Suppressing Bulk Strain and Surface O2 Release in Li-Rich Cathodes by Just Tuning the Li Content

Suppressed Degradation in Semiconducting Lithium-Rich Oxide Cathode Materials via Phase Structure Engineering

Owing to anionic oxygen redox (O redox), cathode materials containing lithium-rich oxides (LROs) exhibit a large discharge capacity exceeding 300 mAh·g-1, in addition to a decent midpoint voltage (∼3.5 V). This makes them viable choices for the fabrication of cathode materials for future development of 500 Wh·kg-1 lithium-ion batteries (LIBs). However, O redox is irreversible. This results in fast degradation of their voltage/capacity during cycling, in addition to a low initial Coulombic efficiency. In this work, we address the problem of degradation during cycling by phase structure engineering (PSE) of Li-rich C2/m and R3̅m through modification of the transition metal (TM) composition. Apart from NCM111, we intentionally incorporate NCM262, NCM523, NCM622, Ni0.75Co0.25, and NCM811 with the Li-rich Li2MnO3 phase domain, so that a series of composite-phase LRO nanocrystals (N20, N50, N60, N75, and N80, respectively) are fabricated, which exhibit an increased midpoint voltage (∼3.8 V) with improved cycling stability. For N75, the voltage fade is suppressed, with retention of 88.07% in voltage and a loss of 1.12 mV per cycle, which results in an increased retention of energy density (63.68%) after 400 cycles at 1C (RT, 2.0-4.8 V). This work provides routes to achieve lithium-rich oxides with high energy density and long lifespan.

Exploring the Mechanism of Surface Cationic Vacancy Induces High Activity of Metastable Lattice Oxygen in Li- and Mn-Rich Cathode Materials

Li- and Mn-rich layered oxides exhibit high specific capacity due to the cationic and anionic reaction process during high-voltage cycling (≥4.6 V). However, they face challenges such as low initial coulombic efficiency (~70 %) and poor cycling stability. Here, we propose a combination of H3BO3 treatment and low temperature calcination to construct a shell with cationic vacancy on the surface of Li1.2Ni0.2Mn0.6O2 (LLNMO). The H3BO3 treatment produces cationic vacancy and lattice distortion, forming an oxidized On- (0 Collapse

Key Words

• Li-rich cathodes
• cationic vacancy
• coulombic efficiency
• metastable state
• oxygen activation

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Grants

• 22108218 the National Natural Science Foundation of China
• 22105059 the National Natural Science Foundation of China

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Affiliation(s)

Tian Zhao School of Chemical Engineering and Technology, Xi'an Jiaotong University, No. 28, West Xianning Road, Xi'an, Shaanxi, 710049, China
Jilu Zhang School of Chemical Engineering and Technology, Xi'an Jiaotong University, No. 28, West Xianning Road, Xi'an, Shaanxi, 710049, China Institute for Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany
Kai Wang School of Materials and Energy, Lanzhou University, No. 222, Tianshui South Road, Lanzhou, 730000, China
Yao Xiao College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, 325035, P. R. China
Qin Wang School of Chemical Engineering and Technology, Xi'an Jiaotong University, No. 28, West Xianning Road, Xi'an, Shaanxi, 710049, China School of Chemical Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, 610065, Chengdu, China
Longfei Li School of Chemistry, Engineering Research Center of Energy Storage Materials, Devices of Ministry of Education, National Innovation Platform (Center) for Industry-Education Integration of Energy Storage Technology, Xi'an Jiaotong University, Xi'an, 710049, Shanxi, China
Jochi Tseng Diffraction and Scattering Division, Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo, 679-5198, Japan
Meng-Cheng Chen Department of Applied Chemistry and Center for Emergent Functional Matter Science, National Yang Ming Chiao Tung University, Hsinchu, 300, Taiwan
Jian-Jie Ma Department of Applied Chemistry and Center for Emergent Functional Matter Science, National Yang Ming Chiao Tung University, Hsinchu, 300, Taiwan
Ying-Rui Lu National Synchrotron Radiation Research Center, Hsinchu, 300, Taiwan
Ishii Hirofumi National Synchrotron Radiation Research Center, Hsinchu, 300, Taiwan
Yu-Cheng Shao National Synchrotron Radiation Research Center, Hsinchu, 300, Taiwan
Xiaoxian Zhao Department of Chemistry, College of Science, Hebei Agriculture University, Baoding, 071001, P. R. China
Sung-Fu Hung Department of Applied Chemistry and Center for Emergent Functional Matter Science, National Yang Ming Chiao Tung University, Hsinchu, 300, Taiwan
Yaqiong Su School of Chemistry, Engineering Research Center of Energy Storage Materials, Devices of Ministry of Education, National Innovation Platform (Center) for Industry-Education Integration of Energy Storage Technology, Xi'an Jiaotong University, Xi'an, 710049, Shanxi, China
Xiaoke Mu School of Materials and Energy, Lanzhou University, No. 222, Tianshui South Road, Lanzhou, 730000, China
Weibo Hua School of Chemical Engineering and Technology, Xi'an Jiaotong University, No. 28, West Xianning Road, Xi'an, Shaanxi, 710049, China Institute for Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany

