Production of Uniformly Sized Gallium-Based Liquid Alloy Nanodroplets Via Ultrasonic Method and Their Li-Ion Storage

materials

Article Production of Uniformly Sized Gallium-Based Liquid Alloy Nanodroplets via Ultrasonic Method and Their Li-Ion Storage

Chenghao Huang 1 , Junjie Zong 1, Xiaodong Wang 1, Qingpin Cao 1, Dongxian Zhang 1,2 and Jian-Zhong Jiang 1,*

1 International Center for New-Structured Materials (ICNSM), Laboratory of New-Structured Materials, State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China; [email protected] (C.H.); [email protected] (J.Z.); [email protected] (X.W.); [email protected] (Q.C.); [email protected] (D.Z.) 2 State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou 310027, China * Correspondence: [email protected]

Abstract: Gallium-based liquid alloys are attractive due to their unique properties, and they can potentially be applied in the field of flexible electronics as coolant materials for nuclear and liquid batteries, due to the high thermal conductivity and excellent fluid properties of liquid metals. However, it is still challenging to fabricate gallium-based liquid alloy nanodroplets with uniform and small size. Here, we performed a systematical study on the influence of various factors affecting the size of nanodroplets. Liquid metal nanodroplets with an average size of 74 nm and narrow size distribution were successfully fabricated. Li-ion half-cells were assembled with eutectic GaIn (eGaIn) nanodroplets as anode active materials, which showed higher specific capacity than the bulk eGaIn

alloy under the same testing conditions. Keywords: Ga-based liquid alloys; nanodroplets; ultrasonic technique; surfactant; centrifugation; Citation: Huang, C.; Zong, J.; Wang, X.; Cao, Q.; Zhang, D.; Jiang, J.Z. Li-ion battery Production of Uniformly Sized Gallium-Based Liquid Alloy Nanodroplets via Ultrasonic Method and Their Li-Ion Storage. Materials 1. Introduction 2021, 14, 1759. https://doi.org/ The concept of liquid alloys has a long history [1]. Generally speaking, in the periodic 10.3390/ma14071759 table of elements, post-transition elements, zinc group elements and their alloys, and Group IA elements and their alloys belong to this kind of emerging materials, such as Ga, Academic Editor: Masato Sone GaIn, Hg, NaK, etc. They have both metallic and liquid properties, such as fluidity, high conductivity, and high thermal conductivity. Compared with amalgam [2] and alkali metal Received: 28 February 2021 elements, gallium-based alloys have attracted much attention due to their low toxicity, low Accepted: 31 March 2021 volatility, and biocompatibility. At present, nanostructured gallium-based liquid alloys, Published: 2 April 2021 e.g., nanodroplets, have many applications in the fields of reconfigurable electronics, soft robotics, and conformable medical devices [3–5]. For example, it has been proved that Publisher’s Note: MDPI stays neutral adriamycin (DOX)-treated ultrasonic gallium-based alloy nanodroplets can be used for with regard to jurisdictional claims in tumor-targeted therapy and enhance the chemotherapy efficacy of tumor-bearing mice [6]. published maps and institutional affil- iations. In addition to biomedical applications, nanodroplets of gallium and its alloys are often used in sensor devices to increase the surface to volume ratio for significant improvement of the sensitivity [7]. Although gallium-based alloy nanodroplets have many applications, the issue of how to prepare these nanodroplets or even high-quality nanodroplets (uniform narrow Copyright: © 2021 by the authors. size distribution and small average size below 100 nm) is still challenging. Microfluidics Licensee MDPI, Basel, Switzerland. produced uniform liquid alloy microdroplets [8–11], but with limited effects on the size This article is an open access article of liquid alloy droplets [12]. Physical vapor deposition was also used to prepare liquid distributed under the terms and conditions of the Creative Commons alloy nanodroplets on various substrates. The most common strategy is to disperse bulk Attribution (CC BY) license (https:// liquid alloys to micro or nano scale by an ultrasonic technique in an appropriate solvent creativecommons.org/licenses/by/ with surfactants. Hohman et al. found that ultrasound treatment could effectively emulsify 4.0/). liquid metal, and assist the formation of liquid metal nanodroplets through sulfhydryl

Materials 2021, 14, 1759. https://doi.org/10.3390/ma14071759 https://www.mdpi.com/journal/materials Materials 2021, 14, x FOR PEER REVIEW 2 of 11

