1Mr Administrator,
Dear colleagues and fellow scientists, dear friends,
Ladies and Gentlemen,

2Although all my colleagues warned me, I am realizing just now that delivering one’s inaugural lecture at the Collège de France is a more emotional experience than I expected it to be. Allow me nevertheless, Mr Administrator and my dear colleagues, to thank you for honouring me with the Annual Chair of Sustainable Development – Environment, Energy and Society, sponsored by the Total group, which I would also like to thank.

3This Chair has a vast programme. My two predecessors, Henri Leridon (2008-2009) and Nicolas Stern (2009-2010), covered respectively the demographic aspect of sustainable development and the development and climate change economics. This year, I will address the question of energy, and more specifically, its storage and conversion.

4My inaugural lecture will however remain more general and will discuss the core issues underpinning crucial socio-economic and global challenges for future generations. Energy is at the heart of our daily concerns and has unquestionably become the vital element of our modern societies, with electricity as a vector and the watt-hour, according to some alarmists, potentially set to become our next monetary unit. But where do we actually stand? This is what I will try to answer in this three-part presentation.

5I will first provide tangible elements with which to understand the current energy situation, its evolution and the consequences for our planet of applying a “laissez-faire” policy. I will then discuss the prospects renewable energies offer. Finally, I will expand on the issue of energy storage by describing advances and perspectives concerning batteries which, as I will show, are largely dependent on chemistry.

1. Current and future energy context

6Our growing dependence on energy is linked to eighteenth century discoveries and the technologies that ensued (Fig. 1). Human beings originally relied on biomass as their sole source of energy, without this impacting on the environment since plants reabsorbed the CO₂ released, through photosynthesis. This situation endured until the end of the 18th century, when James Watt invented the steam engine. It was the first machine able to transform thermal energy into mechanical energy, marking the start of the first industrial revolution with the appearance, amongst other things, of steam trains.

Figure 1

7Industrialization further intensified at the end of the nineteenth century, particularly with the discoveries of the internal combustion engine (by Étienne Lenoir) and of electricity, which led to our growing dependence on fossil fuels. The arrival of new transport technologies and other nineteenth-century advances only further increased our total energy consumption, which has become gigantic, in excess of 1.2 × 10¹⁴ kWh/year, corresponding to an installed capacity of the order of 14 tera-Watts (14 × 10¹² W).

8Energy efficiency, which is often very low in many uses, is a parameter that needs to be taken into consideration. As an example, for 2 joules of light energy supplied by an incandescent lamp, we need 100 joules of primary energy, with 98 joules consumed by the thermoelectric conversion and by distribution. Hence the importance of avoiding, as much as possible, thermoelectric-type conversions, given the limitation imposed by Carnot’s principle.

9Eighty-two percent of the primary energy used worldwide comes from fossil fuels (coal, gas, and oil); the rest comes from renewable energy (sun, wind, sea, and others) and nuclear energy – 11% and 7% respectively. These figures vary across countries, since in France 34% of primary energy is nuclear whereas in China 64% comes from coal (Fig. 2).

Figure 2

10Fossil fuels are not inexhaustible and several models, based on estimates of the growth of current consumption and the reduction of identified reserves, predict that remaining oil, gas, uranium, and coal reserves will only last 41, 62, 64 and 230 years respectively (Fig. 3). Even if these estimates are not exact, note that a mistake of 100 billion tons of oil reserves would only shift the timeframe by +/-10 years. In the space of one or two centuries, we will therefore have used up the fossil reserves that took millions of years to accumulate on our planet. This catastrophic scenario had been announced as early as 1956 by the US geophysicist M. King Hubbert¹, but kept silent until the first oil crisis in the United States in 1970.

Figure 3

11The sectors of primary energy consumption are transport, industry and habitation, accounting for 23, 24 and 42% respectively, with the remaining 11% lost in distribution. Knowing that a litre of gasoline releases 2.5 kg or 1,200 L of CO₂, the transport domain is responsible for 30% of the 26 Gt/year of CO₂ currently emitted, which have contributed to the rise in atmospheric carbon concentration from 250 ppm in 1950 to 370 ppm in 2010.

12Even though other factors, like solar activity, contribute to these high carbon emissions, the latter undeniably influence the Earth’s evolution, both in terms of the level of global warming and of the rise in sea levels, since the two are correlated through a simple expansion effect. Here again, while we seem surprised by these phenomena, some of our predecessors such as Joseph Fourier in 1800 and Swedish chemist Swante Arrhenius in 1850², had foreseen and even calculated a rise in our planet’s global temperature due to an increase in the atmospheric carbon concentration.

