MXene-Based Electrode with Enhanced Pseudocapacitance and

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MXene-Based Electrode with Enhanced Pseudocapacitance and Volumetric Capacity for Power-Type and Ultra-Long Life Lithium Storage Shanshan Niu, Zhiyu Wang, Mingliang Yu, Mengzhou Yu, Luyang Xiu, Song Wang, xianhong wu, and Jieshan Qiu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01459 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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MXene-Based Electrode with Enhanced Pseudocapacitance and Volumetric Capacity for Power-Type and Ultra-Long Life Lithium Storage Shanshan Niu, Zhiyu Wang,* Mingliang Yu, Mengzhou Yu, Luyang Xiu, Song Wang, Xianhong Wu and Jieshan Qiu* State Key Lab of Fine Chemicals, School of Chemical Engineering, Liaoning Key Lab for Energy Materials and Chemical Engineering, Dalian University of Technology, Dalian 116024, Liaoning, China KEYWORDS MXene, molybdenum carbide, lithium-ion batteries, pseudocapacitance, volumetric capacity

ABSTRACT Powerful yet thinner lithium-ion batteries (LIBs) are eagerly desired to meet the practical demands of electric vehicles and portable electronic devices. However, the use of soft carbon materials in current electrode design to improve the electrode conductivity and stability does not afford high volumetric capacity due to their low density and capacity for lithium storage. Herein, we report a strategy leveraging the MXene with superior conductivity and density to soft carbon as matrix and additive material for comprehensively enhancing the power capability, lifespan and volumetric capacity of conversion-type anode. A kinetics-favorable 2D nanohybrid with high conductivity, compact density, accumulated pseudocapacitance and

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diffusion-controlled behavior is fabricated by coupling Ti3C2 MXene with high-density molybdenum carbide for fast lithium storage over 300 cycles with high capacities. By replacing carbonaceous conductive agent with Ti3C2 MXene, the electrodes with better conductivity and dramatically reduced thickens could be further manufactured to achieve 37-40% improvement in capacity retention and ultra-long life of 5500 cycles with extremely slow capacity loss of 0.002 % per cycle at high current rate. Ultrahigh volumetric capacity of 2460 mAh cm-3 could be attained by such MXene-based electrodes, highlighting the great promise of MXene in the development of high-performance LIBs.

Lithium-ion batteries (LIBs) have been recognized as very appealing power source for portable electronic devices (PEDs) and low-emission electric vehicles (EVs) due to the environmental benignity, high energy density and high efficiency.1 The surging demand in these high energy-consuming applications boosts a great deal of interest in the innovation of highperformance electrode materials that can store and deliver more energy efficiently. Among available candidates, the alloying or conversion-type anode materials have attracted great attention because of their high capacity originated from multiple electron involved redox chemistry.2,3 However, their lifetime and charging rate is still far from satisfactory due to sluggish kinetics of solid-state redox process associated with poor ionic/electronic conductivity and huge volume change upon repeated lithium uptaking. Natively conductive and flexible carbonaceous materials (e.g., carbon black, graphene, carbon nanotubes, etc.) have been extensively employed as the matrix materials and/or conductive agent to trickle these issues.4-9 Nevertheless, their presence has introduced fundamental drawbacks to LIBs, including a low


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volumetric capacity as a result of the intrinsically low packing density (< 0.5 g cm−3), limited electrical properties due to the high interparticle resistance and low theoretical capacity for lithium storage, which undoubtedly inhibits the development and application of highperformance LIBs. Recently, two-dimensional (2D) early transitional metal carbides/carbonitrides (e.g., Ti3C2, Ti2C, Nb2C, V2C, Ti3CN and MoC, etc.) with a designation of MXene have triggered great attention in energy-related applications.10-13 The merit of MXene lies in the well combination of a variety of attractive properties such as natively high conductivity, highly hydrophilic properties and rich surface chemistry enabled by grafted chemical groups (e.g., -OH, -O and -F) in kineticsfavorable 2D nanostructure.14 For lithium storage, the MXene not only resemble the pseudocapacitive behavior in supercapacitors, but also exhibit low ionic diffusion barrier for Li+ (0.07 eV), which allows the MXene-based electrodes to be operated at very high current rate (e.g., 36 C).15-19 Moreover, the MXene may hold great promise in high volumetric energy storage because of their superior density to soft carbon materials that have been extensively used as matrix or additive materials in current LIB design.20 Free-standing additive-free MXene electrodes typically possess high densities of 3-4 g cm-3 because carbides are much denser than soft carbon, rendering outstanding volumetric capacitances up to 900 F cm-3 in supercapacitors.21-23 Recently, over 2.5 folds higher volumetric capacities than that of graphite anode (550 mAh cm-3) have been also demonstrated by unexfoliated Ti3C2 MXene decorated with 5 wt.% Sn4+ at a current density of 0.1 A g-1.24 These attractive merits provide great opportunities for enhancing the power output capability and volumetric energy of LIBs, which to some extent is much more crucial than gravimetric energy for practical use.


