Development of high-performance aluminium-ion batteries with low-cost aqueous electrolyte

Abstract

Ion batteries are one of the most promising power sources, among which aluminium-ion batteries show advantages of high theoretical capacity and low cost. An ultrafast rechargeable Al-ion battery was demonstrated with an ionic liquid electrolyte in 2015, showing a high discharge voltage plateau (2.0V) and a good discharge capacity (70 mAh g-1) after 7500 cycles. Since then, Al-ion batteries have been a promising candidate for commercial applications. However, recently reported Al-ion batteries suffer from relatively low voltage, low capacity and expensive electrolytes, leading to less competitiveness compared with the well-developed Li-ion batteries. To lower down the cost, aqueous aluminium-ion batteries (AAIBs) have been proposed recently. However, not many aqueous full batteries have been developed successfully due to the low standard reduction potential of Al3+ in aqueous solutions. Although Al3+ deposition has been reported with the use of expensive Al(OTf)3 electrolyte, apart from the high cost of the electrolyte, the full battery shows a low discharge plateau voltage of 1.2V due to the lack of AlCl4-. Thus, it is highly demanded to develop high-performance AAIBs with low-cost electrolytes. In this thesis, research works on AAIB development have been done to improve battery performance. Moreover, hybrid battery reactions were introduced, which can enhance the battery voltage and capacity, and thus proved to be a practical strategy for the commercial application of AAIB. • A low-cost “water-in-salt” aqueous electrolyte was firstly developed, which expanded the electrochemical stability window. A full Al-ion battery was proposed to further study its ion (de-)intercalation mechanism, which shows a high discharge voltage plateau of 1.85V and a capacity of 165 mAh g-1. This study provides a novel AAIB system, which can be used as a benchmark in subsequent studies. • To solve the low-capacity restrictions on AAIBs, a high-capacity single-wall carbon nanotube cathode was developed as the cathode, which provides a high specific capacity of 600 mAh g-1. Moreover, the complicated multi-ion intercalation mechanism in the battery was studied, where AlCl4-, Al3+ and Cl- enable its high performance, partially revealing the AAIB battery mechanism behind. • To promote the energy density of AAIBs, an AlxMnO2 nanosphere cathode based MnO2/Al battery was developed. Benefiting from the unique α-MnO2 coated Mn2AlO4 structure, a high discharge voltage plateau (1.9V) together with a high Al3+ storage capacity was achieved. This AlxMnO2 cathode would pave the way for cathode design for hybrid batteries. • To solve the relatively low voltage issue, a Na-Al hybrid battery was developed by the addition of Na ion. Thus, a high discharge voltage plateau (2.3V) was obtained due to the Na ion (de-)intercalation on both anode and cathode. The novel multi-ion (de-)intercalation mechanism, especially the Na+ insertion on the NaxAl(OH)3/Al anode, was studied, which offers a full-cell solution for high-voltage hybrid batteries. The above-mentioned research works contributed to the development of AAIBs and provided a preliminary outlook on their practical applications. In the end, future research perspectives on AAIBs were suggested: Building a multi-physics model for the water-in-salt electrolyte based AAIBs and optimizing both cathode and anode materials.