Engineering of cobalt-free Ni-rich cathode material by dual-element modification to enable 4.5 V-class high-energy-density lithium-ion batteries

Highlights

• Ni-rich Co-free cathode by In/Sn dual modification is synthesized.

• In/Sn dual modification inhibits Li⁺/Ni²⁺ mixing and oxygen release.

• In-situ formed LiInO₂ layer stabilizes the cathode/electrolyte interface.

Abstract

Ni-rich Co-free cathodes have attracted extensive attention for high-energy–density lithium ion batteries (LIBs). However, structural and interfacial instability in these cathodes accelerates capacity degradation under high-voltage operation. Herein, Ni-rich Co-free In/Sn dual-element modified cathode (InSn-LiNi_0.85Mn_0.09Al_0.06O_2, InSn-NMA85) was synthesized through a one-step sintering strategy. Dual-element doping along with the in-situ induced LiInO₂ interphase synergistically prolongs the cycle life of the Ni-rich Co-free cathode under high voltage (≥4.5 V) as well as high temperature (≥45 °C). Comprehensive characterizations combined with DFT calculation confirm that In/Sn dual-element modification effectively increases Li⁺/Ni²⁺ mixing energy and oxygen release energy, stabilizes the lattice structure, and improves the electrochemical performance. Meanwhile, in-situ formed coating 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. The modified cathode maintains superior capacity retention of ∼ 100 % and ∼ 90 % within the voltage range of 2.7–4.5 V at 30 °C and 45 °C, respectively, after 100 cycles. The modification strategy enables the Ni-rich Co-free layered NMA85 cathode to deliver comparable battery performance with NCM and NCA cathodes, which provides promising approaches for the application of Ni-rich Co-free cathode in 4.5 V-class high-energy–density LIBs.

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 LiNi_1-x-yCo_xMn_yO₂ (NCM) and LiNi_1-x-yCo_xAl_yO₂ (NCA) (1-x-y > 0.5), with the α-NaFeO₂ structure ($\mathrm{R}\overline{3}$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 (LiNi_0.85Mn_0.09Al_0.06O₂, 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.