SEM images of cross-sectional electrodes of (a,b) pristine NCM and (c,d) CoxB-NCM after 200 cycles of 7 C discharge cycles at 45°C. Change in (c) a-axis and (d) c-axis lattice parameters of CoxB-NCM and pristine NCM under as a function of the charge state.
Motivation for developing advanced Lithium-ion batteries
Comparison of the levelized cost for electric energy during discharge as a function of cycles at one-year frequency. 4 From Steven J. Comparison of national data on the sales of electric vehicles (EVs) with GDP per capita in the EU member states for the full year 2018.8.
Overview of rechargeable Lithium-ion batteries
Function of additive to stabilize the surface of Li-rich cathode materials. a) Degradation phenomena of Li-rich cathode by reactive species. In this regard, graphite is considered to be the most suitable candidate for a possible alternative of Li metal.
Layered lithium metal oxide for cathode materials
Nowadays, Ni-rich materials are highly valued as next-generation cathodes for electric vehicles and smart grids. While most traditional multilayers are stoichiometric materials, there have been many attempts to achieve very high performance and energy densities from Li-excess multilayers.
Obstacles to achieve high-energy density Lithium-ion batteries
Schematic description of charge and discharge state in Ni-rich cathode. a) CSG 90 (structure-modified sample) and (b) CC90 (pristine sample) showing the different volume expansion behavior and following results.60 Reprinted from Kim, U.-H. Schematic showing different interfacial behavior on morphological features of cathode (polycrystalline and single crystalline) during the electrode printing process and electrochemical cycles.58 Reprinted from Kim, J. et al.
Scope and organization of this dissertation
37 Lee, S.et al.Hierarchical atomic surface structure of a manganese-based spinel cathode for lithium-ion batteries. 59 Liu, W.et al.Lithium transition metal oxide with nickel-rich layers for high-energy lithium-ion batteries.
Experimental details
The initial charge-discharge cycle was performed on the assembled cells at a C rate of 0.1 C, and then they were tested with a constant current of 1.0 C charge and 1.0 C discharge. The galvanostatic intermittent titration technique (GITT) was applied using coin half cells, cycling in the voltage range of 3.0-4.45 V at a constant charge and discharge rate of 0.5 C for 8 minutes with a rest time of 1 hour .
Results and discussion
As mentioned above, previous studies attributed (A) improved cyclability of Ni-doped LCO to a “pillar effect” based on the observation of simultaneous (B) suppressed phase transitions at high voltages (i.e., the case showing both A and B). Although its advantage is undeniable, it does not suppress any bulk phase transitions at all (ie, the case shows. A but not B).24,41,42 Third, some studies of bulk doping show suppressed phase transitions, but there were no improvements in high - stress cyclability (ie the case showing B but not A).43So (B) is neither a necessary nor a sufficient condition for (A).
A presumed uniform Ni doping also modifies the surface of LCO by surface segregation, which is crucial for its electrochemical stability. First, the surface of pristine LCO particles has a layered structure (space group R3m, same with bulk phase) as shown by TEM in Figure 2-16. Second, spatially resolved EELS reveals less reduction of Co at the surface of LCNO than LCO after 1st charge and after 100th discharge (Figure 2-17g-i).
The magnified high-angle annular dark field (HAADF) STEM image of pristine LCO (right). b) The High-resolution TEM with corresponding local FFT patterns and magnified HAADF-STEM images collected from different region within 100th-cycle LCO. Third, we performed a floating assay (an established method to evaluate the voltage window of electrolytes)56,57 to investigate the surface reactivity of LCO and LCNO. Compared to (a-c), the SEM image of LCO after 1st floating test (d) shows large amount of impurities accumulated on the surface.
In EDX mapping of the cyclic LCO, an illustrative map of carbon (C) shows the interphase on the surface of active material.
Conclusion
21 Liu, Q. et al. Approaching the capacity limit of lithium cobalt oxide in lithium ion batteries via lanthanum and aluminum doping. 26 Xie, J.et al.Surface engineering of LiCoO2 electrodes using atomic layer deposition for high voltage stable lithium ion batteries. Comparison of the chemical stability of high energy density cathodes of lithium-ion batteries.
Electrolyte reactions with the surface of high-voltage LiNi0.5Mn1.5O4 cathodes for lithium-ion batteries. Dynamic behavior of interphases and its implication on high-energy-density cathode materials in lithium-ion batteries. A review of the characteristics and analyzes of the solid electrolyte interphase in Li-ion batteries.
64 Gauthier, M.et al.Electrode-electrolyte interface in li-ion batteries: current insights and new insights. Reduction of carbonate electrolytes and the formation of a solid-electrolyte interface (SEI) in lithium-ion batteries.
Introduction
We now report a cobalt boride (CoxB) coating that stabilizes NCM and greatly improves its high-speed performance. This CoxB material was chosen based on the following: (i) CoxB is a metallic compound that does not have direct bonds with oxygen32 and would thermodynamically react with oxygen to form stable compounds such as B2O3, Co3O4 and Co4B6O13, implying a strong reactivity between CoxB and NCM surface oxygen. ii) CoxB has exceptional resistance to oxidation even at elevated temperatures (850-950oC)33. This means that although CoxB likes to react with oxygen, the reaction is kinetically self-limiting, probably due to the ability to form a glass of a B2O3-like product at the interface, forming a compact self-healing passive layer34,35.
