<h1 dir="ltr">Introduction</h1>Once a luxury used primarily by early adopters, electric vehicles (EVs) are becoming increasingly commonplace due to rising awareness about carbon emissions and their impact on climate change, supportive government policies, volatile oil prices, and growing cultural acceptance of green energy use. Global EV stock is projected to reach 243 million by 2030 - over 30 times higher than today’s level [<a href="https://www.iea.org/reports/global-ev-outlook-2020" target="_blank">1</a>]. Most major car manufacturers are planning either a complete shift or a transition to EVs [<a href="https://www.nature.com/articles/d41586-018-07585-6?source=post_page-----e0f2661bf61----------------------" target="_blank">2</a>].As demand for electric vehicles increases, so does demand for improved performance, including decreased cost and charge time, increased range and battery life, and improved safety. The cost of EV batteries continues to fall and EV battery technology is advancing rapidly in the form of increasing capacity and energy density [<a href="https://www.iea.org/reports/global-ev-outlook-2020" target="_blank">1</a>,<a href="https://www.nature.com/articles/d41586-018-07585-6?source=post_page-----e0f2661bf61----------------------" target="_blank">&nbsp;2</a>]. However, increases in energy density are accompanied by a growing challenge - safety. In particular, lithium ion batteries - the power source of choice for EVs thanks in part to their high energy density - are susceptible to overheating, and even explosion, and safety incidents have been widely reported [<a href="https://www.mdpi.com/2076-3417/9/12/2483/htm" target="_blank">3</a>,<a href="https://link.springer.com/article/10.1007/s10694-019-00944-3" target="_blank">&nbsp;4</a>]. As EV batteries become more powerful and complex, ever more sophisticated battery management systems are needed to ensure the safe, reliable, and cost-efficient operation of electric vehicles.<h1 dir="ltr">What is a battery management system?&nbsp;</h1>Battery management systems (BMS) are electronic control circuits that closely and accurately monitor and regulate the charging and discharging of rechargeable batteries, serving as the “brains” of an EV battery system. The BMS maintains batteries within their safe operating area and ensures optimal use of the residual energy in batteries, thereby maximizing their safety, reliability, durability, and performance.<h1 dir="ltr">Functions of battery management systems</h1>Battery management systems are central to the safe and efficient operation of electric vehicles and perform a broad array of functions, including:<h2 dir="ltr">Battery monitoring&nbsp;</h2>The BMS monitors the state of an EV battery system through a variety of direct and indirect measures. Current, voltage, and surface temperatures of battery cells are directly measured using sensors. These are in turn used to estimate the battery pack State of Charge (SOC), which indicates the charge level of the battery; the State of Health (SOH), which represents the remaining capacity of the battery pack and indicates when it should be replaced; the internal temperature of battery cells; and other parameters. Accurate voltage monitoring is of particular importance, since voltage deviations due to overcharging, over-discharging, or high power pulses can lead to significantly reduced battery life and safety issues.<h2 dir="ltr">Battery protection&nbsp;</h2>The BMS is also equipped to prevent operations outside the safe operating area of and damage to the battery system, for example due to overcharging, over-discharging, or high temperatures. The action of the BMS can be physical (such as emergency shut down of the battery) or informational (by triggering an alarm or otherwise reporting trouble to the driver).For example, the BMS controls the charger to protect the battery from overheating as well as ensure optimal charge of the battery, thereby prolonging its life and improving capacity utilization. The BMS is also responsible for thermal management of the battery. Battery temperature has a significant impact on battery life and performance, and the BMS controls the pump, fans, coolant (where relevant), and heater to maintain the temperature of the battery pack within its optimum operating range.&nbsp;<h2 dir="ltr">Battery optimization&nbsp;</h2>The BMS also optimizes battery performance via a process called cell balancing. Within an EV battery pack, no two cells are identical, due to manufacturing differences as well as variation in their on-vehicle environment. Weaker cells charge and discharge faster than other cells. Because charging and discharging stop as soon as any cell hits programmed upper and lower capacity limits, the weaker cells cause energy within the other cells to go unused while discharging and limit the capacity of the battery pack while charging. This also results in an increase in the number of battery charge and discharge cycles, which shortens the lifetime of the battery.Cell balancing helps resolve these issues and ensure optimum performance of the battery by equalizing the individual cells’ state of charge. Two types of cell balancing can be used – passive and active.