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Interconnected hollow carbon nanospheresfor stable lithium metal anodesGuangyuan Zheng1, Seok Woo Lee2, Zheng Liang2, Hyun-Wook Lee2, Kai Yan2, Hongbin Yao2,Haotian Wang3, Weiyang Li2, Steven Chu4 and Yi Cui2,5*
For future applications in portable electronics, electric vehicles and grid storage, batteries with higher energy storagedensity than existing lithium ion batteries need to be developed. Recent efforts in this direction have focused on high-capacity electrode materials such as lithium metal, silicon and tin as anodes, and sulphur and oxygen as cathodes. Lithiummetal would be the optimal choice as an anode material, because it has the highest specific capacity (3,860 mAh g1) andthe lowest anode potential of all. However, the lithium anode forms dendritic and mossy metal deposits, leading to serioussafety concerns and low Coulombic efficiency during charge/discharge cycles. Although advanced characterizationtechniques have helped shed light on the lithium growth process, effective strategies to improve lithium metal anodecycling remain elusive. Here, we show that coating the lithium metal anode with a monolayer of interconnected amorphoushollow carbon nanospheres helps isolate the lithium metal depositions and facilitates the formation of a stable solidelectrolyte interphase. We show that lithium dendrites do not form up to a practical current density of 1 mA cm2. TheCoulombic efficiency improves to 99% for more than 150 cycles. This is significantly better than the bare unmodifiedsamples, which usually show rapid Coulombic efficiency decay in fewer than 100 cycles. Our results indicate thatnanoscale interfacial engineering could be a promising strategy to tackle the intrinsic problems of lithium metal anodes.
When interest in secondary lithium batteries began toemerge more than four decades ago1 it was clear that,to make viable Li metal anodes, two fundamental chal-lenges would need to be resolved: (1) accommodating the largechange in electrode volume during cycling (unlike graphite andsilicon anodes, where lithiation produces volume changes of10% and 400%, respectively, Li metal is hostless and its relativevolumetric change is virtually infinite); and (2) controlling the reac-tivity towards the electrolyte (Li is one of the most electropositiveelements)26. Even today, there is still very little control over thethickness, grain size, chemical composition and spatial distributionof the solid electrolyte interphase (SEI), which, together, make thebattery inefficient7,8. One problem lies in the fact that the SEIlayer cannot withstand mechanical deformation and continuouslybreaks and repairs during cycling. As a result, Li metal batterieshave low Coulombic efficiency (8090% for carbonate solventsand 9095% for ether solvents)9 and low cycle life due to therapid loss of Li and electrolyte in the continuous formation/dissolu-tion of the SEI10. A second problem is that Li deposition is notuniform across the electrode surface and can form large dendritesthat cause short circuiting of the battery1113. Third, reactionsbetween the Li metal and the electrolytes are exothermic and largesurface areas can pose risks of overheating (thermal runaway)14.
Considerable effort has been directed at addressing these pro-blems using both solid and liquid electrolytes. As solid electrolytes,polymers and ceramics have been developed for their perceivedability to mitigate dendrite nucleation15,16 and block theirgrowth1720. However, most solid electrolytes have low ionic conduc-tivity, resulting in low power output. Moreover, Li polymer batteriesneed to be operated at elevated temperatures to achieve reasonablepower, at the expense of mechanical stability2022. Ceramic
solid electrolytes with a framework structure, such as Li10GeP2S12and garnet type Li7La3Zr2O12, have been investigated fortheir high Li ion conductivity (1 102 to 1 104 S cm1)19,23,24,but, like their polymer counterparts, interfacial issues remainlargely unresolved25,26.
In the case of liquid electrolytes, a great deal of research hasfocused on using additives2729 together with chemical passivationof the Li metal surface to reduce electrolyte decomposition30,31.However, the thin films formed on the Li metal using thesemethods consist mainly of Li compounds, which are brittle andhave limited cohesion with the metal surface32. Consequently,upon Li deposition, the film surface usually cracks as a result ofvolumetric expansion, exposing fresh Li metal for further reactions(Fig. 1a). Lithium dissolution then takes place, creating pits and cre-vices with low impedance33, and Li ions flow at the defects, leadingto rapid growth of metal filaments and dendrites. Stabilizing theinterface between the Li metal and the electrolyte is therefore keyin improving the cycling performance of Li metal batteries.