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In this work, we propose a concept for rational design of power-type anode materials with high volumetric capacities by coupling the MXene with transitional metal carbides (TMCs). The TMCs is attractive conversion-type anode materials due to the good conductivity, high density and high capacities for lithium storage.25-31 Their persistent hardness further brings additional benefit in tolerance of volume-change induced strain upon lithium insertion/extraction for better electrode stability.32 Specifically, molybdenum carbides are chosen to demonstrate our concept because of their high theoretical capacities (526 mAh g-1 for Mo2C and 993 mAh g-1 for MoC), high density (8.8-9.18 g cm-3) and excellent conductivity (e.g., 1.02 × 102 S cm-1 for Mo2C) originated from the electronic structure resembling to precious metals.33-35 A kinetics-favorable 2D nanohybrid with excellent conductivity, high density, accumulated pseudocapacitance and diffusion-controlled behavior is fabricated by homogenously dispersing ultrafine η-MoC nanocrystallites on 2D sheets assembled from Ti3C2 MXene (denoted as η-MoC/MXene/C). The synergy of MXene and molybdenum carbide nanocrystallites not only reduces the loss of active surface caused by particle aggregation, but also contribute to prominent pseudocapacitive behavior for fast yet stable lithium storage. When evaluated as the anode materials in LIBs, the η-MoC/MXene/C nanohybrids exhibit nearly 100 % capacity retention for 300 cycles with high capacities and excellent high-rate response up to 20 A g-1. By replacing carbonaceous conductive agent with Ti3C2 MXene, the electrodes with better conductivity and greatly reduced thickens could be further manufactured to achieve faster ionic diffusion, lower electrode polarization and higher volumetric capacity. Over 37-40% improvement in capacity retention and ultra-long life of 5500 cycles with extremely slow capacity loss of 0.002 % per cycle is achieved at high current rate. High volumetric capacity of over 2460 mAh cm-3 could be also attained by fully integrating the merits of dense molybdenum carbides and MXene for electrode design.


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RESULTS AND DISCUSSION The η-MoC/MXene/C nanohybrids are synthesized via polymer cross-linked adsorption of Mo-based polyoxometalate (Mo-POM) on MXene and subsequently in-situ conversion to η-MoC at high temperature. Firstly, high-quality Ti3C2 MXene nanosheets with an average lateral size of hundreds of nanometers are made by selectively etching the Al layers in Ti3AlC2 MAX phase with LiF/HCl, followed by the exfoliation under ultrasonic (Figure S1). Afterwards, the surface of the MXene is modified with polyvinyl pyrrolidone (PVP), which may interact with Mo-POM via the electrostatic attraction between polymeric POM anion and protonated PVP chains, and the coordination of metal atoms in POM to strong polar groups of PVP (e.g., >C=O).36 The strong affinity of Mo-POM towards PVP-modified MXene facilitates the homogenous growth and distribution of η-MoC nanocrystallites on MXene via i) the desolvation/dehydration of MoPOM and polycondensation of PVP below 500 oC, which causes a weight loss of ca. 15 wt.%; ii) the decomposition of Mo-POM and PVP to η-MoC at higher temperature, leading to a weight loss of ca. 22 wt.% until 800 oC (Figure S2). A panoramic scanning electron microscopy (SEM) view reveals the η-MoC/MXene/C nanohybrids possess a sheet-like structure with highly wrinkled surface. Their lateral size is much larger than that of pristine MXene nanosheets, implying that they are actually the assembly of individual MXene nanosheets instead of a single crumpled one (Figure 1a). In the absence of MXene as supporting matrix, however, only the aggregates of larger η-MoC particles are yielded (Figure S3). The transmission electron microscopy (TEM) examination indicates the presence of numerous ridges on wrinkled ηMoC/MXene/C sheets (Figure 1c). These ridges represent the boundary region where the individual MXene nanosheets are joined together perhaps driven by a ‘-MXene-PVP-POM-PVPMXene-’ interaction with PVP as a cross-linker (Figure 1d). The formation of assembled MXene


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sheets with large lateral size assures superior electrical contact via enhanced topographical overlapping and interlock between individual units. As a result, rapid yet smooth charge transfer could be facilitated in long range order for faster lithium storage. Closer TEM observation shows the homogenous dispersion of ultrafine η-MoC nanocrystallites particles with tiny size of several nanometers and high density on MXene sheets (Figure 1e). The η-MoC nanocrystallites anchored on MXene have a narrow size distribution in the range of 2-10 nm (Figure S4). Their formation is vitally important to provide sufficient active sites with reduced diffusion barrier for lithium intercalation and redox conversion.37 While The high-resolution TEM (HRTEM) analysis shows that the η-MoC nanocrystallites are embedded in amorphous carbon matrix derived from PVP on MXene sheets. Clear lattice fringes of ca. 0.24 nm is identified for the (006) planes of hexagonal η-MoC (Figure 1f). The presence and homogenous distribution of Ti, Mo and C elements in η-MoC/Ti3C2-MXene/C nanohybrids is validated by elemental mapping analysis (Figure 1g). The content of Mo and Ti in the nanohybrids is ca. 56 and 12.8 wt.%, measured by inductively coupled plasma optical emission spectroscopy (ICP-OES). Accordingly, the content of η-MoC and Ti3C2 in the sample is estimated to be ca. 63 and 15 wt.% in terms of their stoichiometry. The accurate content of PVP-derived carbon is hardly measured owing to the disturbance from the C element in Ti3C2 MXene and η-MoC. Nevertheless, it should be less than 22 wt.% by subtracting the total weight of η-MoC and Ti3C2 in the nanohybrids. The structural characteristics of η-MoC/MXene/C nanohybrids are determined by X-ray diffraction (XRD), as shown in Figure 2a. The diffraction peaks at 37.4o, 39.3o, 61.4o, 74.4o can be assigned to the reflections from the (006), (103), (110) and (116) planes of hexagonal η-MoC phase (JCPDS no. 08-384), respectively. The small peak at around 6.5o is assigned to the (002) plane of Ti3C2 MXene, while the broad peak in the range of 20-30o is ascribed to disordered


