Lithium-ion batteries (LIBs) are crucial for energy storage, and high-energy-density cathodes are key to improving their performance. Nickel-rich layered transition metal oxides (NROs) are promising cathode materials because of their high energy density, cost-effectiveness, and environmental friendliness compared to cobalt-based cathodes. However, the high nickel content in NROs leads to several challenges: 1. **Large Volume Change:** During the charge-discharge process, the significant change in volume causes mechanical stress within the cathode material. This stress leads to particle pulverization, which can cause a decline in capacity and cycling stability. 2. **Oxygen Instability:** The high oxidation states of nickel increase the possibility of oxygen release from the cathode. This results in the formation of oxygen vacancies, which degrade the structure of the cathode material and negatively impact the overall performance of the battery. The formation of oxygen vacancies also causes side reactions between the electrolyte and the cathode, thus leading to poor cycle life. 3. **Phase Transitions:** The charging-discharging process induces phase transitions (layered to spinel to rock-salt phase) within the cathode material. These phase transitions generate additional strain, further contributing to the degradation of the structure and performance. To overcome these problems, researchers have explored different strategies, primarily focusing on surface coating, bulk doping, and single-crystal synthesis. Bulk doping aims to improve the structural stability of NROs by incorporating other high-valent cations into the lattice. While successful to some extent, these methods have shown limited success in mitigating all these issues. A deeper understanding of local structure changes at an atomic level is necessary to better design more effective doping strategies. This study aims to address the limitations of previous doping strategies by introducing tellurium (Te) into the nickel-rich layered cathode to achieve improved performance and better structural stability.

Numerous studies have investigated strategies to improve the stability of high-nickel layered oxide cathode materials. These strategies generally focus on three key areas: 1. **Bulk Doping:** Introducing high-valent cations like Ta5+ or W6+ into the bulk structure has been shown to enhance structural stability and mitigate lattice strain. This strategy has been reported to result in radially distributed particles, which improves the performance of the batteries over long cycles. 2. **Surface Coating:** Coating the cathode particles with protective layers can prevent surface side reactions and enhance stability. This helps to reduce the formation of oxygen vacancies and maintain the structural integrity of the cathode material. 3. **Gradient Materials and Single Crystals:** Synthesizing gradient materials or single crystals reduces the concentration of stress points and minimizes the strain caused by volume changes during cycling. This can reduce the material degradation and improve the overall performance of the battery. However, the understanding of the underlying mechanisms is insufficient, especially at the local structural scale. Previous doping approaches haven't fully addressed the intricate interplay between electronic structure and oxygen evolution, leaving room for improvement in both performance and sustainability.

The researchers synthesized the ultrahigh-nickel layered oxide LiNi0.94Co0.05Te0.01O2 (NC95T) cathode material using a two-step process. First, a coprecipitation method was used to prepare the precursor, Ni0.94Co0.05Te0.01(OH)2. This involved dissolving nickel sulfate, cobalt sulfate, and tellurous acid in deionized water, followed by the addition of a NaOH solution and ammonia as a chelating agent to control the pH and precipitate the desired metal hydroxides. The resulting precursor was then washed, dried and mixed with LiOH·H2O to obtain the desired LiNi0.94Co0.05Te0.01O2. This mixture was then calcined at high temperatures in an oxygen atmosphere. A control sample, LiNi0.95Co0.05O2 (NC95), was synthesized using a similar method without the tellurium. The electrochemical performance of both NC95T and NC95 was evaluated using coin cells and pouch cells with lithium metal or silicon-carbon anodes. Various characterization techniques were employed to investigate the materials' structure and properties, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), 3D atom probe tomography (APT), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), transmission Kikuchi diffraction (TKD), neutron diffraction, in situ XRD, and X-ray absorption spectroscopy (XAS). Density functional theory (DFT) calculations were conducted to understand the electronic structure and oxygen stability. In situ differential electrochemical mass spectrometry (DEMS) measurements were used to monitor gas evolution during the charge-discharge process. The electrochemical testing included cycling performance, rate capability, and energy density measurements. Specifically, for the electrochemical testing, coin cells (2032 type) were assembled in an argon-filled glove box, using lithium metal anodes, Celgard 2320 separators, and a standard electrolyte (1.2 M LiPF6 in EC:EMC). Pouch cells were assembled with Li metal and silicon-carbon anodes. Cycling tests were performed with different current rates and voltage windows to evaluate the capacity, rate capability, and cycling stability at both ambient and elevated temperatures. The XAS experiments (Ni K-edge, Te L-III edge, and O K-edge) examined the oxidation states, local atomic structures, and electronic structure evolution during the charging process. This detailed multi-technique approach provides a comprehensive understanding of the material's behavior.

