Recently, halide materials have emerged as potential inorganic electrolytes for all-solid-state lithium batteries (ASSLB) thanks to their moderate-high ionic conductivity at room temperature and their high-voltage wide electrochemical stability compared to other solid-state electrolytes such as polymers or sulfides, allowing halides to be electrochemically compatible with 4V cathode materials [1]. On another hand, their high ductility and the low-moderate temperatures needed for their synthesis make halides ideal candidates for the upscaling of material preparation, stacking and densification steps on cell manufacturing. Nevertheless, halides must overcome some challenges that might prevent their use such as the stability with the lithium (Li) metallic anode. In contact with Li, the halide undergoes a reduction reaction, leading to the formation of a mixed ionic-electronic SEI [2] until either the lithium electrode or the halide is consumed. As such, understanding the ionic transport and the Li/solid electrolyte interface dynamics is essential to optimize halide-based ASSLB.

In the present work, we report the advantages and the potential of Li₃YCl₄Br₂ (LYCB) halide as solid-state electrolyte. First, LYCB presents a high ionic conductivity at room temperature, greater than 1 mS cm^-1, which can be further improved after a densification step at moderate temperature. The cycling of full cells with a high-voltage NMC622 cathode, LYCB and metallic lithium anode leads to a discharge capacity up to 150 mAh g^-1. Using computational methods, impedance spectroscopy measurements and physico-chemical characterizations, the dynamics at the Li/LYCB interface are evaluated. An increase of the interfacial resistance with time witnesses the formation of a resistive interface between the halide and the Li electrodes. With a good agreement, both simulations and experimentations demonstrate the reduction of LYCB by the Li. However, the tests performed suggest that the evolution of the Li/LYCB interface due to the reactivity between the two components leads to the formation of a SEI which is beneficial to the cycling performances with Li electrodes, enabling the stripping and plating of a symmetric cell during 1000 h applying a current density of 0.1 mA cm^-2 with a capacity of 0.5 mAh cm^-2. The cells show an overpotential as low as 40 mV, one order of magnitude lower compared to other halides in contact with Li electrodes [3,4]. Post-mortem analysis evidence the formation, during cycling, of a multi-layer interface, made of the resulting products issued from the reduction of LYCB, that might protect LYCB from further degradation by the Li electrodes. Overall, this study highlights LYCB as a promising candidate for use as electrolyte Li-metal solid-state batteries.