Table of ContentsIntroductionCharacterization of Lithium Ion BatteriesPVDF+AB
Content Optimization in
Light weight lithium ion batteries with their high energy density have become
an integral part of almost all consumer electronic devices. This energy
storage device finds its latest application in running vehicles. However, only
about 10% of the theoretical capabilities of lithium ion batteries has been able
to be exploited. Therefore, more research is being done to enhance the
formulation and structuring of the anode and cathode materials, shelf life and
cost, chemistry of the materials and the safety features. The electrode of the
battery is constructed by binding micrometer to nanometer size materials using
additives and leaving moving space for the lithium ions. The use of
nanomaterials bound in such a fashion has benefits such as increased battery
capacity and high charging and discharging rate. The PeakForce
TUNA method can be used for characterization of the lithium batteries as
explained in the subsequent sections.
Characterization of Lithium Ion Batteries
The materials used as cathode in the lithium ion batteries are mostly
composite materials, such as L333 -
Li[Ni1/3Mn1/3Co1/3]O2. The L333
particles are bound together using polyvinylidene difluoride (PVDF) and to
improve the electronic conductivity, acetylene black (AB) is also added. In
order to visualize the component distribution and to characterize the conductive
network which connects the L333 particles, the PeakForce
TUNA method was employed. The topography showed the approximate size of L333
particles as 3 to 15µm and that of the PVDF and AB particles as 50nm. Also,
there were two conductivity layers, with the L333 particles which were not
covered by AB+PVDF constituting the lower conducting layer. The layers covered
with PVDF and AB formed the more conductive band. PVDF on its own is not a good
conductor; only when mixed with AB does it conduct, which is due to the
nanoparticles connecting to each other. The conducting layers also displayed
lesser elasticity and adhesion. The uncovered L333 layers are electrically
separated from the electrode and hence do not contribute to the battery’s power.
The current data map has two peaks - one formed by L333 and another by PVDF+AB -
and the conductive network covers 56% area in the map.
Figure 1. PF-TUNA images of a
cathode, on the top row are topography, DMT modulus, adhesion and current maps.
The overlay of a current map on topography is shown on the bottom row. Images
were taken on a Dimension ICON AFM in ambient conditions, with a DDESP probe
(spring constant was calibrated to be 93N/m), 50ìm scan at a DC sample bias of
500mV. Sample courtesy of Dr. Zheng and Battaglia, Lawrence Berkeley National
PVDF+AB Content Optimization in
Another compound cathode material, LiNCA
(LiNi0.8Co0.15Al0.05O2) is being
experimented with to enhance the performance of the lithium ion batteries. Using
TUNA, the effect on characteristics of the composite by varying PVDF+AB
content was studied. Figure 9 shows the experiment results got by varying the
PVDF+AB content by 3.2%, 12.8% and 24%. The ratio of PVDF to AB was 1:0.6.
Figure 2. Bearing analysis of the current maps (not
shown) of LiNi0.8Co0.15Al0.05O2
composite cathode containing 3.2%, 12.8% and 24% PVDF+AB. Sample courtesy of Dr.
Zheng and Battaglia, Lawrence Berkeley National Laboratory.
It was observed that the conductivity increase was proportional to the amount
of AB+PVDF. When 12.8% of the PVDF+AB was added, the conductive network coverage
Figure 3. Plot of the conductive network coverage and
average conductivity (a) and average elastic modulus (b) over 50µm scan area as
a function of the percentage content of PVDF+AB in
LiNi0.8Co0.15Al0.05O2 composite on
the same samples used in Figure 2.
Conductivity increases as the internal resistance of the battery reduces;
therefore the power density of the battery is also increased. Another
observation was that the elastic modulus of the cathode decreased with increase
in the PVDF+AB content. This implied that the cathode became more accommodating
to volume changes that occurred when lithium ions entered the cathode. The
various measurements by the PeakForce
TUNA along with other studies can give the right path towards optimization
of the application of the lithium battery.
To summarize, PeakForce TUNA provided an effective method to study the
cathode materials of the lithium battery. This technique can also be applied to
study anode materials and determine their aging characteristics over time or
during the charging and discharging cycle, during which mechanical degradation
or increase in resistance may happen. PeakForce
TUNA measurements combined with data from other techniques can be used to
optimize results to meet different application requirements.
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This information has been sourced, reviewed and adapted from
materials provided by Bruker AXS.
For more information on this source please visit Bruker