Papers by Abhinav Gaikwad
Flexible lithium ion batteries are necessary for powering next generation of wearable electronic ... more Flexible lithium ion batteries are necessary for powering next generation of wearable electronic devices. In most designs, the mechanical flexibility of the battery is improved by reducing the thickness of the active layers, which in turn reduces the areal capacity and energy density of the battery. The performance of a battery depends on the electrode composition, and in most flexible batteries, standard electrode formulation is used, which is not suitable for flexing. Even with considerable efforts towards the development of flexible lithium ion batteries, the formulation of the electrodes has received very little attention. In this study, we investigate the relation between the electrode formulation and the mechanical strength of the electrodes. Peel and drag test are used to compare the adhesion and cohesion strength of the electrodes. The strength of an electrode is sensitive to the particle size and the choice of polymeric binder. By optimizing the electrode composition, we are able to fabricate a high areal capacity (~2mAh/cm2), flexible lithium ion battery with conventional metal-based current collectors that show superior electrochemical and mechanical performance in comparison to batteries with standard composition.
The interfacing of soft and hard electronics is a key challenge for flexible
hybrid electronics. ... more The interfacing of soft and hard electronics is a key challenge for flexible
hybrid electronics. Currently, a multisubstrate approach is employed, where
soft and hard devices are fabricated or assembled on separate substrates,
and bonded or interfaced using connectors; this hinders the flexibility of the
device and is prone to interconnect issues. Here, a single substrate interfacing approach is reported, where soft devices, i.e., sensors, are directly printed on Kapton polyimide substrates that are widely used for fabricating flexible printed circuit boards (FPCBs). Utilizing a process flow compatible with the FPCB assembly process, a wearable sensor patch is fabricated composed of inkjet-printed gold electrocardiography (ECG) electrodes and a stencil-printed nickel oxide thermistor. The ECG electrodes provide 1 mVp–p ECG signal at 4.7 cm electrode spacing and the thermistor is highly sensitive at normal body temperatures, and demonstrates temperature coefficient, α ≈ –5.84% K–1 and
material constant, β ≈ 4330 K. This sensor platform can be extended to a more sophisticated multisensor platform where sensors fabricated using solution processable functional inks can be interfaced to hard electronics for health and performance monitoring, as well as internet of things applications.
This paper reports on the design and operation of a flexible power source integrating a lithium i... more This paper reports on the design and operation of a flexible power source integrating a lithium ion battery and amorphous silicon solar module, optimized to supply power to a wearable health monitoring device. The battery consists of printed anode and cathode layers based on graphite and lithium cobalt oxide, respectively, on thin flexible current collectors. It displays energy density of 6.98 mWh/cm 2 and demonstrates capacity retention of 90% at 3C discharge rate and ~99% under 100 charge/discharge cycles and 600 cycles of mechanical flexing. A solar module with appropriate voltage and dimensions is used to charge the battery under both full sun and indoor illumination conditions, and the addition of the solar module is shown to extend the battery lifetime between charging cycles while powering a load. Furthermore, we show that by selecting the appropriate load duty cycle, the average load current can be matched to the solar module current and the battery can be maintained at a constant state of charge. Finally, the battery is used to power a pulse oximeter, demonstrating its effectiveness as a power source for wearable medical devices. The number and variety of electronic devices has dramatically increased in the past 5 years and currently there is growing interest in electronic devices with flexible, thin, and large-area form factors. These electronics span a vast range of applications including mobile devices 1 , healthcare 2,3 , smart surfaces 4 , smart packaging 5 , and wearables such as smart watches and e-textiles 6–8. All of these diverse applications require electrical power, and many of them, especially wireless communication and light-emitting devices, require relatively large current pulses on the order of many milliamps. For example, the reported peak current consumption of Bluetooth Low Energy wireless communication in a wearable sensor module was 18 mA 9 , and a smart watch such as the Samsung Gear 2 consumes up to 48 mA during calls 7. For most mobile devices