&nbsp;In passive balancing a bleed resistor is used to dissipate energy from cells with excess charge until all cells have a similar state of charge. While this approach is lower cost, it is also less efficient since excess energy is lost as heat. Active cell balancing is a more complex technique that redistributes charge between cells during the charge and discharge cycles. While it is more efficient, since energy is transferred to where it is needed instead of being lost as heat, it also requires additional components at higher cost. Either method extends battery life and improves safety by preventing damage to battery cells due to overcharging or over-discharging.High precision cell voltage and temperature measurements and robust communication links between cells and to battery controllers can further contribute to optimizing battery performance. By collecting better measurements and communicating them faster, the BMS can be more responsive to battery conditions and enable more efficient use of the battery.<h2 dir="ltr">Communication</h2>The BMS also facilitates communication of information to the driver, for example by triggering an alarm or reporting the state of charge, as well as to other on-board equipment, for example to request changes in vehicle operation in response to monitored battery pack conditions.<h1 dir="ltr">Design of battery management systems</h1>Electric vehicles are powered by hundreds or even thousands of individual battery cells. These cells are connected in modules in series and parallel and placed inside a steel casing to form the battery pack. Battery packs are typically very large, occupying the entire floor area of an EV, and are connected to the BMS and thermal management system. The BMS transmits information from the battery pack to the controller, which converts the DC voltage produced by the battery pack and drives the motor, and the rotational energy from the motor is transferred to the wheels by the transmission system (Figure 1).<img src="https://images.wevolver.com/eyJidWNrZXQiOiAid2V2b2x2ZXItcHJvamVjdC1pbWFnZXMiLCAia2V5IjogImZyb2FsYS8xNTk3MjQzMjU3MTc1LUlMTFVTVFJBVElPTiAyMi0xLTAxICgxKS5qcGciLCAiZWRpdHMiOiB7InJlc2l6ZSI6IHsid2lkdGgiOiA5NTAsICJmaXQiOiAiY292ZXIifX19">Figure 1: Schematic showing layout of BMS, battery pack, controller, transmission, etc. within a car&nbsp;The BMS itself is a combination of hardware and software (Figure 2). Hardware includes sensing microchips, transceivers, power management microchips, and microcontrollers. The sensors measure battery voltage, current, and surface temperature to monitor the condition of individual cells. Transceivers relay information between components. Power management microchips allow the connection and isolation of the battery pack between the charger and load. Microcontrollers receive information from sensors and plug it into algorithms which model internal battery states, such as internal temperature, SOC, and SOH. Once these internal states have been defined, the microcontroller can make decisions, such as optimizing the charging and discharging of the battery or triggering alarm or safety modules.<img src="https://images.wevolver.com/eyJidWNrZXQiOiAid2V2b2x2ZXItcHJvamVjdC1pbWFnZXMiLCAia2V5IjogImZyb2FsYS8xNTk3MjQzMjk1MzYxLUlMTFVTVFJBVElPTiAyMS0wMSAoMSkuanBnIiwgImVkaXRzIjogeyJyZXNpemUiOiB7IndpZHRoIjogOTUwLCAiZml0IjogImNvdmVyIn19fQ==">&nbsp;Figure 2: Schematic showing layout and components of a simplified BMSBattery management systems can be implemented using a variety of architectures. The choice of architecture has a significant impact on the cost, development time, flexibility, and reliability of a BMS. Architectures can be broadly classified as centralized and decentralized (Figure 3). In centralized architectures, a single controller monitors, balances, and controls all battery cells and is connected to them through a large number of wires. This approach is relatively inexpensive but also challenging to expand. In decentralized architectures, which include distributed, master-slave, and modular architectures, the BMS functionality is not concentrated on a single circuit board. These approaches entail fewer wires and are more flexible, but also more expensive.<img src="https://images.wevolver.com/eyJidWNrZXQiOiAid2V2b2x2ZXItcHJvamVjdC1pbWFnZXMiLCAia2V5IjogImZyb2FsYS8xNTk3MjQzNDkzODAwLUlMTFVTVFJBVElPTiAyMi0wMSAoMSkuanBnIiwgImVkaXRzIjogeyJyZXNpemUiOiB7IndpZHRoIjogOTUwLCAiZml0IjogImNvdmVyIn19fQ==">&nbsp;Figure 3: Diagram comparing generic centralized BMS architecture to decentralized/distributed BMS architectureGenerally speaking, decentralized architectures are advantageous because they offer superior measurement precision, because shorter wires are less susceptible to noise; more reliable connections, because shorter wires are less prone to disconnection due to mechanical shocks and vibrations; scalability; and more robust communication between components.&nbsp;Achieving reliable, high performance communication is a key challenge faced by BMS designers. Besides opting for decentralized architectures, designers can improve communication between components (and therefore achieve more accurate, real-time measurements and control) by using microchips which consolidate a variety of functions and controllers with built-in diagnostic functions. Minimizing the number of components, as well as the size of the components which are used, offer the added benefit of giving designers more flexibility and allowing for optimal placement of components within the vehicle.Another challenge BMS can help designers tackle is security. Because EVs (including smart vehicles and autonomous vehicles) connect to the internet, these vehicles run the risk of cyberattacks. BMS security controllers can prevent unauthorized modification of the battery system, protecting the safety of the system as well as privacy and intellectual property.