X-선 결정학 연구를 통한 새로운 이차전지 전극 소재 및 고체 전해질의 개발과 구조적 메커니즘 분석

다양한 기능성 무기 소재들의 특성은 기본적으로 결정 구조와 깊은 관계가 있다. 특히 전지 소재들의 경우 (음극, 양극 및 고체 전해질), 고체 내 양이온의 탈/삽입 및 확산에 기반하는 메커니즘을 통해 구동하기 때문에 해당 소재들의 특성은 구조적 요소들과 밀접한 관계를 갖는다. 따라서 소재의 결정 구조뿐만 아니라, 충/방전 중의 구조적 메커니즘을 이해하고, 최종적으로 구조 정보와 전지 특성 간의 연관 관계를 확립하는 것은 새로운 이차 전지 소재를 개발하는데 있어 기초적이면서도 필수적인 정보를 제공한다. 다양한 무기 소재 분석 법 중 X-선 회절 데이터를 활용한 결정구조 분석은 새로운 전극 및 고체 전해질 소재의 구조를 밝혀내고 작동 메커니즘을 해석하는데 가장 기본적이면서도 강력한 기술이다. 이 논문은 새로운 전극 및 고체 전해질 소재의 개발과, 단결정 및 분말X-선 결정학 분석을 통해 해당 물질들의 작동 메커니즘을 분석한 연구 결과 사례를 다루고 있다. 본 연구에서는 전지 특성과 구조 내 특정 전하 캐리어 이온의 탈/삽입 및 확산 메커니즘을 규명하기 위해 ab inito 구조 결정법, 3차원 bond-valence sum mapping 계산, 자체 제작한 in situ cell을 활용한 충/방전 중 실시간 구조 분석 및 다양한 전기 화학적 분석을 수행하였다. 특히, 일반적으로 단결정 데이터를 얻기 힘든 다양한 전지 소재들의 경우, 분발 X-선 회절 데이터를 사용하는 미지 결정 구조 분석법을 활용해 그 구조와 작동 메커니즘을 해석하였다. (1) 첫 번째 연구는 새로운 암모늄 바나늄 산화물 브론즈 계열의 (NH4)2V7O16를 최초로 합성하고 단결정 X-선 회절 데이터를 통해 그 구조를 결정한 것이다. 새롭게 발견하고 구조를 해석한 (NH4)2V7O16이 리튬 이온 전지와 소듐 이온 전지의 전극 소재로 작동 가능함을 전기화학적 충/방전 실험을 통해 검증하였다. 또한 in situ X-선 회절 데이터를 측정하여 충/방전 중 실시간 구조변화를 관찰하였다. 최종적으로 추가적인 X-선 광전자 분광법과 원소 분석을 통해 리튬 이온의 삽입 (방전) 시 층상 구조 사이에 있던 암모늄이 빠져나오고, 리튬 이온의 탈입 (충전) 시 전해액으로 빠져나왔던 암모늄 이온이 다시 들어가는 충/방전 메커니즘을 확인 하였다. 새로운 암모늄 바나듐 브론즈의 개발과 구조 및 전기화학적 특성에 대한 본 연구 결과는 전극 물질 뿐만 아니라 다양한 기능성 무기 소재로 주목 받고 있는 바나듐 산화물 브로즈 계열 물질들 연구에 대한 새로운 물질 정보를 제공했다는 점에서 학술적인 의의가 있다. (2) 두 번째 연구에서는 Fe2(MoO4)3에 리튬이 탈/삽입 될 때와 소듐이 탈/삽입 될 때의 구조적 메커니즘 차이를 규명하였다. Fe2(MoO4)3는 리튬 이온 전지와 소듐 이온 전지의 전극 소재로 모두 사용 가능하다. 비록 가용 용량이 ~90mAg g-1정도로 낮아 기존 상용화된 양극소재들에 비하면 부족한 성능을 보이지만 해당 물질은 리튬 이온 전지와 소듐 이온 전지에서의 작동 메커니즘이 상이해 최근 몇 년 사이 학계의 많은 관심을 받고 있다. 리튬 이온의 탈/삽입 시에는 single-phase 반응을, 소듐 이온의 경우에는 two-phase 반응을 나타낸다. 몇 연구 팀에서 본 탈/삽입 메커니즘 차이에 대한 원인을 분석하는 연구 결과를 보고하였지만 대부분 양자 계산법을 활용한 예측이었고 해당 결과도 결정학적으로 불가능한 구조를 제시한다는 점에서 신뢰성이 떨어졌다. 본 연구에서는 소듐 이온의 탈/삽입 시 구조 변화를 in situ 측정을 통해 확인하고, 소듐 삽입 후 Na2Fe2(MO4)3 의 구조를 분말 X-선을 통한 미지 구조 결정법으로 밝혀내었다. 해당 결과와 bond valence sum mapping 결과를 복합적으로 활용, 해당 물질에서의 리튬과 소듐 이온의 상이한 탈/삽입 메커니즘이 guest-ion 의 사이즈와 구조 내 탈/삽입 가능한 결정 구조 내 빈 공간 주변의 음이온 (산소 이온) 위치 및 거리와 밀접한 관계가 있음을 밝혀내었다. 결론적으로 본 연구는 다양한 전극 소재 및 고체 전해질에서의 탈/삽입 또는 이온 확산 메커니즘을 해석하고 예측하는 데 있어 X-선 회절 결정 분석의 활용법을 제시해준다. (3) 세 번째 연구는 새로운 소듐 이온 고체 전해질, Na4−xSn1−xSbxS4 (0.02 ≤ x ≤ 0.33) 에 대한 구조 분석과 작동 메커니즘 규명, 그리고 소듐 이온 전도성 측정과 전고체 전지의 고체 전해질로써 활용 가능성에 대한 결과이다. 본 연구는 한양대학교 정윤석 교수님 연구실과 공동으로 진행하였다. 소듐 이온에 대해 부도체인 Na4SnS4에 소량의 Sb 첨가만으로 결정 구조가 완전히 바뀌게 되고 전도성을 갖게 되는 것을 발견하였다. 분말 X-선 회절 데이터를 통한 미지 결정 구조 분석법을 활용해 새로운 고체 전해질Na4−xSn1−xSbxS4의 구조를 결정하였다. 밝혀진 해당 구조 정보와 bond valence sum mapping 을 통해 해당 물질이 소듐 이온에 대해 전도성을 갖는 이유와 구조 내 이온 전도 경로를 규명하였다. 새로운 고체 전해질의 개발과 구조해석을 통한 작동 메커니즘에 대한 명확한 분석 결과는 전고체 전지 기술 연구에 새로운 관점을 제공한다. (4) 네 번째 연구는 차세대 전지 중 하나인 포타슘 이온 전지의 양극 소재에 대한 것이다. rhombohedral prussian blue 계열의 K1.88Zn2.88[Fe(CN)6]2(H2O)5 는 구조 내 커다란 cavity-site에 포타슘 이온이 적절히 위치하고 있는 물질로, 포타슘 이온의 탈/삽입에 유리한 구조적 특성을 갖추고 있다. 이에 근거하여 해당 물질을 포타슘 이온 전지 소재로 활용한 결과 3.9 V 의 높은 구동 전압과 함께 안정적인 충/방전 성능을 나타내었다. 상대적으로 커다란 사이즈를 갖고 있는 포타슘 이온의 탈/삽입에도 불구하고 약 3% 정도의 미미한 단위 셀 부피 변화가 일어남을 in situ X-선 회절 분석을 통해 확인 할 수 있었고 이런 구조적 특성은 안정적인 충/방전 성능의 주된 요인으로 여겨진다. 또한 다양한 SOC 상태의 K1.88Zn2.88[Fe(CN)6]2(H2O)5 (0 ≤ x ≤ 1.88) 분말 X-선 회절 데이터를 기반으로 얻은 Fourier 전자 밀도를 통해 포타슘 이온의 탈/삽입 메커니즘을 원자 분석 수준에서 확인하였다. 본 연구는 포타슘 이온 전지 전극 소재의 충/방전 안정성을 결정하는 구조적 요소에 대한 유용한 통찰력을 제공한다. (5) 마지막 연구는 위와 마찬가지로 차세대 전지 중 하나인 칼슘 이온 전지의 양극 소재에 대한 것이다. 칼슘 이온 전지는 다가 이온 전지 중 하나로 현재 정상적으로 구동 가능하다고 보고된 양극 및 음극 소재의 수가 절대적으로 부족한 굉장히 도적적인 연구 분야 중 하나이다. 본 연구에서는 NASICON 구조를 갖고 있으며 대표적인 소듐 이온 전지 양극 소재인 Na3V2(PO4)3 에서 화학적으로 Na을 일부 제거한 NaV2(PO4)3에 가역적인 칼슘 이온 탈/삽입이 가능하다는 것을 확인하였다. CaI2를 환원제로 사용, 해당 물질에 화학적으로 칼슘을 삽입시켜 성공적으로 CaxNaV2(PO4)3를 합성하였다. 분말 X-선 회절 데이터를 기반으로 얻은 Fourier 전자 밀도 분석을 통해 CaxNaV2(PO4)3의 구조를 해석하였고, 이 결과와 추가적인 원소 분석 결과를 통해 해당 NASICON 구조에 Ca이 삽입되어 있음을 검증하였다. 