today this power is provided by a battery that is designed to be recharged each night using a wired connection. However, as the number of devices grows, so does the need for power sources that can meet their energy demands without frequent wired charging cycles. Minimizing wired charging is particularly important for wireless sensors as well as for health monitoring devices, where the consequences of a user forgetting to plug in the device can be severe. For such applications it is crucial to have a battery with high areal capacity, so that it can store a large amount of energy without being rigid and bulky. Discharge at a high rate is also needed to support the peak current consumption of the load device. Charging would become more convenient if the battery is combined with one or more devices that harvest energy from ambient sources, such as light, thermal, or vibrational energy 4,10–13. Furthermore, taking advantage of the many recent advances in flexible electronics technology, the energy harvester , battery, and load devices should be physically integrated into a single user-friendly flexible package. To date, several flexible thin-film rechargeable battery chemistries and architectures 9,14–18 and energy harvesting technologies 19–22 have been reported. However, an effective energy harvesting and storage system requires not only high-performing individual components, but also good compatibility between components. When designing such a system, it is necessary to consider both average and peak power consumption of the load, as well as the power density that can be harvested from the available ambient energy sources. The type and dimensions of the battery and energy harvester can then be selected in order to power the load efficiently for the desired length of time before wired charging is required. Designing a battery that is mechanically flexible and can also provide the required capacity and discharge rate for wearable and wireless electronics is extremely challenging. Lithium ion batteries have been the choice of
wearable technologies. Wire structured zinc-air and zinc-carbon aqueous batteries with capacities... more wearable technologies. Wire structured zinc-air and zinc-carbon aqueous batteries with capacities of 0.9 and 0.18 mAh cm −1 respectively have been recently demonstrated. However, the primary nature of these batteries is a limitation for garments and jewelry. Silver-zinc battery chemistry is another alternative based on an aqueous electrolyte. In addition to being nonvolatile and rechargeable, the silver-zinc chemistry provides energy density comparable to that of commercially available Li-ion batteries. Nevertheless, its widespread use is hindered by such shortcomings as the high cost of silver, lower operating voltage, and limited cycle life compared to its Li-ion counterparts. Despite of these limitations, it can be a good energy storage alternative for integration with smart garments, where safety and energy density are of the primary importance and the lifetime of the battery is comparable with the lifetime of the garment. To date, there are few reports on wearable primary and secondary silver-zinc batteries with planar configuration. Rechargeable systems show cycling or areal capacity limitations despite of innovative manufacturing approaches. The stretchable silver-zinc battery has a low areal capacity of 0.11 mAh cm −2 while the epidermal tattoo battery with areal capacity of 1.3-2.1 mAh cm −2 is stable only over 13 cycles.
Identification of solvents for dissolving polymer dielectrics and organic semiconductors is neces... more Identification of solvents for dissolving polymer dielectrics and organic semiconductors is necessary for
the fabrication of solution-processed organic field effect transistors (OFETs). In addition to solubility and
printability of a solvent, orthogonality is particularly important when forming multilayer structure from
solutions. Currently, the process of finding orthogonal solvents is empirical, and based on trial-and-error
experimental methods. In this paper, we present a methodology for identifying orthogonal solvents for
solution-processed organic devices. We study the accuracy of Hildebrand and Hansen solubility theories
for building solubility boundaries for organic semiconductor (Poly(2,5-bis(3-hexadecylthiophen-2-yl)
thieno[3,2-b]thiophene (PBTTT) and polymer dielectrics (Poly(methyl methacrylate) (PMMA), Polystyrene
(PS)). The Hansen solubility sphere for the organic semiconductor and polymer gate dielectrics
are analyzed to identify solvents that dissolve PMMA and PS, but are orthogonal to PBTTT. Top gate/
bottom contact PBTTT based OFETs are fabricated with PMMA gate dielectric processed with solvents
that are orthogonal and non-orthogonal to PBTTT. The non-orthogonal solvents swell the semiconductor
layer and increase their surface roughness.
