Consummating ion desolvation in hard carbon anodes for reversible sodium storage.

Hard carbons are emerging as the most viable anodes to support the commercialization of sodium-ion (Na-ion) batteries due to their competitive performance. However, the hard carbon anode suffers from low initial Coulombic efficiency (ICE), and the ambiguous Na-ion (Na+) storage mechanism and interfacial chemistry fail to give a reasonable interpretation. Here, we have identified the time-dependent ion pre-desolvation on the nanopore of hard carbons, which significantly affects the Na+ storage efficiency by altering the solvation structure of electrolytes. Consummating the pre-desolvation by extending the aging time, generates a highly aggregated electrolyte configuration inside the nanopore, resulting in negligible reductive decomposition of electrolytes. When applying the above insights, the hard carbon anodes achieve a high average ICE of 98.21% in the absence of any Na supplementation techniques. Therefore, the negative-to-positive capacity ratio can be reduced to 1.02 for full cells, which enables an improved energy density. The insight into hard carbons and related interphases may be extended to other battery systems and support the continued development of battery technology.

Clarifying the Temperature-Dependent Lithium Deposition/Stripping Process and the Evolution of Inactive Li in Lithium Metal Batteries.

The deposition/stripping behavior of lithium metal is intriguing, and the associated formation of inactive lithium at various temperatures remains elusive, which hinders the practical application of lithium metal batteries. Here, utilizing the variable-temperature operando solid-state nuclear magnetic resonance (SS NMR) technique, we reveal the temperature effects on the lithium microstructure evolution in a carbonate-based electrolyte system. In addition, the mass spectrometry titration (MST) method is used to quantify the evolution of inactive lithium components, including dead lithium, solid electrolyte interface (SEI), and lithium hydride (LiH). Combined SS NMR and MST results show that the morphology of lithium metal is reasonably correlated to the amount of inactive Li formed. At low/ambient temperature, the lithium microstructure has a similar evolution pattern, and its poor morphology leads to a large amount of dead lithium, which dominates capacity loss; however, at high temperature large and dense lithium deposits form with less dead Li detected, and the intensified electrolyte consumption in SEI formation is the major cause for capacity loss. Our phase-field simulation results reveal that the compact lithium deposition formed at higher temperature is due to the more uniformly distributed electric field and Li+ concentration. Lastly, two strategies in forming a dense Li deposit are proposed and tested that show performance-enhancing results.

The applications of solid-state NMR and MRI techniques in the study of rechargeable sodium-ion batteries.

In order to develop new electrode and electrolyte materials for advanced sodium-ion batteries (SIBs), it is crucial to understand a number of fundamental issues. These include the compositions of the bulk and interface, the structures of the materials used, and the electrochemical reactions in the batteries. Solid-state NMR (SS-NMR) has unique advantages in characterizing the local or microstructure of solid electrode/electrolyte materials and their interfaces-one such advantage is that these are determined in a noninvasive and nondestructive manner at the atomic level. In this review, we provide a survey of the recent advances in the understanding of the fundamental issues of SIBs using advanced NMR techniques. First, we summarize the applications of SS-NMR in characterizing electrode material structures and solid electrolyte interfaces (SEI). In particular, we elucidate the key role of in-situ NMR/MRI in revealing the complex reactions and degradation mechanisms of SIBs. Next, the characteristics and shortcomings of SS-NMR and MRI techniques in SIBs are also discussed in comparison to similar Li-ion batteries. Finally, an overview of SS-NMR and MRI techniques for sodium batteries are briefly discussed and presented.

Gas induced formation of inactive Li in rechargeable lithium metal batteries.

The formation of inactive lithium by side reactions with liquid electrolyte contributes to cell failure of lithium metal batteries. To inhibit the formation and growth of inactive lithium, further understanding of the formation mechanisms and composition of inactive lithium are needed. Here we study the impact of gas producing reactions on the formation of inactive lithium using ethylene carbonate as a case study. Ethylene carbonate is a common electrolyte component used with graphite-based anodes but is incompatible with Li metal anodes. Using mass spectrometry titrations combined with 13C and 2H isotopic labeling, we reveal that ethylene carbonate decomposition continuously releases ethylene gas, which further reacts with lithium metal to form the electrochemically inactive species LiH and Li2C2. In addition, phase-field simulations suggest the non-ionically conducting gaseous species could result in an uneven distribution of lithium ions, detrimentally enhancing the formation of dendrites and dead Li. By optimizing the electrolyte composition, we selectively suppress the formation of ethylene gas to limit the formation of LiH and Li2C2 for both Li metal and graphite-based anodes.

Quantifying the Evolution of Inactive Li/Lithium Hydride and Their Correlations in Rechargeable Anode-free Li Batteries.