화학적으로 칼슘이 삽입되는 동안의 구조 변화를 in situ X-선 회절 분석을 통해 실시간으로 관찰한 결과, two-phase 반응에 기반해 칼슘이 삽입됨을 확인하였다. 구조적으로 칼슘 이온의 삽입이 가능하다는 위 결과에 근거해 NaV2(PO4)3 을 유기 전해액을 사용한 칼슘 이온의 양극 소재로 테스트 해본 결과, 고온 (75℃) 에서 초기 5 cycle의 활성화 거친 뒤, 상온에서 가역적인 충/방전 용량 (~116 mAg g-1) 을 확인 할 수 있었다. 본 연구 결과는 굉장히 도전적인 주제로 인식되고 있는 칼슘 이온 전지의 양극 소재 개발에 새로운 가능성을 제시한다. 고차원적인 X-선 회절 분석을 통해, 특정 이온의 탈/삽입으로 구동되는 이차 전지 시스템의 전극 소재 및 고체 전해질을 발견하고 구조적 작동 메커니즘을 규명한 본 논문은 결정 구조와 밀접한 관계가 있는 무기 에너지 소재들의 특성을 이해하고, 이를 바탕으로 새로운 소재 개발 및 성능 개선을 위한 근본적이면서도 필수적인 결정학적 통찰을 제공한다. |Structural information is crucial for creating new functional materials and for understanding and adjusting the properties of materials. Among the various functional materials, the properties of battery materials (cathode, anode and solid electrolyte) are particularly closely related to its structure, because the operation of rechargeable batteries is based on the intercalation and diffusion of charge-carrier ions in solids. Therefore, understanding the structural mechanism during charge/discharge and establishing the structure-properties relationship are basic priorities to improve existing or develop novel battery materials. Crystallographic analysis of X-ray diffraction data associated with electrochemical tests provides the most essential information to bridge the structural characteristics with battery performance. This dissertation discusses the development of new electrode materials and solid electrolytes, and understating their working mechanism through ab initio structure determination via powder X-ray diffraction data as well as single-crystal data. In order to unveil the working mechanism of each material covered in here, 3D bond-valence sum difference map calculation, in situ analysis technique, and various additional chemical (electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared) and electrochemical analyses (galvanostatic test, cyclic voltammetry, kinetic analysis) were conducted. (1) The first section is a discovery of new ammonium vanadium bronze as an electrode material for rechargeable batteries by using single-crystal X-ray diffraction data. The novel ammonium vanadium bronze, (NH4)2V7O6 has been synthesized via a facile hydrothermal method and its structure in the triclinic space group P1 ̅ is determined by single-crystal data. A reversible electrochemical intercalation of Li ions into (NH4)2V7O6 has been evaluated by galvanostatic cycling. It also revealed that the NH4+ accommodated between V–O layer plays a very crucial role in the electrochemical characteristics. On discharge, Li ions are intercalated while the NH4+ is simultaneously de-intercalated from the structure. On charge, a reverse reaction takes placea small amount of NH4+ re-accommodates between the layers, ensuring structural stability and allowing all Li ions to extract. In addition, the electrochemical results suggesting the feasibility of (NH4)2V7O6 as an electrode material for Na-ion batteries are briefly presented. (2) The second study provides a crucial crystallographic rationale to understand the difference in the intercalation mechanism of Fe2(MoO4)3 between lithiation and sodiation by structural analyses using powder X-ray diffraction data. Monoclinic Fe2(MoO4)3 shows distinct structural and electrochemical differences in the intercalation mechanism, depending on the guest ion: a single-phase reaction in a Na-ion cell, but a two-phase reaction in a Li-ion cell. Attempts to understand the difference in the mechanisMaster have been hindered by a lack of structural information on the fully sodiated phase Na2Fe2(MoO4)3 due to its structural complexity and the unavailability of a single crystal. In this work, the crystal