Crystal chemistry and lithium-ion intercalation properties of lithium manganese silicate cathode for aqueous rechargeable Li-ion batteries

Abstract

Lithium manganese silicate (Li₂MnSiO₄) demonstrates rich polymorphism with orthorhombic Pmn2₁ polymorph having better crystalline ordering over orthorhombic Pmnb and monoclinic forms. In this work, Li₂MnSiO₄ nanoparticles with the Pmn2₁ phase were synthesized by hydrothermal synthesis. Powder X-ray diffraction, HRSEM, and HRTEM as well as SAXS investigations revealed crystalline, spherical nanoparticles with average diameters between 14 and 25 nm. The binding energy of surface active Mn²⁺ ions in Li₂MnSiO₄ was observed at 642.9 eV in XPS analysis. SSNMR spectroscopic results revealed isotropic peaks at − 288.5 and 295.32 ppm which are attributed to the hyperfine coupling between the Li nuclei and the unpaired electrons of the Mn²⁺ ions. The electrochemical properties of Li₂MnSiO₄ electrode were studied in various 1 M LiX (X = SO₄²⁻, NO₃⁻ and ClO₄⁻) aqueous electrolytes at a potential window of 0.2–1 V. Studies in the Li₂SO₄ aqueous electrolyte demonstrated better electrochemical properties with increasing peak current values up till the 2000th cycle. Charge and discharge capacities of 77.0 and 57.7 mAh g⁻¹ were obtained at a scan rate of 5 mV s⁻¹ with a Li-ion diffusion coefficient of 1.61 × 10⁻¹⁶ cm² s⁻². A small charge transfer resistance in the Li₂SO₄ electrolyte was also observed in EIS measurements indicating faster charge transfer kinetics. An aqueous Li-ion Swagelok-type full cell was assembled with microcrystalline graphite as anode and Li₂MnSiO₄ as the cathode. 1 M Li₂SO₄ was used as the electrolyte. The cell delivered a charge capacity of 61.8 mAh g⁻¹ and a discharge capacity of 52.5 mAh g⁻¹ at a current load of 1.2 A g⁻¹ with good capacity retention of 94.0% and a Coulombic efficiency of 99.5% over 3300 cycles.

Graphical abstract

SEM image, FTIR, XPS, and discharge capacity versus coulombic efficiency plots of Li₂MnSiO₄ nanoparticles