Electrochemical elaboration of electrodes and electrolytes for 3D structured batteries† Mario Valvo, Matthew Roberts, Gabriel Oltean, Bing Sun, David Rehnlund, Daniel Brandell, Leif Nyholm, Torbj¨ orn Gustafsson and Kristina Edstr¨ om * The challenges associated with the fabrication of three-dimensional (3D) electrode and electrolyte materials for Li-ion batteries are discussed. The basic issues for achieving a solid 3D cell foundation, which can simultaneously offer sufficient electronic conductivity to enable stable cycling, as well as enough compatibility with the incorporation of complementary components, have been addressed. Various electrochemical strategies for elaborating such systems are discussed and critically examined. Several current collector systems are presented including electrochemically prepared Cu and Al nanorods and commercial aperiodic carbon structures. Further electrochemical coating approaches then provide a direct method for the deposition of thin layers of active materials successfully demonstrated here as coatings on both 3D metal structures and commercially available 3D-structured carbon substrates. Enhanced capacities per foot print area are demonstrated for a number of 3D electrode materials, namely polyaniline on reticulated vitreous carbon, Cu 2 O on copper nanorods and TiO 2 on Al nanorods. The crucial points for achieving a thin conformal coating of the corresponding 3D electrode structures with solid polymer electrolytes are also carefully analysed and discussed. In this context electro-polymerisation is proposed as a viable route to form thin electrolyte layers with promising characteristics. The high versatility of electro-polymerisation in combination with the various structures and methodologies adopted here represents a further step towards the development of cost-effective 3D microbattery devices. 1 Introduction Novel micro/nano electromechanical systems (MEMS/NEMS) are currently under development by using rened manu- facturing techniques. Such devices require access to mini- aturised and integrated batteries to be efficiently powered. 1–4 Typically, two-dimensional (2D) microbatteries are employed in these systems, where thin layers of cathode, electrolyte and anode materials are deposited on a planar substrate. However, these tiny devices have power requirements which far outreach those supplied by 2D thin-lm batteries. These cells usually have a foot print area of 5 cm 2 with high power density (7 mW cm 2 ) and low energy density (<1 mW h cm 2 ). 6 The feasibility of producing three-dimensional thin-lm batteries has been discussed 1–5 in the literature, where the ultimate goal is to obtain a high surface area substrate coated with thin layers of cathode, electrolyte and anode materials to enhance the energy density per foot print area, while maintaining a large power density. Long et al. 1 showed that three-dimensional nano- structured Li-ion microbatteries could satisfy the power needs of certain MEMS devices such as smart dust motes, sensors, actuators, as well as miniaturised communication systems and medical implants. The need for novel battery designs has inspired the development of several new 3D structures and coating technologies in order to meet both energy and power requirements. The goal of these particular battery architectures is to have a volume in the range of 1–10 mm 3 and a power density of 1 J (mm 2 per day) 1 (i.e. z 1.16 mW cm 2 ). The investigation and production of microbatteries rely on a complete 3D conguration and assembly of all the components, which is a particularly challenging prospect on the length scales required in such systems and justies the infancy state of this technological area. Moving from 2D towards full 3D microbattery devices indeed raises a number of challenges to overcome before commercialization of such systems can be reached. Different architectures are possible for 3D microbatteries: interdigitated arrangements of nano-rods, trenches, continuous structures (e.g. rod arrays coated with an electrolyte layer where the other electrode lls the remaining volume) and aperiodic sponge designs (e.g. 3D electrodes with a disordered conguration). 1 Recent modelling studies of 3D batteries have shown that the distribution of current in a 3D cell varies considerably between different architectures 7–9 and that this ultimately controls the power that a 3D battery can deliver. In particular, the ions will not necessarily travel in a uniform way between the positive and the Department of Chemistry – ˚ Angstr¨ om Laboratory, Uppsala University, Box 538, SE-751 21, Uppsala, Sweden. E-mail: kristina.edstrom@ kemi.uu.se; Fax: +46 18 513548; Tel: +46 18 4713713 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ta11921a Cite this: J. Mater. Chem. A, 2013, 1, 9281 Received 15th May 2013 Accepted 21st June 2013 DOI: 10.1039/c3ta11921a www.rsc.org/MaterialsA This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. A, 2013, 1, 9281–9293 | 9281 Journal of Materials Chemistry A PAPER Published on 21 June 2013. Downloaded by University of Windsor on 02/08/2013 09:17:47. View Article Online View Journal | View Issue
Electrochemical elaboration of electrodes andelectrolytes for 3D structured batteries†
Mario Valvo, Matthew Roberts, Gabriel Oltean, Bing Sun, David Rehnlund,Daniel Brandell, Leif Nyholm, Torbjorn Gustafsson and Kristina Edstrom*
The challenges associated with the fabrication of three-dimensional (3D) electrode and electrolyte
materials for Li-ion batteries are discussed. The basic issues for achieving a solid 3D cell foundation, which
can simultaneously offer sufficient electronic conductivity to enable stable cycling, as well as enough
compatibility with the incorporation of complementary components, have been addressed. Various
electrochemical strategies for elaborating such systems are discussed and critically examined. Several
current collector systems are presented including electrochemically prepared Cu and Al nanorods and
commercial aperiodic carbon structures. Further electrochemical coating approaches then provide a direct
method for the deposition of thin layers of active materials successfully demonstrated here as coatings on
both 3D metal structures and commercially available 3D-structured carbon substrates. Enhanced
capacities per foot print area are demonstrated for a number of 3D electrode materials, namely
polyaniline on reticulated vitreous carbon, Cu2O on copper nanorods and TiO2 on Al nanorods. The
crucial points for achieving a thin conformal coating of the corresponding 3D electrode structures with
solid polymer electrolytes are also carefully analysed and discussed. In this context electro-polymerisation
is proposed as a viable route to form thin electrolyte layers with promising characteristics. The high
versatility of electro-polymerisation in combination with the various structures and methodologies
adopted here represents a further step towards the development of cost-effective 3Dmicrobattery devices.
