Nowadays, improving the energy output of electrochemical capacitors (ECs) is still a challenge for researchers. In this context, the lithium-ion capacitors (LICs) which exhibit higher energy density than EDLCs, at comparable power, are considered to be very promising energy storage devices. However, the necessary pre-lithiation of the negative graphite electrode constitutes a drawback of these systems. In the first concept of LIC, metallic lithium was used as auxiliary electrode for graphite pre-lithiation [1]. Since this additional electrode complicates the cell construction and may cause safety issues, it has then been proposed to implement a concentrated electrolyte solution as source of lithium ions, yet it resulted in noticeable decrease of electrolyte concentration and ionic conductivity [2]. Therefore, another strategy was to incorporate lithium metal oxide (from which lithium ions can be irreversibly extracted) in the positive activated carbon (AC) electrode. Once the graphite intercalation compound is formed, the LIC can be cycled, while the de-lithiated oxide in the positive electrode is electrochemically inactive [3,4]. Notwithstanding, the use of oxides such as Li₅ReO₆ [3] or Li₅FeO₄ [4] requires a relatively high oxidation potential (up to 4.3 - 4.7 V vs. Li/Li⁺) which may cause parasitic oxidation of the electrolyte.

Recently, we have demonstrated that these drawbacks can be solved by using a renewable salt, namely 3,4-dihydroxybenzonitrile dilithium salt (Li₂DHBN), in combination with activated carbon [5]. Lithium is irreversibly extracted from Li₂DHBN ca. 3.2 V vs. Li⁺/Li, and the residual organic material later dissolves in the electrolyte, while the cell continues to operate as a LIC.

Despite the aforementioned advantages of pre-lithiation through a sacrificial cathodic material to prepare LICs, the possibility of lithium plating when high currents are employed and the low natural abundance of lithium remain some disadvantages of these systems. Therefore, our scientific attention was lately attracted by Na-ion capacitors (NICs), owing to few potential advantages as low cost of sodium, the possibility of using low-priced current collectors made of aluminum for the negative electrode. Our present work is focused on developing a NIC where Na₂S is employed as sacrificial material for sodium insertion into a Sn-P intermetallic negative electrode.

The Sn-P intermetallic was formed by facile high-energy mechanical ball milling in Ar atmosphere. Afterwards, Sn-P was mixed with hard carbon (HC) in the mass ratio 3:7 by ball-milling. A negative electrode coated on copper foil was obtained by mixing HC/Sn-P together with carbon black and CMC binder. A Na₂S/AC positive electrode was prepared by mixing Na₂S with activated carbon (AC), carbon black and PTFE binder. A huge irreversible capacity of 676 mAhg^-1 (theoretical capacity of 687 mAhg^-1) was observed below 3.8 V vs. Na⁺/Na during galvanostatic oxidation/reduction of Na₂S/AC in a cell where metallic sodium was used as counter and reference electrode. A NIC cell was assembled with Na₂S/AC positive electrode, Sn-P/HC negative electrode and a Na metal pin reference electrode. The electrolyte was 1 mol L^-1 NaClO₄ in a mixture of ethylene carbonate (EC) and propylene carbonate (PC) with 1:1 volume ratio. Once pre-sodiation was completed, AC became the active material for electrical double-layer charging, while reversible sodium insertion/deinsertion occurs at the Sn-P/HC electrode. The NIC system demonstrated excellent capacitance stability in the cell potential range 2.2 V - 3.8 V and a high energy density of 49 Wh kg^-1 at 500 mAg^-1. The obtained results confirm that the combination of HC/Sn-P intermetallic negative electrode with Na₂S sacrificial material is a promising solution for a new generation of NICs.

Acknowledgements

The authors acknowledge the Foundation for Polish Science for funding the HYCAP project (research grant TEAM TECH/2016-3/17).

References

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