By introducing halogens to the A/Ga/Q (A = Na, K; Q = S, Se) system, one new chalcohalide namely Na2GaS2Cl was successfully obtained. It crystallizes in the orthorhombic space group Cmcm (63). Na2GaS2Cl has a layered structure consisting of two dimensional [GaS2]∞ layers which are stacked in “face to face” and “back to back” arrays and separated by Na+ and Cl− ions. Interestingly, supertetrahedral building units [Ga4S10] (T2) which are rarely found in metal chalcogenides and metal chalcohalides are formed in this structure. Moreover, the distances of two adjacent layers are around four times larger than the ionic radius of the Na+ ion, which is very likely to provide a perfect environment for the storage and migration of Na+ ions. In particular, the volume concentration of the Na+ cations in this compound is as high as 1.54 × 1022 cm−3. The UV-vis-NIR spectroscopy measurement reveals that the