1 Introduction
Novel micro/nano electromechanical systems (MEMS/NEMS)are currently under development by using rened manu-facturing techniques. Such devices require access to mini-aturised and integrated batteries to be efficiently powered.1–4
Typically, two-dimensional (2D) microbatteries are employed inthese systems, where thin layers of cathode, electrolyte andanode materials are deposited on a planar substrate. However,these tiny devices have power requirements which far outreachthose supplied by 2D thin-lm batteries. These cells usuallyhave a foot print area of �5 cm2 with high power density (7 mWcm�2) and low energy density (<1 mW h cm�2).6 The feasibilityof producing three-dimensional thin-lm batteries has beendiscussed1–5 in the literature, where the ultimate goal is toobtain a high surface area substrate coated with thin layers ofcathode, electrolyte and anode materials to enhance the energydensity per foot print area, while maintaining a large powerdensity. Long et al.1 showed that three-dimensional nano-structured Li-ion microbatteries could satisfy the power needs
tory, Uppsala University, Box 538, SE-751
m@ kemi.uu.se; Fax: +46 18 513548; Tel:
tion (ESI) available. See DOI:
Chemistry 2013
of certain MEMS devices such as smart dust motes, sensors,actuators, as well as miniaturised communication systems andmedical implants. The need for novel battery designs hasinspired the development of several new 3D structures andcoating technologies in order to meet both energy and powerrequirements. The goal of these particular battery architecturesis to have a volume in the range of 1–10 mm3 and a powerdensity of 1 J (mm2 per day)�1 (i.e. z 1.16 mW cm�2).
The investigation and production of microbatteries rely on acomplete 3D conguration and assembly of all the components,which is a particularly challenging prospect on the length scalesrequired in such systems and justies the infancy state of thistechnological area.Moving from2D towards full 3Dmicrobatterydevices indeed raises a number of challenges to overcome beforecommercialization of such systems can be reached. Differentarchitectures are possible for 3D microbatteries: interdigitatedarrangements of nano-rods, trenches, continuous structures(e.g. rod arrays coated with an electrolyte layer where the otherelectrode lls the remaining volume) and aperiodic spongedesigns (e.g. 3D electrodes with a disordered conguration).1
Recent modelling studies of 3D batteries have shown that thedistribution of current in a 3D cell varies considerably betweendifferent architectures7–9 and that this ultimately controls thepower that a 3Dbattery candeliver. In particular, the ionswill notnecessarily travel in a uniform way between the positive and the
negative electrode, as in 2D cells. Accordingly, the materialselection (e.g. structureswithdifferent conductivity, porosity andSEI layer formation) also plays a central role in current distri-bution and there exist a multitude of possibilities to optimisemicrobattery performances by combining specic electrode andelectrolyte materials for certain architectures.
Typically, two main directions are taken in the fabrication ofnovel 3D battery structures: the rst is top-down lithographicmethods,10,11 while the second is bottom-up chemical andelectrochemical techniques.2,3 Lithographic methods generallystart by etching silicon wafers to generate trench structures, onwhich an electrode material can rst be deposited, then coatedwith the electrolyte and nally covered with the other electrodematerial using vapour deposition techniques. This is a powerfulapproach for manufacturing solid-state batteries; however, theresultingmaterials have typical sizes in themicron range, whichrepresents a limit for further down-scaling of the whole batteryarchitecture.1,10,11
Bottom-up electrochemical2 methods can play a key role inthe alternative synthesis and assembly of all the batterycomponents, because they are inexpensive and do not sufferfrom direct limitations on the size of the structures that can beformed. In this paper, we will present a number of simple andlow cost strategies to prepare battery structures, as well as activecoatings using electrochemical routes on different substrates.The challenges related to the fabrication of adequate nano-structured current collectors and their further processing will behighlighted. It will be shown that effective inclusion of thinactive layers can be attained on various 3D supports viastraightforward approaches. We will also discuss the crucialpoints that need to be addressed to uniformly deposit thin solidelectrolytes on complex 3D electrode morphologies, preventingpin-hole formation and providing adequate ionic conductivities.In this context, wewill consider the use of electro-polymerisationto form thin layers of a polymer electrolyte in a simple and directway. Conformal electrolyte coating of the corresponding 3D-structured electrodes can successfully be achieved, thusenabling a fully electrochemical approach to develop the fabri-cation and elaboration of all the various components of 3Dmicrobatteries. Finally, we will outline the remaining challengesand limitations that could arise from theultimate assembly of allthe different parts into whole 3D microbattery architecture.
2 Experimental2.1 Synthesis of 3D structured current collectors
Electrodeposition of aluminium nanorod current collectors wasinitially conducted either potentiostatically12 or galvanostati-cally13 within the pores of an alumina or a polycarbonatetemplate from an ionic liquid electrolyte consisting of 1-ethyl-3-methylimidazolium chloride with an excess of aluminiumchloride salt (1 : 2 molar ratio).14 The galvanostatic route waspreferred over the potentiostatic one, since it offered a bettercontrol of the deposition rate, which was directly proportionalto the applied current density. A dedicated pulsed galvanostatictechnique12 was nally used to avoid the depletion of the elec-trolyte within the pores of the template. The most effective
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deposition scheme also included an initial potentiostatic‘nucleation pulse’ to a highly negative potential, followed byrepeated pulsing between zero current and the depositioncurrent. The experiments were carried out in an argon-lledglove-box (O2 and H2O < 2 ppm), due to the high reactivity ofAlCl3 with water. The templates were either in-house preparedor commercially available nanoporous alumina (AAO, What-man, Anodisc 47, reference 6809 5022), as well as commercialpolycarbonate membranes.
