Flexible, solid-state, ion-conducting membrane with 3D ...

1 Flexible, solid-state, ion-conducting membrane with 3 Dgarnet nanofiber networks for lithium batteriesKun (Kelvin) Fua,b,1, Yunhui Gonga,1, Jiaqi Daib, Amy Gonga,b, Xiaogang Hana,b, Yonggang Yaob, Chengwei Wanga,b,Yibo Wangb, Yanan Chenb, Chaoyi Yanb, Yiju Lib, Eric D. Wachsmana,b,2, and Liangbing Hua,b,2aUniversity of Maryland Energy Research Center, University of Maryland, College Park, MD 20742; andbDepartment of Materials Science and Engineering,University of Maryland, College Park, MD 20742 Edited by Yi Cui, Stanford University, Stanford, CA, and accepted by Editorial Board Member Tobin J. Marks May 4, 2016 (received for review January 10,2016)Beyond state-of-the-art lithium-ion battery (LIB) technology withmetallic lithium anodes to replace conventional ion intercalationanode materials is highly desirable because of lithium shighestspecific capacity (3,860 mA/g) and lowest negative electrochem-ical potential ( V vs.)

2 The standard hydrogen electrode). Inthis work, we report for the first time, to our knowledge, a 3 Dlithium-ion conducting ceramic network based on (LLZO) lithium-ion conductor to provide con-tinuous Li+transfer channels in a polyethylene oxide (PEO)-basedcomposite. This composite structure further provides structural re-inforcement to enhance the mechanical properties of the polymermatrix. The flexible solid-state electrolyte composite membraneexhibited an ionic conductivity of 10 4S/cm at room temper-ature. The membrane can effectively block dendrites in a symmetricLijelectrolytejLi cell during repeated lithium stripping/plating atroom temperature, with a current density of mA/cm2for around500 h and a current density of mA/cm2for over 300 h. Theseresults provide an all solid ion-conducting membrane that can beapplied to flexible LIBs and other electrochemical energy storagesystems, such as lithium sulfur electrolyte|3D garnet nanofibers|polyethylene oxide|ionic conductor|flexible membraneHigh capacity, high safety, and long lifespan are three of themost important key factors to developing rechargeablelithium batteries for applications in portable electronics, trans-portation ( , electrical vehicles), and large-scale energy stor-age systems (1 5).

3 Based on state-of-the-art lithium-ion battery(LIB) technology, metallic lithium anode is preferable to replaceconventional ion intercalation anode materials because of thehighest specific capacity (3,860 mAh/g) of lithium and the lowestnegative electrochemical potential ( V vs. the standardhydrogen electrode), which can maximize the capacity densityand voltage window for increased battery energy density (1).Moreover, the success of beyond LIBs, such as lithium sulfurand lithium oxygen, will strongly rely on lithium metal anodedesigns with good stability to achieve their targeted goals of highenergy density and long cycle lithium metal in organic liquid electrolyte systems facesmany challenges in terms of battery performance and safety. Forexample, lithium sulfur batteries suffer from the dissolution ofintermediate polysulfides in the organic electrolyte that causessevere parasitic reactions on lithium metal surfaces, leading tolithium metal degradation and low lithium cycling efficiency (6).

4 Lithium oxygen batteries have the challenge of chemically in-stable liquid electrolytes on the oxygen electrode that causelimited battery cycling (7). All of these challenges are associatedwith the use of lithium metal in liquid electrolyte battery major associated challenge is lithium dendrite growthon lithium metal anodes, which causes internal short circuitsafter lithium dendrites penetrate through the separator andtouch the cathode. In addition, solid electrolyte interphase(SEI) formation during the uneven lithium deposition will con-tinuously consume Li metal and dry up the electrolyte, leading toan increase of cell resistance and decrease of cell Coulombicefficiency (1, 8). Although extensive studies have been per-formed to address these challenges, Li dendrite and SEI forma-tion are inevitable and mainly caused by the intrinsic problems of thethermodynamically unstable Li with low-molecular weight organicsolvents and the poor strength of formed SEI layers (1).

5 A fundamental strategy to address Li dendrite penetrationand SEI formation is to developa solid-state electrolyte tomechanically suppress the lithium dendrite and intrinsicallyeliminate SEI formation (9 14). Among the different types ofsolid-state electrolytes (inorganic oxides/nonoxides and Lisalt-contained polymers), solid-state polymer electrolyteshave been the most extensively studied (15 21). Polyethyleneoxide (PEO)-based composite electrolytes have attracted themost interest (22, 23). In PEO-based composite, powders areincorporated into a host PEO polymer matrix to influence therecrystallization kinetics of the PEO polymer chains to pro-mote local amorphous regions,thereby increasing the Li salt polymer system s ionic conductivity (15). The addition ofpowders will also improve the electrochemical stability and en-hance the mechanical strength.