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emulsify liquid metal, and assist the formation of liquid metal nanodroplets through sulfhydryl self-assembly [13]. Finkenauer et al. studied the effects of solvents and self-assemblysurfactants on [ 13 the]. Finkenauer formation andet al. stability studied the of gallium effects of-based solvents alloy and nanodroplets surfactants on in thean formationultrasonic andmethod, stability and offound gallium-based that ethanol alloy and nanodroplets octadecanethiol in an were ultrasonic better method,choices [14]. and foundHowever, that ethanolthe nanodroplets and octadecanethiol obtained by were these better methods choices are [14 still]. However, large in thesize, nanodroplets and highly obtaineduneven with by thesea wide methods size distribution. are still large in size, and highly uneven with a wide size distribution.In this work, the effects of various factors on the preparation of gallium-based alloy nanodropletsIn this work, by an the ultrasonic effects of various method factors will be on systematicallythe preparation investigated, of gallium-based especially alloy nanodropletsultrasonic amplitude, by an ultrasonic time, and method surfactant will be systematically concentration, investigated,among other especially factors. ul- In trasonicaddition, amplitude, eutectic GaIn time, (eGaIn) and surfactant nanodroplets concentration, can be applied among as other active factors. electrode In addition,material eutecticfor Li-ion GaIn batteries (eGaIn) owing nanodroplets to their high can theoretical be applied capacity as active and electrode self-healing material properties for Li-ion as batteriescompared owing with to bul theirk high eGaIn theoretical liquid. capacityThus, a and half self-healing-cell Li-ion properties battery usingas compared eGaIn withnanodroplets bulk eGaIn prepared liquid. here Thus, was a half-cell assembled, Li-ion which battery shows using average eGaIn nanodropletsdischarge specific prepared here was assembled, which shows average discharge speciﬁc capacity of 582.6 and capacity of 582.6 and 379.6 mAh g−1 at a current density of 0.1 and 3.0 A g−1, respectively, 379.6 mAh g−1 at a current density of 0.1 and 3.0 A g−1, respectively, which is much better which is much better than bulk eGaIn. than bulk eGaIn.

2. Materials and Methods 2.1. Synthesis of Eutectic GaIn Nanodroplets For the preparation of a eutectic GaIn alloy (eGaIn, 85.8 at.% Ga and 14.2 at.% In), purepure galliumgallium andand indiumindium withwith puritypurity higherhigher thanthan 99.9999.99 at.%at.% werewere used.used. TheThe mixturesmixtures ofof purepure galliumgallium andand indiumindium were were sealed sealed in in quartz quartz tubes tubes under under a vacuuma vacuum of of ~5 ~5× ×10 10−−44 Pa andand heated at 250 ◦°CC for aa fewfew hourshours untiluntil completecomplete meltingmelting inin aa thermostaticthermostatic oiloil bath.bath. AfterAfter melting, the the alloys alloys were were cooled cooled to roomto room temperature temperature and andtransferred transferred into sealed into seal vialsed using vials ausing plastic a transfer plastic transfer pipette for pipette further for use. further The second use. The step second is to prepare step is eGaIn to prepare nanodroplets. eGaIn Innanodroplets. this process, 0.2In g this of eGaIn process alloy, 0.2 and g 20 of mL eGaIn surfactant alloy solution and 20 (octadecanethiol mL surfactant dissolved solution into(octadecanethiol ethanol) are droppeddissolved into into a ethanol) 25 mL beaker. are dropped Nitrogen into gas a was25 mL used beaker. to remove Nitro oxygengen gas forwas 5 used min to remove improve oxygen combined for 5 efficiency min to improve between combined ligand andefficiency liquid between alloy. Then, ligand eGaIn and alloyliquid was alloy. synthesized Then, eGaIn into nanoparticles alloy was synthesized by performing into ultrasonication nanoparticles with by performing the Sonics Vibra-Cellultrasonication Ultrasonic with Processor the Sonics VCX 750 Vibra (Sonics&Materials-Cell Ultrasonic Inc, Newton, Processor MA, VCX USA) 750for different(Sonics&Materials times (1, 2, Inc 4,, 6,Newton and 12, MA, h). Ultrasonication USA) for different processor times was (1, operated 2, 4, 6, and at 750 12 W,h). 20Ultrasonication kHz, and different processor amplitudes was operated (30%, 40%,at 750 and W, 50%20 kHz, of the and maximum). different amplitudes In the ultrasonic (30%, process,40%, and an 50% ice of bath the wasmaximum). used to In mitigate the ultrasonic the temperature process, an fluctuation. ice bath was The used final to solutionmitigate wasthe temperature centrifuged tofluctuation. remove the The surfactant final solution and dropped was centrifuged onto a silicon to remove wafer. Thethe surfactant synthesis processand dropped of eGaIn on nanodropletsto a silicon by wafer. sonicating The synthesis the bulkeGaIn process alloy of precursor eGaIn nanodroplets is schematically by illustratedsonicating inthe Figure bulk 1eGaIn. alloy precursor is schematically illustrated in Figure 1.

FigureFigure 1. Schematic illustration of the fabrication process ofof eutecticeutectic GaInGaIn (eGaIn)(eGaIn) nanodroplets.nanodroplets.

