Lithium-ion Energy Storage. Regular Use Degradation

Transcription

1 Abstract Lithium-ion (Li-Ion) batteries are nearly universally used as the rechargeable energy storage in consumer goods, ranging in applications from cell phones to children s toys. Over the last several years, hybrid and electric vehicle (EV) technology has become the fastest growing and most demanding application of Li-Ion technology. Outlast Technologies developed their Latent Heat Systems material, also known as LHS, to help manufacturers overcome the technological hurdles and rigorous safety requirements for the automotive industry. Background Li-Ion batteries have a well-rounded balance of cost, weight, and capacity. This has led to their widespread adoption in high-tech and lightweight applications. Their success over other rechargeable batteries, in nearly every consumer goods market, has led to a renewed expectation of better battery performance: faster charging and discharging while lasting longer. Along with raised expectations, the rapid growth of Li-Ion technology has opened new visions for what can be made possible through having more efficient energy storage one of which is the widespread adoption of electric vehicles. Global electric vehicle sales have grown at a 32% compound annual growth rate over the last 4 years 1. While the total market for EVs is still a small percentage of the overall vehicle sales, Bernenberg Bank predicted that EV sales will gain a solid foothold by eventually breaking 5% of total sales by This rapid growth does not include the large number of commercial fleet vehicles adopting hybrid and EV technology, for example: UPS s adoption of hybrid technology into some of its fleet 3,Telsa s commercial electric semi-truck 4, and Workhorse Group s electric fleet pickup 3.

2 Lithium-ion Energy Storage This adoption of electric and hybrid vehicles relies heavily on the progress of rechargeable technology demanding longer vehicle range, better performance, and lower costs. Lithium ion batteries are particularly suited for this type of application with properties including: Highest energy density of any mass-produced battery Low maintenance requirements High degree of design flexibility Relatively negligible memory effect Low self-discharge rate Nearly 3x the voltage capacity of the next level batteries at 3.6 V 5 While Li-Ion batteries are the leading edge in rechargeable technology, the increased demands from electric vehicles has pushed the industry to increase energy density, charge/discharge capacity, and storage efficiency in these batteries. Unfortunately, as seen in the consumer goods market 6, these pressures have accentuated some of performance and safety limitations with Li-Ion batteries particularly with the battery packs used in EVs. These packs have three main areas of both performance and safety limitations: regular use degradation, overheating, and packing inconsistencies. Regular Use Degradation During thermal cycling, the charging or discharging causes internal resistance and thermal expansion which causes stress on the materials in the batteries, shortening their useful life.. This puts a small amount of strain on the mechanical and material systems in the battery which is relieved when the battery cools down to its original state. However, this cycle generates a cumulative effect of degrading the materials and putting expansion and contraction strain on mechanical system of the battery. Pouch cells are particularly susceptible to this type of stress, whereas cylindrical cells tend to mitigate this type of life-shortening. Additionally, this thermal cycling accelerates material degradation. When the materials no longer perform with optimal materials, there is a significant loss of recoverable power and capacity. Figure 1 shows the increased cell life due to lower internal degradation due to LHS thermal management.

3 FIGURE 1, Discharge Capacity Retention due to LHS Thermal Management Overheating Thermal runaway is a phenomenon when the battery enters a self-fed cycle of heating and degradation. This results in a catastrophic release of energy usually accompanied by gas venting, sparks, and fire7. In recent years, there have been a few high-profile cases of this happening with popular consumer goods, airplanes, and even electric vehicles. Because Li-Ion batteries have high energy density, thermal runaway is a major safety concern. There are many mechanisms for initiating thermal runaway. To counter-act these problems, manufacturers have implemented various methods of preventing this from happening. Packing Inconsistencies For higher energy applications, like electric vehicles, Li-Ion cells have to be packed together. However, packing cells together adds another dimension to the thermal regulation problem. This not only increases the number of cells that each must be

4 manufactured to navigate around the regular potential problems, but there is the additional problem of localized heating. This is where, due to the packing pattern, a certain cell receives much more extreme thermal cycling degrading that cell quicker. Because of the faster loss of life, the pack then develops both thermal and electrical imbalances, further concentrating degradation on specific cells. The ultimate outcome is the pack loses its efficiency faster than if there was no localized heating. Concentrated degradation Decreased Charge Capacity Unequal voltage draw Hot spot Because the cells are so close, when a single cell is broken down to the point of causing thermal runaway, the resultant heat generation has the potential of initiating thermal runaway to the neighboring cells. As the thermal runaway spreads throughout the pack, the self-fed problems grow significantly because each cell includes a fuel and oxidizer all internally 8. This propagation is known as cascading thermal runaway. Electric and hybrid vehicles are a substantially growing sector of the vehicle market. This rapid growth requires equally innovative support from the rechargeable energy storage industry to produce batteries with higher capacity and better performance. While Li-Ions are the leading energy storage system, they also have a few performance limitations and safety limitations. To overcome these hurdles, Li-Ion manufacturers are working on innovative battery designs and beginning to consider different external systems particularly for increasing efficiency and safety through thermal regulation. Solution Outlast LHS materials are helping manufacturers develop next-generation electric and hybrid vehicles by implementing active thermal regulation in a fireretardant matrix to overcome some of the safety and performance limitations of lithium ion battery packs. Carefully regulating the heat fluctuations in battery

