The world needs more power, preferably in a form that’s clean and renewable. Our energy-storage strategies are currently shaped by lithium-ion batteries – at the cutting edge of such technology – but what can we look forward to in years to come?
Let’s begin with some battery basics. A battery is a pack of one or more cells, each of which has a positive electrode (the cathode), a negative electrode (the anode), a separator and an electrolyte. Using different chemicals and materials for these affects the properties of the battery – how much energy it can store and output, how much power it can provide or the number of times it can be discharged and recharged (also called cycling capacity).
Battery companies are constantly experimenting to find chemistries that are cheaper, denser, lighter and more powerful. We spoke to Patrick Bernard - Saft Research Director, who explained three new battery technologies with transformative potential.
In lithium-ion (li-ion) batteries, energy storage and release is provided by the movement of lithium ions from the positive to the negative electrode back and forth via the electrolyte. In this technology, the positive electrode acts as the initial lithium source and the negative electrode as the host for lithium. Several chemistries are gathered under the name of li-ion batteries, as the result of decades of selection and optimization close to perfection of positive and negative active materials. Lithiated metal oxides or phosphates are the most common material used as present positive materials. Graphite, but also graphite/silicon or lithiated titanium oxides are used as negative materials.
With actual materials and cell designs, li-ion technology is expected to reach an energy limit in the next coming years. Nevertheless, very recent discoveries of new families of disruptive active materials should unlock present limits. These innovative compounds can store more lithium in positive and negative electrodes and will allow for the first time to combine energy and power. In addition, with these new compounds, the scarcity and criticality of raw materials are also taken into account.
Today, among all the state-of-the-art storage technologies, li-ion battery technology allows the highest level of energy density. Performances such as fast charge or temperature operating window (-50°C up to 125°C) can be fine-tuned by the large choice of cell design and chemistries. Furthermore, li-ion batteries display additional advantages such as very low self-discharge and very long lifetime and cycling performances, typically thousands of charging/discharging cycles.
New generation of advanced li-ion batteries is expected to be deployed before the first generation of solid state batteries. They’ll be ideal for use in applications such as Energy Storage Systems for renewables and transportation (marine, railways, aviation and off road mobility) where high energy, high power and safety is mandatory.
In li-ion batteries, the lithium ions are stored in active materials acting as stable host structures during charge and discharge. In lithium-sulfur (Li-S) batteries, there are no host structures. While discharging, the lithium anode is consumed and sulfur transformed into a variety of chemical compounds; during charging, the reverse process takes place.
A Li-S battery uses very light active materials: sulfur in the positive electrode and metallic lithium as the negative electrode. This is why its theoretical energy density is extraordinarily high: four times greater than that of lithium-ion. That makes it a good fit for the aviation and space industries.
Saft has selected and favoured the most promising Li-S technology based on solid state electrolyte. This technical path brings very high energy density, long life and overcomes the main drawbacks of the liquid based Li-S (limited life, high selfdischarge, …).
Furthermore, this technology is supplementary to solid state lithium-ion thanks to its superior gravimetric energy density (+30% at stake in Wh/kg).
Major technology barriers have already been overcome and the maturity level is progressing very quickly towards full scale prototypes.
For applications requiring long battery life, this technology is expected to reach the market just after solid state lithium-ion.
Solid state batteries represent a paradigm shift in terms of technology. In modern li-ion batteries, ions move from one electrode to another across the liquid electrolyte (also called ionic conductivity). In all-solid state batteries, the liquid electrolyte is replaced by a solid compound which nevertheless allows lithium ions to migrate within it. This concept is far from new, but over the past 10 years – thanks to intensive worldwide research – new families of solid electrolytes have been discovered with very high ionic conductivity, similar to liquid electrolyte, allowing this particular technological barrier to be overcome.
Today, Saft Research & Development efforts focus on 2 main material types: polymers and inorganic compounds, aiming the synergy of the physico-chemical properties such as processability, stability, conductivity …
The first huge advantage is a marked improvement in safety at cell and battery levels: solid electrolytes are non-flammable when heated, unlike their liquid counterparts. Second, it permits the use of innovative, high-voltage high-capacity materials, enabling denser, lighter batteries with better shelf-life as a result of reduced self-discharge. Moreover, at system level, it will bring additional advantages such as simplified mechanics as well as thermal and safety management.
As the batteries can exhibit a high power-to-weight ratio, they may be ideal for use in electric vehicles.
Several kinds of all-solid state batteries are likely to come to market as technological progress continues. The first will be solid state batteries with graphite-based anodes, bringing improved energy performance and safety. In time, lighter solid state battery technologies using a metallic lithium anode should become commercially available.