Structure and Conductivity of Lithium Manganese Phosphate Glasses for Lithium Ion Batteries.
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The local structures and conductivities of glass samples with general compositions (50 − x) Li2O: xMnO: 50P2O5 (metaphosphate glasses) and (60 − x) Li2O: xMnO: 40P2O5 (polyphosphate glasses), were studied using a range of techniques including molecular dynamics (MD) simulation of neutron scattering data, a.c. impedance spectroscopy, density measurements, infrared spectroscopy, thermal analysis and X-ray diffraction. For the first system, glasses of the selected composition (50 − x) Li2O: xMnO: 50P2O5 (x = 10.0, 25.0, 33.3, 40.0 and 50.0) were prepared successfully. At low MnO content 50P2O5 (x = 10.0, 25.0 and 33.3), glass transition temperature increases, free volume decreases and activation energy increases, with increasing x value. In the compositional range x ≥ 33.3, the glass transition temperature decreases, free volume increases and activation energy decreases, with the increasing x. The critical composition, x = 33.3, is where the structure is most compact. Manganese cations show both network forming and network modifying behaviour in these systems. MD simulation of neutron pair distribution functions reveals two main competing phenomena: (1) increasing concentration of network forming [MnO2]2− Q4 manganate tetrahedra causes a strengthening of the network leading to higher glass transition temperatures and a more compact structure; (2) reduction in the total number of modifying cations leads to a more open structure and a reduction in cross-link density resulting in a decrease in Tg and increase in free volume. The electrical measurement show that the charge carrier concentration is the main factor controlling the level of ionic conductivity with values in the order of 10-3 S cm-1 at 300 C obtained in the (60 − x) Li2O: xMnO: 40P2O5 system. However, this comes at the expense of a reduced ability to form glasses due to the lower phosphate content.
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