The key findings of this research can be summarized as follows: 1. **Improved Electrochemical Performance:** The LiNi0.94Co0.05Te0.01O2 (NC95T) cathode showed superior electrochemical performance compared to the LiNi0.95Co0.05O2 (NC95) control sample. NC95T exhibited a higher initial discharge capacity (239 mAh g−1 at 0.1 C) and significantly improved cycling stability (94.5% capacity retention after 200 cycles at 0.5 C) compared to NC95 (59.2% retention). At 55°C, NC95T showed 87% capacity retention, while NC95 only retained 33%. The energy density of a pouch cell with a silicon-carbon anode reached 404 Wh kg−1, with 91.2% retention after 300 cycles. 2. **Ordered Superstructure:** Characterization techniques, including XRD, HAADF-STEM, and APT, revealed the formation of a Te-Ni-Ni-Te ordered superstructure within the transition metal (TM) layers of NC95T. This ordered arrangement was found to be more thermodynamically favorable than a random distribution of Te atoms, as confirmed by DFT calculations. This superstructure did not disappear after cycling confirming the stability of the ordered structure. 3. **Grain Refinement:** The introduction of tellurium also resulted in a refined particle morphology compared to NC95. This refinement is believed to have contributed to the improved cycling stability by reducing the mechanical stress generated during volume changes. TKD analysis showed that there is no preferred orientation, unlike previous results with similar dopants. 4. **Enhanced Oxygen Stability:** XAS studies showed that the NC95T cathode demonstrated superior oxygen stability compared to NC95. In NC95, oxygen release occurred above 4.3V (as evidenced by the detection of CO2 gas in DEMS measurements), while the NC95T exhibited no gas release up to 4.6V, suggesting the effective suppression of oxygen redox reactions. The analysis of the electronic structure suggests that this is due to the change in the electronic structure with the presence of the Te and the Te-Ni-Ni-Te superstructure. DFT calculations further supported this observation, showing significantly higher oxygen vacancy formation energy in NC95T compared to NC95. 5. **Strain Mitigation:** In situ XRD and GPA analysis showed that NC95T exhibited significantly lower lattice strain during cycling compared to NC95. This reduction in strain is a consequence of both the grain refinement and the ordered superstructure, which effectively accommodates the volume changes during the charge-discharge process, preventing particle cracking and phase transitions. The strain change of NC95T in the delithiated state was approximately three times lower than that of NC95.

This research successfully demonstrates a multi-effect strategy for enhancing the stability and performance of high-nickel layered oxide cathodes. The introduction of tellurium not only leads to grain refinement but also results in a unique ordered superstructure within the transition metal layer. This unique structure is crucial in mitigating lattice strain, enhancing the stability of the oxygen framework, and preventing irreversible phase transitions. The results directly address the long-standing challenge of balancing energy density and cycle life in Ni-rich cathode materials. The lack of preferred orientation in the grains also suggests that this effect is a new phenomena not seen in previous dopants. The significantly improved cycling performance at both room temperature and elevated temperatures highlight the effectiveness of this strategy for practical applications. The achievement of an energy density exceeding 400 Wh kg−1 is a significant step toward the development of next-generation high-energy-density LIBs. The findings provide valuable insights into the design of advanced cathode materials by highlighting the importance of not only grain size reduction but also precise control of the local atomic arrangement.

This study presents a novel strategy for improving the performance and stability of ultrahigh-nickel layered oxide cathodes by introducing tellurium. The resulting material exhibits exceptional cycling stability and high energy density, outperforming existing nickel-rich cathodes. The key to this improvement lies in the synergistic effects of grain refinement and the formation of a Te-Ni-Ni-Te ordered superstructure, which effectively mitigates lattice strain and enhances the stability of the lattice oxygen framework. This work opens new avenues for designing high-performance cathode materials with improved sustainability, paving the way for the development of next-generation LIBs.

While this study demonstrates the significant advantages of the Te-doped cathode, a few limitations exist. Firstly, the synthesis method could be further optimized for large-scale production. Secondly, while the study provides substantial evidence for enhanced oxygen stability, the precise nature of the interaction between Te and oxygen at the atomic level remains an area for further investigation. Finally, long-term cycling tests at higher current rates are needed to fully assess the long-term performance of the cells under more demanding conditions. Further investigation into different electrolyte compositions and their interaction with the NC95T cathode are also recommended.