Although the current Li-ion battery chemistries appear to be reaching a performance plateau in terms of energy density, presentations given at the recent Portable Power Conference suggest that Li-ion technology is far from running out of steam. As noted in a Li-ion battery tutorial by Brian Barnett and Per Onnerud of R&D company TIAX, which manages the conference, “[Li-ion] technology is still very young in terms of time scales by which one should consider battery technologies and their development.” As indicated by Barnett, Onnerud and others, improvements in cathode and anode materials promise greater capacity for Li-ion cells in the near future. However, the changeover to new cathode and anode designs is complicated by changes in cell voltages and safety issues.
Since they were introduced into the marketplace in 1992, Li-ion batteries have made big gains in energy density. The standard 18650 cylindrical cell, which is commonly used in notebook battery packs, provides a measuring stick. The original cells produced 13 years ago had a capacity around 960 mAh. Today, 18650 cells with as much as 2600 mAh of capacity are in production.* In the near future, these cells may reach 3000 mAh, but achieving that performance is likely to require significant changes in anode and cathode materials.
The latest cells are built using lithium-cobalt-oxide (LiCoO2) cathodes and graphite anodes. The choice of cathode material has remained largely unchanged since the first Li-ion cells were introduced. Improvements to date have been achieved mainly by increasing the packing density and amount of active materials in the cell. And, although some cells employ other cathode materials, as Onnerud noted in the tutorial, lithium cobalt oxide dominates the industry and has set the “standard for capacity, safety and rate,” even while its cost is seen as a limiting factor. As Barnett indicated, the cathode is the most expensive element in the cell.
Now, two categories of new cathode materials are viewed as likely alternatives to lithium cobalt oxide. For higher energy density, nickel-based oxides—which also may include cobalt and manganese in the mix—are expected to be applied in new cell designs. These new cathode materials will make it possible to build “higher capacity cells at lower cost,” according to Onnerud. However, he noted that there are still “important safety issues to resolve.” In addition, nickel-based cathodes may require a higher voltage range (4.4 V to 4.6 V versus the standard 4.2-V end-of-charge voltage) to achieve higher capacity. This latter characteristic would complicate a changeover from existing cells.
Meanwhile, phosphate-based cathodes such as LiFePO4 represent another alternative to lithium cobalt oxide. The phosphate materials offer greater safety and stability, a more environmentally friendly design and lower cost. Onnerud labeled this as a “hot topic” in current cathode materials development, while also noting that phosphate-based cathodes present concerns in terms of high power capability. However, Onnerud noted that recent literature suggests the previous belief that phosphates offered limited power is not true.
New anode developments also are on the horizon. Currently, three types of graphite material are in use in Li-ion manufacturing—artificial graphite, mesophase carbon microbeads (MCMB) and natural graphite. Onnerud said that current research into these carbon materials focuses on reducing cost and improving safety. However, cell makers also are developing new anode materials.
Onnerud pointed to Sony’s Nexelion battery, introduced last February, as an example of a new anode type. The use of a tin-based amorphous material for the anode was partially responsible for Nexelion’s 30% increase in capacity versus conventional Li-ion batteries. (The cell also incorporated a cathode that combined Li-Ni-Co-Mn-based oxide with LiCoO2.) In addition, Onnerud brought up statements from cell makers Sanyo and NEC Tokin, which indicate their interest in silicon-based anode materials.
*An example is Sanyo’s UR18650F, a cell that specifies a typical capacity of 2600 mAh and that is now in production, according to Masatoshi Takahashi.