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carbon derived from PVP. Without PVP, however, the product made by similar way is composed of large aggregate of MoO2 and partially oxidized MXene sheets (Figure S5). Clearly, the PVP plays a vitally important role in the formation of η-MoC/MXene/C structure. It not only acts as the carbon precursor and reductant for converting Mo-POM to η-MoC phase, but also enhances the structural stability of MXene by shielding them with a PVP-derived carbon layer. The formation of PVP-derived disordered carbon is confirmed by Raman spectra, as characterized by the pronounced D and G bands at 1357 and 1595 cm-1 with a relatively low intensity ratio (ID/IG) of 1.02 (Figure 2b).38 Besides these two peaks, the signals from molybdenum carbides are also identified at 817 and 992 cm-1.39 The composition and surface chemical state of ηMoC/MXene/C nanohybrids is analyzed by X-ray photoelectron spectroscopy (XPS). The XPS full-scan survey shows the co-existence of Ti, C and Mo element in the nanohybrids (Figure S6). The high resolution Mo 3d XPS spectrum can be resolved to four pairs of 3d5/2/3d3/2 doublets, including two pairs of pronounced peaks for Mo0 (228.0/231.2 eV) and Mo3+ (228.6/231.8 eV), and the other two pairs of weak signals for Mo4+ (230.2/233.4 eV) and Mo6+ (232.2/235.3 eV)(Figure 2c).40,41 The domination of Mo species with lower valence confirms the formation of molybdenum carbides instead of the oxides. The Ti 2p spectrum can be deconvoluted into four pairs of 2p3/2/2p1/2 doublets for Ti-C (455.0/460.7 eV), Ti2+ (456.3/462.0 eV), Ti3+ (457.3/462.92.0 eV) and Ti-O (458.9/464.7 eV) (Figure 2d).42 The texture of Ti3C2 MXene is well preserved after the loading of η-MoC, as characterized by the strong Ti-C but very weak TiO signals in XPS spectrum. It allows the merits of MXene to be fully utilized to enhance the electrochemical performance for lithium storage. The integration of ultrafine η-MoC nanocrystallites into MXene with highly opened nanostructure renders the resultant nanohybrids with a high Brunauer-Emmett-Teller (BET) specific surface area of around 127 m2 g-1 (Figure


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S7). Compared to the product made without MXene, the η-MoC/MXene/C nanohybrids exhibit broader pore size distribution due to their multi-level structure with hierarchical porosity. Specifically, the PVP-derived disordered carbon and interspace between η-MoC crystallites mainly contribute to the micro/mesopores with a size distribution below 20 nm; while the larger meso/macropores are from the secondary piled pores of η-MoC/MXene/C nanostructure. Such structure is favorable to accelerate the kinetics of mass transport and ionic diffusion across the ηMoC/MXene/C electrodes for fast lithium storage. Four probe tests confirm the excellent electrical conductivity of η-MoC/MXene/C nanohybrids ( ca. 1.2 × 104 S m-1). It is nearly a half of the value of pristine Ti3C2 MXene (ca. 2 × 104 S m-1) but much higher than that of amorphous carbon (1.25-2 × 103 S m-1).43 This advantage is vitally important to achieve low electrode polarization and high utilization of active materials upon lithium storage at high current rate. The electrochemical route of η-MoC/MXene/C electrodes towards lithium storage is monitored by cyclic voltammetry (CV) tests within a cut-off window of 0.01-3.0 V at a scan rate of 0.1 mV s-1 (Figure S8). The irreversible cathodic peak at around 0.58 V at the first cycle is due to the irreversible process such as the formation of a solid electrolyte interface (SEI) film containing Li2CO3 and alkyl carbonates via electrolyte decomposition and the trap of the lithium in crystal lattice.44,45 The phenomenon is common to most anode materials based on redox conversion or alloying mechanism.46 In subsequent cycles, two pairs of cathodic/anodic peak centered at around 1.12/1.40 V and 1.71/2.32 V are identified. The former is due to the reduction of η-MoC to metallic Mo and its reverse process (xLi + MoC ↔ Mo0 + LixC), while the latter is a result of lithium storage in Ti3C2 MXene and absent in CV of MXene-free η-MoC/C.47,48 For ηMoC/MXene/C electrode, the redox peaks at 1.71/2.32 V is much sharper than that of MXene electrode, showing greatly enhanced electrochemical activity of MXene in η-MoC/MXene/C