Electrodepositionof coppernanorodswasperformedbydirectelectrodeposition of copper into the pores of the anodizedaluminamembrane (AAO,Whatman, Anodisc 47, reference 68095022). The membrane was placed on a polished and cleanedcopper foil, and a porous cellulose separator soaked in the elec-trolyte was applied between the alumina and the copper anode.The electrodeposition was carried out using a pulsed cathodiccurrent with a solution of CuSO4$5H2O and (NH4)2SO4 anddiethyl-tri-amine (DETA). This represents a common route for theproduction of copper rods and many other negative electrodematerials deposited on such copper-based structures.15–19
Commercially available 3D substrates were also utilised,namely reticulated vitreous carbon (RVC) and carbon felt (CF)structures. RVC consisted of 40 mm struts of vitreous carbonconnected to each other to form a network in compressed 100ppi (pores per inch) foams (ERG Aerospace engineering). Thecarbon felt (CF) (Sigratherm�, GFA-05) structures were strandsof carbon with a typical diameter of 7 mm, woven together toform a continuous network, similar to a fabric-type material.
2.2 Deposition of electrode materials and battery assembly
Polyaniline (PA) was electrodeposited from an aqueous solutionof 0.1 M aniline and 0.25 M H2SO4 using multiple cycling in thepotential range from �0.2 to 1 V vs. Ag/AgCl. Layers of thiscathode material were respectively deposited on 1 mm-thickstainless steel foil (i.e. taken as reference 2D current collector),100 ppi compressed RVC and 1 mm-thick CF using 20 cycles inthe potential range mentioned above via a standard three-electrode setup with a stainless steel plate counter electrode anda SP240 BioLogic potentiostat. The planar stainless steel, 3DRVC and 3D CF electrodes coated with PA were all dried at 60 �Cunder vacuum before cell assembly. The corresponding cellswere then cycled galvanostatically using an Arbin Instruments(BT 2043) battery cycler.
Cu2O was prepared via spontaneous oxidation of the elec-trodeposited Cu nanorods in contact with the Cu2+ ions in aCuSO4 solution. A pulsed galvanostatic scheme, similar to thatof earlier preparations of 3D Cu nano-architectures,15 wasadopted to form Cu nanorods with typical heights of about 8 mmcovered by an outermost layer of cuprous oxide. The resultingelectrodes were dried in a vacuum at 120 �C prior to batteryassembly. Galvanostatic cycling of the cells was performedusing an Arbin Instruments (BT 2043) battery cycler applying aconstant current density, which yielded a discharge–charge rateof almost 3C between 0.05 and 3.0 V vs. Li+/Li.
TiO2 was deposited onto 3D aluminium substrates withAl nanorods at 250 and 300 �C from a TiI4 precursor, using a
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four-step procedure repeated for 150 times, analogous to theone reported earlier20 at 200 �C. Both planar and 3D-structuredAl supports were coated using this procedure. The growthprocesses were accurately controlled in order to ensure that thesame lm thickness was obtained for the correspondingsubstrates. The electrodes were dried overnight in a vacuumfurnace at 120 �C before battery preparation. The cells werecycled in galvanostatic mode between 1.4 and 3.0 V vs. Li+/Liusing a Digatron BTS-600 instrument (by C-rates ranging fromC/10 to 10C, ESI S2 A and B†).
All the batteries were assembled in vacuum sealed pouchcells in an Ar-lled glove box (O2, H2O < 2 ppm) using lithiummetal as reference and counter electrodes, glass ber or Sol-upore as separators and 1 M LiPF6 in EC/DEC (ethylenecarbonate/diethylene carbonate) as an electrolyte, except for theTiO2 electrode, which was cycled in 1 M LiClO4 PC (propylenecarbonate).
2.3 Polymer electrolyte preparation and testing
Poly(ether amine) functionalized with acrylate groups waselectropolymerised on a LiFePO4 electrode with a surface area of2.57 cm2. This was carried out in a two-electrode cell containingthe LiFePO4 working electrode, an unstirred monomer solutionof poly(ether amine) functionalised with acrylate groups and aPt counter electrode. The lm formation was controlled chro-nopotentiometrically, employing a constant cathodic current of75 mA for 90 minutes. Aer the electropolymerisation, thesubstrate with the polymer coating was rinsed with ethanol andkept under vacuum for 5 hours to remove the solvent.
A chronopotentiometric technique was utilised for thepreparation of coatings of polypropylene glycol diacrylate(PPGDA) in order to form polymeric cross-links on different 3Delectrode surfaces via self-limiting electropolymerisation, withthe 3D substrate as the working electrode and Pt wire as thecounter electrode. The monomer solution containing PPGDAand an ionic conductive salt, e.g. LiTFSI, were rst mixed inethanol. A constant cathodic current of 75 mA was applied untilthe polymerisation stage was reached. The electrolyte coatingformed on the 3D substrates was then rinsed with ethanol toremove the salt and the monomer residuals before furthercharacterisation.
3 Results and discussion3.1 Analysis of possible options for 3D current collectors
Several examples of nanostructured electrode materials withdifferent shapes can be found in the literature.2,3,10 Their energydensity is enhanced by the total area gain of the resultingelectrodes, however, their cycling is oen limited, due to the lowelectronic conductivity of their constitutive elements andcompounds.3,10 There is an obvious need for electrode designswhich can provide good electronic support for the activematerials in order to extend their cycle life. 3D substrates are aneffective means to reach this goal, because a larger portion ofthe active materials is in direct contact with the conductivesupport of these large area current collectors.
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We discuss the following three possible types of currentcollectors that can be used for 3D microbattery purposes. Thegeneral approach proposed in this study is schematicallydepicted in Fig. 1, where the combination of electrochemicalroutes grants direct access to the deposition of both 3D metalstructures and active materials, as well as polymer electrolyteson various types of current collectors, irrespective of theirdifferent features and characteristic shapes.