Traditional printing methods offer the advantage of well-matured technology, high accuracy of dep... more Traditional printing methods offer the advantage of well-matured technology, high accuracy of depositing inks over flexible substrates at high web speeds, and low cost of fabrication. The components of a battery—the current collectors, active layers, and separator—can all be deposited using convention- al printing techniques by designing suitable inks. A combination of low thickness of printed electrodes, flexible packaging, battery architecture, and material properties makes printed batteries flexible. In this paper, we will discuss mate- rial challenges and mechanical limits of flexible printed batteries. We will review several printing techniques and present examples of batteries printed using these methods. In addition, we will briefly discuss other novel non-printed compliant batteries that have unique mechanical form.
Early demonstrations of wearable devices have driven interest in flexible lithium-ion batteries. ... more Early demonstrations of wearable devices have driven interest in flexible lithium-ion batteries. Previous demonstrations of flexible lithium-ion batteries trade off between low areal capacity, poor mechanical flexibility and/or high thickness of inactive components. Here, a reinforced electrode design is used to support the active layers of the battery and a freestanding carbon nanotube (CNT) layer is used as the current collector. The supported architecture helps to increase the areal capacity (mAh cm-2) of the battery and improve the tensile strength and mechanical flexibility of the electrodes. Batteries based on lithium cobalt oxide and lithium titanate oxide shows excellent electrochemical and mechanical performance. The battery has an areal capacity of ≈1 mAh cm-2 and a capacity retention of around 94% after cycling the battery for 450 cycles at a C/2 rate. The reinforced electrode has a tensile strength of ≈5.5–7.0 MPa and shows excellent capacity retention after repeatedly flexing to a bending radius ranging from 45 to 10 mm. The relationships between mechanical flexing, electrochemical performance, and mechanical integrity of the battery are studied using electrochemical cycling, electron microscopy, and electrochemical impedance spectroscopy (EIS).
Mechanically flexible arrays of alkaline electrochemical cells fabricated using stencil printing ... more Mechanically flexible arrays of alkaline electrochemical cells fabricated using stencil printing onto fibrous substrates are shown to provide the necessary performance characteristics for driving ink-jet printed circuits. Due to the dimensions and material set currently required for reliable low-temperature print processing of electronic devices, a battery potential greater than that sourced by single cells is typically needed. The developed battery is a series interconnected array of 10 low resistance Zn-MnO 2 alkaline cells, giving an open circuit potential of 14 V. This flexible battery is used to power an ink-jet printed 5-stage complementary ring oscillator based on organic semiconductors. V C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4810974]
Recent progress in the fabrication of ultra thin silicon ribbons and novel architectures have ena... more Recent progress in the fabrication of ultra thin silicon ribbons and novel architectures have enabled devices that can stretch, bend and twist without mechanical fatigue or changes in operational performance.[1-5] These advances have lead to compliant, conformable electronics for health monitoring and sensing purposes.[6, 7] For true autonomous operations, these devices require an equally accommodating power source. Existing commercially available power sources are too bulky and negate the advantages of these compliant/flexible devices. We demonstrate a stretchable battery with electrochemically active materials embedded in a compliant conductive fabric, which acts as a support for the material. The assembled manganese dioxide (MnO2) - zinc (Zn) stretchable cell with a polyacrylic acid (PAA) based polymer gel electrolyte (PGE) had an open circuit potential (OCV) of 1.5 V and a capacity of 3.875 mAh/cm2. The capacity remained constant when tested under strain as high as 100%. Two cells connected in series continuously powered an LED when stretched to 150% and twisted by 90 degrees.
Folding, conformable electronic devices cannot be realized
until fl exible batteries that match d... more Folding, conformable electronic devices cannot be realized
until fl exible batteries that match device form-factor and power
requirements are developed. Existing fl exible batteries exhibit a
severe limitation due to internal short circuits during flexing.
