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Improved Electrochemical and Thermal Stability of Electrode Materials by Interface Control for Rechargeable Batteries

Improved Electrochemical and Thermal Stability of Electrode Materials by Interface Control for Rechargeable Batteries

  • 이용호 (대학원 화공생명공학과 화공생명공학전공)

초록/요약

Lithium-ion batteries (LIBs) have been widely used as power sources for portable applications, such as cell phones and laptops, due to their high energy density, long cycle life, and the absence of a memory effect. In recent years, several studies have focused on the application of LIBs as power resources in large energy-storage systems, electric vehicles (EVs), and hybrid-electric vehicles (HEVs). Despite their environmental advantages, EVs have limited popularity because of the inferior battery performance and higher cost compared to internal combustion engine vehicles. In this regard, inexpensive batteries with high energy density and power density are required with anode and cathode materials for large energy-storage systems. For EVs to successfully penetrate the mass consumer market, batteries having high energy density, long life, good safety, and low price are needed. In this regard, EV batteries require cathode materials with high energy density to achieve these properties because the most commonly used anode, graphite, can deliver a much higher specific capacity (372 mA h g−1) than available cathodes. To meet the increasingly demanding requirements of maintaining a high energy density at low costs, various cathodes have been proposed and extensively investigated such as Li-rich transition metal oxides (Li1+xMe1-xO2, where Me = Ni, Co, and Mn) and Ni-rich cathode materials owing to their high capacity (>200 mA h g−1), as well as their utilization of cheap and eco-friendly manganese rather than cobalt. However, these cathode experiences poor cycling stability and thermal instability during cycling limit their commercial application in electric vehicles. To solve these obstacle and improve the electrochemical and thermal stability of cathode materials, the various techniques for reducing undesirable behaviors of cathode materials has been made such as surface modification or coating. In Chapter 2, we report the simple surface modification technique of lithium transition metal oxide cathode materials for enhancing the cyclability and thermal stability. In chapter 2.1, the facile surface modification of transition-metal hydroxide precursors with ammonium dihydrogen phosphate was performed by ball-milling before the calcination process. The prepared precursors were mixed with the required amount of lithium hydroxide and then simply calcined to obtain lithium phosphate-coated lithium transition metal oxide cathodes during the one-pot calcination process. A thin, homogeneous Li3PO4 coating is firstly formed on the surface of the precursor owing to the abundance of lithium at a lower-temperature range, and subsequent formation of lithium transition metal oxide is achieved at a higher-temperature range during the calcination process. The Li3PO4-coated cathode electrode with the high loading level over 12 mg cm-1 exhibits a discharge capacity of 106 mA h g−1 at 5C at ambient temperature. Furthermore, it delivers 90% capacity retention after 50 cycles at 60 oC. In chapter 2.2, Ni-rich transition metal hydroxide precursors comprising a Ni-rich core and Ni-less surface region are successfully prepared in this study by a simple treatment process with dilute sulfuric acid. The final cathode materials have a compositional core-shell design, taking advantage of the stable cyclability and high thermal stability of the Ni-less surface layer as well as the high capacity of the Ni-rich core. The cycling stability of this Ni-rich cathode significantly improves after leaching, showing a capacity retention of 82.3% after 150 cycles at a rate of 0.5 C and elevated temperature of 60 oC, much higher than that of a pristine Ni-rich cathode (65.4%). Furthermore, the thermal stability of the prepared Ni-rich cathode improves remarkably after leaching. These results suggest that the prepared cathode meets the energy storage demands of electric vehicles in terms of energy density, power, and cycling life; therefore, it is a promising cathode material for electric vehicle applications. In chapter 2.3, the thermal stability of a fully delithiated cathode, coupled with an inorganic salt, organic solvents, and electrolyte, was evaluated with differential scanning calorimetry (DSC) to address the safety issue of lithium-ion batteries. The DSC experiments revealed that vinylene carbonate (VC)-based systems exhibit decreased thermal stability compared to other coexisting systems. The DSC analysis of the cathode and electrolyte in the presence of VC, employed as additives, confirmed the reduction in thermal stability. Our results suggested that VC additives, which have been recently employed to enhance the electrochemical performance in anodes, negatively affect the thermal properties and safety characteristics of the cathode in lithium-ion batteries. The cost of lithium carbonate has risen steeply since commercialization