K and Cl co-doping for enhanced ionic conductivity and structural stability in Li-rich cathode materials

Since the launching of the first commercial lithium ion batteries (LIBs) in 1991, LIBs have been widely used to power portable electronic devices such as cell phones and laptop computers due to their high energy density. In the past decade, in response to the finite petroleum supply and the associated serious environmental concerns, low emission or even zero emission electric vehicles (EVs) have attracted increasing research and development interests and LIBs have been considered as one of the most potential power sources. However, the widely used layered LiCoO2 cathode material has very limited chance to meet the demand considering its low capacity, high cost and toxicity. Li2MnO3 based layered Li-rich cathode materials as promising cathode candidates of LIBs have attracted much recent attention mainly due to their superior high specific capacity, low cost and high working voltage. To date, although researchers have put much effort to this family of materials, they still face a few serious challenges to overcome in terms of the specific capacity, long-term cycling stability and rate performance. In addition, some fundamental issues are still under debates in the understanding of the crystal structures and the electrochemical reaction mechanisms. This thesis focuses on the development of new high capacity Li-rich cathode materials for LIBs.The first chapter starts with a general introduction of LIBs, followed by a brief summary of the LIBs component, working principle and three types of the well-developed cathode materials in Chapter 2. A detailed review of the recent development on Li2MnO3 based Li-rich cathode materials was also included in Chapter 2. The main objectives and the rationale behind this project are described in Chapter 3. All the experiment details including the material synthesis and characterization, coin cell fabrication and electrochemistry measurement are shown in Chapter 4.Chapter 5 presents a published work on a series of layered-spinel integrated Li-rich cathode materials with controllable capacity. The Co/Mn mole ratio is fixed while the Li/(Mn+Ni) ratio is varied to adjust the ratio of the layered/spinel phases. They exhibit steadily increased specific capacities upon cycling for several dozen of cycles due to the gradual activation of the initial Li-rich layered phase from the surface of the composite particles. Both experimental and computational results suggest that a small amount of Co dopants plays a critical role in the continuous activation process of these materials. In addition, the structural evaluation mechanism is also discussed. Based on this unique feature, controllable discharge capacities of these cathode materials can be achieved in a broad range from 30 to 240 mAh g-1 by deliberately activating the materials at a potential window of 2~4.8 V. Chapter 6 demonstrates a series of low-Co Li-rich cathode materials showing stepwise capacity increase over a few cycles from less than 50 mAh g-1 to around 250 mAh g-1. A systematic analysis on their compositions, crystal structures and the electrochemical performances reveals that the small change of Co content has negligible effect to the crystal structure and morphology, but plays an important role in adjusting the activation rate of the Li2MnO3 phase. In addition, optimized cycling potential window and current rate have been demonstrated to significantly ensure the effective Li2MnO3 activation and good long-term cycling stability.Chapter 7 describes a new class of Li-rich materials Li[Li1/3-2x/3Mn2/3-x/3Nix]O2 (0.09≤x≤0.2) with a small amount of Ni doping as cathode materials for LIBs. Anomalous gradual capacity growth up to tens of cycles due to the continuous activation of the Li2MnO3 phase is observed. Both experimental and computational results indicate that a small amount of Ni doping can promote the stepwise Li2MnO3 activation to obtain increased specific capacity and better cycling capability. On the contrary, excessive Ni will overly activate the Li2MnO3 and result into a large capacity loss in the first cycle. The Li1.25Mn0.625Ni0.125O2 material with an optimized content of Ni has shown a superior high capacity of ~280 mAh g-1 and good cycling stability at room temperature.Chapter 8 proposes a fundamental understanding on the Li2MnO3 activation process. Based on the platform of the low-Ni Li1.87Mn0.94Ni0.19O3 cathode material which exhibits exclusive stepwise capacity increase upon cycling as demonstrated in Chapter 6, the Li2MnO3 activation process was artificially retarded significantly and split into quite a few cycles. A combined study including the powerful in-situ XRD analysis and HAADF-STEM characterization revealed that the oxygen release sub-reaction is much faster than the TM-diffusion reaction. The latter is the key kinetic step to finalize the Li2MnO3 activation and results into the gradual capacity increase. Finally, conclusions and recommendations are presented in Chapter 9 summarising the key findings and achievements of the present work on Li-rich Mn-based cathode materials and also giving insights into the future research and development on this promising cathode material system.