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structure. This phenomena can be ascribed to two reasons: i) the removal of most chemical groups on MXene in η-MoC/MXene/C by annealing at high temperature minimizes the blocking effect on Li transport;49 ii) the η-MoC nanocrystallites anchored on MXene surface may act as the interlayer spacers to prevent the MXene from intersheet aggregation, reducing the loss of electrochemically active surface area. Overall, the CV curves are superimposable after the initial cycle, showing high reversibility of η-MoC/MXene/C electrodes for lithium uptaking. On this basis, the CV tests at various scan rates from 0.2 to 10 mV s-1 are conducted to analyze the kinetics of lithium storage in η-MoC/MXene/C electrodes (Figure 3a). The relationship between the measured current (i) and scan rate (ν) can be described by an equation of i = aνb, where a and b are empirical constants.50 An ideal process controlled by bulk diffusion or capacitive behavior may account for a b value of 0.5 and 1.0, respectively. By plotting log i vs. log ν at various discharge stage (the inset in Figure 3b), the b values of η-MoC/MXene/C electrode are determined to be 0.6-1.0. It indicates a coupled diffusion-controlled and pseudocapacitive process for lithium storage in η-MoC/MXene/C electrodes. The diffusion-controlled process contributes to high capacity via multiple electron involved redox reaction, while pseudocapacitive behavior assures fast charging-discharging via ionic adsorption and redox reaction on the near surface of the nanohybrids.50-52 Accordingly, the current response (i) to a particular voltage (V) at certain scan rate (ν) could be described by sum of the capacitive contribution (k1ν) and diffusion-controlled contribution (k2ν1/2) with an exponential relationship of i (V) = aνb = k1ν + k2ν1/2, where k1, k2, and a stand for a constant, v is the scan rate.50,52 By calculating the value of k1 and k2 at different voltages, the ratio of capacitive and diffusioncontrolled contribution can be quantitatively determined (Figure 3c and Figure S9). For ηMoC/MXene/C electrodes, the lithium storage at slow scan rate below 1 mV s-1 is dominated by


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diffusional process with capacity contribution above 60%. With scan rate increasing from 2 to 10.0 mV s-1, the capacitive contribution rises rapidly from 52% to 75%, which is critical to achieve high power rate under high current rates. Without MXene, the η-MoC/C electrodes exhibit much lower capacitive contribution especially at fast scan rates, revealing the prominent effect of the MXene on lithium storage behavior of η-MoC/MXene/C nanohybrids (Figure 3c and Figure S10). Electrochemical impedance spectroscopy (EIS) is employed to further verify the kinetics of ionic diffusion and charge transfer in η-MoC/MXene/C nanohybrids (Figure 4b). The η-MoC/MXene/C electrodes show a much lower charge-transfer impedance (Rct = 83.8 Ω) than that of MXene-free η-MoC/C electrodes (270.6 Ω), as characterized by a drastically reduced diameter of the semicircle at high-frequency region in Nyquist plots. On the other hand, the ηMoC/MXene/C electrodes exhibit a much larger slope than that of η-MoC/C electrodes at lowfrequency region in Nyquist plots, indicating the greatly reduced Warburg impedance for solidstate ionic diffusion. The Warburg factor (δ) of η-MoC/MXene/C electrodes (δ = 48.6) is decreased by over six times of η-MoC/C electrodes (δ = 307.8). It corresponds to over 36 folds faster lithium diffusion rate in η-MoC/MXene/C with respect to η-MoC/C since the lithium diffusion coefficient (D) is inversely proportional to the square of δ with a relationship of D = R2T2/2A2n4F4C2δ2, where R is universal gas constant, T means the absolute temperature, A is the area of the electrode, n is the number of electrons involved, F is the Faraday constant, C is the concentration of lithium ions.53 The significant enhancement of η-MoC/MXene/C in ionic diffusion kinetics may be attributed to several aspects: i) the η-MoC nanocrystallites with tiny size provide sufficient and extremely shortened diffusion pathway; ii) the MXene reduces the loss of active surface by effectively preventing the η-MoC from aggregation; iii) highly opened 2D nanostructure allows fast accessibility to lithium ions in the electrolyte.


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Apparently, the presence of MXene is critical to enhance the electrochemical properties of ηMoC/MXene/C electrodes in term of the electrical conductivity, lithium storage behavior and ionic/charge transport kinetics. As a result, they exhibit excellent electrochemical performance as the anode materials in LIBs. Figure 4a shows the representative discharge/charge voltage profiles of η-MoC/MXene/C electrodes for the first two cycles at a current density of 0.2 A g-1 within a voltage window of 0.01-3.0 V. A high initial discharge and charge capacities of 1050 and 750 mAh g-1 can be achieved with a relative low initial capacity loss (ICL) of 28.5 %. The ICL is the result of the irreversible process mentioned above, being consistent with the CV results. From the second cycle onwards, the capacities of η-MoC/MXene/C electrodes increases gradually from 757 to 780 mAh g-1 within 300 cycles with a high Coulombic efficiency of around 98-99 % at 0.2 A g-1 (Figure 4b and Figure S11). Strong affinity between the MXene, η-MoC nanocrystallites and PVP-derived carbon imparts the η-MoC/MXene/C nanohybrids with high robustness against the volume change upon repeated cycling. Their structure could be well retained after 300 cycles without appearing degradation (Figure S12). The gradual capacity rise with cycling can be ascribed to the reversible formation of organic polymeric/gel-like layer by electrolyte decomposition, which delivers excess capacity at low potential via a pseudocapacitive effect.54-56 The gradual activation of active materials upon deep cycling may also contribute to this phenomenon.57 To investigate this, the differential capacity vs. voltage curves of 50th, 100th, 200th and 300th cycles are plotted for η-MoC/MXene/C electrodes (Figure S13). The gradually increased intensity of the peaks at around 1.25 and 1.47 V with cycling confirms the enhanced reversibility of the conversion reaction for lithium storage. The pristine Ti3C2 MXene exhibit a low capacities of around 210 mAh g-1 at 0.2 A g-1, which contributes a low capacity of around 30 mAh g-1 to total capacities of η-MoC/MXene/C electrode.