2.2. Material Characterization The nanodroplets were dropped ontoonto a silicon wafer for structural characterization. AfterAfter thethe ethanolethanol volatilized,volatilized, scanning electron microscopy (SEM, Zeiss Supra 55,55, Zeiss,Zeiss, Heidenheim an an der der Brenz, Brenz Germany), Germany equipped) equipped with with an energy an energy dispersive dispersive X-ray X spec--ray trometerspectrometer (EDS) (EDS) was used was to used characterize to characterize the morphology the morphology and composition, and composition, respectively, andrespectively, Nanomeasure and Nanomeasure (Nano Measurer (Nano 1.2, NanomeasurerMeasurer 1.2, Nanomeasurer is developed by is Department developed byof Chemitry,Department Fudan University, of Chemitry, https://nano-measurer.software.informer.com/ Fudan University, https://nano (accessed- onmeasurer.software.informer.co 1 April 2021), Shanghai, China)m/ (accessed software wason 1 usedApril to 2021 calculate), Shanghai the size, China distribution) software of nanodroplets. It was found that all droplets prepared here have the same composition as the bulk liquid alloy.

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was used to calculate the size distribution of nanodroplets. It was found that all droplets Materials 2021, 14, 1759 was used to calculate the size distribution of nanodroplets. It was found that all droplets3 of 11 prepared here have the same composition as the bulk liquid alloy. prepared here have the same composition as the bulk liquid alloy. 2.3. Electrochemical Measurements 2.3. Electrochemical Measurements A slurry was made by mixing the nanodroplets, super P carbon black, and A slurry was made by mixing the nanodroplets, super P carbon black, and polyvinylideneA slurry was fluoride made by(PVDF) mixing binder the nanodroplets, in N-Methyl pyrrolidone super P carbon (NMP black,) solution and polyvinyli- at a mass polyvinylidene fluoride (PVDF) binder in N-Methyl pyrrolidone (NMP) solution at a mass deneratio ﬂuorideof 5:4:1 (PVDF)by stirring binder for in24 N-Methylh. The slurry pyrrolidone was dropped (NMP) onto solution Cu foil at aand mass dried ratio in of a ratio of 5:4:1 by stirring for 24 h. The slurry was dropped onto Cu foil and dried in a 5:4:1vacuum by stirring oven for at 2460 h. °C The for slurry 12 h. was The dropped mass loadingonto Cu foil of the and active dried in mat a vacuumerials (eGaIn oven vacuum◦ oven at 60 °C for 12 h. The mass loading of the active materials (eGaIn atnanodroplets) 60 C for 12 h.on Thethe electrode mass loading is about of the 0.7~1.0 active mg. materials The 2025 (eGaIn-type nanodroplets)on coin was assembled the elec-with nanodroplets)on the electrode is about 0.7~1.0 mg. The 2025-type coin was assembled with trodeeGaIn is nanoparticles about 0.7~1.0 mg.electrode The 2025-type and Li metal coin wasin a assembledglove box withfilled eGaIn with nanoparticlesAr gas (H2O elec-< 1.0 trodeeGaIn and nanoparticles Li metal in aelectrode glove box and ﬁlled Li withmetal Ar in gas a glove (H O

Figure 2. Schematic plot for the synthesis strategy of eGaIn nanodroplets. Figure 2. Schematic plot for the synthesis strategy ofof eGaIn nanodroplets.nanodroplets. 3.1. Ultrasonic Amplitude 3.1. Ultrasonic Amplitude Because ultrasonic amplitude was an important parameter, which provides enough Because ultrasonic amplitude was was an an important important parameter, parameter, which which providesprovides enoughenough kinetic energy to break the bulk eGaIn liquid alloy to form sufficiently small droplets, we kinetic energy to break the bulkbulk eGaIneGaIn liquidliquid alloyalloy toto formform sufﬁcientlysufficiently smallsmall droplets,droplets, wewe first studied the factor of ultrasonic amplitude. Experimental conditions: different ﬁrstfirst studied studied the the factor factor of ultrasonic of ultrasonic amplitude. amplitude. Experimental Experimental conditions: conditions: different different percent- percentages 30%, 40%, and 50% of the maximum amplitude, the same surfactant agespercentages 30%, 40%, 30%, and 40%, 50% of and the maximum50% of the amplitude, maximum the amplitude, same surfactant the sameconcentration surfactant of concentration of 1 mmol/L, the same ultrasonic time of 1 h, and ice bath to keep the 1concentration mmol/L, the of same 1 mmol/L ultrasonic, the time same of 1 ultra h, andsonic ice time bath of to keep1 h, and the temperatureice bath to keep at about the 35temperature◦C. The SEM at about images 35 for°C.the The morphology SEM images of for droplets the morphology prepared of by droplets 30%, 40%, prepared and 50% by temperature at about 35 °C. The SEM images for the morphology of droplets prepared by amplitudes30%, 40%, and are 50% shown amplitudes in Figure are3. shown It is clear in Figure that the 3. sampleIt is clear using that the 30% sample amplitude using has 30% 30%, 40%, and 50% amplitudes are shown in Figure 3. It is clear that the sample using 30% manyamplitude larger has droplets many larger up to droplets 10 µm, and up to 40% 10 amplitudeμm, and 40% further amplitude reduces further the largest reduces size. the amplitude has many larger droplets up to 10 μm, and 40% amplitude further reduces the Usinglargest 50% size. amplitude, Using 50% smaller amplitude, droplets smaller below droplets 1 µm are below formed. 1 μm Thus, are formed. 50% amplitude Thus, 50% is largest size. Using 50% amplitude, smaller droplets below 1 μm are formed. Thus, 50% usedamplitude for further is used tests. for further tests. amplitude is used for further tests.