The electrification of transportation is an important contributor to reducing global carbon dioxide emissions. However, this progress is constrained by anxiety regarding the driving range of vehicles, which is well recognized to originate from the low specific energy of the employed state-of-the-art energy storage devices. Therefore, further promoting the specific energy of lithium-ion batteries (LIBs) is an inevitable need, where the development of cathode materials with high energy densities, i.e. high specific capacity and/or high working voltage, is essential. Accordingly, numerous research efforts are ongoing worldwide, where several materials stand out, including LiCoO2 (LCO), Ni-rich oxides and Li-rich cathodes, mainly because of their potential to deliver high capacities when operating at high voltages. However, the elevated operating voltage turns out to be a double-sided sword for these materials as achieving high specific capacity is always accompanied by the oxygen redox process, which shows unsatisfactory reversibility and has a significant impact on their structure stability and electrochemical performance. Consequently, understanding the failure mechanism of anionic redox chemistry and finding solutions to this issue are crucial for realizing the practical application of these high-voltage materials. Although many studies have been reported on the anionic redox chemistry of different materials, the corresponding reviews have predominantly focused on Li-rich cathode materials. Hence, the reviews on high-voltage LCO and Ni-rich oxides remain incomplete, and a unified understanding of their behavior at high voltages has not been established yet. This lack of comprehensive understanding has hindered the further development and application of high-voltage cathode materials. Thus, this review highlights the similarities and differences in the anionic redox chemistry of LCO, Li-rich and Ni-rich high-voltage cathode materials, emphasizing on a unified mechanistic picture and the related challenges and countermeasures. We aim to provide an outlook for future guidelines in material exploration with anionic redox chemistry, thus unlocking the full potential of high-voltage LIBs for diverse applications.

The effects of multifunctional coating on Li-rich cathode material with hollow spherical structure for Li ion battery

Li-rich oxide cathode is one of the most promising high capacity cathode materials for the next commercialized Li-ion battery application, and its composition can be described as a mixture of Li2MnO3 and Li(TM)O2(TM: Transition metal). Li-rich cathode material has large capacity based on relatively cheap Mn oxide, and it is easy to synthesize. However, the Li-rich cathode material has critical material challenges such as capacity degradation and voltage drop. It has been reported that the main source of the material challenge is related to phase transformation inside the bulk cathode material during charge/discharge processes.In this work, detailed phase transformation mechanism of Li2MnO3 is investigated by combining the experimental and computational approaches to develop fundamental understanding on the atomic scale processes. The structure of Li-rich oxide is a composite of Li2MnO3 and layered Li(TM)O2, and the monoclinic layered structure of Li2MnO3 is known to be the source of the degradation problems. We have observed the evidence of phase transformation in Li2MnO3experimentally, and a theoretical analysis based on density functional theory calculations is combined with experimental data for a systematic comparative study.For experimental study, we have synthesized Li2MnO3 powder by solid state method at low temperature following the literature procedures. Using this powder, we made a coin cell with standard Li reference electrode, and the CV measurement shows the phase transformation during charge/discharge processes. First, basic powder characterization was conducted through XRD, and SEM analyses. Second, coin cell was assembled for cyclic performance test, and charge/discharge profile and cyclic voltammogram were obtained. From the experimental investigation, synthesized Li2MnO3is identified as monoclinic C2/m space group structure and their particle size is around 200~300 nm. The evidences of phase transformation are found from cyclic charge/discharge profile as well as cyclic voltammogram. As reaction cycle is progressed, charge/discharge capacity operated under 4.6 V has a steady increase indicating that, the initial active material is getting transformed to another phase with lower reaction voltage than 4.6 V. As described in Fig. 1, we could observe the changes of charge/discharge profile and reaction voltage fundamentally caused by phase transformation consistent with similar previous experiment studies. As observed before, the first charge voltage is around 4.6 V, but the charge voltage changes to around 3.2 V as cycle goes on. Remarkably, discharge voltage reveals at three different values around 2.8 V, 3.3 V, and 4.0 V, suggesting that there are three different transformed phases inside the active material as the cycle goes on.To understand this and similar experimental observations of phase transformation, we have examined how and why it could happen in terms of thermodynamics and kinetics based on density functional theory investigation. In case of thermodynamic study, phase stability, intercalation voltage, electronic charge, and electronic structures are studied. From phase stability and intercalation voltage analyses, we would estimate when initial structure could be transformed. The electronic charge and partial density of state for both initial and transformed structure (whose Mn ion is migrated) are investigated for structure stability and physical/chemical characters. We found that thermodynamic stability of structures and the changes in bonding characters between Mn and O ions are to the main cause of phase transformation. For kinetic analysis, we investigated the migration barriers of Li and Mn ions in the Li2MnO3framework with controlled delithiation. Based on the kinetic calculation results, we could show the possibility of Li and Mn ion migrations with different Li contents in active material. As shown in Fig 2, not only the possibility and the delithiation effect of phase transformation, but also the detailed Mn migration mechanism could be predicted providing atomic scale explanation of phase transformation. As a result, detailed phase transformation mechanism could be quantitative understood, and it would be possible to suppress such phase transformation based on theoretical studies on material design such as doping on effect the electronic structure analyses.By understanding detailed phase transformation mechanism of Li2MnO3, we are developing material modification strategy to solve capacity degradation and voltage drop problems. The developed strategy will be critically validated by experimental implementation of the designed material modification. Such combined material design and experimental validation approaches will accelerated the high capacity cathode material development based on atomic scale understanding.This work was supported by the Industrial Strategic technology development program(10041589) funded by the Ministry of Knowledge Economy(MKE, Korea)

