Electrodeposition of thin poly(propylene glycol) acrylate electrolytes on 3D-nanopillar electrodes
Introduction
Three-dimensional microbatteries (3DMBs) are considered promising energy storage candidates for powering small-scale electronic devices, such as micro-sensors, micro-electromechanical systems and medical implants [1]. These miniature devices, with volumes of 1-10 mm3, demand both high power and energy densities: the energy per unit area delivery requirements are about 1 J mm−2 over a day, equivalent to a power consumption of around 10 μW [1], [2], [3], [4], [5], [6]. This, in turn, stimulates new ideas for developing novel Li-ion battery designs with enlarged 3D surfaces within a small footprint area. An obvious gain in areal capacity from about 0.1 mAh cm−2 to 1-10 mAh cm−2 could be achieved by developing 3D electrodes for microbattery applications [2], [4], [5], [6]. Various 3D battery configurations have been proposed both theoretically and experimentally [1], [2], [3], not only for the fabrication of 3DMBs, but also applicable for Li-ion batteries with conventional dimensions.
Perhaps one of the most feasible approaches towards constructing a solid-state Li-ion 3DMB whole-cell is through sequential deposition of different solid-state battery components, for example a 3D nano-structured current collector with an electrode material coating arranged in the form of plates, tubes or pillars, a solid electrolyte acting as both an ionic conductor and physical separator, and a second electrode deposited on top of the electrolyte [4], [5], [6]. In order to minimize the electrolyte volume and to promote good interfacial contacts, it is critical to produce a robust and conformal electrolyte with thickness down to micro- and nano-scales. Furthermore, the electrolyte needs to be pinhole-free and mechanically stable to avoid short-circuits during 3DMB cell assembly [1], [2], [3], [4], [5], [6].
Among the different possible electrolyte systems, safe and non-toxic solid polymer electrolytes (SPEs) have been suggested as a replacement to more conventional liquid/gel electrolytes for 3DMB applications [4]. However, the conformal deposition of ionically conductive polymer materials directly onto 3D architectures with large surface areas constitutes a significant challenge. A few LiTFSI/polyether systems produced through UV-induced radical polymerization from acrylate-capped polyether oligomers have recently been reported as potential 3DMB electrolytes [7], [8], but these were found difficult to apply to 3D substrates with very complex surface geometry. In order to form conformal and ultrathin polymer electrolytes (down to ∼10 nm [1]), electropolymerization and/or electrografting (i.e., covalent linkage to solid surfaces) have been considered effective approaches for self-limiting growth of the deposited polymer [9]. When compared with conventional polymer deposition methods through solution casting and UV-initiated polymerization, electropolymerization should be able to achieve improved adhesion at the electrode/electrolyte interfaces by forming a compact polymer layer on the electrode surfaces. Surface modifications of a broad variety of substrates–including metals, metal oxides, different types of carbon and semiconductors–have been thoroughly investigated using electropolymerization of vinyl monomers in aprotic solvents (e.g., acetonitrile and dimethylformamide), such as acrylonitrile [10], [11], [12] and various (meth)acrylate derivatives [13], [14], [15]. Electropolymerization in aqueous solution has also been reported [16], [17], [18]. However, only a few electropolymerized polymer electrolyte systems have so far been reported for microbattery applications [19], [20], [21], [22], [23].
This study aims at exploring an electropolymerized SPE, exemplified by deposition on a 3D nano-pillar architecture. Recent studies using electrodeposition approaches to develop different 3D architectures (e.g., aperodic foam and nanopillar) with electropolymerized polymer electrolyte layers have been summarized by our group [6]. Here, we perform in-depth studies of this electrolyte synthesis strategy and detailed characterizations of the electropolymerized electrolytes derived from poly(propylene glycol) diacrylate. Moreover, as compared to constant current deposition, we demonstrate the beneficial use of pulsed current to form conformal coatings along 3D surfaces. This technique promotes the formation of uniform and completely covering electrolyte layers on long nanopillars with sophisticated 3D geometries. Furthermore, co-deposition of lithium-conductive salt in the electrolyte is achieved during electropolymerization, which simplifies the conventional post-doping of salt. A comparative study on the influence of heat-treatment on the electrolytes is also discussed.
