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High-Rate Nanostructured Pyrite Cathodes Enabled by Fluorinated Surface and Compact Grain Stacking via Sulfuration of Ionic Liquid Coated Fluorides Keyi Chen, Ye Zhang, and Chilin Li ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b06660 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018
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High-Rate Nanostructured Pyrite Cathodes Enabled by Fluorinated Surface and Compact Grain Stacking via Sulfuration of Ionic Liquid Coated Fluorides Keyi Chen†,‡, Ye Zhang†,‡ and Chilin Li†,* †State
Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. Email:
[email protected] ‡Center of Materials Science and Optoelectronics Engineering,University of Chinese Academy of Sciences, Beijing 100049, China. ABSTRACT: Metal-polysulfide batteries are attracting broad attentions as conversion reaction systems of high theoretical energy density and low cost. However, their further applications are hindered by low loading of active species, excess conductive additive and loose (nanostructured) electrode network. Herein, we propose that compact grain stacking and surface fluorination are two crucial factors for achieving high-rate and long-life pyrite (FeS2) cathode enabled by sulfurating ionic liquid wrapped open framework fluorides. Both the factors can accelerate the Li- and Na-driven transport across pyrite-electrolyte interface and conversion propagation between adjacent grains. Such an electrode design enables a highly reversible capacity of 425 mAh/g after 1000 cycles at 1C for Li-storage and 450 mAh/g after 1200 cycles at 2C for Na-storage even under high loading of pyrite grains and ultrathin carbon coating (< 2 nm). Its cathode energy density can reach to 800 and 350 Wh/kg for Li and Na cells respectively under a high power density of 10000 W/kg. The cross-linkage between ionic liquid and fluoride precursors appears to be a solution to the reinforcement of surface fluorination.
Keywords: pyrite FeS2, conversion cathode, ionic liquid, open framework fluoride, fluorinated carbon layer, alkali metal batteries
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Rechargeable Li and Na metal batteries are receiving more attentions recently due to their much higher energy densities than their counterparts of alkali-ion batteries, in view of the extremely high theoretical capacities of Li (3860 mAh/g) and Na (1166 mAh/g) as well as their low reaction potentials.1 However their commercialization is hindered not only by the roughening of anode (or dendrite growth), but also by the lacking of electrochemically stable cathode based on multi-electron transfer or conversion reaction. In the typical metal-O2 and metal-S8 batteries, the molecule cathodes are easy to diffuse or be converted into dissoluble species (e.g. polysulfides, peroxide and superoxide), leading to the difficulty of reaction zone confinement and loss of active species.2 These (intermediate) products likely migrate to the anode side, and cause substantial side reactions and accelerate anode roughening. Therefore it is highly desired to develop conversion-type fluoride and sulfide mineral phases of low solubility, which are expected to outperform conventional intercalation cathode (e.g. LiCoO2) in terms of energy density and cost, and to compete with S8 (or O2) cathode in terms of compaction density and cycling stability.3,4 Among them, inexpensive pyrite FeS2 enables a four-electron transfer with a theoretical capacity as high as 894 mAh/g.4 In FeS2, S-S moieties are diluted in Fe-S lattices, leading to a potential alleviation of polysulfide formation and dissolution during cycling. Different from S8 cathode, FeS2 is electrochemically more stable in ether electrolyte without extra additive (e.g. LiNO3) to reinforce solid electrolyte interface (SEI).5 Furthermore FeS2 crystal itself is much more conductive than S8 molecules and enables the building of internal conductive network (accompanied by the in-situ formation of Fe nanodomains as electron wires) during cycling.6,7 Therefore Li/Na-FeS2 batteries do not require an excess of inactive assistant components in electrode (e.g. redundant carbon framework to adsorb substantial polysulfides) and electrolyte (e.g. highly concentrated salt or additive to suppress polysulfide corrosion towards anode), which however are essential for metal-S batteries.5,8 Nanoparticle synthesis has been widely accepted as a solution to the better kinetics of FeS2 electrode in view of the shortened mass diffusion distance across bulk. However it seems that the improvement on rate performance by particle
