UC Berkeley

As the use of lithium-ion batteries in consumer electronics and electric vehicles has grown, safety issues such as those arising from leakage and flammability of the organic liquid electrolyte have garnered increased attention. By replacing the organic liquid electrolyte with an inorganic solid electrolyte, however, these concerns can be circumvented, thereby improving the safety of the battery system. In addition, solid-state batteries are also expected to possess higher energy density than their conventional counterparts. On the anode side, the high modulus of the solid electrolyte is expected to constrain the growth of Li dendrites, which might enable the use of a Li metal anode. Furthermore, because some solid electrolytes possess a wide electrochemical stability window, high-voltage cathodes (> 4.5 V, vs Li+/Li) may be used to further improve the total energy density.

The superionic conductor is one of the key parts of solid-state batteries. Over the past 20 years, accelerated development of Li superionic conductors has occurred. The ionic conductivities of some of these superionic conductors approach or even surpass those of liquid electrolytes. However, most reported superionic conductors have obvious drawbacks. New superionic conductors that meet all the requirements of solid-state batteries are needed. There are two strategies for exploring new potential superionic conductors: (1) modification the chemical composition based on the crystal structure of known fast Li conductors to further improve the properties or (2) searching for Li conductors with new crystal structures based on the structural features that favor fast Li+ migration. In this dissertation, new superionic conductors are designed and explored by implementing both of these strategies.

Specifically, a strategy is developed to increase the ionic conductivity of sulfide Li-ion conductors through composition modification. Inspired by the wide use of halogens in superionic conductors, we propose that the conductivity could be further improved by substituting halogens with suitable pseudo-halogens. The Li argyrodite system was used to demonstrate the feasibility of this strategy. BH4-substituted Li argyrodite was successfully synthesized and shown to have a room-temperature ionic conductivity of 4.8 mS/cm which is 5 times higher than that of halogen-substituted Li argyrodites. We further discuss the mechanism underlying the enhanced ionic conductivity and find that the faster Li diffusion originates from the weak interaction between Li and BH4. The results provide design strategies for new superionic conductors with pseudo-halogen substitution.

We also present a structural feature that benefits the Li-ion migration in oxide Li-ion conductors. Based on a statistical analysis of the materials in the inorganic materials database, we discovered that the corner-sharing connectivity of the oxide crystal structure framework is more likely to have a distorted lithium environment with higher site energy. Materials with a corner-sharing framework are also usually less compact, which reduces the repulsion from non-lithium cations. Both features lead to a decreased migration barrier and accelerate the Li diffusion. A high-throughput search was performed based on this structural feature, and 10 new oxide Li-ion conductors were predicted. One of them, LiGa(SeO3)2, was successfully synthesized and was shown to have a bulk conductivity of 0.11 mS/cm, in agreement with theory predictions. These findings provide fundamental insights into the physical attributes that govern fast lithium conduction and help project new directions towards the discovery of superionic conductors for all-solid-state batteries.