Aprotic Li-O 2 cells have attracted considerable research interest due to its outstandingly high theoretical specific capacity. However, published discharge capacities vary considerably among different researchers despite only minor differences in the tested cell components. Some research groups observe low discharge capacities and formation of passivating layers of Li 2 O 2 on the electronically conducting cathode support, while other groups report large capacities and toroidal Li 2 O 2 crystals as discharge product. In this study we show that these differences may be due to water and protons, both possible impurities in Li-O 2 cells, having a large effect on discharge capacity and Li 2 O 2 morphology. As evidenced by XRD, FTIR and UV-visible analysis, Li 2 O 2 is still the main discharge product in Li-O 2 cells containing water, and moreover the Li 2 O 2 yield increases with the concentration of water in the electrolyte. On-line electrochemical mass spectrometry was employed to understand the differences in the discharge-charge behavior due to the addition of water and protons. While water seems to get oxidized at high potentials during charge, protons are consumed at the beginning of the discharge leading to a variety of reactive oxygen species and thus to degradation of cell components.

Solid electrolyte materials are crucial for the development of high‐energy‐density all‐solid‐state batteries (ASSB) using a nonflammable electrolyte. In order to retain a low lithium‐ion transfer resistance, fast lithium ion conducting solid electrolytes are required. We report on the novel superionic conductor Li9AlP4 which is easily synthesised from the elements via ball‐milling and subsequent annealing at moderate temperatures and which is characterized by single‐crystal and powder X‐ray diffraction. This representative of the novel compound class of lithium phosphidoaluminates has, as an undoped material, a remarkable fast ionic conductivity of 3 mS cm−1 and a low activation energy of 29 kJ mol−1 as determined by impedance spectroscopy. Temperature‐dependent 7Li NMR spectroscopy supports the fast lithium motion. In addition, Li9AlP4 combines a very high lithium content with a very low theoretical density of 1.703 g cm−3. The distribution of the Li atoms over the diverse crystallographic positions between the [AlP4]9− tetrahedra is analyzed by means of DFT calculations.

Thel ithium phosphidoaluminate Li 9 AlP 4 represents apromising new compound with ahigh lithium ion mobility. This triggered the searchf or new members in the familyo f lithium phosphidotrielates, and the novel compounds Li 3 AlP 2 and Li 3 GaP 2 ,o btained directly from the elements via ball milling ands ubsequenta nnealing, are reported here. It was unexpectedly found through band structure calculations that Li 3 AlP 2 and Li 3 GaP 2 are directb and gap semiconductors with band gaps of 3.1 and 2.8 eV,r espectively.R ietveld anal-yses revealt hat both compounds crystallize isotypically in the orthorhombic space group Cmce (no. 64) with lattice parameters of a = 11.5138(2), b = 11.7634(2)a nd c = 5.8202(1) for Li 3 AlP 2 ,a nd a = 11.5839(2), b = 11.7809(2) and c = 5.8129(2) for Li 3 GaP 2 .T he crystal structures feature TrP 4 (Tr = Al, Ga) corner-and edge-sharing tetrahedra, forming two-dimensional 1 2 TrP 2 3À ½ layers. The lithium atoms are located between and inside these layers. The crystal structures were confirmed by MAS-NMR spectroscopy.Li 2 SiP 2 . [8,11] Interestingly,t he phases Li 8 SiP 4 ,L i 5 SiP 3 (= Li 10 Si 2 P 6 ), Li 2 SiP 2 ,a nd LiSi 2 P 3 are connected by af ormal reduction of the formula by units of Li 3 P. [11] Al ower Li 3 Pc ontent leads to a higherc onnectivity of the tetrahedra.Compared to the related sulfide-basedl ithium ion conductors, [3,6,7,12] the anionic substructure of phosphido-based conductorsc arry one additional charge( formal "P 3À "v ersus a formal "S 2À "), and thus the Li content that is required for chargeb alance is higher.R ecently,w ee xpanded this concept of highly charged tetrahedra to lithium phosphidoaluminates by replacing the centralg roup 14 metal by aluminium. [13] Li 9 AlP 4 contains highly charged[TrP 4 ] 9À tetrahedra andreaches high ionic conductivities of % 3.0 mS cm at room temperature. Besides this first report of astructurally characterized lithium phosphidoaluminate, another compound of the composition Li 3 AlP 2 was mentioned already in 1952 and described with an orthorhombic distorted CaF 2 -type structure, in which the phosphorus atoms form ad istorted cubic close packing, althoughw ithoutr eliable crystallographic data. [14] Twoy ears later,t he corresponding gallium compound Li 3 GaP 2 was also postulated. [15] Despite the poorly characterized structure model, quantum-chemical calculations of Li 3 AlP 2 and Li 3 GaP 2 were performed, anticipating the model of vertex-sharing AlP 4 tetrahedra. [16][17][18] As forl ithium phosphidotetrelates, lithium phosphidoaluminates can also be connected on al ine in a Gibbs composition triangle (Finetti diagram). Li 3 AlP 2 is located on the line in the phase system Li-Al-P connecting Li 3 Pa nd AlP ( Figure S7, Supporting Information) by reducing Li 9 AlP 4 by two units of Li 3 P( Li 3 AlP 2 = Li 9 AlP 4 À2 Li 3 P). Assuming ac harge balanced valence compound, the degree of connectivity of the AlP 4 tetrahedra in Li 3 AlP 2 mustb eh igher, and isolated tetrahedra as observed in Li 9 AlP 4 cannot occur.Here...