The ideal interfacial layer for the Li metal anode needs to bechemically stable in a highly reducing environment, and alsomechanically strong. High flexibility is desired to accommodatethe volumetric expansion of Li deposition without mechanicaldamage. In addition, the ability to control the flow of Li ions withthe SEI inhomogeneities is essential to ensure uniform Li depo-sition33. Here, we describe a flexible, interconnected, hollow amor-phous carbon nanosphere coating with the aim of realizing suchan ideal interfacial layer (Fig. 1b). The advantages of our approachare threefold: (1) amorphous carbon is chemically stable when incontact with Li metal; (2) the thin amorphous carbon layer doesnot increase the impedance to charge transfer, but has a Youngsmodulus34 of 200 GPa, high enough to suppress Li dendrite
1Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025, USA, 2Department of Materials Science and Engineering,Stanford, California 94305-4034, USA, 3Department of Applied Physics, Stanford, California 94305, USA, 4Department of Physics, Stanford University,Stanford, California 94305, USA, 5Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road,Menlo Park, California 94025, USA. *e-mail: email@example.com
ARTICLESPUBLISHED ONLINE: 27 JULY 2014 | DOI: 10.1038/NNANO.2014.152
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology 1
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growth (theoretical calculations have shown that a solid film withmodulus higher than 6 GPa should be sufficient to this end20);and (3) a hollow nanosphere layer is weakly bound to the metalcurrent collector and can move up and down to adjust the avail-ability of empty space during cycling. The top surface, formedfrom the hollow carbon nanospheres, is static and forms a stable,conformal SEI, while Li metal deposition takes place underneath,on the metal current collector. The stable SEI on the carbon nano-sphere surface helps reduce the flow of Li ions towards the regions ofbroken SEI on the metal current collector (Supplementary Fig. 1).
Fabrication of electrodeWe have developed a template synthesis method for fabricating thehollow carbon nanopheres, using vertical deposition of polystyrenenanoparticles (Fig. 2a). A colloidal multilayer opal structure isformed on Cu foil by slowly evaporating a 4% aqueous solution ofcarboxylated polystyrene particles. The highly monodisperse poly-styrene nanoparticles form a close-packed thin film with long-range order (Fig. 2b)35. The polystyrene nanoparticles are coatedwith a thin film of amorphous carbon using flash evaporation ofcarbon fibres (Fig. 2c). The slight morphology change of the
Carbon-spheres thin film
Li+ Li+pheresm Li
Li+ Li+ Li+
Li+ Li+ Li+Li+Li+Li+
Lithium deposition Lithium dissolution Cycling
Figure 1 | Schematic diagrams of the different Li anode structures. a, A thin film of SEI layer forms quickly on the surface of deposited Li (blue). Volumetricchanges during the Li deposition process can easily break the SEI layer, especially at high current rates. This behaviour leads to ramified growth of Lidendrites and rapid consumption of the electrolytes. b, Modifying the Cu substrate with a hollow carbon nanosphere layer creates a scaffold for stabilizingthe SEI layer. The volumetric change of the Li deposition process is accommodated by the flexible hollow-carbon-nanosphere coating.
b c d
e f g
50 nm500 nm
Hollow carbon spheres
Figure 2 | Fabrication of hollow carbon nanosphere-coated electrode. a, Fabrication process for the hollow carbon nanosphere-modified Cu electrode. Leftto right: Polystyrene nanoparticles are first deposited onto the Cu substrate; a thin film of amorphous carbon is coated on top of the polystyrene array usingflash-evaporation of carbon cord; thermal decomposition of the polystyrene template results in the formation of interconnected hollow carbon nanospheres.b,c, SEM images of the carbon-coated polystyrene nanoparticle array at low (b) and high (c) magnifications. The slight morphology change of the carbonnanospheres to a hexagonal shape could be due to the elevated temperature during the carbon-coating process. d, Digital camera image of the as-fabricatedhollow carbon nanosphere thin film after removal of the polystyrene template. e, Cross-sectional SEM image of the hollow carbon nanospheres. f, TEMimage of the hollow carbon nanospheres, with wall thickness of 20 nm. g, SEM image of the hollow carbon nanosphere thin-film peeled off the Cusubstrate. Red dashed line: trace of t