Developing novel binder materials for stable silicon lithium-ion battery performance

Abstract

The Si anode of a Li-ion battery undergoes extreme volume expansions of up to 400% during lithiation and de-lithiation, which leads to a significant build-up of lithiation induced stresses. This results in the fracture of the Si anode material. The fractured Si anode material leads to two phenomenon: 1) Si anode material delaminates from the electrode leading to loss of active material and 2) the fracture exposes fresh Si surfaces, which leads to uncontrolled formation of SEI layer. These two phenomenon occur continuously, causing rapid performance degradation and serves as a major stumbling block to commercialization of the Si anode material. In this thesis, two different designs are reported that uses novel binder schemes to explore and suggest solutions to developing Si microparticle based anodes that exhibit stable electrochemical performances. First, in chapter 2, a Si microparticle based composite anode that allows for relaxation of the diffusion induced stresses in the Si microparticles using self-healing polymers is proposed and investigated. The self-healing polymer and Si microparticle freestanding composite was analyzed for its cyclic stability that confirmed enhanced retention, with the freestanding composite demonstrating 91.8% capacity retention after 100 cycles at C/10 rate. Second, in chapter 3, another composite anode based Si microparticles embedded in combustion reacted, nanoporous ZnO is proposed and investigated. The nanoporous ZnO was shown to undergo conversion reaction with Li ion during lithiation that hence increased the capacity and also was shown to help maintain the coalescence of Si microparticles for enhanced electrochemical cycling. The composite was a binder-less design with an initial capacity of ~3,900 mAh/g at C/20 rate and a reversible capacity of ~1,500 mAh/g beyond 200 cycles at C/5 rate. In addition, the $Li_2O/Zn$ matrix derived from conversion-reacted nanoporous ZnO acted as an effective buffer to lithiation-induced stresses from volume expansion and served as a binder-like matrix that contributed to the overall electrode capacity and stability. Last, in chapter 4, the $Li[NiCoMn]O_2$ cathode material is considered. Although the cathode materials demonstrates significantly less volume expansion as a result of lithiation, the volume expansion is sufficient enough to cause cracks and delamination. A combinatorial analysis of a wide range of $Li[NiCoMn]O_2$ compositions is conducted as a database of the hardness and modulus values before and after cycling is gathered as well as the relationship between structural, mechanical integrity and electrochemical reliability.