<h1 dir="ltr">What’s next for battery management systems?</h1>Demand for improved performance and societal changes are driving new developments in battery management systems. One trend which promises major improvements to the performance and cost of EVs is the advent of wireless BMS. In a typical BMS, components are connected by a large amount of wiring - up to several kilometers long - which adds weight to the EV and makes it less efficient. This wiring is also costly and prone to mechanical failure. Furthermore, the wiring can make it so complex and time consuming to replace an individual battery cell that it makes more sense to simply replace the entire pack. By eliminating the need for wiring and associated components, a wireless BMS helps decrease the weight and size of the BMS, thereby improving vehicle efficiency, extending driving range, and helping make vehicles easier and cheaper to produce and maintain. At the same time, wireless BMS could offer even more accurate and useful information about the condition of the battery cells.Another development is the increasing expectation that BMS components meet the most stringent safety standards. The safety of automotive components is assessed according to the ISO26262 standard, defined by the International Organization for Standardization (<a href="https://www.iso.org/standard/68383.html" target="_blank">5</a>). During the development of hardware and software components, all potential hazardous scenarios are evaluated for a particular component and classified based on the levels of severity, probability of occurrence, and driver controllability. The component is then assigned an Automotive Safety Integrity Level (ASIL), which indicates the level of safety required by the component to function normally without posing any threats to the vehicle. ISO 26262 defines four ASIL levels - A, B, C, and D - where ASIL D refers to the highest level of safety required. Vehicle manufacturers increasingly demand that BMS components are ASIL D compliant. At the same time vehicle designs are becoming increasingly complex, meaning that BMS suppliers need to be more rigorous than ever to meet safety requirements and ensure that EVs can operate safely.Finally, a rapidly evolving and urgently needed development for EVs is fast charging, especially for buses and shared mobility vehicles which need to operate continuously (<a href="https://www.annualreviews.org/doi/full/10.1146/annurev-control-053018-023643" target="_blank">6</a>). With the established AC charging, it can take several hours to fully charge an EV, whereas fast charging enables this within about 20 minutes. Batteries undergoing fast charging require more sophisticated battery management and much more accurate estimations of conditions inside battery cells to prevent overcharging and overheating.<h1 dir="ltr">Conclusion</h1>As electric vehicles become increasingly ubiquitous, so will demand for EVs to offer performance and cost which is comparable to that of traditional vehicles. By improving the safety and reliability of EVs, increasing the efficiency and range of EVs, and prolonging EV battery lifetimes, battery management systems will play a pivotal role in achieving comparable, affordable performance, thereby enabling society to reap the benefits of EVs on a large scale.<hr><h1 dir="ltr">About the sponsor: Infineon Technologies </h1>Infineon Technologies provides semiconductor products for use in a wide range of applications.&nbsp;Infineon Technologies is dedicated to developing integrated circuits and design resources for battery management systems that contribute to more efficient, longer-lasting, and reliable battery-powered applications. To learn more click&nbsp;<a href="https://www.infineon.com/" rel="noopener noreferrer" target="_blank">here.</a><img class="fr-fic fr-dib" src="https://images.wevolver.com/eyJidWNrZXQiOiAid2V2b2x2ZXItcHJvamVjdC1pbWFnZXMiLCAia2V5IjogImZyb2FsYS8xNTk3MjU2MTk0MzQyLUluZmluZW9uLWxvZ28tMDYucG5nIiwgImVkaXRzIjogeyJyZXNpemUiOiB7IndpZHRoIjogOTUwLCAiZml0IjogImNvdmVyIn19fQ==" style="width: 300px;"> <hr><h1 dir="ltr">References</h1><ol><li dir="ltr">Global EV Outlook 2020 – Analysis - IEA [Internet]. IEA. 2020. Available from: https://www.iea.org/reports/global-ev-outlook-2020</li><li dir="ltr">Figueres, C. Nature. [Online]. Available from: https://www.nature.com/articles/d41586-018-07585-6&nbsp;</li><li dir="ltr">Ouyang D, Chen M, Huang Q, Weng J, Wang Z, Wang J. A Review on the Thermal Hazards of the Lithium-Ion Battery and the Corresponding Countermeasures. Applied Sciences. 2019;9(12):2483. &nbsp;</li><li dir="ltr">A Review of Battery Fires in Electric Vehicles [Internet]. Springer. 2020. Available from: http://A Review of Battery Fires in Electric Vehicles &nbsp;</li><li dir="ltr">ISO 26262-1:2018 [Internet]. ISO. 2020. Available from: https://www.iso.org/standard/68383.html &nbsp;</li><li dir="ltr">Xinfan L. Modeling and Estimation for Advanced Battery Management [Internet]. Annual Reviews. 2020. Available from: https://www.annualreviews.org/doi/full/10.1146/annurev-control-053018-023643 &nbsp;</li></ol>

