Solid-state batteries have the potential for higher energy and power, as well as better safety, than conventional lithium-ion batteries; solid-state electrolytes (SSEs) lie at the heart of them. A common issue with the use of SSEs is the poor contact with the electrodes and consequently high ionic resistance within batteries; the resistance becomes more severe in thicker electrodes. This restricts the use of thick electrodes (typically hundreds of micrometres) in applications. On the other hand, the limited amount of active materials loaded in thin positive electrodes reduces the battery energy and power output. Now, Ryoji Kanno and colleagues in Japan report a series of SSEs with the composition of Li9.54(Si1−δMδ)1.74P1.44S11.1Br0.3O0.6 (M = Ge, Sn; 0 ≤ δ ≦ 1) that could enable the use of heavily loaded positive electrodes with a thickness of up to 1 mm.
Starting from a structural analysis of known SSEs such as Li10GeP2S12, Kanno and team deduced two parameters for the design of their electrolyte complex: one based on the configuration entropy of mixing anions and cations, and another based on the ratio of the volume of anions versus that of cations. The aim was to achieve high entropy — for example by partially substituting S in Li10GeP2S12 with other anions — to realize high Li-ion conduction, while maintaining the structural stability. Kanno and team identified a composition, Li9.54(Si0.6Ge0.4)1.74P1.44S11.1Br0.3O0.6, that exhibits an exceptionally high conductivity of 32 mS cm–1 at room temperature. The researchers further demonstrated the use of this SSE composition, paired with a high-energy positive electrode with an ultrahigh loading of 245 mg cm–2, in a full cell. The cell possessed a discharge capacity of 22.7 mAh cm–2 (corresponding to a utilization efficiency of 97% for the active material of the positive electrode), displaying the potential for applications.
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