Batteries: A Green Energy Storage Solution

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Silicon imaged on a nanoscale. Silicon is one of the elements researchers are using to make more powerful batteries.

Like many modern inventions, electricity has become such an integral part of our daily lives that, until recently, we hardly even noticed it. Now, as global warming looms and our electric bills continue to rise, energy in general is weighing on all of our minds. We see that almost every aspect of our lives—from lighting our homes to cooling them—relies on the outlet.

Recently, our increased awareness of environmental and economic concerns associated with fossil-based energy has shifted interest to generating electricity from renewable resources, particularly on wind and solar power. One challenge in using wind and solar power, however, is matching supply with consumption. For example, solar power, or energy from the sun, can only be harnessed during daylight hours. However, consumption of energy in homes is generally higher in the evening. As for wind power, even the windiest areas in the country don’t have a steady breeze every day. Therefore, efficient and effective means to store the electricity generated from these sources will be crucial to the widespread success of using renewable energy.

One obvious solution to energy storage is the rechargeable battery – a technology that has been with us for almost 150 years. Unfortunately, the performance characteristics of current rechargeable batteries limit their wider applications. A typical cell phone battery lasts two to three hours and requires that much time to recharge. Battery power (rate of energy delivery) is too low to make them ideal for use in medium-duty machinery such as lawn mowers. Batteries’ limited energy storage capacity, slow recharge, and maximum deliverable power are also among the major reasons that electric vehicles are not more popular.

Increasing the popularity of electric vehicles, however, is one way more efficient energy storage could make a major impact on the environment and fossil fuel consumption. Consider a scenario in which all passenger cars are powered by batteries. It is estimated that the overall efficiency of converting the energy in gasoline to vehicle motion for a 5-passenger full size sedan is about 16% (close to 12% for city driving and 20% highway driving). In other words, 84% of the energy produced is wasted. An electric vehicle’s efficiency is about 35%, when the electricity is generated from fossil fuel (coal or natural gas). This is due to the high efficiency of modern power plants (45-50%), energy storage in batteries, and electric motors. In other words, greenhouse gas emissions could be reduced by half by using electric cars, even if the electricity is derived from fossil fuel.

To realize the above scenario, major improvements in the charge storage capacity, rate of charge and discharge (i.e. power), life, safety, and cost are needed. An estimate is that a ten-fold improvement of the storage capacity and power would be needed to make the performance of rechargeable batteries competitive with internal combustion engines, making electric and plug-in hybrid vehicles attractive alternatives to conventional cars. With a ten-fold improvement in storage capacity, a car could travel about 400 miles between recharge, a range similar to that provided by a tank of gasoline, and similar improvement in rate of recharge would make complete “refilling” possible in ten minutes or so.

So how can we improve battery technology? To start, it helps to look at how batteries generally work. The most basic battery has three major components—an anode, the negatively charged end, a cathode, the positively charged end, and an electrolyte. The electrolyte is a substance that contains ions, or electrically charged atoms, that react with both the anode and cathode in chemical reactions to produce new compounds. The anode reaction releases an extra electron, which the reaction at the cathode will absorb. However, the extra electron in the anode cannot go back through the electrolyte to the cathode. Instead, the electrons travel through a wire outside of the battery, connected from the anode to the cathode, creating a closed circuit. The item that needs to be powered (such as a light bulb) is included on this circuit in between the anode and the cathode. The flow of the electrons, or electricity, passes through the light bulb, supplying the energy it needs to work. If the electrons cannot flow from the anode to the cathode through a closed circuit, the chemical reactions will stop. This is why batteries that are not powering any kind of device can remained charged for longer periods of time.

Once the cathode or anode runs out of chemical substances, or reagents, to continue the reactions, the battery can no longer supply energy. For a primary cell battery, like an average AA or AAA, this is the end of the road. A secondary cell battery, however, can be recharged by reversing the chemical process. In other words, electricity from another source, like a wall outlet, gives electrons to the anode and removes them from the cathode until the battery is recharged (i.e., until the anode and cathode reagents have been sufficiently replenished).

