Studies On Silicon-base Anode Materials And Safety Of Lithium Ion Batteries

In this thesis, Si-based anode material and cell safety have been systematically investigated surrounding following points. Firstly we evaluated Si-based anode material in full cell and investigated Si-based material structure stability during cycle test. Secondly, the mechanism for capacity fading was further studied, and we found that adhesion force enhancement between film and Cu current collector with PI binder can obviously improve cell cycle stability. Thirdly, according to the investigation, it is found that the interaction on the interface, between cathode and electrolyte, is the critical factor for cathode material thermal stability. We can improve it by particle surface modification, BET optimization and electrolyte functional additive addition. Fourthly, overcharge failure mechanism in Li Ni0.33Co0.33Mn0.33O2//graphite system was studied. Finally, we use different approaches, like internal short simulation, quantitative analysis for oxygen release and hot oven characterization, to study the reason for why overcharge fire.Analysis for Si-based material during cycle was characterized by half coin cell, and result revealed that crystalline Si transformed into amorphous phase. Si particle and Si Ox formed continuous dense bulk structure, and the resultant lithium silicates acted as solid electrolyte interface(SEI) film, which effectively prevented electrolyte from decomposition due to SEI damage and reparation. SSC//graphite composite(2:8, wt%) further improved full cell cycle stability, and capacity retention can remain 80% above after 400 cycles with 0.7C charge and 0.5C discharge condition. This result can meet commercial product requirement. By analyzing cycle capacity fading mechanism, it was found that PI binder could improve the adhesion force from 3.7 N m-1 to 30 N m-1. This helps ensure the film good electronic conductivity, and thus improve cycle stability. Compared with PVDF binder system, we found that most lithium loss happened in delithiation process, as the cycle coulombic efficiency most keeps 100%, indicating good reversibility. TEM result further proved this by SEI film thickness observation.After characterizing several commercial cathode materials thermal stability, Li Fe PO4 was thought as the safety one. In addition, we can improve LixCo O2 thermal stability by Al2O3 surface coating, big size particles and addition of some functional electrolyte additive. Result shows that the interface reaction between active material and electrolyte is the main heat generation source. Any method good for reduction of material reaction activity should be helpful for material thermal stability and cell safety performance.It was observed that the temperature difference, between jelly-roll inside and cell skin, became bigger with increasing charge depth in Li Ni0.33Co0.33Mn0.33O2//Graphite system. We found that the contribution to cell temperature rise from electrolyte decomposition and ohm heat is small. After consideration of cathode delithiation extent and material thermal stability, result revealed that cell thermal run away was mainly caused by violent reaction between high delithiated cathode active material and electrolyte.Investigation of various factors impact on overcharge fire was carried out, it was found that:(1) Cell with hole-cut separator overcharge test result revealed that internal short between cathode and anode film did not trigger overcharge fire;(2) Crystalline structure collapse was not the main reason, as Li Ni0.33Co0.33Mn0.33O2 kept same layer structure by XRD characterization;(3) Quantitative measurement about oxygen release amount was proved not enough to sustain overcharge fire phenomenon;(4) Cell overcharge thermal run away happened when jelly-roll temperature inside reached 132 ℃. Hot oven results revealed that overcharge cell failure was dominated by anode side. With the increasing charge depth, anode thermal stability gradually deteriorated. When cell internal temperature raised up to 135 ℃, anode firstly becomes thermal run away. Meanwhile, outside air rushes in, and then violent fire happened.