The rst type of 3D-structured current collector analysed hereemploys aluminium nanorod arrays, which are prepared byelectrochemical deposition.12,13 Aluminium is a suitablecathode current collector21 for Li-ion batteries, since it does notcorrode in the high voltage range 3.4–4.5 V vs. Li+/Li, wheremost positive electrode materials operate. Another advantagederives from its low density (z2.70 g cm�3), which increases theoverall gravimetric energy density of the battery when comparedto other current collectors, such as stainless steel or nickel, thusbeing attractive for miniaturised architectures based on nano-rods.15,16 Other groups also previously adopted electrodeposi-tion through alumina templates,22,23 though high aspect ratiorods were not immediately accessible.
A comparison between Fig. 2a and b shows that the mosthomogeneous high aspect ratio Al nanorods were obtained viapulsed current deposition rather than applying a constantcurrent. This methodology ensures a more consistent height forthe deposited rods, as well as a uniform coverage of the surfaceof the aluminium substrate.
Alternative routes to directly prepare Al rods without anytemplate using ionic liquids have also been attempted. Thiscauses the formation of low aspect ratio, micron-sized rods (orballs) with large interspacing.24,25
It is worth noticing that the preparation of advanced 3Dcurrent collectors based on high aspect ratio Al structures offersan impressive area gain. In fact, an array of such rods, with atypical height of 5 mm, provides a foot print area enhancementapproximately 20 times higher than a standard planar support.
One drawback of nanoporous alumina (AAO) templates isthat it leads to the formation of nanorods with narrow inter-spacing. This restricts the space available to incorporate thesubsequent layers of a microbattery into a full 3D cell.Conversely in some situations this circumstance is favourable,because it provides a moderate electrode porosity and aneffective packing of the interfaces, thus enhancing the volu-metric energy density.
To increase the interspacing distance and thus yield a moreopen structure we have also used polycarbonate (PC) templates(PC templates provide randomly distributed arrays of poreswith a diameter of about 1 mm and a typical inter-pore distanceof few micrometers). It can be noticed from Fig. 2c and d thatthis resulted in the formation of larger rods with uniformheights and adequate spacing, though displaying an irregularareal distribution. This is clearly compatible and a simplerway to proceed with further deposition of both active layersand electrolytes, not necessarily relying on vapour-phasetechniques.
The second type of 3D current collector that we have exam-ined is based on copper nanorods. Although aluminium can be
Fig. 1 Cartoon of possible approaches for electrochemical synthesis and elaboration of electrodes and electrolytes for three-dimensional battery structures.
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used for the cathode, it is generally inappropriate for the anodiccounterpart, due to its Li-alloying reaction at low potentialsvs. Li+/Li.15 3D copper structures are instead a suitableanodic current collector, where an anode electrode layer can bedeposited.
Considerations analogous to those mentioned above for theAl nanorods hold also in this case. Additionally, for this type ofcurrent collector the area gain can be further increased bydepositing longer Cu rods, e.g. up to 8–9 mm, since the elec-trodeposition of Cu in aqueous solutions is easier and morerobust than that of aluminium in ionic liquids. The excellentelectronic conductivity of Cu is another essential feature thatenables efficient electron transfer in rods with even higheraspect ratios, provided that the thickness of the coated activelayers can be limited to a few tens of nanometers. In thisrespect, a further down-scaling of the Cu structures does notimmediately represent a limitation for the performances of thecorresponding electrodes, where the width of both the sup-porting rods and the active anodic layer can simultaneously bereduced, while increasing the overall surface area.7,8 Nonethe-less, the lack of space for the subsequent components neededin a 3D microbattery design poses the ultimate limitation forthese structures when AAO templates are used, as earliermentioned.
The third type of current collector that we have exploredpossesses a different 3D morphology relying on reticulatedcarbon structures (Fig. 3a and b) and carbon felt strands (Fig. 3c
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and d), which can offer a direct route towards microbatterieswith an aperiodic design.26
These carbonaceousmaterials have large surface areas, due totheir characteristic pores and network-like connections, whichare randomly arranged. Their structures are not as ne as thoseof template-electrodeposited metal nanorods, yet they are stillwithin the length scales generally considered suitable for 3Dmicrobatteries. They intrinsically possess sufficient void space,so that the deposition of a rst electrode layer followed by anelectrolyte and a second electrode can be envisaged. Typical areagains between 20 and 50 are observed for such substrates with athickness of 1 mm. Dedicated electrode materials can readily bedeposited on their surface using electrodeposition and otherchemical methods. Whilst carbon current collectors, in prin-ciple, can be oxidised at potentials above those normally used forcathodematerials, the rate of this or any other corrosion reactionhas been shown to be negligible when used in combination withlithium battery electrolytes.16,27 Furthermore, the use of carbonblack additives in all conventional cathode systems likewiseconrms the stability of this kind of carbon.28 Therefore, carboncurrent collectors are ideally suited for congurations where thecathode is deposited as the primary layer or when high-voltageanode materials, such as Li4Ti5O12 or TiO2, are being employed.
The above substrates represent solid scaffoldings to buildup 3D microbatteries, allowing various strategies to coatboth cathode and anode materials via a number of suitableapproaches.
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Fig. 2 SEMmicrographs of aluminum pillars deposited by constant current (a) and pulsed current (b) into the pores of a commercial aluminamembrane and by pulsedcurrent into the pores of a commercial polycarbonate membrane (c and d).
Fig. 3 SEM micrographs of 100 ppi compressed Reticulated Vitreous Carbon(RVC) structures (a and b) and Carbon Felt (CF) strands (c and d).
