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Rechargeable batteries: a key tool in the fight against global warming

Global views and trends


Reporting on promising developments in storage, delivery capability and cyclability

At a meeting in Paris on 12 December 2015, 195 countries adopted a climate agreement with the long-term goal of keeping global warming to a level “well below 2°C”. To achieve this, the countries undertook to “rapidly reduce their greenhouse gas emissions”. This agreement implies a complete rethinking of global energy production, bearing in mind that it is still largely dependent on fossil fuels, including coal and oil.

Fossil fuel consumption still not falling as a climate chain reaction looms

However, the latest Statistical Review of World Energy published by oil company BP shows that since 2015, total consumption (in tonnes oil equivalent) of coal, oil and natural gas rose by 0.97% in 2016 and by 1.02% in 2017 (-0.98%, 1.82% and 2.88% in 2016 and 0.69%, 1.42% and 2.69% in 2017 respectively). This runs contrary to what could have been legitimately expected. This regrettable observation raises the thorny question of how the Paris Agreement objectives are to be met.

The issue is all the more pressing in that recent studies have shown that climate destabilisation could be subject to an irreversible chain reaction process; in this scenario, a rise in temperatures above a certain threshold, as yet not precisely defined (perhaps in the region of 2°C), would lead to the accelerated melting of the permafrost in glacial regions (including Siberia); the greenhouse gases it contains (CO2 and methane) would be released into the atmosphere, further intensifying global warming through a process as yet not properly understood; this process would result in non-linear change, in fits and starts. It is therefore becoming vital to avoid such tipping points, triggering such fits and starts shocks.

The answer: not just renewable energy generation, but also storage

In this worrying context, the development of renewable energy (solar and wind) is essential. However, these forms of energy are irregular and unstable. One of the challenges that scientific research must quickly resolve is how to store the electricity they produce when conditions are favourable more efficiently and more economically for use when conditions are less favourable.

Hoped-for innovations in this field could be accompanied by the accelerated deployment of installations to produce such forms of renewable energy. The scale of the undertaking is huge! As a study by the Carnegie Institute has shown, for example, the installation of an offshore wind farm in the North Atlantic over an area the size of India could provide humanity with all the energy it consumes.

Various ways of storing energy already exist. Surplus electricity can, for example, be used to move water from a low reservoir to a high reservoir or to electrolyse water with hydrogen and oxygen, which can then be burned or used in a fuel cell, or to compress gases (such as nitrogen) to their liquefaction point. One major area of development will, however, be that of rechargeable batteries.

So what about rechargeable batteries?

A few points to bear in mind: a battery is composed of a cathode to reduce the metals and an anode to oxidise the metals. These electrodes are impregnated with a conducting electrolyte to enable the flow of ions. Batteries are rated in terms of their electric tension, expressed in volts (this is the difference in electric potential between the two electrodes) and by their capacity, expressed in coulombs or ampere hours (Ah). The specific energy, expressed in mAh/g, is the product of the voltage and the capacity and represents the amount of energy the battery can deliver per unit of mass (or volume), from a completely charged state to a completed discharged state. A battery’s lifespan is estimated by its ‘cyclability’, i.e. the number of charge/discharge cycles it can withstand. The nature of the chemical components and materials used determine the level of these variables.

Lithium-ion: state-of-the-art, but not without issues

Currently, lithium-Ion (Li-Ion) batteries are the most cutting-edge type; compared to other types of batteries currently available, they have one of the best specific energies and one of the lowest rates of discharge when not in use; research is ongoing to enhance their performance in terms of both specific energy and cyclability. The most sophisticated types, used in electric cars, could recharge in six minutes, providing 320 km of autonomy.

However, elements such as cobalt (one of the components used as a support matrix in the batteries) and lithium are costly and recycling Li-Ion batteries poses a number of problems, not only technical, but also ecological, due to the toxicity of the metals they contain. This is why other avenues are being explored and should offer more economic, denser, lighter and more powerful electrochemical systems in the years to come.

A look at the future

Three new battery technologies are particularly worth mentioning at this time.

A. sodium-Ion batteries

  • Sodium-Ion (Na-Ion) batteries work in a similar way to Li-Ion batteries, with the main difference residing in the fact that instead of lithium, sodium is intercalated in the electrode materials.
  • The main advantage of a Sodium-Ion battery lies in its overall manufacturing cost, about 30% lower than that of a Li-Ion battery, as sodium is 1,000 times more abundant and therefore less costly than lithium. This technology will not, however, be able to compete with Li-Ion batteries in terms of specific energy (either by weight or volume) and will therefore not be suitable to run electric cars. It may, on the other hand, be used to advantage in stationary applications where the specific energy level is not the determining criterion. It is particularly well-suited for storing surplus electricity generated by renewable energy sources such as solar or wind power.
  • However, Sodium-Ion technologies currently require high temperatures of around 300°C to function, which results in significant loss of performance. The key is to develop Sodium-Ion batteries that can function at room temperature. Research in this field is ongoing.
  • Thanks to the low cost and abundance of its components, Sodium-Ion batteries could enter into mass production by 2022 despite their shortcomings.

B. lithium-sulphur batteries

  • In a Li-Ion battery, the lithium ions are intercalated in the host structures of active materials during charging and discharging. No such host structure exists in a Lithium-Sulphur (Li-S) battery. During discharging, the lithium of the anode is consumed and the sulphur is transformed into various sulphated and lithiated materials. The process is reversed during charging.
  • Li-S batteries contain very light active materials: sulphur for the positive electrode and metallic lithium for the negative electrode. This is why their specific energy is extremely high: four times higher than that of a Li-Ion battery. They are therefore perfectly suited for use in the aeronautical and aerospace industries.
  • Li-S technology still requires research and development to improve its cyclability, which is currently too low, and specific energy. It is unlikely to be ready for widespread use before 2025.

C. ‘all-solid-state’ batteries

  • ‘All-solid-state’ batteries correspond to a veritable technological paradigm shift. In current Li-Ion batteries, ions flow from one electrode to another through the liquid electrolyte; in a ‘solid-state’ battery, the liquid electrolyte is replaced by a solid inorganic composite to diffuse the lithium ions. This concept is far from new, but over the past 10 years, new families of solid electrolytes with high ionic conductivity, similar to that offered by liquid electrolytes, have been discovered, making it possible to overcome a significant technological hurdle.
  • One of the advantages of this new type of battery is increased safety of use as inorganic solid electrolytes are non-flammable, unlike their liquid equivalents. Moreover, the innovative active materials they are made of are high capacity and/or high voltage, making it possible to improve their specific energy, weight and lifespan. They offer a high power-to-weight ratio and could ideally be used in electric vehicles.
  • They should gradually begin to arrive on the market from 2025 onwards, once critical fine-tuning has taken place.

Environmental fund specialists at BNP Paribas Asset Management are closely monitoring the various developments in this niche sector to be in a position to invest in those companies with the best future prospects.

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