The ternary compound CuVS exhibits an excellent performance as anode material for sodium ion batteries with a high reversible capacity of 580 mAh g at 0.7 A g after 300 cycles. A Coulombic efficiency of ≈99% is achieved after the third cycle. Increase of the C-rate leads to a drop of the capacity, but a full recovery is observed after switching back to the initial C-rate. In the early stages of Na uptake first Cu is reduced and expelled from the electrode as nanocrystalline metallic Cu. An increase of the Na content leads to a full conversion of the material with nanocrystalline Cu particles and elemental V embedded in a NaS matrix. The formation of NaS is evidenced by Na MAS NMR spectra and X-ray powder diffraction. During the charge process the nanocrystalline Cu particles are retained, but no crystalline materials are formed. At later stages of cycling the reaction mechanism changes which is accompanied by the formation of copper(I) sulfide. The presence of nanocrystalline metallic Cu and/or CuS improves the electrical conductivity, leading to superior cycling and rate capability.

Nano-crystalline NiFe2O4 particles were synthesized and used as active electrode material for a lithium ion battery that showed a high discharge capacity of 1534 mAh g-1 and charge capacity of 1170 mAh g-1 during the 1 st cycle. X-ray absorption spectroscopy including XANES and EXAFS were used to investigate electronic and local structural changes of NiFe2O4 during the 1 st lithiation and de-lithiation process. As lithium is inserted into the structure, tetrahedral site Fe 3+ ions are reduced to Fe 2+ and moved from tetrahedral sites to empty octahedral sites, while Ni 2+ ions are unaffected. As a consequence, the matrix spinel structure collapses and transforms to an intermediate rock-salt monoxide phase. Meanwhile, the inserted Li is partially consumed by the formation of SEI and other side reactions during the conversion reaction. With further lithiation, the monoxide phase is reduced to highly disordered metallic Fe/Ni nanoparticles with a number of nearest neighbors of 6.0(8) and 8.1(4) for Fe and Ni, respectively. During subsequent de-lithiation, the metal particles are individually re-oxidized to Fe2O3 and NiO phases instead to the original NiFe2O4 spinel phase.

The compound CuCrS2 with a quasi‐layered crystal structure was investigated as room temperature rechargeable sodium‐ion battery electrode. It exhibits excellent performance as anode material with a high reversible capacity of 424 mAh g−1 at 700 mA g−1 after 200 cycles and a capacity retention of 98.6 % compared to the third cycle. Results of ex‐situ X‐Ray diffraction experiments performed at different stages of the discharge process demonstrate that at the beginning of Na uptake, Cu+ cations are reduced to nanosized metallic Cu particles which are expelled from the host lattice. Simultaneously, Na is inserted into the host material leading to the formation of Na0.7Cu0.15CrS2 with significantly expanded interlayer space. Metallic Cu and Na0.7Cu0.15CrS2 coexist at this stage of discharge. Increasing the amount of Na per formula unit leads to successive conversion to X‐ray amorphous Cr, nanocrystalline Na2S and metallic Cu. The formation of highly disordered metallic Cr with domain sizes in the range of few nanometres is revealed by atomic pair distribution function analysis. During the charge process, the nanocrystalline Cu particles are retained and Na0.7Cu0.15CrS2 is at least partially reformed. The finely distributed Cu particles dramatically improve the long‐time stability as evidenced by comparison of the electrochemical behaviour of mere NaCuCrS2.

Iron vanadium sulfide (FeV2S4) was synthesized via a high temperature solid state reaction and was investigated as a cheap anode material for Na and Li ion batteries. Discharge capacities as high as 723 mA h g(-1) (Na) and 890 mA h g(-1) (Li) were found for half-cell measurements at room temperature. The capacity of the Na-FeV2S4 system remained constant at 529 mA h g(-1) after the 10th cycle with an area capacity of 2.7 mA h cm(-2) being very close to that of conventional Li-ion technology.

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