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.
리튬 이온 배터리의 사용이 점차 증가함에 따라 최근 전기 자동차 및 에너지 저장 시스템과 같은 대용량 저장 장치로의 적용이 요구되고 있다. 그러나 $LiCoO_2$, $LiMn_2O_4$ 및 $LiFePO_4$와 같이 현재 리튬 이온 배터리에 사용되는 양극재는 대용량 저장 장치에 필요한 에너지 밀도, 수명 및 비용 측면에서 필요한 성능을 충족시키지 못한다. 이 문제를 해결하기 위해 리튬 이온 배터리의 양극재로서 리튬 과잉 양극재가 대체 물질로서 제안되었다. 리튬 과잉 양극재는 높은 용량과 높은 에너지 밀도를 제공하지만 사이클이 진행될수록 층상에서 스피넬상으로의 구조 상변이 및 비가역적 산소 반응과 같은 문제가 발생하기 때문에 상용화에 어려움이 있다. 위의 문제를 극복하기 위해 본 연구에서는 다양한 물질을 리튬 과잉 양극재 물질에 도핑하는 방법을 진행하였다.
1) 리튬 과잉 양극재 물질에 황 음이온을 기상 반응을 통해 양극재 표면에 선택적으로 도핑하였다. 황 음이온의 도핑을 통해 스피넬상으로의 상변이가 일어날 때 통상적으로 발생하는 $LiMn_2O_4$ 스피넬상이 아닌 전기화학적으로 안정한 $Li_4Mn_5O_{12}$ 스피넬상을 생성함으로서 리튬 과잉 양극재의 전기화학적 성능을 향상시켰다.
2) 리튬 과잉 양극재 물질에 바나듐 양이온을 전구체 코팅 방법을 통해 양극재 표면에 선택적으로 도핑하였다. 표면에 도핑된 바나듐 양이온은 비가역적 산소 반응을 억제하는 역할을 함으로서 비가역적 산소 반응으로 인해 생기는 용량 저하 및 상변이의 가속화를 완화시키며 이로 인해 리튬 과잉 양극재의 전기화학적 성능을 향상시켰다.
3) 리튬 과잉 양극재 물질에 칼륨 양이온을 열처리를 통한 고체 반응 방법을 이용해 양극재 전체에 균일하게 도핑하였다. 칼륨 양이온은 리튬 층에 존재하는 리튬을 대체하며 양극재 물질 전체에 구조적 안정성을 높이는 기둥 효과 및 리튬 층을 넓혀 리튬 이온의 확산 속도를 증가시키는 효과를 동시에 보인다. 이러한 효과를 통해 리튬 과잉 양극재의 전기화학적 성능을 향상시켰다.