Seeking a Safer Future for Advanced Lithium-Ion Battery Technologies

David Lee, CEO, BioSolar
Dr. Sung-Jin Cho, Assistant Professor, Joint School of Nanoscience and Nanoengineering, North Carolina Agricultural and Technical State University

While there are many different battery technologies currently available and in use, the energy storage industry is focused mainly on advancing lithium-ion battery technology because of its overwhelming advantages over other types of batteries commercially available on the market. For instance, compared to other existing batteries, lithium-ion batteries have a substantially higher energy storage density that requires a smaller footprint. This structure minimizes the weight and size of the devices, and its “memoryless” nature makes it more suitable for use in hybrid vehicles that require constant charges and discharges of its batteries in stop and go traffic. It is also important to note that with a low self-discharge property, the battery’s stored electrical energy lasts longer. Though more substantial energy density improvement may be achievable by the so-called “beyond lithium-ion” batteries in the future, lithium-ion battery is expected to stay the mainstream battery chemistry for the next decade or maybe even longer. 

Unfortunately, lithium-ion batteries’ substantially higher energy storage density can pose an increased the threat of battery fire. The battery industry’s approach to making the lithium-ion battery safer has generally been effective, but recent news of battery fire including that of Samsung Galaxy 7 did not help ease the mind of everyday consumers.  After 35 reported incidents of overheating smartphones worldwide, the company decided to recall all of the Galaxy Note 7 smartphones sold, which led to a halt in sales by every US carrier of the Samsung Galaxy Note 7.

While the causes and mechanics behind battery fires are well understood by the scientific  community, there is no one silver bullet that can completely mitigate the problem.  Both scientists and engineers strongly agree that improvement in material technology, as well as consistent safety design practices (packaging, system design, detection and isolation of fire to stop propagation, etc.) will drastically reduce the occurrences of future mishaps. Adding to the pressure to make batteries safer is an overwhelming demand for higher density lithium-ion batteries, and many industries are depending on the advancement of these battery technologies to satisfy the need for higher density. The billion-dollar question here is, “how do we safely advance lithium-ion battery technologies?”

Research at many different levels (material, electrode, cell, module and system integration) are being conducted in search of improving the energy density as well as safety.  BioSolar is currently working on improving both energy density and safety at the material and electrode level through the use of its unique Silicon Nano Alloy material under development.  BioSolar intends to commercialize a Si-based anode material technology to significantly increase the energy density and cycle life of the current lithium-ion batteries.

Addressing the Landscape for Lithium-Ion Battery Technology
While there are many different battery technologies currently available and in use, the energy storage industry is focused mainly on advancing lithium-ion battery technology due to the overwhelming advantages in comparison to other types of batteries commercially available in the market. For instance, compared to other existing batteries, lithium-ion batteries have a substantially higher energy storage density that requires a smaller footprint. This structure minimizes the weight and size of the devices, and its “memoryless” nature makes it more suitable for use in hybrid vehicles that require constant charges and discharges of its batteries in stop and go traffic. It is also important to note that with a low self-discharge property, the battery’s stored electrical energy lasts longer.

Lithium-ion battery is an indispensable device that is comprised of four main components: cathode, anode, electrolyte and separator. Among commercially available battery technologies, lithium-ion battery is the highest capacity energy storage device at this point of time and at least a decade to come. Lithium-ion batteries function as lithium ions commute from cathode to anode for charging and from anode to cathode during discharge while powering up to your device. Ideally, we would like to retain all lithium ions during and after intercalation and de-intercalation processes without any loss, and that would be the perfect battery from our technical point of view.

Many researchers and financial professionals believe energy density improvement with factors of five or more may be possible by advancing lithium–air or lithium-sulfur batteries, ultimately leading to our ability to confine extremely high potential energy in a small volume without compromising safety.  With that in mind, it is important to note that there are fundamental technological barriers that will still have to be overcome at the present time.

Most scientists agree that lithium-air and lithium-sulfur battery technologies will not be commercially dominated within the next decade, thus tempering industry expectations and too often, keeping many in the industry focused on the incremental upgrades to lithium-ion battery technology. Though substantial energy density improvement may be achievable by the so-called “beyond lithium-ion” batteries in the future, lithium-ion battery is expected to stay the mainstream battery chemistry for the next decade or maybe even longer.

There have been many reports of breakthroughs in battery technologies over the years that at the surface, presented viable commercial opportunities to reduce costs. Companies once hailed such promising technologies, only to eventually fail due to cash constraints and an inability to produce results. This has led many within the energy storage and battery development sector to rethink their strategy entirely, focusing on decreasing cost and increasing efficiency within the proven lithium-ion sector, as opposed to entirely new and innovative battery technologies beyond lithium ion.

