MetalâOrganic Frameworks as Electrolyte Additives To Enable

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Metal-Organic Frameworks as Electrolyte Additives to Enable Ultrastable Plating/Stripping of Li Anode with Dendrite Inhibition Fulu Chu, Jiulin Hu, Chenglong Wu, Zhenguo Yao, Jing Tian, Zheng Li, and Chilin Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17924 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 2, 2019

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ACS Applied Materials & Interfaces

Metal-Organic Frameworks as Electrolyte Additives to Enable Ultrastable Plating/Stripping of Li Anode with Dendrite Inhibition Fulu Chu†,‡, Jiulin Hu†, Chenglong Wu†, Zhenguo Yao†, Jing Tian†, Zheng Li*,‡, and Chilin Li*,† †State

Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai

Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, China. Email: [email protected] ‡School

of Materials Science and Engineering, Xiangtan University, Xiangtan 411105, China. Email:

[email protected]

Abstract: Suppressing the extrusion of Li dendrites and alleviating the volume expansion of Li anode during long-term cycling are of great significance to achieve highly reversible Li metal batteries (LMBs) of high energy density potential. However the exploration of facile and effective solutions to smoothen anode surface is still a big challenge. Here we propose a solid additive strategy by blending tailored metal-organic framework (MOF) grains with typical carbonate electrolyte to enable an ultrastable plating-stripping cycling of Li anode for at least 1400 h with evident inhibition of anode roughening and voltage polarization. Zr-based MOF (UiO-66) additive enables the smallest nucleation and plateau overpotentials (~80 mV) during Li plating especially under high current density (2 mA/cm2) and large areal capacity (4 mAh/cm2). The kinetic and cyclic advantages of Li anode modulated by UiO-66 not

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

only

benefit

from

its

intrinsic

features

(high

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surface

area/porosity,

thermal/electrochemical stability), but also from the reinforced solid electrolyte interface with low resistance, which consists of concentrated LiF and robust Zr-O-C moieties. Li-Li4Ti5O12 cell based on MOF additive can achieve a high reversibility for at least 900 cycles.

Keywords: Electrolyte additive, Metal-organic framework, UiO-66, Li dendrite suppression, Li-metal batteries

Introduction New battery technology is becoming increasingly significant in view of large-scale application prospect of electric vehicle and smart grid. Li-ion battery (LIB) has undergone rapid development to address the issue on portable energy storage since it was commercialized by Sony in 1991. Nevertheless, LIBs have met the bottleneck as its popular anode material, graphite, has a limited theoretical specific capacity of 372 mAh/g.1 Rechargeable Li metal batteries (LMBs) are enabled by using metallic Li as anode (instead of graphite) with a much higher specific capacity (3860 mAh/g) and lower electrochemical potential (-3.04 V vs. the standard hydrogen electrode).2 Therefore LMBs based on traditional layered oxide cathodes (e.g. NMC) can achieve an energy density as high as 500 Wh/kg.3 If Li metal anode matches with conversion cathodes (e.g. S, O2 and MFx), the energy density of LMB can even reach to 900 Wh/kg.4,5 However the popularization of Li metal anode is still hampered by various


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challenges, e.g. enormous volume expansion, unstable and easily damaged solid electrolyte interface (SEI) and uncontrollable dendrite growth.6 These shortages likely cause poor Li plating-stripping performance and even safety hazard of cell short circuit. The drastic morphology evolution of Li would frustratingly trigger serious side reactions between extruded lithium of high activity and surrounding electrolyte/salt.7 It leads to the frequent fragmentation, regeneration and accumulation of SEI layers along with the consumption of active lithium and finite liquid electrolyte, accelerating the dry-up of cell. The fracture of extruded Li moieties potentially causes their isolation from the conductive and monolithic Li metal substrate. Such a deactivated Li is called as ‘dead Li’, which enables the increase of cell impedance (or voltage polarization), lowering and fluctuating of coulombic efficiency (CE).8 Electrolyte additive strategy is thought to be a facile solution to the homogenization of Li nucleation spots and then the suppression of dendrite growth.9 It does not require sophisticated technology as the strategies on melted Li infiltration into porous framework, conformal coating as artificial SEI and solid electrolyte (SE) architecture.10-12 The construction of thick SEI or SE can improve the interface stability and rigidity to a certain degree, but with the comprise of conductivity of interface full of grain boundaries. The effect of additive strategy usually stems from the in-situ formation of robust but thin SEI when degradable additive contacts with Li as the cases of LiNO3 and fluoroethylene carbonate (FEC).13,14 However the spontaneous reaction would quickly consume these dissoluble additives, which cannot guarantee a long-term cycling stability of Li plating/stripping. Another type of



