Positive Electrode Material For Li-ion Batteries Research Articles

Feeling the strain: Enhancing the electrochemical performance of olivine LiMnPO[formula omitted] as cathode materials for Li-ion batteries through strain effects

Currently, the improvement of Li-ion batteries (LIBs) is more important than ever, especially due to the increasing spread and rapid progress of electric vehicles and high-tech applications. Materials used for positive electrodes are one of the main constituents of batteries, which typically determine their electrochemical efficiency. Phospho-olivine LiMnPO4 (LMP) material has been regarded as a potential positive electrode material for LIBs due to its outstanding properties. However, it suffers from low electronic and ionic conductivity. Therefore, this work aims to overcome these drawbacks by applying a biaxial strain to LMP. In this regard, we used density functional theory calculations to investigate the effect of biaxial strain on the dynamic and thermal stabilities, structural, electronic, ionic diffusion, electrochemical potential, and defect properties of LiMnPO4 (LMP) structure, as well as on the Average (Mn–O, Li–O and P–O) bond lengths, electrical conductivity, and charge transfer. Moreover, the influence of anti-site defects on the ionic conductivity of LMP compound was evaluated. Our findings suggest that the biaxial tensile strain has a remarkable effect on the rate performance of LMP cathode material. A biaxial tensile strain of +2% reduces the band gap of LMP from 3.51 to 3.41 eV, and ameliorates the diffusion coefficient by 100 times. Furthermore, the migration barrier was calculated to be 0.37 eV for strained (+2%) defective MP, lower than 1.12 eV for unstrained defective MP, indicating that biaxial tensile strain can mitigate the negative effect of anti-site defects on Li-ion migration. These results can prompt us to suggest biaxial tensile strain as a good strategy for improving the electrochemical performance of LMP as cathode material for LIBs. Furthermore, this study may supply insights to cathode material designers and engineers on the importance of an appropriate biaxial strain value to improve the rate performance of LiMnPO4 as cathode materials for LIB batteries.

NAi/Li Antisite Defects in the Li1.2Ni0.2Mn0.6O2 Li-Rich Layered Oxide: A DFT Study

Li-rich layered oxide (LRLO) materials are promising positive-electrode materials for Li-ion batteries. Antisite defects, especially nickel and lithium ions, occur spontaneously in many LRLOs, but their impact on the functional properties in batteries is controversial. Here, we illustrate the analysis of the formation of Li/Ni antisite defects in the layered lattice of the Co-free LRLO Li1.2Mn0.6Ni0.2O2 compound through a combination of density functional theory calculations performed on fully disordered supercells and a thermodynamic model. Our goal was to evaluate the concentration of antisite defects in the trigonal lattice as a function of temperature and shed light on the native disorder in LRLO and how synthesis protocols can promote the antisite defect formation.

Charge/Discharge Reaction Mechanisms of Nanosized Li-Excess Li2TiO3–LiVO2 System

The demand of high-energy Li-ion batteries (LIBs) is rapidly growing for electric vehicles (EVs) to reduce dependence on fossil fuel. Although currently, Ni-rich layered oxides are mainly used as positive electrode materials for LIBs, the cost of Ni is raising because of increase demand of EVs. Therefore, the development of alternative materials without Co/Ni ions is necessary. In this study, vanadium ions with double electron redox reaction (V3+/V5+) are targeted as potential high capacity electrode materials. Stoichiometric LiVO2 is electrochemical inactive due to the phase transition from a layered to rocksalt-type structure.[1] Nevertheless, a Li-excess V-based oxide, Li1.25Nb0.25V0.5O2, with the rocksalt-type structure delivers a large reversible capacity of 250 mA h g-1 with percolative Li migration in the structure. [2] In this study, Li-excess Li2TiO3–LiVO2 binary system is synthesized and tested as potential high-capacity electrode materials. Among the tested samples, Li8/7Ti2/7V4/7O2 (x = 0.33 in x Li2TiO3–(1–x) LiVO2 binary system delivers the largest reversible capacity of approximately 300 mA h g-1 with no capacity degradation for over 100 cycles (Fig. 1). To clarify detailed reaction mechanisms, synchrotron X-ray total scattering was utilized and obtained pair distribution function (PDF) data are shown in Fig. 2. It is revealed that the crystallinity of the sample is lowered and V migration from original octahedral to tetrahedral sites on charging, which is consistent with the finding by X-ray absorption spectroscopy. Small V5+ ions are energetically stabilized at tetrahedral sites. Nevertheless, this process is highly reversible, and the high crystallinity phase with V ions at octahedral sites is obtained after discharging. From these data together with operando XRD data, the origin of the long cycle life for Li8/7Ti2/7V4/7O2 will be discussed in more details.[1] J. B. Goodenough, et al, Mater. Res. Bull., 15, 783-789 (1980)[2] N. Nakajima and N. Yabuuchi, Chem. Mater., 29, 6927 (2017). Figure 1

