Sodium Ion Capacitor Using Pseudocapacitive Layered Ferric Vanadate Nanosheets Cathode

摘要

class="head no_bottom_margin" id="sec1title">IntroductionElectrochemical energy storage (EES) devices play an indispensable role in our daily life owing to their widespread applications in portable electronics, electric vehicles, and large-scale regenerative energy storage systems (, ). Among these, Li-ion batteries (LIBs) offer high energy density (150–200 Wh kg⁻¹) but not enough power density (<1 kW kg⁻¹) (, ). The electrostatic double-layer capacitance (EDLC) can offer transitory high power output (>1 kW kg⁻¹), but the low energy density (<10 Wh kg⁻¹) limits their further applications (, ). The goal of next-generation EES devices is to achieve high energy density close to that of LIBs and high power density close to that of EDLCs (, ). In this case, a method of combining the operation mechanism of both LIBs and EDLCs to simultaneously utilize their individual advantages for the charge storage processes is proposed, that is, the first-generation hybrid ion capacitors (HICs) (). Since 2001, Amatucci and co-workers () constructed a hybrid Li⁺-ion capacitor (LIC) by using an EDLC-type activated carbon (AC) cathode and a nanostructured battery-type Li4Ti5O12 anode, which delivered an energy density up to 20 Wh kg⁻¹ (∼3 times that of a conventional carbon-based supercapacitor). After that, many HICs were developed, such as Nb2O5//AC (), MoS2//AC (), Li3VO4//AC (), and so forth (, ). The energy densities of HICs were gradually improved (approached 140 Wh kg⁻¹), which show great promising applications.The market of lithium-relate EES devices is huge and is rapidly growing; unfortunately, the lithium resource is limited (href="#bib13" rid="bib13" class=" bibr popnode">Deng et al., 2018b). As an emerging technology that could complement current LIBs/LICs, sodium ion storage technology has attracted much attention due to the low cost and wide distribution of abundant sodium resource (href="#bib13" rid="bib13" class=" bibr popnode">Deng et al., 2018b, href="#bib32" rid="bib32" class=" bibr popnode">Luo et al., 2016, href="#bib34" rid="bib34" class=" bibr popnode">Ni et al., 2018, href="#bib38" rid="bib38" class=" bibr popnode">Ren et al., 2017). Having similar configurations as LICs, the general sodium ion capacitors (SICs) are using the AC as cathode and the battery-type or pseudocapacitive oxides/sulfides as anode (href="#bib16" rid="bib16" class=" bibr popnode">Dong et al., 2017, href="#bib44" rid="bib44" class=" bibr popnode">Wang et al., 2017a). Recently, many investigations on the SICs have been published, indicating the wide attention in this area. Several SIC configurations, such as NaTi2(PO4)3//AC (href="#bib48" rid="bib48" class=" bibr popnode">Wei et al., 2017a, href="#bib49" rid="bib49" class=" bibr popnode">Wei et al., 2017b), Nb2O5//AC (href="#bib12" rid="bib12" class=" bibr popnode">Deng et al., 2018a, href="#bib29" rid="bib29" class=" bibr popnode">Lim et al., 2016), TiO2//AC (href="#bib25" rid="bib25" class=" bibr popnode">Le et al., 2017), Ti(O,N)//AC (href="#bib16" rid="bib16" class=" bibr popnode">Dong et al., 2017), and Na2Ti3O₇//AC (href="#bib15" rid="bib15" class=" bibr popnode">Dong et al., 2016), show advantages in high-rate applications. Till now, most of these concepts have used the high surface area of EDLC-type AC as the cathode. The EDLCs utilize non-faradaic electrostatic ion adsorption at the surface or inside pores to store charge, and the storage capacity is very limited (href="#bib5" rid="bib5" class=" bibr popnode">Augustyn et al., 2014a). The battery-type Na₂Fe₂(SO₄)₃ cathode has been used for SIC with enhanced capacity and energy; however, its unstratified reaction kinetics when matched with high pseudocapacitive Ti₂C-MXene anode hindered the rate performance of the full capacitor (href="#bib43" rid="bib43" class=" bibr popnode">Wang et al., 2015). High-capacity cathode materials with excellent rate performance are relatively less explored, but much more are expected. The pseudocapacitance arises when reversible faradaic redox reaction at or near the surface of a material in contact with electrolyte, which delivers much higher capacitance than EDLCs but still with high rate capability (href="#bib5" rid="bib5" class=" bibr popnode">Augustyn et al., 2014a, href="#bib6" rid="bib6" class=" bibr popnode">Augustyn et al., 2014b, href="#bib31" rid="bib31" class=" bibr popnode">Lukatskaya et al., 2016). In this case, an appropriate pseudocapacitive cathode instead of an EDLC-type AC with much enlarged capacity is urgently required. Therefore, much higher energy density, close to battery level, will be expected. However, till date, this kind of cathode, especially for sodium storage, remains