The need to improve electrodes and Li-ion conducting materials for rechargeable all-solid-state batteries has drawn enhanced attention to the investigation of lithium-rich compounds. The study of the ternary system Li-Si-P revealed a series of new compounds, two of which, Li SiP and Li SiP , are presented. Both phases represent members of a new family of Li ion conductors that display Li ion conductivity in the range from 1.15(7)×10 Scm at 0 °C to 1.2(2)×10 Scm at 75 °C (Li SiP ) and from 6.1(7)×10 Scm at 0 °C to 6(1)×10 Scm at 75 °C (Li SiP ), as determined by impedance measurements. Temperature-dependent solid-state Li NMR spectroscopy revealed low activation energies of about 36 kJ mol for Li SiP and about 47 kJ mol for Li SiP . Both compounds were structurally characterized by X-ray diffraction analysis (single crystal and powder methods) and by Li, Si, and P MAS NMR spectroscopy. Both phases consist of tetrahedral SiP anions and Li counterions. Li SiP contains isolated SiP units surrounded by Li atoms, while Li SiP comprises a three-dimensional network based on corner-sharing SiP tetrahedra, with the Li ions located in cavities and channels.
The lithium phosphidosilicates LiSiP and LiSiP are obtained by high-temperature reactions of the elements or including binary Li-P precursors. LiSiP (P2/n, Z = 2, a = 7.2051(4) Å, b = 6.5808(4) Å, c = 11.6405(7) Å, β = 90.580(4)°) features edge-sharing SiP double tetrahedra forming [SiP] units with a crystal structure isotypic to NaSiP and NaGeP. LiSiP (P2/m, Z = 2, a = 6.3356(4) Å, b = 7.2198(4) Å, c = 10.6176(6) Å, β = 102.941(6)°) crystallizes in a new structure type, wherein SiP tetrahedra are linked via common vertices and which are further connected by polyphosphide chains to form unique [SiP] double layers. The two-dimensional Si-P slabs that are separated by Li atoms can be regarded as three covalently linked atoms layers: a defect α-arsenic type layer of P atoms sandwiched between two defect wurzite-type SiP layers. The single crystal and powder X-ray structure solutions are supported by solid-state Li,Si, and P magic-angle spinning NMR measurements.
In the development of high energy density Li-ion batteries, two elements have drawn the interest of the scientific community due to their low cost and high specific capacity: silicon and sulfur. These two elements are considered next generation’s active materials for anode and cathode, respectively. In order to exploit the advantages given by both materials in terms of specific capacity (and thus specific energy), lithium has to be introduced at the time of manufacturing either on the cathode side (as Li2S/Si cell) or on the anode side (as LixSiy/S cell). The Li-Si system has been recently investigated as potential source of lithiated anode materials 1,2, however up to date little is known about their highly exothermal reaction with alkyl carbonate based electrolytes. In this work we address the latter point via in-situ Online Electrochemical Mass Spectrometry (OEMS)3. By monitoring the gas evolution (mainly ethylene) resulting from contacting the metastable phase Li15Si4 and the non-aqueous electrolyte during a simulated electrolyte filling experiment of a Li-ion cell (Fig. 1), it was possible to get fundamental understanding of the challenges that cell manufacturers will have to face when using lithiated anode materials. The cycling stability of Li15Si4 as well as the role played by the total electrode surface area were as well thoroughly investigated.
References: 1) Zeilinger M et al., Chem. Mater. 25 (2013), 4623-4632. 2) Ma R. et al., J. Phys. Chem. Lett. 3 (2012), 3555-3558. 3) Tsiouvaras et al., J. Electrochem. Soc. 160(3) (2013), A471-A477
Figure 1
The title compounds are prepared by high‐pressure reaction of ζ‐Fe2N (obtained from Fe and flowing NH3 at 708 K) and Ni or Co powder (15 GPa, 1473 K, 30 min).
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