(A) study on the improvement in electrochemical properties of Li-excess cathode material for Li-ion batteries = 도핑을 통한 리튬이차전지용 리튬 과잉 양극 소재의 전기화학적 특성 향상에 관한 연구

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The lithium-ion batteries is manly used to most portable electronic devices due to their ability to provide higher energy density compared to other energy storage systems. Recently, large scale applications of lithium secondary batteries, such as electric vehicles and energy storage system, have great interests with the demands on the sustainable energy technology. However, the currently used LIBs, such as $LiCoO_2$, $LiMn_2O_4$ and $LiFePO_4$, do not satisfy the required performance in terms of energy density, life time and cost for the large scale applications. To overcome this issue, the Li-excess layered cathode as a lithium-ion batteries cathode has received significant attention due to the need for high specific energy density. However, these cathode materials are associated with unwanted problems such as surface structural transformations from the layered to the spinel phase and irreversible oxygen redox reaction. To overcome above problems, in this study, various type of dopants have been adopted based on following strategies; 1) surface selective $S^{2-}$ anion doping in the Li-excess layered cathode material to generate the electrochemically stable spinel phase of $Li_4Mn_5O_{12}$, and enhance the cycle performance, 2) substitution of transition metal ions with the $V^{5+}$ cation at the surface of the Li-excess layered cathode to suppress the irreversible oxygen release which deteriorates the structural stability and accelerates the phase transformation, 3) $K^+$ cation that substitute the Li ion act as pillar in the entire layered structure, increasing the diffusion of Li ion and suppressing the bulk dislocation to improve electrochemical performance. 1. Phase transformation from the layered to the spinel phase hinders $Li^+$ ion transport by lattice mismatching and Jahn-Teller distortion. Furthermore, it causes Mn ion dissolution, leading to the formation of an insulating rock-salt phase on the surface. This can deteriorate the electrochemical cycle retention and rate capability, limiting their use in practical applications. In order to address these issues, we dope $S^{2-}$ anion into the surface of the Li-excess layered cathode, to tailor the surface transformation and thus electrochemical performance outcomes. This type of sulfurization strategy can induce the formation of the $Li_4Mn_5O_{12}$ spinel phase, which can relieve the structural incompatibility and Mn dissolution. The S-LLC shows excellent electrochemical performance; the S-doped Li-excess layered cathode has a first specific discharge capacity of 233.7 mAh/g and cycle retention of 95.5% after 200 cycles with good rate capability. 2. Oxygen redox reaction caused by activation of the $Li_2MnO_3$ domain has the crucial role of the high specific capacity. However, it also induces the irreversible oxygen release and accelerates the layered to spinel phase transformation. Here, we show that surface doping of vanadium (V) cation suppresses irreversible oxygen release and undesirable phase transformation. The experimental and theoretical studies indicate that doped $V^{5+}$ cation increases the TM-O covalency and controls the oxidation state of peroxo-like $(O2)^{n-}$ species during delithiation process. The modified material shows discharge capacity 215.8 mAh/g with 90.8% retention after 100 cycle. Furthermore the average discharge voltage drops only by 0.33 V after 100 cycles. This study illustrates the role of $V^{5+}$ cation for control of oxygen activity in the LLC materials for next-generation Li-ion batteries. 3. Structurally stabilized Li-excess layered cathode material with excellent electrochemical performance can be achieved by doping $K^+$ cation into Li slab in the layered structure. The modified material shows superior electrochemical performances with discharge capacity 220.0 mAh/g with 94.3% retention at 0.2 C after 100 cycle. The high C-rate and rate capability of modified material shows discharge capacity 184.1 mAh/g with 83.9% retention at 1 C and 118.3 mAh/g at 5 C, respectively. Furthermore the average discharge voltage drops only by 0.31 V after 100 cycles which indicates the suppressed structural transformation. We find out that the K+ cation in Li slab not only enhances the Li diffusion (4 times larger than the LLC) and but also suppresses the formation of c-direction dislocation in the bulk region. This study illustrates the role of $K^+$ cation in the LLC materials and opens up the possibility for next-generation Li-ion batteries.
Cho, EunAeresearcher조은애researcher
한국과학기술원 :신소재공학과,
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학위논문(박사) - 한국과학기술원 : 신소재공학과, 2020.8,[]


Lithium-ion batteries▼aLi-excess layered cathode▼aDoping▼aPhase transformation▼aOxygen redox reaction▼aLi ion diffusion▼aDensity functional theory; 리튬 이온 배터리▼a리튬 과잉 양극재▼a도핑▼a상변이▼a산소 반응▼a리튬 이온 확산▼a범밀도 함수 이론

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