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degradable additive containing alloyable cation can be converted into dense Li-M (M= In, Sn, Al) alloy protection layer or porous metal network layer (e.g. Mg) with sufficient Li diffusion coefficient on Li anode.15-17 Recently, insoluble solid additives (e.g. LiF, Li3N and Al2O3) appear to enable a more durable effect on suppressing anode dendrite growth.18-20 The direct addition of SEI-reinforced components would not cause serious dilution of their concentration during long-term cycling. However the grain size and shape of these additives are difficultly to be tailored. The lacking of surface decoration and functionalization usually results in a quick and non-uniform precipitation of solid additives on anode substrate, which likely compromises the homogenization of current density. Hu et al. has addressed the importance of sufficient porosity and thin grain self-assembly when adopting polymer C3N4 as electrolyte filler.21 Therefore exploring flexible or deformable additives full of multi-scale pores, functional groups or building blocks is essential to achieve a renaissance of lithium metal anode. In this work, we propose a series of metal-organic framework (MOF) particles as new potential solid additive to reinforce Li dendrite inhibition at anode-electrolyte interface. MOFs as a distinct class of open framework (or porous) materials possess ultrahigh surface area (i.e. loose volume), diverse structural topology and abundant surface functional groups, and have attracted tremendous interests in the fields of gas adsorption, catalysis and biomedicine in the past years.22 These features endow MOFs with superior effect on stabilizing Li plating-striping process and smoothening anode morphology when blending them with commercial carbonate electrolyte consisting of


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1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1. Zr-based MOF additive (UiO-66) displays the best electrochemical performance compared with other typical Cu- and Al-based MOFs (HKUST-1 and MIL-101-NH2). UiO-66-contained system enables an ultralong cycling of Li plating/stripping with a small voltage gap of 75-150 mV up to at least 1400 h under an areal capacity of 1.5 mAh/cm2 based on Li-Li symmetric cells, and up to 200 cycles with an improved Coulombic efficiency (CE) of ~95% for Li-Cu asymmetric cells. The MOF-involved LMBs based on undecorated Li4Ti5O12 electrode can run well for at least 900 cycles. The robustness, porosity and (electro)chemical stability of MOF additives make critical contributions to the superior Li anode performance by motivating the concentration of LiF in SEI, reducing the undesired side reaction, homogenizing the distribution and magnitude of Li+ flux and suppressing the extrusion of Li deposit.

Results and discussion As shown in Figure 1, three typical MOFs of UiO-66, HKUST-1 and MIL-101-NH2 were successfully synthesized by solvothermal method.23-25 The X-ray diffraction (XRD) patterns confirm the pure phase of these Zr-, Cu- and Al-based MOF materials with white, blue and yellow colors respectively. These MOF grains can be well dispersed in carbonate electrolyte, benefiting to the construction of MOF rich SEI. UiO-66

has

a

chemical

formula

Zr6O4-(OH)4(BDC)6,

where

BDC

is

1,4-benzene-dicarboxylate.23 UiO-66 is 12-coordinated, the highest coordination



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reported for MOFs, and has a high decomposition temperature above 500oC and high surface area above 1000 m2/g.26 The inner Zr6O4(OH)4 cores (i.e. Zr6-octahedron nodes) are bridged by carboxylates (-CO2) as organic linker originating from the dicarboxylic acids in an edge-sharing way, leading to the formation of Zr6O4(OH)4(CO2)12 clusters. UiO-66 consists of interconnected ball-like grains with a primary size around 50 nm as observed from the scanning electron microscope (SEM) and transmission electron microscope (TEM) images (Figure 1b, c and S1). Energy dispersive X-ray spectroscopy (EDX) mapping confirms the existence of corresponding elements in UiO-66 (Figure S2). The intergrowth feature of UiO-66 grains is favorable for the homogenization of MOF-rich SEI layer. HKUST-1 shows a totally different grain morphology characterized by well-defined octahedral structure in micro-sized scale. These big octahedral grains are discrete and their precipitation in electrolyte is faster than UiO-66 (Figure 1d-f). HKUST-1 has a chemical formula of [Cu3(BTC)2(H2O)x]n, where BTC is benzene-1,3,5-tricarboxylate.27 This octahedral crystal has a thermal stability up to 240°C and a Brunauer-Emmett-Teller (BET) surface area of ~700 m2/g, which both are lower than those of UiO-66. The overall morphology of MIL-101-NH2 appears to be closer to that of UiO-66, but its grains (~300 nm in size) are evidently full of pores inside as shown in Figure 1h, i and S3, resembling the self-assembly of clusters. This abundant three-dimensional porosity is in accordance with an ultrahigh BET surface area of 2100 m2/g.25 The amino (hydrophilic) functionalization and large cavities (with 1.2-1.6 nm windows) in MIL-101-NH2 appear to be favorable to adjust the interaction


