Recent developments on aqueous sodium-ion batteries

For the extensive utilisation of renewable energy in future, we urgently require new generation of electric energy storage technologies that are low cost, environmentally friendly and sustainable in terms of the electrode and electrolyte materials. Therefore, sodium-ion batteries come back into our horizon. Aqueous sodium-ion batteries exhibit better safety and low cost, providing an excellent prospect for widespread use. Nevertheless, there are still lots of difficulties in developing sodium-storage electrodes and aqueous sodium-ion technology. This review will mainly summarise recent developments of aqueous sodium-ion battery electrodes and the relating electrochemical energy storage reactions. The advantages/disadvantages of typical electrodes as well as future directions in aqueous sodium-ion storage will also be discussed at the end.