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3.2 Deposition of electrode materials on 3D currentcollectors
A number of examples of electrochemical approaches to directlydeposit the active materials onto the 3D current collectors,without the addition of any binder and/or electrically conduc-tive additive, are presented in the following subsections. Wehave focused on simple and low cost methods for preparingconformal cathode and anode layers on these 3D-structured
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substrates to advance the fabrication of microbatteries and todirectly enable the following step of electrolyte deposition.
3.2.1 Positive electrodes. One possible way to proceed is tostart with an initial deposition of a cathode material layer (e.g.LiCoO2,29 CuSx,30 MnO2
27 or LiFePO431,32 on a suitable support.
An interesting alternative to these active compounds is to have apolymer as a cathode material, since this can offer enhancedadaptability for both synthesis and assembly in a 3D cellconguration. In this respect, polyaniline (PA) is a suitablecandidate, which has been used as an electrode material sincethe 1960s.33 It usually operates via an anion insertion mecha-nism and has a theoretical capacity of 148mA h g�1, where mostof the capacity is accessed above 2.8 V vs. Li+/Li. The charac-teristic electronic conductivity of PA, as well as its versatility andthe easy access to porous textures, makes this electro-activepolymer clearly attractive in this context. Furthermore, allconventional oxide-based cathodes (e.g. LiCoO2, LiFePO4,LiMn2O4, etc.) suffer from limited electrical conductivity and itis challenging to immediately synthesise and deposit them inthin layers by electrochemical routes.
Here PA is used as the active cathode material prepared inconformal deposits on aperiodic carbon structures (i.e. RVC andcarbon felt (CF)) via straightforward electrodeposition. ThePA deposits obtained on the RVC and CF substrates are shownin Fig. 4.
The PA on the RVC surface is clearly seen as a rough layer,when compared to the previous smooth carbon surface (seeFig. 3a and b). The PA layer appears to be approximately even,with no exposed areas visible on RVC. In the case of the PA layeron the carbon felt, a rougher coverage of the CF surface can benoticed and akes of the active material can be seen crossingfrom one ber to another. Nevertheless, the deposits look to bereasonably conformal on both the RVC and CF supports. Thecorresponding voltage prole curves of charging and discharg-ing at C/10 are shown in Fig. 5a, where the effect of theincreased surface area for the PA coating on the 3D substratescan be clearly observed.
The capacity per foot print area observed for the PA layer onthe planar stainless steel substrate was 0.01 mA h cm�2,whereas those obtained for the PA coatings on RVC and CF were0.11 and 0.27 mA h cm�2, respectively. Despite the PA coatingon RVC offering reduced performances compared to itsanalogue on CF, these RVC structures can be more advanta-geous than CF in terms of rigidity, which helps in maintainingstructural stability upon cycling and avoiding possible short-circuits. Although the capacity enhancements are signicant,
Fig. 4 SEM images of 100 ppi compressed RVC coated in polyaniline (a and b) andRVC structures, as well as some regions on the CF strands where the PA coating is n
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they are only about half of the values that can be expected on thebasis of the theoretical increase in surface area. This may be dueto electrode thinning in the centre of the structure as a result ofelectrolyte depletion during the electrodeposition process,though this may not be easily observed by SEM.
The PA material was cycled for more than 40 cycles and didnot exhibit any signicant capacity fade (see Fig. 5b). High ratecapabilities were also observed at rates of 10 C for the PA-coatedCF samples, with capacities exceeding 50% of those obtained atC/10. This high performance can be explained considering thehighly conductive porous layers of PA with thicknesses in therange 1–2 mm. Accordingly, the characteristic structure allowsfor good ionic conduction through the pores, as well as effectiveelectronic conduction through the polymer backbone. It isworth mentioning that this situation is oen lacking in thinlm cathodes, where binders and conductive additives are notincluded, thus leading to signicant fading of both the capacityand the rate capability of such electrodes.27
3.2.2 Negative electrodes. Another option is to begin withthe deposition of an anodic material as the rst layer onto acompatible 3D substrate. In this respect, copper stands out as
CF coated in polyaniline (c and d). Note the uniform coverage of polyaniline on theot evenly distributed.
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Fig. 5 Voltage profiles (a) of charge and discharge at C/10 for: stainless steel foilcoated with polyaniline (PA/Foil), 100 ppi compressed RVC coated with polyani-line (PA/RVC) and carbon felt coated with polyaniline (PA/CF). Cycle performanceand rate capability at different C-rates for the same electrodes (b).
Fig. 6 SEM top-view micrograph of Cu2O deposits spontaneously grown on Cunanorods prepared via template-assisted electrodeposition using an AAOmembrane (a). Cycle performance of the 3D electrode incorporating the Cu2Odeposits cycled versus metallic lithium in a half-cell (b).
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an optimal support, because it displays excellent electronicconductivity and does not undergo Li-alloying at low potentialsvs. Li+/Li. Moreover, it is stable with respect to corrosion up to�3.4 V vs. Li+/Li in combination with most of the electrolytesusually employed. Accordingly, several deposition processes ofactive materials such as Fe3O4,15 Sn,17 Si,16 Sb34 and Bi19 havebeen reported on 3D Cu current collectors.
We have earlier presented how a Cu current collector can bedirectly employed for the synthesis of Cu2Sb by electro-depositing a layer of Sb.34,35 This simple approach resulted inCu2Sb electrodes with excellent adhesion to the underlying 3DCu substrate, ensuring stable cycling behaviour36,37 and a higherenergy density18 than that of an analogous 2D conguration.Here, we have further developed this strategy for the depositionof an alternative anode material on these 3D Cu substrates.
In particular, we use the same template-electrodepositionprocess to form, in situ, a thin layer of Cu2O on the coppernanorods. The Cu2O deposits directly generated on the surfaceof the Cu structures during the template-assisted electrodepo-sition can be observed in Fig. 6a as roughening of the rodcontours in the form of grains.
