In solid-state lithium batteries, the organic liquid electrolyte is replaced by a solid ion conductor. The non-flammable, solid nature, as well as the impenetrability of these electrolytes, can prevent common safety issues of Li-ion batteries such as gas production and fires during a short circuit. The different chemistries of such materials may also enable alternative electrode materials such as Li-metal to be used, which can lead to higher energy density batteries.
It is the role of the electrolyte to transport ions between the electrodes, while blocking the electrons. Therefore, the electrolyte needs high ionic conductivity and low electronic conductivity, as well as good chemical and electrochemical stability with the electrode materials.
In a recent study, published in Advanced Energy Materials, researchers have characterised a series of solid electrolytes that could be used in batteries, to understand how their atomic structure influences their conductivity: the ability of lithium ions to move within the material.
The research team looked at a series of materials Li3-3xM1+xCl6 having x values between −0.14 and 0.5, and the different metals, M = Tb, Dy, Ho, Y, Er, Tm. Using neutron diffraction on the Polaris instrument at ISIS and X-ray diffraction they investigated the structures of these materials and determined how they changed when both x and the metal ion are varied. Alongside this structural characterisation, they also measured the lithium conductivity of the materials.
They found that, depending on the value of x, the materials formed either an orthorhombic or trigonal structure, with the orthorhombic forms exhibiting lithium conductivity up to four times higher. They were able to understand why this was the case by using computer simulations, which illustrated the ease of movement of lithium ions parallel to the c-axis of the structure.
Focussing then on the material Li2.73Ho1.09Cl6, they were able to manufacture a solid-state battery cell using NMC811 and indium as cathode and anode materials respectively. This battery cell showed excellent chemical performance at both room temperature, and -10°C.
This detailed insight into the relationship between ionic conductivity, chemical composition, and structure provides an opportunity for the design of new halide-based solid electrolytes. This work will help researchers take one more step on the road towards highly conductive, stable and processable solid electrolytes for solid-state batteries.
The full paper can be found online at DOI: 10.1002/aenm.202103921