structure of Na2Fe2(MoO4)3 have been solved and refined for the first time, using the technique of ab initio structure determination from powder diffraction data. Along with electrochemical and structural characterization, 3D bond valence sum difference map calculations enabled us to ascertain the decisive factors that determine such differences, in terMaster of the interatomic distance and coordination environment of a guest ion. In the case of Na insertion, only a slight expansion of the structure makes the cavity sites of Fe2(MoO4)3 suitable for Na ions, with adequate distances and coordination with surrounding oxygen atoMaster, resulting in a solid-solution-type single-phase reaction. In the case of Li insertion, the cavity sites are so large for a Li ion that a significant structural change involving tilting of the FeO6 and MoO4 polyhedra is required to accommodate the Li ion in a suitable local environment, which does not allow a continuous structural change but results in a two-phase reaction. (3) The following study introduces a novel structure sulfide Na‐ion solid electrolyte, and unveil the sodium diffusion mechanism by using ab initio structure determination process via powder X-ray diffraction data. Sulfide Na-ion solid electrolytes are key to enable room‐temperature operable all‐solid‐state Na‐ion batteries that are attractive for large‐scale energy storage applications. Herein, the discovery of a new structural class of tetragonal Na4−xSn1−xSbxS4 (0.02 ≤ x ≤ 0.33) with space group I41/acd is described. The evolution of a new phase, distinctly different from Na4SnS4 or Na3SbS4, allows fast ionic conduction in 3D pathways (0.2–0.5 Master cm−1 at 30 °C). In the fourth, and final sessions present the development of a new cathode material for next-generation battery candidates, K- and Ca-ion batteries, and the investigation of their working mechanism via structural analysis. (4) K-ion batteries have received attention as an alternative to Li-based batteries due to the earth abundance and low redox potential of potassium metal. Herein, rhombohedral potassium zinc hexacyanoferrate K1.88Zn2.88[Fe(CN)6]2(H2O)5 is demonstrated as a cathode material with a high-voltage and favorable cyclability for non-aqueous K-ion batteries: an initial discharge capacity of 55.6 mAh g−1, an operating voltage of 3.9 V (vs. K/K+), and a capacity retention of ~95% after 100 cycles. The Fourier maps from powder X-ray diffraction data and elemental analysis at each different state-of-charge clearly demonstrate that the potassium-ions intercalation. The operando XRD technique reveal a reversible structural evolution during cycle. The results provide a general insight into the development of electrode materials for potassium-ion batteries. (5) Among the various multivalent ions, Ca has recently begun to receive attention for rechargeable batteries due to its lower redox potential than the other candidate multivalent charge-carrier ions (Mg2+, Zn2+, Al3+). Herein, the feasibility of NASICON-structured NaV2(PO4)3 as a cathode material for Ca-ion batteries is demonstrated. A crystal structure of CaxNaV2(PO4)3 prepared by chemical Ca insertion was identified with elemental analysis, in-situ and Fourier electron densities from powder X-ray diffraction data, suggesting that the NaV2(PO4)3 has a suitable structure as a host for Ca ion intercalation. In our electrochemical test, Ca ions are reversibly intercalated into the NaV2(PO4)3 using an electrolyte with 0.5M Ca(BF4)2 in EC/PC (1:1 v/v). After a few initial cycles for activation at 75℃, NaV2(PO4)3 shows the reversible cycle performance with a discharge capacity of 117 mAg g-1 and favorable capacity retention (99 mAh