13This global warming translates into the disappearance of certain glaciers, the acidification of oceans (demonstrated by coral bleaching), melting icecaps, the rise in sea level (which is a threat to certain islands like those of the Maldives archipelago) and growing damage caused by storms and floods, to name but a few. In a few decades, climate change could completely reconfigure our geography and cause us to relocate industrial and agricultural areas. This situation is also aggravated by the demographic component, judging from the continuous growth of the population which will rise from 6.5 billion to 9 billion in 2050, as well as the ever growing consumption per capita – which puts estimates for the available power necessary for global consumption in 2050 at 28 TW. As a result, if we maintain the “laissez-faire” policy of energy production and use, we will emit 55 Gt CO₂ in 2050 and 75 Gt in 2100, which corresponds to carbon concentrations of 500 to 700 ppm. All models, however conservative, indicate that such concentrations would lead to temperature increases of 2°C (at least) in 2100, which would lead to a 30 cm rise in sea levels.

2. Perspectives surrounding renewable energies

14It is therefore important to find a solution to curb this worrying situation, in other words to ensure that the rate of CO₂ remains the same in 2050 as it currently is. As described by Socolow’s stabilization wedge³, this implies that we will need 14 TW of decarbonized energy sources to avoid carbon emissions of 28 Gt/year. While several strategies may be envisaged (Fig. 4), none of them alone can provide the solution to the problem. Nuclear energy is indeed attractive, but to obtain the missing 14 TW, we would need to build a nuclear reactor capable of generating a GW every day for the next 50 years, not to mention the limited stock of fossil material (for example Uranium-235). Certain countries, including India and China that recently announced the construction of a further 800 thermal power plants, want to continue to believe that coal is a possible solution. Here too, carbon sequestration solutions are still at the experimental stage, through the exploitation of aquifers with an annual capacity of 1 million tons. We are therefore very far from the scale needed to justify the long-term use of coal, which would require having over 20,000 operational aquifers by 2050.

Figure 4

15It is therefore important that we turn to the efficient use of renewable energies with a low carbon footprint – solar, wind, geothermal, biomass, etc. –, which are clean and inexhaustible energies. Except for tidal and geothermal energy, all these sources are linked to solar activity: let me stress that the Sun sends our planet 10,000 times the energy it needs. To be more precise, every 90 min the Earth receives as much energy from the Sun as humanity consumes annually. The problem lies with converting and storing these renewable energies efficiently and at a low cost.

16While different promising research pathways on the conversion of solar energy are currently being explored, it remains highly uncertain how long they will take to come to fruition. These consist, amongst other things, in transforming solar energy into different vectors: (1) electricity, through photovoltaic conversion, (2) hydrogen, through photocatalysis/electrolysis reactions, taking advantage of the semi-conductor/liquid-junction properties to dissociate water into H₂ and O₂, and finally (3) the biofuels obtained through natural photosynthesis or artificial photosynthesis, which is based on the elaboration of systems seeking to mimic the role of hydrogenases.⁴ However, the energetic yields are weak, of the order of 10, 5 and 0.3% respectively for the three cases considered.

17The abundance of energy released by the Sun is such that covering 0.1% of the Earth’s surface with solar cells with a 10% yield would suffice to meet humanity’s current needs. This pathway nevertheless remains a colossal challenge, despite the many existing types of photovoltaic cells (inorganic, organic, or dye-sensitized). Silicon and thin-layer technologies (CdTe, amorphous Si, CGIS) – whose principle is founded on the absorption of a photon by a semiconductor to generate an electron-hole pair dissociated at the p-n junction, thereby generating electricity – are the most advanced, with yields of over 20% for the crystalline Si. Dye-sensitized cells, called Graetzel or DSC cells⁵, have a yield of close to 11%. They function on the principle of plant photosynthesis since, in this case, the creation of electrons and holes is generated through the absorption of light by a pigmented dye (the equivalent of chlorophyll in plants). Lastly, purely organic cells also rely on a P-N junction, this time taking advantage of the acceptor-donor pairs of carefully chosen organic molecules; but their yield currently remains below 5%.

18However, the main issue with photovoltaic energy is still its cost ($0.5/kWh) against that of conventional energies ($0.05/kWh). Government subsidies are thus currently needed to make it competitive. The situation should soon change, as many experts now agree – given recent advances in terms both of yields (~41% for cells with multiple junctions) and of the development of low-cost photovoltaic cells (DSC) – that the cost of the photovoltaic kWh could compete with that of the nuclear kWh by 2030 (Fig. 5).

Figure 5

19Finally, there is another difficulty inherent to renewable energies (solar, wind, and others), which relates to their irregularity, leading to large fluctuations in the energy delivered, as obviously the wind does not blow and the sun does not shine on command. It is therefore imperative to invent new energy storage technologies that can adapt to network applications, so as better to manage our planet’s renewable energy resources, in other words, to invent technologies which are capable of delivering energy when we need it. Similarly, the switch from the thermal to the electric vehicle, with a view to reducing carbon emissions, requires on-board energy to ensure the vehicle’s autonomy; there again, the latter requires effective storage systems.

3. The electrochemical storage of energy

20These two applications – networks and transport – therefore require the storage of energy so as to release it in electrical form. One of the best ways of doing so is by converting chemical energy into electrical energy, since both share the electron as their vector. The electrochemical systems able to perform such a conversion, known under the names of fuel cells, supercapacitors and batteries, will be described later on.