of LIBs, with some reports predicting a shortage of the lithium source in the near future because of the mass renewable energy production requirements of EVs and energy storage systems. In this regard, Sodium-ion batteries (SIBs) are considered to be an alternative to LIBs because sodium exists abundantly in seawater, and its salt can be obtained at a significantly lower price compared with lithium salt. Unfortunately, graphite or silicon materials, which are considered promising anodes in LIBs, cannot be utilized in SIB systems, mostly due to sodium’s larger ionic radius and its lower reactivity with silicon materials compared to lithium. In this regard, alloy materials such as Sn and Sb, which is well known for the anode materials in LIB application, have attracted attention as anode materials for SIB applications due to their high specific capacity. Nevertheless, the huge volume changes during sodiation/desodiation of alloy materials inevitably trigger pulverization of the active material. This leads to rapid capacity fading and safety problems. To solve these obstacle and improve the electrochemical and thermal stability of cathode materials, many attempts have focused on the utilization of inactive materials as buffering matrix such as carbonaceous and inorganic materials to enhance the electrochemical and thermal stability of the alloy-based anode materials. In chapter 3, we report the facile and simple synthesis technique of alloy-based composite materials by employing various inactive materials as buffering matrix for enhancing the cyclability and thermal stability for SIBs. In chapter 3.1, the Sb-embedded silicon oxycarbide (SiOC) composites were simply synthesized via a one-pot pyrolysis process at 900 oC without any additives or surfactants, taking advantage of the superior self-dispersion properties of antimony acetate powders in silicone oil. The structural and morphological characterizations confirmed that Sb nanoparticles were homogeneously embedded into the amorphous SiOC matrix. The composite materials exhibited an initial desodiation capacity of around 510 mA h g-1 and maintained an excellent capacity retention above 97% after 250 cycles. The rate capability test revealed that the composites delivered capacity greater than 453 mA h g-1, even at the high current density of 20 C rate, owing to the free-carbon domain of SiOC material. The electrochemical and post-mortem analyses confirmed that the SiOC matrix with a uniform distribution of Sb nanoparticles provided the mechanical strength without degradation in conductive characteristics, suppressing the agglomeration of Sb particles during the electrochemical reaction. In chapter 3.2, Sb nanoparticles dispersed in a hybrid matrix consisting of aluminum (Al) and carbon, AlC0.75-C were synthesized via one-step high-energy mechanical milling (HEMM) process and assessed as potential anode materials for use in sodium-ion batteries. The introduction of carbon during HEMM led to the formation of individual Sb nanoparticles dispersed in the AlC0.75-C matrix; in the absence of carbon during HEMM, an AlSb alloy was formed. The Sb-AlC0.75-C composite anodes demonstrated better cycling performance as well as higher rate capability compared to an AlSb anode; these improved properties could be due to the well-developed Sb phase, which acts as an electrochemically active nanocrystalline material in the AlC0.75/carbon conductive matrix. Furthermore, when fluoroethylene carbonate (FEC) was added to the electrolyte, the sodium-ion cells exhibited the best electrochemical performances, corresponding to a capacity retention of 83% at 100 cycles at 100 mA g-1 and a high rate capacity retention of 58% at 5000 mA g-1. In addition, the as-prepared Sb-AlC0.75-C composite has a high tap density; thus, its volumetric capacity was approximately three times that of carbon. In chapter 3.3, the thermal behavior of fully lithiated and sodiated Sn electrodes cycled in a MePF6 (Me=Li or Na)-based electrolyte was studied using differential scanning calorimetry (DSC). The sodiated Sn electrode cycled in the NaPF6-based electrolyte showed a thermal reaction with much greater heat generation (1719.4 J g-1) during the first exothermic reaction corresponding to the thermal decomposition reaction of the solid electrolyte interface (SEI) layer, compared to that of the lithiated Sn electrode (647.7 J g-1) in the LiPF6-based electrolyte because of the formation of a thicker surface film on the Sn electrode. The NaPF6-based electrolyte yielded a slightly less conductive and/or thicker SEI layer than the NaClO4-based electrolyte, resulting in the intense thermal decomposition of the SEI layer. The DSC results for the fully sodiated Sn electrode cycled in FEC-containing electrolytes clearly demonstrate that an exothermic reaction corresponding to the SEI decomposition mostly disappears because of the formation of a thermally stable and thin SEI layer on active materials via electrochemical decomposition of FEC. X-ray photoelectron spectroscopy reveals the formation of SEI with a relatively high proportion of NaF, which is known to be a thermally stable inorganic solid at high temperature.

