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전고체 리튬황 전지 복합 양극의 성능 개선

Title
전고체 리튬황 전지 복합 양극의 성능 개선
Other Titles
Performance Enhancement of Composite Cathode for All-Solid-State Lithium-Sulfur Battery
Author
엄민용
Alternative Author(s)
Eom, Min Yong
Advisor(s)
신동욱
Issue Date
2017-02
Publisher
한양대학교
Degree
Doctor
Abstract
리튬 이차전지는 휴대용 전자기기뿐만 아니라 자동차 및 대용량 전력 저장장치에 까지 다양한 산업분야에서의 수요가 지속적으로 증가하고 있고, 이러한 요구조건에 맞는 용량 및 에너지 밀도를 갖추기 위하여 국내외 연구 그룹에서 다양한 종류의 전극 물질을 활용하는 연구가 수행되고 있다. 다양한 전극 물질 중에서, 유황 전극은 높은 이론 용량 (1672 mAh∙g-1)을 기반으로 주목받고 있는 재료 중 하나이다. 이러한 유황 전극은 현재 몇 가지 문제점을 내재하고 있는데, 이는 다음과 같다. 첫째로 리튬이 함유된 음극 물질의 필요이다. 유황 전극은 다른 일반적인 양극 물질과 달리 활물질 구조 내에 리튬 이온이 존재하지 않게 되고, 전지 충/방전에 필요한 리튬을 음극물질에 요구하게 된다. 두 번째 문제점은 다황화물 용해 문제로 전지 충/방전 중 생성되는 다황화물의 일부가 액체전해질에 용해된 후, 음극 표면으로 이동하여 석출됨으로써 쿨롱효율의 감소 및 지속적인 용량감소의 원인으로 지목받고 있다. 따라서 이러한 문제점에 관한 개선이 반드시 필요하며, 본 논문에서는 이러한 문제점을 다음과 같은 방법을 통하여 개선하고자 하였다. Li2S는 리튬황 전지 시스템에서 양극 활물질인 유황 전극 내부에 리튬이온이 완전이 삽입된 상태로, 이러한 Li2S를 양극 활물질로 활용하게 된다면 다양한 음극물질을 적용함과 동시에 리튬황 전지의 용량을 온전히 활용할 수 있게 된다. 다음으로 다황화물 용해와 관련한 문제점을 해결하고자 다황화물 용해가 없는 Li2S-P2S5계 결정화 유리를 전해질 물질로 적용함으로써 문제를 극복하고자 하였다. 이렇게 활용될 Li2S-P2S5계 결정화 유리의 리튬 이온 전도도의 수치가 10-3 S∙cm-1 이상이 필요하였기 때문에 이를 위한 연구 또한 함께 수행하였다. 구체적으로 설명하자면, Chapter 2에서는 Li2S-P2S5계 고체 전해질의 결정화 거동을 분석하여 열처리 공정을 최적화 하였다. 결정화 거동 분석 결과, 170 oC 와 230 oC 에서 각각 최대 핵 생성 및 결정 성장 속도를 나타낸다는 사실을 확인할 수 있었다. 이러한 결정화 거동을 바탕으로 하여 열처리 공정을 최적화한 결과, 전해질의 리튬 이온 전도도가 약 1.9배 향상하는 결과를 얻을 수 있게 되었다. 하지만 이렇게 얻어진 리튬 이온 전도도가 10-3 S∙cm-1에는 다소 부족하였기 때문에, Chapter 3에서는 Li3BO3 물질을 Li2S-P2S5계 결정화 유리 내부에 첨가하여서 전도도의 향상을 모색해 보았다. Li3BO3가 첨가됨에 따라 전해질의 유리 형성 영역이 넓어지고, thio-LISICON II 유사상이 석출하는 것을 확인할 수 있었다. 97(0.78Li2S∙0.22P2S5)∙3Li3BO3 (x=3) 조성에서 가장 높은 리튬 이온 전도도인 1.03×10-3 S∙cm-1 의 전도도를 갖는 고체 전해질을 얻을 수 있었으며 결정 구조 분석 결과, 결정화 유리의 구조가 PS43- 유닛으로 구성되어 있으며, 일부분 부분적인 PO43- 유닛이 존재한다는 사실을 확인할 수 있었다. Chapter 4에서는 용해 후 석출 법을 활용하여서 Li2S-VGCF 나노 복합체를 제조하였고, 이를 통하여 리튬황 전지의 낮은 사이클 특성을 개선해보고자 하였다. 결론적으로 500도에서 열처리된 Li2S-VGCF 나노 복합체가 가장 높은 초기 용량인 469 mhA∙g-1 수준의 용량을 발현한다는 사실을 확인할 수 있었고, Li2S 양극 활물질이 전지 충/방전에 가담하기 위해서는 0.5 V의 과전압이 가해지는 초기 활성화 과정이 필요하다는 사실을 알 수 있었다. 8회 충/방전이 끝났을 때 모든 Li2S 양극 활물질이 활성화 되었고, 전지 용량은 지속적으로 상승하여 600 mhA∙g-1 수준으로 상승하여 20회까지 그 용량이 지속적으로 유지되며 쿨롱 효율이 100%에 가까운 성능을 발현하는 전고체 리튬황 전지를 제조할 수 있었다. Chapter 5에서는 전고체 리튬황 전지의 에너지 밀도를 극대화 하기 위하여 전고체 리튬황 전지에 활용되는 탄소의 양을 최소화 하고자 하였다. Polyacrylonitrile (PAN)을 활용하여 Li2S 표면에 탄소 코팅을 하였고, 양극 활물질의 전자 전도도가 2.39×10-2 S∙cm-1으로 급격하게 상승하는 효과를 확인할 수 있었다. 이렇게 탄소가 코팅된 양극 활물질을 활용하여 전고체 전지를 제조한 결과, 초기 용량이 585 mhA∙g-1 수준으로 나타나는 것으로 확인되었다. 10회의 활성화 과정을 완료하게 된 결과, 730 mhA∙g-1 수준의 용량이 25회까지 감소 없이 유지되는 전고체 리튬황 전지 성능을 확인할 수 있었다. 이러한 전고체 리튬황 전지는 기존에 다른 연구 그룹에서 연구된 전고체 리튬황 전지와 비교하여 복합양극의 용량이 중량당으로는 약 1.3배 향상이 되었으며, 부피당으로는 약 10배의 향상이 되었음을 확인할 수 있었다.|Recently, demands on increase of energy density have been increased to apply large-scale power tools such as electrical vehicles and energy storage systems. The elemental sulfur is one of attractive electrode materials for large-scale batteries due to its inherent high theoretical specific capacity of 1672 mAh∙g-1. Unfortunately, the elemental sulfur has some problems to apply cathode material such as requirement of lithium-containing counter electrode (non-lithium-containing cathode of S) and polysulfide dissolution in liquid