A polymer electrolyte system based on poly(propylene glycol) diacrylate (PPGDA) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt has been investigated. The acrylate end-groups in the monomer used here are expected to be electropolymerized onto the metal oxide substrate surface through the formation of carboxylate-metal oxide bonds and undergo propagation to form a cross-linked polymer network [24]. LiTFSI was used as ionically conductive salt in the polymer matrix during the electropolymerization synthesis. The electrodeposition of 3D Cu-nanopillar substrates was achieved by a template-facilitated electrodeposition method [6]. The Cu substrate undergoes spontaneous oxidation forming a 15 nm Cu2O film, thereby producing a complete 3D anode suitable for Li-ion batteries [6], [25], [26], [27]. Controlled current chronopotentiometry was utilized to fabricate the electrolyte directly onto the 3D Cu2O-coated Cu-nanopillars. Alternatively, pulsed current was implemented to synthesize a conformal and uniform electrolyte layer with controlled thickness on 3D electrodes. The electrochemical characteristics of PPGDA/LiTFSI on 3D Cu2O-coated Cu nano-pillars were studied by cyclic voltammetry and chronopotentiometry. Redox probe analysis was also applied on polymer-coated planar and 3D Cu substrates to examine the presence of pinholes in the electrolytes. The ionic conductivity of the electrolyte was investigated using electrochemical impedance spectroscopy (EIS), and the morphology of the polymer deposits on 3D substrates was analyzed by scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) was utilized to characterize the chemical composition of the coated 3D substrates, especially the content and distribution of polymer and LiTFSI across the surfaces. Furthermore, the degree of polymerization in the electropolymeried electrolytes on 3D Cu foils was measured by attenuated total reflectance/Fourier transform infrared spectroscopy (ATR-FTIR). The influence of heating the electrolyte after electropolymerization will also be discussed.
Section snippets
Materials
All solvents used in the experiments were dried to remove residual water. Ethanol (Solveco, 99.5%) was left over 3 Å molecular sieves for at least one week before use, poly(propylene glycol) diacrylate (Aldrich, Mn ca. 900) (stored in refrigerator) was used as received. LiTFSI (Purolyte) was dried at 120 °C for 12 h in a vacuum oven. Ionic conductivity measurements were carried out on a glassy carbon disk electrode (CH-Instrument, diameter of 3 mm) using Gallium-Indium eutectic (GaIn; Aldrich,
Cyclic voltammetry and chronopotentiometry
The electrochemical reduction of PPGDA/LiTFSI was investigated using a glassy carbon electrode. Typical cyclic voltammograms of 0.16 M PPGDA/LiTFSI ([PO]:[Li+] = 20:1) in ethanol are illustrated and compared with a background voltammogram from a monomer-free solution in Fig. 1. A reduction wave can be observed at around -1.7 V vs. Ag/Ag2O in the background scan, which may attribute to solvent reduction. On the other hand, in the monomer-containing solution there is a distinctive peak at around -1.1
Conclusions
This study has shown the successful application of electropolymerized solid polymer electrolytes on 3D Cu nano-pillars for all solid-state Li-ion microbatteries. Electropolymerized electrolytes based on PPGDA and LiTFSI have been coated on 3D nano-pillar architectures, and pulsed current chronopotentiometry shown to be the preferred electrochemical synthesis method. The SEM micrographs showed that the obtained electrolytes of nano-scale dimensions were conformal and uniformly distributed as
Acknowledgements
This work was supported by the STandUP for Energy project. We wish to thank Dr. Maria Hahlin and Chao Xu, Uppsala University, for assistance with the XPS measurement.
References (37)
- et al.
3-D microbattery electrolyte by self-assembly of oligomers
Solid State Ionics.
(2011) - et al.
Solid polymer electrolyte coating from a bifunctional monomer for three-dimensional microbattery applications
J.Power Sources.
(2013) - et al.