[1] In the present work, we demonstrate a printed alkaline battery fabricated with a polyacrylic acid (PAA)-based polymer gel electrolyte (PGE). The electroactive materials were embedded into a mesh structure providing highly fl exible electrodes. The fl exible alkaline battery showed an open circuit potential of 1.52 V and discharge capacity of 5.6 mAh cm − 2
when discharged at 0.5 mA.The discharge capacity of the printed battery was characterized in bend conditions ranging from 3.81 cm to 0.95 cm bending radii without any decrease in performance compared with the planar state. Two batteries connected in series and bent to a radius of 0.3 cm were able to power a green light-emitting diode (LED)
Additive and low-temperature printing processes enable the integration of diverse electronic devi... more Additive and low-temperature printing processes enable the integration of diverse electronic devices, both power-supplying and power-consuming, on flexible substrates at low cost. Production of a complete electronic system from these devices, however, often requires power electronics to convert between the various operating voltages of the devices. Passive components-inductors, capacitors, and resistors-perform functions such as filtering, short-term energy storage, and voltage measurement, which are vital in power electronics and many other applications. In this paper, we present screen-printed inductors, capacitors, resistors and an RLC circuit on flexible plastic substrates, and report on the design process for minimization of inductor series resistance that enables their use in power electronics. Printed inductors and resistors are then incorporated into a step-up voltage regulator circuit. Organic light-emitting diodes and a flexible lithium ion battery are fabricated and the voltage regulator is used to power the diodes from the battery, demonstrating the potential of printed passive components to replace conventional surface-mount components in a DC-DC converter application.
Energy Technology, Feb 2013
Nanoparticulate printed silver is a core material to flexible, printed circuits. Some commercial ... more Nanoparticulate printed silver is a core material to flexible, printed circuits. Some commercial silvers are of a sufficient purity that
one may consider their use in electrochemical power sources and sensors. We establish an iterative rapid prototyping and
measuring method, printing electrodes, annealing them under temperature conditions from 210 to 280°C, and cycling them in a
microfluidic cell such that the electrolyte becomes the shearing medium. Electrode strength is quantified by the breakage due to
generation of gas-phase oxygen at the electrode. This oxygen generation assisted breaking is found to be a function of the amount
of oxygen generation only, independent of current density and electrolyte flow rate. Silver cured at 280°C for 60 min had highest
strength and required an average of 241.8 mC/mm2
at electrode rupture; curing at 280°C for 20 min required only 203.8 mC/mm2
for failure. Silver strength is quantified as an oxidant storage medium in the forms Ag2O and AgO and as a printed reference
electrode. Ag and AgO have higher shear strength compared to Ag2O. Thus, shear strength of silver oxide electrodes at potentials
of 0.15–0.55 V against a printed silver reference depends on the oxidation stat
The morphology evolution of zinc electrodeposited from alkaline ZnO/KOH is imaged in situ using a... more The morphology evolution of zinc electrodeposited from alkaline ZnO/KOH is imaged in situ using a microfluidic cell. Working
and counter electrodes are in a lateral configuration, separated by a flow channel with a height of 90 m, resulting in quasi-twodimensional
zinc layers. At a flow rate of 0.3 cm/s, zinc packing in the channel is highest at a current density just above the
transition from porous to dense zinc, i 170 mA/cm2. When deposited, compact zinc is approximately 3 times as dense as
porous zinc, as determined by image analysis of the layer. The dense mode invariably leads to ramifications and critical growth,
causing cell shorting. Greater zinc packing is possible at a flow rate of 3.1 cm/s, although flow rates of this order are impractical
for flow-assisted zinc batteries. Ramified zinc tips approach a kinetically limited rate, independent of electrolyte flow rate.
Therefore, increased flow rate cannot control critical growth once it begins. Increased flow rate results in a higher density of
ramified tips at equivalent cell potential. The zinc deposition reaction has a Tafel slope of 130 mV below 10 mA/cm2 and 50 mV
in the second Tafel region 10 mA/cm2. The second Tafel region is relevant to zinc secondary batteries.
