Ionic Transport and Electrochemical Properties of NaSICON-Type Li_{1 + x}Hf_2-xGa_x(PO₄)₃for All-Solid-State Lithium Batteries

NaSICON-Type rhombohedral LiHf₂(PO₄)₃(LHP) is regarded as a quite promising solid electrolyte for future all-solid-state Li-ion batteries. Appropriate aliovalent substitution is, however, necessary to achieve high ionic conductivities. A clear-cut understanding of the substitution effects on microscopic Li⁺ion dynamics is necessary to optimize its conduction properties. To advance in the field, we prepared a series of Ga-bearing Li_1+xHf_2-xGa_x(PO₄)₃(Ga-LHP) samples (x = 0, 0.1,.. 0.4, 1.0) to comprehensively investigate the relationship between composition and Li⁺ion dynamics. ⁷Li and ³¹P NMR helped us characterize the extent of structural disorder introduced through the replacement of Hf by Ga. In a complementary way, we compare our results from broadband conductivity spectroscopy with those obtained from time-domain NMR measurements being sensitive to long-range ion transport and to the elementary Li⁺jump processes. This methodical approach allowed us to trace ion dynamics over a wide length scale. It turned out that the sample with a Ga content x of only 0.1 (89.3% relative density) showed the highest bulk conductivity of 0.45 mS cm^-1(0.24 eV). Importantly, activation energies as deduced from spin-lattice relaxation NMR point to activation energies ranging from 0.15 to 0.23 eV, revealing a rather flat potential landscape to which the ions are subjected in the different forms of Ga-LHP. To test its electrochemical applicability in all-solid-state batteries, we used cyclic voltammetry, Li plating-stripping experiments, and galvanostatic cycling measurements with current densities of up to 0.1 mA cm^-2. An electrochemical stability window of 2.4 to 4.6 V, a critical current density of at least 25 mA cm^-2, and a long cycle life of more than 1900 charge/discharge cycles (60 °C) make Li_1.1Hf_1.9Ga_0.1(PO₄)₃, with a slight amount of Ga incorporated, indeed a highly promising alternative to current solid electrolytes.