Research Progress of Cathode Materials for Lithium-Ion Batteries
INTRODUCTION
Lithium-ion batteries (LIB) in 1991, consumer electronics products have developed rapidly [1,2]. Soon, LIB became the power supply of choice for laptop and mobile phone manufacturers. As smartphones get smaller and have to run more energy demanding applications, LIB has to keep pace. Battery manufacturers have developed more elaborate methods for assembling batteries to provide modern LIBs with high energy density for consumer applications [3,4]. At the same time, scientists have developed better active substances and are constantly improving them. This article focuses on lithium cobalt oxide (LiCoO2, LCO), lithium manganese oxide (LiMnO2, LMO), lithium iron phosphate (LiFePO4, LFP) and ternary cathode materials, which are currently the most researched and used. The latest research progress of cathode materials.
Lithium cobalt oxide: LiCoO2
LiCoO2 was discovered by Professor John B. Goodenough in 1980 and was regarded as a lithium-ion intercalation material at that time. Compared with all other cathode materials, LiCoO2 has many unique advantages, including high Li+/electron conductivity, high compacted density (4.2 gcm−3), and excellent cycle life and reliability. Therefore, LiCoO2 is still the main material for the cathode of lithium-ion batteries at present.
LiCoO2 has high theoretical specific capacity (274 mAhg−1) and high discharge voltage (about 4.2 V vs Li+/Li). However, commercial Li-ion batteries can only deliver half of the theoretical capacity due to the inherent structural instability and interfacial loss of LiCoO2 when the charging voltage exceeds 4.2 V. Yao et al. developed a facile blow-spin synthesis method to achieve precise doping of LiCoO2 particles and simultaneous self-assembled coatings, achieving record performance in existing LiCoO2 cathodes [5]. Due to the steric confinement effect of the blown microfibers, uniform doping of Mn and La in the LiCoO2 matrix and uniform separation of Li-Ti-O on the LiCoO2 surface can be achieved for each batch of samples. The results show that Mn and La co-doping can suspend the intrinsic instability of the LiCoO2 matrix and enhance the Li+ diffusivity. The Ti-based coating can stabilize the interface of LiCoO2 particles at a charging voltage up to 4.5 V. Compared with previously reported LiCoO2 cathodes, the obtained composite LiCoO2 cathode exhibits the best rate performance (1.85 mAhcm−2 at 2C) and the longest cycle stability at 2.04 mAhcm−2 areal capacity (at 83% of capacity maintained at 0.3C over 300 cycles).
Figure 1: Schematic of the spinning technique to fabricate LCO cathodes precisely compounded with Mn and La doping and Ti segregation-inducing nanocoatings
Lithium manganate: LiMnO2
Xia et al. designed a spinel-layered LiMnO2 (SPL-LMO) with a unique interfacial orbital order, namely an approximately orthogonal relationship with respect to the orientation of the MnO6 octahedron at the domain interface [6]. This hetero-structured cathode is realized in situ by a simple electrochemical conversion of Mn3O4. During the electrochemical oxidation, the loss of Mn3O4 in the non-aqueous electrolyte with a small amount of oxygen leads to an anomalous spinel-layered phase transition. The spinel layered heterostructure significantly reduces the Jahn-Teller distortion and Mn dissolution, improving the structural stability of LiMnO2.
The reversible specific capacity of this SPL-LMO cathode is as high as 254.3 mAhg−1, corresponding to about 90% of lithium ion intercalation /deintercalation into LiMnO2 . A high capacity retention of 90.4% was achieved after 2000 cycles, indicating that the proposed interface engineering method is very effective for stabilizing the structure of the Jahn-Teller active electrode material.