Electrolyte optimization, such as using fluoride-bearing electrolytes, is regarded as an effective way to improve the cycle performance of lithium metal batteries (LMBs), but the promotion mechanisms of the electrolytes are in controversy due to the lack of quantitative understanding of the reaction products during cycling. Here, taking several fluorinated electrolytes as models, we use mass spectrometry titration (MST) and solid state nuclear magnetic resonance (NMR) techniques to quantify the evolution of dead Li metal, solid electrolyte interphases (SEI) and lithium hydride (LiH) during cycling. Our quantitative results clearly disclose that lithium difluoro(oxalato)borate (LiODFB) is able to inhibit the formation of SEI and LiH while fluoroethylene carbonate (FEC) mainly inhibits the formation of dead Li metal. Furthermore, we surprisingly observe a linear correlation between LiH and SEI formation, whereas the commonly mentioned lithium fluoride (LiF) shows a weak correlation with either dead Li metal or SEI. Guided by the clear failure mechanism, we can provide a reasonable explanation for the synergistic effect with the combination of LiODFB and FEC from a quantitative perspective. We believe that a quantitative insight of electrolytes on the failure mechanism of LMBs will guide us to explore the functional electrolytes to achieve the practical application of LMBs.

Elastic Lattice Enabling Reversible Tetrahedral Li Storage Sites in a High-Capacity Manganese Oxide Cathode.

The key to breaking through the capacity limitation imposed by intercalation chemistry lies in the ability to harness more active sites that can reversibly accommodate more ions (e.g., Li+ ) and electrons within a finite space. However, excessive Li-ion insertion into the Li layer of layered cathodes results in fast performance decay due to the huge lattice change and irreversible phase transformation. In this study, an ultrahigh reversible capacity is demonstrated by a layered oxide cathode purely based on manganese. Through a wealth of characterizations, it is clarified that the presence of low-content Li2 MnO3 domains not only reduces the amount of irreversible O loss; but also regulates Mn migration in LiMnO2 domains, enabling elastic lattice with high reversibility for tetrahedral sites Li-ion storage in Li layers. This work utilizes bulk cation disorder to create stable Li-ion-storage tetrahedral sites and an elastic lattice for layered materials, with a reversible capacity of 600 mA h g-1 , demonstrated in th range 0.6-4.9 V versus Li/Li+ at 10 mA g-1 . Admittedly, discharging to 0.6 V might be too low for practical use, but this exploration is still of great importance as it conceptually demonstrates the limit of Li-ions insertion into layered oxide materials.

Mitigating the Surface Reconstruction of Ni-Rich Cathode via P2-Type Mn-Rich Oxide Coating for Durable Lithium Ion Batteries.

Ni-rich materials have received widespread attention as one of the mainstream cathodes in high-energy-density lithium-ion batteries for electric vehicles. However, Ni-rich cathodes suffer from severe surface reconstruction in a high delithiation state, constraining their rate capabilities and life span. Herein, a novel P2-type NaxNi0.33Mn0.67O2 (NNMO) is rationally selected as the surficial modification layer for LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode, which undergoes a spontaneous Na+-Li+ exchange reaction to form an O2-type LixNi0.33Mn0.67O2 (LNMO) layer revealed by combining X-ray diffraction and solid-state nuclear magnetic resonance techniques. Owing to the specific oxygen stacking sequence, O2-type LNMO significantly prevents the initial layered structure of NCM811 from transforming to the spinel or rock-salt phases during cycling, thus effectively maintaining the integral surficial structure and the Li+ diffusion channels of NCM811. Eventually, the NNMO@NCM811 electrode yields enhanced thermal stability, outstanding rate performance, and long cycling stability with 80% capacity retention after 294 cycles at 200 mA g-1, and its life span is further extended to 531 cycles while enhancing the mechanical stability of the bulk material.

P2-Na0.67 Alx Mn1-x O2 : Cost-Effective, Stable and High-Rate Sodium Electrodes by Suppressing Phase Transitions and Enhancing Sodium Cation Mobility.

Sodium layered P2-stacking Na0.67 MnO2 materials have shown great promise for sodium-ion batteries. However, the undesired Jahn-Teller effect of the Mn4+ /Mn3+ redox couple and multiple biphasic structural transitions during charge/discharge of the materials lead to anisotropic structure expansion and rapid capacity decay. Herein, by introducing abundant Al into the transition-metal layers to decrease the number of Mn3+ , we obtain the low cost pure P2-type Na0.67 Alx Mn1-x O2 (x=0.05, 0.1 and 0.2) materials with high structural stability and promising performance. The Al-doping effect on the long/short range structural evolutions and electrochemical performances is further investigated by combining in situ synchrotron XRD and solid-state NMR techniques. Our results reveal that Al-doping alleviates the phase transformations thus giving rise to better cycling life, and leads to a larger spacing of Na+ layer thus producing a remarkable rate capability of 96 mAh g-1 at 1200 mA g-1 .