g-1 after 50 cycles) in room temperature (25℃). It is the first report on using NASICON-structured NaV2(PO4) as a cathode material for Ca batteries.openAbstract i List of contents iv List of tables ix List of figures xiv Ⅰ. INTRODUCTION 1 Ⅱ. THEORY 2.1 Crystallography 11 2.1.1 X-ray Diffraction and Crystal Structure 11 2.1.2 Ab initio Structure Determination via Powder XRD data 13 2.1.2.1 Unit cell Determination (Indexing) 15 2.1.2.2 Space group Determination 15 2.1.2.3 Extraction of Diffraction Intensities from the Powder XRD data 15 2.1.2.4 Fourier synthesis electron density and phase problem 16 2.1.2.5 Structure Solution AlgorithMaster 18 2.1.2.6 Difference Fourier Synthesis 19 2.1.2.7 Rietveld refinement 20 2.2 Bond Valence Sum Map 21 2.2.1 Bond Valence 21 2.2.2 Bond Valence Sum 22 2.2.3 Three-Dimensional Bond Valence Sum Mapping 22 2.3 Electrochemistry 23 2.3.1 Electromotive Force 23 2.3.2 Electrode Potential 23 2.3.3 Energy Storage 24 2.3.4 Types of Battery 25 2.4 References 25 Ⅲ. New Class Structure of Ammonium Vanadium Bronze, (NH4)2V7O16 as an Electrode Material for Rechargeable Batteries 3.1 Introduction 28 3.2 Experimental 29 3.2.1 Materials Synthesis 29 3.2.2 Materials Characterization 29 3.2.3 Single-crystal XRD and Structure Determination 29 3.2.4 Electrochemical Characterization 31 3.3 Results and discussion 32 3.3.1 Characterization of (NH4)2V7O16 32 3.3.2 Electrochemical Characterization of (NH4)2V7O16 38 3.4 Conclusions 44 3.5 References 45 Ⅳ. Unveiling the Intercalation Mechanism in Fe2(MoO4)3 as a Cathode Material for Na-Ion Batteries by Structural Determination 4.1 Introduction 51 4.2 Experimental 53 4.2.1 Synthesis 53 4.2.2 Material Characterization 53 4.2.3 Electrochemical Characterization 53 4.2.4 Structural Characterization 55 4.2.5 Structural Determination 56 4.2.6 Bond Valence Sum Maps 56 4.3 Results and discussion 57 4.3.1 Characterization of the Synthesized FMO 57 4.3.2 Electrochemical Preparation of Na2FMO 64 4.3.3 Evolution of Crystal Structure during cycle 64 4.3.4 Structural Determination of Na2FMO 66 4.3.5 XPS Results 81 4.3.6 Na Diffusion Pathways Calculated from BVS-DMaster 83 4.3.7 The Difference between Li and Na Intercalation MechanisMaster 83 4.4 Conclusions 85 4.5 References 86 Ⅴ. New Na superionic conductors Na4-xSn1-xSbxS4 (0.02 ≤ x ≤ 0.33) for all-solid-state Na-ion batteries 5.1 Introduction 90 5.2 Experimental 91 5.2.1 Preparation of Materials 91 5.2.2 Materials Characterization 92 5.2.3 Structure Determination 92 5.2.4 Electrochemical Characterization 93 5.3 Results and discussion 94 5.3.1 Characterization of Na4-xSn1-xSbxS4 (0.02 ≤ x ≤ 0.33) 94 5.3.2 Structural Determination and Ionic Conduction Mechanism 98 5.4 Conclusions 109 5.5 References 110 Ⅵ. Rhombohedral Potassium Zinc Hexacyanoferrate as Cathode Material for Nonaqueous Potassium-Ion Batteries 6.1 Introduction 113 6.2 Experimental 116 6.2.1 Synthesis and electrochemical characterization 116 6.2.2 Structural analysis 117 6.3 Results and discussion 118 6.3.1 Synthesis of potassium zinc hexacyanoferrate 118 6.3.2 Electrochemical characterization 122 6.3.3 Crystal structure 127 6.4 Conclusions 135 6.5 References 135 Ⅶ. New Ca-ion Intercalation Chemistry of NASICON Structure NaV2(PO4)3 for a Cathode Material of Ca-ion Batteries 7.1. Introduction 141 7.2 Experimental 142 7.2.1 Materials Synthesis 142 7.2.2 Materials Characterization 142 7.2.3 Chemical-Calciation 143 7.2.4 Electrochemical Ca-ion Intercalation 144 7.2.5 Structure Determination 145 7.3 Results and discussion 146 7.3.1 Chemical-Calciation Structural Analysis 146 7.3.2 Electrochemical Characterization 151 7.4 Conclusions 152 7.5 References 153 Summary (in Korean) 155DOCTORdCollectio

