Design mechanism of the floor modification utilizing fluorinated carboxylic acids
Fluorinated carboxylic acid reacts with Li by a spontaneous chemical response as illustrated in Fig. 1. On this response, the reactive carboxyl group and Li act because the proton and electron donors, respectively. Fluorinated carboxylic acid reacts quickly with the lowering Li floor by a substitution response, thereby forming lithium fluorinated carboxylate and H2 fuel. The Li floor accommodates a passivation layer primarily composed of Li2CO3 and LiOH, that are shaped throughout its storage in a glove field8. Carboxylic acid is stronger than carbonic acid; thus, fluorinated carboxylic acid can react with Li2CO3 and even with the robust base LiOH and primary Li2O on the Li floor in response to the reactions proven within the supplementary data. In-depth experimental and theoretical calculations are supplied for example the removing of the passivation layer (Supplementary Word S1–S3 and Supplementary Figs. S1–S3). Contemplating these reactions, fluorinated carboxylic acid can be utilized to take away the passivation layers on the Li floor. On the similar time, a lithium fluorocarbon-containing floor layer could also be produced by the floor therapy utilizing fluorinated carboxylic acid to enhance the biking stability of Li steel anodes.
Initially, the impact of the carbon chain size of the fluorinated carboxylic acids on the steadiness of the Li anode was evaluated. Supplementary Fig. S4 reveals the voltage–time curves of the Li/Li symmetric cells containing Li anodes handled with fluorinated carboxylic acids of various carbon chain lengths. The cells containing handled Li anodes demonstrated significantly improved cyclic stabilities. Floor therapy utilizing heptafluorobutyric acid (HFA) with a carbon chain size of 4 had the most effective safety impact on the Li anode. The Li/Li symmetric cell composed of HFA-Li electrodes remained steady even after over 350 instances of the plating/stripping processes. On one hand, carboxylate species with quick carbon chains, that are just like the naturally shaped lithium carboxylates, reminiscent of HCOOLi in methyl formate and CH3(CH2)2COOLi in γ-butyrolactone, within the SEI layers, have restricted protecting impact on the Li anode15. However, long-chain lithium carboxylates are much less versatile as a result of their lengthy alkyl chains17,18. The impact of the C–F useful teams on the steadiness of Li was additionally examined. For this function, Li anodes have been additionally handled utilizing butyric acid (BA) with a carbon chain size of 4. In contrast to HFA, BA accommodates no fluorine substituents. The efficiency of the Li/Li symmetric cell containing HFA-Li electrodes was nonetheless superior to that of the cell with the BA-Li electrodes (Supplementary Fig. S5). C–F useful teams presumably improved the biking stability of Li steel. The mechanism of stability enchancment similar to the presence of C–F teams might be mentioned in additional element within the part on the theoretical calculations. Contemplating these outcomes, HFA-Li was used within the subsequent assessments and characterizations.
Characterization of the lithium carboxylate protecting interface
Infrared (IR) spectroscopy was carried out to confirm the formation of the lithium carboxylate protecting interface on the Li floor (Fig. 2a). The IR indicators of the Li floor have been collected by a diffuse reflection mode (Fig. 2b). The height similar to C = O vibration shifted from 1773.2 cm–1 (carboxylic acid) to 1673.9 cm–1 (HFA-Li), indicating the formation of a steel carboxylate15. Furthermore, the attribute broad –COOH peak in carboxylic acid at roughly 2500–3300 cm–1 disappeared after the response, which signifies the substitution of the –H atoms with –Li atoms19. Moreover, the Li–O peak at 550 cm−1 noticed on the IR spectrum of HFA-Li is indicative of the formation of the Li–O bonds by the response between Li and carboxylic acid. Determine 2c reveals the optical pictures of Li steel anodes earlier than and after the therapy. No obvious change within the coloration and metallic luster of the Li floor was noticed after the therapy with HFA presumably as a result of the shaped interface was comparatively skinny. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) was carried out to check the chemical composition of HFA-Li (Fig. 2nd–f and Supplementary Fig. S6). Ion fragments of (CF3–CF2–CF2)–, (CF3–CF2–CF2COO)–, (CF3–CF2–CF2COOLi)F–, and (CF3-CF2-CF2COO)2Li– appeared within the spectrum with corresponding mass-to-charge ratios (m/z) of 169, 213, 239, and 433, respectively. The looks of those ionic fragments corresponds nicely to the formation of lithium fluorocarboxylate, indicating that the response proceeds on the Li floor as described in Fig. 1. Scanning electron microscopy (SEM) was carried out to check the morphology of the protecting interface on the HFA-Li floor (Fig. 2g, h, and Supplementary Fig. S7). The formation of the protecting interface could be noticed on the highest and cross part views of the HFA-Li electrode. In distinction to flat polymeric coatings, the floor layer was composed of agglomerated small particles. The thickness of the coating was roughly 2 µm (Supplementary Fig. S7).
