Superior Sodium Storage Properties in the Anode Material NiCr₂S₄ for Sodium‐Ion Batteries: An X‐ray Diffraction, Pair Distribution Function, and X‐ray Absorption Study Reveals a Conversion Mechanism via Nickel Extrusion

Abstract

The pseudo‐layered sulfide NiCr2S4 exhibits outstanding electrochemical performance as anode material in sodium‐ion batteries (SIBs). The Na storage mechanism is investigated by synchrotron‐based X‐ray scattering and absorption techniques as well as by electrochemical measurements. A very high reversible capacity in the 500th cycle of 489 mAh g−1 is observed at 2.0 A g−1 in the potential window 3.0–0.1 V. Full discharge includes irreversible generation of Ni0 and Cr0 nanoparticles embedded in nanocrystalline Na2S yielding shortened diffusion lengths and predominantly surface‐controlled charge storage. During charge, Ni0 and Cr0 are oxidized, Na2S is consumed, and amorphous Ni and Cr sulfides are formed. Limiting the potential window to 3.0–0.3 V an unusual nickel extrusion sodium insertion mechanism occurs: Ni2+ is reduced to nanosized Ni0 domains, expelled from the host lattice, and is replaced by Na+ cations to form O3‐type like NaCrS2. Surprisingly, the discharge and charge processes comprise Na+ shuttling between highly crystalline NiCr2S4 and NaCrS2 enabling a superior long‐term stability for 3000 cycles. The results not only provide valuable insights for the electrochemistry of conversion materials but also extend the scope of layered electrode materials considering the reversible nickel extrusion sodium insertion reaction as new concept for SIBs.

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

“…The first reduction occurs at ∼1.94 V (peak 1) and is assigned to the intercalation of Na + into the structure and the extrusion and resulting reduction of Cu + to metallic Cu. The most intense reduction peak at ∼0.48 V (peak 3) is assigned to the reduction of Cr 3+ to Cr 0 in accordance with electrochemical data from Cr 3 S 4 and NiCr 2 S 4 [58] . However, this peak is very broad and potentially superposes smaller signals.…”

“…The most intense reduction peak at ~0.48 V (peak 3) is assigned to the reduction of Cr 3 + to Cr 0 in accordance with electrochemical data from Cr 3 S 4 and NiCr 2 S 4 . [58] However, this peak is very broad and potentially superposes smaller signals. Furthermore, a shoulder at ~0.77 V and a weak signal at ~1.21 V (peak 2) occur, which likely are associated with a (two-step) reduction of P 4 + .…”

Long-Term Stable, High-Capacity Anode Material for Sodium-Ion Batteries: Taking a Closer Look at CrPS4 from an Electrochemical and Mechanistic Point of View

“…During the charging process, an intensive oxidation peak is observed at ∼2.06 V, while a smaller and not well-resolved second oxidation event is detected at a potential of ∼2.55 V. Taking the initial capacity drop (Figure 2b, inset) into account, we suggest a two-step oxidation of phosphorus to its starting oxidation state of P 5+ in the first charge process, while the oxidation of Cr 0 is kinetically hindered. The expected oxidation peak for Cr 0 /Cr 3+ is ∼1.2 V 55 or slightly above at ∼1.5 V, which potentially is the reason for the very small oxidation and reduction peaks in the CV curve (Figure 3b) at this potential. In the second and subsequent cycles, the oxidation process at ∼2.55 V disappears, which might be an indication that phosphorus is only oxidized to P 3+ after the first cycle.…”

“…The reduction of Cr 2+/3+ and Cr 3+ was recently confirmed to occur at a potential of ∼0.4 V for Cr 3 S 4 and NiCr 2 S 4 against sodium. 55 Therefore, we assign the reduction process at ∼0.53 V (peak 3) to the reduction of Cr 2+ to the metallic state. The origin of the reduction peaks occurring at ∼0.83 V (peak 2) and ∼0.26 V (peak 4) remains uncertain.…”

Understanding sodium storage properties of ultra-small Fe3S4 nanoparticles – a combined XRD, PDF, XAS and electrokinetic study

“…S19†) both indicating the presence of X-ray amorphous α-Fe and absence of crystalline Na 2 S. Poor crystallinity of conversion products after few cycles in SIBs was reported for many sulfide materials before. 25,69–71…”