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with Li salt during the Li plating and stripping processes. Its thermal stability (up to 650 K in air) lies between those of UiO-66 and HKUST-1. The corresponding elements in HKUST-1 and MIL-101-NH2 are also confirmed by EDX mapping in Figure S4 and S5 respectively. All the MOF additives take effect on the stabilization of Li anode cycling in Li-Li symmetric cells, but with different polarization retention capability and optimized concentration depending on their morphology and porosity. When adding UiO-66 grains into carbonate electrolyte (LiPF6-EC-DMC), the stable cycling number is extended to 650 h and 700 h for the 0.1 and 0.5 wt% (the mass ratio in electrolyte) additive cases respectively from 500 h for the neat electrolyte under 0.5 mA/cm2 and 1.5 mAh/cm2 (Figure 2a). When further increasing the additive content to 1 wt %, the cycling is prolonged to at least 1400 h without serious polarization. The increase of UiO-66 concentration is favorable for the shrinkage of voltage gap (i.e. voltage difference between plating and stripping steps) to 100 mV or smaller before 1100 h, which is slightly increased to 150 mV after 1400 h. An excess addition of UiO-66 up to 2 wt% leads to a much earlier polarization degradation instead of earlier short circuit phenomena as the cases of less additive. HKUST-1 additive enables a prolonged cycling exceeding 1000 h even under a lower content of 0.1 wt%, and however the voltage gap increases suddenly from 100 mV to 200 mV after 800 h (Figure 2b). The further increase of HKUST-1 concentration (e.g. up to 0.5 and 1 wt%) is prone to accelerate the occurrence of short circuit. This phenomenon is likely caused by the non-uniform current distribution near Li surface as a consequence of



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irregular stacking of large-sized HKUST-1 grains. MIL-101-NH2 additive takes similar effect in a wide concentration range from 0.1 to 1 wt% (Figure 2c). The additive system enables a long cycling exceeding 1300 h with a gradual polarization increase from 100 mV (after 900 h) to 200 mV (after 1300 h). The insensitivity of additive content is associated with the tiny grain size and sufficient porosity of MIL-101-NH2. Based on the optimization result, we will mainly focus on the best UiO-66 additive with a content of 1 wt % for the following electrochemistry and characterization. With the increase of current density and areal capacity to 1 mA/cm2 and 3 mAh/cm2, the Li cycling for neat electrolyte system terminates before 450 h, whereas the cycling can last close to 750 h for the additive system without evident polarization degradation (Figure 2d). After UiO-66 addition, the voltage gap is roughly half of that for the neat electrolyte, and it is less than 100 mV during the stable cycling stage. It gradually increases to 200 mV after 700 cycles, whereas for the additive-free system it has exceeded 200 mV after 400 cycles. Under a higher current density of 2 mA/cm2, the Li plating-stripping can endure a cycling of at least 300 h even with an areal capacity as high as 6 mAh/cm2, whereas the voltage polarization deteriorates at the cycling stage as early as 120 h for the additive-free system based on the same areal capacity (Figure 2e). For HKUST-1 and MIL-101-NH2 additive systems, their cycling performance based on Li-Li symmetric cell at 1 mA/cm2 is worse than UiO-66 with faster increase of voltage polarization, which exceeds 400 mV after 450 h for HKUST-1 or close to 300 mV after 600 h for MIL-101-NH2


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(Figure S6). The polarization advantage of UiO-66 is also seen from the comparison of voltage gap values during the 32-34th cycle (around 200 h), which are respectively 78.4, 84.9 and 94.6 mV for UiO-66, HKUST-1 and MIL-101-NH2. The Li-Cu asymmetric cell architecture is also used to explore the kinetics and reversibility of Li plating/striping depending on different MOF additives (Figure 3). From the first plating-stripping, UiO-66 system shows the lowest nucleation overpotential (ηn) of 27 and 73 mV under both the current densities of 0.5 and 2 mA/cm2, which are respectively 76 and 134 mV smaller than those for neat electrolyte. ηn is estimated as the voltage difference between the lowest value in the preliminary stage of Li plating and the value of the subsequent stable platform. It is used to assess the Li plating energy barrier during the electrochemical heterogeneous nucleation stage, and usually increases with current density due to kinetic limitation.28 Although the ηn value for MIL-101-NH2 is comparable to that of UiO-66 at 0.5 mA/cm2, it becomes evidently larger (310 mV) at 2 mA/cm2. The ηn values (77 and 177 mV) for HKUST-1 are 50 and 104 mV larger than for UiO-66 at 0.5 and 2 mA/cm2 respectively. However the plateau overpotential (ηp = 71 mV, which is estimated as the voltage distance between the plating plateau and zero-volt base line) of UiO-66 system is slightly larger than those (33-43 mV) for HKUST-1 and MIL-101-NH2 at 0.5 mA/cm2, indicating that the possible reduction for the latter two additives into conductive moieties (e.g. metal Cu or Al contained species) are responsible for the lowering of ηp value. However, when increasing the current density to 2 mA/cm2, the ηp performance of UiO-66 system (87.5 mV) has successfully surpassed other two