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목차

Abstract
Abstract in Korean
Contents
List of Tables
List of Figures
Chapter 1. Introduction 1
1.1 Lithium-ion Batteries 1
1.1.1 Principle of Operation 1
1.1.2 Cathode Materials 4
1.1.2.1 Li-rich Layered Oxides 4
1.1.2.2 Ni-rich Layered Oxides 7
1.1.3 Thermal Stability of Cathode Materials 9
1.1.4 Surface Modification Strategy 9
1.2 Sodium-ion Batteries 10
1.2.1 Sodium-ion Batteries 10
1.2.2 Anode Materials 13
1.2.2.1 Hard Carbon Anodes 13
1.2.2.2 Sodium Alloy Anodes 13
1.2.3 Composite Strategy with Inactive Materials 13
1.2.4 Thermal Stability of Alloy Anode Materials 14
Chapter 2. Retarding Interfacial Side-reaction of Layered Lithium Transition Metal Oxide by Surface Modification for High-voltage Lithium-ion Batteries 15
2.1 Suppression of the Undesirable Side Reactions at the Electrolyte/Lithium-rich layered Oxide Cathode Interfaces by Adopting a Facile Surface Modification Method with Li3PO4 for High-capacity Lithium-ion Batteries 15
2.1.1 Introduction 15
2.1.2 Experimental 17
2.1.3 Results and Discussion 19
2.1.4 Conclusions 46
2.2 The Formation of Cathode/Electrolyte Interface Layer by Selective Nickel Leaching on the Surface of Ni-rich layered Cathode for Advanced High-energy and Safe Rechargeable Batteries 47
2.2.1 Introduction 47
2.2.2 Experimental 49
2.2.3 Results and Discussion 51
2.2.4 Conclusions 72
2.3 Influence of Salt, Solvents, and Additives on the Thermal Reaction at Interface between Cathode/Electrolyte of Delithiated Ni-rich layered Cathodes in Lithium-ion Batteries 73
2.3.1 Introduction 73
2.3.2 Experimental 75
2.3.3 Results and Discussion 76
2.3.4 Conclusions 91
Chapter 3. Improving the Interfacial Stability of Alloy-based Anode Materials by Employing Inactive Matrix for Sodium-ion Batteries 92
3.1. Interface Chemistry Engineering of Sb Anode by Employing Silicon Oxycarbide Matrix for High-performance Sodium-ion Batteries 92
3.1.1. Introduction 92
3.1.2. Experimental 93
3.1.3. Results and Discussion 96
3.1.4. Conclusions 128
3.2. Interfacial Properties of the Sb-AlC0.75-C Composite Anodes for High-performance Sodium-ion Batteries 129
3.2.1. Introduction 129
3.2.2. Experimental 131
3.2.3. Results and Discussion 133
3.2.4. Conclusions 152
3.3. Thermal Reactions at Interface between High-Capacity Anode and Non-aqueous Electrolytes in Sodium-ion Batteries 153
3.3.1. Introduction 153
3.3.2. Experimental 154
3.3.3. Results and Discussion 155
3.3.4. Conclusions 169
Chapter 4. References 170
Appendix: List of Publications 189189

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