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Benefited from excellent conductivity, enhanced pseudocapacitive properties and kineticsfavorable structure, the η-MoC/MXene/C nanohybrids exhibit superb rate response against varied current rates. The discharge-charge curves of η-MoC/MXene/C electrodes show a sloping feature without distinct plateau regions at various current rates, confirming the prominent effect of pseudocapacitive behavior on lithium storage (Figure S14). As a result, high capacities of 470 and 430 mAh g-1 could be retained even cycled at high current densities of 10.0 and 20.0 A g-1. These high capacities correspond to 60-65 % of the discharge capacities at 0.2 A g-1 although the current densities increase 100-200 folds higher, respectively. In contrast, the MXene-free ηMoC/C electrodes deliver much lower capacities even at low current densities and lost most of the capacities at high current rate above 10 A g-1 (Figure 4c). After deep cycling at 20.0 A g-1, high capacities of over 700 mAh g-1 could be recovered for η-MoC/MXene/C electrodes by abruptly switching the current density back to 0.2 A g-1, showing the outstanding electrochemical reversibility and electrode robustness. Such a high-rate capability is superior to most TMC-based anodes (Figure 4d).28, 58-64 Moreover, the high-rate capability of η-MoC/MXene/C nanohybrids with different η-MoC content is also investigated (Figure 4c and Figure S15). The sample with higher η-MoC content (e.g., 76 wt.%) suffers from fast capacity decay because of extensive growth and severe aggregation of MXene-free η-MoC particles. While the capacities of the sample with lower η-MoC content (e.g., 34 wt.%) is limited by the lack of sufficient active phase. Furthermore, the combination of MXene and high-density molybdenum carbide imparts the ηMoC/MXene/C nanohybrids with a high density of around 2.6 g cm-3, which makes them particularly attractive for high volumetric lithium storage. High volumetric capacity of 2010 mAh cm-3 could be attained at a current density of 0.2 A g-1, which is nearly 4.5 folds higher than that of graphite anode (550 mAh cm-3) and is comparable to many metal oxide, Si and Sn-based


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anodes (Figure 5a and Table S1).9,

24, 65-76

When cycled at high rate of 10-20 A g-1, high

volumetric specific capacities of 1580-1660 mAh cm-3 may still remain for use, rendering the potential for the storage of more energy in small volume of the cells. The performance of η-MoC/MXene/C electrodes could be further boosted by replacing the extensively used carbon black (CB) with Ti3C2 MXene as the conductive agent for electrode manufacturing. The η-MoC/MXene/C electrodes made with 20 wt.% MXene as the conductive agent exhibit significantly reduced resistance for charge transfer (Rct = 41.2 Ω) than that of the ones made with the same mass ratio of CB (Rct = 83.8 Ω)(Figure 3d), which is favorable to reduce the electrode polarization at high current rate. The enhancement of MXene-based electrodes in conductivity can be ascribed to several reasons: i) the much higher conductivity of Ti3C2 MXene (ca. 2 × 104 S m-1) than that of CB (ca. 1 × 104 S m-1) measured by four probe tests; ii) using denser MXene as auxiliary additive allows the thickness of electrode film to be cut by nearly half of the CB-involved electrodes, which further reduces the internal resistance (Figure 5b); iii) reduced interparticulate resistance by integrating MXene-based 2D active material and conductive agent with enhanced topographical overlapping and superior electrical contact (Figure 5c and Figure 5d). On the other hand, the improvement in electrode thickness further brings the benefit in accelerating the lithium diffusion kinetics of η-MoC/MXene/C electrodes. It manifests as the much lower Warburg factor (δ = 18.5) than that of the CB-involved electrodes (δ = 48.6) (Figure 3d). Compared to CB-involved electrodes, the MXene-based electrodes exhibits smaller hysteresis between the charge and discharge voltage, suggesting accelerated redox conversion with reduced electrode polarization (Figure 4a). As a result, the MXene-based electrodes may deliver around 20 % higher capacities (947 mAh g-1) than that of the CBinvolved electrodes at a current density of 0.2 A g-1 after 300 cycles in LIBs. At high current


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densities of 10.0 and 20.0 A g-1, the capacity retention of MXene-based electrodes (610-650 mAh g-1) is 37-40 % higher than that of CB-involved electrodes. Remarkably, ultra-long life up to 5500 cycles with stable capacity retention of over 90 % and very slow capacity loss of 0.002 % per cycle could be achieved since the 2nd cycle onwards at a high current density of 10 A g-1 for them. Because of the improvement in gravimetric capacities, the volumetric capacity is enhanced to 2460 mAh cm-3 at a current density of 0.2 A g-1 accordingly. To the best of our knowledge, this is among the highest of volumetric-specific capacities based on active material among reported results (Figure 5a and Table S1). Even cycled at a high rate of 10 A g-1, competitive volumetric capacity of above 1560 mAh cm-3 could still remain for use after 5500 cycles (Figure 4e). Considering the total electrode, the MXene-based electrodes with dramatically reduced thickness further exhibit superior volumetric capacity (668 mAh cm-3) to CB-involved electrode (292 mAh cm-3) with similar mass ratio of active material and auxiliary additives. This feature, alongside excellent power output and long life, is very appealing for PED and EV applications where fast charge of high energy in limited battery pack is eagerly desired.

