Charging Voltage Limit Effects on the Electro-Chemical Behavior of High Capacity Manganese-Rich Cathode in Lithium Ion Batteries

Compared with normal cathode materials for lithium ion batteries (LIBs) (such as LiCoO₂, LiMn₂O₄, Li[Ni_1/3Co_1/3Mn_1/3]O₂ and LiFePO₄), the high capacity manganese rich cathode (HCMR) materials (xLi₂MnO₃• (1-x)LiMO₂, M = Ni, Co, Mn) demonstrates the highest discharge capacity (~250 mAh g^-1) along with cost reduction and safety enhancement. This material is therefore considered as a high promising cathode material for the next generation LIBs. However, the charging voltage limits effects on the electrochemical behavior have never been studied. And this material presents complex structural changes upon cycling that are still misunderstood. The purpose of this study is to investigate the charging voltage limit effects on the electrochemical performance and structure of HCMR cathode material.

HCMR electrodes were prepared with a composition of active material (85 wt%), acetylene black conductive additive (7%), and PVdF binder (8%).  The loading is about 1.0 mAh/cm². 2325 coin cells were assembled for electrochemical measurement with Celgard 2400 as separator and lithium metal as counter electrode. Electrochemical measurements were performed using a Maccor battery cycler.

The charge-discharge curves and corresponding cycling performances between 2.0 V and five different upper voltage limits are shown in Fig. 1 and Fig. 2. There is an obvious voltage plateau around 4.5 V during the first charge process, which may be responsible for the activation of the HCMR. The capacities of this material improved with an increase in upper voltage limit up to 4.8 V. For the upper voltage limit of 4.5 V, the capacity of HCMR increased for the first 60 cycles, which may be related to the gradual activation of the HCMR. When the upper voltage exceeds 4.8 V, electrolyte oxidation involving the cathode is believed to cause an increase in resistance that and worse capacity retention. Considering both the discharge capacity and capacity retention, 4.7 V seems to be the best charging voltage limit, which exhibits a discharge capacity of ~240 mAh g^-1and capacity retention of 99.88% after 350 cycles. It is well known that the voltage decreases with cycling. The dQ/dV curves (Fig. 3) indicate that both peak voltage and peak area, which are highly related to the HCMR structure and electrochemical performance, change with cycling and upper voltage limit.

Although an increase in the upper voltage limit can improve capacity, 4.7 V is the best upper cut-off voltage for long life and high energy LIBs containing HCMR. The structure changes will be discussed in further work.