August 2015 • The PCB Design Magazine 51
• Additional power planes need to be
exploited and these need to be split to
accommodate all supplies plus GND.
• The impedance plots project the correct
values of all variables for the chosen
material to achieve the target
impedance.
PCBDESIGN
References
1. Barry Olney Beyond Design columns:
Material Selection for SERDES Design, Material
Selection for Digital Design, The Perfect Stack-
up for High-Speed Design, and Embedded Sig-
nal Routing.
STACKUP PLANNING, PART 3 continues
2. Henry Ott: Electromagnetic Compatibil-
ity Engineering.
3. To download the ICD Stackup and PDN
Planner, visit www.icd.com.au.
beyond design
Barry Olney is managing
director of in-Circuit Design Pty
ltd (iCD), Australia. The compa-
ny developed the iCD stackup
Planner and iCD PDn Planner
software, is a PCB Design
service Bureau and specializes in board level
simulation.
To read past columns, or to
contact olney, click here.
imagine a cell phone that charges in less than
an hour and lasts for three to four days, or an
electric car that runs for hundreds of miles before
needing to be plugged in.
Researchers at the u.s. Department of energy's
Argonne national laboratory are working to make
this dream a reality by developing lithium-ion bat
-
teries containing silicon-based materials. The most
commonly used commercial lithium-ion batteries
are graphite-based, but scientists are becoming in-
creasingly interested in silicon because it can store
roughly 10 times more lithium than graphite.
There's just one problem: current batteries
based on silicon materials don't last long.
The problem lies in the battery's chemistry. The
electrolyte inside the battery transports lithium
ions back and forth
between positive and
negative electrodes as
the battery charges
and discharges.
lithium ions react
with the negative elec
-
trode to form a new
compound, causing
the electrode to ex-
pand, while the elec-
trolyte produces a protective coating called the
solid electrolyte interphase.
"The ideal solid electrolyte interphase should
halt the reaction between the electrode and
electrolyte, while allowing the lithium to come
through," said ilya shkrob, a chemist in the Chem-
ical sciences and engineering Division.
But the coating also needs to expand and con-
tract with the electrode, or else it will crack and the
battery won't work.
"When the protective layer cracks, the elec-
trode surface reacts and consumes the electro-
lyte," shkrob said. "if the electrolyte is completely
consumed, then the battery won't work."
in today's graphite-based lithium-ion batter-
ies, the electrode expands about 10%—a small
enough change that cracks in the coating aren't
an issue.
But the electrode in a silicon-based lithium-ion
battery expands up to 300%. These batteries need
a different electrolyte in order to produce an elas-
tic shell.
The researchers
found that when fluo-
rine is added to eth-
ylene carbonate, the
resulting electrolyte
forms a coating that
can stretch and ac-
commodate the vol-
ume changes in the
electrode.
Protective Shells May Boost
Silicon Lithium-ion Batteries