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Formulating Electrophilic Electrolyte for In Situ Stabilization of 4.8 V Li-Rich Batteries with 100% Initial Coulombic Efficiency

Lithium-rich layered oxide (LLO) cathodes are expected to overcome the energy density limitations, but their applicability is hindered by low initial Coulombic efficiency (ICE) and unstable electrode-electrolyte interphases with sluggish kinetics. Here an elaborate electrophilic electrolyte is proposed that effectively stabilizes the surface lattice oxygen of the LLO cathode, facilitates the formation of dense and fast-ion-transport electrode-electrolyte interphases, and prevents Li-dendrites on the anode. The nucleophilic reaction mechanism driven by the electrolyte enables LLO to exhibit a reversible capacity of 310 mAh g-1 with a record ICE of 100%, as well as impressive 3C fast-charging stability, remarkably superior to that in the basic electrolyte. Using this engineered electrolyte, the assembled 4.5 Ah-class pouch cell of graphite||LLO displays high energy density and remarkable reversibility during cycling, demonstrating wide applicability. This work provides valuable insights and pragmatic strategies in electrolyte chemical engineering for advancing high-energy density and fast-charging batteries.

Mitigating Diffusion-Induced Intragranular Cracking in Single-Crystal LiNi0.5Mn1.5O4 via Extended Solid-Solution Behavior

Single-crystal cathodes have been investigated for their inherent resistance to intergranular cracking due to the absence of grain boundaries. However, these materials exhibit significant intragranular cracking, and the underlying mechanisms remain unclear. In this study, we examined the impact of extended solid-solution reactions on mitigating crack formation in magnesium-doped single-crystal LiNi0.5Mn1.5O4 (Mg-SC-LNMO) cathodes. With Mg acting as a structural pillar, the overall volume change was reduced by nearly 50 %, the two-phase reaction was effectively suppressed, and the Li-ion diffusion coefficient was doubled. Continuum modeling based on experimental observations demonstrates that Mg doping significantly reduces the internal stress induced by lithium diffusion, thereby preserving the mechanical integrity of single-crystal LNMO. This improvement leads to enhanced electrochemical performance and durability. Our study provides new insights into mechanically robust single-crystal cathodes and proposes a design strategy to improve the durability of next-generation Li-ion batteries.

A comprehensive understanding on the anionic redox chemistry of high-voltage cathode materials for high-energy-density lithium-ion batteries

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.

