Battery technologies represent a significant focus for innovators seeking to develop new ways to help us decarbonise our economies and energy systems. If electric vehicles are to take over from their internal combustion engine predecessors, then secondary (rechargeable) batteries must be able to replicate the convenience and range afforded by a tank of petrol or diesel. Similarly, secondary batteries and grid scale energy storage have a part to play in enabling fossil fuel-burning power stations to be replaced by renewables, which may generate electricity intermittently and cannot simply be turned on and off to suit demand. As 21 September is Zero Emission Day, we take this opportunity to highlight some of the areas from which future improvements in battery performance might come.
The lithium-ion battery is a stalwart of present-day consumer electronics and electric vehicles. A lithium-ion battery comprises two electrodes (an anode and a cathode). The electrodes are each made of materials which are capable of reversibly storing lithium between layers in the structures of the materials - typically a lithium composite oxide such as LiCoO2 or LiMn2O4 at the cathode, and graphite at the anode. The battery also contains an electrolyte through which lithium ions can travel, and a separator which prevents electrons from moving between the electrodes inside the battery.
When a lithium-ion battery is used, lithium is oxidised at the anode. That generates electrons which move through a circuit external to the battery, producing the desired current. The oxidised lithium ions, meanwhile, move inside the battery, through the electrolyte, to the cathode. At the cathode they recombine with electrons from the external circuit and are reduced. Eventually the anode will run out of lithium to be oxidised, and the battery needs to be charged before it can be used again. The charging process is the same in reverse: the charger will apply current to the battery so that lithium is oxidised at the cathode. The ions produced at the cathode then move to through the battery to replenish stores of lithium at the anode, so that the battery can be used again.
Drawbacks of lithium battery technology include the possibility for lithium metal to form branch-like "dendrite" structures within the battery, which can grow in the electrolyte to the extent that they damage the separator and cause a short circuit. The most common form of electrolyte in current lithium-ion batteries also suffers from being highly flammable. New electrolyte materials are being developed to solve these problems, with polymer electrolytes being more resistant to dendrite formation, and solid state electrolytes addressing the issue of the flammability of the liquid electrolyte.
Solid state lithium batteries are particularly interesting because of the possibility of using a solid lithium metal anode, rather than materials which merely store intercalated lithium between lattice layers. A solid state electrolyte combined with a lithium anode could provide a dramatic improvement in energy density and fire resistance, as well as lighter weight and faster charge times. However it remains a challenge to prevent dendrite formation at the lithium anode.
As well as making structural changes to improve lithium-ion batteries, promising new battery technologies include those with different chemistries altogether.
Lithium-sulfur batteries have a similar chemistry at the anode as lithium-ion batteries (i.e. the oxidation of lithium to generate lithium ions and electrons during discharge). However, at the cathode the lithium ions are not themselves reduced by incoming electrons from the external circuit. Rather, sulfur at the cathode is reduced, which reacts with the lithium ions to form lithium sulfides. Lithium sulfur batteries are attractive because they have a high theoretical energy density, and because sulfur is abundant, light weight and environmentally friendly.
Lithium-air batteries again use oxidation of lithium at the anode, but at the cathode oxygen is reduced and lithium peroxide is formed. Lithium-air batteries are capable of storing an impressive amount of energy per kilo of lithium but the technology is still at a relatively early stage, and considerable advances are needed before the efficiency and life-cycle characteristics of lithium-air batteries will satisfy the requirements for electric vehicles and grid-scale storage.
All the battery types above rely on the oxidation of lithium at the anode. Other non-lithium chemistries include sodium-ion, potassium-ion and aluminium-ion batteries. Like lithium, sodium and potassium are members of Group I of the periodic table, and the three elements share many properties (including the ability to form ions with a single positive charge). Sodium-ion and potassium-ion batteries are therefore analogous to existing lithium-ion batteries, but in which the lithium is replaced by sodium or potassium. Their key advantage is the greater abundance and lower cost of sodium and potassium as compared to lithium. In addition, sodium-ion batteries may be able to avoid the use of metals such as cobalt, copper and nickel at the cathode, and potassium ion batteries may be capable of faster charging than their lithium-based counterparts.
Aluminium, on the other hand, is a member of Group III of the periodic table. Oxidisation of aluminium at the anode of an aluminium-ion battery therefore generates three electrons for every aluminium ion, which allows for greater energy storage capacity. However, current aluminium-ion batteries suffer from poor electrochemical performance, which will need to be improved before this alternative chemistry is a viable alternative to current lithium-ion technologies.
In summary, there are clearly a multitude of promising avenues being explored to improve on current battery technologies and the next generation of batteries will be an important part of achieving net zero.
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