Attempts to develop rechargeable lithium batteries followed in the 1980s but the endeavor failed because of instabilities in the metallic lithium used as anode material.
Lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest energy density per weight. Rechargeable batteries with lithium metal on the anode (negative electrodes)* could provide extraordinarily high energy densities; however, it was discovered in the mid 1980s that cycling produced unwanted dendrites on the anode.
These growth particles penetrate the separator and cause an electrical short. When this occurs, the cell temperature rises quickly and approaches the melting point of lithium, causing thermal runaway, also known as "venting with flame." A large number of rechargeable metallic lithium batteries sent to Japan were recalled in 1991 after a battery in a mobile phone released flaming gases and inflicted burns to a man’s face.
The inherent instability of lithium metal, especially during charging, shifted research to a non-metallic solution using lithium ions. Although lower in energy density than lithium-metal, Li-ion is safe, provided cell manufacturers and battery packers follow safety measures in keeping voltage and currents to secure levels. In 1991, Sony commercialized the first Li-ion battery, and today this chemistry has become the most promising and fastest growing on the market. Meanwhile, research continues to develop a safe metallic lithium battery.
The energy density of Li-ion is twice that of NiCd, and the high nominal cell voltage of 3.6 V as compared to 1.2 V for nickel systems contributes to this gain. Improvements in the active materials of the electrode have the potential of further increases in energy density. The load characteristics are good, and the flat discharge curve offers effective utilization of the stored energy in a desirable voltage spectrum of 3.7 to 2.80.00V/cell. Nickel-based batteries also have a flat discharge curve that ranges from 1.25 to 1.00 V/cell.
In 1994, the cost to manufacture Li-ion in the 18650** cylindrical cell with a capacity of 1100 mAh was more than $10. In 2001, the price dropped to $2 and the capacity rose to 1900 mAh.
Today, high energy-dense 18650 cells deliver over 3000 mAh and the costs have dropped further. Cost reduction, increase in energy density and the absence of toxic material paved the road to make Li-ion the universally acceptable battery for portable application, first in the consumer industry and now increasingly also in heavy industry, including electric powertrains for vehicles.
In 2009, roughly 38 percent of all batteries by revenue were Li-ion. Li-ion is a low-maintenance battery, an advantage many other chemistries cannot claim. The battery has no memory and does not need exercising to keep in shape. Self-discharge is less than half that of nickel-based systems. This makes Li-ion well suited for fuel gauge applications. The nominal cell voltage of 3.6 V can directly power cell phones and digital cameras, offering simplifications and cost reductions over multi-cell designs. The drawbacks are the need for protection circuits to prevent abuse, as well as a higher price than other battery types.
Types of Lithium-ion Batteries
Similar to the lead- and nickel-based architecture, lithium-ion uses a cathode (positive electrode), an anode (negative electrode) and electrolyte as conductor. The cathode is a metal oxide and the anode consists of porous carbon. During discharge, the ions flow from the anode to the cathode through the electrolyte and separator; charge reverses the direction and the ions flow from the cathode to the anode. Figure 1 illustrates the process.
Li-ion batteries come in many varieties but all have one thing in common — the catchword "lithium-ion." Although strikingly similar at first glance, these batteries vary in performance, and the choice of cathode materials gives them their unique personality.
Common cathode material are Lithium Cobalt Oxide (or Lithium Cobaltate), Lithium Manganese Oxide (also known as spinel or Lithium Manganate), Lithium Iron Phosphate, as well as Lithium Nickel Manganese Cobalt (or NMC)*** and Lithium Nickel Cobalt Aluminum Oxide (or NCA). All these materials possess a theoretical energy density with given limits.
Sony’s original lithium-ion battery used coke as the anode (coal product), and since 1997 most Li-ions manufacturers, including Sony, have shifted to graphite to attain a flatter discharge curve. Figure 2 illustrates the voltage discharge curve of a modern Li-ion with graphite anode and the early coke version with a less stable voltage performance.
Developments also occur on the anode and several additives are being tried, including silicon-based alloys. Silicon achieves a 20 to 30 percent increase in energy density at the cost of lower load currents and reduced cycle life. Nano-structured lithium-titanate as anode additive shows promising cycle life, good load capabilities, excellent low-temperature performance and superior safety, but the energy density is low.
Mixing cathode and anode material allows manufacturers to strengthen intrinsic qualities; however, enhancing one attribute may compromise another. Battery makers can, for example, optimize the energy density (capacity) to achieve extended runtime, increase the power density for improved current loading, extend service life for better longevity, and enhance safety to endure environmental stresses. But there are drawbacks.
A higher capacity reduces the current loading; optimizing current loading lowers the energy density; and ruggedizing a cell for long life and improved safety increases battery size and adds to cost due to a thicker separator. The separator is said to be the most expensive part of a battery.
Manufacturers can attain high energy density and low cost relatively easily by adding nickel in lieu of cobalt, but this makes the cell less stable. While a start-up company may focus on high energy density to gain quick market acceptance, safety and durability cannot be compromised. Reputable manufacturers place high integrity on safety and longevity.
Table 3 summarizes the characteristics of Li-ion with different cathode material. The table limits the chemistries to the four most commonly used lithium-ion systems and applies the short form to describe them. The batteries are Li-cobalt, Li-manganese, Li-phosphate and NMC. NMC stands for nickel-manganese-cobalt, a chemistry that is relatively new and can be tailored for applications needing either high capacity or high loading capabilities. Lithium-ion-polymer is not mentioned as this is not a unique chemistry and only differs in construction. Li-polymer can be made in various chemistries and the most widely used format is Li-cobalt.
Never was the competition to find an ideal battery more intense than today. Manufacturers see new applications for automotive propulsion systems, as well as stationary and grid storage, also knows as load leveling. At time of writing, the battery industry speculates that the Li-manganese and/or NMC might be the winners for the electric powertrain.
Industry’s experience has mostly been in portable applications, and the long-term suitability of batteries for automotive use is still unknown. A clear assessment of the cycle life, performance and long-term operating c
ost will only be known after having gone through a few generations of batteries for vehicles with electric powertrains, and more is known about the customers’ behavior and climate conditions under which the batteries are exposed. Table 4 summarizes the advantages and limitations of Li-ion.
* When consuming power, as in a diode, vacuum tube or a battery on charge, the anode is positive; when withdrawing power, as in a battery on discharge, the anode becomes negative.
** Standard of a cylindrical Li-ion cell developed in the mid 1990s; measures 18mm in diamter and 65mm in length; commonly used for laptops.
*** Some Lithium Nickel Manganese Cobalt Oxide systems go by designation of NCM, CMN, CNM, MNC and MCN. The systems are basically the same.
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., a company that manufactures battery test and diagnostic equipment, and creator of www.BatteryUniversity.com.