Playing Billiards using Protons Generated by Electrolytic Dissociation of Hydrogen for Intercalation and Ion Substitution

TaS₂ is a well-known 2D layered material and intercalation extends its window of opportunity for a wide variety of applications. Achieving homogeneous intercalation while maintaining high crystallinity using liquid phase method is difficult. Chemical vapor transport (CVT), which involves a direct reaction between the guest vapor and host solid requires high temperature to vaporize the guest materials. Na₃V₂(PO₄)₃ (NVP), due to its high redox potential, excellent thermal stability,  and strong (PO₄)³⁻ polyanion networks, is considered as a promising cathode material for Na ion battery. Substitution of K⁺ ions in place of Na⁺ ions is likely to improve the rate capability and cycling stability. Nevertheless, the large difference in size between K⁺ and Na⁺ ions makes such substitution difficult. Researchers at Hokkaido University, Japan, Xi’an University of Technology, China, Kyushu University, Japan, Tokyo University of Science, Japan and Kyushu Institute of Technology, Japan have demonstrated a new synthesis method, referred as proton-driven ion introduction (PDII), which is based on a solid-state electrochemical reaction for intercalation into TaS₂ and ion substitution of Na₃V₂(PO₄)₃.

Intercalation of alkali metal ions (K⁺, Na⁺, and Li⁺) into TaS₂

A single crystal of plate-shaped TaS_­2 was placed on a carbon cathode. Disk-shaped phosphate glass containing alkali metal ions (K⁺, Na⁺, and Li⁺), referred as the ion-source material was placed above the TaS₂ single crystal. The electrodes were arranged inside a chamber with H₂ atmosphere. The schematic of the experimental set-up used for proton-driven ion introduction (PDII) is shown in Fig. 1. Upon application of a high between the needle-shaped anode and the carbon cathode, electrolytic dissociation of H₂ leads to the formation of protons and electrons. The protons are accelerated into the ion-source material. Penetration of protons pushes the alkali metal ions from the top to the bottom side of the ion source material. In order to maintain charge neutrality, the ion-source material releases the alkali metal ions through its bottom side, which reach the surface of TaS₂. Simultaneously, electrons produced by the corona discharge move from the needle-shaped anode to the TaS₂ single crystal placed over the carbon cathode through the electrode. The electric current flows around the circuit enables electrochemically driven intercalation of TaS_­2, according to the following reaction: TaS₂ + xA⁺ + xe⁻ = A_xTaS₂ (A = alkali metal ion).

Fig. 1 Schematic of the experimental set-up used for proton-driven ion introduction (PDII) – intercalation of alkali metal ions into TaS₂

Intercalation of transition metal ions (Cu⁺ and Ag⁺) into TaS₂

Besides alkali metal ions, transition metal ions such as Cu⁺ and Ag⁺ can also be intercalated  into  TaS₂ by  PDII,  using  CuI and  AgI as  the  respective source materials. Phosphate glasses containing Na, CuI, and TaS₂ were stacked on the carbon  cathode in that  order  from the top. The  schematic of the  experimental set-up used for the intercalation of Cu and Ag into TaS₂ by PDII is shown in Fig. 2(a). The phosphate glass is essential to prevent the formation of poisonous  HI gas. Optical image of the top side of CuI (Fig. 2(b)) indicate a change in colouration of CuI following the replacement of Cu⁺ with Na⁺ ions. The protons drive the release of Na⁺ ions from the phosphate glass, which is then substituted for the Cu⁺ ions on the top surface of CuI. At the bottom side, alkali metal ions and Cu ions are homogeneously intercalated into TaS₂. The three black single crystals represent Cu_xTaS₂ while the red products formed around Cu_xTaS₂ are indeed Cu metal (Fig 2(b)).

The supply rate of Cu⁺ ions from CuI and the diffusion coefficient of Cu⁺ ions in TaS₂ determines the nature of intercalation. When the rate of supply of Cu⁺ ions is  much smaller than the diffusion coefficient of Cu⁺ ions, Cu⁺ ions spread effectively throughout the TaS₂. On the contrary, when the rate of supply of Cu⁺ ions is  much larger than the diffusion coefficient, the surplus Cu⁺ ions are precipitated as Cu metal and the migration of Cu⁺ ions in to TaS₂ is prevented. Hence, to achieve a homogeneous intercalation, the supply rate of Cu⁺ ions should be smaller than its diffusion coefficient. Accordingly, a combination of low treatment temperature and high voltage fails to produce homogeneous intercalation of Cu intoTaS₂ even after 50 h. However, a high treatment temperature and a low voltage allow the formation of a homogeneous single crystal of Cu_2/3TaS₂, within a short treatment time of ~ 12 h (Fig. 2(c)).

Fig. 2 (a) Schematic of the Cu intercalation; (b) Optical images of the top and bottom surfaces of CuI after PDII. (c) Conditions under which homogeneous an partial intercalation of Cu into TaS₂ occurs

Ion substitution (K+ in place of Na+) in Na₃V₂(PO₄)₃

Powdered sample of Na₃V₂(PO₄)₃ (NVP) taken in a Al₂O₃ cylinder was placed on the carbon cathode. Potassium-containing phosphate glass (K⁺ source material) was placed over the Al₂O₃ cylinder (Fig. 3). Upon application of the voltage, the protons replace the K⁺ ions in the phosphate glass and push them into NVP. Unlike the intercalation of ions into TaS₂, during ion substitution, the K⁺ ions will not receive electrons between interfaces since NVP is electrically insulating.  Conversely, the Na⁺ ions discharged from the bottom side of NVP receive electrons and precipitates. Cross-sectional optical images (Fig. 3) indicate a change in colouration of the NVP powder to dark green in the upper region following substitution of K⁺ ions in place of Na⁺ ions. Based on the chemical composition (K_1.69Na_1.20V_2.00P_3.88O_13.83), the upper region is referred as K-NVP which is distinctly different from the NVP in lower region (Na_3.18V_2.00P_3.02O_10.86). Post-annealing at 600 °C improved the crystallization of K-NVP. When compared to conventional solid-state reaction, it would be possible to increase the amount of K substitution by 15 times using PDII.

Fig. 3 Schematic of the ion substitution process – K⁺ ion substitution in the Na⁺ site in Na₃V₂(PO₄)₃ and cross-sectional optical images of Na_3−xK_xV₂(PO₄)₃

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Masaya Fujioka et al., Proton-Driven Intercalation and Ion Substitution Utilizing Solid-State Electrochemical Reaction, J. Am. Chem. Soc., Article in press, DOI: 10.1021/jacs.7b09328