Unveiling the Intercalation Mechanism in Fe2(MoO4)3 as an Electrode Material for Na-Ion Batteries by Structural Determination

Monoclinic Fe2(MoO4)3 (FMO) shows distinct structural and electrochemical differences in the intercalation mechanism, depending on the guest ion.(1,2)FMO undergoes a single-phase reaction in a Na-ion cell, but a two-phase reaction in a Li-ion cell. Attempts to understand the difference in the mechanisms have been hindered by a lack of structural information on the fully sodiated phase Na2Fe2(MoO4)3 due to its structural complexity and the unavailability of a single crystal. In this work, we have solved and refined the crystal structure of Na2Fe2(MoO4)3 for the first time, using the technique of ab initio structure determination from powder diffraction data. Along with electrochemical and structural characterization, 3D bond valence sum difference map calculations enabled us to ascertain the decisive factors that determine such differences, in terms of the interatomic distance and coordination environment of a guest ion. In the case of Na insertion, only a slight expansion of the structure makes the cavity sites of FMO suitable for Na ions, with adequate distances and coordination with surrounding oxygen atoms, resulting in a solid-solution-type single-phase reaction. In the case of Li insertion, the cavity sites are so large for a Li ion that a significant structural change involving tilting of the FeO6 and MoO4 polyhedra is required to accommodate the Li ion in a suitable local environment, which does not allow a continuous structural change but results in a two-phase reaction. © 2018 American Chemical Society.1

