Paper-based aqueous Al ion battery with water-in-salt electrolyte

Low-cost, flexible and safe battery technology is the key to the widespread usage of wearable electronics, among which the aqueous Al ion battery with water-in-salt electrolyte is a promising candidate. In this work, a flexible aqueous Al ion battery is developed using cellulose paper as substrate. The water-in-salt electrolyte is stored inside the paper, while the electrodes are either printed or attached on the paper surface, leading to a lightweight and thin-film battery prototype. Currently, this battery can tolerate a charge and discharge rate as high as 4 A g−1 without losing its storage capacity. The charge voltage is around 2.2 V, while the discharge plateau of 1.6–1.8 V is among the highest in reported aqueous Al ion batteries, together with a high discharge specific capacity of ∼140 mA h g−1. However, due to the water electrolysis side reaction, the faradaic efficiency can only reach 85% with a cycle life of 250 due to the dry out of electrolyte. Benefited from using flexible materials and aqueous electrolyte, this paper-based Al ion battery can tolerate various deformations such as bending, rolling and even puncturing without losing its performance. When two single cells are connected in series, the battery pack can provide a charge voltage of 4.3 V and a discharge plateau as high as 3–3.6 V, which are very close to commercial Li ion batteries. Such a cheap, flexible and safe battery technology may be widely applied in low-cost and large-quantity applications, such as RFID tags, smart packages and wearable biosensors in the future.