FigureFigure 3.3. Morphology images of eGaIn nanodroplets under different ultrasonic amplitudes:A (A) Figure 3. Morphology imagesimages ofof eGaIneGaIn nanodropletsnanodroplets under under different different ultrasonic ultrasonic amplitudes: amplitudes: ( (A) 30%) 30% (B) 40%, and (C) 50%. (30%B) 40%, (B) 40% and, (andC) 50%. (C) 50%.

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3.2. Surfactant Concentration 3.2. SurfactantIn the process Concentration of preparation, the role of the surfactant is to adsorb and stabilize dropletsIn the of processeGaIn alloy, of preparation, and hinder thereunion role ofbetween the surfactant droplets. is Experimental to adsorb and conditions: stabilize dropletsdifferent of concent eGaInrations alloy, and of hinder surfactant reunion (1, 2, between 4, and droplets. 6 mmol/L), Experimental the same conditions: ultrasonic differentamplitude concentrations 50% and the of same surfactant ultrasonic (1, 2, 4, time and 6 2 mmol/L), h, and ice the bath same was ultrasonic used to amplitude keep the 50%temperature and the same at about ultrasonic 35 °C. time Figure 2 h, and4A–D ice show bath was the used morphology to keep the of temperaturenanostructured at aboutdroplets 35 ◦asC. measured Figure4A–D by showSEM, thewhile morphology the droplet of size nanostructured distributions droplets are plotted as measured in Figure by4E– SEM,H. As while shown the indroplet Figure size 4A distributions,B and the correspondingare plotted in Figure statistical4E–H. in As Figure shown 4E in,F, Figurerespectively,4A,B and more the dropletscorresponding with smaller statistical size in Figure(below4 E,F, 200 respectively, nm) were observed more droplets for the withconcentration smaller size of 2 (below mmol/L 200, with nm) werean average observed size for of the387 concentrationnm, as compared of 2 mmol/L,to the case with for anthe average concentration size of 387 of nm,1 mmol/L as compared, with toan theaverage case for size the of concentration 451 nm. When of 1 mmol/L, the surfactant with anconcentration average size increases of 451 nm. to 4 When mmol/L the and surfactant 6 mmol/L concentration with the same increases average to 4size mmol/L of 366 andnm, 6the mmol/L amount with of larger the same droplets average with size size of larger 366 nm, than the about amount 1000 of nm larger decrease dropletss. During with size the largerultrasound than aboutprocess, 1000 the nm bulk decreases. liquid alloy During is broken the ultrasound and surfactant process, as thea ligand bulk liquidis absorbed alloy ison broken the bare and surfactant surface to as retard a ligand the is absorbed reunion on of the droplets. bare surface The to higher retard the the reunion surfactant of droplets.concentra Thetion, higher the higher the surfactant the chance concentration, for ligand-covered the higher droplets the chance be smaller for ligand-covered. It seems that dropletsa critical beconcentration smaller. It seems valuethat of about a critical 2–4 concentrationmmol/L is prefer valuered of f aboutor the 2–4 preparation mmol/L isof preferrednanodroplets. for the preparation of nanodroplets.

Figure 4.4. Morphology images and size distributions of eGaIn nanodropletsnanodroplets underunder differentdifferent concen- trationsconcentrations of surfactant: of surfactant: (A,E) 1 ( mmol/L,A,E) 1 mmol/L (B,F) 2, ( mmol/L,B,F) 2 mmol/L (C,G,) ( 4C mmol/L,,G) 4 mmol/L and, ( Dand,H )(D 6, mmol/L.H) 6 mmol/L.