electrolytes. Above-mentioned problems could be solved by following described applications. It is well known to that the Li2S is lithium-intercalated state of lithium-sulfur battery. In other words, the application of Li2S cathode material will be provide a using various counter electrodes. The sulfide based Li2S-P2S5 solid electrolyte has some advantages to apply lithium-sulfur battery system because it has high lithium ion conductivity of over 10-3 S∙cm-1 and completely prevents the polysulfide dissolution issue. According to my strategy, the Li2S-P2S5 glass-ceramics was used as electrolyte material. Moreover, the Li2S-P2S5 solid electrolyte exhibits better safety and reliability than liquid electrolyte due to inherent inflammability and better electrochemical stabilities. More specifically, in Chapter 2, Crystallization kinetics of 78Li2S∙22P2S5 glass was investigated by Differential Thermal Analysis, and the obtained results show that the maximum nucleation rate (I) and crystal growth rate (U) were achieved at 170 oC and 230 oC, respectively. The 78Li2S∙22P2S5 glass-ceramics with improved ionic conductivity was prepared by the optimized heat-treatment, which is consisted of nucleation stage at 170 oC for 30 min, and the following crystal growth stage at 230 oC for 3 h. Lithium ion conductivity was improved by 189 % by this control of crystallization kinetics. In Chapter 3, a lithium ion conductivity of 78Li2S∙22P2S5 glass-ceramics electrolyte was improved by doping of Li3BO3. It turned out that the doping of Li3BO3 enhanced the conductivity by enlarging the glass forming region and promoting precipitation of high lithium ion conductive thio-LISICON II analogue. The 97(0.78Li2S∙0.22P2S5) ∙3Li3BO3 (x=3) glass-ceramics exhibited the highest conductivity (1.03×10-3 S∙cm-1). Structural analysis shows that the samples with Li3BO3 added to the electrolyte were composed of the main structural unit of PS43- with partially modified structural unit of PO43-. In Chapter 4, The Li2S-VGCF (Vapor Grown Carbon Fiber) nanocomposite was prepared via using solution-based technique to address the poor cyclability of lithium-sulfur batteries. The nanocomposite was heat-treated at different temperature to crystallize the Li2S yielding particle sizes of 77 to 93 nm and intimate contact between the Li2S and VGCF. The highest initial capacity of 469 mhA∙g-1 was obtained at 500 °C heat-treatment. The activation of Li2S was observed within the first 8 cycles, and after, the capacity gradually increased up to 600 mAh∙g-1. The optimized cell exhibited excellent cyclic performance through 20 cycles and a coulombic efficiency of ~100 %. In Chapter 5, the low electron conductive Li2S was coated with carbon to improve energy density of cells by enhancement of electrical conductivity. The conductivity of carbon-coated Li2S was dramatically improved to 2.39×10-2 S∙cm-1. The carbon-coated all-solid-state cell shows high initial capacity of 585 mAh∙g-1, which capacity is gradually increased up to 730 mAh∙g-1 at 10th cycles and well maintained for 25th cycles with excellent coulombic efficiency of higher than 99 %. Consequently, the both of gravimetric and volumetric energy density were 1.3 times and 10 times improved than pioneering works, respectively.
