Ionic Transport and Electrochemical Properties of NaSICON-Type Li1 + xHf2 – xGax(PO4)3 for All-Solid-State Lithium Batteries

Abstract

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 Li1+xHf2–xGax(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 Li1.1Hf1.9Ga0.1(PO4)3, with a slight amount of Ga incorporated, indeed a highly promising alternative to current solid electrolytes.