Figure 2: Synthesis of SPL-LMO and SPL-LMO/Mn3O4. a) Electrochemical conversion of Mn3O4 nano wall arrays to SPL-LMO nano wall arrays (CE: counter electrode; RE: reference electrode; WE: working electrode); b) conversion of powdery Mn3O4 electrode to SPL-LMO/Mn3O4 powder electrode
Komarneni prepared orthogonal LiMnO2 nanorods by a simple and economical in situ carbothermal reduction method using nanorod-like MnO2 as a template and manganese precursor[7] .The obtained product has high purity and excellent electrochemical performance. When used as a cathode material, it has a capacity of 165.3 mAhg−1.with a small capacity loss at 0.1 C. After 40 cycles, only 7.4% of the discharge capacity was lost. Moreover, the maximum discharge capacities are 227.5 and 95.3 mAhg−1 at low and high rates of 0.05 and 1.0 C, respectively. Similarly, Liu et al. directly prepared mesoporous orthorhombic LiMnO2 by a one-step flux method [8]. Orthorhombic LiMnO2 prepared by calcination of fluxes LiOH·H2O and Mn2O3 with different molar ratios of Li/Mn mixed , has better lithium storage performance thanks to its unique mesoporous structure. When mesoporous orthorhombic LiMnO2 is used as the cathode of Li- ion batteries, after 50 cycles at a current density of 0.1 C, its maximum discharge capacity is 191.5 mAhg-1 and its reversible capacity is 162.6 mAhg-1 (84.9% retention). These results suggest its potential application in high-performance Li- ion batteries.
Lithium Iron Phosphate: LiFePO4
Hassoun and colleagues report an advanced lithium- ion battery based on a graphene ink cathode and a lithium iron phosphate cathode. By carefully balancing the contents of each component of the battery and suppressing the initial irreversible capacity of the anode over several cycles, the specific capacity of the battery can reach 165 mAhg-1, the estimated energy density is about 190 Whkg-1, and it can be achieved at more than 80 It can run stably under charge and discharge cycles. The components of this battery are low in cost and have potential for expansion [ 9].
Figure 3: SEM diagram, TEM diagram and battery performance diagram of lithium iron phosphate and graphene materials
cost of lithium iron phosphate materials accounts for a large proportion of the entire lithium- ion battery. Therefore, many scholars have focused on recycling lithium iron phosphate and proposed various recycling strategies. Wang et al. proposed a simple, green and effective waste lithium iron phosphate recycling method that combines the charging mechanism of lithium iron phosphate batteries with the slurry electrolysis process [10]. Anionic membrane electrolysis can separate Li and FePO4 without adding chemical reagents. The leaching rate of Li can reach 98%, and more than 96% of Fe can be recovered as FePO4/C. Kinetic analysis indicated that the surface chemical reaction was the controlling step in the slurry electrolysis process. In addition, X -ray diffraction (XRD), X -ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) characterization and thermodynamic leaching mechanism were also analyzed.
Figure 4: Schematic diagram of slurry electrolysis
Ternary material
NCM is a ternary material of nickel-cobalt-lithium-manganese oxide, which has attracted widespread attention due to its stable electrochemical performance and good cycle performance. Raw material costs vary little for different NCM compositions. Therefore, increasing nickel content is an effective way to increase production capacity at the same cost. After previous work focused on more balanced NCM materials, a clear downward trend in the nickel line can now be seen in Figure 5. While NCM523 has become a commercial reality, the higher nickel content presents a number of issues that hinder its success.
1. The highly active Ni4+ will dominate at the end of charging, leading to side reactions with the electrolyte solution. This will eventually lead to depletion of active material and loss of capacity. In addition, as the nickel content increases, the high-temperature stability of the material will decrease, leading to serious safety concerns.
2. Like in pure LNO, high Ni content in NCM leads to Li/Ni cation mixing. This will lead to spinel formation on the surface and eventually capacity fading.
3. Starting with NCM811, prolonged cycling will cause cracks in the secondary grains along the grain boundaries. This results in a continuous increase in surface area and thus loss of more reactive sites.
Figure 5: Phase diagram of LNO, LCO and LMO ternary system
Reference
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