(Invited) Research Development on K-Ion Batteries

In recent years, K-ion batteries (KIBs) have attracted significant attention as potential alternatives to Li-ion batteries (LIBs). Previous studies have developed positive and negative electrode materials for KIBs and demonstrated several unique advantages of KIBs over LIBs and Na-ion batteries (NIBs). Besides being free from any scarce/toxic elements, the low standard electrode potentials of K/K+ electrodes lead to high operation voltages competitive to those observed in LIBs. Moreover, K+ ions exhibit faster ionic diffusion in electrolytes due to weaker interaction with solvents and anions than with Li+ ions; this is essential to realize high-power KIBs. Over the past few years, many potassium insertion materials have been evaluated in K cells. Figure 1 summarizes the yearly number of scientific papers on K batteries. The number of papers on K systems drastically increased in recent years. Thus, we have recently published a comprehensive review to provide a guideline for research on KIBs (1). This talk presents an overview of our recent development on electrode and electrolyte materials.Previous studies on positive electrode materials for KIBs have shed light on the importance of the crystal structures, redox center, and anions in the framework. Although transition metal layered oxides are the most promising positive electrode materials for LIBs and NIBs, we found that typical transition metal layered oxide materials like P2-K x CoO2 exhibit small capacities, low operating potentials, and multiple phase transitions due to K+/vacancy ordering (2). PBAs are one of a promising group of positive electrode materials in terms of energy density, cycle performance, and rate performance (3, 4). Their 3D open framework structure provides suitable channels and interstitial sites for large K+ ion diffusion and insertion. Interestingly, unlike layered transition metal oxides, PBAs tend to exhibit higher K+ ion insertion potentials than those of Li+ and Na+ ions (5). Similar to PBAs, polyanionic compounds with 3D open framework structures are other possible positive electrode material candidates, owing to their high ionic conductivities and high operation potentials. Previous studies have revealed suitable crystal structures, including the KTiOPO4-type structure for K+ ion diffusion (6). Moreover, an inorganic-organic hybrid material called metal organic phosphates open framework (MOPOF) has been demonstrated as a 4 V-class positive electrode for KIBs with high rate capability (7).For negative electrodes, graphite is a promising candidate for KIBs (8). Our current studies are confirming that the physical properties of graphite, such as the d002 and Lc values, affect the electrochemical performance and that the optimum properties should be different from those of the Li system. Therefore, the development of graphite material suitable for K+ insertion is also an interesting and practically important topic. Furthermore, many studies of negative electrode materials have highlighted the properties of SEI, which depend on the electrolyte composition; this is important to demonstrate satisfactory cycles and rate capabilities (1). Therefore, recent studies have been focusing on the development of suitable electrolytes. The use of proper electrolytes, such as superconcentrated KFSA electrolytes (9), dramatically improved the electrochemical performance of the positive and negative electrodes. Thus, we believe that the development of a suitable electrolyte for KIBs will achieve a breakthrough in the research field of KIBs. Based on these results, we will provide insights into electrode reactions and solid ionics and nonaqueous solution chemistry as well as perspectives on the research-based development of KIBs, compared to those of LIBs and NIBs.

Approaching the limits of cationic and anionic electrochemical activity with the Li-rich layered rocksalt Li3IrO4

The Li-rich rocksalt oxides Li2MO3 (M = 3d/4d/5d transition metal) are promising positive-electrode materials for Li-ion batteries, displaying capacities exceeding 300 mAh g–1 thanks to the participation of the oxygen non-bonding O(2p) orbitals in the redox process. Understanding the oxygen redox limitations and the role of the O/M ratio is therefore crucial for the rational design of materials with improved electrochemical performances. Here we push oxygen redox to its limits with the discovery of a Li3IrO4 compound (O/M = 4) that can reversibly take up and release 3.5 electrons per Ir and possesses the highest capacity ever reported for any positive insertion electrode. By quantitatively monitoring the oxidation process, we demonstrate the material’s instability against O2 release on removal of all Li. Our results show that the O/M parameter delineates the boundary between the material’s maximum capacity and its stability, hence providing valuable insights for further development of high-capacity materials.