Simultaneous Regulating the Surface, Interface, and Bulk via Phosphating Modification for High-Performance Li-Rich Layered Oxides Cathodes

Li-rich Mn-based layered oxides (LRMOs) are regarded as the leading cathode materials to overcome the bottleneck of higher energy density. Nevertheless, they encounter significant challenges, including voltage decay, poor cycle stability, and inferior rate performance, primarily due to irreversible oxygen release, transition metal dissolution, and sluggish transport kinetics. Moreover, traditionally single modification strategies do not adequately address these issues. Herein, an innovative "all-in-one" modification strategy is developed, simultaneously regulating the surface, interface, and bulk via an in-situ gas-solid interface phosphating reaction to create P-doped Li1.2Mn0.54Ni0.13Co0.13O2@Spinel@Li3PO4. Specifically, Li3PO4 surface coating layer shields particles from electrolyte corrosion and enhances Li+ diffusion; in-situ constructed spinel interfacial layer reduces phase distortion and suppresses the lattice strain; the strong P─O bond derived from P-doping stabilizes the lattice oxygen frame and inhibits the release of O2, thereby improving the reversibility of oxygen redox reaction. As a result, the phosphatized LRMO demonstrates an exceptional capacity retention of 82.1% at 1C after 300 cycles (compared to 50.8% for LRMO), an outstanding rate capability of 170.5 mAh g-1 at 5C (vs 98.9 mAh g-1 for LRMO), along with excellent voltage maintenance and thermostability. Clearly, this "all-in-one" modification strategy offers a novel approach for high-energy-density lithium-ion batteries.

Resolving the relationship between capacity/voltage decay and the phase transition by accelerating the layered to spinel transition

Lithium-rich cathode materials are some of the most promising choices for lithium-ion batteries due to their excellent energy density (>900 W h kg-1). However, severe voltage/capacity degradation during cycling has seriously hindered the further commercialization of lithium-rich cathode materials. Current research efforts are focused on enhancing their voltage and capacity retention. Here, the coating of FeF3 on specific crystal planes is utilized to achieve a degradation trend that is very different from that of the as-received material. Using this as an entry point, the relationship between voltage and capacity degradation was studied in depth. The oriented coated material undergoes a more drastic phase transition during cycling, yet its voltage decay remains basically the same as that of the original sample (769.6 mV after 200 cycles, compared to 723.5 mV for the original sample). Notably, the capacity retention rate is significantly improved (97% after 200 cycles vs. 75% for the pristine material). These findings suggest that the capacity degradation and the voltage decay do not interact with each other and that the phase transition during cycling does not seem to negatively affect the voltage. This conclusion can also be extrapolated to other oxygen-reducing oxide systems to help understand the relationship between capacity and voltage decay. The modification is generalized and applicable to other cathode materials.

Comparative impact of surface and bulk fluoride anion doping on the electrochemical performance of co-free Li-rich Mn-based layered cathodes

Li-rich Mn-based (LMR) layered oxides are considered promising cathode materials for high energy-density Li-ion batteries. Nevertheless, challenges such as irreversible oxygen loss at the surface during the initial charge, alteration of the bulk structure, and poor rate performance impede their path to commercialisation. Most modification methods focus on specific layers, making the overall impact of modifications at various depths on the properties of materials unclear. This research presents an approach by using doping to adjust both surface and bulk properties; the materials with surface and bulk fluoride anion doping are synthesised to explore the connection between doping depth, structural and electrochemical stability. The surface-doped material significantly improves the initial Coulombic efficiency (ICE) from 77.85% to 85.12% and limits phase transitions, yet it does not enhance rate performance. Conversely, doping in bulk stands out by improving both rate performance and cyclic stability: it increases the specific discharge capacity by around 60 mAh g-1 and enhances capacity retention from 57.69% to 82.26% after 300 cycles at 5C. These results highlight a notable dependence of material properties on depth, providing essential insights into the mechanisms of surface and bulk modifications.

Toward High-Performance Li-Rich Mn-Based Layered Cathodes: A Review on Surface Modifications

Lithium-rich manganese-based layered oxides (LRMOs) have received attention from both the academic and the industrial communities in recent years due to their high specific capacity (theoretical capacity ≥250 mAh g-1), low cost, and excellent processability. However, the large-scale applications of these materials still face unstable surface/interface structures, unsatisfactory cycling/rate performance, severe voltage decay, etc. Recently, solid evidence has shown that lattice oxygen in LRMOs easily moves and escapes from the particle surface, which inspires significant efforts on stabilizing the surface/interfacial structures of LRMOs. In this review, the main issues associated with the surface of LRMOs together with the recent advances in surface modifications are outlined. The critical role of outside-in surface decoration at both atomic and mesoscopic scales with an emphasis on surface coating, surface doping, surface structural reconstructions, and multiple-strategy co-modifications is discussed. Finally, the future development and commercialization of LRMOs are prospected. Looking forward, the optimal surface modifications of LRMOs may lead to a low-cost and sustainable next-generation high-performance battery technology.

