Communication—Enhancing the Electrochemical Performance of Lithium-Excess Layered Oxide Li1.13Ni0.3Mn0.57O2 via a Facile Nanoscale Surface Modification

Journal of The Electrochemical Society, 163 (6) A971-A973 (2016) A971 Communication—Enhancing the Electrochemical Performance of Lithium-Excess Layered Oxide Li 1.13 Ni 0.3 Mn 0.57 O 2 via a Facile Nanoscale Surface Modification Haodong Liu, ∗ Jiajia Huang, Danna Qian, Sunny Hy, ∗∗ Chengcheng Fang, Jian Luo, and Ying Shirley Meng ∗∗,z Department of NanoEngineering, University of California San Diego, La Jolla, California 92093, USA In this work, a facile surface modification with nanoscale equilibrium Li 3 PO 4 -based surface amorphous films (SAFs) has been applied to Lithium-excess layered oxide Li 1.13 Ni 0.3 Mn 0.57 O 2 , which significantly improves the first cycle coulombic efficiency, rate capability, and cycling stability. The nanoscale surface modification can be easily achieved by ballmilling and isothermal annealing. The optimized surface modified Li 1.13 Ni 0.3 Mn 0.57 O 2 is capable of maintaining a high capacity of 201 mAh g −1 after 60 cycles at 55 ◦ C testing with a rate of 1 C. This work provides a facile and scalable surface modification method to improve electrochemical performance of cathode materials for lithium ion batteries. © The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0821606jes] All rights reserved. Manuscript submitted January 5, 2016; revised manuscript received February 16, 2016. Published March 15, 2016. The Lithium-excess layered oxides benefit from an extraordinary high reversible capacity (>280 mAh g −1 ) and is one of the most promising cathode materials for plug-in electric vehicle application. 1–3 However, this high-energy-density material suffers from large irre- versible capacity in its first electrochemical cycle when it is charged to high voltages, namely more than 4.6 V. In addition, its rate capability is yet unsatisfactory for high power application. Moreover, the gradual voltage and capacity degradation upon electrochemical cycling, espe- cially at elevated temperature when side reactions related to the inter- actions with the electrolyte occur more prevalently, represent the most serious technical challenge for this material. 4,5 In the past few years, significant amount of surface modification works have been carried out to protect the surfaces of the Li-excess. Most of the reported surface modifications are performed under solution-based reactions. 6–8 For ex- ample, Bian et al. used LiOH and NH 4 H 2 PO 4 to coat Lithium-excess with Li 3 PO 4 . 7 However, the solution based surface modification adds an additional layer of complexity for preparation of Lithium-excess, which will definitely raise the cost of production. Although Konishi coated high voltage spinel LiNi 0.5 Mn 1.5 O 4 with uniform Li 3 PO 4 via pulsed laser deposition (PLD), this technique requires special equip- ment, which is difficult to realize for large scale production. 9 In this work, we modify the surface of Lithium-excess lay- ered oxide Li 1.13 Ni 0.3 Mn 0.57 O 2 via simple mixing and calcination to form Li 3 PO 4 -enriched and nanometer-thick surface amorphous films (SAFs). 10 Our results indicate that the optimized material shows re- markably improved performance. Experimental The synthesis of the Li-excess is described in our previous publications. 3,6 The procedure for preparing Li 3 PO 4 surface modified Li 1.13 Ni 0.3 Mn 0.57 O 2 (LPLNMO) was adapted from work by Huang et al. 10 0.06 g Li 3 PO 4 (Alfa Aesar, 99.99%) and 3 g Li 1.13 Ni 0.3 Mn 0.57 O 2 were mixed using a planetary ball mill (PM 100, Retsch). The pow- der was calcinated at 500 ◦ C (LP500), 600 ◦ C (LP600), and 700 ◦ C (LP700), respectively, for 5 h in air. Electrochemical test, XRD, SEM and HRTEM characterization are described with details in our previ- ous work. 1,6 Results and Discussion Results.—Figure 1 shows the SEM images of the LNMO and LPLNMOs. All materials show secondary particles with a wide size ∗ Electrochemical Society Student Member. ∗∗ Electrochemical Society Member. z E-mail: shirleymeng@ucsd.edu distribution below 10 μm; the primary particle size distribution ranges from 50 to 200 nm. There were no dramatic changes to the morphology or particle size after Li 3 PO 4 surface modification. HRTEM is used to identify the Li 3 PO 4 coating (SAFs) on LP600. The TEM images show very good layered bulk crystal structure, and amorphous films on the surface of LP600. More than 20 particles are analyzed to examine the amorphous film. As shown in Figure 1i, particles have the amorphous film evenly coated on the surface, with the thickness of ∼2 nm. While the majority of particles exhibit uniform coatings, there are some particles that still show clean surfaces with no indication of surface coatings, as illustrated in Figure 1j. The XRD of LNMO and LPLNMOs are refined and plotted in Figure 2. The major diffraction peaks of these materials are indexed ¯ space group. 1,11,12 No obvious changes were observed to the R 3m in these peaks after Li 3 PO 4 surface modification, suggesting that the layered phase is well retained. The lattice parameters of LNMO are a = 2.8641(3) A and c = 14.253(2) A. Both LP500 and LP600 show no change in lattice parameters, however, LP700 demonstrates ex- panded lattices, which suggests the bulk structure changes under high temperature calcination. Figures 3a–3d illustrate the first charge/discharge voltage profiles of LNMO and LPLNMOs within the voltage window of 2–4.8 V, at 12.5 mA g −1 (C/20). The LNMO delivers a charge capacity of 317 mAh g −1 , and a discharge capacity of 223 mAh g −1 . A low coulombic efficiency of 70.4% has been observed in LNMO. The LP500 demon- strates similar charge capacity of 319 mAh g −1 , but much higher discharge capacity of 261 mAh g −1 . Consequently, the coulombic ef- ficiency of LP500 increases to 81.2%. In contrast, the charge capacity of LP600 decreases to 299 mAh g −1 while the discharge capacity increases to 258 mAh g −1 at the same time. Both factors contribute to a compressed irreversible capacity of 41 mAh g −1 leading to the rel- atively higher coulombic efficiency of 86.3%. Once the calcinations temperature is raised to 700 ◦ C, the LP700 exhibits a high irreversible capacity of 110 mAh g −1 . Figure 3e compares the capacities of these materials at different discharge rates. Among all the materials, LP600 shows the highest coulombic efficiency and best rate capability. The long-term cycling performance of LNMO and LPLNMOs were compared, which is represented in Figure 3f. Over the course of 100 cycles, LNMO, LP500, and LP700 suffer from significant capacity fading. According to the XRD results, the bulk structure of LP700 was changed by high temperature calcination which may accelerate the capacity degradation upon cycling. Although the bulk structure of LP500 is retained, the temperature is not high enough to form an effective coating layer on the layered oxides. 10 Both LNMO and LP600 are cycled for extra 50 times. After 150 cycles, the capacity difference between the LNMO and LP600 increases to 49 mAh g −1 , which is larger than 36 mAh g −1 of the first cycle. Downloaded on 2017-03-31 to IP 137.110.67.168 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use ) unless CC License in place (see abstract).