X-ray photoelectron spectroscopy (XPS) was carried out to analyze the adjustments within the floor composition of the Li anode after the therapy with HFA (Supplementary Fig. S8). No XPS sign similar to C–F was noticed on the F 1s spectrum of Naked-Li. In distinction, a C–F sign was noticed on that of HFA-Li at roughly 688.7 eV20. On the C 1s spectrum of Naked-Li, peaks at 284.8, 287.1, and 289.8 eV have been recorded, which correspond to C–C, C–O, and carbonates (O–C=O), respectively. After the HFA therapy, new peaks at 288.3, 291.2, and 293.6 eV have been noticed on the C 1s spectrum, which could be attributed to C=O, C–F, and C–F3, respectively. The depth of the peaks similar to the C–O and C=O teams elevated after the therapy. These outcomes additional affirm the profitable formation of a lithium carboxylate protecting layer on the Li floor after HFA therapy.
Impact of HFA-Li on suppressing Li dendrite development
The Coulomb effectivity (CE) of the Li striping/plating course of is a vital parameter of the cycle efficiency of battery programs. For the reason that lithiophilic protecting interface of the HFA-Li anode was shaped in situ on the Li floor, the CE can’t be decided by the Li/Cu cell technique. On this examine, an improved take a look at technique utilizing skinny Li foil was employed, which is nearer to the sensible cycle circumstances of the LMBs. Determine 3a reveals the measured voltage profiles of the HFA-Li and Naked-Li anodes in a 1 M LiPF6 in EC/EMC (v/v = 3:7) electrolyte with 5.0 wt% FEC. The schematic of the plating/stripping processes can be introduced. The common worth of the CE of Naked-Li was solely 94.7% (Fig. 3b). This means a big irreversible capability loss presumably as a result of era of Li dendrites throughout biking, which ultimately grew to become lifeless Li after Li striping21. In distinction, the HFA-Li anode registered a a lot larger CE (99.3%), which additional confirms the uniform Li+ ion flux and Li deposition, and inhibited dendritic Li development throughout biking of HFA-Li. To assist the above and to realize extra perception into the origin of the capability loss, mass spectrometry titration (MST) method was additional carried out to tell apart the contribution of lifeless Li0 (metallic lifeless Li steel wrapped by SEI) and SEI-Li+ (SEI parts) within the CE take a look at (Fig. 3c, d)22,23,24. The gathered lifeless Li0 and SEI-Li+ of HFA-Li throughout CE take a look at have been 0.118 and 0.148 mAh cm−2, respectively, whereas these of Naked-Li have been 1.470 and 0.636 mAh cm−2. Particularly, the gathered lifeless Li0 of Naked-Li occupies 70% of the irreversible capability loss, demonstrating that metallic lifeless Li steel is the principle supply of capability loss throughout Li plating/stripping. Whereas after the HFA therapy, SEI-Li+ grew to become the principle supply of irreversible capability loss. The proportion of lifeless Li0 decreased to 44% and the irreversible capability loss was considerably decreased (from 2.106 to 0.266 mAh cm−2). This end result signifies that the Li deposition conduct of HFA-Li was successfully regulated after HFA floor therapy, which minimized the curvature of the microstructure and achieved a uniform morphology. Consequently, lifeless Li0 attributable to remoted Li particles trapped within the SEI throughout Li stripping was considerably decreased21.
Li/Li symmetric cells have been used to judge the electrolyte-blocking characteristic of the HFA-Li coating. Supplementary Fig. S9 reveals the impedance evolution of Naked-Li/Naked-Li and HFA-Li/HFA-Li symmetric cells over time in a 1 M LiPF6 in EC/EMC (v/v = 3:7) electrolyte with 5 wt% FEC.The HFA-Li symmetric cell exhibited a low and steady interfacial impedance over the whole cell resting time. In distinction, the impedance of the Li/Li symmetric cell assembled utilizing the Naked-Li electrodes enhance sharply after the cell assembling to 32 h (from ~110 Ohm·cm2 to ~190 Ohm·cm2).