Therefore, although the reactive wetting ensures full surface coverage and good adhesion between the CoxB and NCM, it does not consume oxygen from the NCM lattice. The passivation layer would kinetically suppress the penetration/loss of oxygen through this coating layer, and the interfacial polyanionic borate glass also contains Li-alkali metal supplied with the NCM, making it a mixed ionic and electronic conductor (MIEC). iii) CoxB coating can be easily synthesized at room temperature, which eliminates the need for any subsequent high-temperature treatments that could introduce additional complexity and/or defects into the already highly optimized NCM synthesis pathway. iv) Cobalt boride has been used to coat metal parts to improve their corrosion and wear resistance. Thus, its mechanical properties should be good in the sense that it should not easily crumble or break at the nanoscale. Remarkably, we will show that the synthesized CoxB coating not only completely covers the surface of secondary NCM particles, but also penetrates into the GBs between primary particles with zero wetting angle, which we will abbreviate as.
This is comparable to the complete wetting of GBs by liquid metal (e.g. liquid Ga in aluminum GBs) and intergranular amorphous nanofilms in ceramics. Considering the similar crystal structure, redox-active transition metal species, and/or microstructure, we believe that the investigated CoxB infusion can be directly applied to many other cathode materials for LIBs, including LiNi1−x−yCoxMnyO2, LiNi1−x−yCoxAlyO2, LiCoO2, and Li - /Mn-rich cathodes.
Experimental details
In situ differential electrochemical mass spectrometry (DEMS) measurements were performed on Swagelok-type cells between 3.0 and 4.4 V (vs. Li/Li+), and details are described elsewhere39. After rate testing, the cells were charged/discharged at 1.0 C using constant current (CC)–constant voltage (CV, threshold 0.05 C) mode for a further 100 cycles (cycles 41–140 in the total number of cycles), to evaluate the cycling stability between 3.0 and 4.4 V (vs. Li/Li+) at 25oC. Galvanostatic intermittent titration (GITT) measurements were then performed after the first (41st cycle in total number of cycles) and last (140th cycle in total number of cycles) cycles of the 1.0 C cycle, between 3.0 and 4.4 V (compared to Li/Li+) with a titration step at 0.5 C for 8 minutes and a relaxation step for 1 hour.
To evaluate the high-speed cycling stability, the cells were charged at 0.5 C and discharged at 5.0/7.0 C. Symmetrical Li/Li cells were collected in the glove box with 80 μL of the prepared electrolyte for each cell. The cathode loading level was 13.5 mg cm-2 on each side of the double-sided coated Al sheet.
The anode loading level was 8.2 mg cm-2 on each side of the double-sided coated Cu foil. The cycling voltage window was 2.8–4.3 V, and two formation cycles were performed at 0.1 C before long-term cycling of 500 cycles at 1 C.
Results and discussion
XPS binding energies with assigned species and curve fitting results for CoxB-NCM and pristine NCM in Figure 3-4g. In addition, additional experiments were also performed for pristine NCM and CoxB-NCM with a discharge rate of 5 C at 45 °C (Figure 3-18), again demonstrating the superior electrochemical performance of CoxB-NCM. As shown in Figure 3-12e, CoxB-NCM has an impressive capacity retention of 95.0% (compared to 79.2% for pristine NCM) and a high CE over 500 cycles (more detailed electrochemical performance is shown in Figure 3-19) .
Charge-discharge profile of the first cycle (i.e., formation cycle) and corresponding differential capacitance versus voltage (dQ/dV) plot of CoxB-NCM and pristine NCM at 0.1 C between 3.0 V and 4 .4 V (vs. Li/Li+) at 25 °C. Superior electrochemical performance of CoxB-NCM over pristine NCM. (a) Rate tests and 1 C cycling of CoxB-NCM and pristine NCM in the voltage range 3.0−4.4 V versus galvanostatic gap titration technique (GITT) measurements on CoxB-NCM and pristine NCM after certain cycles during cycling 1 C in Figure 3-12a.
The discharge–discharge curves of (b) pristine NCM and (c) CoxB-NCM after specific cycles are also shown. Third, by differential electrochemical mass spectrometry (DEMS) measurements in Figure 3-20h, we found a much smaller release of gases (CO2 and O2) during the first charge cycle of CoxB-NCM than that of the original NCM. In comparison, LMA paired with CoxB-NCM has a smooth surface (Figure 3-29d) and a dense SEI layer (Figure 3-29e).
XPS spectra of Mn 2pan and Co 2po LMA paired with CoxB-NCM and pristine NCM after 200 cycles of 7 C-discharge cycling at 45 °C.
Conclusion
16 Kim, J.et al. A highly stabilized nickel-rich cathode material by nanoscale epitaxy control for high-energy lithium-ion batteries. 17 Yan, P.et al.Tailoring the grain boundary structures and chemistry of Ni-rich layered cathodes for improved cycle stability of lithium-ion batteries. 29 Yoon, M.et al. Discovery of nickel chemistry in stabilizing high-voltage cobalt-rich cathodes for lithium-ion batteries.
30 Zhu, Z.et al.Gradient Li-rich oxide cathode particles immunized against oxygen release by molten salt treatment. 36 Masa, J.et al.Amorphous cobalt boride (Co2B) as a highly efficient non-costly catalyst for electrochemical water splitting: oxygen and hydrogen evolution. 39 Park, J.-H.et al.Effect of residual lithium rearrangement on Ni-rich layered oxide cathodes for lithium-ion batteries.
46 Jiang, B. et al. A mesoporous non-noble metal boride system: synthesis of mesoporous cobalt boride by strictly controlled chemical reduction. 63 Seo, D.-H. et al. The structural and chemical origin of the oxygen redox activity in layered and cation-disordered Li-excess cathode materials.