largely unexploited.Layered transition metal oxides (TMOs) with two-dimensional channels for Na⁺ ion intercalation are the most promising sodium storage materials (href="#bib13" rid="bib13" class=" bibr popnode">Deng et al., 2018b, href="#bib21" rid="bib21" class=" bibr popnode">Han et al., 2015, href="#bib52" rid="bib52" class=" bibr popnode">Yabuuchi et al., 2014). Recently, different phases and nano-morphologies of TMOs have shown attractive sodium storage performance (href="#bib11" rid="bib11" class=" bibr popnode">Dall’Agnese et al., 2015, href="#bib18" rid="bib18" class=" bibr popnode">Fang et al., 2017, href="#bib20" rid="bib20" class=" bibr popnode">Guo et al., 2017, href="#bib23" rid="bib23" class=" bibr popnode">Hwang et al., 2017, href="#bib37" rid="bib37" class=" bibr popnode">Raju et al., 2014, href="#bib41" rid="bib41" class=" bibr popnode">Su and Wang, 2013, href="#bib45" rid="bib45" class=" bibr popnode">Wang et al., 2017b). However, they rarely demonstrate pseudocapacitive storage abilities, especially for the ones used as cathodes. Wei et al. reported that the V₂O₅·nH₂O xerogel with large interlayer spacing (∼11.53 Å) delivered a high capacity up to 338 mAh g⁻¹ and an impressive pseudocapacitive sodium storage behavior (href="#bib46" rid="bib46" class=" bibr popnode">Wei et al., 2015a). However, the layered vanadium oxide xerogel structure was unstable during long-term cycles. The vanadates, derivatives of vanadium-based materials, with increased electronic conductivity and enlarged/stabilized layer structure without blocking ion diffusion show promising electrochemical performance (href="#bib17" rid="bib17" class=" bibr popnode">Durham et al., 2016). Sodium vanadate (Na_1.25V₃O₈ and Na_1.1V₃O_7.9) showed excellent cyclability and rate capability as a sodium storage cathode (href="#bib14" rid="bib14" class=" bibr popnode">Dong et al., 2015, href="#bib53" rid="bib53" class=" bibr popnode">Yuan et al., 2015). Among the families of vanadates, ferric vanadates are abundant and natural. The Kazakhstanite phase [Fe₅V₁₅O₃₉(OH)₉·9H₂O, known as Fe-V-O], one of the ferric vanadates, has a large layered spacing (d₀₀₂ = 10.51 Å), which is beneficial for sodium diffusion. However, the detailed charge storage mechanism and reaction kinetics are unclear yet.For anode materials, hard carbon (HC) is regarded as the most promising material (href="#bib27" rid="bib27" class=" bibr popnode">Li et al., 2018, href="#bib22" rid="bib22" class=" bibr popnode">Hou et al., 2017, href="#bib52" rid="bib52" class=" bibr popnode">Yabuuchi et al., 2014). HC has a “house-of-cards” structure, containing graphite-like microcrystallites and amorphous carbon. Liu and co-workers recently confirmed the “adsorption-intercalation” sodium storage process in HC materials (href="#bib36" rid="bib36" class=" bibr popnode">Qiu et al., 2017). In detail, the Na⁺ ions first absorb on the active sites on the HC surface, leading to a sloping voltage profile (above 0.2 V versus Na⁺/Na). Then, Na⁺ ions intercalate into the graphite-like microcrystallites, showing a flat voltage plateau (below 0.2 V versus Na⁺/Na). The electrochemical performance and the contribution of capacity from the adsorption and intercalation regions are related to the microstructure (href="#bib22" rid="bib22" class=" bibr popnode">Hou et al., 2017, href="#bib36" rid="bib36" class=" bibr popnode">Qiu et al., 2017). In this work, we utilize the capacitive adsorption mechanism of HC anode and match it with the high-rate pseudocapacitive cathode to assemble an SIC with both high energy and high power densities.Herein, the sodium storage performance of the layered Fe-V-O nanosheets (NSs) cathode is investigated, which delivers a high reversible sodium storage capacity up to 229 mAh g⁻¹ at 0.25 C (1C = 200 mA g⁻¹), excellent rate capability, and cycling stability. The pseudocapacitive sodium storage mechanism is further demonstrated by ex situ characterizations and detailed electrochemical kinetics analysis. Furthermore, we assemble an SIC, utilizing the high pseudocapacitive Fe-V-O cathode and capacitive adsorption HC anode (href="#bib22" rid="bib22" class=" bibr popnode">Hou et al., 2017, href="#bib36" rid="bib36" class=" bibr popnode">Qiu et al., 2017), which shows remarkable electrochemical performance. Owing to the pseudocapacitive Fe-V-O cathode having much higher capacity than that of AC, the assembled HC//Fe-V-O SIC delivers a high energy density of ∼194 Wh kg ⁻¹, which is very close to battery level. Meanwhile, the SIC shows excellent high average power density up to 3,942 W kg⁻¹ with a high energy density of 32 Wh kg⁻¹. This work demonstrates that pseudocapacitive cathodes are promising candidates for the next-generation SICs with both high energy and power density.