The long-term cycle stabilities of Li/Li symmetric cells assembled utilizing the HFA-Li and Naked-Li electrodes have been additionally evaluated. Determine 3e, f reveals the voltage–time curves of the Li/Li symmetric cell at a present density of 1.0 mA cm–2 and a capability of 0.5 mAh cm–2 in 1 M LiPF6 in EC/EMC (v/v = 3:7) with 5.0 wt% FEC. The HFA-Li symmetric cell exhibited glorious stability even after 1400 h. In distinction, the cell potential of the Li/Li symmetric cell assembled utilizing the Naked-Li electrodes started to fluctuate in the course of the early levels of the steadiness testing and elevated sharply after 200 h solely. The above phenomenon could be defined by the next causes: On one hand, as beforehand mentioned, the Li deposition charge on the Li floor varies at totally different factors on the substrate as a result of inhomogeneous composition of the inherent SEI layer25. In the course of the Li plating course of, the inner Li dendrites constantly grew on the Naked-Li floor. However, in the course of the Li stripping course of, the Li dendrites generated massive quantities of lifeless Li species, which consumes the restricted electrolyte and lively Li, and hinders the next Li ion transport as a result of spatial obstruction4. Consequently, this regularly will increase the impedance and, ultimately, the overpotential. In distinction HFA-Li exhibited a decrease overpotential than Naked-Li throughout biking. And the cell potential of the HFA-Li symmetric cell within the carbonate electrolyte was steady, indicating that the expansion of the Li dendrites throughout biking was successfully suppressed. Particularly, the efficiency of the HFA-Li symmetric cell fabricated on this examine is healthier than these of beforehand reported Li/Li symmetric cells in carbonate- and ether-based electrolytes (Supplementary Desk S1).
Determine 3g–n reveals the SEM pictures of the Li/Li symmetrical cells containing HFA-Li and Naked-Li electrodes after 100 and 200 cycles. The formation of Li dendrites on the Naked-Li floor was obvious. The dendrites exhibited a unfastened porous construction, which is indicative of the non-uniform deposition of Li. In distinction, the HFA-Li floor was very flat and compact. From the cross-sectional SEM picture of HFA-Li, the deposited Li layer was uniform, additional confirming that the Li fluorinated carboxylate interface promotes uniform Li deposition and successfully inhibits Li dendrite formation.
Fluoroethylene carbonate (FEC) is an efficient SEI-forming additive that may improve the steadiness of Li steel anodes26. Herein, the efficiency of the HFA-Li anode in a carbonate electrolyte with out FEC was additionally investigated. Supplementary Fig. S10 reveals the voltage–time curves of the Li/Li symmetric cell in a 1 M LiPF6 in EC/EMC (v/v = 3:7) electrolyte. At 1.0 mA cm–2 and 0.5mAh cm−2, HFA-Li exhibited 350 h of biking. At 2.0 mA cm–2 and 1.0 mAh cm−2, HFA-Li could be cycled 220 h (Supplementary Fig. S11). In distinction, the Naked Li anode could be cycled for under 120 and 40 h, respectively. As well as, HFA-Li registered a CE towards the Li plating/stripping course of as excessive as 93.71% (Supplementary Fig. S12). The worth of the CE of the Naked-Li anode was solely 78.93%. The above outcomes indicated that HFA-Li nonetheless reveals higher electrochemical efficiency in carbonate electrolytes with out FEC. And the addition of FEC within the electrolyte significantly improved the efficiency of the HFA-Li anode. As for Naked-Li, when FEC was launched into the electrolytes, regardless of the next formation of a LiF-rich SEI layer, the constraints imposed by the inside SEI layer compromise the steadiness of Li steel anode25,26. After HFA therapy, the native SEI layer is faraway from the HFA-Li floor, thereby forming a uniform lithium fluorocarboxylate containing layer. The LiF-rich SEI layer shaped on the floor in the course of the subsequent FEC biking decomposition can additional improve the safety impact on Li steel anode as within the mechanism proven in Supplementary Fig. S13.
