Facile synthesis and phase stability of Cu-based Na₂Cu(SO₄)₂·xH₂O (x = 0-2) sulfate minerals as conversion type...

Mineral exploration forms a key approach for unveiling functional battery electrode materials. The synthetic preparation of naturally found minerals and their derivatives can aid in designing of new electrodes. Herein, saranchinaite Na₂Cu(SO₄)₂ and its hydrated derivative kröhnkite Na₂Cu(SO₄)₂·2H₂O bisulfate minerals have been prepared using a facile spray drying route for the first time. The phase stability relation during the (de)hydration process was examined synergising in situ X-ray diffraction and thermochemical studies. Kröhnkite forms the thermodynamically stable phase as the hydration of saranchinaite to kröhnkite is highly exothermic (-51.51 ± 0.63 kJ mol^-1). Structurally, kröhnkite offers a facile 2D pathway for Na⁺ ion migration resulting in 20 times higher total conductivity than saranchinaite at 60 °C. Both compounds exhibited a conversion redox mechanism for Li-ion storage with the first discharge capacity exceeding 650 mA h g^-1 (at 2 mA g^-1vs. Li⁺/Li) upon discharge up to 0.05 V. Post-mortem analysis revealed that the presence of metallic Cu in the discharged state is responsible for high irreversibility during galvanostatic cycling. This study reaffirms the exploration of Cu-based polyanionic sulfates, which while having limited (de)insertion properties, can be harnessed for conversion-based electrode materials for batteries.