N/S codoped carbon microboxes with expanded interlayer distance toward excellent potassium storage

Highlights

• N/S codoped carbon microboxes is prepared via a carbonization-etching process.

• The NSC delivers super-long cycle life and extraordinary rate ability.

• The excellent performance is attributed to the large interlayer spacing of NSC.

Abstract

Carbonaceous materials for potassium-ion batteries (PIBs) are quite attractive for the cost-effective feature. However, the cycle stability and rate performance of carbonaceous anode materials based on PIBs are limited, for the difficulty of accommodating large K ions (1.38 Å). Thus, the nitrogen (N) and sulfur (S) codoped carbon microboxes (NSC) with increased interlayer spacing is prepared via a carbonization-etching process. Unlike the traditional graphite, in which the K ions are difficult to insert into the restricted interlayer spacing, the NSC displays extremely large interlayer spacing of 0.412 nm, making the K ions more flexible during the insertion/extraction process in NSC. Moreover, ex-situ TEM study reveals that the layer structure of NSC is well-remained during the potassiation/depotassiation process. As a result, the NSC delivers super-long cycle life (180.5 mAh g⁻¹ at 500 mA g⁻¹ after 1000 cycles) and extraordinary rate ability (155.6 mAh g⁻¹ at 2 A g⁻¹), which makes the NSC a promising anode material for PIBs.

Introduction

Owning to the fast consumption of traditional fossil fuels, development of cost-effective and environmental-friendly energy storage system become urgently necessary [1], [2], [3]. Although conventional lithium-ion batteries (LIBs) has been widely applied in electric vehicles (EV), hybrid electric vehicles (HEVs) and electronic devices (ED), high uneven distribution and limited lithium resource on the earth could hinder the large-scale application of LIBs [4], [5]. Thus, exploring new-style energy storing device based on the earth-abundant resource is really needed. Potassium ion batteries (PIBs) and sodium ion batteries (SIBs) have attracted considerable interests due to the abundant nature of potassium and sodium resources (2.09 wt% and 2.36 wt% for K and Na in the earth crust, respectively), and their similar intercalation process to LIBs [6]. However, the redox potential of K⁺/K (−2.92 V vs. standard hydrogen electrode (SHE)) is much lower than that of Na⁺/Na (−2.71 V vs. SHE) and comparable to that of Li⁺/Li (−3.01 V vs. SHE) [7]. The lower redox potential for K⁺/K compared to Na⁺/Na suggests a higher cell voltage of PIBs. These outstanding features make the PIBs a promising candidate for the next generation energy storage system. Thus, vast of attempts of cathode (such as P3-type K_0.5MnO₂ [8] and P2-type K_0.65Fe_0.5Mn_0.5O₂ [9]) and anode materials (such as N/O dual-doped hard carbon [10] and N-doped carbon nanofibers [11]) on PIBs have been explored and displayed outstanding electrochemical performance.

As we all know, carbonaceous anode materials have been widely used in LIBs due to their effective cost and encouraging cycling stability [12], [13], [14], [15], [16]. However, it may can‘t work when applied in PIBs for the large radius of K ions (1.38 Å) [17], [18], which makes the insertion/extraction of K ions into/from the carbonaceous anode materials much harder than that of Li ions (0.76 Å) and lead to a worse electrochemical performance. Currently, many literatures have demonstrated that the electrochemical properties of carbonaceous materials in PIBs could be improved by doping N for that they have plenty of edges and defects, which could make the K ions adsorbed on them instead of inserting into their interlayers [19]. Generally, the adsorption behavior (pseudocapacitance effect) can accelerate K ions transportation rate and has no influence to the volume effect during charge-discharge process, which is helpful to increase the rate performance and cycling property [20]. Thus, these N-doped carbonaceous materials has achieved impressive electrochemical properties for the vast of pseudocapacitance effect [21]. However, the technological challenge on how to facilitate the insertion-extraction process of K ions in the carbonaceous materials is still existence for that the interlayer space of carbonaceous materials are quiet limited. So, an enhanced interlayer is urgently needed. S-doped, as an effective way to enhance the interlayer of carbonaceous materials, has been extensively investigated in multifarious electrochemical energy storage [22], [23]. But few attention has been focused on the application of S-doped carbonaceous materials in PIBs for that the doping of S may lower the electron transfer rate [24], [25], [26], [27]. Thus, preparing S-doped carbonaceous materials with advanced conductivity may be a viable option to heighten the electrochemical performance of carbonaceous anode materials in PIBs.

In past decades, metal-organic frameworks (MOFs), formed by the linking organic and metal sites, is quite attractive for its various structure, tunable properties and multiple applications [28], [29], [30], [31]. Herein, a N,S codoped carbon microboxes (NSC) derived from Co-based Metal-organic frameworks (MOFs) has been fabricated via an effectively carbonization-etching method as an anode materials for PIBs. Owning to the S (enhancing the interlayer space) and N (increasing the conductivity) codoped nature, in which the S-doped feature take the dominant position, the NSC delivered a distinguished cycling ability and outstanding rate performance. This work reveals that the NSC may be a promising candidate for anode in PIBs.