Synthesis and Characterization of High-capacity Li-rich Nickel Manganese Oxide Cathode for Lithium-ion Batteries
Date Issued
2014
Date
2014
Author(s)
Chen, Wen-Chin
Abstract
The future development of active materials for lithium ion battery is expected to proceed toward two major directions, namely reducing material cost and increasing electrode energy density. Among the reported cathode materials so far, layered lithium-rich manganese-transition metal oxide composite cathode (abbreviated as LrMOs), xLi2MnO3‧(1-x)Li(Mn, M)O2 (M= Mn, Ni, Co), possesses specific capacity of >250mAh/g with an average redox potential near 4V and therefore potential energy density nearly ~1000 WH/kg, which is much higher than those of Li2MnO4, LiNi0.33Co0.33Mn0.33O2 and LiFePO4. In this dissertation, this is the first time that the micro-structural evolution of the resulting LrMOs during calcinations was investigated mainly by transmission X-ray microscopy (TXM). Thus a radially distributed pore pattern, less pore tortuosity and small grain size produced by intermediate heating history, favor the rate performance of the composite oxide cathode due to reduced charge-transfer resistance and enhanced Li ion solid-state diffusivity.
Moreover, the microstructures and electrochemical performance are shown to strongly depend on the transition metal composition on the surface of LrMOs particles. the electrodes containing either Mn- or Ni-rich surface cathode lead to capacity loss, while Ni-rich cathode exhibits much lower discharge capacity and poorer rate capability than the one with Mn-rich because of its high charge-transfer resistance and high degree of Ni/Li cations mixing. Nevertheless, the presence of Ni-rich out-layer suppressed the phase transformation from layer to spinel and associated voltage fading during cycling. The results provide a new strategy to develop advanced lithium-rich layered cathode materials from the viewpoints of metal composition on the surface of LrMOs particles.
In the other hand, two methods in this dissertation are proposed to enhance the initial coulombic efficiency, rate capability and cycling stability of a highly packed (2.5 g/cm3) LrMOs cathode for lithium ion batteries. First, graphene nanosheets (GNSs) additive into the LrMOs electrodes can reduce electrode polarization and enhance specific capacity and rate performance, and also promote the formation of solid-electrolyte interface (SEI). This study is the first to identify that such an adverse effect is caused by a graphene additive. the results showed that a GNSs additive content of approximately 100 ppm is optimal for achieving both rate and cycle-life enhancements.
In the other modification, a conductive polymer, Poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonate) (abbreviated as PEDOT:PSS), was coated on the LrMOs particles by a liquid mixing and low temperature process. It revealed that the resistance of LrMOs powder can be substantially reduced by four orders through 2 wt.% PEDOT:PSS coating, while it have limited improvement of powder resistance when the amount of PEDOT:PSS is above 2 wt.%. For electrochemical test, the presence of PEDOT:PSS film can drive fast electron transport through the surface of the oxide particle and provide efficient conducting network inside the electrode, leading to enhanced rate performance and increased specific capacity for highly packed LrMOs microsphere electrode. Furthermore, the PEDOT:PSS-coated LrMOs exhibited less initial capacity loss and good cycling stability due to inhibited SEI formation under high potentials.
Subjects
鋰離子電池
富鋰層狀氧化物
形貌變化
石墨烯
導電高分子
SDGs
Type
thesis
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