High Potassium Storage Capability of H 2

Potassium-ion batteries (KIBs) are one of the potential candidates for large-scale energy storage devices with low cost due to the abundance of potassium resources. However, the development of cathode materials with high capacity and structural stability has been a challenge due to the difficulties of intercalation of the large size of K-ions into host materials. In this work, H2V3O8 (or V3O7⋅H2O) is reported as a new cathode material for KIBs. It shows reversible potassium-intercalation behavior with the first discharge capacity of 168 mAh g−1 at 5 mA g−1 and an average discharge voltage of ∼2.5 V (vs. K/K+) in 0.5 M KPF6 in EC/DEC (1:1 v/v). The specific capacity increases up to 181 mAh g−1 for the third cycle and gradually decreases with 75% of the capacity retention after 100 cycles. The chemical formula of the potassiated phase is K1.77H2V3O8. However, scan-rate dependent cyclic voltammetry and elemental analyses suggest that ∼28% of the capacity comes from the surface K ions on the H2V3O8 particles; thus, the bulk-intercalated phase can be formulated as K1.27H2V3O8. The crystal structure is stable during the electrochemical cycling, keeping the structural water, confirming that H2V3O8 can be considered as one of the high-capacity cathode materials for KIBs. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim1

Silver vanadium bronze, beta-Ag0.33V2O5: crystal-water-free high-capacity cathode material for rechargeable Ca-ion batteries

Calcium-ion batteries (CIBs) are getting increasing attention as post-lithium-ion batteries owing to their theoretical and potential advantages in terms of energy density and cost-effectiveness. However, most of the reported cathode materials suffer from low capacity or cyclability in dried nonaqueous electrolytes. So far, all of the materials with high capacity (>100 mA h g−1) contain crystal water, which was considered to be crucial to the structural stability, enabling facile Ca diffusion. Here, we report β-Ag0.33V2O5as a high-capacity cathode material for CIBs without crystal water. After the initial activation process, the material exhibited a capacity of 179 mA h g−1at approximately 2.8 V (vs.Ca2+/Ca) in the ninth cycle and showed a modest cycling performance. The capacity is the highest among the Ca cathode materials without crystal water reported to date. We revealed that the activation process was caused by a replacement reaction between the silver and calcium ions. This material demonstrates that crystal water is not an essential component of CIB electrode materials for a high capacity, stimulating the ongoing research for developing higher-performance materials. © The Royal Society of Chemistry 2021.FALS

Ammonium Vanadium Bronze, (NH4)2V7O16, as a New Lithium Intercalation Host Material