Currently, two most important and challenging subjects related to lithium-ion batteries are increasing the energy density and reducing the charging time. Increased energy density, which is corresponding to increased battery specific capacity (mAh/g), means that the device will stay on longer before having to be recharged. Reduced charging time means that the device will be ready for use in shorter time after each discharging cycle.  There are many other technical challenges associated with improving lithium-ion batteries, but these are the top priorities to overcome in the near future.

Lithium-Ion Battery Safety – What is Being Done.  Issues Remaining|
Lithium-ion batteries have substantially higher energy storage density which translates into smaller footprint to minimize the weight and volume of the devices, but its high energy storage density also poses increased threat of battery fire. The battery industry’s approach to make lithium-ion battery safer has been generally effective.  Unfortunately, recent news of battery fires such as with the Samsung Galaxy did not help ease the mind of everyday consumers.  After 35 reported incidents of overheating smartphones worldwide, the company has decided to recall all of the Galaxy Note 7 smartphones sold – and every US carrier eventually ended up halting the sales of the Samsung Galaxy Note 7 as well.

The causes and mechanics behind battery fires are well understood by the scientific community, but there is no one silver bullet that can completely mitigate the problem.  Scientists and engineers strongly agree that improvement in material technology and consistent safety design practices (packaging, system design, detection and isolation of fire to stop heat propagation, etc.) will drastically reduce the occurrences of potential future mishaps. Adding to the pressure to make batteries safer, there is an overwhelming demand for higher density lithium-ion batteries.  Many industries depend on the advancement of lithium-ion battery technologies to satisfy their need for higher density, so the billion-dollar question here is, “how do we safely advance lithium-ion battery technologies?”

Over the years, the battery industry has identified what the common causes of lithium-ion battery fires (i.e. electrical shorts as well as impact and mechanical deformation) are through cell and system level failure analysis as well as by studying the causes of the thermal runaway at the material, cell, and system level. To date, the battery industry has focused on detecting thermal runaway and manage the cell temperature with the use of some form of Battery Management System (BMS) which often rely on embedded sensors in the battery cell or module to collect data and perform operational diagnostics.  Research activities also include using mathematical modeling and computational analysis of data obtained from embedded sensors.

Industry Efforts to Improve Lithium-Ion Battery Safety
A battery’s thermal behavior is characterized and modeled to predict the thermal runaway.  Modeling for safe batteries involves thermal management in the form of active or passive cooling.  Provisions are then made in the structure of the battery system, mechanical designs, as well as the material design. The objective is to stop the progression of the thermal runaway beyond the starting point either by using shut off or fire suppressants as part of battery packaging materials. Academic researchers are also working on the thermal stability of the battery’s electrolyte.  In the long run, use of non-flammable solid state electrolyte (SSE) is expected to reduce the incidences of fire in lithium-ion batteries significantly.

Different types of fire safety provisions need to be made depending on the type of battery applications.  In the case of electric vehicles, for example, battery fires are often caused by crash and fault. High emphasis is placed on stopping the fire at the source (by short detection and fuse cut) and on slowing down the propagation of fire beyond the source (by effective insulation between cells).  Overall, an electric vehicle’s safety design usually involves tests that simulate crashes (often by driving a nail into the battery) and selective use of insulation materials as thermal pulse barrier.  To mitigate possible shorts, fuse-link cell connections are used instead of simple electrical wiring.

At the system level of battery safety design, the objectives are again to prevent fire as well as to isolate unfortunate occurrences of fire at the source.  This involves preventing cell level thermal runaways as well as the spread to entire pack/system.  All of these efforts, however, involve a steep learning curve, and knowledge accumulates slowly over time.  The insulation between cells is designed either to have high thermal conductive behavior to release heat to the outside of the battery system, or to have low thermal conductive properties to prevent heat being released to nearby components. In addition to a passive approach of prevention and isolation of fire, another possible approach is to have an active fire suppression provision. Due to the nature of electricity, however, a water-based fire suppression system cannot be used in general.

For single cell battery system such as cell phone batteries, however, the battery management system can only sense electrical current flow volume and temperature rise, limiting the ability to detect fire. As such, most of the efforts, to date, has been placed on packaging, added margin on separator thickness, and using less volatile electrolyte.

Academic Efforts on Improving Battery Safety
Scientists and engineers are working on various aspects of battery safety. For instance, academic research on battery safety is focused on developing a more intelligent way of monitoring, sensing, predicting, and administering a proper reaction (shut off, mitigation, or slowdown of heat activities). Recent efforts surrounding improving safety include:

  1. Mathematical modeling of the thermal behavior of batteries based on collected data
  2. System level computer simulation to help pinpoint how and what type of fuses to install between cells to isolate spot incidence.
  3. Developing new materials to be used as retardant battery packaging materials

Coating materials for electrodes for reducing the chance of fire stemming from the formation of dendrites.

  1. Additionally, some are studying all possible types of physical damage to the battery and how they affect the ignition and propagation of battery fire.

Academic research on solid state electrolyte (SSE) is also active. It is generally believed that use of solid state electrolyte will be the most effective solution to drastically reduce potential battery fire, but the safety should be achieved without compromising other important properties of electrolytes since changing one parameter may change all other important battery performances.