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3.3.3.3. UltrasonicUltrasonic TimeTime ExperimentalExperimental conditions:conditions: underunder thethe samesame surfactantsurfactant concentrationconcentration of of 2 2 mmol/L mmol/L andand ◦ thethe samesame ultrasonicultrasonic amplitudeamplitude of of 50%,50%, thethe samesame iceice bathbath keepskeeps temperaturetemperature at at about about 35 35 °CC duringduring thethe ultrasonicultrasonic process,process, differentdifferent ultrasonicultrasonic times of 2, 4,4, 66 andand 1212 hh werewere selected.selected. SEMSEM morphologymorphology characterization characterization and and droplets droplets statistics statistics results results are are shown shown in Figurein Figure5A–H 5A–. ItH. is It foundis found that that with with increasing increasing the the ultrasonic ultrasonictimes times fromfrom 2 to 12 12 h, h, the the average average size size of ofdroplets droplets is clearly is clearly reduced reduced from from 387 387nm to nm 276 to nm, 276 respectively. nm, respectively. For the For ultrasonic the ultrasonic time of time12 h, ofas 12shown h, as in shown Figure in 5D Figure,H, the5D,H, overall the overallmorphology morphology of droplets of droplets is relatively is relatively uniform uniformwith alm withost none almost above none 1 aboveμm. It 1canµm. be It concluded can be concluded that, in the that, whole in the ultrasonic whole ultrasonic process, process,before the before liquid the alloy liquid reaches alloy the reaches equilibrium the equilibrium of crushing of and crushing re bonding, and re the bonding, longer thethe longerultrasonic the ultrasonictime, the more time, the the crushing more the process, crushing and process, the smaller and the the smaller droplet the size. droplet Therefore, size. Therefore,prolonging prolonging the time is thea favourable time is a favourable factor in the factor ultrasonic in the ultrasonicprocess to processobtain small to obtain size smalldroplets. size droplets.

FigureFigure 5.5. MorphologyMorphology images images and and size size distributions distributions of of eGaIn eGaIn nanodroplets nanodroplets under under different different ultrasonic times:ultrasonic (A,E times:) 2 h (B (,AF),E 4) h2 (hC (,BG,)F) 6 4 h h and (C,G (D) ,6H h) 12and h. (D,H) 12 h.

3.4.3.4. TemperatureTemperature inin SolutionSolution ItIt shouldshould bebe notednoted thatthat duringduring ultrasonicultrasonic oscillation,oscillation, thethe temperaturetemperature ofof thethe solutionsolution increases,increases, indicating indicating thethe local local temperature temperature at liquid at liquid alloy alloy could could be higher. be higher. Thus, Thus, temperaturetemperature variability variab shouldility should be examined. be examined. Experimental Experi conditions:mental conditions: the same surfactant the same concentrationsurfactant concentration 2 mmol/L, 2 ultrasonic mmol/L, ultrasonic amplitude amplitude 50%, and the50% total, and ultrasonic the total ultrasonic time 6 h; thetime total 6 h; ultrasonic the total ultrasonic time is achieved time is viaachieved three differentvia three multiplyingdifferent multiplying ultrasonic ultrasonic “on-off” modes,“on-off” i.e., modes, (1) 24 i.e., repeats (1) 24 ofrepeats 15 min of “on”15 min and “on” 5 min and “off”, 5 min(2) “off”, 12 repeats(2) 12 repeats of 30 minof 30 “on” min

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“on” and 5 min “off”, and (3) 6 repeats of 60 min “on” and 5 min “off”. Ice bath is always applied. SEM morphology characterization and droplet size distributions are plotted in Figure 6. From Figure 6A–C, large-sized droplets of about 1.5 μm are observed for the single ultrasonic duration of 60 min. When the single ultrasonic duration is 30 min, the large-sized droplet decreases to about 1 μm. When the single ultrasonic duration is further Materials 2021, 14, 1759 6 of 11 shortened to 15 min, the overall morphology becomes uniform and the fraction of small droplets largely increases. The same results are also reflected in the size distributions of droplets in Figure 6E–G with the average droplet size of 357 nm for 60 min, 312 nm for 30 andmin, 5a minnd 224 “off”, nm andfor 15 (3) min 6 repeats. Under of the 60 mincondition “on” andof constant 5 min “off”.total ultrasonic Ice bathis time, always the applied.shorter the SEM single morphology ultrasonic characterization duration, the higher and droplet the proportion size distributions of small are droplets, plotted the in Figurelower 6 the. From fraction Figure of large6A–C, droplets large-sized, and droplets the smaller of about the overall 1.5 µm droplet are observed size. It forcan the be singleunderstood ultrasonic that duration if the time of 60 is min. too When long, the single ultrasonic ultrasonic oscillation duration will is increase 30 min, the the large-sizedtemperature droplet of the decreasessolution. However, to about 1 aµ m.short When single the ultrasonic single ultrasonic duration duration means relatively is further shortenedlower temperature to 15 min, of the the overall solution, morphology which can becomes reduce uniformthe probability and the of fraction reunion of events small dropletsbetween largelydroplets. increases. The same results are also reﬂected in the size distributions of dropletsIn addition, in Figure we6E–G alsowith compared the average the total droplet ultrasonic size of time 357 nmof 6 forh with 60 min, 12 h 312 under nm forthe 30same min, single and 224ultrasonic nm for duration 15 min. Under (15 min the), conditionas shown ofin constantFigure 6C total,D,G,H ultrasonic. It is found time, that the shorterwhen the the ultrasonic single ultrasonic time increases duration, from the 6 higher to 12 h, the the proportion average droplet of small size droplets, does not the further lower thedecrease, fraction but of largerather droplets, increases and from the smaller 228 to the 288 overall nm. dropletThese results size. It canindicate be understood that it is thatimportant if the timeto ensure is too relatively long, the lower ultrasonic temperature oscillation of the will solution, increase and the the temperature single ultrasonic of the solution.duration However, should not a short be too single long ultrasonic for the duration preparation means of relatively eGaIn nanodroplets lower temperature by the ofultrasonic the solution, technique. which can reduce the probability of reunion events between droplets.

