Achieving high-power capability of a battery requires minimizing the overall resistance of the electrochemical system. For lithium-ion batteries, much effort has been devoted to minimize the ionic diffusion resistances and electronic resistance associated with the electrode active materials. In the typical electrode configuration, the layer containing the active material is supported on a metallic current collector. The interface between the current collector and active layer imposes additional resistance to charge transfer within the electrode. The advancement in material synthesis technologies has reduced the ionic and electronic resistances associated with the active materials to the point that they become competitive to the other resistance sources. Thus, the significance of the electronic resistance at the active layer/current collector (AL/CC) interface is worthy of re-examination.
Carbon-coated Al current collectors were prepared by two different coating processes: high-temperature thermal chemical vapor deposition (HT-CVD) and low-temperature chemical vapor deposition (PA-CVD). At least two beneficial effects are anticipated to result from the C-coating: 1) the C-coating removes the native surface oxide layer on the metal current collectors, and 2) the C-layer is hydrophobic in nature and helps to improve the interfacial bonding. Both effects are expected to reduce the AL/CC interfacial resistance. The ultimate goal is to develop a viable C-coating process of the current collector in order to improve the overall power performance and/or cycle life of the electrode of Li-ion batteries.
The strategy for manufacturing a large area of C-coated Al foil consisted of two parts. For the first part, due to the designed plasma coating system, the plasma process was easily scaled up under high vacuum. With the same operation parameter as the one to produce PB-Al, the length of the product was increased to more than 1.5 m. The second part is to scale up the thermal treatment, which is more difficult because of its large space occupation and the need of extra-low oxygen content in the furnace. To overcome this problem, a roll-calcination process was created. By winding plasma carbon- coated Al foil (PB-Al) tightly on a metal roll, not only was the occupied space compressed dramatically, but the interfaces protected each other from the surroundings and inhibited the oxide layer formation in high temperature.
The presence of insulating oxide layers at the pristine Al current collectors would impose significant resistance to current flow across the interface. Surface modification by the plasma process is set up along with suitable thermal treatment. The schematic diagram of the C-coated Al is shown in the figure. There are some high conductive regions penetrating through the native alumina oxide layer, and a high conductive carbon coating on the surface makes the surface hydrophobic. When used as current collector, a hydrophobic surface can increase the contact of the surface of active material and the current collector, and the electron transfer can simultaneously be enhanced. This low resistance successfully results in better rate capacity, low polarization, and better cycle life.
The relation of the thickness of the carbon coating to performance was studied. With the same treatment but different carbon amount, the one with the thin carbon layer gets worse performance of the electrode. However, increasing the thickness of carbon to get a visible color change, a high-conductive channel will be formed on the surface and provide a positive effect on the performance of the electrode. The scaling up of this surface modification process proved to be feasible by overcoming the oxidation problem after high-temperature treatment.
This work was done by Nae-Lih Wu of National Taiwan University for the Air Force Research Laboratory. AFRL-0230