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systems. The neat electrolyte has the worst ηp performance for both the current densities. The advantage of UiO-66 system also lies in the higher CE value, which is 90% for the first cycling and much larger than those (70 and 73 % at 0.5 mA/cm2, 78 and 65 % at 2 mA/cm2) for the neat and HKUST-1 contained electrolytes. The low initial CE for HKUST-1 is likely caused by its big grain size, which retards the smooth stripping of deposited Li. The existence of rich pores and cluster-like particles in MIL-101-NH2 is beneficial to the improvement of its initial CE, which is comparable to that of UiO-66. From these results, it is concluded that UiO-66 additive enables the most superior Li cycling performance in terms of initial CE, nucleation and plateau overpotentials especially under a high current density. The small ηn value for UiO-66 additive tends to induce a homogeneous Li nucleation and good dendrite inhibition effect, consistent with the results based on Li-Li symmetric cell and morphology of cycled Li surface discussed later. The CE value in the UiO-66 additive system is quickly increased to above 95% from the second cycle and stabilized at ~95 % for 200 cycles at 0.5 mA/cm2 and 1 mAh/cm2, which is high for carbonate system (Figure 3e and S7).17 In contrast, the additive-free system requires the activation of five cycles in order to uplift the CE to above 90%. However the CE value cannot reach to 95% during the following cycling and drops to below 90% after 100 cycles. The voltage hysteresis between plating and stripping is remarkably decreased to less than 100 mV from the second cycle after UiO-66 addition, and is quite stable with a slight increase to 100 mV after 200 cycles (Figure 3f). However the hysteresis value for the neat electrolyte is always above 100


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mV and close to 200 mV after 100 cycles. The comparison effect is even more remarkable when increasing the current density to 2 mA/cm2 and areal capacity to 4 mAh/cm2 (Figure 3g and h). The CE value for UiO-66 system is stabilized at 95% after two cycles and still above 90% after 60 cycles under such a high areal capacity. For the control system, the cycling merely maintains 10 cycles with a low CE value around 85%. The voltage hysteresis for UiO-66 system does not degrade at much higher 2 mA/cm2, and can still be below 100 mV before 40 cycles and slightly exceeds 100 mV after 60 cycles. However it is located between 300 and 350 mV during the short ten cycles for the additive-free system. HKUST-1 additive enables a stable cycling performance of Li-Cu asymmetric cell comparable to that by UiO-66 at 0.5 mA/cm2 (Figure S8). The smaller voltage hysteresis (~50 mV) is in accordance with the tendency of plateau overpotential at this current density as mentioned before. However at higher 2 mA/cm2, the CE value never exceed 95% and the stable cycling terminates after 45 cycles for HKUST-1 additive. MIL-101-NH2 additive system shows a low CE less than 95% at 0.5 mA/cm2, which drops to below 90% after 130 cycles (Figure S9). Higher current density (2 mA/cm2) triggers the improvement of CE, indicating a potential evolution of MIL-101-NH2 based SEI. High Li-ion fluxing likely accelerates the decomposition/reduction of this Al-based MOF. The electrochemical impedance spectra (EIS) based on Li-Li symmetric cell is used to detect the evolution of interfacial resistance (Ri) at different cycling stages under 0.5 mA/cm2 and 1.5 mAh/cm2 (Figure S10). In the high-frequency region of Nyquist plots, the Ri value can be obtained from the size of semicircle, which contains



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the resistance of SEI layer and the charge transfer resistance between electrode and electrolyte.14,29 The resistance of bulk electrolyte (Rb) is estimated from the intercept value before the starting of semicircle. Note that the addition of UiO-66 and HKUST-1 significantly promotes the decrease of Ri for the fresh cell (< 45 ·cm2) as well as the cycled cells compared with the additive-free and MIL-101-NH2 systems, for both which the fresh Ri values are close to 1000 ·cm2. Their Ri values continuously decrease with the progress of cycling, and are stabilized at 5-10 ·cm2 for both the former additives. Repeating Li plating does not remarkably fluctuate the Rb values, which are stabilized at ~5 ·cm2. Although the interface conductivity is drastically improved after multiple cycles, the interface and electrolyte resistances are still larger for additive-free and MIL-101-NH2 systems at the corresponding cycling stages. Moreover, their Rb values are less stable and become smaller especially for the case of MIL-101-NH2 additive, further indicating a potential decomposition of this Al-based MOF and precipitation of Al or Li-Al alloy. The quite small interface resistance with high stability (after 10 cycles) for UiO-66 additive system is responsible for the ultralong Li metal cycling. The reversibility of Li plating-stripping is also reflected from the SEM morphology roughness of cycled Li surface (Figure 4). UiO-66 additive enables a smoothest Li surface after 60 cycles at 1 mA/cm2 with a high areal capacity of 3 mAh/cm2. The Li surface shows an overall compact morphology without the formation of any dendrite-like grains and well-defined grain boundaries. The magnified SEM discloses a shallow nanoporous texture, which should be modulated by the ultrafine