The most widely used rechargeable battery is lead-acid battery, which is found in practically all conventional vehicles. It is heavy and has low storage capacity. The lithium (Li) ion battery is the most advanced rechargeable battery and is used in portable computers and communication devices (cell phones). In a Li ion battery, the anode is made of Li metal atoms intercalated, or inserted, in between layers of graphene, sheets of carbon atoms that make graphite. The cathode is made of layers of cobalt metal ions joined with oxygen atoms. During use, a Li metal atom in the anode is converted, or ionized, to a positive Li ion (Li+). This reaction releases an electron that travels from the anode to the cathode through an external circuit and performs work, such as powering a computer. Meanwhile, the Li ion exits the anode and travels through the electrolyte of the battery to the cathode, where it reacts with the cobalt, oxygen, and the electron. This chemical process is reversed during charging.

The limitations of the Li ion batteries can be seen by taking a closer look at their individual chemical components. The power is limited by how fast lithium can diffuse into and out of the anode and cathode, as well as the resistance to ion flow in the electrolyte. The amount of charge that can be stored per unit volume or weight of the batteries is limited in the anode by how many Li metal atoms can be accommodated by the graphite. The carbon atoms that make up graphite stabilize the lithium. If so much lithium is added so that the carbon cannot stabilize it, the lithium atoms start to bond together, forming lithium “whiskers.” Lithium is more reactive this way. In fact, if these whiskers form in a leaking battery and they become exposed to air, the lithium can react and cause a fire.

One direction of development is to replace graphite with another material that has higher Li storage capacities, meaning it can stabilize more lithium atoms. Silicon is a very promising material—each silicon atom can stabilize four lithium atoms. However, Li atom diffusion in silicon is much slower than in graphite, lowering the battery’s power. This is because the structure of silicon is so different than the structure of graphite. As discussed earlier, graphite is made of sheets of carbon atoms called graphene. The bonds between these sheets are weak, allowing the lithium to move between them easily. Silicon atoms, however, are arranged in tightly bonded chunks, making it difficult for lithium to access the silicon atoms in the middle. Instead, Li metal atoms have to push apart Si atoms that are otherwise bonded together to diffuse through. The deeper the Si atom is located, the more difficult it is for Li to reach it. Furthermore, pushing the Si atoms apart introduces stress to the Si chunks, causing them to fracture and degrading the battery performance.

Scientists are trying to overcome this by forming very small nanostructures (nanometer-size structures) of silicon, so that none of the silicon atoms are far from the structure surface. Then, Li atoms would need to travel only a short distance to access all the silicon atoms. Also, it should be easier for the nanostructures to accommodate the expansion during insertion of atoms and contraction when the Li ions exit. This has been demonstrated successfully in laboratories using nanometer-sized spherical or rod-shape structures, and thin coatings. However, the physical properties of these structures make them inherently less stable, meaning that they cannot maintain their original shape and charge following a number of recharge cycles. For example, if a cell phone battery last three hours the first time you use it, it may only last one hour after you charge/recharge it fifty times. This creates a whole new problem for researchers.

Other ideas explore different metal ions that release more than one electron each in a charge-discharge cycle. Any metal ion that releases two electrons instead of one doubles the charge storage capacity. However, this solution may sacrifice the battery’s voltage. Voltage is often likened to water pressure—it’s directly proportional to the force that pushes electrons from place to place. Voltage is determined by the difference between the anode’s tendency to give up an electron, which is lower, and the cathode’s tendency to accept it, which is higher. If a metal ion were to give up two electrons at the anode, it would likely be less willing to do so for the second electron than the first, which would lower the battery’s voltage.

Researchers throughout the country continue to tackle these problems in their labs, in the hopes to create better energy storage solutions. The development of this new generation of rechargeable batteries could lead to significant changes in the ways we power our lives, especially if households and even communities further embrace installation of solar panels and wind turbines to supplement their electricity supply, or exclusively drive electric cars. As the harmful effects of greenhouse gas emissions and global warming become increasingly evident in our environment, the need to use energy cleanly and efficiently will become even more crucial. The development of more effective batteries is just one way researchers are working towards a more sustainable energy model.

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