21Although it recently received extensive media coverage, the battery/electric vehicle association is not new: it goes back to the end of the nineteenth century, when the “Jamais Contente”, equipped with lead-acid batteries with an autonomy of 89 km, reached 109 km/h. Furthermore, let me highlight that in 1900, in the USA, there were 1,500 electric cars for 1,500 thermal cars, and that in 1914, 30% of cars there were still electric. Why did they virtually disappear?

22There are two prevailing reasons: first, the abundance of easily usable fossil fuels, and second, until recently, the limited performance of electrochemical systems. While a lot of progress has been made, the limited autonomy afforded by batteries is still what stalls the rapid spread of the electric vehicle (Fig. 6).

Figure 6

23The electrochemical systems currently developed for this application (fuel cells, supercapacitors, batteries) are all comprised of two electrodes, positive and negative, soaked in an electrolyte, and only differ in their functioning.

24The fuel cell functions on the reverse principle to that of water electrolysis⁶: electricity is produced through the oxidation of a dihydrogen (H₂) electrode, associated with the reduction on the other electrode of an oxidant such as the oxygen in air, with the recombination producing water. This is an “open” system, fuelled from the outside, and therefore not directly electrically rechargeable.

25Supercapacitors⁷ are based on the capacitive properties of an electron-ion double layer at the electrolyte-electrode interfaces, with a capacitance per unit of area that is up to millions of times greater than that of ordinary capacitors.

26Finally accumulators, commonly known as “batteries”, can release/store electric energy through reversible oxidation-reduction reactions that can take place within the constituent materials of their electrodes.⁸

27We owe the discovery of the chemical generation of electricity to Alessandro Volta, and the way it occurred, as is often the case in research, to critical thinking. Following the observation, in 1781, that the frog muscle contracts when in contact with a pair of two different metals, in this instance copper and iron, Luigi Galvani concluded that the frog muscle produces electricity. In 1787, Volta confirmed this observation, but provided a different explanation: it is the presence of two different metals, soaked in the muscular liquid (which is an ion carrier) that is responsible for the electricity produced. To demonstrate this two years later, he assembled a pile of metallic zinc and copper discs, each separated by discs of felt soaked in a saline solution: the first “pile”, called “Volta pile”, was the one able to provide current.

Figure 7

28Over the course of the two centuries following this discovery, numerous battery technologies appeared, in particular with the lead-acid, nickel-cadmium, nickel-metal hydride, lithium-polymer and lithium-ion systems. The comparison of these different systems (Fig. 7) shows that lithium-based technologies offer the highest performance in terms of energy density. This is due to the fact that lithium is not only one of the most electropositive metals (in other words which facilitates high potentials) but also one with a small molar mass (which is advantageous in terms of gravimetric capacity).

29Another problem relating to battery technology concerns the slow pace of advances in the domain of energy storage (five-fold increase in energy density in 200 years) when compared to those in information storage governed by Moore’s law (doubling of storage capacity in 18 months). This slow pace has often been highlighted in the media, which abound with expressions such as “research in the domain of batteries advances at the speed of a glacier” or “the timescale of micro-electronics is much shorter than that of batteries, where performances are still limited by… CHEMISTRY”. Let me stress, in our defence, that elaborating new electrochemical energy storage systems, whichever they may be, is a multidimensional problem, which mainly finds its complexity in (1) the formulation of the electrodes, which include active matter, a binder and an electronic conductor, (2) the choice of formulation of the liquid, gel or polymeric electrolyte, but especially (3) the command of the macroscopic/microscopic interfaces, which are the key elements of any electrochemical system. It therefore goes without saying that the exercise going from the design of the electron material to its use in an accumulator requires a multidisciplinary approach, in which chemistry in all its forms – inorganic, organic or surface chemistry – has a crucial role to play. I here describe this approach through the example of lithium-ion accumulators.

30Like any electrochemical system, a lithium-ion accumulator (its design and commercialization date back to the eighties and the nineties respectively) is constituted of two electrodes soaked in an electrolyte. Its specificity, however, lies in the fact that it uses (1) a non-aqueous electrolyte and (2) existing composites with variable rates of lithium ions inserted, as positive and negative electrodes.⁹ These insertions/dis-insertions occur at different redox potentials, hence the difference in the accumulator’s potential (expressed in Volts). The number of lithiums exchanged per mass unit of the electrode material constitutes its gravimetric capacity, given as

C(Ah/kg) = (26.8 x)/M

where x is the number of Li⁺ inserted and M the molar mass of the material.

31In spite of this simple formula, the search for better electrode materials – which constitute the heart of any electrochemical system and govern the quantity of energy stored by the system and the power available – remains complex. This research is subject to the vagaries of chemistry, which does not supply effective electrode materials on demand. This lack of materials, coupled with the difficulties surrounding the command of interfaces, is precisely what has stalled technological advances in the sectors of portable electronics, electric transport and other applications.

