Binder-free LiCoO2-based composite cathodes produced by cold sintering for lithium–ion batteries

Renewable energy sources, such as solar energy, tide energy, and hydroelectric energy, are attracting numerous attentions on the grounds that they are able to effectively alleviate the effect of global warming induced by CO2 emission. Owing to the intermittent nature of these renewable energy sources, energy storage devices are needed to store surplus energy and release the energy in demand. Lithium-ion batteries (LIBs) as energy storage

devices have stimulated a surge of interests due to the high energy density, extended longevity, and low-discharge rate. LiCoO2(LCO)-graphite is the first commercialised materials system for LIBs and is being widely used. However, most of the current research endeavours focus on improving the power density of LIB rather than energy density, especially volumetric energy density. This thesis presents an account of a novel sintering technique, termed cold sintering process (CSP), leading to the fabrication of high-performance LCO/graphene composite cathodes which significantly improve the volumetric energy density of LIBs whilst still maintain its appealing electrochemical properties, such as high working potential and promising rate capability.

In this thesis, the concept of CSP feasibility on densification of monolithic LCO ceramics via a dissolution-precipitation process at ultralow temperature and high pressure with

appropriate processing time was first verified. CSP variables were investigated and optimised in achieving high bulk density and preferable materials properties for CSP-ed

LCO. With the integration of multi-walled carbon nanotubes (CNTs) or microwave reduced graphene oxide (MWrGO) into CSP-ed LCO matrix, a thick binder-free composite cathode was achieved in one-step synthesis at temperature as low as 250 ℃ for the first time, which is impossible to realise in conventional high-temperature sintering methodology because of inevitable carbon decomposition. The abovementioned MWrGO was synthesised from graphene oxide using microwave irradiation at a superfast

heating rate (within seconds) and was incorporated in the dense CSP-ed LCO body to compensate the adverse effects induced by sintered ceramics, such as intrinsically low electrical conductivity, rigid LCO matrix and slow Li+ ion diffusion in dense ceramics, due to its high electrical conductivity, planar character and porous structure. With the systematic investigation in this project, the property-structure relationship of MWrGO was thoroughly elucidated and found that the quality of MWrGO can be carefully tuned

by controlling microwave power and inert atmosphere to meet the specific expectation of high-performance electrode for LIBs.

The merits of MWrGO to the electrochemical performance of thick LCO composite cathode were demonstrated as compared to CNTs. The addition of 1 wt% MWrGO in

LCO matrix (LCO/1%MWrGO) provided a comparable electrical conductivity to the addition of 3 wt% CNTs (LCO/3%CNTs). Owing to less addition of carbon materials and absence of binder/current collector, the LCO/1%MWrGO cathodes delivered a superior volumetric discharge capacity of ~501.5 mAh cm-3 in the first cycle and a promising capacity retention of 80.9% after 40 cycles with respect to ~459 mAh cm-3 and 62% retention after 40 cycles for LCO/3%CNTs. More notably, the LCO/1%MWrGO cathodes can still afford ~50 mAh g-1 at 1 C and ~30 mAh g-1 at 2 C, whereas no capacities

were attained for LCO/3%CNTs cathodes at current rates > 1 C.

This research ultimately advances the understanding of the processing of pressure-assisted sintered electrode composites for use in LIBs. The main scientific and technical contributions of this project are as follows:

1. The concept of CSP feasibility on fabrication of LCO-based electrode expands the materials spectrum of CSP applicability.

2. The realisation of high energy density without compromising other electrochemical properties expands the field for binder-free electrode fabrication exploration beyond

conventional strategies and structural motifs. This promising electrochemical performance of binder-free electrodes is expected to bring them a step closer to use

in practical LIBs.

3. The versatility of CSP opens up the possibility of ‘all-in-one-step’ densification of two materials with substantial dissimilar physical properties to produce advanced

functional devices (such as solar panel, supercapacitors, and sensors) but not limiting to LIBs, demonstrating CSP is a valuable asset within ceramic processing toolbox for

both research and industrial production.