6 As studied in previous work, thefillers can be either non Li+-conductive nanoparticles, such asAl2O3(15), SiO2(24), TiO2(25), ZrO2(26), and organic polymerspheres (23), or Li+-conductive nanoparticles, such as (22), tetragonal Li7La3Zr2O12(27), and (PO4)3(28). Developing nanostructured fillers is an essential approach toincrease the ionic conductivity of polymer composite electrolytesbecause of the increased surface area of the amorphous region andimproved interface between fillers and polymers. Typically, 1 DSignificanceThis work describes a Flexible, solid-state, lithium-ion conductingmembrane based on a 3D ion-conducting network and polymerelectrolyte for lithium batteries. The 3D ion-conducting network isbased on percolative garnet-type nanofibers, which enhance the ionic conductivity ofthe solid-state electrolyte membrane at room temperature andimprove the mechanical strength of the polymer electrolyte.

7 Themembrane has shown superior electrochemical stability to highvoltage and high mechanical stability to effectively block lithiumdendrites. This work represents a significant breakthrough toenable high performance of lithium contributions: K.(K.)F., , , and designed research; K.(K.)F., , , , and performed research; K.(K.)F., , , , , , , , ,and analyzed data; and K.(K.)F. and wrote the authors declare no conflict of article is a PNAS Direct Submission. is a guest editor invited by the (K.)F. and contributed equally to this whom correspondence may be addressed. Email: or article contains supporting information online 7099|PNAS|June 28, 2016|vol. 113|no. fillers, based on perovskite-type lithium-ion , were shown by Cui and coworkers (22)to enhance the ionic conductivity of the polymer composite elec-trolyte.

8 This enhanced ionic conductivity was because the nano-wire fillers provide extended ionic transport pathways in thepolymer matrix instead of an isolated distribution of nanoparticlefillers in the polymer electrolyte (22). However, the agglomerationof ceramic fillers may remain, and it will become a challenge for itsmixing with polymer to fabricate uniform solid polymer electrolyteon a large scale. To solve this challenge, in situ synthesis of ce-ramic filler particles with high monodispersity in polymer elec-trolyte was recently reported (29). By in situ synthesizingnanosized SiO2particles into PEO Li salt polymer, the reportedsolid polymer electrolyte exhibited an ionic conductivity 10 5S/cm at 30 C, which needs additional improvementto achieve a higher ionic conductivity at room on our understanding, therefore, creating a continuousnanosized network with interconnected long-range ion transportand controlling a minimum/nonfiller agglomeration are the maindirections to design high ionic-conductive polymer this work, we have successfully developed a 3D ceramicnetwork based on garnet-type (LLZO) nano-fibers to provide continuous Li+transfer channels in PEO-basedcomposite electrolytes as all solid ion-conducting membranes forlithium batteries.

9 Here, we select garnet-type lithium-ion conductingceramic as the inorganic component because of several desiredphysical and chemical properties, including (i) high ionic con-ductivity approaching 10 3S/cm at room temperature withoptimized element substitution, (ii) good chemical stabilityagainst lithium metal, and (iii) good chemical stability againstair and moisture (11, 30, 31). Fig. 1 shows the schematic structureof the 3D LLZO polymer composite membrane . The LLZO po-rous structure consists of randomlydistributed and interconnectednanofibers, creating a continuous lithium-ion conducting Li salt PEO polymer is then filled into the porous 3D ceramicnetworks, forming the 3D garnet polymer composite from conventional methods to prepare polymer electro-lytes, the 3D garnet polymer composite membrane does not need tomechanically mix fillers with polymers; instead, we can directly soaka preformed 3D ceramic structure into Li salt polymer solutions toget the desired polymer composite electrolyte hybrid structure, thussimplifying fabrication process and avoiding the agglomerationof and DiscussionFig.

10 2 schematically shows the procedure to synthesize flexiblesolid-state garnet LLZO nanofiber-reinforced polymer compos-ite electrolytes. As shown in Fig. 2A, garnet LLZO nanofiberswere prepared by electrospinning of polyvinylpyrrolidone (PVP)polymer mixed with relevant garnet LLZO salts followed by thecalcination of the as-prepared nanofibers at 800 C in air for 2 the drum collector of the electrospinning setup, a thin non-woven fabric was covered to collect the schematic fabrication of fiber-reinforced polymer com-posite (FRPC) lithium-ion conducting membrane using the 3 Dporous garnet nanofiber network is shown in Fig. 2B. A PEOpolymer mixture with Li salt, such as bis(trifluoromethane)sulfo-nimide lithium salt (LiTFSI), is prepared. Then, the Li salt PEOpolymer is reinforced by the 3D nanofibers to form a compositeelectrolyte, which can be called FRPC electrolyte with filler-containing polymer electrolyte, the FRPC electrolyte membrane maintains the framework of 3D garnetnanofiber networks and is believed to have a better mechanicalproperty because of the continuous nanofiber structure thatenhances the integrity of polymer of the as-spun PVP garnet salt nanofibers andcalcinated garnet nanofibers were characterized by SEM asshown in Fig.