Figure 6.6. Morphology images and size distributions distributions of of eGaIn eGaIn nanodroplets nanodroplets under under different different single single ultrasonic durations:durations: ((AA,E,E)) 66 repeats repeats of of 60 60 min min “on”, “on” (,B (B,F,)F 12) 12 repeats repeats of of 30 30 min min “on”, “on” (C, (,CG,)G 24) 24 repeats ofrepeats 15 min of “on”,15 min and “on” (D,H and) 48 (D repeats,H) 48 ofrepeats 15 min of “on”. 15 min “on”.

In addition, we also compared the total ultrasonic time of 6 h with 12 h under the same single ultrasonic duration (15 min), as shown in Figure6C,D,G,H. It is found that when the

ultrasonic time increases from 6 to 12 h, the average droplet size does not further decrease, but rather increases from 228 to 288 nm. These results indicate that it is important to ensure relatively lower temperature of the solution, and the single ultrasonic duration should not be too long for the preparation of eGaIn nanodroplets by the ultrasonic technique. Materials 2021, 14, x FOR PEER REVIEW 7 of 11 Materials 2021, 14, 1759 7 of 11

3.5. Centrifugal Process 3.5. Centrifugal Process After all the above-mentioned efforts, droplet size distribution is still wide. To obtain After all the above-mentioned efforts, droplet size distribution is still wide. To obtain high-quality liquid metal nanodroplets (i.e., uniform with smaller size distribution and high-quality liquid metal nanodroplets (i.e., uniform with smaller size distribution and smaller average size), we also tested various centrifugation speeds to separate the original smaller average size), we also tested various centrifugation speeds to separate the original solution. The experimental conditions were as follows: the concentration of surfactant was solution. The experimental conditions were as follows: the concentration of surfactant was 2 mmol/L, the ultrasonic time was 6 h, and the ultrasonic amplitude was 50%. The 2 mmol/L, the ultrasonic time was 6 h, and the ultrasonic amplitude was 50%. The different differentcentrifugal centrifugal speeds used speeds were used 500 were r/min, 500 800 r/min, r/min, 800 1000 r/min, r/min, 1000 2000r/min, r/min, 2000 r/min, 5000 r/min, 5000 r/minand 8000, and r/min8000 r/min to centrifugate to centrifugate the the original original solution solution step step by by step, step, in in whichwhich the upper upper solutionsolution of of centrifugation centrifugation was was used used for for the the next next step. SEM SEM morphology characterization andand droplet droplet size statisticsstatistics areare shown shown in in Figures Figures7 and 7 and8, respectively. 8, respectively. It is clearIt is thatclear the that large the largesize dropletssize droplets are screened are screened at low at speed. low speed. When theWhen centrifugal the centrifugal speed is speed 5000 r/min,is 5000 ther/min, size thedistribution size distribution becomes becomes uniform uniform with an with average an average size of aboutsize of 122about nm, 122 and nm, for and 8000 for r/min, 8000 r/min,the size the distribution size distribution gets even gets smallereven smaller with anwith average an average size ofsize 74 of nm. 74 nm. It is obviousIt is obvious that thatcentrifugation centrifugation is beneﬁcial is beneficial to separate to separ nanodropletsate nanodroplets with differentwith different sizes. However,sizes. However, due to duenanodroplets to nanodroplets with different with different sizes having sizes differenthaving different mass fractions, mass fractions, the yield the is low yield when is low the whencentrifugation the centrifugation speed is low speed because is low the because fraction the of fraction larger particlesof larger inparticles the sample in the is sample smaller. isThe smaller. yield The is large yield for is large the size for the ofabout size of 300–500 about 300 nm.–500 Above nm. Above 2000 r/min,2000 r/min, the higherthe higher the thecentrifugation centrifugation speed, speed, the lower the lowerthe yield. the Hence, yield. smaller Hence,and smaller narrower and size narrow distributioner size distributionof liquid eGaIn of liquid alloy nanodropletseGaIn alloy cannanodroplets be fabricated can by be an fabricated ultrasonic by technique an ultrasonic using techniqueoptimized using parameters. optimized parameters.

