Developing Sb and Sb Based Chalcogenides Anode Material for Lithium Ion Batteries

鋰離子電池的銻基以及銻基氧族化合物負極材料的開發

Student thesis: Doctoral Thesis

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Award date19 Aug 2019

Abstract

In the present thesis, several Sb based materials are investigated as anode material for lithium ion batteries. Sb is one of the most promising next-generation anode material to replace commercial graphite because of its high capacity and moderate operation voltage. The specific capacity of Sb is 660 mAh g-1, which is about 2 times higher than graphite, while the lithiation potential is 0.85 V, which avoid the dendrite formation and enhance the safety issue during fast charging.

I start our study with commercial bulk Sb to evaluate the electrochemical behaviour of this material. I find that the drastic capacity fading is ascribed to the mechanical failure induced by large volume expansion and lithium trapping inside the host Sb material. Cycle performance is significantly enhanced by the reduction of the particle size, but capacity still decreases along cycles. To solve the problem, I design a surface polyimide (PI) coating on Sb particles combined with Carboxymethyl cellulose (CMC) binder system. Surface PI coating effectively prevent the particle from cracking, while the CMC binder further enhance the connection between particles via an ion dipole interaction between CMC and PI. With this PI-CMC system, Sb electrode with 9.4% PI coating exhibits a high reversible capacity of 580 mAh g-1 at 1 A g-1 after 100 cycles. The rate performance of the electrode can be further improved by adding 5% acetylene black (AB) during the PI coating process. Even at a current rate of 20 C (13.2 A g-1), a highly reversible capacity of 380 mAh g-1 can be obtained. The superior high-rate capability and excellent stability of our Sb electrode are also verified by full cell tests with LiFePO4 cathode.

PI-CMC system improves the cycle performance significantly, but PI coating requires a high temperature of 350 ℃ for polymerization, which increases the cost significantly. In addition, PI can accommodate the Li+ with low reversibility, which decreases the 1st cycle coulombic efficiency. I then attempt to find substitute to replace the PI, which could provide enough strength without high temperature heating process. Inspired by slime toy, polyacrylic acid (PAA) with borax cross-linking polymer system (PAA-Borax) is investigated as binder for Sb anode material. I systematically vary the amount of crosslinker borax to understand the correlation between the electrochemical performance and mechanical strength via several characterization techniques including indentation, in-situ dilatometer and X-ray photoelectron spectroscopy (XPS). Indentation is performed to understand the mechanical property of the binders. XPS is performed to understand the chemical crosslinking between PAA and Borax. The electrode using PAA with 5% borax (PAA-5 Borax) as binder deliver a comparable performance compared with PI-CMC system.

To further explore Sb based material, we study the Sb with active S or Se matrix, because these materials usually provide higher specific capacity via both conversion and alloying reaction with lower theoretical volume expansion compared with Sb metal. In addition, these chalcogenide materials exhibit better performance than Sb, but the mechanism of the improvement is not clear. I therefore like to understand the role of S and Se in the charge-discharge mechanism of the materials so that we can select a suitable phase to stabilize the Sb material. With the help of several characterization techniques including Raman, XRD and dilatometry, we find that the poor stability of Sb electrode originates from the irreversible phase transformation between crystalline Sb and Li3Sb. Addition of S or Se into the matrix can inhibit such phase transformation, thus improving capacity retention. The stabilization effect is stronger for Se matrix than S matrix, because it enables the re-formation of Sb-Se bonds upon delithiation and prevents phase segregation. In addition, Se matrix reduces volume change of the electrode during charge and discharge, preventing electrode cracking and resulting in good mechanical reversibility.

To further develop promising Sb2Se3 material, I constructed a uniformly distributed carbon/Sb2Se3 cluster via ball-mill technique. All the Sb2Se3 particles are closely encapsulated with carbon. The addition of carbon can effectively enhance the interaction between Sb and Se and buffer the volume expansion, thus preventing the phase segregation and cracking even after extended cycles. With an addition of 20% acetylene black, the Sb2Se3 material exhibits an excellent cycle performance with a charge capacity of 545 mAh g-1 at 250 mA g-1 after 1000 cycles, corresponding to a high capacity retention of 98.9%. A full cell with LiFePO4/Sb2Se3-C delivers a stable capacity of 340 mAh g-1 after 300 cycles, verifying the effectiveness of the enhancement.