Electrochemical efficiency of the Li||NMC811 full cells
Determine 4a–c reveals the total cell take a look at outcomes recorded utilizing a LiNi0.8Co0.1Mn0.1O2 cathode (NMC811) with a excessive space loading of 20 mg cm–2 and 50-μm thick Li foil anode. The HFA-Li | |NMC811 full cell demonstrated a steady cycle of over 300 cycles with 83.2% capability retentions. Though the electrolyte-induced resistive SEI constructed up throughout Li plating/striping, resulting in a rise in polarization, particularly within the early stage of battery biking, HFA-Li homogenized the Li ion flux, which in flip induces uniform Li deposition and minimized the incidence of tortuous lithium dendrites. In consequence, no appreciable capability drop and cell sudden demise have been noticed throughout biking. In distinction, the unstable interface between Naked-Li and electrolyte accelerated the publicity and development of recent lithium dendrites, resulting in the quick depletion of Li reservoir. The capability of the Naked-Li||NMC811 full cell quickly decayed after solely 50 cycles. The continual formation of lifeless Li within the Naked-Li anode and steady consumption of the restricted electrolyte in the course of the biking severely affected the ion transport, which led to the abrupt discount within the capability of the Naked-Li | |NMC811 cell. The noticed fast capability decay is indicative of the instability of the interface between the Naked-Li anode and electrolyte. Particularly, the efficiency of the HFA-Li | |NMC811 full cell fabricated on this examine is healthier than these of beforehand reported Li | |NMC811 full cells in carbonate- and ether-based electrolytes (Supplementary Desk S2).
Electrochemical impedance spectroscopy (EIS) measurements have been additionally carried out to judge the change within the electrode/electrolyte interface properties of Naked-Li and HFA-Li after a long-term cycle. Determine 4d–f reveals the EIS assessments profiles of the Li||NMC811 cells after 2, 50, and 100 cycle of the total cell testing. All spectra include a semicircle at excessive and medium frequencies, respectively, and a diagonal line at low frequencies, which correspond to the cost switch resistance (Rct), the impedance of SEI movie (RSEI) and Warburg impedance, respectively. Particularly, The worth of Z′ on the highest frequency is outlined as Rs, which correspond to interface impedance inflicting by aspect response and dendrites development4,27. After the preliminary two cycles, the values of Rct, RSEI, and RS for HFA-Li and Naked-Li have been comparable. Because the cycle proceeded to 50 and 100 cycles, the Rct, RSEI, and RS of each HFA-Li and Naked-Li regularly elevated. Nonetheless, it may be noticed that the values of Rct and RSEI are nonetheless shut to one another after cycles, whereas the Rs of Naked-Li are considerably bigger than that of HFA-Li, indicating that the cells assembled by Naked-Li endure extra extreme inside aspect reactions and dendrites development throughout biking.
Lithiophilic protecting interface of the HFA-Li anode
To probe the vital impact of HFA-Li on uniform lithium-ion flux and the lithiophilic efficiency of Li steel anode, the interactions of HFA-Li, BA-Li, and single Li ion with the Li substrate have been evaluated by density useful concept (DFT) calculations. Determine 5a, b and Supplementary Fig. S14 reveals the steady configurations and corresponding cost density distinction of BA-Li, HFA-Li and Li ion on the floor of Li (100). The blue and yellow areas symbolize cost loss and accumulation, respectively (Fig. 5a, b). The absorption of HFA-Li has a big affect on the digital state and cost distribution of the encompassing space, resulting in an electron switch from Li to O atoms surrounding the area15. The adsorption energies of HFA-Li and BA-Li on a Li substrate have been decided to be −2.11 eV and −1.99 eV, respectively, after absolutely optimizing the construction. For comparability, the Li adsorption power of the Li substrate was decrease at −1.61 eV (Supplementary Fig. S14). The PDOS (Projected density of state) of adsorbed Li on Li substrate with BA-Li and HFA-Li have been additionally evaluated primarily based on DFT calculations (Fig. 5c, d). When HFA-Li was launched, the O atom of HFA-Li interacted considerably with the Li adatom in comparison with the introduction of BA-Li. The excessive orbital hybridization of Li-s and O-2p states signifies the robust interactions between HFA-Li and adsorbed Li on Li substrate. The adsorption power of the Li substrate for HFA-Li was additionally larger than that for BA-Li. This may be attributed to the robust electron-withdrawing impact of the C–F useful teams, which additional promotes the electron switch and enhances the HFA-Li adsorption capability28. And the robust adsorption power of HFA-Li with Li floor can suppress the longitudinal development of Li and facilitate the lateral development of Li to attain uniform Li deposition29. Owing to their robust interplay with the Li substrate, the O atoms within the carboxylic acid group can act as nucleation websites for Li deposition to homogenize Li ion flux, promote uniform Li deposition, and consequently enhance the lithiophilic character of the interface15,16. Subsequently, the carboxyl and C–F useful teams of HFA-Li have been presumably vital to make sure uniform Li+ ion flux and Li deposition, and suppress Li dendritic development throughout biking.
