Lithium Manganese Sulfates as a New Class of Supercapattery Materials at Elevated Temperatures

To make supercapattery devices feasible, there is an urgent need to find electrode materials that exhibit a hybrid mechanism of energy storage. Herein, we provide a first report on the capability of lithium manganese sulfates to be used as supercapattery materials at elevated temperatures. Two compositions are studied: monoclinic Li₂Mn(SO₄)₂ and orthorhombic Li₂Mn₂(SO₄)₃, which are prepared by a freeze-drying method followed by heat treatment at 500 °C. The electrochemical performance of sulfate electrodes is evaluated in lithium-ion cells using two types of electrolytes: conventional carbonate-based electrolytes and ionic liquid IL ones. The electrochemical measurements are carried out in the temperature range of 20-60 °C. The stability of sulfate electrodes after cycling is monitored by in-situ Raman spectroscopy and ex-situ XRD and TEM analysis. It is found that sulfate salts store Li⁺ by a hybrid mechanism that depends on the kind of electrolyte used and the recording temperature. Li₂Mn(SO₄)₂ outperforms Li₂Mn₂(SO₄)₃ and displays excellent electrochemical properties at elevated temperatures: at 60 °C, the energy density reaches 280 Wh/kg at a power density of 11,000 W/kg. During cell cycling, there is a transformation of the Li-rich salt, Li₂Mn(SO₄)₂, into a defective Li-poor one, Li₂Mn₂(SO₄)₃, which appears to be responsible for the improved storage properties. The data reveals that Li₂Mn(SO₄)₂ is a prospective candidate for supercapacitor electrode materials at elevated temperatures.