LiFePO 4 -Coated LiNi 0.5 Co 0.2 Mn 0.3 O 2 Cathode Materials with Improved High Voltage Electrochemical Performance and Enhanced Safety for Lithium Ion Pouch Cells

Abstract

The applicable nickel-rich cathode materials (LiNi 0.5 Co 0.2 Mn 0.3 O 2 , NCM) under high cutoff voltage for high energy density lithium ion batteries shows a broad prospect, but creates enormous challenges for safety. In this work, the LiFePO 4-coated LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM-LFP) composites were prepared on a large scale by the high-speed dispersion and mechanical fusion method, attempting to improve the electrochemical performance and safety of pouch full cells worked under a high cutoff voltage of 4.5 V. Performances have been greatly enhanced, especially for the heat generation during the charge process: the temperature rise of a 5Ah NCM-LFP/MCMB (mesocarbon microbeads) pouch cell was 18.7 • C, much lower than that of a NCM/MCMB pouch cell (42.7 • C). Also, no thermal runaway was observed of NCM-LFP/MCMB pouch cells at 100% state of charge (SOC) during the extrusion and metallic nail penetration tests. The results of ex-situ impedance measurement, high magnification transmission electron microscope (HR-TEM) and in-situ X-Ray Diffraction (XRD) during heating revealed that the enhanced high voltage safety was contributed to the suppressed structure evolution from layer to spinel to rock salt, as a result of a dense and uniform LFP protective layer on NCM. The development of new energy industries, especially electrified vehicles, requires the support of highly efficient energy storage with both high energy and safety. 1,2 Lithium ion battery is considered to be the best solution because of its continuous improvements and commercial applications. 3,4 High specific energy lithium ion battery depends on cathode materials with high specific capacity. 5 Li[Ni 1-x-y Co x Mn y ]O 2 materials are candidate materials with higher theoretical capacity of 200 mAh g −1 than that of traditional cathode materials, such as LiCoO 2 , LiMn 2 O 4 and LiFePO 4. 6-8 Among them, LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM) shows a reversible capacity of 160 mAh g −1 with the voltage range from 2.8 to 4.3 V (vs. Li/Li +), which can be increased to 190 mAh g −1 as the cutoff voltage up to 4.6 V (vs. Li/Li +). 9 However, side reactions are more likely to occur at high voltage, rendering growth of thick solid electrolyte interface (SEI) film on the electrode surface, which increases the impedance of batteries and blocks Li + transmission. 10 In addition, irreversible phase transformation of cathode structure can be accelerated at high voltage, 11,12 accompanied by oxygen release and greatly heats. Given the frequent electric vehicle security accidents, how to manage the safety risk of NCM based lithium ion batteries at high cutoff voltage is urgent for practical application. 13 Recent study indicates an applicable solution toward the issue mentioned above, that is, building the composite with core-shell structure constructed by two or more lithium insertion/de-insertion compounds. 14,15 A central role has been played against the defects of each individual component by the emergence of distinctive lithium ion transmission path between the core and shell components. LiMnPO 4-coated LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) 16 and LiFePO 4-coated NCM cathodes 17 as well as related materials 18 have been constantly reported for advanced performance, such as high discharge capacity , promoted rate and cycling performance. In our previous work, 19 LiFePO 4-coated NCA cathode was designed to enhance the thermal stability. Accordingly, building a stabilized olivine phase LiMPO 4 = These authors contributed equally to this article. z E-mail: tangwp@sina.cn (M = Fe, Mn, Co, etc.) coating layer has been proved to be an effective approach to enhance properties of Ni-rich layered oxides. Nevertheless , most of these works focused on the electrochemical performance or the thermal stability of cathode materials, but less attention was paid on the safety behaviors of full cells with a capacity higher than 1 Ah, especially at high cutoff voltage. Herein, the high voltage safety as well as electrochemical performance of NCM and NCM-LFP cathode materials were studied via 5 Ah pouch cells. NCM-LFP composites in large quantities were successfully synthesized by high speed dispersion and mechanical fusion method. During the preparation, LFP nanoparticles can be totally dispersed by shear force, and then introduced to the surface of NCM by mechanical friction. The NCM-LFP based full cell was demonstrated to show satisfactory electrochemical and safety performance under a high cutoff voltage of 4.5 V. That was attributed to the LFP coating layer, which had a significant impact on the structure transformation of NCM and the growth of SEI film on the electrode surface. Experimental Synthesis.-NCM and LFP (Wei Xu Co., Ltd., China) pristine materials were mixed to obtain NCM-LFP composites by a high speed dispersion and mechanical fusion machine (Wuxi Fuan Powder Equipment Co., Ltd., China) for 15 min at a speed of 400 r min −1. The NCM-LFP samples with different LFP contents (5wt%, 10 wt%, 15 wt%, 20 wt%) were prepared. Materials characterization.-XRD (Rigaku D/max-2600PC, Japan) with Cu-Kα radiation source was applied for the in-situ observation of structure evolution. Samples were scanned over the 2θ range of 10-80 • with a scan rate of 2 • min −1 under different temperatures ranged from 25 to 585 • C with a heating rate of 20 • C min −1. The particle morphologies were characterized using a scanning electronic microscope (SEM, S-4800XL, HITACHI, Japan) and HR-TEM (JEM-2010F, Japan). The thermal stability of NCM and NCM-LFP cathode materials was observed by differential scanning calorimetry) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 141.52.160.164 Downloaded on 2019-02-03 to IP