To research the wettability of the Naked-Li and HFA-Li surfaces in carbonate electrolytes, contact angle measurements have been carried out (Fig. 5e). The contact angle of the Naked-Li floor (26.2°) is way larger than that of HFA-Li (12.5°). HFA-Li exhibited higher wettability than Naked-Li presumably as a result of presence of polar teams on its floor, which facilitated its interplay with the polar electrolyte13. Subsequently, HFA therapy improved the affinity of the Li floor with the carbonate-based electrolyte. Increased affinity and wettability promote environment friendly Li ion transport, which homogenizes the distribution of the Li ions close to the Li anode and reduces the inner impedance of the cell16.
To straight observe the impact of HFA therapy on the Li deposition conduct, in situ electrochemical optical microscopy was carried out whereby an optical electrochemical cell manufactured from a polytetrafluoroethylene chamber with a quartz window was used (Supplementary Fig. S15). Determine 5f reveals the real-time optical pictures of the in-situ Li deposition on the HFA-Li and Naked-Li substrates at a present density of 5 mA cm–2 taken after 0, 2, 4, 6, 8, and 10 min. As well as, movies of the dynamic Li deposition course of are additionally included within the Supporting Info (Supplementary Motion pictures 1 and a pair of). Li dendritic development can already be noticed after 2 min of plating on the Naked-Li floor. Because the deposition course of progressed, the dendritic development intensified, which shaped inhomogeneous and huge Li dendrites on the Naked-Li floor. Finally, moss-like areas with erratically distributed Li dendrites have been shaped. Because of the variation of the chemical composition of the pristine SEI alongside the Naked-Li floor, the kinetics of Li deposition differ at totally different factors on the substrate, ensuing to the non-uniform flux of Li ions and selling the expansion of the Li dendrites30. In distinction, the expansion of Li dendrites on the HFA-Li floor was successfully suppressed. Li dendrites weren’t shaped on HFA-Li even after 10 min of deposition. Solely a gradual change within the luster of the HFA-Li floor was noticed, which could be attributed to the uniform deposition of advantageous Li grains all around the HFA-Li substrate. As such, Li ions could be deposited uniformly on the Li steel floor after HFA therapy. HFA therapy removes the inherent passivation layer on the Li floor whereas producing a brand new synthetic SEI protecting interface. The formation of the lithiophilic interface modified the floor power of the anode and in flip, the power panorama of ion deposition, thereby reworking the Li deposition conduct on the Li floor. Determine 5g reveals the Tafel plots of the HFA-Li and Naked-Li anodes. Supplementary Fig. S16 reveals the corresponding voltage–present curves. The change present density (i0) is obtained from the Tafel equation21. The worth of i0 of HFA-Li (0.463 mA cm–2) is larger than that of Naked-Li (0.150 mA cm–2), which suggests that the cost switch processes are sooner on the electrode/electrolyte interface of HFA-Li than these on Naked-Li.
Electrochemical impedance spectroscopy (EIS) was carried out at totally different temperatures to check the impact of HFA therapy on the cost switch resistance of HFA-Li and Li deposition activation energies. EIS was additionally carried out on Li anodes modified utilizing totally different HFA concentrations (Supplementary Fig. S17). Li/Li symmetric cells have been assembled utilizing the HFA-Li and Naked-Li electrodes. EIS measurements have been recorded earlier than biking (Fig. 5h and that i). Desk S3 and S4 summarize the values of Rs and Rct obtained by equal circuit becoming (Supplementary Fig. S18). The diameters of the semicircles within the Nyquist plots of the HFA-Li anode obtained at totally different temperatures have been smaller than these of Naked-Li, indicating that HFA therapy decreased the Rct of Li steel anode. HFA-Li registered a better worth of i0 and decrease Rct, which signifies that the power barrier for Li deposition on the HFA-Li floor was decrease than that on the Naked-Li substrate. To research the mechanism additional, the activation energies (Ea) for Li deposition on the HFA-Li and Naked-Li floor have been calculated31. The decrease activation power of the HFA-Li floor (49.25 kJ mol–1) than the Naked-Li pattern (55.26 kJ mol–1) displays the decrease Li deposition power barrier on HFA-Li and suggests the robust lithiophilic character of the HFA-treated Li anode (Fig. 5j).