32In the early nineties, aware of our planet’s energy challenges, I decided to leave my studies in the domain of supraconductivity in order to devote myself to the electrochemical storage of energy and, more specifically, to lithium-ion technology, which was still in its infancy. It thus offered ample research opportunities, as it was devoid of preconceptions. Alternatives to classical approaches, both in terms of the material and the configuration of the cells of the reaction mechanisms, were needed to overcome existing technological hurdles, if not to encourage technological breaks.

33I will now describe three examples of research activities I have carried out since 1990, in order to illustrate my contribution to the development of more effective Li-ion accumulators for application in electric vehicles. I will first present the research linked to insertion chemistry, which has allowed for the commercialization of lithium-ion LiMn₂O₄/C technologies with liquid and plastic electrolytes. I will then describe the contribution of nanomaterials and the nanostructuring of electrodes, in order to highlight the importance of moving away from tradition and exploring new paths. Finally, I will discuss the research currently pursued within the framework of sustainable development to elaborate materials through new eco-efficient or bio-inspired syntheses, to use electrode materials from biomass obtained through green chemistry, and to develop new technologies.

3.1. Li-ion LiMn₂O₄/C technology

34In the early nineties, the Li-ion battery commercialized by Sony was based on the use of LiCoO₂ as a positive electrode. At the time, however, the LiMn₂O₄ spinel was appealing because of its high redox potential (4.2 V), though offering mediocre electrochemical performances, particularly with a migration of the Mn of the positive electrode towards the negative electrode due to a dissolution problem linked to electrolyte and structural instability problems.¹⁰

35Since the electrolytes that had been used until then were not stable at high potential, we sought to isolate others. Based on DFT calculations linked with solvation, dielectric constant and viscosity considerations, we elaborated a new electrolyte which combined cyclic and acyclic carbonates associated with LiPF₆; the latter proved stable up to 5 V and is currently commercialized worldwide under the name LP30. It thus made it possible to explore the domain of potentials above 4.3 V, to discover new electrode materials such as CoO_2, and to optimize the LiMn₂O₄ spinel. Another issue inherent to LiMn₂O₄,, which is also linked to a non-stoichiometry, lies with the existence of a structural transition associated with the presence of a Mn⁺³ concentration greater than 1. This triggers a distortion of the octahedron and a change in the volume of more than 10% of the crystal lattice. As this repeated change of volume with charges-discharges was detrimental to the cycling, it was important to find a solution. Based on structural considerations, it was possible, either with a Li excess on Mn sites or with a double cationic (Al for Mn) and anionic (O for F) substitution, to reduce the Mn⁺³ concentration in the material, thereby avoiding the structural transition. Once the volume change had been eliminated, sustained reversible capacity was achieved at room temperature. These efforts, coupled with coating methods, led to the liquid electrolyte Li-ion LiMn₂O₄/C battery on which the Alliance project by Renault and Nissan is based.

36In order to expand the field of applications of this technology, however, its configuration still needed to be modified, so as not to be restricted to cylindrical forms only, and rather work with flat and flexible configurations. The process used until then, which drew on polymer chemistry, consisted in mixing a polymer of the polyethylene oxide type, for example, with a plasticizer such as propylene carbonate that contains Li salt to form a stable polymer film. However, all these attempts resulted in a sticky film that was sensitive to air, and had to be reticulated through physical and chemical means, making it all complex and commercially unfeasible.

37To bypass this problem, my colleague Paul Warren and I decided to exploit a particularity of plastic materials: their capacity to contain plasticizers within their networks that can be extracted by the appropriate solvents. As a result, thanks to the reasoned choice of the PVDF-HFP copolymer, which consisted in adjusting the molecular weight, the quantity and the distribution of HFP through the elaboration of a polymerization process, we obtained mechanically stable plastic films. We subsequently found the trick to be to extract the plasticizer by washing it with ether so as to obtain a porous membrane. The latter is then soaked in the electrolyte which, in turn, acts as a plasticizer, thus allowing for the elaboration of a plasticized electrolyte sheet.

38One of the advantages of the PVDF-HFP matrix is that, by virtue of its HFP composition, the proportions of crystalline and amorphous areas can be adjusted, making it possible to regulate the quantity of electrolyte it contains and adjust its ionic conductivity for energy or power applications. Using the same protocol, plastic sheets of positive and negative electrodes were obtained. The assembly of these three films through a lamination process at a moderate temperature thus afforded, for the first time, a flexible plastic battery, with a variable shape and capacity, which is currently being developed and commercialized under the name of plastic lithium-ion battery PLiON^TM. This innovation¹¹, which was first and foremost the fruit of a multidisciplinary approach drawing on a diverse chemistry, gave new impetus to Li-ion technology.