As EV batteries become more powerful and complex, ever more sophisticated battery management systems are needed to ensure the safe, reliable, and cost-efficient operation of electric vehicles.

The pivotal role of battery management systems on the performance of electric vehicles

Battery life is one of the most important considerations when designing wireless products. If it is&nbsp;too short, users will have to charge or change batteries often, leading to frustration and possibly less use of the product. But as well as making sure your battery has enough capacity, you need to make sure your product is using it effectively.<h2>Battery cells and batteries explained</h2>A battery cell is simply a controlled and contained chemical reaction which releases electrons. The reaction speeds up and slows down based on how much current is drawn from the system. Broadly speaking, the type of electrodes will determine the voltage (V) of the cell, the amount of electrolyte will determine the capacity (milliampere hours – mAh) and the size of the electrodes and design of the cell will determine the Internal Resistance (IR).A battery should technically refer to multiple cells connected either in series (to double the voltage) or parallel (to double the capacity) but in most applications the distinction is unimportant and both battery and cell are used interchangeably. They can be tiny, such as in a hearing aid, or bulky, such as in an electric car. Finally, batteries can also be either single-use or rechargeable.<h2>The effect of voltage</h2>The maximum voltage of a battery is fixed by the materials used in the electrodes. The voltage of a typical CR2032 Lithium Coin Cell is 3V. As the charge in the battery decreases, the IR increases and the effective voltage decreases. Most electronics will have a minimum operating voltage, known as the Functional End Point (FEP). Once this is reached, the battery is effectively dead.<h2>How current draw affects battery capacity</h2>The manufacturer of a battery will give data on the nominal capacity, i.e. the capacity the battery will have under perfect conditions. For a typical CR2032 cell, a constant draw of 0.5mA or less will get pretty much all of the stated capacity. This is similar to how an old unconnected analogue watch uses its cell.If we increase to a constant draw of 2mA, we only get around 85% of the capacity. Increase this to 3mA the capacity drops to as low as 60%.Modern electronic devices don’t have a constant current draw. They have a low background draw interspersed with a high peak draw. This high draw can reduce the effective capacity further. This is due to a combination of the IR, which causes an immediate voltage drop whenever a high current is drawn, and the chemical reaction. Put simply, the reaction can’t keep up. The longer the pulse, the more damage is done to the battery. The battery eventually recovers, if given the time to catch up.A short pulse of peak current, say 7mA (typical of a Bluetooth Low Energy radio) for 2 millisecond (ms) followed by an idle period of 25ms is well within the limit of a CR2032 battery to get the best possible use of its capacity. Repeated peak currents higher than 10mA can cause a permanent capacity reduction to the CR2032 battery, and must be avoided.<h2>Minimizing current draw to maximize capacity</h2>There are a number of things you can do to avoid battery related problems.First of all, choose the components that operate at the lowest voltage and have the lowest peak and average current consumption. If you do this, you will be able to utilize the battery’s full capacity.Next, ensure that you only use current when you need to. For example, reducing the duration of radio communications as much as possible and increasing the idle time will greatly increase efficiency. Electronic components often use a large amount of current initially, which then falls once they’re up and running. If you alter the order and timing of when components power up, you can minimize the peak current and hence prolong the battery life.Once you’re settled on what the peak and average current consumption of your application will be, it’s time to pick the right battery for the job.<h2>The right battery for the job</h2>The lower the current consumption, the more batteries you can choose from, in order to find the right match between price, availability, battery lifetime and performance.Test your battery use in realistic situations to ensure they perform as planned in the real world and not just under laboratory conditions.<h2>Smooth out the current drawn with a super capacitor</h2>If your application’s peak current consumption is too high for the desired battery, you can remedy some of it by using a super capacitor in parallel with the batteries.A super capacitor will smooth out the current drawn from the battery by acting as a buffer and help retain more of the optimum battery capacity. Capacitors will also overcome the effects of IR in the battery if its base IR is too high for your application. As additional components will increase the cost of your product, this should be carefully considered before designing in a super capacitor.<h2>Extend battery life in ultra low power wireless applications</h2>If you get your battery choice wrong, at best your product will perform poorly and at worst, it could even explode. Needless to say, this leads to bad will among consumers, expensive product recalls, compensation claims, and reputational damage which can be impossible to repair.Designing a product with great software, great components and the most suitable battery are the building blocks of success with the Internet of Things.<hr>This article was first published on Nordic's&nbsp;<a href="https://blog.nordicsemi.com/getconnected" rel="nofollow" target="_blank"></a><a href="https://blog.nordicsemi.com/getconnected?utm_campaign=2021%20wevolver" rel="noopener noreferrer" target="_blank">Get Connected Blog</a>.

Battery life is one of the most important considerations when designing wireless products. If it is too short, users will have to charge or change batteries often, leading to frustration and possibly less use of the product