Cyclic voltammetry of zinc plated from flowing alkaline zincate electrolyte with a bismuth additi... more Cyclic voltammetry of zinc plated from flowing alkaline zincate electrolyte with a bismuth additive showed a marked mass transport effect during metal layer deplating. This bismuth was added as Bi 2 O 3 and had a saturated concentration of 26 ppm bismuth. Using a small, transparent window flow cell the mechanism was studied in situ using synchrotron X-rays. X-ray microdiffraction revealed that the metal-electrolyte interface was bismuth rich, and bismuth behaved in a manner similar to a surfactant. Transmission X-ray microscopy revealed that in the presence of bismuth additive, 5 μm raised features on the metal layer were preferentially dissolved during deplating. However, macro-morphology experiments demonstrated that at 26 ppm a detrimental bismuth buildup occurred over many cycles. By reducing additive concentration to 3 ppm a metal layer was planarized compared to a no-additive control, while avoiding the bismuth buildup. These findings suggested that 3 ppm bismuth could be used to planarize zinc metal layers such as those in flow-assisted zinc batteries. However, concentration will need to be well-controlled.
Flexible electronics require flexible energy storage, and electrochemical batteries are currently... more Flexible electronics require flexible energy storage, and electrochemical batteries are currently the strongest option for such devices. We further our previous investigation, beginning to add quantitative analysis to the composite mechanical/electrochemical performance of printed electrodes. The presented work will explain the principles of microfluidic stress analysis and how it provides insight into the operating conditions of real microbatteries.
Nanoparticulate printed silver is a core material to flexible, printed circuits. Some commercial ... more Nanoparticulate printed silver is a core material to flexible, printed circuits. Some commercial silvers are of a sufficient purity that one may consider their use in electrochemical power sources and sensors. We establish an iterative rapid prototyping and measuring method, printing electrodes, annealing them under temperature conditions from 210 to 280°C, and cycling them in a microfluidic cell such that the electrolyte becomes the shearing medium. Electrode strength is quantified by the breakage due to generation of gas-phase oxygen at the electrode. This oxygen generation assisted breaking is found to be a function of the amount of oxygen generation only, independent of current density and electrolyte flow rate. Silver cured at 280°C for 60 min had highest strength and required an average of 241.8 mC/mm 2 at electrode rupture; curing at 280°C for 20 min required only 203.8 mC/mm 2 for failure. Silver strength is quantified as an oxidant storage medium in the forms Ag 2 O and AgO and as a printed reference electrode. Ag and AgO have higher shear strength compared to Ag 2 O. Thus, shear strength of silver oxide electrodes at potentials of 0.15-0.55 V against a printed silver reference depends on the oxidation state.
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Papers by Abhinav Gaikwad
hybrid electronics. Currently, a multisubstrate approach is employed, where
soft and hard devices are fabricated or assembled on separate substrates,
and bonded or interfaced using connectors; this hinders the flexibility of the
device and is prone to interconnect issues. Here, a single substrate interfacing approach is reported, where soft devices, i.e., sensors, are directly printed on Kapton polyimide substrates that are widely used for fabricating flexible printed circuit boards (FPCBs). Utilizing a process flow compatible with the FPCB assembly process, a wearable sensor patch is fabricated composed of inkjet-printed gold electrocardiography (ECG) electrodes and a stencil-printed nickel oxide thermistor. The ECG electrodes provide 1 mVp–p ECG signal at 4.7 cm electrode spacing and the thermistor is highly sensitive at normal body temperatures, and demonstrates temperature coefficient, α ≈ –5.84% K–1 and
material constant, β ≈ 4330 K. This sensor platform can be extended to a more sophisticated multisensor platform where sensors fabricated using solution processable functional inks can be interfaced to hard electronics for health and performance monitoring, as well as internet of things applications.
