Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 38915−38921
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Facile Mn Surface Doping of Ni-Rich Layered Cathode Materials for Lithium Ion Batteries Woosuk Cho,† Young Jin Lim,‡ Sun-Me Lee,† Jong Hwa Kim,‡ Jun-Ho Song,† Ji-Sang Yu,† Young-Jun Kim,*,§ and Min-Sik Park*,‡
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Advanced Batteries Research Center, Korea Electronics Technology Institute, 25 Saenari-ro, Bundang-gu, Seongnam 13509, Republic of Korea ‡ Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin 17104, Republic of Korea § SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, 2066 Seobu-ro, Suwon 16419, Republic of Korea S Supporting Information *
ABSTRACT: A facile Mn surface doping process is proposed to improve the thermal and structural stabilities of Ni-rich layered cathode materials (Ni ≥ 80%) for lithium-ion batteries in electric vehicles. Herein, we demonstrate that the surface structure of the Ni-rich layered cathode materials can be stabilized by the introduction of a thin Mn-rich surface layer. This layer effectively reduces the direct exposure of the highly reactive Ni on the surface of the cathode materials, thus enhancing thermal stability and mitigating side reactions associated with highly reactive Ni that causes the loss of reversible capacity. In practice, the Mn surface-doped Ni-rich layered cathode material exhibits a high specific capacity with an improved cycling stability even at a high temperature (60 °C). We believe that our simple approach offers more opportunities to upscale production without any extra caution. KEYWORDS: Li-ion battery, cathode materials, surface doping, thermal stability, electrochemistry
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contributes to the power characteristics.9−11 Furthermore, the specific capacity can be effectively improved by increasing the Ni concentration because Ni is mainly associated with a reversible capacity based on the double redox reaction of Ni2+/ Ni4+ in the given structure.12−14 In practice, Ni-rich layered cathode materials (Ni ≥ 80%) exhibit a high specific capacity (>190 mA h g−1).1,12,13 Furthermore, they can be produced by a conventional coprecipitation process, which makes them suitable for mass production.15,16 Despite the benefits and research into Ni-rich layered cathode materials, they still suffer from thermal and structural instabilities.17,18 These induce significant dissolution of the transition metals and oxygen evolution during operation at elevated temperatures, leading to rapid capacity fading during cycling.19 Such instabilities mainly originate from the presence of directly exposed Ni at the surface because they are vulnerable to moisture as well as tend to form a rock-salt NiO phase.13,14,20,21 In addition, the degree of Li/Ni disorder increases with the increasing Ni content, resulting in a
INTRODUCTION The rapid expansion of the electric vehicle (EV) market has drawn particular emphasis to the importance of reliable power sources. Among various energy storage systems, a rechargeable lithium-ion battery (LIB) is highlighted as the most promising candidate for EV applications owing to its high-energy density and excellent power characteristic.1−3 For successful implementation, the energy density of the current LIBs should be further improved to extend the driving distance to ensure user convenience.4−6 Because the specific capacity and operating voltage of the cathode materials are the predominant factors for determining the energy density of LIBs, much research attention has been given to the development of advanced cathode materials.6,7 Over the last few decades, layered cathode materials (e.g., LiMO2, M = transition metals) have been widely used for commercial LIBs. Compared with other cathode materials, they have many benefits, such as high Li+ diffusivity, high reversibility, and low cost of mass production.8 In particular, the electrochemical performance of these cathode materials can be easily tuned by adjusting the composition of the transition metals. Mn is considered as a key element for maintaining the structural stability, whereas Co mainly © 2018 American Chemical Society
Received: August 11, 2018 Accepted: October 18, 2018 Published: October 18, 2018 38915
DOI: 10.1021/acsami.8b13766 ACS Appl. Mater. Interfaces 2018, 10, 38915−38921
Research Article
ACS Applied Materials & Interfaces significant loss of Li+ diffusivity.22 Furthermore, when synthesized at a relatively low temperature (∼800 °C), a considerable quantity of Li residue is left at the surface, which increases the impedance during Li+ insertion and extraction.23−25 To overcome these technical limitations, additional structural modifications are necessary to achieve stable cycle performance with improved rate capability and safety. In this respect, intensive research has been conducted to enhance the thermal and structural stabilities of Ni-rich layered cathode materials.15,26−28 Recently, Sun et al. proposed a full-gradient Ni-rich NCM composed of a Ni-rich core surrounded by a Mn-rich shell, which showed superior electrochemical performance.29 The outer Mn-rich shell could minimize the side reactions associated with Ni, such as the formation of impurities (e.g., NiO) and structural disorder at the surface, allowing a high reversible capacity and long-term stability. However, it required extra caution during the synthesis to ensure reliability and reproducibility of the materials.29 Otherwise, Lu et al. claimed that surface doping would give surface stability comparable to that of bulk-doped cathode materials.30 In practice, the inevitable capacity loss induced by bulk doping can be minimized by controlling the surface doping with heteroatoms.31,32 Taking into account the advantages of both the promising approaches, we developed a simple Mn surface-doping process to resolve the current problems associated with Ni-rich layered cathode materials. Additional Mn surface doping induces the formation of a thin Mn-rich surface layer, which plays a significant role in mitigating direct exposure of Ni at the surface, to effectively reduce the undesirable side reactions associated with Ni.33 Moreover, by introducing only a thin Mn-rich surface layer, the bulk properties of the cathode materials are not significantly affected.29 Herein, we demonstrate the positive effects of the Mn surface doping through various structural and electrochemical investigations. Our approach offers practical guidelines for developing a highly reliable Ni-rich layered cathode material.
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Electrochemical Measurements. For evaluation of the electrochemical properties, electrodes were fabricated by a conventional slurry coating process. A slurry was prepared by mixing of an active material (95 wt %), a conducting agent (Super P, 2 wt %), and a binder (polyvinylidene fluoride, 3 wt %) dissolved in N-methyl-2pyrrolidone solution. After coating the slurry on the aluminum foil, the electrodes were dried for 12 h at 120 °C, then pressed. The mass of the active material and the area of the electrode were 12 mg and 1.13 cm2, respectively. Coin-type (CR2032) half-cells were subsequently assembled with a counter electrode (Li metal foil) and a porous polyethylene separator in a dry room with a controlled dew point (