Improve battery life in Ultra Low Power wireless applications

The rapid development of flexible and wearable electronics is giving rise to an exciting range of applications, from smart watches and flexible displays&mdash;such as smart phones, tablets, and TV&mdash;to smart fabrics, smart glass, transdermal patches, sensors, and more. With this rise, demand has increased for high-performance flexible batteries. Up to now, however, researchers have had difficulty obtaining both good flexibility and high energy density concurrently in lithium-ion batteries.&nbsp;<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1578070497445-fig1a-1b-newsite.jpg">Schematic of the structure and the fabrication process of Yang&#39;s spine-like battery. (a) Schematic illustration of bio-inspired design, the vertebrae correspond to thick stacks of electrodes and soft marrow corresponds to unwound part that interconnects all the stacks. (b) The process to fabricate the spine-like battery, multilayers of electrodes were first cut into designed shape, then strips extending out were wound around the backbone to form spine-like structure.&nbsp;<style type="text/css"></style><style type="text/css"></style>Images And Video Courtesy: Yuan Yang/Columbia Engineering&nbsp;A team led by Yuan Yang, assistant professor of <a href="http://apam.columbia.edu/materials-science-and-engineering-0" target="_blank">materials science and engineering</a> in the <a href="http://apam.columbia.edu/" target="_blank">department of applied physics and mathematics</a> at <a href="http://www.engineering.columbia.edu/" target="_blank">Columbia Engineering</a>, has developed a prototype that addresses this challenge: a Li-ion battery shaped like the human spine that allows remarkable flexibility, high energy density, and stable voltage no matter how it is flexed or twisted. The <a href="http://onlinelibrary.wiley.com/doi/10.1002/adma.201704947/full" target="_blank">study</a> is published today in <a href="http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1521-4095" target="_blank">Advanced Materials</a>.&ldquo;The energy density of our prototype is one of the highest reported so far,&rdquo; says Yang. &ldquo;We&rsquo;ve developed a simple and scalable approach to fabricate a flexible spine-like lithium ion battery that has excellent electrochemical and mechanical properties. Our design is a very promising candidate as the first-generation, flexible, commercial lithium-ion battery. We are now optimizing the design and improving its performance.&rdquo;<iframe src="https://www.youtube.com/embed/JGpzSF0p1Xg?&wmode=opaque" frameborder="0" allowfullscreen="" class="fr-draggable" style="width: 1115px; height: 621px;"></iframe>Video demonstrating flexibility of Yang&#39;s lithium-ion battery powering a smart watch, in flat, flexed, and twisted configurations. <style type="text/css"></style><style type="text/css"></style>Images And Video Courtesy: Yuan Yang/Columbia Engineering&nbsp;Yang, whose <a href="http://blogs.cuit.columbia.edu/yanggroup/" target="_blank">group</a> explores the composition and structure of battery materials to realize high performance, was inspired by the suppleness of the spine while doing sit-ups in the gym. The human spine is highly flexible and distortable as well as mechanically robust, as it contains soft marrow components that interconnect hard vertebra parts. Yang used the spine model to design a battery with a similar structure. His prototype has a thick, rigid segment that stores energy by winding the electrodes (&ldquo;vertebrae&rdquo;) around a thin, flexible part (&ldquo;marrow&rdquo;) that connects the vertebra-like stacks of electrodes together. His design provides excellent flexibility for the whole battery.&ldquo;As the volume of the rigid electrode part is significantly larger than the flexible interconnection, the energy density of such a flexible battery can be greater than 85 percent of a battery in standard commercial packaging,&rdquo; Yang explains. &ldquo;Because of the high proportion of the active materials in the whole structure, our spine-like battery shows very high energy density&mdash;higher than any other reports we are aware of. The battery also successfully survived a harsh dynamic mechanical load test because of our rational bio-inspired design.&rdquo;<iframe src="https://www.youtube.com/embed/CDJpnqT_Y0c?&wmode=opaque" frameborder="0" allowfullscreen="" class="fr-draggable" style="width: 1115px; height: 630px;"></iframe>Video of the dynamic mechanical load test demonstrating flexibility of Yang&#39;s lithium-ion battery. <style type="text/css"></style><style type="text/css"></style>Images And Video Courtesy: Yuan Yang/Columbia Engineering&nbsp;Yang&rsquo;s team cut the conventional anode/separator/cathode/separator stacks into long strips with multiple &ldquo;branches&rdquo; extending out 90 degrees from the &ldquo;backbone.&rdquo; Then they wrapped each branch around the backbone to form thick stacks for storing energy, like vertebrae in a spine. With this integrated design, the battery&rsquo;s energy density is limited only by the longitudinal percentage of vertebra-like stacks compared to the whole length of the device, which can easily reach over 90 percent.The battery shows stable capacity upon cycling, as well as a stable voltage profile no matter how it is flexed or twisted. After cycling, the team disassembled the battery to examine the morphology change of electrode materials. They found that the positive electrode was intact with no obvious cracking or peeling from the aluminum foil, confirming the mechanical stability of their design. To further illustrate the flexibility of their design, the researchers continuously flexed or twisted the battery during discharge, finding that neither bending nor twisting interrupted the voltage curve. Even when the cell was continuously flexed and twisted during the whole discharge, the voltage profile remained. The battery in the flexed state was also cycled at higher current densities, and the capacity retention was quite high (84 percent at 3C, the charge in 1/3 of an hour). The battery also survived a continuous dynamic mechanical load test, rarely reported in earlier studies.&ldquo;Our spine-like design is much more mechanically robust than are conventional designs,&rdquo; Yang says. &ldquo;We anticipate that our bio-inspired, scalable method to fabricate flexible Li-ion batteries could greatly advance the commercialization of flexible devices.&rdquo;

