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

Lithium manganese silicate (Li 2 MnSiO 4 ) demonstrates rich polymorphism with orthorhombic Pmn2 1 polymorph having better crystalline ordering over orthorhombic Pmnb and monoclinic forms. In this work, Li 2 MnSiO 4 nanoparticles with the Pmn 2 1 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 Mn 2+ ions in Li 2 MnSiO 4 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 Mn 2+ ions. The electrochemical properties of Li 2 MnSiO 4 electrode were studied in various 1 M LiX (X = SO 4 2− , NO 3 − and ClO 4 − ) aqueous electrolytes at a potential window of 0.2–1 V. Studies in the Li 2 SO 4 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 cm 2 s −2 . A small charge transfer resistance in the Li 2 SO 4 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 Li 2 MnSiO 4 as the cathode. 1 M Li 2 SO 4 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 Li 2 MnSiO 4 nanoparticles