Polymorphism in Li₄Zn(PO₄)₂ and Stabilization of its Structural Disorder to Improve Ionic Conductivity

Abstract

Realization of the vulnerability of current rechargeable battery systems drives the research of solid electrolytes. In the search for a new Li ion conductor, we explore the rich crystal chemistry of Li 4 Zn(PO 4) 2 which presents a low temperature monoclinic (α-) and a high temperature orthorhombic (β-) polymorph. We solved the crystal structure of the β-phase and found that it has a disordered Li/Znsublattice while showing the largest conductivity; however it could not be stabilized at room temperature by quenching. We discovered that the partial substitution of Zn 2+ with Ga 3+ in Li 4-x Zn 1x Ga x (PO 4) 2 first leads to an intermediate β' phase. Increasing the Ga content to 0.5 mol pfu. enables to stabilize the pure β-phase at room temperature, which exhibits a conductivity by several orders of magnitudes higher than the pristine sample. The crystal structures of the new β'/β-phases have been solved to elucidate the conduction mechanism, which confirms the high sensitivity of ionic conductivity on disorder.

Structural Polymorphism in Na4Zn(PO4)2 Driven by Rotational Order–Disorder Transitions and the Impact of Heterovalent Substitutions on Na-Ion Conductivity

“…First, we tried ball-milling the α-Na 4 Zn(PO 4 ) 2 sample, as ball-milling can increase disorder by repeated particle fracturing and welding. 26 Indeed, the γ-polymorph could be obtained after ball-milling for 2 h. The SXRD pattern of the obtained powder shows significant broadening of the peaks, which could be nicely fitted (Figure S4) with the structural model of the γ-Na 4 Zn(PO 4 ) 2 polymorph deduced earlier and with a peak broadening consistent with crystallite size of 21.7(2) nm and a microstrain of 49.7(4)%.…”

“…Overall, the crystal structures of the Na 4 Zn-(PO 4 ) 2 polymorphs are quite different from those of Na 3 PO 4 and from the one adopted by its Li-counterpart Li 4 Zn-(PO 4 ) 2 . 26,36 Stabilization of the High-Temperature Polymorphs. Since γ-Na 4 Zn(PO 4 ) 2 presents structural features (PO 4 rotational disorder and cationic vacancies) that could enable a good Na-ion conductivity, we tried to stabilize this polymorph at RT by deliberately creating disorder in the system.…”

“…A classical way to promote ionic conduction in a crystalline framework is to create vacancies or interstitials. ,− This was extensively adopted for Li-ion conductors, either by chemical substitutions or by stabilizing high-temperature disordered phases (HTP) at room temperature (RT) via quenching. For example, the high-temperature cubic phase of garnet-type Li 7 La 3 Zr 2 O 12 (LLZO) could be stabilized at RT by aliovalent substitution (with Ga, Al, and Ta, etc.…”

High-throughput computational screening for solid-state Li-ion conductors

“…We miss the prototypical garnet structure Li 7 La 3 Zr 2 O 12 because of failures (drifts of the constant of motion) during the pinball dynamics. We find also many NASICONs such as Li 3 Sc 2 P 3 O 12[119], Li 3 In 2 P 3 O 12[27], LiZr 2 P 3 O 12[26], LiTi 2 P 3 O 12[120], and Li 4 ZnP 2 O 8[121]. We find in the screening both oxide and sulphide argyrodytes, namely Li 6 PS 5 I[122], Li 6 PClO 5 and Li 6 PBrO 5…”

“…LiZr 2 P 3 O 12 [26], LiTi 2 P 3 O 12 [120], and Li 4 ZnP 2 O 8 [121]. We find in the screening both oxide and sulphide argyrodytes, namely Li 6 PS 5 I [122], Li 6 PClO 5 and Li 6 PBrO 5 [23], and Li 5 PS 4 Cl 2 [66].…”