the fabrication of solution-processed organic field effect transistors (OFETs). In addition to solubility and
printability of a solvent, orthogonality is particularly important when forming multilayer structure from
solutions. Currently, the process of finding orthogonal solvents is empirical, and based on trial-and-error
experimental methods. In this paper, we present a methodology for identifying orthogonal solvents for
solution-processed organic devices. We study the accuracy of Hildebrand and Hansen solubility theories
for building solubility boundaries for organic semiconductor (Poly(2,5-bis(3-hexadecylthiophen-2-yl)
thieno[3,2-b]thiophene (PBTTT) and polymer dielectrics (Poly(methyl methacrylate) (PMMA), Polystyrene
(PS)). The Hansen solubility sphere for the organic semiconductor and polymer gate dielectrics
are analyzed to identify solvents that dissolve PMMA and PS, but are orthogonal to PBTTT. Top gate/
bottom contact PBTTT based OFETs are fabricated with PMMA gate dielectric processed with solvents
that are orthogonal and non-orthogonal to PBTTT. The non-orthogonal solvents swell the semiconductor
layer and increase their surface roughness.
until fl exible batteries that match device form-factor and power
requirements are developed. Existing fl exible batteries exhibit a
severe limitation due to internal short circuits during flexing.
[1] In the present work, we demonstrate a printed alkaline battery fabricated with a polyacrylic acid (PAA)-based polymer gel electrolyte (PGE). The electroactive materials were embedded into a mesh structure providing highly fl exible electrodes. The fl exible alkaline battery showed an open circuit potential of 1.52 V and discharge capacity of 5.6 mAh cm − 2
when discharged at 0.5 mA.The discharge capacity of the printed battery was characterized in bend conditions ranging from 3.81 cm to 0.95 cm bending radii without any decrease in performance compared with the planar state. Two batteries connected in series and bent to a radius of 0.3 cm were able to power a green light-emitting diode (LED)
one may consider their use in electrochemical power sources and sensors. We establish an iterative rapid prototyping and
measuring method, printing electrodes, annealing them under temperature conditions from 210 to 280°C, and cycling them in a
microfluidic cell such that the electrolyte becomes the shearing medium. Electrode strength is quantified by the breakage due to
generation of gas-phase oxygen at the electrode. This oxygen generation assisted breaking is found to be a function of the amount
of oxygen generation only, independent of current density and electrolyte flow rate. Silver cured at 280°C for 60 min had highest
strength and required an average of 241.8 mC/mm2
at electrode rupture; curing at 280°C for 20 min required only 203.8 mC/mm2
for failure. Silver strength is quantified as an oxidant storage medium in the forms Ag2O and AgO and as a printed reference
electrode. Ag and AgO have higher shear strength compared to Ag2O. Thus, shear strength of silver oxide electrodes at potentials
of 0.15–0.55 V against a printed silver reference depends on the oxidation stat
and counter electrodes are in a lateral configuration, separated by a flow channel with a height of 90 m, resulting in quasi-twodimensional
zinc layers. At a flow rate of 0.3 cm/s, zinc packing in the channel is highest at a current density just above the
transition from porous to dense zinc, i 170 mA/cm2. When deposited, compact zinc is approximately 3 times as dense as
porous zinc, as determined by image analysis of the layer. The dense mode invariably leads to ramifications and critical growth,
causing cell shorting. Greater zinc packing is possible at a flow rate of 3.1 cm/s, although flow rates of this order are impractical
for flow-assisted zinc batteries. Ramified zinc tips approach a kinetically limited rate, independent of electrolyte flow rate.
Therefore, increased flow rate cannot control critical growth once it begins. Increased flow rate results in a higher density of
ramified tips at equivalent cell potential. The zinc deposition reaction has a Tafel slope of 130 mV below 10 mA/cm2 and 50 mV
in the second Tafel region 10 mA/cm2. The second Tafel region is relevant to zinc secondary batteries.