3.2. Nanomaterials’ contribution

39I chose this second example with a view to showing the importance of challenging tradition in research if we wish to move forward. Since the onset of Li-ion technology, research on the subject had established a set of specifications for the selection of the ideal electrode material, that is, a material which is a good electronic and ionic conductor, with an open structure and a micrometric size. These criteria of course limited the number of candidates and only three composites had survived this screening: LiCoO₂, LiNiO₂ and LiMn₂O₄ with, in each case, a capacity limitation of the order of 0.5 electrons per transition metal, which caused technology to stagnate.

40When faced with such a situation in science, one must wander off the beaten track and explore new routes. We thus envisaged, successfully, the possibility of moving to nanomaterials, thereby adding size as a third parameter in addition to structure and composition, to play on the electrochemical performances of electrode materials. Given this evolution, one can legitimately wonder why it took so long for the shift to the nanometric scale to extend to the world of energy storage and, more specifically, that of Li accumulators. For several years, other technologies have been making the most of nanomaterials, which are behind the spectacular advances currently witnessed in microelectronics. The reason for it is simple, and involves the catalytic reactions (in other words the deterioration of the electrolyte) that occur on the surface of electrode materials that come into contact with the electrolyte: they compete with the redox reactions that occur at the heart of the particle and define capacity. These two seemingly opposed worlds can nevertheless converge if we consider nanomaterials with a redox potential that corresponds to the electrolytes’ domain of stability or, in the opposite case, if we command the nanomaterial/electrolyte interface through the use of protective layers obtained with physical and chemical coating techniques. It is through the use of these intermediaries that nanomaterials are now appearing in Li-ion technologies.¹²

41The coupled coating/morphology nanometric approach also affords a better use of the electrode. This can be simply explained by likening the functioning of an electrode to that of the brain, where all neurons must be properly connected-irrigated for it to function properly. Coating the nanoparticles with a uniform carbon layer within the electrode thus provides the electronic percolation to access all the active matter. As for the particle’s nanometric character, it reduces the travel time of an ion, from the heart to the surface of the material, by a factor of 100 when the radius is divided by 10 for example – with the two combined affording a better electrode kinetics.

42Nanometric and nanostructured materials now benefit Li-ion technology in many ways. Amongst other things, the shift to the nanometric scale has allowed for:

1. the transformation of a once neglected material, such as LiFePO₄, into one of the electrode materials that is most prized by battery manufacturers, due to its abundance and low cost – two very important criteria in the current context of sustainable development. Li-ion LiFePO₄/C batteries, commercialized since 2006 by the company A123, have now prevailed as the prime technology for electric vehicle and network applications;

2. the resolution of a twenty-year-old problem concerning silicon- or tin-based negative electrodes, which show alloy reactions with lithium (Li_xSi_y) that led to significant volume changes. Thanks to the relaxation of constraints afforded by the nanoparticle state, Si and Sn electrodes, which combine a nanometric character and carbon coating, could be produced, leading to the development of the NEXELION technology, marketed in 2005;

3. finally, going beyond the conventional insertion/disinsertion processes of well-established lithium ions, by shedding light on a new reaction mechanism called a conversion. According to this new mechanism, the electrochemical reduction of a binary oxide (CoO) by lithium leads to the formation of a composite electrode formed of nanoparticles of metallic Co inserted into a Li₂O matrix which, by virtue of its nanometric character, can be oxidized upon recharging. The reversible switch from Co⁺² to Co is therefore associated with the absorption/desorption of two Li by a transition metal (3d) – instead of a maximum of 1 for insertion reactions – affording access to electrode materials with an exacerbated capacity. Note that this reversibility, until now unknown, has proved universal; it can occur in metals or anions of virtually any nature with, moreover, the possibility to adjust its potential. However, unlike previous technologies, Li-ion systems with a conversion electrode are still at the development stage, due to difficulties encountered with regard to their energy efficiency.

43In light of all the advances described above, Li-ion technology, by virtue of its assembly flexibility and performances (~210 Wh/kg and ~700 Wh/L respectively), which doubled in the space of twenty years, has prevailed on many markets. A question nevertheless remains regarding its evolution and its successful use in electric vehicles. We therefore need to identify critical points for this application.

44Besides the security aspect, which is non-negotiable, there are grounds for doubling the energy density and cutting the cost by half. These objectives will mainly require working on the positive electrode material, which not only governs the battery’s energy density, but is also its costliest element, ahead of the separator. This therefore takes us back to the search for new materials, which will have to be grounded in a different context from the one that once prevailed, that of sustainable development, for which the lifecycle analysis of the material or of the electrochemical systems is becoming crucial.

3.3. The Li-ion battery against the backdrop of sustainable development

45It is necessary to look back at the battery assembly stages – the extraction of the minerals, their purification and the elaboration of the electrode as well as its end of life recycling, all energy intensive – to understand the “energy-guzzling” aspect of the process (Fig. 8). The lifecycle analysis of current electrode materials shows that their production is energy-costly and that their recycling is also far from neutral. Studies show that 280 kWh are needed, producing 80 kg of CO₂, to manufacture an Li-ion accumulator capable of storing 1 kWh.¹³ Hence we need to develop more sustainable and more ecological Li-ion or other accumulators, and to promote eco-compatible storage.

