Design and Characterisation of Solid Electrolytes for All-Solid-State Lithium Batteries

The development of all-solid-state lithium batteries, in which the currently used liquid electrolytes are substituted for solid electrolyte materials, could lead to safer batteries offering higher energy densities and longer cycle lifetimes. Designing suitable solid electrolytes with sufficient chemical and electrochemical stability, high lithium ion conduction and negligible electronic conduction remains a challenge. The highly lithium ion conducting LiBH4-LiI solid solution is a promising solid electrolyte material. Solid solutions with a LiI content of 6.25%-50% were synthesised by planetary ball milling and annealed at 140 °C. Their crystal structure was investigated using powder x-ray diffraction and their ionic conductivity was measured using impedance spectroscopy. The ionic conductivity is found to exceed 0.1 mS/cm at 30 °C and 10 mS/cm at 140 °C. The formation of defect-rich microstructures during ball milling is found to significantly influence the conductivity of the samples. The long-range diffusion of lithium ions was measured using quasi-elastic neutron scattering. The solid solutions are found to exhibit two-dimensional conduction in the hexagonal plane of the crystal structure, with the formation of Frenkel pairs playing a large role. The charge and discharge performance of all-solid-state batteries with LiBH4- LiI as an electrolyte is reported for the first time. Lithium titanate (Li₄Ti₅O₁₂) was used for the positive electrode and lithium metal for the negative electrode. The electrochemical stability of LiBH₄-LiI is found to be limited to 3 V. The all-solid-state cells reach 81% of their theoretical discharge capacity at 60 °C and a discharge rate of 10 μA, but a capacity fade of 1.6% per charge-discharge cycle and a large overvoltage are observed. Impedance spectroscopy results show a strong correlation between changes in the discharge capacity of the cells and changes in the cell resistance over 200 cycles. This may be due to a possible formation of a passivating areas in the cell as well as contact issues between the electrode-electrolyte interfaces. The crystal structure and ionic conductivity of the LiBH₄-Ca(BH₄)₂ composite were also studied. No formation of a solid solution is observed and the ionic conductivity is lower than that of pure, ball milled LiBH4. Heat treatment of the samples leads to the formation of a small amount of defect-rich, electronically conducting CaH2 with a cubic crystal structure. Its formation has an effect on the measured conductivity of the samples and increases the risk of an internal short-circuit. This reveals a more general issue that must receive attention in further research on solid electrolytes.