Pulverization-Tolerance and Capacity Recovery of Copper Sulfide for High-Performance Sodium Storage

Abstract

Finding suitable electrode materials is one of the challenges for the commercialization of a sodium ion battery due to its pulverization accompanied by high volume expansion upon sodiation. Here, copper sulfide is suggested as a superior electrode material with high capacity, high rate, and long-term cyclability owing to its unique conversion reaction mechanism that is pulverization-tolerant and thus induces the capacity recovery. Such a desirable consequence comes from the combined effect among formation of stable grain boundaries, semi-coherent boundaries, and solid-electrolyte interphase layers. The characteristics enable high cyclic stability of a copper sulfide electrode without any need of size and morphological optimization. This work provides a key finding on high-performance conversion reaction based electrode materials for sodium ion batteries.

Keywords: capacity recovery; pulverization tolerance; semi‐coherent interfaces; sodium ion batteries; transmission electron microscopy.

Figure 1

Capacity recovery of CuS nanoplates. a) Low magnification TEM image of pristine CuS nanoplates and corresponding selected area electron diffraction (SAED) pattern (scale bar, 200 nm). b) Cyclic performance of CuS nanoplates at 0.2 C and 3 C during 300 cycles. c) Nyquist plot from EIS result of CuS nanoplates during 200 cycles at 0.2 C. Semi‐circles in high frequency and high‐medium frequency regions are associated with R _SEI and R _ct, respectively. R _E corresponds to ohmic resistance. CPE_SEI and CPE_ct indicate constant phase elements in SEI and charge transference.

Figure 2

Ex situ observation of CuS nanoplates disintegration. a) Schematic model demonstrating the disintegration in CuS nanoplates. Low magnification TEM images and corresponding SAED patterns of b) pristine CuS (scale bar, 200 nm) and desodiated CuS nanoplates (scale bar, 100 nm) after c) 20 cycles, d) 50 cycles and e) 240 cycles at 0.2 C. Inset graph in (d) shows size distribution of CuS nanograins. TEM images of the SEI layers on the surface of Na_xCuS after f) 20 cycles (scale bar, 20 nm) and g) 240 cycles (scale bar, 10 nm).

Figure 3

Stress‐induced overpotential and corresponding stress profiles of CuS nanoplates in conjunction with in situ diffraction pattern changes during the first cycle. a) The stress‐induced overpotential and the stress profiles from the experiment during the first cycle at 0.2 C and 3 C. In situ observation of diffraction pattern changes from b) a single pristine CuS nanoplate to c) CuS/Na_xCuS and to d) fully sodiated phases (Na₂S/Cu). CuS experiences elastic deformation upon initial Na insertion until it reaches to yield point. After touching the yield strength, plastic deformation begins with further increase of the stress. However, the intercalation phase still retains a single spot, meaning that disintegration rarely occurs in intercalation stage. Once the stress reaches to ultimate strength, it is relieved by forming Na₂S grains through the conversion reaction, showing the diffuse diffraction spots. The yield point becomes much higher at 3 C, which originates from yield's strength variation on strain rates. The yield strength dramatically increases once it becomes larger than a critical strain rate.36 The stress relaxation point moves forward at 3 C due to the reaction limited intercalation reaction.15 Large sodium insertion into CuS induces coexistence of the intercalation and the conversion phases. Although the intercalation reaction initiates first, the conversion reaction occurs before the intercalation reaction is finished. Finally, the intercalation area caught up soon by the conversion area at high current density.15 As a result, stress is relieved earlier at 3 C than at 0.2 C. η_stress and σ corresponds to stress‐induced overpotential and stress, respectively.

Figure 4

HR‐TEM observation of grain boundaries and phase interfaces in Na_xCuS. a) Schematic model demonstrating grain boundaries and phase interfaces formations in Na_xCuS phases. HR‐TEM images of grain boundaries formed by b) different Na inserting orientation (scale bar, 2 nm) and c) stress relaxation during the conversion reaction (scale bar, 2 nm). HR‐TEM images of phase interfaces between d) the intercalation (Na₃(CuS)₄) and the conversion (Na₂S) phases (scale bar, 5 nm), and between e) Na₂S and Cu (scale bar, 5 nm). Diffuse FFT spots of Na₂S and Cu in (c,d) indicate that a number of Na₂S and Cu grains are mis‐orientated from one another. GB1 and GB2 in the schematic model correspond to grain boundaries formed in the intercalation and the conversion reaction, respectively. The HR‐TEM images are Wein‐filtered.

Figure 5

Electrochemical performance of bare bulk CuS. a) SEM image of bulk CuS (scale bar, 100 µm), and its b) C‐rate capability from 1 C to 5 C and c) cyclic performance at 1 C and 5 C during 1000 cycles. d) EIS result obtained from bulk CuS within the frequency range between 1000 kHz and 0.1 Hz at amplitude of 10 mV. Inset graph in (d) is magnified high frequency region.