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Beyond Conventional Cathode Materials for Li-ion Batteries and Na-ion Batteries Nickel fluoride conversion materials and P2 type Na-ion intercalation cathodes /

Abstract

The Li-ion battery is one of the most important rechargeable energy storage devices due to its high energy density, long cycle life, and reliable safety. Although the performances of Li-ion batteries have been improved dramatically, the limit in terms of the energy density still needs to be resolved to meet the growing demands for large-scale mobile devices. Choosing the cathode material is the most pivotal issue in achieving higher energy, since the energy density is directly correlated to the specific capacity of the cathode. Intercalation-based cathode materials have been widely utilized in commercial products; however they yield a limited capacity due to restricted crystallographic sites for Li-ions. In this thesis, the NiF₂ and NiO doped NiF₂/C conversion materials , which display substantially greater capacities, are intensively studied using various synchrotron X-ray techniques and magnetic measurements. The enhanced electronic conductivity of NiO doped NiF₂/C is associated with a significant improvement in the reversible conversion reaction. While bimodal Ni nanoparticles are maintained for NiO doped NiF₂/C upon the discharge, for pure NiF₂ only smaller nanoparticles remain following the 2nd discharge. Based on the electronic conductivity, it is demonstrated that the size of Ni nanoparticles is associated with the conversion kinetics and consequently the reversibility. Although Li-ion batteries offer the highest energy density among all the secondary batteries, the amount of the reserves and the cost associated with the Li sources are still a concern. In the second part of the thesis, P2 type Na₂/₃[Ni₁/₃Mn₂/₃]O₂ is investigated to understand the structural stability in the Na-ion batteries. Significantly improved battery performances are obtained by excluding the phase transformation region. In addition, the structural evolution of the P2- Na₀.₈[Li₀.₁₂Ni₀.₂₂Mn₀.₆₆]O₂ is tracked by in situ technique and revealed no phase transformation during the cycling. It is identified that the presence of Li-ions in the transition metal layer allows increased number of Na- ions after charging maintaining the P2 structure. The design principles for the P2 type Na cathodes are proposed on the basis of our understanding; eventually an advanced cathode material is achieved for high energy Na-ion batteries

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