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Title Page 2

Abstract 5

Contents 7

Authorship Attribution Statement 13

Ⅰ. Research Background 14

1.1. Introduction of lithium-ion batteries (LIBs) 14

1.2. Prussian Blue Analogues (PBAs) 31

1.3. References 39

Ⅱ. Prussian Blue Coated Separator to Prevent Crossover of Transition Metal Ions 46

2.1. Introduction 46

2.2. Experimental methods 48

2.2.1. Preparation of Prussian Blue coated separator (PB @ separator) 48

2.2.2. Physical and chemical analysis of PB @ separator 48

2.2.3. Electrochemical analysis 48

2.3. Results and discussion 50

2.3.1. Synthesis of PB @ separator 50

2.3.2. Electrolyte affinity of PB @ separator 51

2.3.3. Electrochemical analysis of PB @ separator 51

2.4. Conclusion 53

2.5. References 67

Ⅲ. Fast Lithium-ion diffusion in Prussian Blue for lithium metal hybrid anodes 69

3.1. Introduction 69

3.2. Experimental methods 71

3.2.1. Preparation of Prussian Blue coated graphite (GrPB) 71

3.2.2. Preparation of electrodes 71

3.2.3. Physical and chemical analysis of PB @ separator 71

3.2.4. Operando observation of anode by using optical microscopy 72

3.2.5. Electrochemical analysis 72

3.3. Results and discussion 73

3.3.1. Synthesis of PB coated graphite (GrPB) 73

3.3.2. Improving lithium diffusion within electrode 73

3.3.3. Electrochemical analysis under various conditions 74

3.3.4. Morphology of plated lithium within the GrPB 75

3.4. Conclusion 77

3.5. References 94

Ⅳ. Electrolyte Design for Prussian Blue in Lithium-ion Batteries 95

4.1. Introduction 95

4.2. Experimental methods 97

4.2.1. Synthesis of PBAs cathode materials 97

4.2.2. Preparation of electrodes 97

4.2.3. Electrochemical tests 97

4.2.4. Characterization 97

4.3. Results and discussion 99

4.3.1. CEI formation on PBAs 99

4.3.2. Electrolyte design for improving reaction kinetics 100

4.3.3. Mechanism studies depending on the solvation structure of lithium ions 101

4.4. Conclusion 104

4.5. References 119

Ⅴ. Conclusion 121

List of Tables 12

Chapter Ⅰ 12

Table 1.1. Formal potentials of PBAs on potassium ion insertion 36

Chapter Ⅳ 12

Table 4.1. Ionic conductivity of each electrolyte 113

List of Figures 8

Chapter Ⅰ 8

Figure 1.1. Scheme of severe global climate changes on earth 19

Figure 1.2. Diagram of the global demand for LIBs 20

Figure 1.3. Scheme of Lithium-ion battery system 21

Figure 1.4. Cell degradations in LIBs 22

Figure 1.5. Scheme of HF formation and transition metal ion dissolution mechanism 23

Figure 1.6. Intergranular and intragranular crack of Ni-rich transition metal oxide materials 24

Figure 1.7. Scheme of comparing reactions on the anode with/without transition metal component 25

Figure 1.8. Overview of reaction types of lithium-ion battery anodes and plot of specific energy vs... 26

Figure 1.9. Scheme of cell failure mechanism of high energy density lithium-ion batteries anode, a.... 27

Figure 1.10. Scheme of heterogeneity in different length scale 28

Figure 1.11. Lithium composition map ofindividual platelet particles and limitations of inter-and... 29

Figure 1.12. Scheme of reaction heterogeneity of graphite anode and strategy for reducing... 30

Figure 1.13. Scheme of structure of Prussian Blue analogues (PBAs) 34

Figure 1.14. Characteristics of PBAs that accommodate various types of ions. a. Cyclic voltammetry... 35

Figure 1.15. Diagram of PBAs architecture modification strategies 37

Figure 1.16. Scheme of effect of interstitial water within the PBAs to cation insertion 38

Chapter Ⅱ 9

Figure 2.1. Scheme of the NCMgraphiite full cell with bare separator and PB coated separator... 54

Figure 2.2. Scheme of synthesis of PB@separator 55

Figure 2.3. Contact angle analysis of bare separator and O₂ plasma treated separator with droplet of... 56

