the industry through the RS2E (see fig.2).
Figure 1 : Abundance of certain chemical elements used in batteries (copyright: Researchers' work / Nature Materials)
PERSONALIZED MEDECINE FOR RECHARGEABLE BATTERIES?
Another challenge will be to extend the lifetime of battery packs for electric vehicles (they are composed of smaller batteries2, or cells, which may experience failures that endanger the overall health of the pack) and, above all, to give a second life to these "packs" of batteries. The "second life" corresponds to the reuse of a rechargeable battery pack on a less demanding use after a significant reduction of its autonomy (generally around 20%). A promising market that already interests the electric vehicle industry3,4. But here too questions arise: how to determine the value of the "used" battery and its actual "health"?
At the laboratory level, methods such as NMR, MRI, EPR, TEM, GC/MS, TGA, XPS have experienced dramatic advances in recent years and allow to observe for reactions such as the outgassing happening in the electrolyte, the lithiation/delithiation fronts, or even to enter into the secret of interfaces such as the SEI (solid electrolyte interface).
Figure 2 : sodium-ion battery assembled by CEA and CNRS (copyright : Vincent Guilly/Liten/CEA/CNRS)
Problem: these spectacular advances 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 (a format of industrial battery cells) to have a live and easy access to parameters such as temperature, pressure... They call to launch significant research efforts in the field in order to develop such "passive", non-destructive and usable in industry methods. In case of failures, the two scientists even propose to “cure” by an external intervention or by using self-healing materials. Is a new paradigm opening up?
2. In a Tesla car we need around six thousand 18650 battery cells to assemble the “battery pack”
Sustainability and in situ monitoring for battery development
CP Grey & JM Tarascon
Nature Materials, 20/12/2016, DOI : 10.1038/NMAT4777