Engineering of cobalt-free Ni-rich cathode material by dual-element modification to enable 4.5 V-class high-energy-density lithium-ion batteries
Graphical abstract
Ni-rich Co-free In/Sn dual-element modified cathode (InSn-LiNi0.85Mn0.09Al0.06O2, InSn-NMA85) was synthesized through one-step sintering strategy. In/Sn dual-element modification effectively inhibits Li+/Ni2+ mixing and oxygen release, stabilizes the lattice structure and improves the electrochemical performance. Meanwhile, an in-situ formed coating layer of LiInO2 effectively protects the cathode from redundant cathode–electrolyte side reactions, preserves the layered phase, and further inhibits the generation of microcracks after cycles.
Introduction
With the ever-rising demand for high-energy–density lithium-ion batteries (LIBs) nowadays, the development of high-voltage cathode materials with high capacity has attracted considerable attention [1], [2], [3]. Ni-rich cathode materials, including LiNi1-x-yCoxMnyO2 (NCM) and LiNi1-x-yCoxAlyO2 (NCA) (1-x-y > 0.5), with the α-NaFeO2 structure (m, space group) have high capacity and excellent electrochemical kinetic characteristics, and show great potential in reducing cost and improving energy density [4], [5], [6]. Co has been considered as an indispensable component because it efficiently reduces Li+/Ni2+ mixing, facilitates 2D diffusion in the Li+ plane, and improves the rate capability of the cathode [7], [8]. However, Co resources are scarce, expensive, and highly toxic, which motivates researchers to research on Ni-rich Co-free cathode materials [9]. Currently, Ni-rich Co-free cathodes still face problems including surface residual alkali, deficient rock salt phase formation and high-order Li+/Ni2+ mixing [10], [11], [12]. These issues severely damage the stability of cathodes, leading to structural collapse, transition metal dissolution, irreversible phase transformation, and lattice oxygen release, as a result, capacity dramatically degrades. Meanwhile, high temperature and high cut-off voltage operating conditions are necessary to realize a wide range of environmental practicability and elevated energy density. However, the structural stability of Ni-rich Co-free cathodes is poor when cycling at high temperature and high cut-off voltage. The high valence nickel ions are prone to irreversible reaction with the electrolyte, and the surface will be gradually corroded by the electrolyte. As a result, the layered structure is gradually destroyed from the inside out and the electrolyte is gradually consumed, eventually leading to rapid battery failure [13]. So far, the application of the Ni-rich Co-free cathodes is still quite challenging.
Doping is a low-cost and efficient strategy to alleviate the above-mentioned problems and enhance the structural stability and electrochemical performance of NCM and NCA cathodes [14], [15], [16]. Usually, the incorporation of high valence ions (such as Zr4+, Ti4+, Ta5+, Nb5+, W6+, Mo6+) can form strong interaction with lattice oxygen. It effectively prevents the release of lattice oxygen during cycling and induces the formation of lithium vacancies, thus greatly improving the diffusion efficiency of lithium ions [17], [18]. Sn4+ has larger ion radius and higher valence state, which can effectively regulate lattice parameters and reduce lithium ion diffusion barrier to improve lithium ion diffusion efficiency, and it is a candidate element to inhibit the contraction of lattice parameters in the c direction under high charge state, inhibit the migration of nickel ions, and reduce the release of oxygen [19]. Additionally, dual-ion co-doped Ni-rich cathodes exhibit improved performance due to synergetic improvement effect compared with single ion doping [20], [21]. However, there are few studies on the synergistic effect of Sn4+ and other transition metals ions in Ni-rich cathodes, especially in the Ni-rich Co-free cathode system for high voltage (≥4.5 V) and high temperature (≥45 °C) applications. Besides, the engineered surface structure of cathode, which is robust and stable, also plays a key role in reducing the cathode/electrolyte interfacial side reactions and improving the cyclability of the cathode materials [22]. In-situ formed stable interphase is quite attractive to enable highly reversible cathode, and various materials, like Li3PO4 [23], Li2MnO3 [24], and Li2SiO3 [25] have been utilized to establish stable interphase on the cathode. Moreover, it is imperative to understand the modification mechanism of the Ni-rich Co-free cathode by cation doping.
In this work, we report a one-step sintering strategy to introduce In/Sn dual-element modification in the Ni-rich Co-free cathode (LiNi0.85Mn0.09Al0.06O2, NMA85) and get the InSn-NMA85 cathode material. Structural evolutions of NMA85 and InSn-NMA85 cathodes during cycling are investigated by high-resolution transmission electron microscopy (HRTEM), in-situ XRD, and DFT calculations, etc. to reveal the modification effects at the atomic scale. In/Sn dual-element modification results in homogeneous doping in the NMA85 cathode, which not only effectively inhibits Li+/Ni2+ mixing and stabilizes the lattice oxygen to mitigate the Ni dissolution in the Ni-rich Co-free cathode, but also improves the lithium ion diffusion efficiency and synergistically improves the electrochemical performance of NMA85. Meanwhile, the dual-element modification simultaneously facilitates an in-situ formed LiInO2 coating layer on the surface. The in-situ formed LiInO2 layer successfully protects the NMA85 particles from the erosion of detrimental species generated by electrolyte decomposition and stabilizes the cathode/electrolyte interface. The irreversible phase transformation of NMA85 and redundant cathode/electrolyte interfacial side reactions during cycling are greatly suppressed by the synergistic effect of the in-situ formed LiInO2 layer together with the doping in the lattice. This work provides an insightful strategy on the development of Co-free Ni-rich cathodes for the high-energy–density rechargeable lithium battery with prolonged cycle life.
Section snippets
Synthesis and characterizations
The InSn-NMA85 cathode was synthesized via a one-step sintering strategy. Briefly, as shown in Figure S1, the LiNi0.85Mn0.09Al0.06(OH)2 with spherical particles of ∼ 11 μm was selected as the precursor. The modified cathode was synthesized by calcinating uniformly mixed Ni0.85Mn0.09Al0.06(OH)2, LiOH·H2O and In2O5Sn, denoted as InSn-NMA85. As shown in Fig. 1a, In2O3 in In2O5Sn reacts with LiOH and forms LiInO2 layer on the surface of materials (In2O3 + 2LiOH → 2LiInO2 + H2O) [26]. In addition,
Conclusion
In summary, we synthesized a Ni-rich Co-free In/Sn dual-element modified cathode through a one-step sintering strategy. Comprehensive structural and electrochemical characterizations provide clear insights into the correlation between the microstructure of Ni-rich cathodes and the corresponding capacity performance and mechanism. In/Sn dual-element modification improves the structural and interfacial stability in the Ni-rich Co-free cathode. Specifically, the In/Sn dual-element modification not
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by Guangdong Basic and Applied Basic Research Foundation (2022A1515010486), China Postdoctoral Science Foundation (2021M691750), Shenzhen Science and Technology Program (JCYJ20210324140804013, RCBS20200714115000219-Doctoral Startup Project), and Tsinghua Shenzhen International Graduate School (QD2021005N, JC2021007).
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These authors contributed equally to this work.