Figure 2.4. SEM images of PB@separator without and with O₂ plasma treatment 57

Figure 2.5. SEM and EDS analysis of bare separator and PB@separator. a. topview of bare... 58

Figure 2.6. XRD analysis of PB@separator and PB powder. Asterisked peaks of the PB@separator... 59

Figure 2.7. Electrolyte affinity tests of bare separator and PB@separator with organic electrolyte, a.... 60

Figure 2.8. Time-dependent electrolyte uptake test with bare separator and PB@separator 61

Figure 2.9. Full cell cycle test with both separator under harsh condition (0.5C-rate and 2.0-4.6V cut... 62

Figure 2.10. Rate performance test of full cells with bare separator and PB@separator 63

Figure 2.11. Lithium symmetric cell test with bare and PB@separator and SEM images after cycling 64

Figure 2.12. SEM EDS analysis and TOF-SIMS analysis of anode surface after full cell cycle under... 65

Figure 2.13. SEM EDS analysis of separator surface after full cell cycle under harsh condition 66

Chapter Ⅲ 9

Figure 3.1. Scheme of lithiation and artificial lithium plating on GrPB and graphite electrode 78

Figure 3.2. The one step process for synthesis of Prussian Blue coated graphite 79

Figure 3.3. TGA analysis of GrPB and PB powder 80

Figure 3.4. XRD analysis of PB and GrPB powder and high magnification TEM image of GrPB... 81

Figure 3.5. SEM images of graphite coated with PBAs (CoHCFe, MnHCFe, and NiHCFe) 82

Figure 3.6. Lithiation of Gr and GrPB electrode observed by operando optical microscopy and its... 83

Figure 3.7. Voltage profile of GrPB half-cell at initial two cycles under 2V cutoff 84

Figure 3.8. Voltage profile of graphite half cell and voltage profile of GrPB lithium metal hybrid... 85

Figure 3.9. Rate performance test of Gr and GrPB half-cell with lithium plating step under 2V cutoff 86

Figure 3.10. Long-term cycle ability and its voltage profile of Gr and GrPB electrode with artificial... 87

Figure 3.11. Long-term cyclability and voltage profiles of NCM622 full cell with N/P ratio of 0.67 88

Figure 3.12. Voltage profiles of Gr and GrPB half-cell at 0.1C-rate and rate capability test of Gr and... 89

Figure 3.13. SEM analysis of surface of Gr and GrPB after lithium plating cycles 90

Figure 3.14. SEM and EDS analysis of cross section view of Gr and GrPB electrode after artificial... 91

Figure 3.15. SEM and EDS analysis of cross-section view of graphite electrodes (weak calendering... 92

Figure 3.16. SEM and EDS analysis of cross-section view of weak calendering GrPB anode at... 93

Chapter Ⅳ 10

Figure 4.1. Scheme of the effect of electrolyte engineering for PBAs structure in LIBs 105

Figure 4.2. XRD and TEM analysis of Pristine state of PB. a. XRD of PB. b. low magnification TEM... 106

Figure 4.3. Scheme of formation of polymeric CEI layer on PBAs surface 107

Figure 4.4. Electron beam weakness of PBAs and polymeric layer, a. Pulverization of PB under HR-... 108

Figure 4.5. cryo-TEM image of PB in HCO after 100th cycle 109

Figure 4.6. Chemical characterization of CEI on PB in each electrolyte after cycles to form CEI layer.... 110

Figure 4.7. Electrochemical test of PB in each electrolyte and cryo-TEM images of PB in HCA (c and... 111

Figure 4.8. PB dissolution at low concentration electrolyte 112

Figure 4.9. Effect of electrolyte design to electrochemical performance of PB. a-c are differential... 114

Figure 4.10. Average working potential of PB in each electrolyte 115

Figure 4.11. Electrochemical analysis of NiHCFe in each electrolyte, a. Differential capacity analysis... 116

Figure 4.12. Electrochemical analysis of NiHCFe in 1m aqueous electrolytes (Li ion, Na ion, K ion)... 117

Figure 4.13. Voltage profile of NiHCFe in each electrolyte after each cycle 118

초록보기

 The increasing demand for energy storage systems (ESS) and electric vehicles (EV) has led to a surge of interest in lithium-ion batteries due to their high energy density and power density. Researchers have been focusing on improving various components of the lithium-ion batteries to achieve higher energy density with longer cycle life and low cost.

