Lithium ion batteries (LIBs) are the dominant power sources for portable electronics and electric vehicles. However, the relatively low theoretical capacity of graphite anode (372 mAh g-1) hinders the enhancement of energy density of LIBs. Therefore, exploiting the anode materials with high capacity is drawing increasing attention.  

Among various anode materials, Al (aluminum) is a promising candidate due to its excellent conductivity, high theoretical capacity, low discharge potential, natural abundance, and especially low cost. However, Al-based anodes are usually investigated in half cells or full cells with low cathode areal density (<2 mg cm-2), which are far from practical requirements. 

A research team led by Prof. TANG Yongbing at the Shenzhen Institutes of Advanced Technology (SIAT) of the Chinese Academy of Sciences, reported a research paper named " Uniform Distribution of Alloying/Dealloying Stress for High Structural Stability of Al Anode in High Areal Density Lithium Ion Battery " on Advanced Materials, which improved the cycling performance of Al-based batteries with a high areal density cathode. 

In precedent achievements, the team developed a new high-efficiency and low-cost lithium-ion battery configuration, which used an integrated design of aluminum foil to replace the graphite anode and the Cu current collector of conventional LIBs, thus conventional anode materials were omitted from this design. Owing to the elimination of conventional anode materials, the dead weight and dead volume can be greatly reduced, thus the energy densities of this battery are further improved. Nevertheless, this integrated anode also confront with the problem of cycling stability when assembled with a high areal density cathode. 

Recently, the team found that the cracking and pulverization of the Al anode could be attributed to the uneven charge/discharge reaction along the boundaries of pristine Al, which led to the stress concentration and ultimate failure of Al anode. Therefore, it is possible to extend the lifetime of Al anode via uniform distribution of the alloying/de-alloying stress.  

In this reported work, Prof. TANG and his cooperators promoted an inactive (Cu) and active (Al) co-deposition strategy to homogeneously distribute the alloying sites and disperse the stress of volume expansion, which is beneficial to obtain the structural stability of the Al anode (namely Cu-Al@Al). Owing to the homogeneous reaction and uniform distribution of stress during charge/discharge process, it is demonstrated that the full battery of Cu-Al@Al assembled with a high LiFePO4 cathode areal density of 7.4 mg cm-2 achieved a capacity retention of ~88 % over 200 cycles, which is the best performance of Al anodes in full batteries with such a high areal density of cathode.  

This work suggested that this inactive/active design provides a viable way to solve the problem of Al anodes and offer possibility for Al anodes towards practical applications. 

Fig. (a) Fabrication process for the inactive (Cu)/active (Al) layer by simultaneous deposition. (b) The 3D structure of Cu-Al@Al electrode. (c) Cross-section SEM images of the Cu-Al@Al and EDS element mapping. (d) Low and high resolution TEM of Cu-Al nanocomposite layer. (Image by Prof. TANG)