Characterization of Lithium Ion Batteries Using the PeakForce TUNA Method

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[Ni_1/3Mn_1/3Co_1/3]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 Li[Ni_1/3Mn_1/3Co_1/3]O₂ composite 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 Laboratory.

PVDF+AB Content Optimization in LiNi_0.8Co_0.15Al_0.05O₂ Cathode

Another compound cathode material, LiNCA (LiNi_0.8Co_0.15Al_0.05O₂) is being experimented with to enhance the performance of the lithium ion batteries. Using PeakForce 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 LiNi_0.8Co_0.15Al_0.05O₂ 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 neared completion.

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 LiNi_0.8Co_0.15Al_0.05O₂ 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.

Conclusions

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.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

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