Solid electrolyte materials are crucial for the development of high‐energy‐density all‐solid‐state batteries (ASSB) using a nonflammable electrolyte. In order to retain a low lithium‐ion transfer resistance, fast lithium ion conducting solid electrolytes are required. We report on the novel superionic conductor Li9AlP4 which is easily synthesised from the elements via ball‐milling and subsequent annealing at moderate temperatures and which is characterized by single‐crystal and powder X‐ray diffraction. This representative of the novel compound class of lithium phosphidoaluminates has, as an undoped material, a remarkable fast ionic conductivity of 3 mS cm−1 and a low activation energy of 29 kJ mol−1 as determined by impedance spectroscopy. Temperature‐dependent 7Li NMR spectroscopy supports the fast lithium motion. In addition, Li9AlP4 combines a very high lithium content with a very low theoretical density of 1.703 g cm−3. The distribution of the Li atoms over the diverse crystallographic positions between the [AlP4]9− tetrahedra is analyzed by means of DFT calculations.

Lithium-ion conductors are currently tested for their possible usage in all-solid-state lithium-ion batteries. In order to design high-performance solid electrolytes, the fundamental understanding of the relationships of the atomic structure and the transport properties such as temperature-dependent ionic conductivity is a basic prerequisite. Therefore, systematic investigations of closely related structures are essential. Phosphide-based materials are promising candidates for solid electrolytes, and recently, we have shown that the superionic conductor Li9AlP4 with an ionic conductivity of 3 mS cm–1 at room temperature can be obtained by the substitution of Si by Al in Li8SiP4. Now, we present the heavier gallium homologue Li9GaP4, which reveals a similarly high superionic conductivity of 1.6 mS cm–1 and a low activation energy. Li9GaP4 is easily accessible via ball milling of the elements and subsequent annealing at quite moderate temperatures. The single-crystal X-ray structure determination reveals that Li9GaP4 is isotypic to Li9AlP4 and crystallizes in the cubic space group P4̅3n (no. 218) with a lattice parameter of a = 11.868(1) Å. Temperature-dependent single-crystal X-ray diffraction reveals that lithium is not located at the center of the octahedral voids of the slightly distorted cubic close packing of P atoms but occurs with split positions. Impedance spectroscopy and temperature-dependent static 7Li NMR experiments reveal activation energies of 36 and 25 kJ mol–1, respectively.