In the cathode part, high nickel transition metal oxides have shown potential as candidates for high energy density due to their high capacity and operating voltage. However, these materials suffer from transition metal elution and dissolution due to hydrogen fluoride attacks on the cathode, leading to severe capacity decay. Additionally, dissolved transition metal ions can migrate through commercial polyolefin-type separators and deposit on the anode surface, catalyzing the formation of lithium dendrites that can penetrate the separator and cause internal short circuits. Although several strategies, such as doping, surface coating, and single crystallization, have been proposed to address these issues, they have not been completely solved. In addition, due to the high prices of raw materials like Ni and Co used in transition metal oxides, there is extensive research focused on reducing their content and developing cheaper cathode materials. LFP, made from inexpensive iron, is gaining attention for its affordability.

In the anode part, graphite anode is typically used due to its stability and low operating potential. However, graphite has relatively low specific capacity, and lithium metal can plate on the surface of graphite. Silicon has been studied for the next generation of lithium-ion battery anodes because of its high specific capacity. However, severe volume expansion and continuous irreversible side reactions have not been completely solved. To address this, researchers have started mixing graphite with small amounts of silicon to achieve a higher specific capacity. Another candidate for high energy density anode material is the lithium metal anode, which has a several times higher specific capacity than graphite. However, significant challenges remain, including irreversible side reactions and the growth of lithium dendrites.

This study proposes the use of Prussian Blue (PB) and its analogues (PBAs) to enhance lithium-ion battery performance. PBAs are a type of metal-organic framework material composed of transition metals linked by cyanide ligands and it have primarily been researched as a cathode material for Na and K ion batteries. PBAs has large interstitial channels of 0.32㎚, which are sufficient for the insertion of multivalent ions. PBAs can be synthesized by the coprecipitation method and easily coated on other materials, making PB a cost-effective and easy to apply material for battery applications. Some papers reported that Prussian blue traps multivalent ions, and this characteristic works as a sieve in high nickel cathode batteries. In addition, the polar and lithiophilic characteristics of PB help lithium-ion movement at the PB-coated surface in lithium-ion batteries. Therefore, this study aims to investigate the potential of PB as a coating material and low-cost cathode material in lithium-ion batteries.

The first strategy is coating the polyolefin-type separator with a PB nanolayer. Despite its many merits, the polyolefin-type separator allows transition metal ions to travel to the cathode-to-anode side, causing lithium dendrite growth and internal short circuits. The PB nanolayer, homogeneously coated on the separator surface using a dip coating method, enhances the hydrophilicity of the separator, improving electrolyte uptake performance, and increasing the transference number of lithium ions, thus enhancing their movement and lifetime. As a result, the PB nanolayer effectively traps transition metal ions from the high nickel cathode and prevents lithium dendrite growth and internal short circuits.

The second strategy entails coating graphite particles with a PB nanolayer. This coating inhibits lithium dendrite formation on the anode surface and enhances lithium-ion mobility within the electrode. PB has demonstrated higher anodic capacity compared to graphite anodes, leading to increased energy density. Additionally, when the fully lithiated anode is overcharged, lithium deposits within the anode rather than on its surface, as seen with graphite anodes. This reduces lithium dendrite formation and enhances cell lifespan and safety.

The third strategy is designing an electrolyte to utilize Prussian Blue, a low-cost cathode material, for lithium-ion battery cathodes. By blending organic and aqueous electrolytes into a hybrid electrolyte, we altered the solvation structure to mitigate the slow ionic conductivity and sluggish kinetics oforganic electrolytes. Additionally, the ethylene carbonate(EC) ring opening formed a robust polymeric cathode electrolyte interphase(CEI) layer on the PB surface, preventing active material dissolution and reducing structure distortion, thus enhancing structural stability. The EC ring opening also removed the water content in the PB structure, that inhibits kinetics, thereby activating low-spin Fe and achieving higher capacity.

In conclusion, utilizing PB through the proposed strategies holds significant potential for enhancing the performance and safety of lithium-ion batteries. This study aims to investigate the use of cost-effective PB to reduce battery cell degradation and enhance the stability of battery operation, thereby contributing to the development of next-generation energy storage systems.

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