Impedance Analysis of High Capacity Manganese Rich Cathode in Lithium Ion Batteries

The high capacity manganese rich (HCMR) cathode material (xLi 2 MnO 3 • (1-x)LiMO 2 , M = Ni, Co, Mn) demonstrates the highest discharge capacity (~250 mAh g -1 ) along with potential cost reduction and safety enhancement among all the high energy cathode materials considered for lithium ion batteries (LIBs).. However, the practical application of HCMR is prohibited due to, among other things, the high impedence at low states of charge (SOC), as shown in the PITT test in Fig. 1. The purpose of this study is to investigate the source of the high impedence for HCMR cathode materials. HCMR electrodes were prepared with active material (85 wt%), acetylene black conductive additive (7 wt%), and PVdF binder (8 wt%). The loading of HCMR in the cathode is 1.1 mAh/cm 2 . 2325 coin cell hardware was used for electrochemical measurements, assembled with Celgard 2400 as separator and lithium metal as counter electrode. EIS tests were performed using a VMP3 electrochemical workstation. The EIS results are shown in Fig. 2, and the results of detailed impedance analysis are shown in Fig. 3. At lower SOCs, the HCMR exhibits a greater diffusion impedance (R diff ), while the charge transfer impedance (Rct) rises quickly and then decreases. Both sources of resistance are much higher than the ohmic impedance (R L ) and the another high-frequency source associated with wither the anode or an SEI on the cathode, (R SEI ). In all, we can conclude that both the solid state ionic diffusion and electrochemical charge transfer offer nearly equal contribution to the high impedance of HCMR at low SOCs (between 3.6 and 3.2 V). Therefore, stratagies that may increase the ionic diffusion coefficient ( e.g . chemical doping), decrease the ionic diffusion length, or increase the rate of electrochemical reaction must be adopted to solve the high impedance issue at low SOC. Reducing the particle size would have the effect of simultaneously reducing the solid state transport length and increasing the area for charge transfer, but may come at the expense of increased dissolution of the transition metals (not studied here) from the active material. Figure 1