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The reaction of the Cu2O electrode is different from that ofLi-alloying materials (e.g. Cu2Sb), since the reduction of Cu2Ocauses the formation of copper nanoparticles embedded in amatrix of Li2O, due to a conversion reaction.38 Cu2O nano-particles are formed upon subsequent oxidation, while the Li2Omatrix is dissolved. Such a process yields a surprising overallreversibility, particularly considering the dramatic structuralchanges involved in the reactions. Although cuprous oxide has atheoretical capacity of 375 mA h g�1, larger capacities can in factbe expected for 3D electrodes, as a result of the signicantdouble layer charging effect39,40 at the extensive Cu/Li2O inter-faces generated by the conversion reaction, as well as the largersurface area of the corresponding support. Conversely, capac-ities exceeding their theoretical values have also been explainedin terms of electrolyte decomposition, yielding an organic, gel-like layer.41 Another attractive characteristic of Cu2O is its smallvolume change (i.e. 22%) upon lithiation and de-lithiation,which is signicantly limited compared to those of other tran-sition metal oxides and most Li-alloys. Accordingly, themechanical strain induced by the conversion and de-conversion
Fig. 7 TEM micrographs of ALD-deposited TiO2 on Al nanorods at 200 �C;overview of the Al rods with a deposited layer on the surface (a) and a highresolution image of the cross-section of a TiO2-coated Al rod (b). The dotted linesserve as a guide for the eyes to locate the different interfaces.
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of the outermost Cu2O layer can be further alleviated by theunderlying 3D Cu structure, thanks to its large surface contactand the void between the high aspect ratio nanorods.
The capacity per foot print area of this 3D Cu2O electrode, aswell as its retention upon cycling, is very good, as shown inFig. 6b. It is noticeable that a capacity of about 0.14 mA h cm�2
was obtained aer 50 cycles even at a relatively high rate ofcharge–discharge. This excellent performance suggests that theadhesion between the Cu2O layer and the Cu nanorods wasgood, as it can usually be expected for surface oxide terminationof most metals. Furthermore, the electrochemical millingduring repeated cycling resulted in the generation of smallerand smaller particles, which facilitated the charge transfer andincreased the contribution due to double layer charging. Thepolarization between discharge and charge was, however, stillsignicant (as for all the transition metal compounds), due tothe particular reaction paths, involving nucleation of Cu orCu2O nanoparticles with high surface energies.
Lithium insertion in TiO2 is another possible alternative toLi-alloying and conversion materials that may result in a moreefficient and reliable anode operation. Even though thespecic gravimetric capacity of TiO2 is lower than that of Cu2O,this material has the advantage of experiencing a low strainduring lithium insertion and de-insertion, due to the top-otactic nature of this reaction, delivering reversibly most of itscapacity at a relatively high voltage (�1.4 V vs. Li+/Li). Thislowers its energy density as an anode material, however, itmakes its operation safer, because it can be charged and dis-charged quickly without undergoing dendrite formation,which generally occurs at potentials close to that of lithium,avoiding at the same time most of the problems connectedwith SEI formation at low voltages with an Al current collector,since its operational voltage is far from the potential rangewhere Al is unstable due to Li-alloying. Consequently, theoverall gravimetric energy density of the resulting electrodecan still be improved, since copper (rCu z 8.96 g cm�3) is nolonger necessary as a support.
Our initial experiments used atomic layer deposition (ALD)to coat the aluminium nanorods with a thin lm of TiO2,because direct electrodeposition of this compound would likelyinterfere with the stability of Al in the pH ranges usuallyemployed for such an electrochemical approach. ALD is apowerful method, since it allows for conformal coating ofintricate structures with tuneable thickness for the depositedlayers. A precise value of the amount of deposited material canthus be obtained. The TiO2 was deposited onto the 3D Alsubstrates at 250 and 300 �C and the results from this study arecompared to an earlier one, which was performed at 200 �C.39
The different deposition temperatures inuenced themorphology, crystallinity and homogeneity of the respectivecoatings. The deposition of TiO2 at 200 �C resulted in a uniformcoverage of the Al nanorods, even in the narrow inter-spacingsat their bases (see the TEM image in Fig. 7a). A conformal,amorphous TiO2 layer of about 15 nm was formed onto a z 5nm-thick lm of native Al2O3 that passivated the Al nanorodsupon air exposure, prior to the ALD process (see the HR-TEMmicrograph in Fig. 7b).
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However, at higher deposition temperatures crystalliteswith an anatase structure were formed, which were grown inan amorphous matrix of the same TiO2 material. The presenceof these crystalline structures was conrmed by XRD (seeFig. S1 in the ESI†). The crystallite growth also modied thesurface coverage of the rods, which became rougher. Islands ofcrystals were found on the Al structures for the substratesprepared at 250 and 300 �C and it was difficult to obtain auniform coating of TiO2 in the space between the rods. Thecorresponding substrates coated at different temperatureswere subsequently tested. A stable capacity was achieved in allthe cases for the planar current collectors, with the highest
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capacity delivered from the substrate coated at 300 �C. Thislikely suggested that the larger the fraction of crystallineanatase present in the planar coating, the higher the respectivecapacity. These results are consistent with previous studies ofpowder samples,42,43 which indicate that crystalline TiO2 canstore more lithium ions than its amorphous counterpart in 2Dconventional laminated electrodes, though it should also bereminded that small nanoparticles of TiO2 can signicantlyextend their lithium storage beyond the theoretical limit (i.e. x> 0.5 in LixTiO2).44 However, the cycling of the coated 3D Alsubstrates displayed a reverse trend here, where the prepara-tion carried out at lower temperatures yielded the highestcapacities, as shown in Fig. 8.