FigureFigure 7. 7. MorphologyMorphology images images of eGaIn of nanodroplets eGaIn nanodroplets under different under centrifugal different centrifugal speeds: (A) speeds:500 r/min(A) 500, (B r/min,) 800 r/min (B) 800, (C r/min,) 1000 r/min (C) 1000, (D) r/min, 2000 r/min (D) 2000, (E) r/min,5000 r/min (E), 5000 and r/min,(F) 8000 and r/min. (F) 8000 r/min.

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FigureFigure 8.8.Size Size distributionsdistributions of of eGaIn eGaIn nanodroplets nanodroplets under under different different centrifugal centrifugal speeds: speeds: (A ()A 500) 500 r/min, (r/minB) 800, ( r/min,B) 800 (r/minC) 1000, (C r/min,) 1000 r/min (D) 2000, (D) r/min, 2000 r/min (E) 5000, (E) r/min,5000 r/min and,( andF) 8000 (F) r/min.8000 r/min.

BasedBased on on the the above above analyses, analyses, the the eGaIn eGaIn nanodroplets nanodroplets exhibit exhibit smaller smaller and narrowerand narrower size distribution,size distribution, which which can be ca attributedn be attributed to the following to the following reasons: reasons: (1) High amplitude(1) High amplitude used can provideused can more provide energy more to theenergy liquid to the metal liquid and metal a higher and chance a higher for chance the breaking for the ofbreaking droplets. of Consequently,droplets. Consequently, smaller eGaIn smaller nanoparticles eGaIn nanoparticles are fabricated are fabricated in Figure 3in. (2)Figure The 3. thiolate (2) The surfactantthiolate surfactan can effectivelyt can effectively prevent the prevent oxidation the oxidation of the nanodroplets of the nanodroplets and assist and droplet assist cleavagedroplet cleavage to the nanoscale to the nanoscale [13]. Although [13]. Although the droplet the size droplet will decrease size will with decrease increased with surfactantincreased concentration,surfactant concentration, the higher concentration the higher concentration has less effect has on less the furthereffect on reduction the further of thereductio size [14n of]. Hence,the size a [ moderate14]. Hence, concentration a moderate couldconcentration be used. could (3) Similar be used. to the (3) surfactant Similar to concentration,the surfactant averageconcentration, diameters average will decreasediameters with will increased decreasesonication with increased time. sonication However, heattime. will However, accumulate heat aswill the accumulate ultrasound as time the increases,ultrasound which time willincreases, increase which the nanodropletwill increase size.the nanodroplet Hence, it is difﬁcult size. Hence, to reduce it is the difficult droplet to size reduce further the with droplet prolonged size further ultrasonic with timeprolong dueed to ultrasonic the balance time between due to breakingthe balance and between merging breaking of droplets and merging [15,16]. (4)of droplets Due to particle[15,16]. size(4) Due distribution to particle in size the distribution ultrasonic treated in the samples,ultrasonic nanodroplets treated samples, with nanodroplets narrow size distributionwith narrow can size be distribution obtained through can be highobtained centrifugation through high speed, centrifugation but the yield speed, is low. but the yield is low. 3.6. eGaIn Nanodroplets as Anode for Li-Ion Battery 3.6. eGaInGa and Nanodroplets In metals canas Anode form for alloys Li-Ion with Battery Li. When they are used as the anode active materials for a Li-ion battery, they could have higher theoretical capacity than commercial Ga and In metals can form alloys with Li. When they are used as the anode active graphite anodes (372 mAh g−1). Hence, a Li-ion half-cell was assembled to evaluate materials for a Li-ion battery, they could have higher theoretical capacity than commercial the potential application of the eGaIn nanodroplets prepared here. Figure9A shows the graphite anodes (372 mAh g−1). Hence, a Li-ion half-cell was assembled to evaluate the schematic illustration for the structure and the charge storage mechanism of Li-ion half-cell. potential application of the eGaIn nanodroplets prepared here. Figure 9A shows the After full discharge, the liquid alloy is lithiated to form a solid alloy, and its volume expands. schematic illustration for the structure and the charge storage mechanism of Li-ion half-

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cell. After full discharge, the liquid alloy is lithiated to form a solid alloy, and its volume expands. During charging, lithium ions are released and the solid alloy gradually returns Duringto the initial charging, liquid lithium state. ionsFigure are 9B released,D exhibit ands thethat solid the alloyaverage gradually discharge returns specific to the capacity initial liquidof the state. eGaIn Figure nanodroplets9B,D exhibits is 582.6, that the532.8, average 466.5, discharge 439.1, 402.7 specific, and capacity 379.6 mAh of the g−1 eGaIn at the − nanodropletscurrent densities is 582.6, of 0.1, 532.8, 0.2, 466.5,0.5, 1.0, 439.1, 2.0, and 402.7, 3.0 and A g 379.6−1, respectively, mAh g 1 at meaning the current excellent densities rate − ofperformance 0.1, 0.2, 0.5, (65.2% 1.0, 2.0, after and the 3.0 current A g 1 ,density respectively, increased meaning 30 times). excellent As shown rate performancein Figure 9C, (65.2%the discharge after the specific current capacities density increased of these three 30 times). cycles As at shown 0.1 A g in−1 Figureare 889,9C, 623.1, the discharge and 597.5 −1 −1 specificmAh g−1 capacities, respectively. of these The threefirst cycle cycles shows at 0.1 high A g dischargeare 889, specific 623.1, and capacity 597.5 mAhdue to g the, respectively.existence of Thean irreversible first cycle shows reaction high and discharge the formation specific capacity of an SEI due layer. to the Moreover existence, the of ancontours irreversible of the reaction discharge and and the charge formation curves of are an SEIsimilar layer. to Moreover,those previously the contours reported of [ the17– discharge20], representing and charge thecurves gradual are lithiation/delithiation similar to those previously of Ga/In reported through[17– 20 the], representing intermediate thealloy. gradual lithiation/delithiation of Ga/In through the intermediate alloy.

