High-Capacitance Pseudocapacitors from Li⁺ Ion Intercalation in Nonporous, Electrically Conductive 2D Coordination Polymers

Abstract

Electrochemical capacitors (ECs) have emerged as reliable and fast-charging electrochemical energy storage devices that offer high power densities. Their use is still limited, nevertheless, by their relatively low energy density. Because high specific surface area and electrical conductivity are widely seen as key metrics for improving the energy density and overall performance of ECs, materials that have excellent electrical conductivities but are otherwise nonporous, such as coordination polymers (CPs), are often overlooked. Here, we report a new nonporous CP, Ni 3 (benzenehexathiolate) (Ni 3 BHT), which exhibits high electrical conductivity of over 500 S/m. When used as an electrode, Ni 3 BHT delivers excellent specific capacitances of 245 F/g and 426 F/cm 3 in nonaqueous electrolytes. Structural and electrochemical studies relate the favorable performance to pseudocapacitive intercalation of Li + ions between the 2D layers of Ni 3 BHT, a charge-storage mechanism that has thus far been documented only in inorganic materials such as TiO 2 , Nb 2 O 5 , and MXenes. This first demonstration of pseudocapacitive ion intercalation in nonporous CPs, a class of materials comprising thousands of members with distinct structures and compositions, provides important motivation for exploring this vast family of materials for nontraditional, high-energy pseudocapacitors.

Dual‐Ion Intercalation and High Volumetric Capacitance in a Two‐Dimensional Non‐Porous Coordination Polymer

“…We previously reported the synthesis of Ni 3 BHT, wherein Ni 2+ ions and BHT linkers are arranged in a densely‐packed 2D structure with an inter‐layer spacing of 3.45 Å (Figure 1) and SSA of 25 m 2 g −1 [39] . The sulfur atoms on the BHT moieties each coordinate with two Ni 2+ ions in square‐planar fashion to yield an extended π‐d conjugated framework with high electrical conductivity of 5 S cm −1 , sufficient for a potential use in ECs.…”

“…The distinction between non‐porous CPs and porous CPs, also known as MOFs, is important here: the latter have seen greater interest in the context of ECs because they naturally exhibit higher SSA, [34–38] while non‐porous CPs have largely been overlooked in this sense. Here, we show that Ni 3 (benzenehexathiol) (Ni 3 BHT), a non‐porous two‐2D CP that was recently shown to intercalate Li + , [39] reversibly intercalates not just cations, but also anions, thereby qualifying as a rare dual‐ion intercalation electrode material. Detailed electrochemical studies reveal a unique intercalation behavior in Ni 3 BHT that involves charge‐transfer events, often referred to as pseudocapacitive, under cathodic potentials, and purely capacitive behavior under anodic bias.…”

Dual‐Ion Intercalation and High Volumetric Capacitance in a Two‐Dimensional Non‐Porous Coordination Polymer

“…We previously reported the synthesis of Ni 3 BHT,wherein Ni 2+ ions and BHT linkers are arranged in ad ensely-packed 2D structure with an inter-layer spacing of 3.45 (Figure 1) and SSA of 25 m 2 g À1 . [39] Thes ulfur atoms on the BHT moieties each coordinate with two Ni 2+ ions in square-planar fashion to yield an extended p-d conjugated framework with high electrical conductivity of 5Scm À1 ,sufficient for apotential use in ECs.H aving highlighted the intriguing ability of Ni 3 (BHT) to intercalate Li ions,w es ought to explore its interaction with other mono and multi-atomic ions spanning both reductive and oxidative polarizations.Aqueous solutions of several inorganic and organic chlorides,s ulfates,a nd nitrates were used as the electrolytes for this study.T ests were performed in at hree-electrode cell configuration and the applied potentials are reported with respect to the open circuit potentials (OCP). This representation with OCP denotes the applied bias as either positive or negative with respective to the steady-state potential, allowing for ad irect comparison of cation (or anion) sorption in different electrolytes.N evertheless,p otentials are also given with respect to Ag/AgCl for comparison purposes.…”

“…[24,25] This ongoing discussion notwithstanding,b oth descriptions make it clear that intercalation electrodes for EC must have good electronic conduction, swift ion-transport pathways,a nd weak (if any) energy interactions with the electrolyte components.T he emerging class of 2D coordination polymers (CPs) meet these difficult criteria, and additionally offer tremendous compositional diversity by accommodating abundant metal ions and organic linkers in p-d conjugated 2D layers. [26][27][28][29][30][31][32][33] Thed istinction between non-porous CPs and porous CPs,also known as MOFs,isimportant here:the latter have seen greater interest in the context of ECs because they naturally exhibit higher SSA, [34][35][36][37][38] while non-porous CPs have largely been overlooked in this sense.H ere,w es how that Ni 3 (benzenehexathiol) (Ni 3 BHT), an on-porous two-2D CP that was recently shown to intercalate Li + , [39] reversibly intercalates not just cations,b ut also anions,t hereby qualifying as ar are dual-ion intercalation electrode material. Detailed electrochemical studies reveal aunique intercalation behavior in Ni 3 BHT that involves charge-transfer events, often referred to as pseudocapacitive,u nder cathodic potentials,a nd purely capacitive behavior under anodic bias.…”

Application of Ionic Liquids for Batteries and Supercapacitors

“…are usually used as pseudocapacitive electrode materials [ 2 , 14 , 15 , 16 , 17 ]. Pseudocapacitor electrodes can store energy via reversible faradic redox reactions or ion intercalation into layered structures of electrodes at the electrode/electrolyte interface, and they can reach high specific capacitance and energy density compared to EDLCs [ 18 , 19 , 20 ]. Hybrid supercapacitors can store charge in the combination of EDL and redox reactions.…”