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shrinkage strategy is still limited.9 The capacity degradation still exists especially when the upper cut-off voltage exceeds 2.4 V, since S molecules are prone to be formed at the charged state and then convert to dissoluble polysulfides in the following cycles.4 Actually the strategies of surface coating and solid state architecture appear to be more effective than particle downsizing in terms of capacity preservation, which benefits from the inhibition of polysulfide shuttle and active species loss. The embedding of FeS2 into thick carbon or polyacrylonitrile (PAN) matrix enables a significant improvement of cycling and/or rate performance even though without intentional nano-engineering.10,11 It benefits from the formation of robust external conductive network and the compaction of internal conductive network. However too thick coating would compromise the ratio of active species. The solid state architecture based on Li2S-P2S5 electrolyte can avoid the undesired step of polysulfide dissolution, but at the cost of solid-solid interface degradation during cycling.12 Moreover, the poor penetration of solid electrolyte components requires an extra blending with mixed conductive phase at the cathode side in order to reinforce the conversion reaction kinetics.13 Although many efforts have been done to address the capacity sustainability of FeS2, the true determining factor for kinetic upgrade is still not well disclosed. In this work, we propose that grain interconnection and surface doping are the dominant factors to push FeS2 into high-rate and long-life conversion cathode even with the cut-off voltage up to 3 V. Two submicron-scale FeS2 samples (denoted as P-FeS2 and H-FeS2) are synthesized by thermal sulfuration of ionic liquid (IL) coated open framework fluorides, pyrochlore FeF3·0.5H2O and hexagonal tungsten bronze (HTB) FeF3·0.33H2O respectively.14-16 The grain stacking manner highly depends on the morphology of fluoride precursors and residual amount of IL. Furthermore, an ultrathin fluorinated carbon coating (< 5 nm) is in-situ constructed on FeS2 surface by simultaneous carbonization of IL. The C-F moiety is potentially beneficial for the acceleration of mass (Li+ or Na+) transport across cathode-electrolyte interface as well as the reinforcement of interface stability.17 Pyrochlore-derivative FeS2 with more intimate grain interconnection can enable an achievement of extremely high rate
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(>10C) and long life (at least 1000 cycles) for Li- and Na-batteries. Such a performance outperforms those already reported,5,8,9-11,18-21 and endows FeS2 as a competitive cathode candidate for future Li/Na metal batteries. Our results also indicate that intentional nano-sizing, substantial coating, cut-off voltage limitation and solid state architecture are not indispensable for high-performance FeS2 systems.
Results and discussion Figure 1 illustrates the synthesis processes of P-FeS2 and H-FeS2. FeF3·0.5H2O and FeF3·0.33H2O as precursors were firstly synthesized in 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) IL ambience based on buttom-up and top-down methods using Fe(NO3)3·9H2O and FeF3·3H2O precursors respectively according to our previous reports.14,16 For the preparation of pyrochlore precursor, Fe(NO3)3·9H2O is firstly dissolved in BmimBF4 to release free Fe3+, which then combines with free F(i.e. fluorination) produced from the hydrolysis of BF4- anions, leading to the precipitation of FeF3·0.5H2O. For the preparation of HTB-type precursor, FeF3·3H2O undergoes an IL-induced slow dehydration process and topotactic solid-solid phase transformation to FeF3·0.33H2O under thermal condition. Both the methods enable a IL capping on fluoride surface or IL wetting at grain boundaries during the dehydration process. When these open framework fluorides are thermally sulfurated into pyrite FeS2, the residual IL is simultaneously pyrolyzed into carbon coating layer. As a comparison, we also prepared a C-free FeS2 (denoted as A-FeS2) by sulfurating commercial anhydrous FeF3. The sulfuration process can be expressed like this: 4FeF3·xH2O (s) + 5S2 (g) → 4FeS2 (s) + 2SF6 (g) + 4xH2O (g).19 Note that carbon yield derived from IL has been recently reported, however mainly based on few cyano/nitrile-containing ILs.22 The traditional BmimBF4 is difficult to be residual after pyrolysis. Its chemical interaction with fluoride as support platform provides an opportunity for BmimBF4 serving as carbon source. Both the hydrated fluorides show aggregate morphology with ultrafine nanoparticles (~10 nm in size) bounded by IL interlayer as shown in the transmission electron microscope (TEM) images of Figure S1 and S2. The pyrochlore nanocrystals