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nanoparticles of UiO-66 dissociated from the original aggregate in electrolyte. The high surface area (>1000 m2/g) and hierarchical porosity are also responsible for the altering of Li plating pathway.26 The compact and dendrite-free morphology is also observed from the cross-section of cycled Li anode, focusing on the interface region between plated Li and unreacted Li as well as the near-surface region (Figure S11). Therefore it is concluded that there is no underlying dendrite hidden in the deep region of plated Li. The EDX mapping confirms the existence and uniform distribution of Zr-MOF components (e.g. Zr, O and C) as well as decomposed products of Li-salt (F and P) on the cycled Li surface (Figure 4e). Since ZrO2 has been thought as a filler with excellent mechanical strength for polymer electrolyte,30 the combination of Zr-O-C and LiF is expected to achieve an effective SEI reinforcement and dendrite inhibition. The spatial distribution of MOF components appears to be even more homogeneous than F element, indicating that insoluble solid additive does not always compromise the uniformity of its deposition at solid-liquid interface compared with liquid or dissolvable additive. After much longer 200 cycles at 1 mA/cm2 with a capacity of 1 mAh/cm2, the cycled Li surface still remains compact and roughly uniform (Figure 5). The granular texture is more evident with the increase of cycling number. The chemical stability of MIL-101-NH2 rich SEI is relatively vulnerable to the repeated Li-plating as indicated from the above-mentioned electrochemical characterization. This is also implied by the evolution of Li-surface morphology in different regions (Figure S12). The potential coverage of Al-MOF with the highest surface area and finest primary particles blurs the Li grain boundaries. The less



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coverage appears to lead to a clearer observation of grain boundaries, which is one of the reasons for the earlier performance degradation than UiO-66. HKUST-1 additive enables another textured morphology, consisting of nano-sized (~100 nm) Li grains, which are further adhered together to form larger aggregates (Figure S13). However these Li surfaces modulated by MIL-101-NH2 and HKUST-1 additives are still not highly corrugated due to the absence of Li dendrites, which are inclined to generate in the additive-free electrolyte (Figure 4d). X-ray photoelectron spectra (XPS) of cycled Li surface with different depth further disclose the desired modulation effect of UiO-66 additive on SEI components (Figure 6). For a better identification and analysis, the fitting and separation of XPS peaks are roughly based on the profiles of spectra. Li 1s curve is fitted into two sets of peaks. The dominant peak at higher binding energy (BE) involves the contributions from LiF (55.7 eV), Li2CO3/LiOH (55 eV) and Li2O (54.2 eV).31 After etching the sample from the neat electrolyte for 10s, this peak shifts towards a higher BE value, indicating a higher fraction of LiF beneath the surface.32 The addition of UiO-66 enables a further peak shift to the position of higher BE. This tendency is more pronounced for the etched sample from the additive system, where the peak position is very close to that of LiF. The evolution of Li 1s spectra indicates that UiO-66 additive can trigger the generation of more LiF in SEI. The other peak is around 52.6 eV, corresponding to the formation of LixC.32 F 1s spectra show the similar intensification of LiF signal at 685 eV by adding UiO-66 or etching surface layer.31 On the other hand, the addition of UiO-66 can suppress the degradation of Li-salt anion (PF6-) as


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indicated from the weakening of the shoulder peak at 683 eV. Instead, a new peak appears at 687-688 eV, and it is likely caused by the fluorination of Zr-MOF moieties (e.g. C-F or Zr-F).33,34 This peak becomes weaker after etching. From the O 1s spectra, UiO-66 additive promotes the formation of Li2CO3 (at 532 eV) at the cost of the dilution of Li2O (at 528.5 eV),17 agreeing with the positive displacement of Li 1s peaks. C 1s spectra are made up of four peak signals assigned to CO32- (290 eV), ROCOOLi (289 eV), (CH2CH2O)n (287 eV) and LixC (282-283 eV), apart from the dominant C-C peak.32 The ratio of CO32- and (CH2CH2O)n increases with the addition of UiO-66, whereas the proportion of LixC correspondingly decreases. This result is in accordance with that of Li 1s and O 1s spectra. In conclusion, UiO-66 additive enables an effective mitigation of undesired side reactions between Li and electrolyte from the decreased residuals of decomposed anion and LixC. Meanwhile, the LiF content in SEI is increased from ~4.5 mol% for the neat system to ~14 mol% for the additive reinforced system. The enrichment of LiF induced by UiO-66 additive is also confirmed by the comparison of EDX images of cycled Li anode surface for additive-free LiPF6-EC-DMC system (Figure S14). The unprotected surface is evidently inhomogeneous and uneven, and therein the F content is less with less uniform distribution. The intimate contact of sufficient LiF and Zr-O-C(-F) is favorable for the construction of robust SEI (benefiting from Zr-oxide) with small surface diffusion barrier (benefiting from LiF), which takes effect on improving the reversibility of LMBs as discussed in the following.18,35 The residual of UiO-66 grains in separator cannot be ruled out, and it should play the role mainly on the mechanical



