Lithium-ion Tutorial Highlights the Old and the New
by Donald Georgi
The beginning tutorial at Power 2001 has become a classic. Each year, Prof. Jeff Dahn of Dalhousie University presents the Lithium-ion Battery Tutorial and Update to both new arrivals to the technology and the old guard which eagerly looks forward to his next presentation. Maybe his easy style is conducive to learning or maybe his personal experiences in the commercial pursuit of rechargeable Lithium batteries makes him a more interesting presenter than the ordinary tenured tutor from the ivory towers. Whatever his formula for success, the Tutorial ignites the creativity of Power 200X, setting standards for the presenters of the following days to match.
As his presentation title states, Dr. Dahn’s content is a combination of a tutorial and an update. The tutorial describes the problem of plating lithium on the anode during recharge. As recharge cycles increase, uneven lithium metal surfaces form, breaking down the passivation coating, thus resulting in thermal runaway above 120 0C.
Anode intercalation to the rescue
The concept of replacing the solid lithium metal anode with a carbon electrode is an elegant solution because it allows Lithium atoms to be placed in atomic spaces where Lithium shares electrons with the host, eliminating the uneven metal surfaces (as long as the cell is not overcharged). The result is a much safer battery. Today, the most common form of anode is graphite.
Separators and electrolytes
Electrolytes are primarily selected to effectively transport the Lithium-ion to the cathode during discharge; the conductivity selected for these electrolytes is about 100 times smaller than that used in Nickel-cadmium cells. The consequence of this low conductivity requires the electrodes to be very thin, (in the order of 100 micrometers), resulting in the need for complex coating and winding equipment which increases manufacturing costs.
While the chemical reactions and intercalations are the same as with liquid Lithium-ion electrolytes, polymer cells use either a plasticized separator or polymer with liquid electrolyte. These materials act both as separator and ion conductor between the electrodes. With the thin construction and flexible container of polymer cells, gas generation and swelling could be a problem, but with the addition of a compound referred to as ‘BETI,’ which also facilitates the oxidation at the cathode and reduction at the anode, the problem is reduced.
Liquid Lithium-ion cells also require a separator which provides both a separation and a safety function. Made of either a polyethylene or polypropylene, these materials are formed with microporous holes in the material. When a thermal problem creates heat, the holes in the separator melt shut, interrupting the battery circuit and hopefully stopping the runaway. Some polymer cells also use these shutdown separators.
The thirst for greater electronic performance in portable devices continues to place greater demands on the energy storage capabilities of batteries. Smaller cell phones have become popular because they fit conveniently in the pocket or purse. Shrinking the battery means shrinking the talk time unless battery energy increases. Some who never learned that battery chemistry does not follow Moore’s Law deride battery technology as falling behind in needed energy storage. ‘Imagineering’ would have pinhead size batteries power 3G phones for weeks, months or years. The creative minds of PDA designers have been able to tailor device performance with simple primary Alkaline cells, but as convergence adds cellular requirements, the PDAs will also fall victim to the power hungry dragon demands.
Cathode materials have evolved from lithium cobalt oxide with energy density around 140 mAh/g. Safer cells use anode materials made of lithium manganese oxide produced by E-One/Moli and NEC. The penalty for the safer cells is that energy density is reduced to 120 mAh/g. Combining nickel with a lithium cobalt oxide produces a less expensive cell than cobalt and increases the energy density to 180 mAh/g.
Dr. Dahn’s presentation does not skirt the issue of higher energy density in the future. Research is pursuing nanoscale alloys for anode materials which offer hope based on the specific capacity increases of lithium compounded with other metals. Lithium carbon intercalation material has a specific capacity of 372 mAh/g, while lithium lead boosts that figure to 550 mAh/g, lithium aluminum to 993 mAh/g and lithium silicon to a whopping 4,200 mAh/g. Don’t go out to corner the sandy beach sources yet because the alloying, while having such a large capacity in principle, has a reversible mechanism for charge and recharge which in reality implements a radical structure that is not reversible. Today, rays of hope shine in the performance of a 3M silicon tin electrode to demonstrate over 1200 mAh/g. In testing at 50 cycles, there has been no degradation in volumetric capacity of 6,000 Ah/l. Projecting the use of the 3M alloy in an 18650 cell shows current graphite anodes with cobalt cathodes at 6.8 Wh, and using alloy anodes with nickel cobalt oxide cathodes, 12 Wh is possible. This material is not yet available commercially because problems of bulk manufacturing still need to be solved.
Others such as Mitsui have reported carbon coated silicon performance out to 1000 mAh/g.
Because lithium is a reactive element, safety concern is very important. Overcharge of a cell will aggregate lithium metal on the anode and lead to thermal runaway. Overcharge happens when charging electronics fail. Battery cell or pack mounted thermal fuses can ‘open’ circuits as thermal runaway begins. Internal pressure fuses can also open the circuit if overcharge begins outgassing.
As energy density increases, the focus on safety becomes more important, and complacency over the past excellent safety record cannot be accepted. Dr. Dahn points out that the ability of the cell to transfer heat away to the environment is critical, giving the advantage to thin flat cells over cylindrical large cells.
Evaluation of cell safety must also improve with better oven testing and ‘smart nails’ for better penetration tests. Cells must also be tested for: forced overcharge, discharge, shock, vibration, crush, fire exposure, impact and short circuiting.
One of the tools to improve safety may be in electrolyte additives. Sony has a redox shuttle to provide overcharge protection. Larger cell safety may benefit from phosphates.
Looking to make a few million dollars, new age companies presently are pursuing miniature fuel cells which ideally would have an ‘engine’ smaller than present batteries, with refillable fuel tanks using methanol or ethanol. (See BD # 66-1.) These millions of dollars would come at the expense of reduced Lithium-ion battery sales. The target for the fuel cells to power electronic devices is a two to three times improvement in ‘energy density’ over today’s Lithium-ion batteries. If during the fuel cell development time, the Lithium-ion chemistry can evolve as anticipated, the energy density may be 2X or more than that of today, making the fuel cell’s cost competitive position weaker. Other factors such as battery low impedance, demonstrated safety, high reliability, gas gauging, docking station recharge convenience, and ever falling prices make the future of Lithium-ion quite respectable for the next decade.
Early in a new technological offering, the global options are not yet visible or accepted. Maybe there is a hybrid configuration with fuel cells, batteries, photovoltaics and even supercapacitors, thus providing the ideal power source of the developing 21st century. Whatever transpires, Power 20XX attendees will look forward to hearing Jeff kickoff those meetings with his classic tutorial.