Inorganic solid electrolytes, in comparison with their liquid counterparts, have more potential in varioust ypes of batteries due to their dual roles of ion transportation and separation. For all-solid-state batteries, solide lectrolytes bring several advantages, [1][2][3][4][5][6] such as enhanced safety,i ncreased energy density,s olid device integration, and packaging;t hese expand the operation temperature range and potentially improve cycling stabilitya nd lifetime. Moreover,i norganic solid electrolytes are also beneficial for lithium-ion batteries and lithium-air batteries, in which they functiona se ither surface modification layers or lithium-ion conductors. [7,8] In principle, ideal solid electrolytes are expected to have several features: [9][10][11][12][13][14] 1) fast ion dynamics and negligible electronic conductivity (minimum ionic conductivity of 10 À4 Scm À1 at room temperature for practical consideration);2 )a wide electrochemical potential window for battery cycling;3 )ane xceptional mechanical strength to suppress lithium dendrite growth;4 )excellent thermal stability during the cycling processes;and 5) asimple and low cost synthetic process for large-scale applications.Generally,i norganic lithium superionic conductors are divided into three categories:o xides, sulfides, and phosphates. Suc-cessfule xamples in oxidesi nclude garnet oxides, [15,16] perovskite-type oxides, [17,18] and antiperovskite oxides. [19][20][21] Sulfide solid electrolytes include Li 2 SÀP 2 S 5 , [22,23] Li 3 PS 4 , [24][25][26] Li 7 P 3 S 11 , [27,28] Li 7 PS 6 ,a nd Li 6 PS 5 X( X = Cl, Br). [29][30][31] Popular phosphate solid electrolytes include sodium superionic conductor (NaSICON)structured lithium-ion conductors, [32][33][34][35][36] such as LiTi 2 (PO 4 ) 3 (LTP), Li 1 + x Al x Ti 2Àx (PO 4 ) 3 (LATP), and Li 1 + x Al x Ge 2Àx (PO 4 ) 3 (LAGP). Many exciting discoveries of these materials have been summarized in important review papers. [2,6,[37][38][39][40] NaSICON-type solid electrolytes, such as LATP and LAGP, have marked advantages compared with sulfides and oxides: they display chemicals tabilityi na ir and/or water,a re low cost and low toxicity,a nd have great electrochemical stability with the added benefito fe asy preparation. [2,36,38] They also exhibit attractive ionic conductivities of 10 À4 -10 À3 Scm À1 at room temperature. Such features have recently caused renewed interest in these NaSICON-structured materials and much effort has been devoted to the investigation of this type of solid electrolyte, especiallyL ATPa nd LAGP.T his manuscript reviewsr ecent progress in LATP and LAGP solid electrolytes, with as pecific focus on synthetic approaches and their effects on the crystal structures, conductive properties, and applications. Herein, general synthetic methods for LATP and LAGP solid electrolytes are first introduced, followed by ac omparison of the crystal structures, phase purities, and ionic conductivities that result from different approaches. Later,t he applications of LATP and LAGP in ful...