(Digital Presentation) Lithium Transition Metal Nitride Li7MnN4 (LMN) As Competitive Negative Electrode Material for Li-Ion Battery: Synthesis Optimization and High Energy Density of the NMC/Li5.3MnN4 Full Cell

Abstract

Li-on battery (LIB) is an important technology which is widely used in portable electronic devices, electric vehicles (EV) and other energy storage applications. Commercial LIB has multiple choices as positive electrode materials such as layered transition metal oxides (LiCoO2, LiNi1-y-zMnyCozO2...), spinel oxides LiNixMn2-xO4 as well as olivine LiFePO4. Conversely, the selection of negative electrodes is mainly limited to graphite or Li4Ti5O12 (LTO). Graphite is inexpensive and delivers large capacity but suffers from the formation of solid electrolyte interphase (SEI) as well as Li dendrites formed at high rate, leading to low rate capability and security problems [1]. Li4Ti5O12 (LTO) on the other hand, is able to circumvent the problems of graphite thanks to its higher working potential (1.5 V vs Li+/Li) and minimal structural change during lithiation [2]. However, LTO has a lower energy density than graphite due to a higher working potential and a moderate specific capacity (∼150 mAh g− 1 and 120 mAh g− 1 at 1C and 5C rate, respectively). Therefore, there is a strong need of researching large-capacity insertion-based negative electrode materials working in the 1.0 < V ≤ 1.5 V voltage range to design new generation high energy density full cells.Transition-metal nitrides are considered to be among the most promising class of anode materials for LiBs [3]. Within this family, Li7MnN4 (LMN) [4] with an anti-fluorite 3D structure has received great attention due to its large specific capacity of 280 mAh g-1, excellent cycle stability and appropriate working potential of 1.2 V. We previously showed this material prepared at high temperature exhibits very large particle size [4]. Therefore, a crucial post-synthesis ball-milling step was required to benefit from the maximum capacity and high rate capability. However, this ball-milling step is hard to reproduce due to its dependence on many instrumental factors such as jar geometry, ball/material mass ratio [4].In this work, an optimization of the synthesis conditions of LMN is proposed and new key parameters controlling the particle size distribution (PSD) are identified, allowing the suppression of the post-synthesis ball-milling process. Thanks to the specific morphology attained when using our optimized synthesis conditions, the as-synthesized LMN material is able to deliver larger capacity at higher rate (265 mAh g− 1 and 160 mAh g− 1 at 1C and 5C rate, respectively). These capacity values are the best to our knowledge and compete with that of benchmark LTO. Furthermore, the lower working potential of LMN (1.2 V, i. e. 0.35 V lower than LTO) is expected to provide larger energy density in a full cell device compared to LTO.To carry this argument further, NMC/LMN full cell is constructed for the first time with LiNi0.6Mn0.2Co0.2O2 and pre-delithiated LMN (Li5.3MnN4). This NMC/LMN coin cell is applied for galvanostatic cycling at different current densities while a 3-electrode cell using metallic Li as reference electrode is used to clarify potential changes during the charge-discharge process. We show the NMC/LMN full cell replicates the electrochemical performance of NMC/Li half-cell in the 3.2 V - 2 V potential window. These results prove the feasibility and compatibility of the NMC/LMN full cell and the suitability of delithiated LMN as negative electrode material. Remarkably, the maximum energy density of the NMC/LMN full cell, of 256 Wh/kg(based on total active materials mass loading), is 30% to 50% higher than that exhibited by a NMC/LTO full cell.[1] T. Waldmann, B. I. Hogg, and M. Wohlfahrt-Mehrens, “Li plating as unwanted side reaction in commercial Li-ion cells – A review,” J. Power Sources, vol. 384, no. November 2017, pp. 107–124, 2018.[2] T. Ohzuku, A. Ueda, and N. Yamamoto, “Zero‐Strain Insertion Material of Li [ Li1 / 3Ti5 / 3 ] O 4 for Rechargeable Lithium Cells,” J. Electrochem. Soc., vol. 142, no. 5, pp. 1431–1435, 1995.[3] J. M. Tarascon and M. Armand, “Issues and challenges facing rechargeable lithium batteries,” Mater. Sustain. Energy A Collect. Peer-Reviewed Res. Rev. Artic. from Nat. Publ. Gr., vol. 414, no. November, pp. 171–179, 2010.[4] E. Panabière, N. Emery, S. Bach, J. P. Pereira-Ramos, and P. Willmann, “Ball-milled Li7MnN4: An attractive negative electrode material for lithium-ion batteries,” Electrochim. Acta, vol. 97, pp. 393–397, 2013. Figure 1