A new type of ammonium vanadium bronze, (NH4)2V7O16, was synthesized by the hydrothermal method. The triclinic crystal structure (P1¯) is successfully identified by the single-crystal X-ray diffraction method. The layered structure is similar to that of other vanadium bronzes but with an unprecedented stoichiometry and crystal structure. The structure is composed of a stack of V7O16 layers along the c axis, and two NH4 + ions occupy the interlayer space per formula unit. Each ammonium ion is hydrogen-bonded to four lattice oxygen atoms, resulting in a stable structure with a large interlayer space, thus enabling the intercalation of various guest ions. Lithium ions are electrochemically intercalated into (NH4)2V7O16, with an initial discharge capacity of 232 mAh g-1 and an average discharge voltage of 2 V (vs Li/Li+). Upon the first discharge, lithium ions are inserted, whereas ammonium ions are extracted. Upon charging, a reverse reaction takes place. However, only a fraction of the extracted ammonium ions are reaccommodated. Despite the small quantity, the reinsertion of ammonium ions contributes crucially to the structural stability, improving the electrochemical performance. These results could provide a general understanding of the intercalation mechanism of host materials containing ammonium ions. In addition, (NH4)2V7O16 intercalates Na+ ions reversibly, implying a potential capability as a host material for other guest ions. © 2020 American Chemical Society.1

Double-Sheet Vanadium Oxide as a Cathode Material for Calcium-Ion Batteries

Calcium-ion batteries (CIBs) are theoretically considered one of the potential post-lithium-ion battery technologies. However, only a few host materials are known to intercalate Ca ions. Herein, we demonstrate the use of double-sheet vanadium oxide V2O5 ⋅ 0.63H2O as a high-performance cathode material for CIBs. The vanadium oxide was synthesized via an electrochemical oxidation process on a graphite foil substrate. The material exhibited a high reversible capacity of 204 mAh g−1 at a 0.1 C rate in an aqueous electrolyte, with an average discharge voltage of 2.76 V vs. Ca/Ca2+, and a capacity retention of 86% after 350 cycles. The reaction mechanism can be described as a combination of diffusion-controlled intercalation and surface-limited pseudo-capacitance reactions. This study provides a new type of Ca host material, motivating further development of new CIB cathode materials. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim1

Electrochemical Exchange Reaction Mechanism and the Role of Additive Water to Stabilize the Structure of VOPO 4 ⋅2 H 2 O as a Cathode Material for Potassium-Ion Batteries

VOPO 4 ⋅2 H 2 O is demonstrated as a cathode material for potassium-ion batteries in 0.6 m KPF 6 in ethylene carbonate/diethyl carbonate, and its distinct exchange reaction mechanism between potassium and crystal water is reported. In an anhydrous electrolyte, the cathode shows an initial capacity of approximately 90 mAh g −1 , with poor capacity retention (32 % after 50 cycles). In contrast, the capacity retention dramatically improved (86 % after 100 cycles) in a wet electrolyte containing 0.1 m of additive water. VOPO 4 ⋅2 H 2 O contains two types of water (structural and crystal). Upon discharge, potassium ions are intercalated whereas the crystal water is simultaneously de-intercalated from the structure. Upon charging, a completely reverse reaction takes place in the wet electrolyte, resulting in high stability of the host structure and excellent cyclability. However, in the anhydrous electrolyte, some portion of the extracted crystal water molecules cannot be reinserted into the host structure because they are distributed over the anhydrous electrolyte. Keeping some concentration of water in the electrolyte turns out to be was the key to achieving such high reversibility. The potassium ions (90 %) and proton or hydronium ions (10 %) seem to be co-intercalated in the wet electrolyte. This work provides a general insight into the intercalation mechanism of crystal-water-containing host materials. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim1

Rhombohedral Potassium-Zinc Hexacyanoferrate as a Cathode Material for Nonaqueous Potassium-Ion Batteries

Rhombohedral potassium-zinc hexacyanoferrate K 1.88 Zn 2.88 [Fe(CN) 6 ] 2 (H 2 O) 5 (KZnHCF) synthesized using a precipitation method is demonstrated as a high-voltage cathode material for potassium-ion batteries (PIBs), exhibiting an initial discharge capacity of 55.6 mAh g -1 with a discharge voltage of 3.9 V versus K/K + and a capacity retention of 95% after 100 cycles in a nonaqueous electrolyte. All K ions are extracted from the structure upon the initial charge process. However, only 1.61 out of 1.88 K ions per formula unit are inserted back into the structure upon discharge, and it becomes the reversible ion of the second cycle onward. Despite the large ionic size of K, the material exhibits a lattice-volume change (3%) during a cycle, which is exceptionally small among the cathode materials for PIBs. The distinct feature of the material seems to come from the unique porous framework structure built by ZnN 4 and FeC 6 polyhedra linked via the CN bond and a Zn/Fe atomic ratio of 3/2, resulting in high structural stability and cycle performance. © 2019 American Chemical Society.1