CONCLUSIONS In summary, we report a facile strategy for the fabrication of 2D η-MoC/MXene/C nanohybrids with excellent electrical/ionic conductivity, kinetics-favorable structure and high density by coupling ultrafine η-MoC nanocrystallites with assembled Ti3C2 MXene sheets with large lateral size. The introduction of MXene plays multiple roles in enhancing the electrode conductivity, immobilizing the η-MoC nanocrystallites and accelerating the pseudocapacitance for lithium storage. The η-MoC/MXene/C electrodes exhibit excellent high-rate response up to 20 A g-1 and stable capacity retention for 300 cycles with high capacities via an accumulated


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diffusion-controlled and pseudocapacitance mechanism. Replacing the carbon black by MXene as the conductive agent leads to more conductive and thinner electrode film. As a result, the electrochemical performance of η-MoC/MXene/C nanohybrids could be further boosted to achieve exceptional high-rate capability and ultra-long cycle life of 5500 cycles at high current rate of 10 A g-1. Remarkably, high volumetric capacities that competitive to Si or Sn-based materials could be attained by successful integration of high-density molybdenum carbide and MXene with superior density to soft carbon.

EXPERIMENTAL SECTION Synthesis of Ti3C2 MXene. The Ti3C2 MXene was synthesized by etching 1.0 g of Ti3AlC2 powder in a mixture solution of LiF (1.32 g) and hydrochloric acid (6 M, 20 mL) for 24 h at 35 °C. After several centrifugation-rinsing cycles with deionized (DI) water, the products were dispersed in 500 mL of DI water and kept under ultrasonic for 3 h. The dark green supernatant was collected by centrifugation at 2000 rpm for 1 h and was stored at 4 oC in N2-filled bottles before use. Synthesis of η-MoC/MXene/C Nanohybrids. In a typical run, 0.15 g of phosphomolybdic acid hydrate was slowly added into a suspension of Ti3C2 MXene (1.9 mg mL-1, 20 mL) and PVP (K30, 5 mg mL-1). The mixture solution was kept under stirring for 24h at ambient condition. After several centrifugation-rinsing cycles with DI water, the products were annealed at 800 oC for 6 h in Ar flow with a ramp rate of 2 oC min-1. For compassion, the samples with various η-MoC content were also prepared by varying the initial amount of phosphomolybdic acid hydrate used. The η-MoC/C samples were produced by the similar approach in the absence of MXene.


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Material Characterization. The morphology of the samples was characterized with fieldemission scanning electron microscopy (FESEM, FEI NanoSEM 450) and transmission electron microscopy (TEM, Tecnai G2 20). Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2400 type X-ray spectrometer with Cu Kα radiation (λ = 1.5406 Å). Raman analysis were carried out on Thermo Fisher Scientiﬁc DXR Raman Microscope using laser excitation at 532 nm. The surface characteristics of the samples were investigated using Thermo ESCALAB 250 X-ray photoelectron spectrometer (XPS). The textural properties of the samples were measured by Micrometrics ASAP 2020 Surface Area and Porosity Analyzer at 77 K. The content of the elements in the sample was measured by coupled plasma optical emission spectroscopy (ICP-OES, Optima 2000DV, PerkinElmer, Inc.). The electrical conductivity of all the samples was measured by a ST2722 four probe tester under a constant pressure of 20 MPa. The density of the active material is the monolith density determined by Archimedes principle with a balance (Mettler Toledo XSE205DU) equipped with accessories. Lithium-ion Battery Test. The tests were conducted using CR2016 coin cells with pure Li foil as the counter and reference electrode at room temperature. The working electrode consists of an active material, conductive agent (e.g., Super P carbon black or Ti3C2 MXene) and PVDF in a weight ratio of 8:1:1. The electrolyte used is 1.0 M LiPF6 in a 50:50 (w/w) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). The mass loading of the electrodes is around 1.5 mg cm-2. Cell assembly was carried out in an Ar-filled glovebox with the moisture and oxygen below 1.0 ppm. The galvanostatic charge/discharge tests were performed using a LAND CT2001A battery tester at different current densities within a cut-off voltage window of 0.01-3.0 V. The specific capacities were calculated based on the total mass of the active materials. Cyclic voltammetry (CV) studies were conducted using a CHI 760D electrochemical