FigureFigure 9.9. ((A) Schematic illustration illustration for for the the structure structure and and the the charge charge storage storage mechanism mechanism of of Li Li-ion-ion half-cell.half-cell. (B)) Charge–dischargeCharge–discharge curves at at different different current current densities densities and and ( (CC)) the the first ﬁrst three three charge charge–– discharge curves at 0.1 A g−1 of eGaIn nanodroplets. (D) Cycling stability tests and (E) rate- discharge curves at 0.1 A g−1 of eGaIn nanodroplets. (D) Cycling stability tests and (E) rate-capability capability tests of eGaIn nanodroplets and bulk eGaIn alloy. (F) Cycling performance of eGaIn tests of eGaIn nanodroplets and bulk eGaIn alloy. (F) Cycling performance of eGaIn nanodroplets. nanodroplets. The excellent performance of the eGaIn nanodroplets was further veriﬁed by com- parison with the bulk eGaIn alloy. The eGaIn nanodroplets exhibit at least 80% higher

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reversible capacity than bulk eGaIn alloy at various current densities (Figure9E). It is worth noting that the bulk eGaIn alloy has almost no performance at high current density. In addition, Figure9E shows that the performance of the bulk eGaIn alloy begins to decline rapidly after 20 cycles, and there is almost no performance after 60 cycles. Correspondingly, the performance of the eGaIn nanodroplets starts to decline after 40 cycles and stabilizes at around 151 mAh g−1, meaning the eGaIn nanodroplets have better cycle stability than bulk eGaIn liquid. Figure9F further confirms the excellent cycle stability of the eGaIn nanodroplets. The long-term cycle test shows a stable specific capacity retention of the eGaIn nanodroplets after stabilization of 88.6% (100–600 cycle). Based on the above analyses, the eGaIn nanodroplets exhibit better performance, which can be attributed to the following possible reasons: (1) The nanodroplets increase the exposure area of the electroactive material, and make more materials participate in the reaction. (2) The nanodroplets’ structure not only facilitates easy electrolyte access, but also reduces the ion diffusion transport distance, making the ion diffusion fast within active materials. (3) Bulk eGaIn alloy has a huge volume expansion (about 160%) after full lithiation, causing the bulk material to fall off the current collector. The eGaIn nanodroplets are beneficial to relieve the stress induced by lithiation/delithiation of eGaIn. The carbon black is also beneficial to protect the nanodroplets deposited on the bottom from falling off the current collector.

4. Conclusions The preparation process of liquid eGaIn alloy nanodroplets has been systematically investigated by an ultrasonic method, especially the effects of ultrasonic amplitude, the concentration of surfactant, ultrasonic oscillation time, ultrasonic duration, and rotation speed. In order to obtain droplets with small size, the experimental conditions should be optimized as follows: high amplitude, moderate concentration of surfactant, long ultrasonic time, and short duration of single ultrasonic oscillation. Successive centrifugation of the original solution is required to further achieve high-quality uniform liquid eGaIn alloy nanodroplets with narrow size distribution and small average size below 100 nm. Fur- thermore, we applied eGaIn nanodroplets as an anode active material for Li-ion batteries, which shows higher speciﬁc capacity than bulk eGaIn under various current densities.

Author Contributions: J.Z.J. designed the research project; C.H. and J.Z. contributed to experimental design, investigation, methodology and writing. J.Z.J., X.W., Q.C. and D.Z. contributed to data analysis and discussion. All authors have read and agreed to the published version of the manuscript. Funding: National Natural Science Foundation of China (U1832203 and 11975202), National Key Research and Development Program of China (2017YFA0403400), Natural Science Foundation of Zhejiang Province (LY15E010003 and LZ20E010002). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: Financial support from the National Natural Science Foundation of China (U1832203 and 11975202), National Key Research and Development Program of China (2017YF A0403400), Natural Science Foundation of Zhejiang Province (LY15E010003 and LZ20E010002), and the Fundamental Research Funds for the Central Universities is gratefully acknowledged. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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