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reinforcement of separator, which also contributes to the extension of Li anode life more or less. The Li-Li4Ti5O12 (Li-LTO) cells based on UiO-66 system display an ultralong cycling life without serious capacity fading and CE degradation (Figure 7). A reversible capacity of 150 mAh/g is achieved at 0.5C with a high CE of 100 %, and it is still preserved at 120 mAh/g even after 900 cycles. At 1C, the capacity is highly stabilized at 120 mAh/g for at least 500 cycles. In contrast, the capacity for the neat electrolyte drops to below 120 mAh/g after merely 80 cycles at 0.5 C, due to the easy roughening of Li anode free of additive protection (Figure 4d). Note that, after hundreds of cycles, the overpotential increase in galvanostatic curves is observed for 1 wt% UiO-66 contained electrolyte, indicating an influence of viscosity property on the less conductive commercial LTO electrode (Figure S15). We also performed the electrochemical cycling based on the higher voltage conversion cathode of FeS2. This additive method still takes effect on stabilizing the capacity retention of Li-FeS2 cell, which shows a highly reversible discharge capacity around 600 mAh/g (Figure S16). However the adoption of cathodes working above 4 V requires the assistance of high-voltage-stable buffer layer between separator and cathode to extend the stable electrochemical window (e.g. the polymer interlayer Poly(N-methyl-malonic amide) as reported by Zhou et al.).36 Anyway, the Li anode stabilized by MOF additive is helpful to improve the capacity retention of high voltage cathodes. The intrinsically high specific surface area, cavity/surface decoration and flexible grain geometry endow MOF materials as solid additive with the capabilities to


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modulate the distribution of ionic flux, to homogenize the local electric field at Li-electrolyte interface and to reinforce the Li dendrite inhibition.37,38 Thermally and chemically stable Zr-based MOF (UiO-66) is resistant to the electrochemical reduction by repeated Li plating or to the decomposition by heat release especially under high current density.39 The structural integrity of MOF benefits to the stabilization of SEI, smoothening of Li metal morphology and extension of Li plating-stripping cycling. MIL-101-NH2 and HKUST-1 with redox element (e.g. Al and Cu) and poorer thermal stability (150-250oC lower than that of UiO-66) do not guarantee a smaller interface resistance even though with the formation of metal or alloy phase (e.g. Li-Al) by over-lithiation,40 since the decomposition of MOF likely catalyzes the side reaction of electrolyte, increases the energy barrier for Li nucleation and roughens the Li surface. It seems that central metal type and thermal (or chemical) stability play more important roles than grain morphology, surface area, inner porosity and hydrophilic decoration (e.g. amino group) for MOF additives. The dual reinforcement of SEI by robust Zr-MOF grains of high dispersion and in-situ formed LiF layer of high content is responsible for the ultrastable Li-Li symmetrical and Li-LTO cells.18,41 However the comprehensive factor or synergic effect on Li dendrite inhibition still requires more investigations from the abundant recipes of MOF materials.

Conclusion In summary, a MOF additive strategy is proposed to suppress Li dendrite growth at LMB anode by blending different MOF grains with typical carbonate electrolyte. The



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center metal in MOF, grain morphology/size and additive concentration play an important role to modulate the anode texture evolution during electrochemical cycling. Zr-based MOF (UiO-66) additive is confirmed to enable an ultrastable plating-stripping of Li anode for at least 1400 h without serious degradation of voltage polarization. UiO-66 also enables a kinetic improvement of Li plating as indicated from the smallest nucleation and plateau overpotentials (~80 mV) especially under high current density (2 mA/cm2) and large areal capacity (4 mAh/cm2). The reinforcement of SEI (consisting of concentrated LiF and robust Zr-O-C moieties) modulated by highly porous and electrochemically stable UiO-66 additive is responsible for the superior performance of Li metal anode and Li-Li4Ti5O12 cells (for at least 900 cycles). The implantation of rich MOF materials as electrolyte additive opens an effective pathway to achieve smooth Li plating behavior.