Electrochemical lithium intercalation chemistry of condensed molybdenum metal cluster oxide: LiMo4O6

The electrochemical lithium-ion intercalation properties of molybdenum metal-cluster oxide LixMo4O6 (0.33 ≤ x ≤ 1.0) in an organic electrolyte of 1.0 M LiPF6 in ethylene carbonate/dimethyl carbonate (1:2 v/v) were characterized for the first time. Li0.66Mo4O6 (tetragonal, P4/mbm, a = 9.5914(3) Å, c = 2.8798(1) Å, V = 264.927(15) Å3, Z = 2) was prepared via ion-exchange of indium and lithium ions from InMo4O6 (tetragonal, P4/mbm, a = 9.66610(4) Å, c = 2.86507(2) Å, V = 267.694(2) Å3, Z = 2), which was first synthesized from a stoichiometric mixture of In, Mo, and MoO3 via a solid-state reaction for 11 h at 1100 °C. Then, Li0.33Mo4O6 was obtained via electrochemical charge of the electrode at 3.4 V vs. Li. The electrochemical lithium-ion insertion into Li0.33Mo4O6 occurs stepwise: three separate peaks were observed in the cyclic voltammogram and three quasi-plateaus in the galvanostatic profile, indicating a complicated intercalation mechanism. However, examination of the structural evolution of LixMo4O6 during the electrochemical cycle indicated a reversible reaction over the measured voltage range (2.0–3.2 V) and x range (0.33 ≤ x ≤ 1.00). Despite the excellent electrochemical reversibility, LixMo4O6 showed poor rate performance with a low capacity of 36.3 mAh g−1 at a rate of 0.05 C. Nonetheless, this work demonstrates a new structural class of lithium cathode materials with condensed metal clusters and 1D tunnels, and provides a host material candidate for multivalent-ion batteries. © 2017 Elsevier Inc.

Electrochemical Zinc-Ion Intercalation Properties and Crystal Structures of ZnMo6S8 and Zn2Mo6S8 Chevrel Phases in Aqueous Electrolytes

The crystal structures and electrochemical properties of ZnxMo6S8 Chevrel phases (x = 1, 2) prepared via electrochemical Zn2+-ion intercalation into the Mo6S8 host material, in an aqueous electrolyte, were characterized. Mo6S8 [trigonal, R3, a = 9.1910(6) Å, c = 10.8785(10) Å, Z = 3] was first prepared via the chemical extraction of Cu ions from Cu2Mo6S8, which was synthesized via a solid-state reaction for 24 h at 1000 °C. The electrochemical zinc-ion insertion into Mo6S8 occurred stepwise, and two separate potential regions were depicted in the cyclic voltammogram (CV) and galvanostatic profile. ZnMo6S8 first formed from Mo6S8 in the higher-voltage region around 0.45-0.50 V in the CV, through a pseudo two-phase reaction. The inserted zinc ions occupied the interstitial sites in cavities surrounded by sulfur atoms (Zn1 sites). A significant number of the inserted zinc ions were trapped in these Zn1 sites, giving rise to the first-cycle irreversible capacity of ∼46 mAh g-1 out of the discharge capacity of 134 mAh g-1 at a rate of 0.05 C. In the lower-voltage region, further insertion occurred to form Zn2Mo6S8 at around 0.35 V in the CV, also involving a two-phase reaction. The electrochemical insertion and extraction into the Zn2 sites appeared to be relatively reversible and fast. The crystal structures of Mo6S8, ZnMo6S8, and Zn2Mo6S8 were refined using X-ray Rietveld refinement techniques, while the new structure of Zn2Mo6S8 was determined for the first time in this study using the technique of structure determination from powder X-ray diffraction data. With the zinc ions inserted into Mo6S8 forming Zn2Mo6S8, the cell volume and a parameter increased by 5.3% and 5.9%, respectively, but the c parameter decreased by 6.0%. The average Mo-Mo distance in the Mo6 cluster decreased from 2.81 to 2.62 Å. © 2016 American Chemical Society.