Mechanism of electropolymerisation of methyl methacrylate and glycidyl acrylate on stainless steel
Electrochim. Acta.
(2002) - et al.
a Louati, Synthesis of acrylic polymer networks by electroinitiated polymerization
Prog. Org. Coatings.
(2004) - et al.
Electropolymerization of copolymer electrolyte into titania nanotube electrodes for high-performance 3D microbatteries
Electrochem. Commun.
(2011) - et al.
In situ growth of polymer electrolytes on lithium ion electrode surfaces
Electrochem. Commun.
(2009) - et al.
An electrochemical quartz crystal microbalance study of poly(acrylonitrile) deposition initiated by electrogenerated superoxide
Electrochim. Acta.
(2013) - et al.
On the origin of the spontaneous potential oscillations observed during galvanostatic deposition of layers of Cu and Cu2O in alkaline citrate solutions
J. Electroanal. Chem.
(2006) - et al.
Plasma-induced graft-polymerization of polyethylene glycol acrylate on polypropylene films: chemical characterization and evaluation of the protein adsorption
J.Colloid Interface Sci.
(2010) - et al.
Modelling electrode material utilization in the trench model 3D-microbattery by finite element analysis
J.Power Sources.
(2010)
Finite element modelling of ion transport in the electrolyte of a 3D-microbattery
Solid State Ionics.
Modelling polymer electrolytes for 3D-microbatteries using finite element analysis
Electrochim. Acta.
Three-dimensional battery architectures
Chem. Rev.
Three-dimensional electrodes and battery architectures
MRS Bull.
Multifunctional 3D nanoarchitectures for energy storage and conversion
Chem. Soc. Rev.
Electrodeposition as a tool for 3D microbattery fabrication
Electrochem. Soc. Interface.
3D lithium ion batteries—from fundamentals to fabrication
J. Mater. Chem.
Electrochemical elaboration of electrodes and electrolytes for 3D structured batteries
J. Mater. Chem. A.
Cited by (19)
Enhanced ionic conductivity and mechanical properties via dynamic-covalent boroxine bonds in solid polymer electrolytes
2020, Journal of Membrane ScienceCitation Excerpt :The 3D SPEs fabricated by UV-initiated polymerization exhibited good electrochemical stability and electrode surface adhesion. They also prepared a conformal and uniform electrolyte layer consisting of poly(propylene glycol) diacrylate and LiTFSI with controlled thickness on 3D-nanopillar electrodes through cathodic electropolymerization [55]. The electrolyte coated on the nano-pillar electrodes can act as a template for further deposition of electrode.
Modeling 3D-microbatteries based on carbon foams
2018, Electrochimica ActaCitation Excerpt :The key attributes of these electrodes are related to shorter Li-ion diffusion path lengths, increased working electrode surface area, and prolonged cell life due to limited mechanical stresses from Li+ insertion and removal [4]. To date, a variety of electrode architectures [5–8], novel electrode morphologies [9,10], active materials [11,12] and fabrication techniques [13–15] for 3D-MB-s have received considerable attention [16–19]. Among the fabrication techniques considered, photolithography of vertically-standing micropillars and the electrodeposition of arrays of nano-rods through porous membranes constitutes one of the first techniques to be explored [20].
Electrochemical and chemical modifications of electrode surfaces and interphases for Li-Ion batteries
2018, Encyclopedia of Interfacial Chemistry: Surface Science and ElectrochemistryThermal Simulations of Polymer Electrolyte 3D Li-Microbatteries
2017, Electrochimica ActaSalt concentration effects on mechanical properties of LiPF<inf>6</inf>/poly(propylene glycol) diacrylate solid electrolyte: Insights from reactive molecular dynamics simulations
2016, Electrochimica ActaCitation Excerpt :LiPF6 molecules at a ratios of Li:EO = 1:16/1:32 (EO = ethylene oxygen atoms) were added to the system during equilibration. The maximum concentration corresponds approximately to that used in a previous experimental study [1]. The unit cell was expanded 2 × 2 × 2 times (size effects are considered in Section 3.2).