Figure 8

46A futuristic situation, within the framework of sustainable development (Fig. 9), would consist in using precursors from biomass to produce electrochemically active materials, drawing on the concepts of green chemistry, which could subsequently be recycled at a low temperature with carbon emissions that will be reabsorbed by plants via photosynthesis. Reaching this ideal situation requires long-term work which we have undertaken, and three current approaches of which I here present, still drawing on the rich and varied aspects of chemistry. They concern respectively (1) the eco-efficient elaboration of inorganic electrode materials, (2) the use of renewable organic electrodes, as well as (3) the use of new storage systems (lithium-air).

Figure 9

3.3.1. The eco-efficient synthesis

47The electrode materials currently found in Li-ion batteries are mainly developed with ceramic methods. These methods, which use high temperatures to ensure the spread of the reactants needed for the growth of new phases, are highly energy-intensive and, what is more, lead to polydisperse materials. In our search for eco-efficient syntheses¹⁴, we therefore preferred to employ low-temperature methods (“soft chemistry” methods), mainly using solution syntheses (Fig. 10), thus revisiting well-known processes such as the hydrothermal or solvothermal process for the synthesis of monodisperse LiFePO₄ powders at temperatures of 250°C only, or even developing new processes such as the ionothermal and biomineral processes, described further on.

Figure 10

48As its name suggests, the ionothermal process uses ionic liquids, which are simply salts made of organic cations and anions which, unlike our cooking salt (NaCl), are liquid at room temperature. These ionic liquids offer many advantages, including a virtually neutral vapour tension, which means that syntheses can be carried out in an open flask and not in an autoclave as with hydro(solvo)-thermal methods. Moreover, they are uninflammable and possess good solvation properties, while also remaining stable up to 300°C. They have a very rich chemistry, since the number of ionic liquids that can exist is estimated, based on cationic-anionic combination, at 15,000 (1,000 of them have already been synthesized). Finally, they are easy to use as they can be recovered through the classical organic approach. Given all these qualities, it was then surprising that the ionothermal approach had never been used in inorganic synthesis, which in the past had often drawn on the melted inorganic salt method to develop new phases.

49Wandering off the beaten tracks, thanks to the use of ionic liquids, we showed that it was possible to prepare electrode materials such as LiFePO₄, with a controlled size and morphology for reaction times of 24 hours, while working below 250°C and under atmospheric pressure. For the sake of comparison, temperatures of 700°C and reaction times of several days are needed with the ceramic approach. Taking full advantage of ionic liquid, which serves as both the reaction environment and the structuring agent, electrode materials already known (Li₂FeSiO₄, Na₂FePO₄F, etc.) were synthesized at a temperature of 500°C, lower than with the ceramic method, while also making it possible to steer the growth (and therefore the morphology) of the particles.

50Last but not least, these ionothermal methods enabled us to prepare a whole new family of fluorosulfates¹⁵ of the AMSO₄F formula (A = Li, Na ; M = Fe, Co, Ni), which until then could not be prepared with ceramic or solution methods for the simple reason that these phases are not stable in aqueous environments and decompose at temperatures above 350°C. Among these new composites, iron fluorosulfate, comprised of abundant and cheap elements, presents attractive electrochemical properties as a battery electrode material with, in particular, a 3.6 V potential and a 140 mAh/g capacity, thus seriously competing with LiFePO₄, currently the most prized electrode material for electric vehicles. Discovered only a year ago, there are no less than twenty members to this family, some of which (ZnFeSO₄F) present sufficient ionic conductivities to serve as solid electrolytes in lithium-ion batteries. The ionothermal approach thus opens up a whole new domain of possibilities in inorganic synthesis, not only for electrode materials but also for magnetic and ferroelectric materials.

51The use of ionic liquids allowed us to lower the temperatures of electrode material syntheses down to 200°C; the question, however, remained of knowing whether these materials could be prepared at room temperature.

3.3.2. Bio-inspired synthesis

52Generally, when chemists search for new synthesis methods they turn to the chemistry of life, as many of our predecessors did in the past, such as Jean-Baptiste de Lamarck, if we can go by this citation:

the most important discoveries of [...] nature have nearly always sprung from the examination of the smallest objects which she contains.¹⁶

53In fact, nature abounds with examples illustrating its capacity to produce calcite or silica nanomaterials, through bio-mineralization carried out by micro-cellular organisms, which therefore involve highly efficient catalytic systems, in particular, enzymes.

54In light of earlier works on calcite, in collaboration with F. Guyot at Paris-Sorbonne University (Paris IV), we tried to synthesize LiFePO₄ through bio-mineralization, in line with the reaction written below, by which the bacteria (specifically its urease enzyme) could hydrolyse the urea molecule and thus produce the basicity of the environment necessary for the precipitation of LiFePO₄.

