Phase Transformations and Phase Segregation during Potassiation of Sn_xP_y Anodes

Abstract

K-ion batteries (KIBs) have the potential to offer a cheaper alternative to Li-ion batteries (LIBs) using widely abundant materials. Conversion/alloying anodes have high theoretical capacities in KIBs, but it is believed that electrode damage from volume expansion and phase segregation by the accommodation of large K-ions leads to capacity loss during electrochemical cycling. To date, the exact phase transformations that occur during potassiation and depotassiation of conversion/alloying anodes are relatively unexplored. In this work, we synthesize two distinct compositions of tin phosphides, Sn 4 P 3 and SnP 3 , and compare their conversion/alloying mechanisms with solid-state nuclear magnetic resonance (SSNMR) spectroscopy, powder X-ray diffraction (XRD), and density functional theory (DFT) calculations. Ex situ 31 P and 119 Sn SSNMR analyses reveal that while both Sn 4 P 3 and SnP 3 exhibit phase separation of elemental P and the formation of KSnP-type environments (which are predicted to be stable based on DFT calculations) during potassiation, only Sn 4 P 3 produces metallic Sn as a byproduct. In both anode materials, K reacts with elemental P to form K-rich compounds containing isolated P sites that resemble K 3 P but K does not alloy with Sn during potassiation of Sn 4 P 3 . During charge, K is only fully removed from the K 3 P-type structures, suggesting that the formation of ternary regions in the anode and phase separation contribute to capacity loss upon reaction of K with tin phosphides.

Alkali-Ion Intercalation Chemistry and Phase Evolution of Sn4P3

“…The Sn−P bond length for potassiated Sn 4 P 3 is determined by the first peaks of RDFs (from 0.31 to 0.27 nm) and CN Sn−P (from 2.7 to 2.1), which may be related to the Sn 3 P phase (obtained from the Materials Project Database). Based on the K−Sn bond length, from 0.28 nm (KSn 4 P 3 ) to 0.24 nm (K 10 Sn 4 P 3 ), the calculated CN K−Sn is from 4.3 to 2.4, possibly belonging to the KSnP phase (structure obtained from Ells), 15 as judged from the NMR results (Figure S6c). (1) (5)…”

“…15,32,33 Afterward, the intensity of the 31 P NMR peak at about −250.2 ppm appears until the end of C0.5 V, related to the K 3 P phase. 15 The peak pattern at the C2.0 V state is similar to that of the pristine material, indicating that the intermediates produced in the preceding process have almost recovered to the initial Sn 4 P 3 at the end of the charge.…”

“…When the electrode is discharged to 0.1 V in KIBs, KP (−228.6 ppm) and K 4 P 6 (∼−119.0 ppm) phases are formed, along with the presence of amorphous P (∼11.9 ppm) and KSnP (∼3 ppm) phases. 15,32,33 Afterward, the intensity of the 31 P NMR peak at about −250.2 ppm appears until the end of C0.5 V, related to the K 3 P phase. 15 The peak pattern at the C2.0 V state is similar to that of the pristine material, indicating that the intermediates produced in the preceding process have almost recovered to the initial Sn 4 P 3 at the end of the charge.…”

“…Second, there is no general consensus on the intermediate phases formed during the transition process after the insertion process. For instance, Guo has identified the existence of K–Sn phases, while Ells suggested the occurrence of a new ternary KSnP-type environment instead of K–Sn phases. , It was supported by NMR analysis, where the position and intensity of the signal representing metallic Sn remain constant in the transition reaction, indicating the formed metallic Sn does not participate in the reaction.…”

2023 roadmap for potassium-ion batteries

“…In this way we can computationally probe the phase diagrams of complex potassium containing compounds, including polyanionic and non-oxide based anions, with designer element choice, where factors such as earth abundance and technoeconomic considerations can be taken into account. This approach has begun to be used in the Li-ion and Na-ion fields [196][197][198], and recently has been demonstrated for the ternary K-Sn-P composition space [199,200].…”