The explanation for this result stems from the need for auniform coating on the substrate in order to provide goodcontact of the active layer with the current collector and to allowfor effective cycling of the electrode material. The formation ofthe crystallites at 250 and 300 �C reduced the uniformity of thecoating and, although this enhanced the exposed surface, it alsoincreased the local contact resistance of the TiO2 islands grownon the Al2O3-passivated interface of the Al nanorods. Therefore,their corresponding electrochemical performance was signi-cantly impaired.
The need for homogenous coating is thus important forthese Al structures and it clearly becomes crucial in moreelaborated systems, where a larger number of interfaces are tobe integrated for the various electrode and electrolyte compo-nents of a 3D cell.
3.3 Coating of polymer electrolytes on the 3D-structuredelectrodes
The next step towards a full 3D microbattery is to cover thedeposited electrode with a suitable electrolyte layer. The lattermust denitely coat the surface of the active material and coverany exposed areas of the underlying current collector, so thatwhen a second electrode is included in the design, no shortcircuits occur. Moreover, the electrolyte layer must be thinenough to reduce the path lengths of the ions, in order to ensurehigh power densities for the resulting 3D battery systems.
Fig. 8 Comparison of the cycling performance of TiO2 deposited via ALD atdifferent temperatures on Al nanorods. The 3D electrodes were discharged andcharged galvanostatically between 1.4 and 3.0 V vs. Li+/Li.
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Reducing the layer thickness without compromising itsmechanical properties – so that it can simultaneously act as aseparator – constitutes a major challenge for most electrolytes.So far, all solid electrolytes for Li-ion batteries suffer from lowionic conductivities, in the range of 10�4 to 10�6 S cm�1. Anumber of examples of ceramic andglassy-type electrolytes, suchas Li0.35La0.55TiO3 (LLT),45 or LiPON sputtered from Li3PO4 ontoLiCoO2 pillars,46–48 as well as lithographic Si,49 have been repor-ted for 3D batteries. The resulting battery cycling has not beensuccessful so far, due to the brittleness and the poor adhesion ofthese electrolytes to the electrode materials. A silane-basedmaterial has also been tested for the same purpose. However, itproved to work merely as a separator, rather than as an electro-lyte.23 Although PVdF-based systems can be used, they require aliquid component to function as an electrolyte.44
There are certain advantages of using polymer electrolytesystems and trying to reduce the amount of embedded liquidcomponents, or eliminating them completely – i.e. to form asolid polymer electrolyte. Polymer electrolytes are exible, yetrobust and are oen more electrochemically stable than theirliquid counterparts. Some polymers also display good surfaceadhesion properties, but it is generally difficult to get conformalcoatings on complex structures.
Here, we discuss and compare some initial attempts todeposit thin solid polymer electrolytes onto 3D electrodestructures via a chemical method, showing subsequently thebenets of adopting an alternative electrochemical route topursue this aim.
Initially our work focused on the use of self-assemblingsurfactants based on amines combined with oligomeric poly-propylene oxide (PPO) diacrylate groups to address this issue.The amine groups have surfactant properties and formhydrogen bonds with oxygen-rich surfaces, while PPO-dia-crylates can crosslink on exposure to UV radiation, yielding apolymer with good mechanical stability.50,51 These electrolytesdisplayed good surface coverage on electrodes made of LiFePO4
powders, forming layers with a thickness of a few micrometers,enabling galvanostatic cycling for more than 50 cycles. Thesame polymer electrolyte has also been successfully used incombination with Cu2Sb 3D-microbattery anodes.18,51
We began investigating electropolymerisation routes,instead of UV cross-linking to continue improving the perfor-mance of the corresponding electrolytes, so as to enable ahigher degree of integration for the electrode/electrolytecomponents towards a full 3D cell architecture. This electro-chemical technique is a viable route to further advance thedeposition of thin lms of polymer electrolytes, because thethickness of the layer can be sensibly reduced and a bettercontrol of the coating process can be directly achieved. Exam-ples of suitable polymers for 3D-microbatteries include poly-acrylonitrile52 and polyphenylene oxide,53 which could bedeposited with limited thicknesses (e.g. <25 nm). However,these polymers still need a liquid electrolyte component, suchas LiPF6 in EC:DEC, in order to function properly, raisingunavoidably other issues.
Electropolymerisation of electronically insulating polymers isattractive, because it forms very thin insulating lms, namely
monomers, which undergo polymerisation at their reactionsites, as a result of anelectrochemical redox reaction. Thismeansthat the deposition process occurs in a targeted position andincludes a convenient self-limiting step. The rst example ofelectropolymerisation of a true solid polymer electrolyte, whichalso displays surfactant properties and thus ensures good
Fig. 9 SEM micrograph of an uncoated LiFePO4 powder electrode (a) and coatedamine) functionalized with acrylate groups (b).
Fig. 10 SEM micrographs of polyaniline-coated RVC foam without a polymer electrd) showing the successful deposition of a conformal polymer electrolyte layer onto
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stability on the electrode surface, is presented in Fig. 9, where aconventional powdered material has been used as the prelimi-nary electrode target.
In Fig. 9 it can be seen that a thin electrodeposited layer ofthe polymer electrolyte was successfully formed on the surfaceof the LiFePO4 particles.
with a thin layer of the polymer electrolyte by electropolimerisation of poly(ether
olyte being deposited (a and b) and with an electropolymerised electrolyte (c andthe polyaniline cathode.
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Fig. 11 SEMmicrographs of Cu micro-rods grown by template-assisted electrodeposition via a PCmembrane (a) and covered by an electropolymerised electrolyte (b).
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The electrolyte layer covered the particles in a homogeneousway, following their surface morphology and contours, as it canbe noticed by comparing Fig. 9a (uncoated LiFePO4) and Fig. 9b(coated LiFePO4).