5 packs increases the life span of the battery and has the unique capability of preventing cascading thermal runaway. LHS battery matrixes are engineered to be integrated thermal regulators. The matrixes are custom-designed blocks that integrate into the fire-retardant battery housing unit (Figure 2) while maximizing the use of the LHS s unique latent heat technology the driver behind the LHS thermal regulation. Figure 2, LHS Battery Matrix Absorbing Energy Engineered to absorb and dissipate heat in a controlled manner, the LHS material uses latent heat technology to regulate the surface temperatures within a battery pack. Latent heat technology uses specially formulated polymers designed to change phases when heated to the right temperature. During this transition, the polymers absorb a large amount of heat without raising the temperature. This is why materials have standardized transition temperatures like how water has a set boiling temperature at 100 C even when the heat source continues to pour in energy.

6 Figure 3, Charge/Discharge Cycles with and without LHS Thermal Management Matrix Outlast LHS materials are made from a fine-tuned amount of organics with a lower melting temperature, known as phase change materials (PCMs), confined within a thermoset polymer structure. This allows the material to absorb a lot of heat while maintaining its structural integrity. Improving Performance Thermally regulating battery packs improves the performance of battery packs by enabling faster sustained charge/discharge cycles while reducing battery chargeloss. These are key performance metrics for Li-Ion batteries, especially for creating the next-generation of electric vehicles which will have to meet extremely tough consumer demands for mass adoption, including increased range and decreased charge time. Outlast LHS materials are designed to provide active support for helping manufactures develop new technologies to overcome the barriers for widespread adoption.

7 Consumer Outlast LHS solution Requirements 9 >300 miles on a single Increased battery life charge ~5 minute charge Lowered temperature profile Increased Battery Life Outlast LHS is helping EV and hybrid manufacturers overcome the barrier for long-range vehicles through providing technology needed to increase long-term battery life. Like any rechargeable battery, lithium ion batteries degrade after cycling through charging and discharging thousands of times reducing charge capacity. Most products, particularly EVs, will go through a charge and discharge cycle multiple times in a week. This degradation is exacerbated by the large amount of energy discharged during regular driving conditions and variable weather conditions. Unlike gasoline and diesel-powered vehicles, electric and hybrid vehicles will have to take long-term battery life into consideration. Long-term reduction in charge capacity presents a massive hurdle for EV and hybrid vehicle manufacturers as the 300-mile benchmark needs be sustained over the lifetime of the vehicle. Regardless of the initial charge capacity, the reduction in battery-life has the capability to be degraded to where the 300-mile requirement is no longer feasible. This is where some thermal management using Outlast LHS material has been proven to help. By regulating and dampening out the extreme thermal swings associated with cyclical use, Outlast LHS can help increase the long-term effective charge as shown in Figures 1 and 3. In addition, using the LHS battery matrix has a cumulative effect. The battery matrix thermally isolates the cells and homogenizes the thermal profile of the battery cutting off the hot-spot degradation cycle as seen in Figure 3. Outlast LHS battery matrixes help reduce the thermal load on individual cells while improving overall pack efficiency. Lowered Temperature Profile Rapid charging has a barrier, which is being lowered through utilizing LHS latent heat technology. Even cutting-edge Li-Ion batteries have a limitation on the

8 maximum temperature they can be at while charging. When this temperature is exceeded, the battery is quickly degraded sometimes to the point of safety problems. This is why most chargers are designed to slow down before reaching full capacity 10. Unfortunately, this process slows down the rate at which the vehicle can be charged, particularly if the battery already has an elevated temperature due to discharging or hot weather conditions. 45 C is the maximum temperature for cycling before dramatically faster degradation occurs 11. This temperature has the potential to occur when external factors are combined with the heat generated from charging. To protect the batteries integrity, EV chargers have had to elongate the charging cycle. Even some of the most widely utilized advanced rapid chargers, Tesla Superchargers, take 30 minutes to fully charge cars optimized for the station 12. This is why manufacturers are looking into other ways of reducing the required charge-time. Outlast is helping with this through smoothing the thermal spike from charging. While some latent heat systems can be depleted meaning the latent heat organics are saturated with heat before charging due to external circumstances, Outlast battery matrixes are fine-tuned to transform at the right temperature. The LHS is designed to provide a good buffer around a designated temperature without becoming saturated too early. Providing Safety Outlast LHS battery matrixes help improve the overall safety of battery packs reducing the likelihood of thermal runaway and preventing cascading thermal runaway. A thermal runaway event is caused by failure in an individual cell that reacts and begins breaking down the internal battery structures. This causes a thermal chain-reaction, creating a self-propagating cycle of rapid heating and deterioration. A few examples of causes for failure are: Exposure to excessive temperatures Short-circuiting Surges in both charging and discharging current Hot spots in large packs Improper electrical connections