55The change of colour of the reaction environment shortly after the bacteria are introduced appeared to be an indicator of the reaction’s existence. This was confirmed by high-resolution microscopic measurements, which indicated the growth of a bio-film generated by the bacteria, containing inorganic phase nanodomains which, after 24 hours of reaction, proved to be the nucleation centre for the growth of thin needles – needles that turned out to be LiFePO₄, based on the analysis of electron diffraction patterns.

56Pursuing this bio-inspired approach, I cannot help but mention the recent work carried out at MIT by Dr. A. Belcher¹⁷, who successfully assembled batteries using viruses. The virus is used, in this case, as a form for the growth of nanostructured electrodes. More specifically, the authors practice viral genetic engineering to endow the virus with protein links able either to fix themselves onto metallic elements (substrates), or to adapt to the grafting of organic groupings favourable to the growth of an oxide or a phosphate. With this virus multi-functionalization approach, different peptidic groups can thus be elaborated and used for the elaboration of electrodes. This type of genetic programming process, which requires only very few post-synthesis stages, offers new opportunities to revive materials that were previously neglected.¹⁸

57Those who take an interest in the chemistry of life will have to acknowledge that most energy mechanisms draw on redox mechanisms involving organic molecules. It therefore seemed interesting to use organic molecules as electrode materials and thus offer a vegetal alternative to the mineral approach followed until now. This is what one of my colleagues, P. Poizot, attempted. To do so, he revisited the chemistry of monocyclic oxycarbons, which was over a hundred years old. Surprisingly, these conjugated systems, in which all carbon atoms are linked to oxygen-forming ketone functions, had never been tested with Li. One of them, dilithium rhodizonate (Li₂C₆O₆), proved reactive¹⁹, as hoped for in view of the presence of four ketone functions. It reacted reversibly with four lithiums, providing double the capacity of LiFePO₄.

58Apart from these attractive performances, this approach is beneficial in that it fits perfectly within the framework of sustainable development: Li dirhodizonate can be prepared from myo-inositol, which is a natural product widely found in plants in the form of an easily exploitable derivative, phytic acid (it represents 8 % of the mass of maize maceration liquid).

59Following this path and taking advantage of the wealth of organic chemistry, we were able to identify several organic molecules with carbonyl, carboxylate or ester functions that can react reversibly with Li, and we were thus able to establish a scale of potentials for these organic composites. Among them, dilithium terephthalate²⁰ (Li₂C₈H₄O₄) – which has an aromatic core and can be simply prepared via an acid-base reaction at room temperature using the corresponding acid (C₈H₆O₄) – reacts reversibly at 0.7 V with 2 Li, thereby providing neighbouring capacities to those of the carbon electrodes currently used.

60Based on the same principles as those described in the case of inorganic composites, we were then able to build the first eco-compatible Li-ion organic battery, by coupling the most oxidising composites with the most reducing ones. This battery shows very good cyclelife performance, irrespective of the temperature; its energy density, however, is still limited.²¹ The fact nevertheless remains that this type of battery presents the ideal life cycle given its low carbon footprint, due to the fact that (1) the electrodes come from biomass, and (2) these materials can be recycled quickly using the sun, an abundant and cheap source.

3.3.3. Li-air technology

61While this is an elegant approach from an ecological point of view, it cannot provide the autonomy sought for electric vehicle applications. Despite the latest advances discussed so far, there is still a fifteen-fold gap between the useful energy provided by gasoline combustion (2,500 Wh/kg, taking Carnot efficiency into account) and that provided by a battery. This brings us to the last question that needs to be raised, regarding the possibility of increasing the energy density of lithium batteries whilst maintaining environmentally-friendly storage. There may be some hope stemming from metal-air systems, especially the lithium-air system which is currently very popular among car manufacturers. These accumulators use a Li metal electrode as a negative electrode and an air electrode as a positive electrode, consisting of a catalyst deposited on highly porous carbon material; the functioning of this electrode is in some respects close to that of the oxygen electrode of fuel cells. When current is supplied (discharge), the oxygen drawn from the outside reduces, and a superoxide ion O₂⁻ forms which, with the Li, makes LiO₂; the latter is unstable and transforms into solid Li₂O₂ which fills the pores of the electrode.²² Upon recharging, the reverse mechanism takes place, the only difference being that it does not involve the ion O₂⁻. Based on theoretical calculations, the Li-air technology could provide energy densities of 3,500 Wh/kg, which represents approximately fifteen times more than that of lithium-ion accumulators. However, in order to make such systems operational, many technological obstacles surrounding energy efficiency and long-term cycling must be overcome. Though it goes without saying that some of the concepts that I have discussed so far will be applicable to Li-air technology, they will not suffice.