Schematic of the structure and the fabrication process of Yang's spine-like battery. (a) Schematic illustration of bio-inspired design, the vertebrae correspond to thick stacks of electrodes and soft marrow corresponds to unwound part that interconnects all the stacks. (b) The process to fabricate the spine-like battery, multilayers of electrodes were first cut into designed shape, then strips extending out were wound around the backbone to form spine-like structure. Images And Video Courtesy: Yuan Yang/Columbia Engineering

Shaped like a spine, new design enables remarkable flexibility, high energy density, and stable voltage no matter how it is flexed or twisted

Engineers Develop Flexible Lithium Battery for Wearable Electronics

Drones are being used for a growing number of purposes. Their designs are ever more efficient, and techniques for flying them are being further refined all the time. But drones still have the same weak point: their battery. This is particularly true of propeller drones, which are popular for information-gathering purposes in dangerous or hard-to-reach regions. These drones can fly for only around 15 minutes at a time because their engines quickly burn through their batteries. One way of addressing this limitation &ndash; without weighing the drones down &ndash; would be to recharge them while aloft using a power beaming system: an energy-rich laser beam that is guided by a tracking system and shines directly on photovoltaic cells on the drones&rsquo; exterior.Several labs around the world, including in the US, have been working on this idea in recent years. LakeDiamond, an EPFL spin-off based at Innovation Park, has now demonstrated the feasibility of using a high-power laser for this purpose. What&rsquo;s more, LakeDiamond&rsquo;s laser emits a wavelength that cannot damage human skin or eyes &ndash; the issue of safety is paramount, since the system is meant for use with civilian drones. LakeDiamond&rsquo;s technology is built around diamonds that are grown in the company&rsquo;s lab and subsequently etched at the atomic level.World record for powerDespite appearances, standard laser beams are not as straight as they seem: as they travel, they expand ever so slightly, leading to a loss in density as they go. But LakeDiamond&rsquo;s system produces a laser beam with a wavelength of 1.5 &micro;m that, in addition to being safe, can travel much farther without losing strength. &ldquo;Systems developed by other companies and labs, often for military applications, employ lasers that are more powerful and thus more dangerous for humans,&quot; says Pascal Gallo, CEO of LakeDiamond. His company took the opposite tack: their technology transforms the rays emitted by a simple low-power diode into a high-quality laser beam. Their beam has a larger diameter, and its rays remain parallel over a longer distance &ndash; in this case up to several hundred meters.In LakeDiamond&rsquo;s laser, the light produced by a diode is directed at a booster composed of reflective material, an optical component and a small metal plate to absorb the heat. The breakthrough lies not with this set-up, which already exists, but with the fact that the emitted beam is only a few dozen watts strong. The secret is using a small square lab-grown diamond as the optical component, as this delivers unparalleled performance. LakeDiamond&#39;s system holds the world record for continuous operation using a wavelength in the middle of the infrared range &ndash; it delivers more than 30 watts in its base configuration. &ldquo;That&rsquo;s equivalent to around 10,000 laser pointers,&rdquo; adds Gallo.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1571294491761-1920x1080+%282%29.jpg" style="width: 766px;" class="fr-fic fr-dib">The lab-grown diamonds&rsquo; key properties include high transparency and thermal conductivity. Achieving those things &ndash; and mastering the nano-etching process &ndash; took the researchers over ten years of development. LakeDiamond grows its diamonds through a process of chemical vapor deposition, an approach that ensures their purity and reproducibility. The surfaces of the resulting tiny square diamonds are then sculpted at the nano level using expertise developed in Niels Quack&#39;s lab at EPFL (<a href="https://actu.epfl.ch/news/carving-diamonds-for-optical-components/" target="_blank">read the EPFL article on this topic</a>). Thanks to their inherent properties and etched shapes, the diamonds are able to transfer heat to a small metal plate that dissipates it, while at the same time reflecting light in such a way as to create a laser beam.&ldquo;To achieve greater power &ndash; say to recharge a larger drone &ndash; these lasers could easily be operated in series,&rdquo; says Nicolas Malpiece, who is in charge of power beaming at LakeDiamond. The company&rsquo;s remote recharging system works in the lab but will require further development and refinement before it&rsquo;s ready for field use. What would happen if a drone flies behind an obstacle and is cut off from its laser energy source? Several approaches to this problem are currently being explored. A small back-up battery could take over temporarily, or, for information-gathering missions over rough terrain for example, the drone could simply return to within range of the laser in order to top up its battery.This energy transmission system is also interesting for other areas of application. It can for example be used for charging and transmitting data to satellites. The development of the system is included in a support program of the Swiss Space Office, which began on 1 November and runs for two years.

A small lab-grown diamond measuring a few millimeters per side could one day enable civilian drones to be recharged in mid-flight through a laser. Thanks to the diamond, the laser beam can remain strong enough over a long distance to recharge photovoltaic cells on the drones’ surface. This system, which poses no threat to human health, is being developed by EPFL spin-off LakeDiamond. It could also be used to transmit both power and data to satellites and has just been included in the ten projects supported for two years by of the Swiss Space Office.

