In our effect to develop a lithium secondary battery with high energy density, aluminum sulfide (Al2S3) was studied for use as an active material. The measured initial discharge capacity of Al2S3 was ca. 1170 mAh g(-1) at 100 mA g(-1). This corresponds to 62% of the theoretical capacity for the sulfide. Al2S3 exhibited poor capacity retention in the potential range between 0.01 V and 2.0 V. mainly due to the structural irreversibility of the charge process or Li extraction. XRD and Al and S K-XANES analyses indicated that the surface of Al2S3 reacts reversibly during charge and discharge processes, while the core of Al2S3 showed structural irreversibility because LiAl and Li2S were formed from Al2S3 at the initial discharge and remained as they were afterward. (c) 2010 Elsevier B.V. All rights reserved.