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workstation between 0.01-3.0 V at a scan rate of 0.1 mV s-1. The electrochemical impedance spectroscopy (EIS) measurements were carried out using a CHI660D workstation by applying the AC amplitude of 5 mV over the frequency range of 100 kHz to 0.01 Hz.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. More SEM, TEM, XPS, TGA, XRD, nitrogen adsorption-desorption isotherms and electrochemical data of the η-MoC/MXene/C and controlled samples. A comparison of ηMoC/MXene/C anode with recently reported anodes in electrochemical performance. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC, No. 51522203, 51772040), Fok Ying Tung Education Foundation (No. 151047) and Xinghai Scholarship of Dalian University of Technology. REFERENCES (1)

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(65) Lu Z.; Liu, N.; Lee, H.-W.; Zhao, J.; Li, W.; Li, Y.; Cui, Y., Nonfilling Carbon Coating of Porous Silicon Micrometer-Sized Particles for High-Performance Lithium Battery Anodes. ACS nano 2015, 9, 2540-2547. (66) Wang, X.; Lv, L.; Cheng, Z.; Gao, J.; Dong, L.; Hu, C.; Qu, L., High-Density Monolith of N-Doped Holey Graphene for Ultrahigh Volumetric Capacity of Li-Ion Batteries. Adv. Energy Mater. 2016, 6, 1502100. (67) Xia, F.; Kim, S. B.; Cheng, H.; Lee, J. M.; Song, T.; Huang, Y.; Rogers, J. A.; Paik, U.; Il Park, W., Facile Synthesis of Free-Standing Silicon Membranes with Three-Dimensional Nanoarchitecture for Anodes of Lithium Ion Batteries. Nano Lett. 2013, 13, 3340-3346. (68) Gowda, S. R.; Pushparaj, V.; Herle, S.; Girishkumar, G.; Gordon, J. G.; Gullapalli, H.; Zhan, X.; Ajayan, P. M.; Reddy, A. L. M., Three-Dimensionally Engineered Porous Silicon Electrodes for Li Ion Batteries. Nano Lett. 2012, 12, 6060-6065. (69) Han, F.; Li, D.; Li, W. C.; Lei, C.; Sun, Q.; Lu, A.-H., Nanoengineered PolypyrroleCoated Fe2O3@C Multifunctional Composites with an Improved Cycle Stability as Lithium-Ion Anodes. Adv. Funct. Mater. 2013, 23, 1692-1700. (70) Wan, Y.; Xu, X.; Liu, J.; Sha, Y.; Chen, Y.; Li, L.; Xue, G.; Wang, X.; Zhou, D., A ColdFlow Process for Fabricating a High-Volumetric-Energy-Density Anode for Lithium-Ion Batteries. Adv. Mater. Technol. 2017, 2, 1600156. (71) Jeong, M.-G.; Du, H. L.; Islam, M.; Lee, J. K.; Sun, Y.-K.; Jung, H.-G., SelfRearrangement of Silicon Nanoparticles Embedded in Micro Carbon Sphere Framework for High-Energy and Long-Life Lithium Ion Batteries. Nano Lett. 2017, 17, 5600-5606.


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(72) Dong, Y.; Wang, B.; Zhao, K.; Yu, Y.; Wang, X.; Mai, L.; Jin, S., Air-Stable Porous Fe2N Encapsulated in Carbon Microboxes with High Volumetric Lithium Storage Capacity and a Long Cycle Life. Nano Lett. 2017, 17, 5740-5746. (73) Liang, J.; Xi, K.; Tan, G.; Chen, S.; Zhao, T.; Coxon, P. R.; Kim, H.-K.; Ding, S.; Yang, Y.; Kumar, R. V.; Lu, J., Sea Urchin-Like NiCoO2@C Nanocomposites for Li-Ion Batteries and Supercapacitors. Nano Energy 2016, 27, 457-465. (74) Ryu, J.; Hong, D.; Shin, S.; Choi, W.; Kim, A.; Park, S., Hybridizing Germanium Anodes with Polysaccharide-Derived Nitrogen-Doped Carbon for High Volumetric Capacity of Li-Ion Batteries. J. Mater. Chem. A 2017, 5, 15828-15837. (75)

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Figure captions Figure 1. (a, b) SEM images of η-MoC/MXene/C nanohybrids; (c) TEM image of ηMoC/MXene/C nanohybrids with highly wrinkled surface; (d) TEM image of the ridge region on the surface of η-MoC/MXene/C nanohybrids; (e) TEM image revealing the uniform dispersion of ultrafine η-MoC nanocrystallites on MXene sheets; (f) HRTEM image of η-MoC nanocrystallites on MXene surface; (g) elemental mapping showing the homogenous distribution of Ti, Mo and C elements in η-MoC/MXene/C nanohybrids. Figure 2. (a) XRD patterns and (b) Raman spectra of η-MoC/MXene/C, η-MoC/C and Ti3C2 MXene; (c) Mo 3d and (d) Ti 2p XPS spectra of η-MoC/MXene/C nanohybrids. Figure 3. (a) CV curves of η-MoC/MXene/C electrodes at different scan rates; (b) b values plotted against the voltages applied on η-MoC/MXene/C electrodes for cathodic scans. The inset is the current response plotted against scan rate for η-MoC/MXene/C electrodes at different voltages; (c) the percentage of capacitive contribution to total capacity of η-MoC/MXene/C and η-MoC/C electrodes at different scan rates; (d) EIS spectra of η-MoC/MXene/C, η-MoC/C and Ti3C2 MXene electrodes with CB (η-MoC/MXene/C+CB) or Ti3C2 MXene

