Many of us are aware of a number of technologies that have followed some variant of "Moore's Law" for growth in long-term performance, but advances in battery technology have been more modest. The increased presence of power-hungry portable devices (e.g., smart phones, personal digital assistants and their military counterparts) as well as the push to clean hybrid or all-electric vehicles has intensified the focus on — and public and private sector investment in — battery chemistry and development. This article highlights recent developments in lithium battery technologies that may advance the current state of the art and meet the increasing energy needs of our customers..
A battery is a device that converts stored chemical energy, in the form of metals and electrolytes, into electric current through internal reactions at the battery's positive and negative terminals. Performance of any battery is dependent on three technology areas: the chemistry that generates the electrons, the electrodes that provide half of the reaction and collect and distribute the electrons, and the electrolytes that provide the remaining chemistry and the internal pathway for the electron flow.
Electrical current begins to flow when a load is applied connecting the two battery terminals. Without the load providing the path from the negative to the positive terminal, the chemical reaction does not take place and the battery remains charged. A single unit of a battery, commonly called a cell, will have a characteristic voltage range between charged and discharged states based on the electrochemical properties of the materials used and the specific reactions that occur in the electrolytic solution between the two terminals.
There are basically two types of batteries. A primary battery is one where the energy is exhausted after the active materials are consumed (e.g., carbon-zinc, silver oxide and alkaline). A secondary battery is one where the active materials can be regenerated by charging (e.g., lead acid, lithium ion, nickel cadmium or NiCd, nickel metal hydride or NiMH).
The specific materials used within a battery control its voltage; each different reaction has a characteristic voltage value. Take a car battery as an example. A single cell of a typical automotive lead-acid battery has a negative plate made of lead (Pb) and a positive plate made of lead dioxide (PbO2), both of which are placed into a strong electrolyte solution of sulfuric acid (H2SO4) and water (H2O). When placed in aqueous solution, the sulfuric acid separates into a hydrogen ion (H+) and a hydrogen sulfate ion (HSO4–). During battery discharge, a reaction takes place at the negative terminal, where the lead combines with the hydrogen sulfate ion to create lead sulfate (PbSO4), a hydrogen ion and two electrons (e–) that drive the load (starting the engine). At the positive terminal during discharge, lead dioxide (PbO2), hydrogen sulfate ions (HSO4–), hydrogen ions (H+) plus the returning electrons from the negative terminal create lead sulfate on the lead dioxide plate and water. As the battery discharges, both plates build up lead sulfate, and the HSO4– concentration decreases in the electrolyte solution. This reaction generates a characteristic voltage of -0.356 volts at the negative plate and +1.685 volts at the positive plate, or about 2 volts per cell, so by combining six cells in series, a standard 12-volt battery is formed. To recharge, current is applied to the battery from the alternator with the additional electrons reacting to regenerate lead, lead dioxide, and hydrogen sulfate ions. Figure 1 summarizes these chemical processes.
Advances in Battery Technology
Figure 2 identifies several milestones in the evolution of battery technology. The introduction of lithium-based batteries in past decades was revolutionary in that battery performance rapidly improved after years of attaining only small incremental gains over the universal lead-acid technology.
Why lithium? Conventional, commercially available battery technologies typically have energy densities on the order of tens of watt-hours per kilogram. This is dictated by the chemistry used as well as the ionic transport media — the electrolytes. Lithium is a highly reactive element with the additional advantage that its ionic size (atomic number 3) is relatively small compared with other elements; this facilitates ionic transport. In order to utilize the stored chemical energy in an element or compound, the reaction with oxygen or other reactants needs to be controlled, and paths of electrons and ions need to be separated. Consequently, reaction rates are limited by ionic conductivity through the electrolyte. In lead-acid automotive batteries the ionic species is lead traveling through a sulfuric acid electrolyte. Since the liquid allows fast ionic conduction, these batteries can produce great power for, as an example, starting the engine. The downside, however, is that the chemicals are quickly depleted and the reaction slows. Therefore, the stored energy tends to be low, and the battery needs to be recharged to reverse the reaction and restore the level of stored energy.
With an atomic number of 3, lithium is the lightest of all metals. The electrodes of a lithium-ion battery are made of a lithium compound, (e.g., lithium phosphate) and carbon, so they are generally much lighter than other types of rechargeable batteries of the same size. Lithium is also a highly reactive element (located on the far left of the periodic table of elements), meaning that a lot of energy can be stored in its atomic bonds, resulting in a very high energy density. A typical lithium-ion battery can store 200 watt-hours of energy in 1 kilogram of battery versus the automotive lead-acid battery, which can store about 30 watt-hours per kilogram.
Lithium-based batteries' higher energy density brings with it greater challenges to contain and control the chemical reaction. The first lithium battery experiments conducted in Japan and the U.S. were failures due to the explosive nature of the compounds used. The end result is a compromise that sacrifices performance for safety, an approach that utilizes lithium not in its elemental form, but in compound form. In this way, the explosive nature of pure lithium can be controlled, but at the expense of reduced energy storage.
Application in Hybrid Power Systems
While they find common application in portable devices, lithium batteries play an important role as energy storage devices in hybrid power systems being developed at Raytheon. Raytheon designed, and is now testing, hybrid power systems using advanced technology lithium-ion battery energy storage with solar, wind and generator inputs to provide power for forward-operating equipment in support of the warfighter. These systems are designed to provide power surety as well as significant reduction in fuel usage, resulting in fewer fuel sorties, thus lowering the casualty rate, reducing maintenance, and lowering total cost of ownership. Environmentally ruggedized batteries based on lithium with long-life, deep-discharge capability, high-efficiency, and high power and energy densities are instrumental in realizing the advantages inherent within these hybrid power systems.
Lithium-ion batteries have made tremendous inroads in the commercial market, and their use in providing the driving power in automotive applications has now become possible. Boston Power's lithium-ion batteries are a good example. Their rechargeable batteries, based on a proprietary lithium compound, produce energy densities of about 180 Wh/kg, and power densities of about 440 W/kg. These batteries are commercially available and well suited for long missions. Another promising lithium-ion variant is offered by A123 for the automotive market. These batteries are based on lithium iron phosphate nanotechnology, which creates an extremely large surface area on the electrodes for the chemical reaction to take place, and results in high power densities up to 2,000 W/kg. The large surface area provides for quick discharge to accelerate a vehicle and fast recharging.
Several companies continue development of a pure lithium-based battery. Success in this will open up many applications, and it will be a breakthrough in the automotive world.
One company that has not given up on pure lithium is California-based Polyplus. It has developed a method to contain pure lithium in a solid electrolytic capsule that controls the violent reaction of lithium and oxygen. An experimental cell from Polyplus recently set a new record in energy density of 1,200 Wh/kg. The company's next challenge is to increase the power density of their system.
The quest for more powerful and energetic batteries continues, and the available energy of lithium is still not fully tapped (lithium has an energy density potential of ~12,000 Wh/kg, close to gasoline at ~13,300 Wh/kg). Consequently, another performance leap is anticipated for the near future. Figure 3 highlights ongoing developments in the three technology areas of chemistry, electrodes and electrolytes. Successful development and merging of these technologies could achieve an energy density of ~3,000 Wh/kg and a power density of ~2,000 W/kg.
Tony Marinilli, Peter Morico, Bart VanRees
Contributor: Steve Klepper