Drones are being used for a growing number of purposes. Their designs are ever more efficient, and techniques for flying them are being further refined all the time.

Using diamonds to recharge civilian drones in flight

Lithium dendrites grow, shrink and break away from lithium metal electrodes. Image: Dasgupta Lab, University of Michigan

Watching electrodes degrade could help battery researchers take lithium to the next level.

A window into battery life for next-gen lithium cells

CHAMPAIGN, Ill. &mdash; Batteries that charge faster and have greater capacity could boost portable electronic devices and electric cars. A new method to simultaneously test stress and strain in battery electrodes gives researchers a window into the mechanical, electrical and chemical forces within lithium-ion batteries. The method revealed an unexpected point of stress in the charging cycle, which could guide development of better batteries.University of Illinois&nbsp;<a href="http://www.chemistry.illinois.edu/" target="_blank">chemistry</a> professor&nbsp;<a href="http://www.chemistry.illinois.edu/faculty/agewirth.html" target="_blank">Andrew Gewirth</a>,&nbsp;<a href="http://www.matse.illinois.edu/" target="_blank">materials science and engineering</a> professor&nbsp;<a href="http://www.matse.illinois.edu/directory/profile/n-sottos" target="_blank">Nancy Sottos</a> and graduate students Elizabeth Jones and Hadi Tavassol published&nbsp;<a href="http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4708.html" target="_blank">their work</a> in the journal Nature Materials.&ldquo;We are finding limitations that are preventing the material from operating properly,&rdquo; Gewirth said. &ldquo;We are trying to understand what those limitations are and then develop materials and ways of processing materials that overcome them.&rdquo;A lithium-ion battery works by transferring lithium ions between two electrodes: the cathode and anode. When the battery is charged, lithium goes into the anode, which causes the material to expand. When the battery is discharged, the lithium leaves the anode, causing it to contract. However, because the material is constrained within the battery cell, it cannot truly expand and contract as it would when unconstrained, which causes mechanical stress within the material, Sottos said. Most methods of studying battery operation focus on electrochemistry, overlooking this mechanical response, she said.&ldquo;The relationship between the electrochemistry and the development of stress and strain is not well understood,&rdquo; said Sottos. &ldquo;The stress and strain can actually change the way a battery functions, and usually in a negative way &ndash; like the capacity will fade, or it will cause small cracks that cause reactions you don&rsquo;t expect.&rdquo;The researchers combined two techniques to get a more holistic picture of the mechanical forces during battery charging and discharging. They measured the strain, or how much the material expands, by seeding the anode with fluorescent nanoparticles and precisely measuring their movement as the battery went through charging cycles. Meanwhile, they measured stress, or how much the material bends or buckles, using an advanced optical technique.&ldquo;There have been other ways of looking at batteries &ndash; electrochemical methods, circuit methods, techniques involving light &ndash; but no one has done a comprehensive mechanical probe the way that we have,&rdquo; Gewirth said. &ldquo;Stress and strain have been measured separately, but no one has even looked at them together before.&rdquo;The researchers have dubbed their holistic measurement &ldquo;electrochemical stiffness,&rdquo; defined as the ratio of stress to strain.The measurement has already revealed one surprising finding: Right before the lithium is taken up by the anode, there is a spike in stress.&ldquo;We didn&rsquo;t expect there would be a barrier or an impediment to the lithiation process,&rdquo; Gewirth said. &ldquo;First, it shows us where the impediment is for charging. Now that we know where it is, we can work on ways to fix it. Second, it gives us a better idea of how the battery actually works.&rdquo;The electrochemical stiffness measurement technique is not limited to lithium-ion batteries, the researchers say, but can be used on a wide variety of ion-battery materials. Now that they have studied the anode, they are using the technique to study the cathode, which has its own properties and behaviors, Sottos said.&ldquo;These experiments are giving insight that it&rsquo;s not just an electrochemical issue; it&rsquo;s a mechanical issue,&ldquo; Sottos said. &ldquo;We think we are beginning to understand some of the factors that are hindering faster battery charging. We may be able to identify things that let us charge faster and have higher capacity, with less capacity fade over time.&rdquo;The Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, supported this work. Sottos also is affiliated with the&nbsp;<a href="http://beckman.illinois.edu/" target="_blank">Beckman Institute for Advanced Science and Technology</a> at the U. of I.

Illinois professors Nancy Sottos and Andrew Gerwith developed a method to comprehensively measure the mechanical stress and strain in lithium-ion batteries. It revealed a point of stress in charging that, if addressed through new methods or materials, could lead to faster-charging batteries.   Photo by L. Brian Stauffer

CHAMPAIGN, Ill. — Batteries that charge faster and have greater capacity could boost portable electronic devices and electric cars.