Figure 11

62However, through the command of nanostructuring, in collaboration with P. Bruce’s team at the University of St-Andrews, we were able to elaborate electrodes supported by carbon containing dispersed and nanometric α-MnO₂, able to cycle on 5 or 6 cycles with energy densities 5 to 6 times greater than that of lithium-ion positive electrodes.²³ The challenge for this Li-air technology, however, remains its high polarization, and especially the presence of superoxide ions O₂^-, which are highly nucleophile, and which decompose virtually all the electrolytes that have been used until now. One of the directions we explored in order to overcome this technological challenge led us once again to the chemistry of life, since superoxide is the best known radical in our human body and we know how to reduce its activity by using superoxide dismutase or other enzymatic approaches. Such approaches, that reproduce by mimicry the human body’s chemical syntheses, are currently in use.

63The three examples discussed above once more highlight the crucial importance of materials in the search for new electrochemical storage systems. However, despite the opening of new paths, there is still a long way to go. Lithium-ion technology has reached a sufficient degree of sophistication to serve electric vehicle applications, if the variety of vehicle models announced by many manufacturers who will be using Li-ion batteries is anything to go by. This technology can also serve network applications since systems of 2 to 4 MW, built by the US company A123, are now available. However, these technologies still need to be improved, particularly in terms of energy density (currently of the order of 150 Wh/kg) and cost (about €500/kWh), which should respectively be doubled and halved, something that does seem possible. Furthermore, other technologies are already shaping the outline of our future scientific research (Fig. 11). They offer highly attractive environmental features, like organic electrode systems, or real potentialities with the Li-air system that can triple, if not quadruple current energy densities.

64So overall such systems are complex and the time we have to provide an electrochemical storage solution is limited. To tackle these issues efficiently there is a pressing need to gather and federate research, at both national and EU levels. And without being utopic, but lucidly aware of the size of the scientific challenges ahead of us, we can even call for global cooperation. In view of this, we can only celebrate the creation, on 2 July 2010, of the first national research and technology network on electrochemical energy storage by the Higher Education and Research Minister. This network is supported by the CNRS, the Ministry and the CEA, and brings together researchers, engineers and industrial actors. Its aim is to federate French research around common objectives, to ensure technology transfers and make our country a global leader in the environmental protection race.

65Finally, coming from the Jules-Verne University of Picardie, I could not end this presentation without mentioning the one of a kind visionary that was Jules Verne. Over a century ago, he had already anticipated sustainable development and energy problems; here are extracts from The Mysterious Island and Twenty Thousand Leagues under the Sea.

66The first touches on the issues of limited fossil fuel resources:

“However”, resumed Gideon Spilett, “you do not deny that some day the coal will be entirely consumed?”
[...]
“For how long a time?” asked the reporter.
“For at least two hundred and fifty or three hundred years.”
“That is reassuring for us, but a bad look-out for our great grandchildren!” observed Pencroft. ²⁴

67The second is equally relevant to today’s reality as it highlights the need to develop new chemistries, like the use of sodium, to withstand possible lithium shortages which many newspapers, albeit too alarmingly, have constantly been writing about recently. Although I did not have the time in this lecture to expand on this question, note that there is currently intense research being carried out on sodium technologies. Sodium is abundant and cheap and, while its molar mass is three times greater than lithium’s, and though it affords lower potentials of 0.3V and therefore a priori to systems with less mass energy, it is again becoming a highly coveted element for accumulators.

68Jules Verne had therefore identified a near-future technology!

“Sodium?”
“Yes. [...] Only the sodium is consumed, and the sea itself provides me with more. Also, sodium batteries must be regarded as producing the most energy.”²⁵

69As Carlos Dossi used to say, “fools pave the way that wise men then take”. Without my predecessor’s visionary qualities to project you two hundred years into the future, I will conclude by citing this thought by Nathan Lewis on the United States, which can easily be transposed to a global scale:

In the United States, we daily spend overall more money on buying fuel at the pump than we devote annually to research on solar energy, whereas at this very moment the sun is providing us with all the energy our planet uses in a year. Alas, we know neither how to capture it, nor to convert it nor to use it efficiently.

70As for storing energy, we have to acknowledge that all the energy processes of the living are based on redox pairs involving organic molecules (for example, Krebs’ cycle). Why could we not one day see to it that they also serve to fuel our domestic appliances as well as our cars? Does the importance of the challenge we are tackling and of the societal problem of energy storage not call for such risk taking? Pursuing this objective at the interface of several disciplines is vital in order to develop the technology that will allow us to reduce carbon emissions into our atmosphere, while producing enough clean energy for our world to continue to function. Beyond describing problems, progress and future trends, the teaching that will follow this lecture will primarily seek to be educational, so that participants may acquire the necessary knowledge and tools to contribute to this energy adventure on which the future of our planet depends.

71Without all the brilliant and talented researchers with whom I have had the privilege of interacting in my years in the United States, at the Bellcore laboratories and the Bell telephone laboratories, and upon my return to France, at the LRCS or ALISTORE, as well as during brief collaborations, many of the scientific directions described above could not have been exploited. I extend my deepest gratitude to all of them, and dedicate this lecture to them.