Engineering Surface Vacancy to Stabilize High-Voltage Battery Cathodes

Abstract

In this issue of Chem, Cao and co-workers report a stabilization strategy for high-voltage lithium-ion battery cathodes down to the atomic scale. Using LiNi0.5Mn1.5O4 as a model, the authors demonstrated a protocol for precisely modulating the interstitial lattice sites on the material surface, showing the promising capability to improve electrochemical cyclability. In this issue of Chem, Cao and co-workers report a stabilization strategy for high-voltage lithium-ion battery cathodes down to the atomic scale. Using LiNi0.5Mn1.5O4 as a model, the authors demonstrated a protocol for precisely modulating the interstitial lattice sites on the material surface, showing the promising capability to improve electrochemical cyclability. Extensive efforts have been committed to improving the energy density of lithium-ion batteries (LIBs) so as to meet the ever-increasing demand for energy storage.1Tarascon J.M. Armand M. Issues and challenges facing rechargeable lithium batteries.Nature. 2001; 414: 359-367Crossref PubMed Scopus (16247) Google Scholar, 2Shi Y. Zhou X. Yu G. Material and structural design of novel binder systems for high-energy, high-power lithium-ion batteries.Acc. Chem. Res. 2017; 50: 2642-2652Crossref PubMed Scopus (200) Google Scholar However, as a capacity-determining component for LIBs, high-energy cathode materials usually suffer from the inherent problem of reduced stability.3Goodenough J.B. Kim Y. Challenges for rechargeable Li batteries.Chem. Mater. 2010; 22: 587-603Crossref Scopus (7918) Google Scholar, 4Ma J. Hu P. Cui G.L. Chen L.Q. Surface and interface issues in spinel LiNi0.5Mn1.5O4: insights into a potential cathode material for high energy density lithium ion batteries.Chem. Mater. 2016; 28: 3578-3606Crossref Scopus (240) Google Scholar Notably, cathode-electrolyte interfacial side reactions and transition-metal (TM) ion-dissolution-related structural degradation have been well recognized and studied.4Ma J. Hu P. Cui G.L. Chen L.Q. Surface and interface issues in spinel LiNi0.5Mn1.5O4: insights into a potential cathode material for high energy density lithium ion batteries.Chem. Mater. 2016; 28: 3578-3606Crossref Scopus (240) Google Scholar, 5Lee K.T. Jeong S. Cho J. Roles of surface chemistry on safety and electrochemistry in lithium ion batteries.Acc. Chem. Res. 2013; 46: 1161-1170Crossref PubMed Scopus (208) Google Scholar The importance of stabilizing these high-energy cathode materials has been widely acknowledged for the development of the next generation of LIBs. For achieving a long-life cathode, the key issue lies in the material’s surface, whose instability directly leads to reduced overall electrochemical performance. Two strategies have been previously employed to handle the instability issues for cathode materials. The first is to provide a chemically inert coating layer to protect the surface of cathode materials by suppressing the surface corrosion and the electrolyte decomposition, and the second is through the reinforcement of the lattice framework via cationic doping.5Lee K.T. Jeong S. Cho J. Roles of surface chemistry on safety and electrochemistry in lithium ion batteries.Acc. Chem. Res. 2013; 46: 1161-1170Crossref PubMed Scopus (208) Google Scholar Recently, a surface-doping strategy has been reported to help stabilize cathode materials.6Lu J. Zhan C. Wu T. Wen J. Lei Y. Kropf A.J. Wu H. Miller D.J. Elam J.W. Sun Y.K. et al.Effectively suppressing dissolution of manganese from spinel lithium manganate via a nanoscale surface-doping approach.Nat. Commun. 2014; 5: 5693Crossref PubMed Scopus (222) Google Scholar, 7Piao J.-Y. Duan S.-Y. Lin X.-J. Tao X.-S. Xu Y.-S. Cao A.-M. Wan L.-J. Surface Zn doped LiMn2O4 for an improved high temperature performance.Chem. Commun. (Camb.). 2018; 54: 5326-5329Crossref PubMed Google Scholar Combining the advantages of surface coating and bulk-level doping, the surface-doping strategy highlights the effectiveness of surface modification in stabilizing battery cathode materials.6Lu J. Zhan C. Wu T. Wen J. Lei Y. Kropf A.J. Wu H. Miller D.J. Elam J.W. Sun Y.K. et al.Effectively suppressing dissolution of manganese from spinel lithium manganate via a nanoscale surface-doping approach.Nat. Commun. 2014; 5: 5693Crossref PubMed Scopus (222) Google Scholar In this issue of Chem, Cao and co-workers report an elegant strategy called surface-vacant-site occupation (SVSO),8Piao J.-Y. Sun Y.-G. Duan S.-Y. Cao A.-M. Wang X.-L. Xiao R.-J. Yu X.-Q. Gong Y. Gu L. Li Y. et al.Stabilizing cathode materials of lithium-ion batteries by controlling interstitial sites on the surface.Chem. 2018; 4: 1685-1695Abstract Full Text Full Text PDF Scopus (54) Google Scholar which provides an alternative perspective for remedying the instability of high-energy cathode materials. The secret to success lies in controlling the ionic transport on the electrode-electrolyte interface by manipulating the vacant lattice sites, given that the cation diffusion across the interface is highly relevant in the structural degradation of cathode materials.4Ma J. Hu P. Cui G.L. Chen L.Q. Surface and interface issues in spinel LiNi0.5Mn1.5O4: insights into a potential cathode material for high energy density lithium ion batteries.Chem. Mater. 