A promising Li-rich high-capacity cathode material (xLi2MnO3·(1-x)LiMn0.5Ni0.5O2) has received much attention with regard to improving the performance of lithium-ion batteries in electric vehicles. This study presents an environmental impact evaluation of a lithium-ion battery with Li-rich materials used in an electric vehicle throughout the life cycle of the battery. A comparison between this cathode material and a Li-ion cathode material containing cobalt was compiled in this study. The battery use stage was found to play a large role in the total environmental impact and high greenhouse gas emissions. During battery production, cathode material manufacturing has the highest environmental impact due to its complex processing and variety of raw materials. Compared to the cathode with cobalt, the Li-rich material generates fewer impacts in terms of human health and ecosystem quality. Through the life cycle assessment (LCA) results and sensitivity analysis, we found that the electricity mix and energy efficiency significantly influence the environmental impacts of both battery production and battery use. This paper also provides a detailed life cycle inventory, including firsthand data on lithium-ion batteries with Li-rich cathode materials.

Atomistic structure and stability of Li-rich high-entropy layered oxide cathode materials are studied. A significant structural change including Li/Ni interchange, TM migration, and secondary phase formation leading to capacity fading is found.

Recently, spinel-layered integrated Li-rich cathode materials have attracted great interest due to the large enhancement of their electrochemical performances. However, the modification mechanism and the effect of the integrated spinel phase on Li-rich layered cathode materials are still not very clear. Herein, we have successfully synthesized the spinel-layered integrated Li-rich cathode material using a facile non-stoichiometric strategy (NS-LNCMO). The rate capability (84 mA h g-1vs. 28 mA h g-1, 10 C), cycling stability (92.4% vs. 80.5%, 0.2 C), low temperature electrochemical capability (96.5 mA h g-1vs. 59 mA h g-1, -20 °C), initial coulomb efficiency (92% vs. 79%) and voltage fading (2.77 V vs. 3.02 V, 200 cycles@1 C) of spinel-layered integrated Li-rich cathode materials have been significantly improved compared with a pure Li-rich phase cathode. Some new insights into the effect of the integrated spinel phase on a layered Li-rich cathode have been proposed through a comparison of the structure evolution of the integrated and Li-rich only materials before and after cycling. The Li-ion diffusion coefficient of NS-LNCMO has been enlarged by about 3 times and almost does not change even after 100 cycles indicating an enhanced structure stability. The integration of the spinel phase not only enhances the structure stability of the layered Li-rich phase during charging-discharging but also expands the interslab spacing of the Li-ion diffusion layer, and elongates TM-O covalent bond lengths, which lowers the activation barrier of Li+-transportation, and alleviates the structure strain during the cycling procedure.

A cation and anion dual-doping strategy in novel Li-rich Mn-based cathode materials for high-performance Li metal batteries

High-capacity Li-rich cathode materials can significantly improve the energy density of lithium-ion batteries, which is the key limitation to miniaturization of electronic devices and further improvement of electrical-vehicle mileage. However, severe voltage decay hinders the further commercialization of these materials. Insights into the relationship between the inherent structural stability and external appearance of the voltage decay in high-energy Li-rich cathode materials are critical to solve this problem. Here, we demonstrate that structural evolution can be significantly inhibited by the intentional introduction of certain adventive cations (such as Ni2+) or by premeditated reservation of some of the original Li+ ions in the Li slab in the delithiated state. The voltage decay of Li-rich cathode materials over 100 cycles decreased from 500 to 90 or 40 mV upon introducing Ni2+ or retaining some Li+ ions in the Li slab, respectively. The cations in the Li slab can serve as stabilizers to reduce the repulsion between the two neighboring oxygen layers, leading to improved thermodynamic stability. Meanwhile, the cations also suppress transition metal ion migration into the Li slab, thereby inhibiting structural evolution and mitigating voltage decay. These findings provide insights into the origin of voltage decay in Li-rich cathode materials and set new guidelines for designing these materials for high-energy-density Li-ion batteries.

Surface modification of boron cobalt complexes to enhance cycling performance of cobalt-free Li-rich cathode materials

Theoretical insight into oxygen vacancy formation in Li1.25Ni0.5Mn0.25O2 cathode material

Due to its better physical and electrochemical properties, Li2MoO3 was proposed to replace Li2MnO3 for constructing new Li-rich cathode materials. However, the molybdenum (Mo)-ion shuttling between the Li layer and the Mo layer upon electrochemical Li-extraction raises concerns on the structural stability of the Mo-based Li-rich materials. In this article, the nudged energy band method was applied using first-principles calculations to understand the reason for the Mo-ion migration and to sieve substituent elements for Mo from a number of transition metals. Molecular dynamics calculations were performed to simulate the kinetic properties of the pristine and transition metal substituted Li2MoO3. On the basis of these calculations, antimony (Sb) was proposed as a substituent to enhance the structural stability of Li2MoO3 and improve its rate performance.

Surface F-doping for stable structure and high electrochemical performance of Li-rich Mn-based cathode materials

Structural insights into composition design of Li-rich layered cathode materials for high-energy rechargeable battery

Enhanced electrochemical performances of layered-spinel heterostructured lithium-rich Li1.2Ni0.13Co0.13Mn0.54O2 cathode materials