9 Poor fail-safe software Mechanical destruction, penetration, or impact Preventing Thermal Runaway The unique material properties of the LHS battery matrixes reduce the likelihood of a thermal runaway event by regulating the individual cell temperatures. However, if a cell were to enter thermal runaway, the battery matrix isolates the incident and prevents any cascading effect. Electrical and LHS products work together to prevent thermal runaway. Unlike electrical systems, LHS matrixes actively cool the battery cell mechanically by melting the latent heat organics to absorb heat. However, this cooling effect is limited by the heat saturation levels of the organics: meaning the electrical systems responsiveness in cutting off the electrical flow is vital. While both systems can consistently and effectively prevent thermal runaway, combining the two systems allows for a dynamic and actively responsive cooling system. While the combination of LHS and electronic safety systems is highly effective, short circuiting disrupts the system, and induces thermal runaway. Outlast LHS is electrically insulative. This helps reduce the likelihood of short-circuiting. This is key because sometimes the short-circuit is too close to the battery for the feedback systems to react, like one located close to the battery pack (insert diagram of short circuit). Once a battery cell reaches its critical temperature, the degradation will lead to a self-fed cycle. This means that even if the electronic feedback systems could be deployed, it would not be able to cut off the cycle in time to prevent the latent heat organics from being saturated from the constant influx of heat therefore allowing thermal runaway. Preventing Cascading Once a cell enters thermal runaway, there is nothing that can be done inside the pack to halt the runaway. However, LHS matrixes are designed to isolate the cell to prevent any cascading effect. This requires a set of material properties, which are engineered into each matrix see below table:

10 Need Description Property Impact on Cascading Effect Heat spreading from Low thermal Thermally isolates cells, cell to cell during conductivity reducing the heat thermal runaway spread to nearby cells. leads to cascading. Rapid heat spreading This most commonly has been proven to happens through cause problems for conduction as most similar technology that battery packs do not uses heat spreading to have thermally reduce pack heat. insulative systems in place. Stop heat from spread Stop flames from spreading heat Divert excess energy from surrounding cells Gas release during thermal runaway can be flammable. This can lead to flames which are able to heat up surrounding cells quickly. Lots of heat is generated in a short amount of time. The large amount of heat energy released from the cell has to be either absorbed by the contents of the pack or released. Most packs are sealed, meaning the contents of the pack absorb the majority of the energy. Fire retardant, UL94-V0 High thermal load capacity Slows down, if not stop altogether, any fire spreading throughout the pack. This allows the latent heat organics, rather than the battery cells, can absorb the majority of the excess energy in a sealed pack. Outlast LHS battery matrixes are engineered to prevent any cascading effect. Isolating a thermal runaway event requires each system of material properties to work together. Through stopping the heat from flowing and absorbing excess

11 heat, each matrix can prevent cascading thermal runaway. Figure 4 below shows the thermal curves of nail penetration testing. The trigger cell exceeds 700 o C, whereas none of the adjacent cells propagate or exceed 120 o C. Figure 4, Thermal Curves of Nail Penetration Test. Thermal runaway can lead to catastrophic, and memorably dramatic, consequences resulting in costly recalls and loss of brand equity. Electric and hybrid vehicle manufacturers are aware of this potential threat. One popular brand had problems with road debris punctures causing a cascading thermal runaway. They fixed the initial problem by protecting the batteries from road debris better but it cost them an estimated $2.4B in market value 12.

12 The spectacle of thermal runaway brings negative attention to the already closely watched new technology. This is compounded by the viral nature of the internet and how rapidly information spreads. These types of stakes have prompted manufacturers to look to innovative solutions, prevent the major thermal runaway events. Outlast is helping lead the innovative technologies rising to the challenge. Conclusion Outlast LHS matrixes are being integrated into electric and hybrid vehicles to protect consumers while increasing overall battery performance. Safety and performance are equally intense factors for electric and hybrid vehicle manufacturers. As with any new technology, particularly in the automotive industry, adopting strict safety measures help them keep their customers and their brands safe while increasing the overall performance of their vehicles. However, being a relatively new technology, EVs have increased pressures for performance and are under intense public scrutiny. Because of the high market demands, electric and hybrid vehicle manufacturers are turning innovative solutions like Outlast LHS battery matrixes. These are helping manufactures reach their goals both in terms of safety and performance as outlined below. Property Smooths thermal curves Thermally isolates individual cells Stops thermal spikes Fire retardant High heat absorption Electrically insulates cells Problem(s) addressed Long-term charge capacity, Electronic safety temperature buffer Cascading thermal runaway Reduced charge time, Thermal spikes during operations and emergencies Cascading thermal runaway Cascading thermal runaway Helps prevent accidental shorting Outlast is helping the rapidly growing sector or EVs and hybrid vehicles, the fasted growing users of lithium ion batteries, overcome some of the limitations of this

13 method of energy storage. By uniquely applying latent heat technology, Outlast battery matrixes help improve the safety of battery packs while improving their overall performance. Contact Outlast to learn how latent heat technology can help you overcome your application s challenges. [1] [2] [3] [4] [5] [6]

14 [7] [8] /Day_2_Fundamental_Science_4/2_Hewson_FS_4.pdf [9] mark.pdf [10] [11] [12]