A Combined Sodium Intercalation and Copper Extrusion Mechanism in the Thiophosphate Family: CuCrP₂S₆ as Anode Material in Sodium‐Ion Batteries

Abstract

The layered hypothiophosphate CuCrP2S6 was identified as promising candidate for the application as anode in sodium‐ion batteries exhibiting an appropriate electrochemical performance (409 mAh g−1 after 200 cycles @ 1 A g−1). The electrochemical long‐term experiments suggest that the reaction mechanism changed upon repeated discharging and charging. The initial discharge and charge process was studied by X‐ray diffraction evidencing that at the early discharge stages, Na+ was intercalated into the interlayer space accompanied by a simultaneous reduction of Cu+ to metallic Cu, which was extruded from the host. At later stages of discharge, a conversion of the intermediate phase to Cu0, amorphous Cr0 and P0 embedded in nanocrystalline Na2S occurred. After recharging, only reflections of nanocrystalline NaF could be identified, which is part of the generated solid electrolyte interphase (SEI) layer. In contrast to observation made in literature, elemental Cu seems to be oxidized during the charge process again.

Influence of the Cation on the Reaction Mechanism of Sodium Uptake and Release in Bivalent Transition Metal Thiophosphate Anodes: A Case Study of Fe2P2S6

“…Shortly summarizing, we were able to demonstrate that the anodes Ni 2 P 2 S 6 , CrPS 4 and CuCrP 2 S 6 each follow individual reaction pathways instead of a universal mechanism (see Eq. (1)–(3)) [24–26] . While this might be unexpected at first, these results are quite sound when considered carefully.…”

“…The second NMR resonance in the discharged state with a chemical shift of À 25 ppm can be explained by SEI components, such as NaF, which is a common observation for certain electrolyte salts. [24][25][26][27][28]30] It is noteworthy that the NMR spectra presented here do not exhibit a very good resolution and some NMR resonances are slightly shifted. This is largely because the samples that were examined here are not ideally suited for such experiments, primarily because they contain ferromagnetic Fe nanoparticles and the induced local magnetic fields at these particles potentially influence the resolution and chemical shift of the NMR resonances.…”

“…[24]). The second NMR resonance in the discharged state with a chemical shift of −25 ppm can be explained by SEI components, such as NaF, which is a common observation for certain electrolyte salts [24–28,30] …”

“…While such observations can have several reasons, [24,25,[36][37][38] it is most likely that a changing electrochemical reaction during long periods of cycling is responsible in this case. The successive changing of discharge/charge profiles (see ESI; Figure S2) during extended cycling is an obvious indication of a changing long-term mechanism, which is a common observation for MTP compounds [24][25][26] as well as for other conversion electrodes. [27][28][29]36,38,39] For further electrochemical characterization, rate capability experiments (Figure 2c) were conducted applying increasing current rates (0.2, 0.5, 1.0, 2.0, 3.0 and again 0.2 A g À 1 ) for 10 cycles each.…”

“…Most certainly no atomic phosphorus (but likely some sort of P 0 ; e. g. polymeric P) is formed, which is why these reactions should be viewed as formal equations. [24][25][26] Although all of the studied MTPs follow individual reaction mechanisms, it is very likely that anodes of the same category, e. g. M II TPs, M 2 P 2 S 6 , indeed react according to a universal mechanism. Hence, we focused the research on this assumption and studied Fe 2 P 2 S 6 anodes.…”