Phosphorus Electrodes in Sodium Cells: Small Volume Expansion by Sodiation and the Surface‐Stabilization Mechanism in Aprotic Solvent

Abstract

The electrode performance of amorphous phosphorus in aprotic Na cells is examined. Amorphous phosphorus is electrochemically reduced in the Na cells with a three‐electron redox process, crystallizing into Na3P. NaP bonds in Na3P have high covalent characteristics. Therefore, the molar volume of Na in Na3P is anomalously small in comparison to other Na–metal alloys that have been used as negative electrode materials. The theoretical volumetric capacity, calculated at full volume expansion of amorphous phosphorus electrodes through sodiation, is expected to be 27 % larger than that of metallic sodium. However, experimentally, it is found that severe electrolyte decomposition results in insufficient reversibility of the electrode materials, owing to the highly reactive Na3P surface. Electrode reversibility is successfully improved by utilizing an electrolyte additive, fluoroethylene carbonate (FEC). The surface chemistry of phosphorus in the aprotic solvent with sodium salts is examined by hard/soft X‐ray photoelectron spectroscopy (PES). PES studies reveal that FEC effectively stabilizes the solid–electrolyte interphase (SEI), containing monovalent phosphorus species and sodium fluoride, and thus electrolyte decomposition is partly suppressed by the relatively stable SEI formed on the surface of phosphorus particles.

Review—Practical Issues and Future Perspective for Na-Ion Batteries

“…The effects of additives on the electrode performance might change in NaPF 6 solution, which is under investigation by our group. As recently reported, FEC addition in 1 mol dm −3 NaClO 4 (Aldrich, ≥ 98%) in EC : DEC (= 1:1 v/v, PANAX ETEC, Korea), 1 mol dm −3 NaPF 6 in EC : DEC and 1 mol dm −3 NaClO 4 in PC (Kishida Chemical) electrolyte solutions effectively improved cycle stability of alloy compounds, Sn 4 P 3 , 46 Sb/C nanocomposite 41 and phosphorus, 51 respectively. Alloy compounds usually induce large volume expansion and shrinkage during Na insertion and extraction, respectively.…”

Electrochemical Na-insertion/Extraction Properties of Sn–P Anodes

“…Komaba et al have succeeded in bringing out an NIB anode of hard carbon a reversible capacity of 250-300 mA h g ¹1 by a reversible Na-insertion into its nanopores. 1 For a further high capacity, elemental phosphorous (P) 2 and tin (Sn) 3 are promising candidates of anode materials because these elements show high theoretical capacities (P, 2596 mA h g ¹1 ; Sn, 847 mA h g ¹1 ) based on alloying/dealloying reactions. Anodes consisting of elemental P or Sn, however, generally show deterioration of an active material layer and a resulting capacity decay by several ten cycles because these elements exhibit significant volume increase in their Na-storage (Na 15 Sn 4 , 525%; Na 3 P, 490%).…”

High‐Performance and Low‐Cost Sodium‐Ion Anode Based on a Facile Black Phosphorus−Carbon Nanocomposite

“…Further SEI formation in the following cycles was limited, possibly aided by the use of FEC in the electrolyte. [35,36] A high capacity of 1381 mAh g -1 was retained after 100 cycles which accounts for 90.5 % of the capacity from the first cycle.The Rate capability of the electrode (Figure 3b) has been studied at various current rates from 0.1 to 2.0 A g .These high rate capabilities indicate that the electrical conductivity has been greatly enhanced throughout the electrode by the conducting carbon network that enables good electronic charge transport. A sloped voltage plateau at ~ 0.6 V originating from the irreversible SEI formation is observed in the 1 st sodium ion insertion of the BP-C electrode (Figure 3c); it disappears in the following cycles.…”

“…Further SEI formation in the following cycles was limited, possibly aided by the use of FEC in the electrolyte. [35,36] A high capacity of 1381 mAh g -1 was retained after 100 cycles which accounts for 90.5 % of the capacity from the first cycle.…”