Redox Couple Strategy for Improving the Oxygen Redox Activity and Reversibility of Li- and Mn-Rich Cathode Materials

The specific capacity of Li- and Mn-rich layered oxide (LMROs) cathodes can be enhanced by the oxidation of lattice oxygen at high voltages. Nevertheless, an irreversible oxygen loss emerges with cycling, which triggers interlocking surface/interface issues and results in the fast deterioration of cycling performance. Herein, we prepare a surface modified LMRO electrode by one step doctor-blade casting and introducing a benzoquinone species DBBQ redox couple. The electrochemical test shows that the DBBQ-modified electrode has a high reversible capacity (>320 mAh g-1) and excellent rate performance, while the cyclic stability has been significantly improved as well. The capacity retention reaches as high as 93.3% after 500 cycles at 1 C. Mechanism analysis shows that DBBQ can not only play a redox couple in LMROs which achieves the adsorption and reduction of surface oxygen gas but also significantly enhance anionic redox in the bulk, thus realizing extraordinary capacity.

Band Structure Engineering Promotes Anionic Redox Reversibility of Cobalt-Free Li-Rich Layered Oxides Cathodes

Li-rich layered oxides cathodes (LLOs) have prevailed as the promising high-energy-density cathode materials due to their distinctive anionic redox chemistry. However, uncontrollable anionic redox process usually leads to structural deterioration and electrochemical degradation. Herein, a Mo/Cl co-doping strategy is proposed to regulate the relative position of energy band for modulating the anionic redox chemistry and strengthening the structural stability of Co-free Li1.16Mn0.56Ni0.28O2 cathodes. The incorporation of Mo with high d state orbit and Cl with low electronegativity can narrow the band energy gap between bonding and antibonding bands via increasing the filled lower-Hubbard band (LHB) and decreasing the non-bonding O 2p energy bands, promoting the anionic redox reversibility. In addition, strong covalent Mo─O and Mn─Cl bonding further increases the covalency of Mn─O band to further stabilize the O2 n- species and enhance the reversible distortion of MnO6 octahedron. The strengthening electronic conductivity, together with the epitaxial structure Li2MoO4 facilitates the fast Li+ kinetics. As a result, the dual doping material exhibits enhanced anionic redox reversibility and suppressed oxygen release with increased cyclic stability and excellent rate performance. This strategy provides some guidance to design high-energy-density LLOs with desirable anionic redox reversibility and stable crystal structure via band structure engineering.

Gradient Interphase Engineering Enabled by Anionic Redox for High-Voltage and Long-Life Li-Ion Batteries

Intelligent utilization of the anionic redox reaction (ARR) in Li-rich cathodes is an advanced strategy for the practical implementation of next-generation high-energy-density rechargeable batteries. However, due to the intrinsic complexity of ARR (e.g., nucleophilic attacks), the instability of the cathode-electrolyte interphase (CEI) on a Li-rich cathode presents more challenges than typical high-voltage cathodes. Here, we manipulate CEI interfacial engineering by introducing an all-fluorinated electrolyte and exploiting its interaction with the nucleophilic attack to construct a gradient CEI containing a pair of fluorinated layers on a Li-rich cathode, delivering enhanced interfacial stability. Negative/detrimental nucleophilic electrolyte decomposition has been efficiently evolved to further reinforce CEI fabrication, resulting in the construction of LiF-based indurated outer shield and fluorinated polymer-based flexible inner sheaths. Gradient interphase engineering dramatically improved the capacity retention of the Li-rich cathode from 43 to 71% after 800 cycles and achieved superior cycling stability in anode-free and pouch-type full cells (98.8% capacity retention, 220 cycles), respectively.