Novel Conductive Carbon as an Additive Material for Enhancing Performance of Lithium-Ion Batteries

摘要

Because of highest energy density among all ot practical batteries, lithium-ion batteries (LIBs) are employed as power sources for a variety of systems in portable device, xEVs and so on. In particular, the xEV's market has been rapidly expanding due to worldwide GHG Emission Standard and regulations. The present LIB cells for xEVs do not always have enough performances not only on energy and power density, but also on durability under severe condition in use for vehicles.In order to improve the durability, various approaches have been suggested. As one of the ideas from aspect of material-base, surface coating of active materials has been conducted to prevent interfacial side reaction of the active materials, which is an economic and feasible method. Actually, many reports have described that the surface coating by inorganic materials like Al[2]0[3] (Refl) and also by carbon materials (Ref.2) has good effect on cycle stability even under high temperature. The inorganic material coating, however, brings about lower electronic conductivity in its electrodes due to insufficient electronic pass formation among the active materials particles, and eventually lower power density of the cells. So, the conductive carbon coating will be preferable to enhance the electrochemical performance with keeping better cycle stability. Therefore, we have been focusing on a novel carbon material with uniform coating behavior, and successfully achieved developing a unique conductive carbon called NH Carbon. In this poster presentation, we will report the good performances of the NH Carbon for cathode and also anode. Experimental LiNi[l/3]Co[l/3]Mn[l/3]O[2] (NCM) was used as cathode active material. The composition of cathode electrode was adjusted in NCM (96wt. %), NH Carbon (NHC) or common conductive carbon (CCC) (2wt. %), and PVDF (2wt. %) as binder. The slurry was prepared by kneading those materials in NMP. To evaluate coating states, Raman spectra of the electrodes were measured. The coating states were estimated from relative intensity ratio of D band at around 1350 cm[-l] arise from graphitic carbon to NCM peak at around 600 cm[-l]. For charge-discharge cycle test, laminate cells were fabricated using general graphite anode, polypropylene separator, and 1 M LiPF[6] / ethylene carbonate and dimethyl carbonate mixture (1:1) with vinylene carbonate (lwt%). Cycle test was carried out at current density of 1C between 4.3-3.0 V. The electrochemical impedance spectrum was measured on symmetric cell with two cathodes of SOC 50%. Results and discussion The relative intensity ratio (D band / NCM band) in Raman spectrum of the NHC and CCC electrodes were 1.05 and 0.11, respectively. The much higher relative intensity in the NHC electrode means a higher coverage of the NHC on the surface of NCM. This result suggests that the NHC has a superior coating ability on the surface of active materials though the CCC exhibit little or no coating behavior. The unique ability of the NHC could provide the following three advantages. 1) Enhancement of cycle ability due to suppression of interfacial side reactions. Actually, capacity retention of the NHC and CCC cells after testing of 200 cycles were 95.3 % and 74.1%, respectively. 2) Reducing of DCIR in electrode. The electrochemical impedance spectroscopy showed a lower charge-transfer resistance of the NHC electrode compared with the CCC one. The good result is attributed to facilitation of Li ion insert/removal at interface by the uniformly coated NHC. 3) Enhancement of the electrode density. The NHC electrode showed a higher density (3.78 g/cc) compared with the CCC one (3.63 g/cc). Whereas the CCC behaves as a steric hindrance in the electrode, the NHC do not interfere a packing due to the coating layer formation on the NCM surface. In addition, we found that the NHC has the slimier effects in using Si anode. In our poster, we will also discuss on the effect.