Experimental Section Materials: Li foils, Cu foils (25 μm in thickness) and Li4Ti5O12 were bought from Hefei KEJING Materials Technology CO., LTD. Carbonate solvents like ethylene carbonate (EC) and dimethyl carbonate (DMC) were purchased from Sigma Aldrich. Lithium hexafluorophosphate (LiPF6) was obtained from Alfa Aesar. Glass fiber from Whatman was used as separator. All these materials were used as received in an Argon filled glove box. Preparation of MOF Materials: Typical solvothermal method was adopted to synthesize MOF grains. For the preparation of zirconium-based MOF (UiO-66),23 57


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mg 1,4-benzenedicarboxylic acid (0.343 mmol) and 80 mg zirconium chloride (0.343 mmol) were dissolved in 20 ml N,N-dimethylformamide (DMF) and dispersed uniformly by ultrasonic treatment for 5 min at room temperature. The solution was then transferred to a 50 mL Teflon-lined stainless steel autoclave reactor and therein maintained at 120 ℃ for 48 h. After the solvothermal reaction, the white product was cooled down for centrifugation, and washed with DMF for several times. The collected solid was suspended in methanol overnight, and finally the white powder was dried and dehydrated at 120 ℃ for 24 h. For the preparation of copper-based MOF (HKUST-1),24 49.1 mg (0.234 mmol) of 1,3,5-benzenedicarboxylic acid was dissolved in 250 ml ethanol, and 108.6 mg (0.466 mmol) of copper nitrate hydrate (Cu(NO3)2 ·2.5H2O) was dissolved in 250 ml deionized water. Both the solutions were mixed in the same proportion for 30 min, and then transferred into the Teflon reactor. The reaction was heated at 110 ℃ for 18 h. The obtained blue powder was cooled to room temperature for washing and centrifugation, and then dried and dehydrated at 110 ℃. For the preparation of aluminum-based MOF (MIL-101-NH2),25 271.7 mg (1.5 mmol) of 2-aminoterephthalic acid was dissolved in 60 ml DMF in the 100 ml round bottom flask and therein heated at 110 ℃. Then 724 mg (3 mmol) of aluminum chloride hexahydrate (AlCl3 ·6H2O) was divided into seven parts, which were added after every 15 min with the stirring at 110 ℃ for 3 h. The solution was kept at this temperature for additional 16 h without stirring. The product was cooled to room temperature, filtered and washed three times with DMF and ethanol, respectively.



Page 20 of 33

Then the precipitate was kept in ethanol at 90 ℃ for 16 h and washed with ethanol again. The final yellow solid was dried and dehydrated at 120 ℃ for 12 h. Physical Characterization: X-ray diffraction (XRD) patterns of the synthetic powder samples were recorded by using D2 PHASER powder X-ray diffractometer with Cu Kα radiation. The diffraction 2-theta angle ranges from 5° to 60° with a scanning step of 0.02°/s. Transmission electron microscopy (TEM) images of MOF grains were obtained through a JOEL-2100F electron microscope, operated at 200 kV. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) of MOF powders and cycled Li anode surfaces (or cross-section) were observed on the FEI Magellan 400 of Nanolab Technologies Corporation, where the excitation voltage is up to 30 kV. X-ray photoelectron spectroscopy (XPS, ESCAlab-250 of Thermo Fisher Company) was used to detect the surface element and bonding information of cycled Li anode. For the preparation of cycled Li metal anode for ex-situ characterization, the Li||Li symmetric cells were disassembled in the glove box. Then the cycled Li anodes were taken out from the cells and washed several times with DMC before measurement. Electrochemical measurement: The electrochemical polarization, Coulombic efficiency (CE) and cycle performance of cells based on Li metal anode were tested on Land CT2001A. The Li plating/stripping process was observed based on the architecture of CR2025-type Li||Li symmetric coin cells, where the constant current charge-discharge mode was used from 0.5 to 2 mA/cm2 with an areal capacity of 1.5 to 6 mA h/cm2 (that is, charging for 3 h and discharging for 3 h in every


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plating-stripping step). The electrolyte contain 1 M LiPF6 in EC:DMC (1:1 in volume ratio) with MOF additive of different concentration. In these cells, the electrolyte loaded separator (Celgard 2400) was sandwiched between two pieces of lithium foils. The CE was measured based on the configuration of Li||Cu asymmetric cells (copper foil as working electrode and Li metal foil as reference and counter electrode) with the similar constant current test conditions, where the discharge duration was 2 h and the charging cut-off voltage was 0.5 V. Li metal batteries (LMBs) with Li4Ti5O12 or FeS2 as cathode (the weight ratio of active species, Super P carbon and PVDF binder is 7:2:1) and Li foil as anode were assembled to examine the effect of MOF as additive, and they were run at different rate in a voltage range of 1.0-2.5 V or 1.0-3.0 V respectively. Impedance measurement based on Li||Li symmetric cells at different cycling stages was done by using a Solartron frequency analyzer (1260-1296) in a frequency range of 10-2 to 5×106 Hz.

