Abhinav M. Gaikwad

Highly Stretchable Alkaline Batteries Based on an Embedded Conductive Fabric

Recent progress in the fabrication of ultrathin 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/fl exible 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 (MnO 2 ) 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/cm 2 . 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. In the past decade, stretchable electronics with a wide variety of functionality such as biological sensors, [ 8 ] solar cells, [ 9 ] polymer light-emitting devices, [ 10 ] transistors, [ 11 , 12 ] active matrix displays, [ 13 , 14 ] and photo-detectors [ 15 ] have been demonstrated. While there has been progress on power sources with similar mechanical properties, there is still a signifi cant gap. Previously, stretchable supercapacitors based on SWNT deposited on stretched PDMS, [ 16 ] CNTs embedded in fabric [ 17 ] and conducting polymer on compliant substrates [ 18 ] have been demonstrated, but these devices are suited to short term energy storage and cannot be used to power standalone devices. A stretchable MnO 2 -Zn primary battery with a stretchable carbon oil current collector was demonstrated, however the discharge capacity decreased by ∼ 55% when the battery was stretched by 50%. The discharge profi le showed a high ohmic potential drop (I × R) at the start of discharge for the strained battery. Drop in potential can be accounted by decrease in electrical conductivity of the carbon oil current collector and loss in electrical contact in the electrode when stretched. [ 19 ]

HIGHLY FLEXIBLE PRINTED ALKALINE BATTERIES BASED ON MESH EMBEDDED ELECTRODES

A flexible battery and a method to form the flexible battery include forming an anode by embedding an anode type electro-active material within a mesh material and associating an anode current collector with the anode. Similarly a cathode is formed by embedding a cathode type electro-active material within a mesh material and a cathode current collector is associated with the cathode. An electrolyte is located between the anode and cathode, and the arrangement is sealed.

A flexible high potential printed battery for powering printed electronics

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-MnO2 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.

High-performance flexible energy storage and harvesting system for wearable electronics.

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/cm2 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.

A Lateral Microfluidic Cell for Imaging Electrodeposited Zinc near the Shorting Condition

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/cm 2 . 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/cm 2 and 50 mV

Screen printed passive components for flexible power electronics.

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.

Electrochemical-Mechanical Analysis of Printed Silver Electrodes in a Microfluidic Device

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 0 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.

Understanding the Effects of Electrode Formulation on the Mechanical Strength of Composite Electrodes for Flexible Batteries

Flexible lithium-ion batteries are necessary for powering the 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 made toward 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 tests 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 were able to fabricate a high areal capacity (∼2 mAh/cm2) flexible lithium-ion battery with conventional metal-based current collectors that shows superior electrochemical and mechanical performance in comparison to that of batteries with standard composition.

Further studies in the electrochemical/mechanical strength of printed microbatteries

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.