Another similar approach employed polypropylene glycoldiacrylate (PPGDA) as a cross-linkable agent to form polymericcross-links directly on 3D electrode surfaces via self-limitingelectropolymerisation. This new preparation enabled conformalcoating of complex 3D-structured electrodes with an ultra-thin electrolyte layer, having a thickness down to a hundrednanometers and offering compatibility with different 3D cellarchitectures.
The rst example of a 3D substrate coating is shown inFig. 10, where the polymer electrolyte could be successfullydeposited on the previously coated cathodes having polyaniline(PANI) as the active material on RVC structures.
The polymer electrolyte layer uniformly covered the surfaceof the RVC/PANI-coated electrode, retaining the surface featuresassociated with the intricate structure of the substrate.
The electropolymerised electrolyte was also applied ontoelectrodeposited Cu micro-rods employing short-pulsed chro-nopotentiometry. The SEM micrographs in Fig. 11 illustrate thesuccessful deposition of a conformal electrolyte coating with athickness of roughly 500 nm.
It is worth pointing out that also in this case the coverage ofthe electrolyte layer was very effective, since it enabled envel-oping individual microstructures with a high aspect ratio.
Therefore, the novel combination of these straightforwardelectrochemical approaches provides a powerful means toadvance the fabrication of 3D microbatteries. In fact, optimalcoating of the 3D electrodes can be achieved, because thethickness of the electrolyte could be further controlled byvarying the composition of the monomer solution and theelectropolymerisation time. More important, the preliminaryresults in Fig. 10 and 11 clearly demonstrate that the appliedmethodology not only improves the contact of various materialsat their respective interfaces, but also leaves sufficient space inthe structures to host other cell components.
As a nal remark, we should keep in mind that other parts(i.e. a second electrode and its associated electrical contact)
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still need to be included in the previous fabrication processesto end up with a full 3D microbattery device. Electrode back-lling, especially in open structures, like those of foams, maybe a possible option to embed a second active material ontothe electrolyte layer, whereas the subsequent electricalcontact could be realised, in principle, by more conventionalmethods (e.g. physical or chemical vapour depositiontechniques).
Nevertheless, other challenges may arise from technicalhurdles in achieving the required homogeneity and adhesionproperties for the subsequent layers, as well as risks of possibleshort-circuiting of the two electrodes during the last steps of thefabrication and assembling processes.
4 Conclusions
The approaches we have demonstrated in this paper focus onsimple, low cost electrodeposition and electropolymerizationmethods, which can be employed and combined to prepare 3D-structured electrodes with signicantly higher energy densitiesper foot print area than those corresponding to analogous 2Dsystems.
The results that we have presented here highlight severalimportant points about the fabrication of Li-ion microbatteriesaccording to the selected bottom-up approaches, namely:
- A range of current collector substrates having a largesurface area are required as a solid base for 3D cell design; wehave highlighted the use of 3D Al, Cu and carbon currentcollectors, which offer various options to deposit a whole rangeof possible electrode/electrolyte arrangements via severalroutes;
- What works for 2D congurations might not work in thesame way for 3D architectures, especially when the typicallength scales involve sub-micrometric features and the variousmaterial interfaces are closely spaced;
- TiO2 preparation on 3D Al current collectors demonstratedthat the amorphous material outperformed the crystallinematerial, since a good interfacial contact between the nanorodsand the active material is crucial to access the capacity of TiO2
in these nanometric lms directly in contact with native Al2O3;
- Materials such as Cu2O, which typically has limited cycleperformance in conventional preparation of planar electrodes,work extremely well when directly wired to an underlying 3D Cucurrent collector;
- Polyaniline suffers from poor volumetric capacity whenused in conventional electrodes, whereas in a 3D system itprovides an attractive and functional cathode option;
- Careful bespoke electrolyte coating methods need to bedeveloped at the same time to match the downscaling of theelectrode structures and to allow further electrode deposition.
We have shown how the electrolyte coating can be targetedand grown in thin conformal layers by using surfactant mole-cules and electrodeposition methodologies to ensure effectivecoverage of the peculiar surfaces of different 3D electrodes,allowing further manipulation towards a full 3D cell assembly.
The subsequentdeposition stepsofa secondactive componenton the electrolyte layer, as well as the ultimate fabrication of anelectrical contact for such a material, could involve even greaterchallenges than those encountered in the initial stages of thepreparation of these 3D-structured electrode/electrolyte systems.
Nevertheless, such key challenges also provide an opportu-nity for the development of many newmaterials and approachesto achieve operative 3D microbatteries, advancing at the sametime sub-micron fabrication techniques.
Although we have focused on the development of thesesystems for specic applications in microbatteries, the presentndings also have important implications for possible large scalemanufacture of advanced Li-ion batteries, since themethods canbe readily up-scaled. The resulting 3D electrodes will have betterpower and energy densities than their conventional 2D counter-parts. Finally, it should be highlighted that the bottom-upapproaches that we have presented here open up new pathstowards cost-effective miniaturisation of these systems, whileoffering perspectives of larger throughputs than those achievableby top-down approaches, such as lithography, whose practicalapplications will be mainly limited to the microbattery range.
Acknowledgements
Seng Kian Cheah, Mattis Fondell, Anders Harsta and MatsBoman, Uppsala University, are greatly acknowledged regardingthe support with the ALD synthesis and Jun Lu, now at Link-oping University, for the TEM measurements. I-Ying Liao isacknowledged for the electropolymerisation synthesis work.Professor John Owen is acknowledged for kindly supplying thecarbon foam and felt substrates. The ALISTORE-ERI isacknowledged as an important scientic community. Thefunding from the Swedish Energy Agency (STEM), The SwedishResearch Council (VR), StandUp for energy and KIC EITInnoenergy (STORAGE) is hereby recognized. The partnerswithin the EU FP7 program Superlion are likewise gratefullyacknowledged.
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