Supporting Information Experimental detail, SEM, TEM and EDX mapping of UiO-66, MIL-101-NH2 and HKUST-1, Li plating/stripping of Li||Li symmetric cells with HKUST-1 and MIL-101-NH2, voltage profiles of Li||Cu asymmetric cells with UiO-66, HKUST-1, MIL-101-NH2 and free of MOF additive, Coulombic efficiency and voltage hysteresis based on Li||Cu asymmetric cells with HKUST-1, MIL-101-NH2 and free of MOF additive, impedance spectra of Li||Li symmetric cells with different MOF additives or free of additive, SEM images of cycled Li surface morphology modulated by



Page 22 of 33

MIL-101-NH2 and HKUST-1, voltage curves of Li/LTO cells with UiO-66 and free of additive. This material is available free of charge via the Internet at http:// pubs.acs.org. Acknowledgements This

work

was

supported

by

National

Key

R&D

Program

of

China

(2016YFB0901600), National Natural Science Foundation of China (51772313, U1830113), “Hundred Talents” Program of Chinese Academy of Sciences and “Thousand Talents”Program of Shanghai.

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(a)

(b)

(c)

(d)

(e)

(f)

(h)

(g)

(i)

Figure 1. XRD patterns of (a) UiO-66, (d) HKUST-1 and (g) MIL-101-NH2. Insets: corresponding crystalline structure diagrams, photographs of synthesized MOF powders, corresponding additive-contained electrolyte solutions and their dispersion on separators. SEM images of (b) UiO-66, (e,f) HKUST-1 and (h) MIL-101-NH2. TEM images of (c) UiO-66 and (i) MIL-101-NH2.



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(a)

(b)

(c)

(d)

(e)


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Figure 2. Li plating/stripping performance of Li||Li symmetric cells based on LiPF6-EC-DMC system containing (a) UiO-66, (b) HKUST-1 and (c) MIL-101-NH2 of different concentrations under a current density of 0.5 mA/cm2 with a constant areal capacity of 1.5 mAh/cm2. Insets: corresponding voltage curves near performance degradation stages for the non-optimized additive systems. Li plating/stripping performance comparison for the cases free of additive and containing 1.0 wt% UiO-66 grains (d) at 1 mA/cm2 with a capacity of 3 mAh/cm2 and (e) at 2 mA/cm2 with a capacity of 6 mAh/cm2. Insets: corresponding voltage profiles of Li||Li symmetric cells during 32-34 cycles at 1 mA/cm2 or during 19-20 cycles at 2 mA/cm2.



Page 28 of 33

(a)

(c)

(b)

(d)

(f)

(e)

(g)

(h)

Figure 3. The first Li plating-stripping curves of Li||Cu asymmetric cells in LiPF6-EC-DMC system with or without MOFs under (a) 0.5 mA/cm2-1 mAh/cm2 and (b) 2 mA/cm2-4 mAh/cm2 protocols. Insets: the magnified plating processe to estimate


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the nucleation overpotential (ηn) and plateau overpotential (ηp) for respective electrolyte system. Histograms of the comparison of corresponding (c) ηn and (d) ηp values under different current density. Comparison of Coulombic efficiency as a function of cycle number in LiPF6-EC-DMC system with 0 and 1.0 wt% UiO-66 as additive (e) at 0.5 mA/cm2 with a capacity of 1 mAh/cm2 and (g) at 2 mA/cm2 with a capacity of 4 mAh/cm2. Corresponding voltage hysteresis as a function of cycle number at (f) 0.5 and (h) 2 mA/cm2. (a)

(b)

(c)

(d)

(e)

Figure 4. SEM images of cycled Li surface morphology at Li plating stage based on Li||Li symmetric cells after 60 cycles at 1 mA/cm2 with a capacity of 3 mAh/cm2 for (a,



Page 30 of 33

b, c) 1wt% UiO-66-contained and (d) additive-free LiPF6-EC-DMC systems. (e) EDX element mapping of Zr, F, C, P and O on the cycled Li surface, indicating the existence of UiO-66 component and decomposed products of Li-salt.

(a)

(b)

(c)

(d)

Figure 5. (a) Overview and (b-d) magnified SEM images of cycled Li surface morphology at Li plating stage based on Li||Li symmetric cells after 200 cycles at 1 mA/cm2 with a capacity of 1 mAh/cm2 for 1wt% UiO-66-contained LiPF6-EC-DMC systems.


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(a)

(b)

(c)

(d)

Figure 6. XPS spectra of (a) Li 1s, (b) F 1s, (c) O 1s and (d) C 1s for the cycled Li anode at plating stage for both UiO-66-contained and additive-free LiPF6-EC-DMC systems after five plating/ stripping cycles at 0.5 mA/cm2. The cycled Li surface is detected before etching or after etching for 10 s. The fitting of spectra is presented, including the assignment of corresponding peaks.



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(a) (c)

(b)

(d)

Figure 7. Discharge capacity and Coulombic efficiency of Li/LTO cells as a function of cycle number based on UiO-66-contained LiPF6-EC-DMC systems at (a) 0.5C and (b) 1C, and based on additive-free system at (d) 0.5C. (c) Corresponding charge-discharge curves of Li/LTO cell at different cycling stages at 0.5C UiO-66-contained system.


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TOC figure


MetalâOrganic Frameworks as Electrolyte Additives To Enable

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