Method opens a window on how stress and strain affect battery performance

Additive manufacturing, otherwise known as 3D printing, can be used to manufacture porous electrodes for lithium-ion batteries&mdash;but because of the nature of the manufacturing process, the design of these 3D printed electrodes is limited to just a few possible architectures. Until now, the internal geometry that produced the best porous electrodes through additive manufacturing was what&rsquo;s known as an interdigitated geometry&mdash;metal prongs interlocked like the fingers of two clasped hands, with the lithium shuttling between the two sides.Lithium-ion battery capacity can be vastly improved if, on the microscale, their electrodes have pores and channels. An interdigitated geometry, though it does allow lithium to transport through the battery efficiently during charging and discharging, is not optimal.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1568214668191-0730-em-rahul-battery.png" style="width: 766px;" class="fr-fic fr-dib">Lattice architecture can provide channels for effective transportation of electrolyte inside the volume of material, while for the cube electrode, most of the material will not be exposed to the electrolyte.Rahul Panat, an associate professor of mechanical engineering at Carnegie Mellon University, and a team of researchers from Carnegie Mellon in collaboration with Jonghyun Park, assistant professor of mechanical and aerospace engineering at Missouri S&amp;T have developed a revolutionary new method of 3D printing battery electrodes that creates a 3D microlattice structure with controlled porosity. 3D printing this microlattice structure, the researchers show in a paper <a href="https://www.sciencedirect.com/science/article/pii/S2214860418302379#bib0095" rel="noopener" target="_blank">published in the journal Additive Manufacturing</a>, vastly improves the capacity and charge-discharge rates for lithium-ion batteries.&ldquo;In the case of lithium-ion batteries, the electrodes with porous architectures can lead to higher charge capacities,&rdquo; says Panat. &ldquo;This is because such architectures allow the lithium to penetrate through the electrode volume leading to very high electrode utilization, and thereby higher energy storage capacity. In normal batteries, 30-50% of the total electrode volume is unutilized. Our method overcomes this issue by using 3D printing where we create a microlattice electrode architecture that allows the efficient transport of lithium through the entire electrode, which also increases the battery charging rates.&rdquo;<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1568214704238-Screenshot_2019-09-11+3D+printing+the+next+generation+of+batteries.png" style="width: 766px;" class="fr-fic fr-dib"> The additive manufacturing method presented in Panat and Park&rsquo;s paper represents a major advance in printing complex geometries for 3D battery architectures, as well as an important step toward geometrically optimizing 3D configurations for electrochemical energy storage. The researchers estimate that this technology will be ready to translate to industrial applications in about two to three years.The microlattice structure (Ag) used as lithium-ion batteries&rsquo; electrodes was shown to improve battery performance in several ways such as a fourfold increase in specific capacity and a twofold increase in areal capacity when compared to a solid block (Ag) electrode. Furthermore, the electrodes retained their complex 3D lattice structures after forty electrochemical cycles demonstrating their mechanical robustness. The batteries can thus have high capacity for the same weight or alternately, for the same capacity, a vastly reduced weight, which is an important attribute for transportation applications.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1568214728313-0730-em-rahul-battery-microscope.png" style="width: 766px;" class="fr-fic fr-dib">SEM images of 3D printed electrodes for Li-ion batteries used for electrochemical cycling in the researchers&rsquo; study. Image taken from top of microlattice electrodes with height of about 250mm.The Carnegie Mellon researchers developed their own 3D printing method to create the porous microlattice architectures while leveraging the existing capabilities of an Aerosol Jet 3D printing system. The Aerosol Jet system also allows the researchers to print planar sensors and other electronics on a micro-scale, which was deployed at Carnegie Mellon University&rsquo;s College of Engineering earlier this year. This novel 3-D printing method could be applied to battery technology with a collaboration with the Missouri university, where researchers developed 3-D battery design and analysis, cell fabrication technology, and electrochemical analysis via simulation and modeling.Until now, 3D printed battery efforts were limited to extrusion-based printing, where a wire of material is extruded from a nozzle, creating continuous structures. Interdigitated structures were possible using this method. With the method developed in Panat&rsquo;s lab, the researchers are able to 3D print the battery electrodes by rapidly assembling individual droplets one-by-one into three-dimensional structures. The resulting structures have complex geometries impossible to fabricate using typical extrusion methods.&ldquo;Because these droplets are separated from each other, we can create these new complex geometries,&rdquo; says Panat. &ldquo;If this was a single stream of material, as in the case of extrusion printing, we wouldn&rsquo;t be able to make them. This is a new thing. I don&rsquo;t believe anybody until now has used 3D printing to create these kinds of complex structures.&rdquo;This revolutionary method will be very important for consumer electronics, the medical devices industry, as well as aerospace applications. This research will integrate well with the biomedical electronic devices, where miniaturized batteries are required. Non-biological electronic micro-devices will also benefit from this work. And on a bigger scale, electronic devices, small drones, and aerospace applications themselves can use this technology as well, due to the low weight and high capacity of the batteries printed using this method.The team, which also includes mechanical engineering Ph.D. student Mohammad Sadeq Saleh and postdoctoral researcher Jie Li at Missouri S&amp;T, is also working on creating more complex three-dimensional structures, which can simultaneously be used as structural materials and as functional materials. For example, a part of a drone can act as a wing, a structural material, while simultaneously acting as a functional material such as a battery.

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