(η-

MoC/MXene/C+MXene) as the conductive agent over the frequency ranging from 100 kHz to 0.01 Hz. The inset is the real part of the complex impedance vs. w-1/2 for these electrodes. Figure 4. (a) Discharge-charge voltage profiles of η-MoC/MXene/C electrodes with CB (ηMoC/MXene/C+CB) or Ti3C2 MXene (η-MoC/MXene/C+MXene) as the conductive agent for the first two cycles at 0.2 A g-1; (b) cycling performance of η-MoC/MXene/C+MXene and ηMoC/MXene/C+CB electrodes at 0.2 A g-1; (c) high-rate capability of η-MoC/MXene/C+MXene and η-MoC/MXene/C+CB electrodes at varied current densities of 0.2-20 A g-1. As compassion,


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the rate performance of η-MoC/MXene/C with various η-MoC content and MXene-free ηMoC/C is also evaluated under similar conditions with CB as the conductive agent; (d) a comparison

between

reported

TMC

anodes

and

η-MoC/MXene/C+MXene

or

η-

MoC/MXene/C+CB anodes in high-rate capability; (e) long-term cycling stability of ηMoC/MXene/C+MXene electrodes at high current rate of 10 A g-1. All the electrochemical tests are conducted between 0.01-3.0 V. Figure 5. (a) Top view of η-MoC/MXene/C electrode with Ti3C2 MXene as the conductive agent; (b) top view of η-MoC/MXene/C electrode with CB as the conductive agent; (c) the cross section of η-MoC/MXene/C electrodes with Ti3C2 MXene or CB as the conductive agent; (d) a comparison between reported anodes and η-MoC/MXene/C with CB or MXene as the conductive agent in volumetric capacity.


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ACS Nano

Figure 1. (a, b) SEM images of η-MoC/MXene/C nanohybrids; (c) TEM image of η-MoC/MXene/C nanohybrids with highly wrinkled surface; (d) TEM image of the ridge region on the surface of η-MoC/MXene/C nanohybrids; (e) TEM image revealing the uniform dispersion of ultrafine η-MoC nanocrystallites on MXene sheets; (f) HRTEM image of η-MoC nanocrystallites on MXene surface; (g) elemental mapping showing the homogenous distribution of Ti, Mo and C elements in η-MoC/MXene/C nanohybrids. 129x118mm (300 x 300 DPI)


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Figure 2. (a) XRD patterns and (b) Raman spectra of η-MoC/MXene/C, η-MoC/C and Ti3C2 MXene; (c) Mo 3d and (d) Ti 2p XPS spectra of η-MoC/MXene/C nanohybrids. 129x103mm (300 x 300 DPI)


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ACS Nano

Figure 3. (a) CV curves of η-MoC/MXene/C electrodes at different scan rates; (b) b values plotted against the voltages applied on η-MoC/MXene/C electrodes for cathodic scans. The inset is the current response plotted against scan rate for η-MoC/MXene/C electrodes at different voltages; (c) the percentage of capacitive contribution to total capacity of η-MoC/MXene/C and η-MoC/C electrodes at different scan rates; (d) EIS spectra of η-MoC/MXene/C, η-MoC/C and Ti3C2 MXene electrodes with CB (η-MoC/MXene/C+CB) or Ti3C2 MXene (η-MoC/MXene/C+MXene) as the conductive agent over the frequency ranging from 100 kHz to 0.01 Hz. The inset is the real part of the complex impedance vs. w-1/2 for these electrodes. 129x100mm (300 x 300 DPI)


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Figure 4. (a) Discharge-charge voltage profiles of η-MoC/MXene/C electrodes with CB (ηMoC/MXene/C+CB) or Ti3C2 MXene (η-MoC/MXene/C+MXene) as the conductive agent for the first two cycles at 0.2 A g-1; (b) cycling performance of η-MoC/MXene/C+MXene and η-MoC/MXene/C+CB electrodes at 0.2 A g-1; (c) high-rate capability of η-MoC/MXene/C+MXene and η-MoC/MXene/C+CB electrodes at varied current densities of 0.2-20 A g-1. As compassion, the rate performance of η-MoC/MXene/C with various η-MoC content and MXene-free η-MoC/C is also evaluated under similar conditions with CB as the conductive agent; (d) a comparison between reported TMC anodes and η-MoC/MXene/C+MXene or ηMoC/MXene/C+CB anodes in high-rate capability; (e) long-term cycling stability of η-MoC/MXene/C+MXene electrodes at high current rate of 10 A g-1. All the electrochemical tests are conducted between 0.01-3.0 V. 119x119mm (300 x 300 DPI)


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ACS Nano

Figure 5. (a) Top view of η-MoC/MXene/C electrode with Ti3C2 MXene as the conductive agent; (b) top view of η-MoC/MXene/C electrode with CB as the conductive agent; (c) the cross section of η-MoC/MXene/C electrodes with Ti3C2 MXene or CB as the conductive agent; (d) a comparison between reported anodes and η-MoC/MXene/C with CB or MXene as the conductive agent in volumetric capacity. 140x104mm (300 x 300 DPI)


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Table of cotent image 82x33mm (300 x 300 DPI)


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MXene-Based Electrode with Enhanced Pseudocapacitance and

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