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appear to be more intimately bound into spherical secondary particles with a size of ~250-300 nm, which are further adhered with each other to form broader pyrochlore network.14 Numerous HTB nanocrystals are interconnected into micro-sized porous monolith.16 The linkage manner of precursor grains and distribution of IL interlayer are highly associated with the morphology of pyrite products. After sulfuration, the X-ray diffraction (XRD) patterns display the peaks corresponding to pure pyrite phase (JCPDS Card no. 42-1340) for P-FeS2, H-FeS2 and A-FeS2 samples (Figure S3), indicating a successful conversion from fluoride to sulfide. From the scanning electron microscope (SEM) images of Figure 2a, c and S4, P-FeS2 presents a compact grain stacking with grain size of 100-200 nm, whereas the grains of H-FeS2 are coarser in a broad particle size range of 200-600 nm. The roughening for the latter leads to a loose grain interconnection characterized by dominant point contact as opposed to the facet contact substantially existing for the former. The sufficient contact between grains can promote the charge and mass transport across grain boundaries and therefore is beneficial to the overall electrochemical performance as discussed later.6 The grain size distribution and grain contact situation for A-FeS2 are comparable to those for H-FeS2 (Figure S5). The element distribution mapping by energy dispersive X-ray spectroscopy (EDS) is disclosed in Figure 2b and d, where the C content at particle surface is evidently higher for H-FeS2 than for P-FeS2 (with a C/FeS2 ratio of 6.76 wt%). The existence of carbon coating on H-FeS2 is also indicated from the color contrast in dark-field image as well as from the line-scan analysis at selected region (Figure 2e). The attenuation of carbon layer is potentially beneficial to the merging of adjacent grains as shown in P-FeS2. From the (high revolution) TEM images of Figure 2f-i, one can note that the carbon layer on H-FeS2 lattice planes (e.g. with a d-spacing of 2.21 Å assigned to (211) plane) is homogeneous with a thickness of 3-4 nm, which is thicker than that (1-2 nm) on P-FeS2. The grain aggregation appears to be more serious for P-FeS2, agreeing with the observation of compact grain stacking in SEM. X-ray photoelectron spectroscopy (XPS) was performed to further confirm the surface species and chemical bonding of different FeS2 samples (Figure 3). Typical S
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2p spectra consist of two sets of doublet peaks (S 2p3/2 and S 2p1/2) corresponding to Fe–S and S–S bonds. They are fitted based on 2:1 area ratio of S 2p3/2 to 2p1/2 and 1.2 eV splitting space. For P- and H-FeS2, the peaks for Fe-S shift to the higher binding energies (at 163.6 and 162.4 eV for S 2p1/2 and 2p3/2) compared with those of A-FeS2 (at 163 and 161.8 eV for S 2p1/2 and 2p3/2), which is very close to the values of commercial FeS2.23 The peaks for S-S bond do not undergo the corresponding displacement, and are located at 164.3 and 163.1 eV for S 2p1/2 and 2p3/2. The positive displacement of Fe-S peaks should be associated with the F transfer and doping into surface carbon layer. The residual of higher electronegative F would influence the charge density of surface Fe-S moiety.24 The F doping into C is also indicated from the existence of C-F peaks in F1s and C1s spectra (at 687.4 and 289 eV respectively) with a F/C ratio of 2.5 mol%.25 The Fe-F signal at 685 eV is negligible in F1s, indicating a complete sulfuration of bulk fluorides.15 Note that the fraction of S-S moiety is higher for P- and H-FeS2 than for A-FeS2. This phenomenon implies that surface CFx can retard the escape of S-S moiety from FeS2 lattices and the formation of S-S deficient surface layer during pyrolysis. The potential adsorption of excess S molecules as interlayer between CFx and FeS2 cannot be ruled out.26 The attenuation of carbon coating in P-FeS2 is also confirmed by the weakening of C peak and its dopant peaks (e.g. C-F and C-N). The existence of C-N bonds (e.g. at 400.7 eV for graphitic-N and at 398.9 eV for pyridinic-N in N1s) derives from the pyrolysis of imidazole group in IL residue.27 The galvanostatic charge-discharge of various submicron-scale FeS2 samples for Li-storage was measured by using an electrolyte consisting of 1.0 M lithium bis(trifluoromethane)sulfonamide (LiTFSI) dissolved in diglyme (DGM), denoted as LiTFSI-DGM. The initial lithiation of pyrite is thought to involve a two-stage reaction (Figure S6), including FeS2 + 2Li+ +2e- → Li2FeS2 and Li2FeS2 + 2Li+ +2e- → 2Li2S + Fe0.4 The appearance of single plateau at 1.5 V indicates that both the reactions proceed simultaneously due to the relatively slow diffusion of Li+ into pyrite. During the first charge, there are two delithiation plateaus located at 1.75 and 2.4 V respectively. The low-voltage plateau is expected to be caused by the reactions Fe0 +
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2Li2S → Li2FeS2 + 2Li+ +2e- and Li2FeS2 → Li2-xFeS2 + xLi+ + xe- (0.5< x