The Right Way to Approach Advancing Battery Technologies
Though there is no one silver bullet to make lithium-ion battery safer, the general consensus of the scientific and engineering community is that smart and diligent safety design practice will be the most important factors to prevent future mishaps.  Scientists and engineers strongly agree that improvement in material technology, as well as consistent safety design practices (packaging, system design, detection and isolation of fire to stop propagation, etc.) will drastically reduce the occurrences of future mishaps.

Research at many different levels (material, electrodes, cells, batteries, manufacturing process, etc.) needs to be conducted in the pursuit of improving the energy density and charging speed.  At the material level, BioSolar’s current focus, high capacity cathode material along with an appropriate anode material must be chosen. The amount of active material in battery electrode slurry composition should be maximized while still meeting other requirements.  At the electrode level, maximizing loading of electrode is highly critical to improve energy density.

Most existing lithium-ion battery cathode materials have a theoretical capacity of higher than 270 mAh/g at 4.7-4.8V, but we have not been able to reach these numbers yet.  In reality, we have not even tried to get there due to a multitude of technical issues associated with the high operating voltage.  At this point of time, we currently utilize only about 55 percent of existing material’s theoretical capacity.  We definitely need to continue searching for new materials with higher theoretical capacity, but at the same time, we should expand our investigation on how to extract more capacity out of the existing materials.  I believe it is possible for us to extract more than 70 percent of the theoretical capacity from the existing the material currently in use.

Typically, charging speed is dependent upon the anode material and its electrode design. It has been proven that selection of high-dimensional structured anode material plays an important role in determining the charging speed.  At the material level, it is desirable to have more lithium ions pass through all three (X, Y, and Z) dimensions, whereas certain anode materials that allow just two (X and Y) dimensions are not as desirable. Therefore, the most important first step is to choose the best anode material and after choosing the anode material, we can further improve the charging speed with a thinner electrode design that accelerates lithium-ion diffusion as well as electron transfer from the cathode to the anode.

BioSolar’s Si-Alloy Anode Technology
Silicon (Si) is one of the most promising anode materials that is being considered for next generation, high energy and high power lithium-ion batteries. Graphite is currently the most widely used anode material, but Si has attracted extensive attention because of its natural abundance, non-toxicity, and very high theoretical specific capacity of nearly 4,200 mAh/g – about 10 times more capacity than conventional graphite anodes.

However, Si anodes suffer from huge capacity fading and tremendous volume changes during lithium-ion charge/discharge cycles. The strains due to the huge volume change actually pulverize the Si material and eventually lead to electrode shattering and delamination, which adversely affect the battery performance and cycle life. These are the primary challenges to the commercial use of Si for battery anodes, which BioSolar intends to overcome.

Existing Si-based Anode Approaches
To overcome the issues with Si, popular attempts over the recent years included fabrication of Si into nanoparticles, porous structures, composites with carbon or composites with intermetallic flexible matrix, and many others.

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Nanoscale Si particles have been shown to reduce volume expansions and prevent particle cracking, which in turn improves battery life and allows for a faster charging battery. Nevertheless, Si nanoparticles are expensive and not industry scalable yet.

Silicon Oxide (SiOx) based systems used for dilution of Si expansion stresses offer high capacity and long cycle life but require high-temperature synthesis process. They also suffer from low conductivity which requires carbon coating and surface treatments.

Si and carbon composite systems can mechanically protect the Si particles by a surrounding carbon matrix which maintains the electrical integrities. They can also be synthesized using low-cost milling process and can be incorporated into existing graphite electrode systems. However, this approach has low matrix strength to prevent Si expansions, which results in short battery life.

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BioSolar is currently focused on developing a unique and highly effective material and processing solutions to take maximum advantage of Si’s full capacity using Si-Alloy.  Our goal is to demonstrate a working high capacity anode for battery manufacturers to include in their pursuit and creation of the ultimate high capacity, high power, and fast charging and discharging lithium-ion battery.

BioSolar intends to commercialize a Si based anode material technology to significantly increase the energy density and cycle life of the current lithium-ion batteries. BioSolar’s target is to design Si-Alloy materials with a capacity objective of 1,500 mAh/g, which translates into to an anode with a capacity density of 1,000 mAh/cm3 and a cell level volumetric energy density of 1,000 Wh/L. As a point of reference, the lithium-ion batteries in a current Tesla Model S electric vehicle have an energy density of 676 Wh/L.

We’re currently funding a sponsored research program to develop our anode technology. The lead inventor of the technology is North Carolina A&T State University assistant professor Dr. Sungjin Cho, a lithium-ion battery expert with an extended career in the battery industry prior to joining the University.

David Lee is CEO at BioSolar, a company developing breakthrough technology to double the storage capacity, lower the cost and extend the life of lithium-ion batteries.

 

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