Low cost and low density chloride solid electrolyte for all solid state cathode with high active material ratio

Chloride solid electrolytes (SEs) have attracted widespread attention due to their high room-temperature ionic conductivity and excellent cathode compatibility. However, the conventionally selected central metal elements (e.g., In, Y and Ta) are usually rare and heavy, inevitably causing the high cost and high density of the obtained chloride SEs. Here, by choosing abundant and light Mg and Al as central metal elements, we develop a cheap and low density Li_1.2Mg_0.95Al_0.3Cl₄ SE for high active material ratio in all solid state cathode. Partial replacement of Mg²⁺ by Al³⁺ in the framework yields vacancies and lowers the non-lithium metal ions occupancy at Mg/Li co-occupied 16d site, effectively relieving the blocking effects by Mg²⁺ in the pristine spinel Li_2–2xMg_1+xCl₄. Thus, a significantly improved room-temperature conductivity of 3.08 × 10^–4 S·cm^–1 is achieved, two orders of magnitude higher than that of Li_1.2Mg_1.4Cl₄. More attractively, its low density of only 1.98 g·cm^–3 enables low SE mass ratio in cathodes (only 16 wt.%) with still effective electrolyte/cathode contact and lithium-ion conduction inside. When charged to potential of 4.30 V, the as-fabricated Li_1.2Mg_0.95Al_0.3Cl₄-based solid lithium battery with uncoated NCM523 cathode can be cycled for over 100 cycles with a capacity retention of 86.68% at room temperature.