When portable device designers develop the
latest gadgets with the most advanced features, much thought goes into
the battery technology that powers these small, yet complex products.
For over a decade, lithium-ion cobalt-oxide has been the technology of
choice for mobile device manufacturers because it offers high energy
density, which translates into more run-time for the end users. During
the past two years there have been several recalls of lithium-ion cobalt
oxide battery packs, including cells from a major Japanese manufacturer.
It is becoming increasingly clear that engineering controls and safety
devices have not been sufficient to prevent safety recalls. Today's
newest phosphate based lithium-ion chemistry may have the ability to
make lithium-ion batteries safer than ever before.
Battery Options: What You Should Know
The latest lithium-ion battery technologies
represent the pinnacle of a development process that has progressed from
the humble lead-acid battery through Nickel-Cadmium (NiCd) and
Nickel-Metal Hydride (NiMH) batteries. When compared to these older
battery chemistries, lithium-ion technology offers several advantages,
including: higher energy density, longer cycle life, no memory effect
and materials that are more environmentally friendly.
In 2006 lithium ion batteries were
forecasted to reach a production rate of tens of millions of cells per
month, with a high annual growth rate. Most lithium ion cells use
lithium cobalt oxide and are targeted at small portable electronic
devices, where energy density is critical but safety, thermal stability
and cost have not been determining factors to date. Other technologies
like lithium manganese oxide have been shown to be more thermally stable
and are less expensive, but have failed to replace lithium cobalt oxide
in these applications. In light of recent safety recalls, lithium
cobalt oxide's safety issues represent a hidden cost in small format
batteries, while new applications such as large format batteries raise
safety requirements that rule out its use.
Identifying New Chemistries
New phosphate-based lithium ion cathode
materials have been identified, such as Valence Technology, Inc.'s
Saphion® lithium-ion technology. Due to the high available material
specific capacity of this chemistry compared to cobalt and manganese
oxides, Saphion® lithium-ion cathode material is currently regarded as a
very promising cathode material because of its enhanced thermal
stability, for small as well as large platform applications. Its low
cost, non-toxicity, excellent thermal stability, safety characteristics
and very good electrochemical performance add to an already long list of
desirable criteria required for a viable cathode material.
With Saphion phosphate cathode material, the
strong covalent bonding between the oxygen and the phosphorus forms a
strong polyanion unit in the phosphate ion that allows for greater
stabilization of the structure compared to layered oxides, e.g.
lithium-ion cobalt oxide, wherein the oxygen is more weakly bound. The
large phosphate polyanion also enlarges the free volume of the host's
interstitial space available for lithium. The phosphorus-oxygen-metal
bonding helps to stabilize the redox energies of the metal cation and
the structure, allowing a relatively fast ion migration. Consequently,
oxygen atoms are a lot harder to extract from Saphion phosphate cathode
materials. Under abuse conditions, there is a much less likelihood of
oxygen liberation from the structure due to phosphate decomposition.
Only under extended and extensive heating (typically > 800ºC) can
decomposition to a Nasicon related phase, in part, (without oxygen
release) occur. This is important to note because it further illustrates
the ability of Saphion phosphate cathode materials to remain stable even
in the harshest conditions, thus avoiding any uncontrollable thermal
excursions.
 Upon removal of lithium, lithiated cobalt
oxide undergoes a nonlinear expansion of the unit cell. This is
particularly important for battery safety in that it affects the
structural integrity of the material, and hence its safety. Removing all
of the lithium available in a Saphion phosphate cathode material causes
no structural modification. In fact, the structures of the fully
lithiated and de-lithiated phases are very similar. This confirms that
the thermal stability of the Saphion phosphate cathode material even
fully depleted of lithium is still far better than the partially de-lithiated
lithium-ion cobalt oxide.
With higher energy density batteries
available, safety is of paramount concern for consumer batteries and
more advanced safety technology is required. Insight into the behavior
and thermal stability of the cathode in the charged state is essential
in determining the overall safety of the final cell. With this in mind,
there are methods available for evaluating the safety of cells under
abuse conditions. Creating an over-charge condition, for example, may
lead to a thermal runaway (or excessive heat), which can cause a
combustion reaction in the battery because of the presence of flammable
solvents and vapor mixtures in the cell. This situation would make the
battery unsafe for consumer use.
Power Safety for the Future
Fundamental properties of Saphion phosphate
cathode material make for an intrinsically safer cathode material for
lithium ion applications. When fully charged, no excess lithium is left
in the cathode (unlike lithium-ion cobalt oxide where 50% still
remains). Lithium cobalt oxide material has a high resilience to oxygen
loss, which can result in a significant exothermic event upon heating.
Consumers are becoming more reliant on their portable devices than ever
before but do not stop to think about inherent risks associated with the
batteries powering them. This may change in view of recent publicity on
the recall of portable device batteries. In view of recent events and
changes in consumer attitude, it will become the burden of the designer
to recognize and incorporate safety along with the most stable power
sources available to ensure the optimal use of such portable devices.
Lithium-ion batteries continue to be the technology of choice for small
electronics but, as illustrated, their continued evolution and the
introduction of new large format applications require a closer look at
the technology inside the battery and the important differentiators
among the choices for cathode materials.
 M.Y.
Saidi, with Valence Technology, Inc.,
holder of several patents related to the phosphate cathode material in
Saphion® lithium-ion technology. |
