Compliant battery design strategy for wearable power sources with high degree of flexibility and stretchability.

to a bending radius of less than 1 cm without reaching its fracture limit (≈1% strain). [ 10,11 ] Electronics fabricated on soft polymeric substrate can conform over curvilinear surfaces and form the building blocks for wearable electronics. Powering these devices while maintaining the fl exibility and form-factor of the device is a challenge. Towards this goal there have been numerous attempt to develop thin, fl exible and stretchable supercapacitors, [12][13][14][15][16][17] energy harvesters, [18][19][20][21][22] and batteries. [ 14, Due to their high energy and power density, fl exible rechargeable batteries are an essential part of a power module. [ 53 ] Commercially available batteries are typically rigid due to their packaging and thick electrode stack. Flexible batteries require that all of the key components (current collector, active layer, separator, packaging) be bendable. Such devices are currently fabricated by using a thin conductive layer supported on a non-conductive substrate as a current collector, on which a thin active layer (20-60 µm) is printed. The areal capacities of fl exible lithium-ion batteries have been comparatively low (0.05-0.20 mAh cm −2 vs 1-2 mAh cm −2 for traditional systems) as the active layers are printed thin to reduce the electrode degradation during fl exing and the inactive layers associated with supporting the current collectors increase the thickness of the battery. [ 26,35,41,44,54 ] Here we demonstrate a fl exible rechargeable lithium-ion battery with an areal capacity of ≈1 mAh cm -2 . We use lithium cobalt oxide (LCO) and lithium titanate oxide (LTO) as the positive (cathode) and negative (anode) electrodes, to form a battery with a nominal potential of ≈2.5 V. [55][56][57] The fl exible battery with CNT as the current collector had neglible drop in capacity after electrochemically cycling the battery for 450 cycles at C/2 rate. The areal capacity (mAh cm −2 ) of the battery was increased by printing active layers as thick as 150 µm while improving their mechanical property by embedding the active layers inside a fi brous support. The fi brous membrane binds the active layers while carrying the stress associated with fl exing. The reinforced electrodes have a tensile strength of ≈5.5-7.0 MPa, an order of magnitude higher than conventional non-fl exible electrodes, making the electrode resistant to cracking and mechanical fatigue. The battery maintained capacity during electrochemical cycles under fl ex conditions and after undergoing repeated fl exing cycles. ElS was used to analyze the structural changes Early demonstrations of wearable devices have driven interest in fl exible lithium-ion batteries. Previous demonstrations of fl exible lithium-ion batteries trade off between low areal capacity, poor mechanical fl exibility 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 h...

Ultrasonic analysis was used to predict the state of charge and state of health of lithium-ion pouch cells that have been cycled for several hundred cycles. The repeatable ultrasonic trends are reduced to two key metrics: time of flight shift and total signal amplitude, which are then used with voltage data in a supervised machine learning technique to build a model for state of charge (SOC) prediction. Using this model, cell SOC is predicted to ∼1% accuracy for both lithium cobalt oxide and lithium iron phosphate cells. Elastic wave propagation theory is used to explain that the changes in ultrasonic signal are related to changes in the material properties of the active materials (i.e., elastic modulus and density) during cycling. Finally, we show the machine learning model can accurately predict cell state of health with an error ∼1%. This is accomplished by extending the data inputs into the model to include full ultrasonic waveforms at top of charge. A key component of an electric vehicle is the battery management system (BMS), which is responsible for controlling the operating conditions on a given battery cell (or stack of cells) in order to optimize the performance and lifetime of the full battery system. The most effective battery management systems must be able to track battery state of charge (SOC), state of health (SOH) and cell failure, including early prediction of catastrophic failure. Despite the importance of this task, being able to reliably determine SOC, SOH and failure at low cost still presents a significant challenge. A range of methods exist at present, however the simplest methods can prove inaccurate, and more complex methods are not suitable for low-cost, in-operando SOC determination. 1-3For instance, in its most common implementation, SOC prediction consists of voltage monitoring (direct measurement) combined with coulomb-counting (book-keeping).1 This can present challenges for a variety of reasons. First, for voltage measurements, the flatness of voltage readings over the majority of battery capacity, especially for lithium iron phosphate (LFP) cells, presents difficulties. 4 Furthermore, voltage fade, changing cell impedances, and varying discharge rates impact measured voltage, obscuring true SOC. Second, coulomb counting is also an inexact science, as discharge rate, environmental factors such as temperature, and cell degradation can all impact the actual capacity for any given discharge. This can lead to a cycle of abuse, whereby discharge conditions lead to an incorrect estimate of SOC, and therefore the cell becomes inadvertently over-discharged. This causes damage to the cell, which leads to further inaccuracy in the SOC prediction, resulting in continued over-discharging and cell damage. Effectively, a battery "death-spiral" ensues.One method to further increase the accuracy of battery management systems is to introduce a technique that can directly measure the physical state of the battery to enhance the determination of SOC, SOH, and cell failure, especially when applied i...

A Bi2O3 in β-MnO2 composite cathode material has been synthesized using a simple hydrothermal method and cycled in a mixed KOH-LiOH electrolyte with a range of concentrations. We show that, at a KOH:LiOH molar ratio of 1:3, both proton insertion and lithium insertion occur, allowing access to a higher fraction of the theoretical capacity of the MnO2 while preventing the formation of ZnMn2O4. This enables a capacity of 360 mAh/g for over 60 cycles, with cycling limited more by anode properties than traditional cathodic failure mechanisms. The structural changes occurring during cycling are characterized using electron microscopy and in situ synchrotron energy-dispersive X-ray diffraction (EDXRD) techniques. This mixed electrolyte shows exceptional cyclability and capacity and can be used as a drop-in replacement for current alkaline batteries, potentially drastically improving their cycle life and creating a wide range of new applications for this energy storage technology.

This study investigates the evolution of material and electrochemical properties in commercial lithium-ion batteries during cycling. Results indicate that as-received batteries undergo a post-formation break-in period, which is signified by an initial, rapid evolution of the battery's properties before stabilizing. Break-in corresponds to non-chemical crosstalk, whereby physical changes in the negative electrode affect the electrochemical performance of the positive electrode. These findings demonstrate how interplay between components during early cycles can affect the future battery performance.