Recently, demands on increase of energy density have been increased to apply large-scale power tools such as electrical vehicles and energy storage systems. The elemental sulfur is one of attractive electrode materials for large-scale batteries due to its inherent high theoretical specific capacity of 1672 mAh∙g-1. Unfortunately, the elemental sulfur has some problems to apply cathode material such as requirement of lithium-containing counter electrode (non-lithium-containing cathode of S) and polysulfide dissolution in liquid electrolytes. Above-mentioned problems could be solved by following described applications. It is well known to that the Li2S is lithium-intercalated state of lithium-sulfur battery. In other words, the application of Li2S cathode material will be provide a using various counter electrodes. The sulfide based Li2S-P2S5 solid electrolyte has some advantages to apply lithium-sulfur battery system because it has high lithium ion conductivity of over 10-3 S∙cm-1 and completely prevents the polysulfide dissolution issue. According to my strategy, the Li2S-P2S5 glass-ceramics was used as electrolyte material. Moreover, the Li2S-P2S5 solid electrolyte exhibits better safety and reliability than liquid electrolyte due to inherent inflammability and better electrochemical stabilities. More specifically, in Chapter 2, Crystallization kinetics of 78Li2S∙22P2S5 glass was investigated by Differential Thermal Analysis, and the obtained results show that the maximum nucleation rate (I) and crystal growth rate (U) were achieved at 170 oC and 230 oC, respectively. The 78Li2S∙22P2S5 glass-ceramics with improved ionic conductivity was prepared by the optimized heat-treatment, which is consisted of nucleation stage at 170 oC for 30 min, and the following crystal growth stage at 230 oC for 3 h. Lithium ion conductivity was improved by 189 % by this control of crystallization kinetics. In Chapter 3, a lithium ion conductivity of 78Li2S∙22P2S5 glass-ceramics electrolyte was improved by doping of Li3BO3. It turned out that the doping of Li3BO3 enhanced the conductivity by enlarging the glass forming region and promoting precipitation of high lithium ion conductive thio-LISICON II analogue. The 97(0.78Li2S∙0.22P2S5) ∙3Li3BO3 (x=3) glass-ceramics exhibited the highest conductivity (1.03×10-3 S∙cm-1). Structural analysis shows that the samples with Li3BO3 added to the electrolyte were composed of the main structural unit of PS43- with partially modified structural unit of PO43-. In Chapter 4, The Li2S-VGCF (Vapor Grown Carbon Fiber) nanocomposite was prepared via using solution-based technique to address the poor cyclability of lithium-sulfur batteries. The nanocomposite was heat-treated at different temperature to crystallize the Li2S yielding particle sizes of 77 to 93 nm and intimate contact between the Li2S and VGCF. The highest initial capacity of 469 mhA∙g-1 was obtained at 500 °C heat-treatment. The activation of Li2S was observed within the first 8 cycles, and after, the capacity gradually increased up to 600 mAh∙g-1. The optimized cell exhibited excellent cyclic performance through 20 cycles and a coulombic efficiency of ~100 %. In Chapter 5, the low electron conductive Li2S was coated with carbon to improve energy density of cells by enhancement of electrical conductivity. The conductivity of carbon-coated Li2S was dramatically improved to 2.39×10-2 S∙cm-1. The carbon-coated all-solid-state cell shows high initial capacity of 585 mAh∙g-1, which capacity is gradually increased up to 730 mAh∙g-1 at 10th cycles and well maintained for 25th cycles with excellent coulombic efficiency of higher than 99 %. Consequently, the both of gravimetric and volumetric energy density were 1.3 times and 10 times improved than pioneering works, respectively.
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http://dcollection.hanyang.ac.kr/jsp/common/DcLoOrgPer.jsp?sItemId=000000099323http://repository.hanyang.ac.kr/handle/20.500.11754/124983
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GRADUATE SCHOOL[S](대학원) > FUEL CELLS AND HYDROGEN TECHNOLOGY(수소·연료전지공학과) > Theses (Ph.D.)
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