2016; 28: 3578-3606Crossref Scopus (240) Google Scholar Taking the high-voltage LiNi0.5Mn1.5O4 (LNMO) material as an example, the authors show that inserting Al3+ ions into a proper proportion of vacant 16c sites on the surface of LNMO particles results in a desired surface chemistry that helps to combat the surface degradation. The SVSO strategy involving a two-step synthesis is outlined in Figure 1A. In the first step, a uniform and continuous AlPO4 coating layer is formed on the surface of LNMO particles. The coating layer is amorphous, and the thickness is able to be adjusted with nanometer accuracy. In the second step, a post-heating process is applied to promote a solid-state reaction between AlPO4 and LNMO, leading to the final product, denoted as Al-LNMO. Notably, the ability to build the uniform coating layer is essential for achieving a convincing model system, which is the precondition of identifying the surface structure of Al-LNMO and finding the unique mechanism for surface stabilization. The authors conducted careful structural characterization to better understand the surface structure of Al-LNMO. They used aberration-corrected scanning transmission electron microscopy in a high-angle angular dark-field (STEM-HAADF) mode, which directly images the atomic-scale structure, to record the atomic arrangement along the [110]spinel direction. They observed that the heavy atomic columns were arranged into characteristic diamond shapes, which confirms the survival of the spinel framework after the introduction of Al3+. Interestingly, the STEM-HAADF images revealed a distinct difference between the surface and the body in atomic configuration. Figure 1B shows extra-bright columns in the center of the diamond shapes on the surface of Al-LNMO, indicating the formation of a modified surface layer with their 16c sites partially occupied. The thickness of the surface layer was around 10 nm, and electron energy-loss spectroscopy revealed that the atoms standing in the 16c sites were Al3+ ions. Beyond the depth of 10 nm, the diamond shapes remained hollow as before (Figure 1C), confirming that the surface modification did not affect the inner part of the Al-LNMO sample. The authors evaluated the effectiveness of the SVSO strategy by examining the electrochemical properties of Al-LNMO. When comparing the cycling performance of Al-LNMO and pristine LNMO, although the two samples showed similar charge-discharge curves at the first cycle, the authors observed major differences after long-term cycling. After 150 cycles, the pristine LNMO sample exhibited both obvious decay in specific capacity and a substantial change in working voltage, which originated from the severe Mn dissolution and electrolyte decomposition.9Pieczonka N.P.W. Liu Z. Lu P. Olson K.L. Moote J. Powell B.R. Kim J.-H. Understanding transition-metal dissolution behavior in LiNi0.5Mn1.5O4 high-voltage spinel for lithium ion batteries.J. Phys. Chem. C. 2013; 117: 15947-15957Crossref Scopus (462) Google Scholar On the contrary, Al-LNMO exhibited almost the same charge-discharge profiles before and after long-term cycling, and the capacity retention was improved from 85.4% to 97.6% as a result of the surface modification. The improved structural stability was experimentally verified in various post-cycling characterizations. Intriguingly, the insertion of Al3+ also led to a boost in rate capability, which the authors understood as the formation of a much more stable cathode-electrolyte interface that was absent in untreated LNMO. Moreover, the surface treatment reduced the migration probability of TM ions while imposing no adverse effects on Li+ diffusion, as confirmed by theoretical simulations. All of the above evidence suggests a highly effective means of structurally engineering previously problematic cathode materials. Cao and co-workers’ SVSO strategy opens up a promising route for the stabilization of high-voltage cathode materials of LIBs. In particular, the demonstrated surface-modification mechanism, which directly modulates the vacant lattice sites on the surface without sacrificing the mobility of Li+, is rather distinctive and possesses high practical significance. Insight into the crystal structure and thermodynamics operating at the atomic scale in the synthesis process are needed for extending the current strategy to various other cathode materials. It will be equally important to explore variations of the strategy that might allow more choices of occupying atoms to combine structural stabilization and performance enhancement. Lastly, Cao and co-workers’ strategy could also provide an alternative mechanism for tuning the surface chemistry of functional materials in different fields, where material properties are critically determined by surface atomic structure. Stabilizing Cathode Materials of Lithium-Ion Batteries by Controlling Interstitial Sites on the SurfacePiao et al.ChemMay 24, 2018In BriefA surface-vacant-site-occupation (SVSO) strategy is reported to be very effective at stabilizing the high-voltage cathode. The precise control of the interstitial lattice sites on the surface is able to modulate the movement of metal ions by suppressing the diffusion and dissolution of transition metal ions while maintaining the mobility of Li+, resulting in a very stable cathode with excellent cyclability and rate capability. Full-Text PDF Open Archive